Download Kozlowski, T.T. 2002. Physiological ecology of natural regeneration

Forest Ecology and Management 158 (2002) 195±221
Physiological ecology of natural regeneration of harvested and
disturbed forest stands: implications for forest management
T.T. Kozlowski*
Department of Environmental Science, Policy, and Management, University of California, Berkeley, CA 94720, USA
Received 24 July 2000; accepted 26 November 2000
Abstract
Forests are indispensable as sources of harvested products and a variety of services. Unfortunately forests throughout the
world, and especially in the tropics, are threatened by natural and human-induced disturbances. Regeneration of harvested or
otherwise severely disturbed forest stands typically occurs in four stages: (1) stand initiation and regeneration stage, (2)
thinning or stem exclusion stage, (3) transition or understory regeneration stage, and (4) steady-state or old-growth stage.
Blocks to stand regeneration may occur in each of these stages. Revegetation of severely-disturbed forests often is very slow
and unpredictable because of complex interactions among propagules as well as site and climatic conditions. Stand
regeneration depends on an abundant and viable seed supply, a suitable medium for seed germination, favorable environmental
conditions, and capacity for sprouting or layering of some species. Massive losses of seeds in seed banks occur because of seed
aging, failure of seed germination, predation, diseases, and death of seeds. Mortality of seedlings also is high, especially in the
cotyledon stage of development, because of low reserves of carbohydrates and mineral nutrients. Growth of older seedlings,
saplings, and mature trees is inhibited by air and soil pollution, drought, ¯ooding, soil compaction, insect attacks, and diseases.
After canopy closure occurs in a developing forest stand changes in species composition are traceable to competition among
plants for light, water, and mineral nutrients. Following severe disturbance of a multi-aged, mixed forest, a mixed-species
stand of fast growing, generally short-lived trees typically occupies a site and is succeeded by species that dominated the stand
before the disturbance occurred. After a minor disturbance gaps form in the canopy and are recolonized. If plant succession in
a developing stand runs its full course the resulting old-growth forest is impacted by frequent minor disturbances and is
maintained in an oscillating steady-state in which stand establishment, thinning, gap formation, and colonization recur. In
some regions frequent ®res maintain forest stands in a subclimax stage of development. Natural regeneration of harvested or
otherwise severely disturbed stands likely will be too slow and unpredictable to provide all the forest products and services
required by increasing population growth. Hence, greater emphasis on several concurrent strategies will be needed, including
heavy emphasis on arti®cial regeneration of disturbed forests; conservation of the remaining tropical forests; expansion of
plantations, agroforestry systems, and forest reserves; expansion of tree improvement programs; more intensive and improved
forest management; and expanded research with particular emphasis on seed biology, responses of tree species and genotypes
to environmental stresses, tradeoffs between bene®cial and harmful effects of environmental stresses on tree species and
genotypes, genetic engineering, potential effects of global warming on forests, agroforestry systems, models of stand
productivity; and appraisal by remote sensing of injury to forest ecosystems. # 2002 Elsevier Science B.V. All rights
reserved.
Keywords: Stand regeneration; Seed production; Seed germination; Competition; Succession; Management strategies
*
Present address: 2855 Carlsbad Blvd., S-326 Carlsbad, CA 92008, USA.
0378-1127/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 8 - 1 1 2 7 ( 0 0 ) 0 0 7 1 2 - X
196
T.T. Kozlowski / Forest Ecology and Management 158 (2002) 195±221
1. Introduction
Forests are enormously important to mankind. They
not only supply essential harvestable products (e.g.
wood, food, medicines, oils, gums, resins, and
tannins), but also ornament landscapes; regulate
climate, hydrology, mineral cycling, and soil erosion;
create wildlife habitats; and purify the air and water
(Gordon, 1996; Ferguson, 1996; Chapin et al., 1998;
Kaufman et al., 1999). Unfortunately, the goods and
services that forests can supply are threatened by a
wide range of abiotic and biotic stresses. For example,
disturbances of forests by excessive harvesting,
insects, diseases, drought, ¯ooding, pollution, ®re,
and soil compaction often result in catastrophic losses
of both forest products and services (Kozlowski and
Pallardy, 1997b; Kozlowski, 2000).
There is particular world-wide concern about the
high rate of loss of tropical forests, largely for wood,
farming, and ranching (Ramakrishnan, 1995; Potter,
1999). Skole and Tucker (1993) estimated current
deforestation in the Amazon Basin alone to be
380,000 km2 per year. About half of the destruction
of tropical forests has been attributed to farming and
one-fourth to cutting for lumber (Myers, 1996).
Estimates of changes in the world's forested areas
show a net loss of 56.3 million ha during 1990±1995.
This value re¯ects a decrease of 65.1 million ha of
forest in developing countries and an increase of 8.8
million ha in developed countries (FAO, 1997).
In contrast to temperate forests most of the mineral
pool in tropical forests is stored in the trees rather than
in the soil (White, 1983; Jordan, 1985). Hence, ``slash
and burn'' agriculture in the tropics inevitably leads to
serious losses of nutrient capital. During repeated
cultivation of cutover tropical forest lands nutrient
pools are progressively depleted in harvested food
plants and by leaching and volatilization. As a result,
these previously-forested lands rapidly become unproductive, degraded, eroded, and waterlogged. Conversion of tropical forests to pastures also adversely
affects the nutrient capital of these lands (Jordan, 1985;
Medina, 1991). Several investigators have con®rmed
that extensive deforestation in the tropics is followed
by soil erosion, reduction in rainfall, lowering of the
water-holding capacity of soil, increased ¯ooding, siltation of streams, and emission of CO2 to the atmosphere,
with the latter effect contributing to potential global
warming (Ramakrishnan, 1995; Houghton, 1995, 1998;
Salati, 1997). Once a tropical forest is cleared of trees
regeneration to primary forest may take many decades
(Kozlowski, 1979).
Temperate-zone forests are much more resilient than
tropical forests to disturbance. When only some of the
trees in a temperate forest are harvested, mineral losses
typically are replaced by decomposing organic matter,
weathering of soil minerals, N ®xation, and atmospheric deposits. Even clear-cutting of temperate forests on long rotations and with slash left behind may not
seriously deplete the nutrient capital of a site (Kozlowski et al., 1991; Kozlowski and Pallardy, 1997a).
Plantations of forest trees can appreciably compensate for losses of harvestable products from disturbed
natural forests (Dekker-Robertson and Libby, 1998).
Nevertheless, as the world's population grows, plantations alone are unlikely to satisfy human needs for all
the bene®ts that forest ecosystems will be expected to
supply. For example, there is concern about potential
decreases in biomass production by second and
subsequent rotations of plantations in Europe (Wormald, 1992), Australia (Keeves, 1996), and China (Ding
and Chen, 1995; Sun Cuiling et al., 1997). Plantations
often lead to reduced energy ¯ow through ecosystems,
less nutrient cycling than in natural forests, and loss of
mineral nutrients (Springett, 1976; Feller, 1978; Ewel
et al., 1991). Short rotations of fast-growing trees in
plantations often result in loss of soil fertility (Boyle,
1975; Evans, 1992). Plantations also provide conditions for spread of insect pests and diseases (Speight
and Speechly, 1982; Zobel and Talbert, 1984; Zobel
et al., 1987; Day and Leather, 1997; Dick, 1998).
Moreover, most plantations lack the biodiversity
needed for providing the raw materials that are essential for human activities (Pimental et al., 1992, 1997;
Old®eld, 1995; Solbrig et al., 1996; Reaka-Kudla et al.,
1997). Loss of biodiversity will lead to current or
potential irreplaceable losses of foods, ®bers, and
medicinal plants. Environmental stresses imposed on
mixed-species forests may decrease biodiversity if
they (1) alter genetic diversity within populations, (2)
decrease reproductive potentials of species, (3) reduce
crop yields or products of natural vegetation, or (4)
adversely affect ecosystem structure and function
(Barker and Tingey, 1992).
There is a compelling need for establishing more
forest reserves on productive sites in order to protect
T.T. Kozlowski / Forest Ecology and Management 158 (2002) 195±221
wild genetic resources (Noss and Cooperrider, 1994;
McNeely, 1994). Unfortunately many forest reserves
have been and are being eliminated from unreserved
lands. Furthermore, many existing reserves were
established on unproductive sites characterized by
low diversity and with relatively few important species
of forest trees (Lindenmayer and Franklin, 1997).
Hence, serious questions have been raised about reliance on existing forest reserves for conserving the
important values of forest ecosystems (Franklin, 1993).
Combinations of herbaceous and woody plants in
agroforestry systems are important, especially in the
tropics, for increasing yields of forest products, thus
relieving pressure for harvesting of natural forests
(Torres, 1983; Nair, 1989). Although approximately
2000 species of woody plants have shown some
promise in agroforestry systems (Von Carlowitz,
1985) much needs to be learned about selection of
appropriate species and genotype mixtures as well as
management practices for agroforestry systems.
Because of the foregoing considerations this review
will emphasize that (1) natural regeneration of disturbed forest ecosystems is very slow and fraught with
problems, and (2) impending increases in population
growth will mandate a variety of concurrent managerial initiatives to supply the essential products and
services that will be expected of forest ecosystems.
