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Forest Ecology
1. Resources - Factors

Light
Only a small fraction of incoming solar energy (<0.1%) is converted by plants into chemical potential
energy as plant (dry) mass.
When light intensities are becoming very low, leaf and plant respiration cause more loss (CO2
release) than gains (CO2 fixation in plant matter), respiration exceeds assimilation. Thereby, plant
productivity, but also plant species diversity, diminishes with decreasing light intensity.
However, shade plants (eg in a shaded environment on the forest ground) or shade leaves are
adapted to low-light conditions; they are characterized by a lower compensation point, so that
carbon assimilation occurs already at lower light intensities than in sun-exposed leaves.

Warmth / Temperature (PuUl p 252-256)
It’s mainly the temperature regime which restricts and conducts growth performance of ecosystems
and their plants, provided sufficient moisture and other resources (CO2, nutrients, light) are
available. However, growing season length and annual growth of plants not only depend on
temperature features, but also on the photoperiodic regime, which may influence events such as
growth cessation and release of dormancy.
Active growth of plants is generally possible within a range of 5-40°C. (253; Evans p 189 tb 8.1)
Vegetation growth outside the tropics is however season-dependant and restricted to the frost-free
season. (253; Evans p 189 tb 8.1)
Under constant environmental conditions, net photosynthesis (and growth and ecosystem carbon
storage) increases to an optimum at approximately 20°C and then decreases steeply, meanwhile
respiration (and ecosystem carbon loss) of plants and soil continues to increase with rising
temperatures.
Temperature optima of higher plants, provided sufficient supply of resources, are around 25-30°C.
Temperate forest species (eg Fagus) require mean daily air temperatures of about 5°C in order to
initiate active life processes; on the other hand, surface soil temperatures above 45°C (such as at
open sites and on dark soil surfaces) are harmful for many seedlings and surface fine roots. In many
temperate species (eg Pinus, Picea) root growth increases proportional to temperature from 3°C 26°C, above which it tends to diminish and cease.
At temperatures between 10-15°C middle-altitude tree species perform best, meanwhile at higher
temperatures (20-25°C) low-latitude (1000-1500m) species may perform best. High altitude species
and provenances are best adapted not only to cold temperatures, but also high wind speeds, and
the specific radiation and air pressure features typical for high altitudes.
Forests are generally absent, where the mean temperature of the warmest month falls below +10°C
or where the temperature sum above +5°C is less than 350 degree days. (253)
In addition, different stages of the life cycle of trees (eg pollen and seed development) require
minimum heat sums; specific plant life cycle stages are are sensible to frost. (253)
Maximum and minimum temperatures as well as the growing season length (better reflected by
“temperature sums”) limit the specific vegetation types which occur; tree species may require a
minimum growing season length and accumulated warmth (growing degree days GDD) during the
growing season (PuUl fig 6.13-woodward).
Within a geographic region, occurrence and distribution of natural forest communities is closely
dependent on both, “temperature sums” as well as (annual) “precipitation sums” (PuUl 6.15).
Across Europe, accumulated temperature maps (PuUl fig. 6.14-cramer) have both, some similarity
with maps of the highest monthly temperature, as well as with maps of the distribution of
vegetation types.
Minimum and maximum temperatures as well as minimum ‘growing degree days’ (e.g. temperature
sums > 0 °C) are identified for many of the temperate tree species/taxa (PuUl tb 6.4).
Many temperate tree species (like Fagus) require 5°C of mean daily air temperatures (annual
average); Fagus’ best height growth occurs at 7°C annual mean temperatures
-Sufficient warmth is needed to reach flowering and maturing of seeds, in some cases, such
conditions are only occurring rather rarely (eg Tilia)
COLD
dormancy:
- Dormancy: Many actively growing plants of the temperate zone may be injured or killed by near
freezing point temperatures during the active growth phase; but during “dormancy” (cold rest or
break phase), many of them are adapted to survive temperatures of -25°C or lower.
The exogenously driven dormancy, called “quiescence”, prevents buds from bursting. The
endogenously directed dormancy ‘break phase’ is called “rest”.
Seed dormancy of temperate zone species often requires temperatures between 0-5°C, alpine
species require freezing temperatures.
Under normal winter conditions with a shortened photoperiod, deciduous trees undergo
physiological changes and reach the rest phase –the occurring chilling temperatures allow for the
necessary hormonal and physical changes in the buds; after completion of the rest phase, the buds
and the plant are ready for a new growth cycle.
If the cold/chilling requirements have been completely fulfilled, rising spring temperatures will allow
for bud burst, but these are susceptible to eventual temperature drops (“late frost”)
cold requirement:
Some species, like Fraxinus or some Pinus, require cold temperatures to prepare biochemical or
physical processes in the seed, so that germination can occur; other species from warmer regions
may require warmth to initiate germination. (PuUl p254)
-“Chilling requirement”: Many tree species-esp. deciduous- occurring in the temperate forest zone
require a period of (winter-)cold temperatures close to the freezing point, in order to complete the
rest phase of dormancy and enable normal renewed growth in the following growing season. Picea,
Pinus sylv. and Acer platanoides have cold requirement between -2.5°C - 0°C or below (coldest
month mean temperature); Tilia cordata, Fagus, Fraxinus, Quercus petraea require chilling at only +
5-6.5°C or below (PuUl p254)
bud development:
-After becoming dormant in autumn, buds no longer grow actively, but can easily be forced (eg if
they are subjected to cold or heat, long photoperiods, N-fertilisation, shock or gibberelic acid
treatment.
In temperate zone plants, bud dormancy can be initiated when low temperatures (eg < 5°C) occur
and bud dormancy can be broken if sufficient chilling/freezing/cold has happened – but this also
largely depends on the species, genotype, weather condition of the previous season and the bud’s
position in the tree crown.
The time of bud burst is however dictated eg by the number of chill days (eg < 5°C) during winter
and warm temperatures during spring (eg accumulated day-degrees > 5°C).
cold tolerance & hardiness:
-Cold tolerance is different among different provenances of tree species and also depends on their
different development stage. Trees native to warmer (southern, lower) regions do not develop
enough hardiness to cold; they have difficulties to survive early cold weather, they de-harden too
quickly or they can be even killed by sub-freezing temperatures.
-Consequently, minimum temperatures tolerated by wood species differ widely, high tolerance is
shown by deciduous conifers and broadleaved species of the polar region Larix, Betula, Populus;
very low tolerance by species of the broad-leaved raingreen and evergreen species of the
tropics/subtropics, some of them don’t even resist temperatures under + 12°C (PuUl p253)
-Frost hardiness: Frost hardiness is the ability not to be damaged despite extracellular ice
formation; a series of molecular, metabolic and physiological processes are preparing plant cells for
resisting winter cold. Plant cells may accumulate sugars or diminish their water content in order to
avoid intra-cellular ice formation – usually, only extra-cellular ice formation is tolerated by plants.
In order to avoid damage, plants have to acquire frost hardiness before the onset of the cold period.
But only healthy, nutrient-balanced plants, can become fully hardened to very low temperatures.
Picea abies needles may show frost tolerance down to -38°C (PuUl p254)….
early and late frost:
-Early and late frost, different from winter frost, affect plant growth during the plant’s moments of
highest sensitivity, eg bud burst and at the beginning of the extensional growth period –in early
spring-.
Meanwhile some tropical species don’t even resist temperatures below + 12°C, species from cold
regions may resist growing season temperatures of -5°C (without ice formation), but may become
sterile at lower temperatures.
Some Quercus and Fagus species can partially compensate for shoot damage generated by (late)
frost, while only few conifer species may substitute lost shoots. Young shoots of northern species
(Picea, Pinus, Populus, Betula) are slightly sensitive to frost injury, meanwhile other species,
characterized by early sprouting or not adapted to cold climates are more susceptible to frost injury
(Fagus, Tilia, Fraxinus, Quercus petraea, Castanea, Platanus, Juglans).
The progress of the cold damage depends on a range of factors, eg degree and duration of cooling,
the plant’s ontogenic state and its state of acclimatization; the progress of cold damage also
depends on a number of boundary conditions, eg speed of temperature change, soil temperature,
air humidity, light ingress during and after chilling(PuUl p255).
XXX-flowering induction, florescence (PuUl p255)

