Forestry & Forest Sciences · Forest Ecology & Biology · also Forest Ecology, Forest Ecology and Ethnobotany, Forest Ecosystem (PRC)
Forests as ecosystems: biotic and abiotic components, energy flow, productivity, nutrient cycling, succession, disturbance and the population and community ecology that silviculture depends on.
In the exams: IFoS Forestry Paper II Section B (forest ecology and ethnobotany); ICAR AIEEA-PG Group 09 (ecosystem, succession, climax); ICAR AICE-JRF 59 Unit 1; PRC Forest Ecosystem subject; GATE EY Section 1
Forest ecology is the study of forests as living systems: the organisms that make them up, the physical environment they live in, and the flows of energy, water and nutrients that connect them. It explains why a forest grows where it does, how much it produces, how it recovers after fire or felling, why some trees regenerate under shade and others only in gaps, and how animals pollinate flowers and carry seeds. Silviculture is often called applied forest ecology, because every silvicultural decision, from the choice of system to the timing of a thinning, works with or against these ecological processes.
This subject covers ecosystem concepts, energy flow and food webs, primary productivity and biomass, the cycles of carbon, nitrogen, phosphorus and water, the population ecology of trees, community structure and diversity, succession and climax, disturbance regimes and gap dynamics, plant-animal interactions, the field methods used to describe vegetation, and ethnobotany and traditional ecological knowledge. It builds on and links to other subjects: silviculture principles covers stand development and self-thinning from the silviculturist's angle; forest inventory covers sampling designs; forest types and biogeography covers the classification of forests; forest hydrology covers watershed processes. Where these overlap, this manual gives the ecological principle and points to the other subject for detail.
Forest ecology is examined in the UPSC Indian Forest Service Forestry optional (forest ecology and ethnobotany in Paper II), in ICAR's AIEEA-PG and AICE-JRF forestry papers (ecosystem, succession, climax and related units), in the Philippine Foresters licensure examination's Forest Ecosystem subject, and in the ecology sections of GATE Ecology and Evolution. Questions range from definitions and classical theories to calculations of productivity, diversity indices and vegetation parameters.
The term ecosystem was introduced by the British ecologist A. G. Tansley in 1935 for a community of organisms together with its physical environment, considered as a single functioning system. An ecosystem has structure, the kinds, numbers and arrangement of its organisms and the physical features of its environment, and function, the processes of energy flow, nutrient cycling, water movement and regulation that link them. Ecosystems have no fixed size: a single log, a stand, a watershed or a whole forest region can be treated as one, depending on the question.
A forest is an ecosystem dominated by trees, whose size and longevity give it distinctive properties: a deep, layered canopy that intercepts most of the light; large stores of carbon and nutrients in wood and soil; a microclimate under the canopy that is cooler, more humid and less windy than the open; and a strong influence on water and soil far beyond its boundaries.
The abiotic (non-living) components are the physical and chemical environment. Climate, the long-term pattern of radiation, temperature, precipitation, humidity and wind, sets the broad limits of forest distribution and productivity; forests generally need enough water for trees to outgrow competition from grasses and shrubs, which is why they give way to savanna and grassland in drier climates and to tundra where the growing season is too short or cold. Soil supplies water, nutrients and anchorage, and its depth, texture, chemistry and drainage shape which species grow. Topography modifies climate and soil through aspect, slope and elevation. Water bodies, fire and the inorganic nutrients cycling through the system are also abiotic components.
The biotic (living) components are grouped by their role in energy flow. Producers, or autotrophs, mainly green plants, fix solar energy by photosynthesis; in forests, trees dominate production but understorey plants, epiphytes, mosses and algae also contribute. Consumers, or heterotrophs, obtain energy by eating other organisms: primary consumers (herbivores) eat plants; secondary and tertiary consumers (carnivores) eat animals; omnivores eat both. Decomposers, mainly fungi and bacteria, break down dead organic matter and release nutrients, aided by detritivores such as earthworms, termites, millipedes and mites that fragment litter. In forests, decomposers process most of the plant production, because only a small fraction of leaves and wood is eaten alive.