2. Stages of stand regeneration
Natural revegetation of harvested or otherwise disturbed forest stands typically occurs in four sequential
stages, including (1) a stand initiation and regeneration stage, (2) a thinning or stem exclusion stage, (3) a
transition or understory regeneration stage, and (4) a
steady-state or old-growth stage (Oliver, 1981; Oliver
and Larson, 1996). Each of these stages exhibits
some unique characteristics. Disturbances during any
of these stages may induce physiological dysfunctions in trees and result in failure of adequate stand
regeneration.
2.1. Stand initiation and regeneration stage
Following tree harvesting or disturbances forest
stands regenerate through interactions among propagules, including seeds in seed banks and those
197
dispersed into a site as well as sprouting or layering
of residual trees, and soil and climatic conditions
(Kozlowski, 1971).
2.1.1. Seed banks
Regeneration of forest stands may occur from seeds
stored in the soil, or in serotinous (late-to-open) cones
in forest canopies. Some species (e.g. Pinus contorta,
P. banksiana, P. rigida, P. clausa, P. attenuata, and
Picea mariana) store viable seeds in cones that remain
unopened while still attached to trees for many years.
When the resinous material on serotinous cones is
destroyed by forest ®res the cone scales open and the
seeds are dispersed (Kozlowski et al., 1991; Kozlowski
and Pallardy, 1997b).
Both wind and animals play variable roles in
establishing seed banks in disturbed forest stands. In
temperate deciduous forests the seeds of Acer are
dispersed by wind; those of Fagus by mammals or
birds. In tropical forests dispersal of seeds of lianas
depends more on wind than does dispersal of tree
seeds. Dissemination of seeds by wind also occurs
more commonly in dry tropical forests than in wet
tropical forests (Leck, 1995). Most of the winddispersed seeds in forests are those of canopy trees or
vines (Keay, 1957).
Wind-dispersal distances vary greatly among seeds
of different tree genera in accordance with the weights
and structural modi®cations of seeds such as hairs
(Salix, Populus, Platanus), wings (certain gymnosperms, Acer, Alnus, Betula, Dalbergia, Erythrina,
Liquidambar, and Tilia). Whereas the heavy seeds of
Pinus pinea are shed almost vertically, the small,
large-winged seeds of Pinus radiata are disseminated
for great distances (Wilgen and von Siegfried, 1986).
The birds and mammals that carry seeds into
disturbed forest stands improve germination capacity
by partially digesting the seed coats. As the structural
complexity of a forest stand increases, seed dispersal
by birds becomes progressively important. Insects and
bats also affect seed production by acting as pollinators
(Darley-Hill and Johnson, 1981; Crawley, 1983; Leck
et al., 1989; Leck, 1995; Waring and Running, 1998;
Withgott, 1999).
The full potential of seed banks for regenerating
cutover and disturbed forest stands is never realized
because of massive losses of seeds and young seedlings.
Such losses have been attributed to seed aging, failure of
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T.T. Kozlowski / Forest Ecology and Management 158 (2002) 195±221
seeds to germinate, predators, pathogens, seed redispersal, and seed and seedling mortality (Leck, 1995).
After a heavy seed rain vast numbers of seedlings often
emerge but most of them are desti-ned to die shortly.
Forest trees face their greatest mortality risk in the
ungerminated embryo stage of the seed or cotyledon
stage of seedling development. Many seeds fail to
germinate because of dormancy; others because of
unfavorable environmental conditions. Mortality of
seedlings in the cotyledon stage is particularly high
because of their low reserves of carbohydrates and
mineral nutrients. Hence, even mild environmental
stresses often lead to seedling mortality (Kozlowski,
1976, 1979). Extremely high seedling mortality (up to
90%) during the ®rst year after emergence has been
reported for Pseudotsuga menziesii (Gashwiler, 1967;
Tappeiner and Helms, 1971), Eucalyptus spp. (Jacobs,
1955), Pinus thunbergii (Tazaki et al., 1980), and Acer
saccharum (Hett and Loucks, 1971).
2.1.2. Seed production
Greatest success in regeneration of forest stands
almost always occurs after years of unusually heavy
seed production (Smith, 1995). Accumulation of seeds
in seed banks varies appreciably depending on the size
of the seed crop and regularity of seed production by
different species of plants. For example, Abies grandis
and Pinus monticola produce relatively small seed
crops, whereas Tsuga heterophylla produces large
crops. Some species (e.g. Juglans nigra and Liriodendron tulipifera) have large seed crops at irregular
intervals; others (e.g. A. saccharum and Fraxinus
americana) at fairly regular intervals. In Alaska
Betula papyrifera produces large seed crops once
every 4 years on average; Picea glauca once every 10±
12 years (Perry, 1995). Some species (e.g. Magnolia
grandi¯ora) produce good seed crops annually.
To assist regeneration of cutover stands a few seed
trees often are left behind. Seed trees are selected for
high seed production by having large crowns, a high
live-crown ratio (percent of length of stem with large
branches) and strongly tapering stems (Kozlowski
et al., 1991).
2.1.3. Seed longevity
The life span of seeds of different species, which
varies from a few days to years, in¯uences their
potential for stand regeneration. Relatively short-lived
seeds of temperate zone woody plants include those of
Aesculus, Betuia, Carya, Populus, Quercus, Salix,
Taxus, and Ulmus. Quercus acorns and Liquidambar
seeds typically do not remain viable in the soil for
more than 1 year, whereas Liriodendron seeds retain
viability for up to 8 years (Little, 1974). Seeds of many
tropical woody plants are very short-lived, including
those of Cinchona, Cocos, Coffea, Erythroxylum,
Hevea, Macadamia, and Thea (Kozlowski and Pallardy, 1997b). By comparison Albizia and Hovea seeds
may remain viable for over 100 years (Osborne, 1980).
It has been estimated that seeds of the chaparral plant,
Ceanothus velutinus, may retain viability in forest
litter for over 500 years. On sites where chaparral
shrubs have died, because of shading or old age, C.
velutinus stimulates the dormant seeds to germinate
(Zavitkovski and Newton, 1968).
During seed aging an initial decrease in germination
capacity is followed by susceptibility to attacks by
microorganisms, progressively shorter emerging radicles, and failure of cotyledon emergence as preludes
to seed cling mortality. As seeds senesce their capacity
for synthesizing proteins, lipids, RNA, enzymes, and
repair systems declines, and injury to cell membranes
and chromosomes increases (Harrington, 1972;
Osborne, 1980; Roos, 1982; DeCastro and Martinez-Honduvilla, 1984; Priestley, 1986; Pukachka and
Kuiper, 1988).
2.1.4. Seed germination
The germination of seeds involves resumption of
embryo growth and seed coat rupture, followed by
seedling emergence. During germination the radicle
typically elongates ®rst and enters the soil. The thin
seed leaves (cotyledons) of most gymnosperms and
angiosperms emerge above ground (epigeous germination). Exceptions are the ¯eshy cotyledons of some
angiosperms (e.g. Quercus, Juglans, Hevea) which
remain below ground (hypogeous germination).
Epigeous cotyledons are important largely as photosynthetic organs and sometimes also for food storage.
However, their storage function varies appreciably
among species. Whereas the cotyledons of Acer
negundo and Robinia pseudoacacia store large
amounts of carbohydrates, those of Ailanthus altissima and Fraxinus pennsylvanica store only small
amounts of carbohydrates but large amounts of lipids
(Marshall and Kozlowski, 1976b).
T.T. Kozlowski / Forest Ecology and Management 158 (2002) 195±221
Seed germination is characterized by cell division
and expansion in the growing points of the embryo as
well as intense physiological activity. An increase in
seed hydration is accompanied by accelerated respiration (Vertucci and Leopold, 1987) and release of
hormones that stimulate synthesis of enzymes
involved in digestion of foods in storage tissues.
Starches are hydrolyzed to sugars; fats to glycerol and
fatty acids, with most of the latter converted to sugars
by way of acetyl coenzyme A. Reserve proteins are
broken down by proteolytic enzymes to soluble N
compounds. The products of hydrolysis are then
translocated to growing points where existing tissues
expand and new tissues are produced. Simple
carbohydrates are converted into cellulose; pectic
compounds and lignin into new cell walls; amino acids
and amides into the protein framework and enzymes of
new protoplasm (Kozlowski and Pallardy, 1997a,b).
2.1.5. Seed dormancy
Because seeds of most species of woody plants
exhibit some degree of dormancy they may not
germinate promptly under ostensibly favorable environmental conditions. Long postponement of seed
germination often accounts for failure of adequate
regeneration of forest stands.
Seeds may be dormant for a variety of reasons,
including (1) seed immaturity, (2) seed coat impermeability to water and/or oxygen, (3) seed coat resistance
to embryo growth, (4) metabolic blocks in the embryo,
and (5) various combinations of these. Sometimes
viable seeds without primary dormancy lapse into a
state of secondary dormancy when exposed to unfavorable environmental regimes (Mayer and PoljakoffMayber, 1989).
Physiological maturity of seeds is associated with
maximum germination capacity. Seeds usually are
physiologically mature when resources from the
mother plant have stopped moving into seeds. A
variety of indicators have been used to judge maturity
of seeds and fruits. These include color changes in
fruits and cones (Schopmeyer, 1974), critical embryo
size (Ching and Ching, 1962; Mercier and Langlois,
1992), amounts of stored seed sugars or fats (Rediske
and Nicholson, 1965), and conductivity of seed
leachates (Sahlen and Gjelsvik, 1993).