Water, water uptake, water restriction (“water stress”)
All active life in plant and animal cells require water, and fresh wood plant tissues may contain 4060% of water, herbaceous plant tissues even 80-90%.
The water circulating as freshwater makes only a tiny portion of the Earth’s total water reserves,
but average water vapor of 3mm rainfall at any given time is enough to drive life processes of the
land ecosystems.
In the Caucasus region more than 2/3 of terrestrial evapotranspiration is likely to come from plant
transpiration and not more than 15% from open soil and water surfaces, a similar amount is from
immediate evaporation of rainfall or air moisture intercepted by plant & other surfaces.
Some ‘poikilohydrous’ plants such as funghi, lichens, certain algae and some mosses depend mainly
on the humidity of the environment, meanwhile ‘homoiohydrous’ or higher plants possess the
capacity of internal water storage, which makes them independent from short-term fluctuations in
external humidity.
Ecosystems show a maximum potential evapotranspiration at a given site, when there is a complete
plant coverage and no limitation of water supply and water uptake.
Forests, due to their high leaf area per surface area, and their high rooting depth, are the
ecosystems with highest evapotranspiration. In the temperate zone, they may release some 50% of
the precipitation sums, while grass surfaces may show some 30-50% of evapotranspiration (in the
same climate zone). As forests are aerodynamically rough systems, their interception rates are
higher than those of aerodynamically smooth surfaces (eg grasslands). Mixed forests show
interception rates of 15-30%, evergreen coniferous forests 20-35%, in tropical evergreen forests
interception may be as high as 35-70%.
On a global scale, growth and productivity of ecosystems and plants is limited by restricted water
supply (water deficits) more than by any other factor, and the balance between water ingress
(above&belowground) and evapotranspiration is decisive for which plant functional types and
species occur at a given site. Precipitation is the key determinant of tree growth, severe droughts
thereby may trigger decline of the dominating tree cover.
Evapotranspiration includes transpiration from plants, evaporation from the bare soil and
interception (evaporation of rainfall and dew intercepted by the plant canopy). The change of
water’s phase from liquid water to gaseous water vapor (below the boiling point) takes place at or
near the respective surface.
More than 90% of higher plant transpiration goes through stomata (pores), and less than 10%
through the plant’s / leaves’ waxy cuticle (its ‘skin’). Stomata opening is regulated by environmental
factors such as water supply, CO2 concentration, temperature (closure occurs at 30-35°C), air
currents and humidity, etc.
Only when stomata are open, gas exchange (ingress of CO2, H2O, release of H2O, O2) can occur,
affecting nutrient transport and photosynthesis. Although open stomata pores only make up 1% of a
leaf’s surface area, a leaf transpires 50-70% the amount of evaporation from an open water surface,
which is due to high lengths of border lines along the open stomata.
Transpiration allows water and nutrient/element uptake and transport in the plant; it allows plants
to cool their surfaces. Photosynthesis itself only requires only little water, but open stomata are
needed by most plants for CO2 uptake from the air so that photosynthesis is not diminished, carbon
can be ‘assimilated’, and energy production and transport can take place.
It’s solar radiation cast on leaf surfaces, often also a reduction in air humidity, which creates a
demand of water to be transpired: this water deficit is responded by water uptake from roots (when
water is available) which occurs together with nutrient uptake, through root tips and via the waterconducting tissues – in woody plants, living sapwood tissue.
Water stress (lack of water) at the root level causes stomatal closure, and where water restriction is
frequent (eg semiarid regions), only certain type of perennial or woody species and plant functional
types may tolerate this restraining environmental condition and reproduce themselves.
In the temperate zone, it’s mainly the older leaves which are more susceptible to stress situations
like that and, in consequence, are shed earlier by the plant/tree. On the other hand, xylem
embolism, the collapse of water conducting cells and cavitation, the formation of intercellular
cavities due to enlargement of intercellular spaces, are frequently caused by water stress and/or
frost drought (stomata opening in warm air and high irradiance, meanwhile frozen water in soil or
conducting/sapwood plant tissue).
In forest trees, the initiation of water movement starts in the morning in the crown, then in the
stem and the roots. At midday, transpiration losses usually lead to a slight reduction of the xylem
