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37
Managing and Directing
Natural Succession
Steve Whisenant
Key Points to Retain
Carefully designed silvicultural strategies
can accelerate growth, influence the direction of succession, increase the goods and
services provided, or enhance diversity.
Directing natural processes toward land use
goals requires an understanding of the
processes driving succession.
Tools for managing and directing natural
succession should be used as an imitation of
natural processes rather than as a substitute
for them.
1. Background and
Explanation of the Issue
After regeneration begins on previously forested sites, carefully designed silvicultural
strategies can accelerate growth, influence the
direction of succession, increase the goods and
services provided, or enhance diversity.327 Selecting proper treatment options requires an
understanding of the factors limiting successional change and increases in desired species.
These treatments should be designed to assist
natural processes rather than fight them. This is
most likely to occur when forest restoration
plans (1) consider and remove the underlying
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Lamb and Gilmour, 2003.
causes rather than the symptoms of degradation; (2) are based on an understanding of succession and threshold barriers that must be
overcome through designed interventions; and
(3) stimulate the desired successional behaviour with minimal interventions.
1.1. Consider Underlying Causes
Halting Natural Succession
Many forest restoration programmes fail
because they do not address the underlying
causes of degradation.A number of social, political, and economic factors are often the underlying cause of forest loss or degradation. It is
equally important to identify the biophysical
barriers to recovery through natural successional
processes. For example, livestock may contribute to degradation in some situations but be
an important part of the recovery plan in other
circumstances. Forests limited by excessive fire
and invasive grasses may benefit from cattle that
reduce fuel loads until the tree canopy begins to
close. In contrast, forests limited by livestock
that consume high percentages of developing
seedlings benefit more from livestock exclusion
than from control of unsustainable harvest of
wood and nonwood forest products.
1.2. Understand Natural Succession
and Potential Threshold
Barriers
Having stimulated natural regeneration processes that establish forest species (see previous
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chapter), it is necessary to manage and direct
succession processes toward the desired objectives. It is important to promote continued
development of the vegetation to conserve soil,
nutrient, and organic resources; restore fully
functional hydrologic, nutrient cycling and energy flow processes; and create self-repairing
landscapes that provide the goods and services
necessary for biophysical and socioeconomic
sustainability. Different stages of degradation
require management actions that focus on different processes. Severely degraded sites
require early repair of hydrologic, nutrient
cycling, and energy capture and transfer
processes. As the vegetation increases in
biomass and stature, it reduces abiotic limitations of the site by improving soil and microenvironmental conditions. Directing natural
processes toward land use goals requires an
understanding of the processes driving succession. The rate and direction of succession is
influenced by the availability of species, the
availability of suitable sites, and by differential
species’ performance.
Previous land use has important and potentially long-lasting impacts on the rate and direction of natural succession.328 Natural succession
on abandoned farms and pastures is limited and
directed by the available seed bank, sprouting
ability of remaining stump and root systems,
seed immigration, soil type and condition, and
climatic conditions.329 Natural recovery occurs
most rapidly and completely following abandonment of pastures that were cleared by hand
and received little weeding and light grazing.
These areas benefit from diverse seed banks,
nearness to seed sources, and sprouting from
stumps and roots. Moderately grazed pastures
are much less productive and diverse due to the
loss of grazing intolerant species, diminished
seed banks, and less organic matter in the upper
soil horizons. Heavily grazed, mechanically
cleared pastures are far more likely to remain
dominated by grasses and forbs following abandonment, since they are completely dependent
on seed immigration for successional development. Frequent burning prior to abandonment
328
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Uhl et al, 1988.
Kammesheidt, 2002.
reduces the density of tree seed and sprouts.
Large treeless areas are unattractive to most
birds and bats that disperse small seeds.
Monkeys and ground-dwelling mammals that
disperse large-seed, late successional species
are even more prone to avoid open areas. Thus,
perching sites provided by isolated trees can
accelerate succession.