Forests are also structured vertically. A typical forest has an emergent layer in some tropical forests, a main canopy, one or more understorey layers of smaller trees, a shrub layer, a herb layer, the forest floor of litter and humus, and the soil with its roots and organisms. Each layer has its own light, temperature and humidity and its own set of organisms, which is one reason forests hold so many species.
Ecosystem services are the benefits people obtain from ecosystems. The Millennium Ecosystem Assessment (2005) grouped them as provisioning services (timber, fuelwood, food, fodder, medicines, water), regulating services (climate regulation through carbon storage, regulation of water flow and quality, erosion control, pollination, pest regulation), cultural services (recreation, spiritual and aesthetic values, education) and supporting services (soil formation, nutrient cycling, primary production) that underlie the others. Forests supply all four kinds. Many regulating and cultural services are not sold in markets, so they are often undervalued when forests are converted to other uses; their valuation is covered in forest economics and valuation. Thinking in terms of ecosystem services helps foresters explain why ecological processes such as nutrient cycling and pollination matter to people who never visit the forest.
Energy enters ecosystems as sunlight, is fixed by producers, passes through consumers and decomposers, and leaves as heat; it flows one way and is not recycled, unlike nutrients. At each transfer from one trophic level (feeding level) to the next, most energy is lost as heat in respiration, as uneaten material and as undigested waste. R. L. Lindeman's work on trophic dynamics in 1942 established this framework. Transfer efficiency between trophic levels is commonly around 10 per cent, the so-called ten per cent rule, though measured values range roughly from 5 to 20 per cent. If producers in a forest make 10,000 units of energy available, herbivores might incorporate about 1,000 and first-level carnivores about 100. This loss limits the number of trophic levels to usually four or five.
A food chain is a sequence of organisms each feeding on the one before; a food web is the network of interconnected chains in a real ecosystem. Ecosystems have two main kinds of chain. In the grazing food chain, living plants are eaten by herbivores, which are eaten by carnivores: leaves eaten by caterpillars eaten by birds eaten by hawks. In the detrital food chain, dead organic matter is consumed by decomposers and detritivores, which are eaten by their predators: leaf litter consumed by fungi and springtails eaten by spiders and beetles eaten by birds and shrews. In forests the detrital chain is dominant: most of the energy fixed by trees, including nearly all wood and most leaves, enters the detrital pathway, while herbivores consume only a small fraction of production except during insect outbreaks.
Ecological pyramids show the relative size of trophic levels. A pyramid of numbers counts individuals; in forests it is often inverted at the base, because a single large tree supports thousands of insects. A pyramid of biomass shows the mass of living organisms at each level; in forests it is upright, with a very large mass of trees and a small mass of animals. A pyramid of energy shows the energy flow through each level per unit time; it is always upright, because energy is lost at every transfer. Examiners often ask which pyramid can be inverted in a forest: the pyramid of numbers.
Food webs also show the importance of particular species. Keystone species have effects on the ecosystem out of proportion to their abundance: large predators that regulate herbivores, figs that supply fruit when little else is available, and ecosystem engineers such as termites and elephants that change the physical environment.
Primary production is the fixation of energy or carbon by producers. Gross primary production (GPP) is the total fixed by photosynthesis. Net primary production (NPP) is GPP minus the respiration of the plants themselves (autotrophic respiration, Ra); it is the production available for growth, reproduction, litter and consumers. Net ecosystem production (NEP) is NPP minus heterotrophic respiration (Rh) by animals and decomposers; it is the net carbon gain or loss of the ecosystem. Over large areas and long periods, losses from disturbances such as fire and harvest are also subtracted, giving net biome production. If a forest has a GPP of 30 tonnes of carbon per hectare per year and autotrophic respiration of 16, its NPP is 14; if heterotrophic respiration is 11, its NEP is 3 tonnes of carbon per hectare per year, a net sink. The ratio NPP/GPP, the carbon use efficiency, is often around one half in forests, though it varies with age and site.
Productivity is a rate (per unit area per unit time); biomass or standing crop is the amount present at one time. A mature forest can have a very large biomass but a low NEP, because its large respiration and decomposition nearly balance its production.