Dormancy associated with impermeability of seed
coats is common in many legumes (e.g. Robinia spp.,
199
Gleditsia spp.) and some non-legumes (e.g. Tilia spp.,
Malus spp., and Pinus strohus). Examples of tropical
species with impermeable seed coats include Podocarpus spp., Parkia javanica and Sindora coriacea
(Sasaki, 1980a,b).
Seed dormancy is most commonly physiological
and involves failure of morphologically mature
embryos to germinate. Embryo dormancy appears
to be regulated by complex interactions of hormonal
and other internal factors in the embryo and surrounding tissues. These interactions are in¯uenced by availability of light, water, and suitable temperature (Khan
and Samimy, 1982; Battaglia, 1989). Physiological
embryo dormancy often cannot be reversed by the
same environmental conditions that induced it.
In some species failure of seeds to germinate results
from both restriction of a thick pericarp on expansion
of the embryo and also on physiological embryo
dormancy. Such double dormancy may occur in Ilex
spp., Rosa spp., Taxus spp., Tilia spp., Cladrastis
lutea, Pinus sabiniana, P. cembra, and P. albicaulis
(Jackson and Blundell, 1963; Kozlowski and Pallardy,
1997b). In seeds of some species of Acer, including
A. ginnala, A. negundo, A. pseudoplatanus, and A
velutinurn, seed dormancy has been attributed to hard
seed coats; in other species (A. platanoides, A.
saccharum, and A. tataricum) to embryo dormancy
(Pin®eld and Dungey, 1985; Pin®eld et al., 1987;
Pin®eld and Stutchbury, 1990). Dormancy of Quercus
nigra acorns was caused by mechanical strength of the
pericarp, chemical inhibition of embryo growth by the
pericarp, and slow imbibition of water (Peterson,
1983).
2.1.6. Seedling development
During the earliest phases of seed germination cell
division occurs throughout the embryo. Shortly thereafter cell division becomes localized in the apices of
shoots and roots of young seedlings. A system of
branches soon forms by development of leaves, nodes,
and internodes from shoot apical growing points.
Additional branches are produced later as a result of
expansion of apical meristems in the lateral buds of bud
clusters located at stem and branch apices, and also
from elongation of adventitious and dormant buds
distributed over the stem and branches. A taproot or
primary root develops from the root apical meristem.
The ®rst root either elongates and branches profusely
200
T.T. Kozlowski / Forest Ecology and Management 158 (2002) 195±221
or dies back. Many lateral roots arise from a deep
pericycle layer (Kozlowski and Pallardy, 1997a,b).
Reserve foods in the seed (which may include
variable amounts of carbohydrates, fats, and proteins)
sustain the growing embryo until epigeous cotyledons
and/or foliage leaves develop suf®ciently to provide
photosynthetic products to meristematic sites. Heavy
seeds usually store large amounts of carbohydrates
and sustain young seedlings better during environmental stresses than do small seeds (Buckley, 1982;
Jones et al., 1994).
2.1.7. Vegetative propagation
Revegetation of some species in cutover or disturbed
stands occurs by sprouting from stumps or roots
(Kozlowski and Pallardy, 1997a; Hoffmann et al.,
2000). The sprouts that arise from root collars and
lower parts of stems emanate from dormant buds that
grow outward just under the bark. They typically
develop into shoots only when growth of the tree crown
is disturbed. Such sprouts account almost entirely for
reproduction of some broad-leaved forests (e.g. Quercus spp.) in the eastern US (Braun, 1950). In Australia
recruitment of seedlings after ®re was closely correlated with sprouting capacity (Benwell, 1998). Most
chaparral species produce many stump sprouts. Not all
do, however, and various species grow in combinations
of intermixed sprouting and non-sprouting species
(Biswell, 1974). Some species of broad-leaved trees
regenerate largely by root sprouts (root suckers) that
emanate from adventitious buds on roots. Such buds
arise anew from cambial tissues. Both B. papyrifera and
Populus tremuloides sprout abundantly after ®re and
grow much faster than seed-producing conifers during
the ®rst few years (Ahlgren, 1974; Dickmann and
Stuart, 1983). Reproduction by root suckers also may
occur in Fagus, Ailanthus, Liquidambar, Robinia, and
Nyssa (Kozlowski et al., 1991). Although regeneration
by sprouting is relatively unimportant in gymnosperms,
a few species (e.g. Sequoia sempervirens, Pinus rigida,
P. echinata, P. serotina, and Taxodium distichum) may
sprout proli®cally, especially after ®re (Kozlowski et al.,
1991). On some sites the capacity to form adventitious
roots or reproduce by root suckers enables early
successional species to tolerate the sedimentation that
accompanies ¯ooding (Yarie et al., 1998).
Some species regenerate by both sexual and
vegetative reproduction. In the Mediterranean region
many woody plants survive ®re by sprouting from
dormant buds on the root crown and also from
adventitious buds on lateral roots, stems, and old
shoots. Regeneration also is supplemented by ®restimulated seed germination. Naveh (1974) classi®ed
woody plants of Israel into (1) obligatory resprouters
that depend entirely on vegetative propagation (e.g.
Quercus coccifera, Q. calliprinos, and Pistacea
lentiscus), (2) facultative resprouters that regenerate
by both sprouting and seed germination (e.g. Calycotome villosa, Poterium spinosum, Erica arborea,
Arbutus andrachne, and A. unedo).
Some species are propagated by formation of roots
on branches (layering). Although not very common,
layering has been described for a number of genera of
woody plants including Abies, Acer, Chamaecyparis,
Cornus, Clethra, Picea, Taxus, and Viburnum (Lutz,
1939). Layering also is common in chaparral shrubs at
high elevations where the weight of snow forces
branches and stems down to the ground. Important
shrubs that layer readily include Ceanothus cordulatus, Arctostaphylos patula, C. velutinus, and Quercus
vaccinifolia (Biswell, 1974). In the Rocky Mountains
dwarfed Picea engelmannii trees near the timber line
reproduce by layering of their lower branches (Wardle,
1968). P. mariana and Thuja occidentalis in peat
swamps may also reproduce by layering.
2.1.8. Environmental regulation of stand
regeneration
2.1.8.1. Site characteristics. Wide differences among
seedbeds in physical characteristics, supplies of water
and mineral nutrients, and temperature often account
for variations in regeneration of disturbed forest stands.
When revegetation occurs from seeds the species that
appear depend heavily on physical characteristics of
the surface on which the seeds land. The most important determinants of success in reestablishment are the
capacity of the surface medium to supply water and the
amount of light that reaches the young seedlings. The
denser the soil medium the better is its capacity to
supply water to both seeds and germinants (Smith,
1995).
Mineral soil generally is a good seedbed because of
its high water in®ltration capacity, favorable aeration,
and good hydraulic contact between soil particles and
seeds (Winget and Kozlowski, 1965). Litter and duff
T.T. Kozlowski / Forest Ecology and Management 158 (2002) 195±221
usually are less suitable because they do not warm
rapidly and impede penetration of radicles, thereby
preventing contact with mineral soil (Facelli and
Pickett, 1991). Sphagnum moss and decayed wood
usually are good seedbeds because of their high waterholding capacity (Place, 1955). Although P. menziesii
seeds germinated readily on a variety of seedbeds if
adequate moisture was available, only those seedlings
with roots that rapidly contacted the mineral soil and
were exposed to direct solar radiation survived
(Weaver, 1974). Bumper seed crops of Pinus strobus
in Minnesota forests every 3±5 years were followed by
emergence of very many young seedlings. However,
most of these seedlings died because their roots could
not penetrate the thick dry humus to the wet mineral
soil and because of their intolerance of heavy shade
(Ahlgren, 1974).
Mild compaction of soil may bene®t stand regeneration by improving capillary movement of water to
seeds. Severe compaction, which is much more
common, inhibits both seed germination and seedling
growth, and also induces early seedling mortality.
Compaction of soil is common in recreation areas such
as picnic sites, timber harvesting sites, fruit orchards,
agroforestry systems, and tree nurseries (Kozlowski,
1999, 2000).
Compaction induces a variety of changes in soil
structure and hydrology. It increases soil bulk density
(mass per unit volume) (Purser and Cundy, 1992; Jim,
1993; Miller et al., 1996), decreases total pore volume
(Currie, 1984) and the proportion of macropore space
(Willatt and Pullar, 1983; Huang et al., 1996; Cassel,
1983; Malmer and Grip, 1990; Kramer and Boyer,
1995), increases water runoff (Kang and Lal, 1981;
Nortcliff et al., 1990; Anderson and Spencer, 1991),
and decreases soil aeration (Greacen and Sands, 1980;
Wolkowski, 1990; Stepniewski et al., 1994; Horn et al.,
1995). The many changes in soil structure and
hydrology associated with compaction often lead to
severe physiological dysfunctions in both seeds and
seedlings. Water absorption usually is inhibited by
plants and leaf water de®cits follow. The rate of
photosynthesis is reduced by both stomatal and
nonstomatal inhibition (Kozlowski, 1999).