and stem cross-surface area (‘basal area’); during the afternoon and at night, these water stores are
filled again. Transpiration depends stronger on the difference between the water status of the tree
and the ambient air (‘vapor pressure deficit’), than on temperature alone. Thereby warm, dry air
during the growing season leads to either high transpiration (under sufficient water supply) or
water-deficits and drought damage (restricted water supply) of forest and other vegetation. (tab
6.5)
Although plants possess certain prevention and repair instruments (photo-inhibition, stomatal
closure, leaf construction, leaf orientation, etc.) to respond to high levels of light and heat, water
supply can prevent or diminish such effects while water stress usually accelerates them.
At optimal (25°C) or suboptimal (15°C) temperatures photosynthetic capacity only decreases at
relatively low ‘relative’ water contents in the leaves (60%), meanwhile at over-optimal temperatures
(35°C) already at levels of 80% of ‘relative’ (complete) water content, photosynthetic capacity is
reduced.
Plant growth, ie cell growth and leaf growth, depend on the irreversible extension of cells and their
cell walls. This happens only if they are water/liquid saturated and the required cell turgor (cell
pressure) is reached. Loss of cell turgor therefore means decreased leaf growth. Water shortage
means: restricted leaf and shoot growth, reduced shoot/root growth ratio, and under extreme
water shortage- wilting, leaf senescence and abscission, and shedding of transpiring leaf area. But
plants have a certain possibility to adapt to water deficits and avoid loss of tissues. After extreme
water shortage trees may however react by increasing the non-water conducting xylem area (‘dead’
heartwood surface area) above normal, a process which cannot be quickly revolved.
Plants already respond to mild soil water-supply restrictions, even when plant-water relation is
conserved. Plants may respond to mild water restrictions with less foliage, greater apical (‘toporiented’) dominance of branching patterns, and smaller leaves. At mild water restrictions, growth
of rootles (increasing chances of water uptake) and long roots (searching for water) may increase,
meanwhile photosynthetic activity and shoot growth may become sharply reduced. Mild water
shortage thereby leads to higher root/shoot ratios (ie more assimilate allocation towards roots than
shoots), indicating that water restriction favors root growth above shoot growth.
Within an ecosystem, the soil represents a mid-to long-term store of water, whose accessibility to
plants depends on rooting depths and intensity of the plants, but also on soil texture and soil water
content in different depths. Soil evaporation of water, in addition, depends on the vertical soil
temperature gradient. When water is no longer accessible for roots (at high soil water-pressure
deficits, -15 bar), ‘water stress’ occurs and water uptake becomes impeded, even if root systems
and rootlets are in healthy conditions. Thereby, not only water-, but also nutrient uptake for plants
is inhibited.
Water excess (eg due to flooding or excess rainfall) and insufficient water drainage in soils (i.e. in
clayey and/or compacted soils) can cause waterlogging and insufficient aeration (O2 deficiency) of
the underground portion of the plants, their active fine root systems, and if prolonged, leading to
decay of active fine roots. This has strong implications for water and nutrient uptake and other plant
life processes.
Drought periods often affect shoot elongation, and thereby competitiveness among tree species. In
determinate-growth species (eg Fagus, Picea; for example, Castanea is in-determinate) drought
during bud development commonly reduces shoot-growth during the following season.
In climates with pre-dominant winter-rain (like the Mediterranean, and the Caucasus region), the
balance between growing season of sufficient daily mean temperatures (>5°C), available soil water
and rainfall controls the effects of droughts.
fruit production
Water restriction also influences the balance between vegetative and reproductive growth (Koz
Pall); these are negatively correlated: A heavy crop of fruits, cones and seeds is associated with
reduced vegetative growth in the same or following year (or even years). Reproductive growth can
be favored by subjecting trees to drought during the early stages of fruit development to inhibit
vegetative growth, later continuing with normal irrigation. Short periods of drought at critical times
can induce formation of flower buds and break dormancy of flower buds in some species.
Water deficits may may induce flowering directly or by inhibiting shoot flushing (and thereby
limiting young leaves’ capacity to inhibit floral induction).
Water shortage after fruit harvest induces abundant return bloom in well irrigated plants. Fruit
yields of some species may even be increased by withholding (briefly suspending) irrigation during
the period of shoot elongation.