Among terrestrial ecosystems, tropical rain forests have the highest NPP and biomass, followed by tropical seasonal, temperate and boreal forests. Widely cited global summaries, such as those of Whittaker and Likens, give mean NPP values of roughly 2,000 grams of dry matter per square metre per year for tropical rain forest, around 1,200 for temperate deciduous forest and around 800 for boreal forest, with wide ranges within each type, and mean biomass of tropical rain forest of the order of 45 kilograms per square metre (about 450 tonnes per hectare). These figures are approximate and vary with site, age and method. Productivity rises with water, warmth, nutrients and the length of the growing season, and in a given stand with leaf area up to the point where the canopy is closed.
In young stands NPP rises as the canopy develops, peaks around canopy closure, and often declines in older stands; explanations include rising respiration, nutrient limitation, hydraulic limits on tall trees and changes in allocation. This decline is one reason growth and yield tables show a culmination of increment, as covered in forest mensuration and growth and yield modelling.
Biomass is measured by harvesting sample trees and weighing their parts, and more often estimated from allometric equations that predict tree biomass from diameter, height and wood density, applied to inventory plots; below-ground biomass is estimated with root-to-shoot ratios or root excavation. NPP is estimated by summing biomass increment, litterfall, losses to herbivores and fine-root turnover over a year. Eddy covariance towers measure the net exchange of carbon dioxide between the forest and the atmosphere continuously, giving NEP directly, and partition it into GPP and ecosystem respiration with models. Satellite remote sensing estimates GPP and NPP over large areas from measures of absorbed light and vegetation indices. Each method has errors and assumptions, and combining them gives the most reliable picture.
Carbon enters forests as carbon dioxide fixed in photosynthesis and is stored in living biomass, dead wood, litter and soil organic matter. It returns to the atmosphere through respiration by plants, animals and decomposers, and through fire. Forests hold a large share of the carbon in terrestrial vegetation and soils; tropical forests store much of theirs in biomass, whereas boreal forests store most in soils and peat. When forests are cleared or degraded, their carbon is released; when they grow or are restored, they take carbon up, which is why forests are central to climate policy, covered in forests, climate and carbon.
Nitrogen is abundant in the atmosphere as dinitrogen gas but must be fixed into usable forms. Biological nitrogen fixation is carried out by bacteria: Rhizobium and related bacteria in root nodules of legumes, Frankia actinomycetes in nodules of actinorhizal trees such as alders and casuarinas, cyanobacteria in lichens and free-living soil bacteria. Lightning and industrial fixation, and deposition of nitrogen pollution, add more. In the soil, organic nitrogen in litter is converted to ammonium by decomposers (ammonification, part of mineralisation); ammonium is oxidised to nitrite by bacteria such as Nitrosomonas and then to nitrate by bacteria such as Nitrobacter (nitrification). Plants take up ammonium and nitrate. Nitrogen is lost by leaching of nitrate, by denitrification (bacteria reducing nitrate to nitrogen gases in waterlogged soils) and by fire. In many temperate and boreal forests nitrogen is the main limiting nutrient, and forests retain it tightly.
The Hubbard Brook experiment in New Hampshire, led by Likens and Bormann, showed this retention dramatically: when a small watershed was cleared and regrowth was suppressed, nitrate concentrations in the stream rose sharply, because uptake by plants had stopped while mineralisation and nitrification continued.
Phosphorus has a sedimentary cycle with no significant gaseous phase. It enters ecosystems mainly by weathering of rock and is cycled between soil, plants and litter; it is lost by erosion and leaching, and in old, highly weathered soils much of it becomes bound to iron and aluminium oxides in forms plants cannot use. Phosphorus is therefore the commonest limiting nutrient in many lowland tropical forests on old soils, which depend heavily on rapid recycling from litter and on mycorrhizal fungi that extend the roots' reach. Other nutrients, such as potassium, calcium and magnesium, also come from weathering and atmospheric inputs and cycle through litter.