2.1.8.2. Drought. The initial requirement for seed germination is imbibition of enough water to catalyze
essential physiological processes in the embryo. Very
201
small amounts of water, usually only two to three times
the weight of the seed, are necessary to soften seed coats
and stimulate metabolic processes. After germination
is completed, progressively greater amounts of water
must be absorbed as the amount of foliage and transpirational water loss increase (Koller, 1972).
Most viable seeds (except those with impermeable
seed coats or impermeable layers under the seed coats)
can imbibe enough water for germination from soil at
®eld capacity (Kaufmann, 1969). However, water
absorption and germination capacity of seeds generally decrease rapidly as the soil dries below ®eld
capacity (Satoo, 1966). Nevertheless, the tolerance of
tree seeds to desiccation varies widely. Seeds of
Quercus spp., Salix spp., Taxodium spp., and Ulmus
spp. are more intolerant of desiccation than seeds of
Pinus spp., Liquidambar spp., or Eucalyptus spp.
(Kozlowski et al., 1991). The effect of drought on seed
germination also varies with the temperature regime
(Scifres and Brock, 1969). On most sites the majority
of young tree seedlings die from desiccation during
periodic droughts.
2.1.8.3. Flooding. Inundation of soil deprives seeds
of the oxygen necessary for respiration. Hence, seed
germination of many species is postponed or prevented by flooding (Kozlowski, 1985a; Kozlowski
and Pallardy, 1997b). However, the germination
capacity of flooded seeds varies appreciably among
species and genotypes. Usually soaking of seeds of
many upland species for hours to a few days stimulates germination; soaking for longer periods inhibits
germination (Kozlowski, 1997). Seeds of some wetland species (e.g. Salix nigra) germinate readily
under water (DeBell and Naylor, 1984). The seeds
of Nyssa aquatica and T. distichum may retain
viability when submerged for as much as 2 years.
Seeds of these species typically germinate during dry
periods when the soil surface is exposed (Smith and
Linnartz, 1980).
Once seedlings emerge their survival often depends
on rapid growth and capacity to protrude above water
level. In a ¯oodplain forest species with abundant food
reserves in the large cotyledons (e.g. Q. nigra) had
high survival rates, whereas seedlings of small-seeded
species (e.g. Acer rubrum, Ulmus americana) and low
cotyledonous food reserves showed low survival
(Streng et al., 1989).
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2.1.8.4. Temperature. Both breaking of seed dormancy and subsequent germination are very responsive to temperature. In nature embryo dormancy in
seeds of most temperate zone woody plants is broken
by prolonged exposure to near-freezing temperature.
Thereafter, much higher temperature regimes are
required to stimulate the metabolic activity necessary for germination (Mayer and Poljakoff-Mayber,
1989). Dormancy of seeds with hard seed coats
sometimes can be broken with high temperatures.
For example, seed germination of some shrub species
depends on fire to rupture the seed coat or melt seed
coat waxes. Both heat and the action of fire scarified
the seed coats of Ceanothus integerrimus, thereby
increas-ing the capacity of the seeds to imbibe water
(Biswell, 1974). A portion of the seed crop of some
shrubland species (e.g. Adenostoma fasciculatum)
germinated after a short period of ripening while the
rest of the crop required heat to germinate (Christensen,
1995).
Optimal temperatures for germination vary appreciably for seeds of different species. They are much
higher for seeds of tropical species than those of
temperate zone species (Bradbeer, 1988; Bewley,
1994). Seeds of many species will germinate over a
wide temperature range (above a critical lower level).
Examples are seeds of P. contorta (Critch®eld, 1957)
and Picea engelmannil (Kaufmann and Eckard, 1977).
However, there are exceptions and seeds of some
species (e.g. Acer platanoides) germinate over only a
narrow temperature range (Roe, 1941).
2.1.8.5. Solar radiation. Both seed germination and
early seedling development are variously influenced
by light intensity, light quality, and photoperiod. Seeds
of most temperate-zone woody plants do not have a
rigid light intensity requirement for germination, but a
few do. For example, seeds of Betula and Pinus
require very low illumination for germination. In
forests dominated by Tsuga canadensis availability
of light influenced seedling regeneration. Betula spp.
seedlings were particularly sensitive, with low light
reducing emergence by 43%, decreasing seedling
growth by 99% and survival by 94%. Shading had
less effect on performance of T. canadensis and P.
strobus (Catovsky and Bazzaz, 2000). The seeds of
many tropical species also require some light for
germination (Hall and Swaine, 1980; Whitmore,
1983; Orozco-Segovia et al., 1993). Hence, seeds
buried in soil or seeds in litter of these species may
undergo enough shading to impede their germination.
Seed germination of some species is also in¯uenced
by photoperiod. Whereas earliest and greatest total
germination of most light-sensitive seeds occurred in
daily light periods of 8±12 h, germination of Eucalyptus sp. seeds was especially high in 8 h days and of
Betula sp. seeds in 20 h days (Olson et al., 1959; Jones,
1961).
Wavelength also has some effect on germination of
seeds of several species including Betula pubescens,
Pinus palustris, P. strobus, P. thunbergii, and P.
virginiana. The effect of wavelength is regulated by
the phytochrome pigment system, with germination
stimulated by red light and inhibited by far-red light.
Both metabolism and cell division in embryos are
accelerated by red light and arrested by far-red light
(Nyman, 1961; Taylorson and Hendricks, 1976; Hart,
1988). Seed germination sometimes is inhibited by the
low red to far-red ratio of light transmitted by the litter
layer on the forest ¯oor. However, stimulation of
germination by red light varies considerably with
prevailing temperature and seed hydration (Toole
et al., 1961).
The importance of light intensity for seedling
growth is emphasized by the necessity of maintaining
high rates of photosynthesis by seedlings very early in
their development. Germination of pine seeds is
typically followed by sequential development of
cotyledons, primary needles, and secondary needles.
Development of primary needles depends on cotyledon photosynthesis; development of secondary needles on photosynthate produced by the primary
needles (Sasaki and Kozlowski, 1968a,b, 1970). When
light intensity was low immediately after Pinus
resinosa seeds germinated only few primary needles
and no secondary needles emerged. When these
shade-stressed seedlings were then exposed to higher
light intensities the primary needles readily expanded
and were followed by emergence and development of
a normal complement of secondary needles.
The importance of cotyledon photosynthesis for
development of seedlings of broad-leaved trees was
shown by suppression of seedling growth after the
cotyledons of R. pseudoacacia were excised or
cotyledon photosynthesis was inhibited by a chemical
such as 3-[3,4-dichlorophenyl] 1-1, dimethylurea
T.T. Kozlowski / Forest Ecology and Management 158 (2002) 195±221
(DCMU) (Marshall and Kozlowski, 1974, 1976a,
1977).
2.1.8.6. Pollutants and agricultural chemicals.
Regeneration of forest stands often is adversely
affected by a wide array of chemicals in the air and
soil (Mudd and Kozlowski, 1975; Keller, 1983;
Kozlowski, 1985b,c). Such compounds include
some gaseous and particulate pollutants, herbicides,
pesticides, growth retardants, and antitranspirants.
Arrested stand regeneration may be associated with
lowered seed production and inhibition of seed
germination and plant growth, as well as accelerated
plant mortality. These responses often follow injury to
plants by chemicals as well as metabolic dysfunctions
(Sasaki and Kozlowski, 1968c), with seedlings in the
cotyledon stage of development especially vulnerable.
Plant growth generally is arrested more by combinations of chemicals than by single chemicals
(Kozlowski and Kuntz, 1963; Kozlowski and Torrie,
1965; Waisel et al., 1969; Kozlowski and Clausen,
1970; Wu and Kozlowski, 1972; Constantinidou and
Kozlowski, 1979a,b; Suwannapinunt and Kozlowski,
1980; Norby and Kozlowski, 1981a,b,c; Olofinboba
and Kozlowski, 1982; Shanklin and Kozlowski,
1984, 1985a,b; Kozlowski, 1985d, 1986c).
The major environmental pollutants include sulphur
dioxide (SO2), ozone (O3), ¯uorides, oxides of nitrogen (NOx), peroxyacetyl nitrates (PAN), and particulates (e.g. soot, cement-kiln dusts, lead particles,
magnesium oxides, foundry dusts, and sulphuric acid
aerosols (Mudd and Kozlowski, 1975; Kozlowski,
1980b, 1985c; Kozlowski and Constantinidou,
1986a,b).
Pollutants sometimes reduce seed production and
inhibit seed germination and plant growth (Constantinidou et al., 1976; Kozlowski, 1986b,c). The size of
seed crops often is lowered after leaf injury by
pollutants inhibits synthesis of carbohydrates and
growth regulators. Pollutants may also alter mechanisms of ¯owering and fruiting and injure reproductive
structures (Kozlowski et al., 1991; Kozlowski, 2000).
Some pollutants lower the rate of photosynthesis by
occluding stomatal pores, inducing stomatal closure,
promoting disintegration of chlorophyll, and altering
synthesis of photosynthetic enzymes (Mudd and
Kozlowski, 1975; Norby and Kozlowski, 1982). In
the long term total photosynthesis is lowered by a
203
reduction of leaf surface associated with leaf necrosis, abscission, and inhibition of leaf formation and
expansion (Sasaki and Kozlowski, 1966; Barnes,
1972; Carlson, 1979; Lamoreaux and Chaney,
1978a,b; Heath, 1980; Eckert and Houston, 1980;
Coyne and Bingham, 1982; Reich, 1983, 1987;
Lorenc-Plucinska, 1978, 1982, 1988; Kozlowski and
Constantinidou, 1986a,b; Oleksyn and Bialobok,
1986; Reich and Amundson, 1985; Tsukahara et al.,
1985; Shanklin and Kozlowski, 1985a).