Carbon Dioxide, uptake
The carbon dioxide content of the atmosphere has a direct effect on photosynthetic activity in most
plants (“C3”) plants. During much of Earth history, CO2 contents were much higher than today, eg 710 times during the Cretaceous (145-65 M yr ago), during the last 200 000 yrs these have been
between 100-300 ppmv, since the beginning of industrialization, atmospheric CO2 has risen to 400
ppmv.
Elevated atmospheric CO2 is known to cause less opening of stomata, therefore resulting in less
transpiration per assimilated carbon; a unit of growth thereby will result in less water ‘loss’ via
transpiration. CO2 is also called an ‘anti-transpirant’ for plants. However, plants and ecosystems may
adapt to increasing carbon dioxide levels, eg by reducing number or size of stomata per surface
area.
Different plant species (C3, C4), species groups (eg herbaceous and tree species) but even individual
species of the same family (eg Fagus – Castanea) respond differently to carbon dioxide and its
changes in the atmosphere, either by different photosynthetic pathways (C3-C4 / C4-plants have the
capacity to internally concentrate CO2), different rates of photosysnthesis (herbaceous having higher
rates than tree species), or a more (Fagus) or less (Castanea, Picea) direct response of
photosynthetic activity to atmospheric CO2 concentration (‘partial pressure’). But it is not clear, how
and how much CO2 influences competition and competitiveness between plant and tree individuals
and species.
But photosynthesis is not only influenced by the atmospheric CO2 concentration; water and light,
but also temperature, nutrient supply and wind conditions are crucial for photosynthesis.
The overall outcome of photosysnthesis, ie net-photosynthesis and net-plant and -ecosystem
production, are also influenced by other plant-internal processes, such as photorespiration (during
photosysnthesis), dark respiration, leaf-, wood- and root-respiration