Most nutrients taken up by trees return to the soil in litterfall and through decomposition. The speed of decomposition depends on temperature and moisture, on litter quality (litter with a high carbon-to-nitrogen ratio or high lignin content decomposes slowly) and on soil organisms. A simple index is the decomposition constant k, annual litterfall divided by the mass of the forest floor at steady state; its inverse is the mean residence time of organic matter on the forest floor. A tropical forest with litterfall of 8 tonnes per hectare per year and a forest floor of 4 tonnes per hectare has k = 2 and a residence time of half a year; a boreal forest with litterfall of 2 tonnes and a forest floor of 40 tonnes has k = 0.05 and a residence time of 20 years. This explains thin litter layers in humid tropics and thick humus layers in boreal forests.
Water cycles through forests by precipitation, interception by the canopy (part of which evaporates directly), throughfall and stemflow to the ground, infiltration into the soil, uptake by roots and transpiration, drainage to groundwater and runoff to streams. Forests usually use more water than grassland or crops because of their deep roots and large leaf area, but they reduce peak flows and erosion and improve water quality. Cloud forests can gain water by intercepting fog. Watershed processes are treated in forest hydrology and watershed management.
Most forest trees form mycorrhizas, symbiotic associations between roots and fungi in which the fungus receives carbon from the tree and supplies water and nutrients, especially phosphorus and nitrogen, from a much larger volume of soil than roots alone could reach. Ectomycorrhizal fungi form a sheath around fine roots and a network between root cells; they are typical of pines, oaks, beeches, dipterocarps, eucalypts and many other temperate and some tropical trees, and can access organic forms of nitrogen and phosphorus. Arbuscular mycorrhizal fungi penetrate root cells and are typical of most tropical broadleaved trees, teak and many herbs. The type of mycorrhiza influences nutrient cycling, litter decomposition and how forests respond to nitrogen deposition. Fungal networks can link the roots of neighbouring trees, though claims about large transfers of resources between trees through these networks remain debated.
A tree population renews itself through regeneration: seed production, dispersal, germination and establishment, or vegetative sprouting. Regeneration can come from the seed rain (seeds arriving each year), the soil seed bank (dormant seeds stored in the soil, typical of pioneer species), the seedling bank (suppressed seedlings waiting under the canopy, typical of shade-tolerant species) or sprouts from stumps and roots. Recruitment is the entry of new individuals into a defined size class, such as trees reaching 10 cm diameter, and in demographic studies it is balanced against mortality to judge whether a population is growing or declining. The ecology of regeneration from the silviculturist's viewpoint is covered in silviculture principles.
Most trees die young. A forest tree may produce millions of seeds over its life, but only one, on average, needs to replace it. Tree populations therefore show a survivorship curve of the type III kind: very high mortality among seeds and seedlings, then much lower mortality among established trees. If 10,000 seedlings establish on a hectare and only 100 remain as trees after 50 years, 99 per cent have died. Causes include seed predation, pathogens, herbivory, drought, competition for light and nutrients, fire and wind.
Mortality can be density-independent, affecting individuals regardless of crowding (a drought, a storm), or density-dependent, rising with crowding (competition, pathogens spreading among neighbours). The Janzen-Connell hypothesis, proposed independently by Daniel Janzen and Joseph Connell in the early 1970s, holds that seeds and seedlings close to their parent, or at high density of their own species, suffer more from specialist enemies such as insects and fungi; this favours establishment of other species near each tree and is one explanation for the high tree diversity of tropical forests. Evidence supports the mechanism in many forests, though its strength varies.
In crowded even-aged stands, competition kills the smaller trees as the survivors grow, a process called self-thinning. Yoda and colleagues in 1963 described the self-thinning rule: on logarithmic axes, mean plant mass rises with a slope of about −3/2 against density as the stand self-thins, so when density halves, mean mass rises by about 2 raised to 1.5, or 2.8 times. Reineke's stand density index, described in stand tending, expresses a related relation between tree number and diameter. The exact slopes vary among species and are debated, but the principle that a site can support only a limited total size of trees underlies thinning practice.
Species differ in life-history strategy. Pioneer species, in the classification of Swaine and Whitmore (1988), germinate and establish only in gaps or open ground, grow fast, produce many small seeds often stored in the soil, and are short-lived; non-pioneer or climax species can establish and survive in shade and are usually slower-growing and longer-lived. The older r and K selection concepts contrast species favoured by rapid reproduction in unstable conditions with those favoured by competitive ability in crowded, stable conditions; they are simplifications but help explain contrasts such as those between Macaranga or Trema and dipterocarps.