Salinity inhibits seed germination not only by
lowering the osmotic potential of the soil solution),
thus inhibiting water absorption by seeds, but also by
toxicity to the embryo (Khan and Ungar, 1984; Zekri,
1993). Whereas seed germination of nonhalophytes
typically is arrested by 0.5% salt solutions, seeds of
certain halophytes remain viable when exposed to vary
strong salt solutions and germinate after the salt stress
is relieved (Kozlowski, 1997). Considerable variation
in effects of salinity on germination of seeds of
different species of Casuarina was demonstrated by
Clemens et al. (1983). Salinity may affect plant
succession. Various species have different salt tolerances because salty soils vary in nutrieni balances, and
toxic concentrations of different ions can develop, thus
adversely in¯uencing the soil processes essential for
uptake of N and P (Swanson et al., 1998).
Some plants release toxic chemicals (allelochems)
to the soil. These compounds may adversely affect
seed germination and growth of neighboring plants
(HytoÈnen, 1992). Allelocheins include phenolic acids,
coumarins, quinones, terpenes, essential oils, alkaloids, and organic cyanides. These potentially toxic
compounds are released by exudation, decay of plant
tissues, leaching, and volatilization (Rice, 1974,
1984).
Water-soluble toxic substances have been found in
leaves, ¯owers, fruits, and/or roots of some chaparral
species (e.g. Salvia arctostaphylos and Adenostoma
fasciculatum). During ensuing rains the toxins are
carried into the soil where they inhibit seed germination and growth of plants. After late-summer ®res
destroy the toxins, regeneration of herbaceous plants
and shrubs typically follows (Biswell, 1974).
Although many laboratory experiments have phytotoxicity of allelochems their ecological signi®cance
has been debated because accumulation of these compounds in forests is modi®ed by soil moisture as well
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T.T. Kozlowski / Forest Ecology and Management 158 (2002) 195±221
as their destruction by soil micro¯ora (Kozlowski,
1995). Zackrisson and Nilsson (1992) and Nilsson
(1994) demonstrated allelopathic effects of Empetrum
hermaphroditum on regeneration of Pinus sylvestris
stands in Sweden. Although appreciable amounts of
allelochems were found in frozen soil, these compounds were detoxi®ed by microorganisms when
ground ice was not present.
A number of agricultural chemicals, including some
insecticides, fungicides, herbicides, and antitranspirants are useful to indispensable in regenerating forest
stands. However, regeneration sometimes is retarded
by toxicity of high dosages or incorrectly applied
chemicals. Sometimes these chemicals alter plant
metabolism, arrest seed germination, and impede
seedling growth as a prelude to early seedling
mortality (Kozlowski and Kuntz, 1963; Kozlowski
and Torrie, 1965; Sasaki and Kozlowski, 1968b;
Waisel et al., 1969; Kozlowski and Clausen, 1970; Wu
and Kozlowski, 1972; Olo®nboba and Kozlowski,
1982; Kozlowski, 1986a,c). Reduction in the rate
of photosynthesis has been attributed to certain
insecticides (Ayers and Barden, 1975), fungicides
(Kozlowski and Keller, 1966, 1968; Keller, 1983),
growth retardants (Roberts and Domir, 1983), and
antitranspirants (Olo®nboba et al., 1974; Davies and
Kozlowski, 1975).
Symptoms of herbicide toxicity include inhibition
of seed germination, chlorosis of seedlings, impeded
seedling growth, and seedling mortality. Abnormal
morphogenic changes such as fusion of cotyledons
and swelling of stems often occur. Herbicide toxicity
varies greatly with the stage of seedling development;
herbicide dosage; method of herbicide application;
and environmental conditions before, during and after
herbicide application (Kozlowski and Kuntz, 1963;
Winget et al., 1963; Kozlowski et al., 1967; Sasaki and
Kozlowski, 1968b,c; Sasaki et al., 1968; Kozlowski
and Sasaki, 1970; Wu et al., 1971; Wu and Kozlowski,
1972; Kozlowski, 1986a,b).
Both ®lm-type and metabolic antitranspirants have
been used to conserve water in plants and sometimes
to reduce injury from insects, fungi, and salt spray
(Kozlowski, 1986b). In some cases ®lm-type antitranspirants (e.g. oil, silicones, plastics, latex and
resins) were bene®cial in reducing water loss and
increasing plant growth (Davenport et al., 1972;
Chalmers et al., 1983). However, some ®lms caused
chlorosis, leaf lesions, leaf abscission, and plant
mortality. Certain antitranspirants variously lowered
the rate of photosynthesis when they accumulated in
antestomatal chambers and prevented diffusion of
atmospheric CO2 to the leaf mesophyll cells (Davies
et al., 1974; Lee and Kozlowski, 1974; Davies and
Kozlowski, 1975). Solarova et al. (1981) concluded
that ®lm-type antitranspirants reduced transplanting
shock in trees but were a problem because they were
less permeable to CO2 than to water vapor.
Several metabolic antitranspirants (e.g. abscisic acid,
succinic acids, phenylmercuric acetate, and atrazine)
reduced transpirational water loss by promoting
stomatal closure (Davies and Kozlowski, 1975) but
some were phytotoxic. For example, decenylsuccinic
acid not only injured P. resinosa needles and inhibited
bud development but also induced early mortality of
seedlings (Kozlowski and Clausen, 1970). Phenylmercuric acetate closed stomata of B. papyrifera leaves but
also caused chlorophyll breakdown (Waisel et al.,
1969). Furthermore, there is the added danger of
releasing mercury compounds into the atmosphere.
2.2. Thinning or stem exclusion stage
The most prevalent feature of this stage, beginning
with canopy closure, is accelerated mortality of trees
and changes in the dominant trees of monocultures or
variations in species in mixed-species stands. These
changes often have been attributed to competition
among plants for solar radiation, water and mineral
nutrients. In Costa Rica the poor growth of Cedrela
odorata in mixture resulted from early onset of severe
competition with Cordia alliodora and Hyeronima
alchorneoides (Manalled et al., 1998).
2.2.1. Competition for resources
In forest stands the light intensity typically
decreases progressively from above the canopy to
the forest ¯oor. For example, less than 5% of the light
intensity was transmitted to the forest ¯oor in a 150year-old Fagus grandifolia stand (Trapp, 1938). In a
50-year-old L. tulipifera forest most light reached the
forest ¯oor in the spring before the leaves begun to
expand; the least light during short autumn days while
the trees still retained all their leaves (Hutchinson and
Matt, 1977). The light intensity that penetrates a forest
canopy may be only 10±15% in open, even-aged Pinus
T.T. Kozlowski / Forest Ecology and Management 158 (2002) 195±221
stands and less than 1% in some tropical rain forests
(Spurr and Barnes, 1980).
Understory plants of dense forest stands are
exposed to prolonged dim and diffuse irradiance
punctuated by short periods of sun¯ecks. The duration
of sun¯ecks may vary from seconds to under 2 min in
tropical forests to ``sunpatches'' of 1 h or more in open
coniferous forests. Sun¯ecks sometimes comprise up
to 80% of the total irradiance at the forest ¯oor
(Chazdon and Pearcy, 1991).
Physiological responses, growth, and survival of
many understory plants are appreciably in¯uenced
by sun¯ecks (Pearcy, 1983). Examples include
Euphorbia forbesii and Claoxylon sp. (Oberbauer
et al., 1988). Growth increases due to sun¯ecks may
be counteracted by de®cits of water or mineral
nutrients, as well as high leaf temperatures (Pearcy
et al., 1994).
Increased growth of understory plants exposed to
sun¯ecks is associated with stimulation of photosynthesis as shown for Fraxinus excelsior, Fagus
sylvatica, and Ilex aquifolium (Harbinson and Woodward, 1984) and several rain forest species (Pearcy
et al., 1985; Chazdon and Pearcy, 1986). The
photosynthetic responses to sun¯ecks are affected
by light conditions experienced by plants before a
sudden increase in irradiance as well as by the extent
of the increase in irradiance. Hence, sun¯ecks exert a
carryover stimulation of photosynthesis after it is
induced by light (Pearcy et al., 1985). For three
understory species (Fagus crenata, Acer ru®nerve,
Daphniphyllum humile) in a F. crenata forest in Japan
the period of photosynthetic induction was several
times longer when irradiance was increased from
darkness to 800 mmol m2 s 1 than when irradiance
was increased from darkness to 1500 mmol m 2 s 1
(Han et al., 1999). Fluttering of upper canopy leaves of
P. tremuloides was followed by an increase in the
number of sun¯ecks and in enhanced photosynthesis
in the lower canopy (Roden and Pearcy, 1993).
The importance of competition for water was
emphasized by greater availability of water and higher
growth rates of the residual trees following thinning of
P. resinosa and P. menziesii stands (Sucoff and Hong,
1974; Aussenac and Granier, 1988). Leaf hydration of
P. contorta trees was higher in thinned than in
unthinned stands (Donner and Running, 1986).