1.3. Design Minimal Interventions
to Achieve Goals
Will the site recover within an acceptable time
frame in the absence of active restoration
efforts? If so, will it provide the desired combination of goods and services? Answers to these
key questions may be found by examining two
types of reference sites. Selecting reference
sites that have not been damaged provides an
approximation of the potential goods and services. Reference sites that have been similarly
damaged and allowed to recover naturally for
different periods of time provide important
information on the presence or absence of barriers to recovery. This provides critically important information about the passive intervention
option. Active management interventions may
be required where invasive species, damaged
ecosystem processes, or other limitations halt
natural recovery.
If the site is not seriously degraded and seed
sources are adequate, the first few years of succession will be dominated by herbaceous vegetation and shrubs.This will typically be followed
by early succession tree species and midsuccessional tree species will gradually become more
dominant. In lowland humid forests, biomass
peaks of early successional species occur at
around 10 years. Mid-successional species may
reach their peak biomass at 15 to 30 years, but
remain dominant for many decades. These successional changes occur more slowly in less
humid or very degraded environments.
Improving the management of ecosystem
consumption (timber or wood harvest) is
usually effective on relatively intact sites. Sites
dominated by grasses may require vegetation
control of the existing vegetation. This may be
done with fire, herbicides, or mechanical or biological control methods. It may be necessary to
37. Managing and Directing Natural Succession
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add some species through seeding or transplanting. Denuded or depauperate sites that
can neither stabilise nor achieve management
objectives require enrichment plantings.
2. Examples
2.1. Restoring Dry Tropical Forests
to Anthropogenic Grasslands
in Guanacaste National Park,
Costa Rica
Anthropogenic fire converted the dry tropical
forest of Costa Rica to grasslands that continued to burn frequently. A programme begun in
the 1980s effectively stopped fire and allowed
the natural reforestation by trees. The initial
forests, of species with wind-blown seed, rapidly
covered the landscape. As these trees grew
larger, seed-dispersing birds and mammals
increasingly moved through the site and added
new species to the developing forest.330 This is
an excellent example of removing barriers to
natural succession and then allowing natural
processes to operate over many decades to
return an increasingly diverse forest to the
landscape.
2.2. Plantation Trees as Nurse
Plants to Increase
Regeneration of Native Species
Tree plantations can sometimes facilitate the
return of native vegetation. In Puerto Rico, tree
plantations improved soil and microenvironmental conditions enough to facilitate the
natural immigration of native species.331 The
plantation also accelerated the return of native
species by attracting animals that brought additional seed. Tree plantations in the moist and
wet tropics do not remain monocultures
because native trees invade the understorey
and penetrate the canopy of the exotic species.
Unless site damage is extreme, native forests
eventually dominate. Where damage is more
severe, the resulting forests are likely to
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Janzen, 1988.
Aide et al, 2000.
Figure 37.1. Previously mined site in Hungary that
has undergone natural regeneration for about 30
years. (Photo © Steve Whisenant.)
become a combination of native and exotic
species.
2.3. Spontaneous Regeneration of
a Mine Site in Hungary
Mining is a drastic alteration of site conditions
and processes. Planting trees on these sites is
expensive and risky, thus they are often abandoned to natural processes. A mine site in
Hungary received no active replanting, but 30
years following the cessation of mining, it shows
numerous signs of spontaneous regeneration of
herbaceous and woody vegetation (Fig. 37.1).
The abundance of natural vegetation in the surrounding landscape provides seed sources. This
site will take many more decades for recovery,
but natural processes are operating in the
absence of new disturbances.
3. Outline of Tools
Tools for managing and directing natural succession should be used as an imitation of
natural processes rather than as a substitute for
them. The tools described in the previous
chapter focus on influencing natural regeneration. They remain appropriate throughout
succession, but here is a list of tools for manipulating existing vegetation:
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Patience: Time can be used as a tool. Wait for
signs and expression of successional trajectory. Understanding what drives and limits
succession will make it easier to recognise the
probable direction of successional change
and the potential vegetation for that area.