A community is the set of populations of different species living together in an area. Its structure includes its physiognomy (overall appearance, such as tall evergreen forest or open woodland), its vertical stratification into layers, its horizontal pattern (random, clumped or regular distribution of individuals), its composition (which species are present) and dominance (which species make up most of the biomass or basal area). Raunkiaer's life-form classification groups plants by the position of their perennating buds during the unfavourable season: phanerophytes (buds well above ground, trees and shrubs), chamaephytes (buds near the ground), hemicryptophytes (buds at the soil surface), cryptophytes (buds below ground) and therophytes (annuals surviving as seed). The proportions of life forms, the biological spectrum, reflect climate: tropical humid forests are dominated by phanerophytes, while deserts have many therophytes.
Species diversity has two components: richness, the number of species, and evenness, how equally individuals are distributed among them. Whittaker distinguished alpha diversity (within a site or community), beta diversity (the change in species composition between sites) and gamma diversity (the total diversity of a landscape or region). The Shannon index H′ is minus the sum over species of each species' proportion times the natural logarithm of that proportion; it rises with both richness and evenness. The Simpson index, the sum of squared proportions, measures dominance, the probability that two individuals drawn at random belong to the same species; its complement (one minus the index) or its reciprocal is used as a diversity measure. Pielou's evenness is H′ divided by the natural logarithm of the number of species, giving a value from 0 to 1.
For a community with three species in proportions 0.5, 0.3 and 0.2, H′ = −(0.5 ln 0.5 + 0.3 ln 0.3 + 0.2 ln 0.2) = 1.03; the Simpson index is 0.25 + 0.09 + 0.04 = 0.38, so the Simpson diversity (1 − D) is 0.62; Pielou's evenness is 1.03 ÷ ln 3 = 0.94. The same calculations are used in biodiversity inventory, covered in forest inventory and sampling.
The number of species recorded rises with the area sampled, steeply at first and then more slowly. The species-area relation is often described by the power law S = cA^z, where S is the number of species, A the area and c and z constants, the form associated with Arrhenius and developed by Preston and by MacArthur and Wilson in island biogeography. On logarithmic axes it is a straight line of slope z. Values of z commonly fall roughly between 0.15 and 0.35, lower for samples within continuous areas and higher for isolated islands or habitat fragments. With z = 0.25, doubling the area increases species by 2^0.25 = 1.19, or about 19 per cent, and a tenfold loss of area reduces species by about 44 per cent in the long run. Such relations are used, with caution, to predict species losses from habitat destruction and to decide the minimum sample area in vegetation surveys, where the species-area curve shows the area beyond which few new species are added.
Ecological succession is the directional change in species composition and structure of a community over time. Primary succession begins on newly exposed surfaces with no soil, such as bare rock, lava flows, sand dunes or glacial moraines; pioneer organisms such as lichens and mosses start soil formation and are followed by herbs, shrubs and trees over centuries. A succession starting in water and proceeding through submerged, floating and reed-swamp stages to woodland is a hydrosere; one starting on dry rock or sand is a xerosere. Secondary succession follows disturbance of an existing community where soil remains, such as abandoned fields, logged or burned forests; it is much faster because seeds, roots and soil are present. Each sequence of communities is a sere, and each stage a seral stage.
Several mechanisms drive succession. Connell and Slatyer (1977) proposed three models: facilitation, in which early species modify conditions to favour later ones (such as nitrogen-fixing pioneers enriching soil); tolerance, in which later species simply tolerate the conditions created by earlier ones and outlast them; and inhibition, in which early species resist invasion until they die or are damaged. Real successions involve all three.
The climax is the relatively stable community at the end of succession. Frederic Clements (1916) proposed the monoclimax theory: in a given climatic region all successions converge on a single climatic climax, and the community behaves like a superorganism. Henry Gleason's individualistic concept (1926) argued that communities are chance assemblages of species responding individually to the environment, not organisms. A. G. Tansley's polyclimax theory recognised several stable climaxes in a region, determined by soil, topography, fire and animals as well as climate. R. H. Whittaker's climax-pattern hypothesis treated climax vegetation as a continuous pattern varying along environmental gradients.