Competition for soil water also was shown between
205
Abies concolor saplings and chaparral shrubs (Conard
and Radosevich, 1982), between P. menziesii trees and
Ceanothus velutinus shrubs (Petersen et al., 1988), and
between Pinus ponderosa seedlings and several
species of grasses (Elliott and White, 1987).
Competition for mineral nutrients among individual
woody plants as well as among woody and herbaceous
plants is well documented (Bell, 1968; Atkinson and
Johnson, 1979; Haynes and Goh, 1980; Connor, 1983;
Van den Driessche, 1984). Both N and P concentrations were lower in closely spaced Pseudotsuga
menziesii trees than in widely spaced trees (Cole
and Newton, 1986). In F. sylvatica forests both trees
and shrubs competed with herbaceous plants for Fe, N,
K, and P (Olsen, 1961).
As plant competition intensi®es some trees express
dominance and grow rapidly while others grow slowly
and occupy low canopy positions. The competitive
capacity of the more suppressed trees declines
progressively and their growth rates decrease accordingly. Between the stem initiation stage and thinning
stage growth ef®ciency (stem wood production per
unit of leaf area) decreases greatly, sometimes by more
than 90% (Waring and Running, 1998). Eventually the
most suppressed trees die.
The leaf area index of a regenerating forest stand
becomes maximal during the thinning stage. Net
addition of foliage to the canopy stabilizes and there is
a transfer of leaf area from suppressed trees to more
dominant ones (Waring and Running, 1998).
Competition is especially intense within monocultures (intraspeci®c competition) where resource
requirements are more or less similar for all trees. In
mixed-species stands the differences among species
in depths of rooting and in patterns of absorption
of mineral nutrients may account appreciably for
variations in availability of resources (Schulze et al.,
1994).
Because forest stands generally are quite variable
the impacts of competition on plant community
structure may be in¯uenced to considerable degree
by a variety of environmental factors. As emphasized
by Perry (1995) (1) species may not need to compete
when their growth is largely in¯uenced by predation,
disturbance, or climatic perturbation (e.g. when
herbivores and pathogens increase biodiversity by
lowering the capacity of one species to dominate
others), (2) competing species may actually bene®t
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T.T. Kozlowski / Forest Ecology and Management 158 (2002) 195±221
each other by either decreasing or eliminating the
adverse effects of plant competition (e.g. by improving the soil or microclimate, reducing the in¯uence of
other competing plants, discouraging predators,
catalyzing bene®cial components in the root zone,
and attracting pollinators) (Hunter and Aarssen, 1988)
(3) trees may avoid competition through specialization. For example, early-colonizing N-®xing plants
compete for resources, but later bene®t each other by
increasing soil fertility.
Much rigid emphasis on the importance of plant
competition alone in structuring ecosystems evolved
from mathematical models that portrayed interacting
plant species as if they lived in a constant environment and were isolated from other species in the
system (Perry, 1995). Stone and Roberts (1991)
concluded that up to 40% of the interactions among
species were advantageous when removed from the
community context in which they occurred. Only in
a nearly constant environment are species adversely
affected by other species. Hence, species often
depend on each other in ways that are not always
obvious, thus greatly modifying the adverse effects of
plant competition.
2.3. Transition or understory regeneration stage
Dominant features of this stage are death of some
overstory trees, resulting in formation of gaps in the
canopy and reintroduction of understory vegetation.
Formation of canopy gaps allows more solar radiation
to reach the forest ¯oor and enhance growth of trees
that were suppressed in the previous (thinning) stage.
Gap sizes vary appreciably. Small gaps form often
because of death or windthrow of a few trees; the less
frequent large gaps are traceable to various disturbances, including ®res, hurricanes, and insect attacks.
In temperate forests very small gaps are ®lled by
extension of branches of surrounding trees; larger gaps
close by growth of saplings and sprouts within a gap.
Large gaps very often are colonized by early shadeintolerant species and later replaced by shade-tolerant
species. Or species of both groups may colonize a gap
more or less concurrently, with the faster-growing,
short-lived pioneer species eliminated after a period of
dominance (Runkle, 1985).
In tropical forests the majority of gaps are small
even though most of the total gap area is in a few large
gaps. Very small gaps typically are ®lled by growth of
branches of adjacent trees; somewhat larger gaps by
growth of seedlings that were suppressed prior to
canopy opening, and by sprouting; still larger gaps by
plants from seeds in soil banks. The largest gaps are
®lled mostly by plants produced from seeds brought in
primarily by animals and wind after the gaps formed
(Bazzaz, 1984; Swaine and Whitmore, 1988; Whitmore, 1989).
Well-constructed models are potentially useful for
studying species dynamics and predicting effects of
combinations of factors that may in¯uence growth and
development of disturbed forest ecosystems (Kaufmann and Linder, 1996; Landsberg and Gower, 1997).
Botkin et al. (1972) pioneered in appraising tree
mortality and regeneration of gaps in forests by a
model that simulated vegetation dynamics. Many
modi®ed gap models were subsequently introduced to
evaluate such aspects of stand growth dynamics as
recruitment of propagules, seed germination, and tree
mortality (e.g. Cattelino et al., 1979; Dale et al., 1985;
Bonan, 1989; Bossel, 1991; Huston, 1991; Solomon
and Bartlein, 1992; Shugart et al., 1992; Friend et al.,
1993; Pacala et al., 1993, 1996; Korol et al., 1995,
1996; Roberts, 1996a,b; Keane et al., 1996). Gap
models usually incorporate such site variables as
species lists and maximum tree sizes, diameter and
height growth of trees, entry of young trees into the
simulation, mortality data, and growth-limiting potentials of available resources (Waring and Running,
1998).
2.4. Steady-state or old-growth stage
Salient characteristics of this stage are continuation
of a series of successional stages that began in the
previous stage that may culminate in an old-growth
climax forest.
2.4.1. Succession
Disturbed plant communities tend to regenerate to a
previous condition if they are not exposed to
additional stresses. Whereas competition among trees
in a monoculture does not involve succession,
competition in a mixed-species forest stand progresses
toward a more-or-less stable climax community with
high capacity for tolerating the stresses of competition. However, an old-growth community often is not
T.T. Kozlowski / Forest Ecology and Management 158 (2002) 195±221
achieved because of the long time required for
succession to run its full course and the strong
likelihood of interruption by recurrent disturbances
(Kozlowski, 1995; Perry, 1995). Examples of forest
succession to old-growth forest are replacements of
early-colonizing stands of Pinus taeda and P. echinata
of the southeastern US by Quetcus-Carya stands as the
overstory Pinus species die when they become
overmature in 70±80 years. In the northeastern US
fast-growing shade-intolerant Prunus pensylvanica
trees dominate forest stands in early succession and
are replaced by slower-growing more shade tolerant
species. Following early land clearance for agriculture
in Massachusetts the abundance of long-lived shade
tolerant species (T. canadensis, F. grandifolia, and
Betula alleghaniensis) declined sharply. These were
replaced by species more tolerant of disturbance (P.
strobus, Castanea dentata) which in turn were
succeeded by Quercus spp., A. rubrum, and Betula
sp. (Fuller et al., 1998).
Composition of the early colonists during succession
depends on the severity of disturbance of forest stands.
Early successional species usually are better represented than late successional species in seed banks
(Leck, 1995). Hence, after severe stand disturbance the
early-colonizing, fast-growing, usually short-lived
species dominate a site and later are replaced by
species that were the dominants prior to disturbance.
The replacement species may arise from seeds, sprouts,
or residual plants that survived the disturbance. After
minor disturbances gaps form and are recolonized.
In addition to the effect of severity of disturbance
the composition of the early-successional species is
in¯uenced by the time of year and length of time
between disturbances (Perry, 1995). In Alaska most
®res occur in June and July, when ripe seeds of P.
tremuloides and P. balsamifera are shed, well before
those of B. papyrifera and P. glauca. On a wide variety
of sites ®re in midsummer is less likely to be followed
by regeneration through abundant sprouting than ®re
occurring during the dormant season. Production of
stump sprouts and root suckers depends on the level of
stored carbohydrates in plants, which typically is
much higher in the dormant season than in midsummer when trees have depleted most of their
carbohydrate reserves during shoot elongation and
leaf expansion (Clark and Liming, 1953; Bachelard
and Sands, 1968; Kozlowski, 1992). Sprouting of
207
Miconia albicans and Clidemia sericea after ®re was
highly correlated with the amount of root starch
(Miyanishi and Kelman, 1986). The sprouting capacity of Eucalyptus viminalis depended on the amount
of starch in the sapwood (Bamber and Humphreys,
1965). Abundance of root suckers of P. tremuloides
also depended on stored carbohydrates (Schier and
Zasada, 1973).
The species composition of a pioneer forest community often varies with the time between disturbances. For example, when ®res occur frequently P.
contorta precedes establishment of T. heterophylla.
However, T. heterophylla may succeed itself if the
interval between ®res is so long that P. contorta has
already died out (Perry, 1995).