Knowledge of potential successional pathways:
Understanding how forest vegetation recovers following disturbances is a critical aspect
of directing natural successional processes.
Know what prevents improvement and remove that limiting factor.
Fencing: Where livestock delay, limit, or prevent successional development, fences that
restrict livestock entry are one method for
increasing seedling development. This may
only be necessary until the seedlings grow out
of reach of the livestock (or fences may also
be more permanent for continued benefits).
Direct removal of invasive species: Invasive
species may be killed or removed with herbicides, mechanical treatments, or hand removal to release native species. These tools
may be expensive or very labour intensive, so
their practicality is often limited to small or
high priority sites.
Reducing invasive species with shade: Shadeintolerant invasive species are most effectively managed with tree species and
management strategies that accelerate the
occurrence of closed canopies. For example,
establishing forests on fire-prone grasslands
requires the prevention of fires until the
forest canopy effectively excludes the
grasses.
Thinning to reduce density or alter species’
composition: Selective thinning may be used
to provide products and income while
increasing growth rates of the remaining
trees. It may also be used to encourage
regeneration and growth of certain desired
species while reducing the abundance of
more common species.
Enrichment plantings: Sites with no regeneration of shade-requiring late successional
species may necessitate enrichment plantings
under the canopy of earlier successional
species. Enrichment plantings add species to
sites where they are unlikely to enter through
natural processes. They are most useful
where the desired species, or suite of species,
are neither present nor found in adjacent
forests.
4. Future Needs
Priority areas for further development are:
Policies that encourage the development of
natural, diverse forests: Government policies
can accelerate destruction of natural forests
or they can be crafted to encourage the
development of natural and managed forests
that combine production and conservation
functions and reduce pressures on natural
forests of high conservation value.
Improved understanding of successional processes and barriers to natural recovery: There
are numerous gaps in our knowledge of succession and ways in which we might encourage and direct those processes. Many factors
drive succession and similar impacts may
have dramatically different results in different ecosystems. A more mechanistic understanding of the factors limiting or accelerating
succession would greatly improve our predictive ability in new situations.
Novel strategies for payment of landscape
forest restoration: New ways to fund forest
restoration are essential. Programmes to
plant trees are more easily funded than those
designed to encourage and manage natural
regeneration. This is unfortunate because
natural succession often occurs more rapidly
and at less risk than artificially planted
forests.
References
Aide,T.M., Zimmerman, J.K., Pascarella, J.B., Rivera,
L., and Marcano-Vega, H. 2000. Forest regeneration in a chronosequence of tropical abandoned
pastures: implications for restoration ecology.
Restoration Ecology 8(4):328–338.
Janzen, D.H. 1988. Tropical ecological and biocultural restoration. Science 239:243–244.
Kammesheidt, L. 2002. Perspectives on secondary
forest management in tropical humid lowland
America. Ambio 31:243–250.
37. Managing and Directing Natural Succession
Lamb, D., and Gilmour, D. 2003. Rehabilitation and
Restoration of Degraded Forests. IUCN, Gland,
Switzerland, and Cambridge, UK, and WWF,
Gland, Switzerland.
Uhl, C., Buschbacher, R., and Serrao, E.A.S. 1988.
Abandoned pastures in Eastern Amazonia. I. Patterns of plant succession. Journal of Ecology
76:663–681.
Additional reading
Ashton, M.S. 2003. Regeneration methods for dipterocarp forests of wet tropical Asia. Forestry
Chronicle 79:263–267.
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Feyera, S., Beck, E., and Lüttge, U. 2002. Exotic trees
as nurse-trees for the regeneration of natural tropical forests. Trees 16:245–249.
Parrotta, J.A. 1995. Influence of overstory composition on understory colonization by native species
in plantations on a degraded tropical site. Journal
of Vegetation Science 6:627–636.
Whisenant, S. 1999. Repairing Damaged Wildlands:
A Process-Oriented, Landscape-Scale Approach.
Cambridge University Press, Cambridge, UK.