Indian forestry, following Champion and Seth, uses terms for communities held below or diverted from the climatic climax: a preclimax is a stage short of the climax because of drier or poorer conditions, a postclimax a stage beyond it because of wetter or better conditions; edaphic and topographic climaxes are set by soil and landform; biotic climaxes and disclimaxes are maintained by grazing, fire or human use; a subclimax is a long-persisting seral stage, such as many fire-maintained forests.
Retrogression, or retrogressive succession, is change towards simpler, less productive or lower-biomass communities. It can be caused by degradation: repeated fire, heavy grazing, lopping, over-cutting and soil erosion can convert closed forest to open woodland, scrub and finally grassland or bare land, as described for many South Asian forests in which degradation stages lead from high forest through scrub to grassland and barren land. Retrogression can also occur naturally on very old soils without catastrophic disturbance: long-term chronosequences in New Zealand, Hawaii, Australia and elsewhere show forest biomass and productivity declining after thousands of years as phosphorus is lost or becomes unavailable. Recognising retrogression matters for management: degraded stages may not recover by succession alone if seed sources, soil or the disturbance regime have changed, and active restoration may be needed, as covered in restoration of difficult sites.
A disturbance is a relatively discrete event that disrupts ecosystem or community structure and changes resources or the physical environment: fire, windstorm, flood, landslide, insect outbreak, disease, or human felling. Ecologists describe the disturbance regime of a forest by the type of disturbance, its frequency or return interval, its size, its intensity (the physical force, such as fire energy release) and severity (the effect on organisms, such as the proportion of trees killed), its seasonality and its interaction with other disturbances. The fire rotation, the time needed to burn an area equal to the whole area, is a common measure: if on average 2 per cent of a landscape burns each year, the fire rotation is 100 ÷ 2 = 50 years. Disturbance regimes shape the age structure, composition and diversity of forests, and many forests depend on them; suppressing a regime can change a forest as much as intensifying it.
The intermediate disturbance hypothesis, associated with Joseph Connell (1978), proposes that diversity is highest where disturbance is intermediate in frequency or intensity, because very frequent disturbance allows only tolerant species and very rare disturbance allows competitive exclusion. It is widely taught but its generality is debated.
Fires are classified as ground fires, burning organic soil and peat; surface fires, burning litter and undergrowth; and crown fires, spreading through the canopy. Fire regimes range from frequent low-intensity surface fires, as in many dry pine and dry deciduous forests, to infrequent high-severity crown fires, as in many boreal and some temperate conifer forests. Species adapted to fire have thick bark, the ability to resprout from roots or stems, serotinous cones that release seed after fire, or seeds stimulated to germinate by heat or smoke. Fire ecology and management are covered in forest fire science.
Wind disturbance ranges from single treefalls to cyclones, hurricanes and storms that flatten large areas; its effects depend on tree height, rooting, soil and stand structure. Insect outbreaks, such as bark beetles in conifer forests and defoliators such as the teak defoliator, can kill or weaken trees over large areas and are often linked to drought and to stands of uniform age and species. Pathogens, from root rots to introduced diseases, act as chronic or catastrophic disturbances.
In many forests, especially humid tropical and old temperate forests, the commonest disturbance is the death of single trees or small groups, which creates canopy gaps. A. S. Watt's 1947 paper on pattern and process described forests as mosaics of patches in different phases of a cycle of gap formation, regeneration, building and maturity. Gap-phase regeneration is the filling of gaps by advance regeneration, seedlings from the seed bank or seed rain, sprouts and lateral growth of neighbouring crowns. Gap size matters: small gaps favour shade-tolerant species already present as seedlings, while large gaps let pioneers establish. Gap dynamics maintain the coexistence of species with different light requirements and are imitated by selection and group selection systems, described in silvicultural systems.