Succession in disturbed tropical forests to reestablishment of primary forest is very slow. Following
clear-cutting of primary forest a dense growth of
weeds, shrubs, vines, and young trees typically
emerges. Fast-growing, very-short-lived trees then
become dominant and are succeeded by slowergrowing, longer-lived species. Succession progresses
slowly until the climax species are reestablished,
sometimes only after hundreds of years. If the course
of succession is interrupted by additional disturbances, more mineral nutrients are lost from the
soil, thus rendering the site unsuitable ®r even the
early successional tree species (Kozlowski, 1979;
Kozlowski et al., 1991).
Many early studies in the tropics gave little
attention to the importance of secondary forests
(woody revegetation after removal of much or all of
a primary forest) in biomass production and carbon
¯ux. It has now been well documented that secondary
forests play an important though variable role in
carbon sequestration in tropical forest ecosystems.
The rate of biomass production differs appreciably
with the forest type, age of the secondary forest,
history of use of cleared areas, number of times an area
has been cleared, and soil fertility (Uhl et al., 1988;
Moran et al., 1994; Fearnside, 1996; Nelson et al.,
2000). Carbon accumulation rates in tropical moist,
seasonal, and open forests (which varied from rain
forest types to dry wooded savannas) were about 5, 4,
and 3 t ha 1 per year, respectively (Houghton et al.,
1991). Accumulation of biomass in a disturbed
Belgian Congo forest varied from 19 t ha 1 per year
for the ®rst 8 years to only 2.5 t ha 1 per year during
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T.T. Kozlowski / Forest Ecology and Management 158 (2002) 195±221
the next 9 years (Lugo and Brown, 1982). In Para,
Brazil abandoned pastures that had been lightly grazed
accumulated biomass at approximately 10 t ha 1 per
year whereas pastures that had been moderately
grazed produced biomass at about half that rate
(Uhl et al., 1988).
2.4.2. Mechanisms of succession
Successional trajectories are in¯uenced by two
primary factors: (1) the species that comprise the
initial colonists on a disturbed site, and (2) the impacts
of the initial and succeeding dominants on subsequent
conditions and events (Perry, 1995). Regulation of
growth and survival of seral plants during succession
is complex. It typically includes aspects of facilitation
and inhibition and is affected by interactions among
resource availability, climate, animals, pathogens,
mycorrhizal fungi, and other microbes. Furthermore,
the nature of the interactions changes over time
(Connell and Slatyer, 1977; Perry, 1995).
According to the facilitation model the early
successional species modify the site and render it
more suitable for establishment and growth of later
successional plants. Site modi®cation is accomplished
by weathering of rocks, accumulation of mineral
nutrients and carbon, and provision of energy that
allows plants to build soil and absorb mineral
nutrients. An example of facilitation is the ``island
effect'' characterized by establishment and growth of
plants near nurse trees or shrubs. Nurse trees may
shelter seedlings from environmental extremes, act as
centers for seed dispersal (e.g. by birds), and provide
local sites with enriched soil. Plant islands also may
facilitate establishment of late-arriving species by
in¯uencing soil chemistry, biology, and structure.
Early colonizers also may facilitate colonization of
succeeding species by providing a legacy of mycorrhizal fungi (Perry, 1995).
The inhibition model is based on changes in
availability of resources during plant succession
(Tilman, 1985). Once the early colonizing plants
obtain space and available resources, they either arrest
invasion by subsequent colonists or suppress those
present. Sometimes early colonists comprise a dense
monoculture that slows or precludes invasion by
other species. For example, in South America trees do
not readily invade abandoned pastures because of
inadequate seed dispersal to grasslands; consumption
of tree seeds and seedlings by rodents; competition
between grasses and trees for solar radiation, water,
and mineral nutrients; and mortality of tree seedlings
by recurrent burning of grasslands (Nepstad et al.,
1990).
2.4.3. Characteristics of old growth forests
Should the course of succession be completed, the
resulting old-growth forest has become multi-aged in
contrast to the even-aged character of earlier successional communities. The dominant trees of old-growth
forests show little height growth, whereas diameter
growth continues. As a result live biomass reaches a
maximum and stabilizes in the old-growth stage.
Species composition, community height, and structural diversity also maximize in the old-growth stage,
even though the number of plant and animal species
together may be higher in the stand initiation stage
(Oliver and Larson, 1996). In general growth ef®ciency of all the trees in an old-growth forest tends to
be lower than it was in earlier successional slages.
Old-growth forests are impacted by frequent minor
disturbances (including death of some old trees,
windthrows, or ground ®res) and often less-frequent
harsh disturbances such as severe crown ®res. Hence,
they are not completely stable, but are maintained in
an oscillating steady-state and consist of mosaics of
patches of various sizes and ages. In such forests the
processes of establishment, thinning, gap formation,
and recolonization recur. When old-growth forests are
impacted by the infrequent major disturbances they
are maintained in a steady-state only within large
landscapes (Perry, 1995).
In many regions succession to an old-growth forest
is arrested by frequent severe ®res. For example, in the
southeastern US subclimax P. taeda, and P. echinata
forests are maintained by ®re (Komarek, 1974). In the
western US P. menziesii is a major ``®re-climax''
species (Kozlowski et al., 1991). In the northeastern
US P. tremuloides, Pinus banksiana, and P. mariana
are maintained on many sites by ®re. Adaptations of
®re-tolerant species include stimulation by ®re of seed
dispersal, seed germination, and ¯owering, as well as
seed storage in the soil or on plants (in serotinous
cones), thick bark, and capacity for sprouting (Gill,
1981; Gill and Groves, 1981; Hanes, 1988; Biswell,
1989; Kozlowski et al., 1991; Kozlowski and Pallardy,
1997a,b).
T.T. Kozlowski / Forest Ecology and Management 158 (2002) 195±221
3. Conclusions
Accelerating rates of destruction of forest ecosystems, especially in the tropics, often result in catastrophic losses of the harvestable forest products and a
variety of services essential for maintaining Earth's
life support systems. Natural regeneration of disturbed
mature forests to a predisturbance condition often is
slow, unpredictable, and fraught with dif®culties.
Because of the added demands made on forests by a
rapidly growing human population, reliance on natural
regeneration is questionable. Hence, much emphasis
is needed on abetting natural regeneration with arti®cial
methods, including direct seeding with high-quality
seed, outplanting of superior planting stock, and
intensive site preparation (Smith, 1995). However,
ef®cient regeneration of disturbed forest stands can
provide only a partial solution to meeting the nearfuture demands for more forest products and services
inevitably associated with a burgeoning population
growth. To not only regenerate and accelerate growth
of disturbed forests but also to maintain their sustainability and biodiversity several concurrent initiatives
are needed including: (1) preservation of the remaining tropical forests for their wild genetic resources.
Especially needed is better understanding of ways in
which biodiversity and ecosystem functioning are
coupled (Dasgupta et al., 2000). Evolution of strategies
for achieving sustainability of tropical forests will
require cooperation among forest biologists, social
scientists, and economists; (2) expansion of plantations
(Zobel and Talbert, 1984; Evans, 1992; DekkerRobertson and Libby, 1998); (3) establishment of large
forest reserves on carefully selected sites for preservation of wild genetic resources (Noss and Cooperrider,
1994; McNeely, 1994); (4) expansion and international support of agroforestry system (Vergara and Nair,
1983; Nair, 1998); (5) improved forest management
(e.g. timing of tree harvesting to coincide with good
seed years; long rotations; decreasing human-induced
environmental stresses such as pollution, drought,
¯ooding, ®re, and soil compaction which are avoidable to various degrees) (Kozlowski, 2000); more
intensive management of plantations, forest reserves,
and agroforestry systems (e.g. greater emphasis on
irrigation, fertilizer application, site selection, and site
preparation) (Kozlowski et al., 1991; Kozlowski and
Pallardy, 1997b); (6) expansion of tree improvement
209
programs (e.g. selection of trees in small populations to
maximize the rate of improvement in one or a few
characters; mating of selected parents in different
combinations; selection of superior genotypes followed
by propagation in seed orchards for testing, breeding,
and seed production) (Guries, 1995); (7) expansion of
research with focus on both regeneration and preservation of forest ecosystems as well as accelerated
productivity (Landsberg and Gower, 1997; Kozlowski,
2000). We do not know enough about many disturbed
forest ecosystems to regenerate them rapidly and
ef®ciently. Hence, much more research is needed on
how to maximize yields of forest products and services
while maintaining sustainability and biodiversity of
forest stands. Determinations of how physiological
dysfunctions are induced in trees by environmental
stresses will require a long-term commitment to
ecological research (Likens, 1983, 1985, 1987; Franklin, 1987; Lindenmayer and Franklin, 1997). Some
examples of priority research areas include the
following:
1. Seed biology: this includes indices of seed
maturity, control of seed quality, variations in
dormancy of seeds of various species and
genotypes, causes of seed aging, effects of
environmental stresses on seed development, and
factors in¯uencing seed vigor.
2. Seedling biology: this includes physiological
determinants of growth of seedlings (especially
in the cotyledon stage of development), effects of
environmental stresses on seedling vigor, and
causes of mortality of seedlings beyond the
cotyledon stage of development.
3. Responses of various species and genotypes to
disturbances: this includes controlled environment
research on impacts of environmental changes,
stresses, and their interactions on physiological
processes, growth, and survival of woody plants,
and effects of environmental preconditioning in
plant responses to disturbances (Norby and
Kozlowski, 1981a,b,c; Kozlowski and Huxley,
1983; Shanklin and Kozlowski, 1985a,b).