Nutrients N P K Ca Mg Micronutrients, Nutrient uptake, Nutrient restrictions
In addition to Carbon, Hydrogen and Oxygen, which constitute the main part of plant dry matter,
some 13 chemical elements are considered essential for the normal growth and development of
trees. (YoungGiese 105; tb 5.1)
Elements absorbed in relatively large amounts are called macronutrients: N P K Ca Mg S
Essential elements absorbed in small but important quantities are called micronutrients: Fe Mn B Cu
Mo Zn Cl
Nitrogen is mainly present in organic form in the top organic layer and humus of forest soils from
where it is absorbed by roots as ammonium (NH4+) or nitrate (NO3-), facilitated by the action of
bacteria in the process of nitrogen mineralization
Phosphorus is present in organic forms and (secondary) inorganic phosphate compounds, and can
best be taken up under near-neutral pH conditions.
Potassium, Calcium and Magnesium are mainly provided by weathering of soil minerals, and hold
available as exchangeable, water soluble mono/divalent cations on the soil-mineral surfaces. Sulfur
can be present in organic and mineral forms and can be taken up as exchangeable and water-soluble
sulfate.
Micronutrients are either present in mineral forms or as complexes with organic matter; they may
be in short supply especially in sandy or organic soils, but also in intensively-cropped soils.
Nutrient supply and uptake depends chiefly on the condition of roots and their environment, such
as soil moisture, temperature, aeration; the pH in the soil environment; the humus condition of the
forest floor and the mineral soil; the availability of exchange surfaces and sites for nutrient storage
in the clay-mineral fraction of the soil minerals.
Many tree species try to re-locate nutrients within the tree, eg before leaf and branch shedding
occurs, especially when nutrients are in short supply.
Environmental stress such as pollution, leaching, surface runoff, erosion, gaseous losses and product
(wood) removal confront input of nutrients into the ecosystems, eg by weathering of minerals,
atmospheric deposition or application of fertilizers.

Liebig’s law of the Minimum (Young fig 6.5)
In any ecosystem, and also in forests, performance of tree and other plant species is, in the first
place, governed by the availability of different required resources (light, water, nutrients).
These resources are supplied via soil and air through root tips and leaf stomata, under intermediate
weather (i.e. temperature) conditions.
The plant and the tree react most strongly to the improvement of the supply of the resource, which
is the main limiting (“minimum”) factor at a certain time.
Taking into account the balance of the above mentioned factors, the diversity of the environmental
conditions and the responding plant communities, can be understood.
2. Processes

growth, respiration, net photosynthesis

competition for resources

age-dependant processes

renewal
o
death, decomposition
o
leaf and branch shedding and self pruning
o
regeneration, from seeds & dispersal, migration
o
regeneration, vegetative, from root sprouts, stool sprouts (coppice) and cuttings
o
3. Structure and dynamics

succession (structure in time)

triggers of succession (abiotic, biotic & age)

tree dispersal (PuUl 265)

vertical ecosystem structure

horizontal ecosystem structure
o
gap-effects
o
edge-effects
o
shelterwood

spring snow pack (275)

north-facing, south-facing aspects
4. Diversity