Many forest trees depend on animals to carry pollen. Pollination syndromes are sets of flower traits associated with particular pollinators. Bee-pollinated (melittophilous) flowers are often blue, yellow or white, scented, with nectar guides and landing platforms. Bird-pollinated (ornithophilous) flowers are often red or orange, tubular, unscented and rich in dilute nectar; Bombax ceiba and Butea monosperma are examples visited by birds in South Asia. Bat-pollinated (chiropterophilous) flowers open at night, are pale, large, strongly scented and produce much nectar; durian, Kigelia and many baobabs are bat-pollinated. Moth- and butterfly-pollinated flowers have their own traits. Wind pollination (anemophily) prevails in conifers, oaks, birches and many temperate trees, which produce large amounts of light pollen and small, inconspicuous flowers.
Figs and their fig wasps are a classic obligate mutualism: each fig species is pollinated by one or a few species of tiny wasp that breed only inside its inflorescence. Because figs fruit at different times of year, they supply food to many animals when other fruit is scarce, which makes them keystone resources in tropical forests. In many tropical forests, most tree species are animal-pollinated and occur at low density, so they depend on pollinators that travel between scattered individuals; logging and fragmentation that reduce tree density can reduce seed set and genetic diversity.
Seed dispersal carries seeds away from the parent, reducing competition and enemy attack near it (the Janzen-Connell effect), allowing colonisation of new sites and gaps, and connecting populations. Wind dispersal (anemochory) uses wings and plumes, as in many conifers, dipterocarps, maples and Terminalia. Animal dispersal (zoochory) includes endozoochory, in which fruits are eaten and seeds pass through the gut of birds, bats, monkeys, elephants and other animals, and epizoochory, in which seeds with hooks or sticky coats attach to fur or feathers. Scatter-hoarding animals, such as jays, squirrels and agoutis, bury seeds of oaks, pines and nuts and fail to recover some, which then germinate. Ants disperse seeds with fatty appendages (myrmecochory). Water disperses mangroves and many riverside trees (hydrochory); mangroves such as Rhizophora are viviparous, their seedlings developing on the parent before falling.
Large frugivores such as hornbills, primates, elephants and fruit bats disperse large-seeded trees that small animals cannot move. Hunting and habitat loss that remove these animals (defaunation) can reduce dispersal and regeneration of large-seeded trees, changing forest composition over time, which links wildlife conservation directly to forest regeneration.
Herbivores eat leaves, shoots, bark, roots, flowers and seeds. Most herbivory in forests is by insects and removes a modest fraction of leaf area in normal years, but outbreaks can defoliate whole stands. Browsing deer, elephants and livestock can prevent regeneration of palatable species and shape forest structure, which is why fencing and grazing control are central to regeneration in many regions. Seed predators can destroy most of a crop in non-mast years. Plants defend themselves with thorns, tough leaves and chemicals such as tannins and alkaloids. Mutualisms below ground, with mycorrhizal fungi and nitrogen-fixing bacteria, are equally important to forest functioning.
Vegetation is described by sampling. A quadrat is a sample plot of defined size and shape, usually square or rectangular; quadrat size is chosen to suit the vegetation, larger for trees and smaller for shrubs and herbs, and is often guided by the species-area curve, the minimal area beyond which few new species are added. Nested quadrats use small plots for herbs, larger ones for shrubs and saplings and the largest for trees. A transect is a line or strip along which observations are made: a line transect records plants touching or intercepted by a line, and a belt transect is a strip of fixed width, often divided into quadrats. Transects are useful across gradients, such as from valley to ridge or from forest edge to interior. Permanent plots, such as large forest dynamics plots in which every tree above a small diameter is mapped and remeasured, are used to study demography and diversity over decades.
Plotless methods estimate density from distances. In the point-centred quarter method of Cottam and Curtis (1956), the area around each sample point is divided into four quarters and the distance to the nearest tree in each is measured; the mean of these distances, squared, gives the mean area per tree, so density is one divided by the square of the mean distance. With a mean distance of 4 m, the mean area per tree is 16 square metres and density is 10,000 ÷ 16 = 625 trees per hectare.
Several standard measures describe each species. Frequency is the percentage of quadrats in which the species occurs; a species present in 12 of 20 quadrats has a frequency of 60 per cent. Density is the number of individuals per unit area; 36 individuals counted in 20 quadrats of 100 square metres each (0.2 hectare in total) give a density of 180 per hectare. Abundance is the number of individuals per quadrat of occurrence; here 36 ÷ 12 = 3. Dominance or cover is usually measured for trees as basal area per unit area, and for herbs as the percentage of ground covered.