4. Tradeoffs between bene®cial and harmful effects
of environmental extremes on various species and
genotypes: not all environmental stresses are
harmful to plant ecosystems. In the past much
preoccupation with the deleterious impacts of
210
T.T. Kozlowski / Forest Ecology and Management 158 (2002) 195±221
environmental extremes obscured some of their
potentially bene®cial effects which sometimes
might be incorporated in forest management
plans. Examples include some effects of drought,
¯ooding, and ®re.
Wherever woody plants grow their development
and survival depend on adequate water supplies. Water
de®cits adversely affect several physiological processes, resulting in loss of vegetative and reproductive
growth, as well as in injury and mortality of plants
(Kozlowski, 1968, 1972, 1979, 1982a, 1983, 1996).
Nevertheless, slowly increasing droughts often induce
physiological adjustments in woody plants that protect
them from the adverse responses that inevitably occur
following abrupt imposition of drought. Hence,
seedlings preconditioned by exposure to mild water
stress in forest nurseries undergo less growth inhibition and injury from transplanting and drought than
seedlings not previously drought-stressed (Clemens
and Jones, 1978; Abrams, 1988; Mexal and South,
1991; Kozlowski and Pallardy, 1997b). Exposing fruit
trees to water stress during a critical early period of
fruit development to inhibit vegetative growth,
followed by normal irrigation, favored formation of
¯ower buds and increased fruit yield (hence larger
seed crops) in several species (Menzel, 1983; Southwick and Davenport, 1986; Menzel and Simpson,
1990). A period of drought also stimulated breaking of
dormancy of ¯ower buds (Barbera et al., 1985;
Shalhevet and Levy, 1990) and increased fruit yields
(Chalmers et al., 1983, 1984; Mitchell et al., 1984,
1986).
The deleterious effects of ¯ooding on woody plants
have been well documented. They include physiological dysfunctions, inhibition of seed germination and
growth of plants, as well as plant injury and mortality
(Kozlowski, 1982b, 1984a,b,c; Newsome et al., 1982;
Tang and Kozlowski, 1982a,b, 1983; Kozlowski and
Pallardy, 1984; Tsukuhara and Kozlowski, 1986; Reid
and Bradford, 1984; Sena Gomes and Kozlowski,
1986, 1988; Yamamoto and Kozlowski, 1987a,b;
Kozlowski and Pallardy, 1997b; Lopez and Kursar,
1999). However, ¯ooding sometimes has bene®cial
effects that can be judiciously used in forest management plans. For example, ¯ooding for short periods
during the dormant season may stimulate subsequent
vegetative growth or favor reproductive growth over
vegetative growth. ``Greentree reservoirs'', shallow
impoundments during the autumn and winter, sometimes have been advantageously created to inhibit
vegetative growth and thereby stimulate reproductive
growth of woody plants in order to provide food (mast)
for wildfowl (Frederickson and Reid, 1990). In
addition, ¯ooding of soil during the growing season
induces rapid stomatal closure in many woody plants
(Pereira and Kozlowski, 1997; Kozlowski, 1980a,b,
1982b, 1997; Tang and Kozlowski, 1982b,c; Pezeshki
and Chambers, 1985), thereby preventing absorption
of gaseous air pollutants and reducing pollution
damage (Norby and Kozlowski, 1983). Closure of
stomata of ¯ooded plants has been attributed to
hormonal signals transmitted from roots to leaves
(Zhang and Davies, 1990; Else et al., 1996). Flooding
also protects wetlands, resets ecosystem trajectories,
and in¯uences decomposition of organic matter and
nutrient cycling. Furthermore, ¯ooding often maintains high diversity of riparian forests, supplies
mineral nutrients, and transports sediments needed
for maintenance of downstream ecosystems (Bayley,
1995; Postel and Carpenter, 1997; Haeuber and
Michener, 1998; Yarie et al., 1998).
Early installation of dams and levees was based
largely on reducing variability in water supplies and
channel positions of rivers (Sparks et al., 1998).
Unfortunately the elimination of natural ¯ooding
regimes caused some irreversible and undesirable
downstream results. Preoccupation with bene®cial
effects of dam construction along rivers often obscured some harmful effects such as loss of species and
declining biodiversity (Dudgeon, 2000). Decreased
downstream groundwater leads to inhibited regeneration of pioneer species and mortality of mature trees
(Stromberg et al., 1996; Rosenberg et al., 2000).
Arrested plant regeneration typically alters the course
of succession to less productive communities (Nilsson
and Berggren, 2000). In Alberta, Canada, a decrease in
Populus angustifolia, P. balsamifera, and P. deltoides
was attributed to downstream drought following river
regulation (Rood et al., 1994). In Australia mortality
of Eucalyptus largi¯orens was associated with greatly increased downstream salinization due to reduced ¯ooding frequency in the Murray River (Jolly
et al., 1993). Reexamination of early ¯ood management plans led to realization of a need for preserving forested ¯oodplains which depend on an annual
T.T. Kozlowski / Forest Ecology and Management 158 (2002) 195±221
advance and recession of ¯oods (Bayley, 1995).
Conservation of biodiversity will require preservation
of the remaining intact forested ¯oodplains and
fostering of more natural ¯ooding disturbances
(Michener et al., 1998; Toth et al., 1998). The complexities of the effects of ¯ooding on different
ecosystems and interactions with land use point up
a need for integrated approaches to balance ecologically bene®cial and socially unacceptable effects of
¯ooding. Cooperation of interdisciplinary teams
from biological, physical, and social sciences may
be essential to develop holistic plans for managing
¯oods (Michener and Haeuber, 1998; Swanson et al.,
1998).
The effects of ®re on forested ecosystems range
from disastrous to bene®cial. Harmful effects include
changes in physical, chemical, and biological properties of soils (e.g. reduction in water holding capacity;
soil compaction; soil erosion; loss of mineral nutrients
by volatilization, convection, and leaching; destruction of plants; changes in species composition by
inhibiting plant succession, and increasing pollution)
(Kozlowski and Ahlgren, 1974; Chandler et al., 1983;
Pyne, 1984; Pyne et al., 1996; Kozlowski and Pallardy,
1997b).
Over the years forest management objectives have
evolved from completely excluding ®re to tolerating
many natural ®res because of their importance in
perpetuating forests. However, many catastrophic
forest ®res also emphasized a need for incorporating
prescribed burning in forest management planning.
Much interest has been shown in such bene®ts of
prescribed burning as removal of accumulated fuels
(thereby preventing catastrophic ®res), increase in
water yield, control of insects and diseases, preparation of seedbeds, and release of seeds from serotinous
cones (Kayll, 1974; Chandler et al., 1983; Biswell,
1989). Burning of peat greatly improved seedbeds
and regeneration of P. mariana stands (Chrosciewicz,
1976). Both N mineralization and nitri®cation in the
forest ¯oor of a P. ponderosa stand were increased
by prescribed burning (White, 1986). Use of prescribed burning requires knowledge of ®re ecology,
experience, and very competent supervision (Biswell,
1989).
1. Genetic engineering: studies of effects of gene
transfer from one species to another on resistance
2.
3.
4.
5.
211
of plants to insects, diseases, biocides, and environmental stresses (Fillatti et al., 1987, 1988;
McCown et al., 1991; Hamerschlag and Litz,
1992; Kozlowski and Pallardy, 1997b).
Potential effects of global warming: mechanisms
by which temperature increases may in¯uence
physiology and growth of forest ecosystems
(Mooney, 1991); determination of interactive
effects of CO2 and other climatic variables on
productivity of forest ecosystems (Norby et al.,
1996); effects of exposure of large forest ecosystems to elevated CO2 on canopy development,
below-ground processes, and interactions with
other environmental factors and stresses (Hendrey,
1992; Mauney et al., 1992; Rogers et al., 1992;
Kozlowski, 2000).
Agroforestry systems: studies on how spatial and
temporal arrangements of plants affect resource
pools as a basis for selecting species and
genotypes that will maximize sharing of resources
and sustain high yields of harvested products
(Huxley, 1985, 1987; Brewbaker, 1987; Nair,
1989; Owino, 1996; Rhoads, 1996±1997); physiological interactions among herbaceous and
woody plants that will optimize use of radiant
energy, water, and mineral nutrients (Kozlowski
and Huxley, 1983; Huxley, 1987, 1996; Sanchez,
1995; Nair, 1998).
Modeling: construction of models to estimate
stand productivity; calculation of ¯ows of carbon,
water, and mineral nutrients through forest
ecosystems (Landsberg and Gower, 1997). Models
may be useful in dealing with complex processes
in forest stands and in better understanding the
impacts of the factors that in¯uence growth and
development of forest ecosystems (O'Hara and
Vallapil, 1995, 1999; Kaufmann and Linder, 1996;
Dale and Van Winkle, 1998; Koltenberg and
O'Hara, 1999; O'Hara et al., 1999).
Remote sensing: monitoring of spectral signatures
to quantify injury to forest ecosystems (Skole and
Tucker, 1993; Holmgren and Thuresson, 1998).
Acknowledgements
The valuable assistance of Pavel and Joy SÏvihra in
manuscript preparation is gratefully acknowledged.
212
T.T. Kozlowski / Forest Ecology and Management 158 (2002) 195±221
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