Relative values express each species as a share of all species: relative density is the species' density as a percentage of total density; relative frequency is its frequency as a percentage of the sum of frequencies of all species; relative dominance is its basal area as a percentage of total basal area. The importance value index (IVI), widely used in Indian and tropical phytosociology, is the sum of these three relative values for a species; across all species the IVI totals 300. Species with the highest IVI are the dominants used to name the community, such as a sal-dominated or teak-dominated forest.
Reliable vegetation data need a clear objective, a sampling design that avoids subjective placement of plots (covered in forest inventory and sampling), correct identification backed by voucher specimens deposited in a herbarium, standard measurement rules, recording of location by GPS, and data management that preserves raw records. Ordination and classification methods, such as cluster analysis and detrended correspondence analysis, are used to group samples into community types and relate them to environmental gradients. Results should be reported with the sampling effort and methods, so that later surveys can repeat them and detect change.
Ethnobotany is the study of the relationships between people and plants: how cultures name, classify, use, manage and value plants for food, medicine, fibre, fuel, construction, ritual and other purposes. The term was coined by the American botanist J. W. Harshberger in 1895. In forestry, ethnobotany documents the uses of forest plants by forest-dwelling and local communities, many of which are non-timber forest products, and helps understand how communities have shaped forests through selective use, burning, planting and protection.
Ethnobotanical methods include semi-structured interviews, participant observation, free listing, walks in the forest with knowledgeable people, and the collection of voucher specimens so that local names can be linked to scientific names. Quantitative indices summarise data: the use value of a species is the number of uses reported for it divided by the number of informants, and the informant consensus factor, from Trotter and Logan (1986), measures agreement among informants on plants used for a category of ailment: the number of use reports minus the number of taxa used, divided by the number of use reports minus one. With 50 use reports for a category citing 11 species, the consensus factor is (50 − 11) ÷ 49 = 0.80, indicating high agreement. Ethical research requires prior informed consent of communities and individuals, respect for their wishes on what is published, and fair sharing of any benefits.
Traditional ecological knowledge (TEK), also called indigenous or local knowledge, is the cumulative body of knowledge, practice and belief about the relationships of living beings with one another and with their environment, handed down through generations, as described by Fikret Berkes. It includes knowledge of species, seasons, soils and fire, as well as institutions that regulate use. Examples in South Asia include sacred groves, patches of forest protected by communities for religious reasons, known by local names such as kavu in Kerala, devrai in Maharashtra, oran in Rajasthan and sarna in Jharkhand, which often conserve old-growth trees and rare species; the Bishnoi community of Rajasthan, whose protection of trees and wildlife is remembered in the Khejarli sacrifice of 1730 to protect khejri trees; and rotational shifting cultivation systems with long fallows. Elsewhere, Aboriginal fire management in Australia and community forest rules in many countries are studied and increasingly incorporated into management.
TEK and scientific knowledge are complementary: TEK offers long-term, place-specific observation and practice, while science offers controlled testing and generalisation. Co-management arrangements, such as joint forest management in India and community forestry in Nepal, rely on both.
Traditional knowledge of plants has commercial value, especially for medicines and cosmetics, and its use without consent or benefit has been called biopiracy. The Convention on Biological Diversity (1992) recognises the sovereign rights of states over genetic resources and the importance of traditional knowledge, and its Nagoya Protocol on access and benefit sharing, adopted in 2010 and in force from 2014, sets rules for prior informed consent and mutually agreed terms. India's Biological Diversity Act of 2002 created the National Biodiversity Authority, state biodiversity boards and local biodiversity management committees, which prepare People's Biodiversity Registers documenting local knowledge and resources. A frequently cited example of benefit sharing is the agreement involving the Kani community of Kerala, whose knowledge of the plant arogyapacha (Trichopus zeylanicus) led to a herbal product, with a share of the proceeds going to a community trust. Foresters working with communities should understand these rules and the ethical obligations that come with documenting traditional knowledge.
A student should be able to define ecosystem components and processes, draw food chains and pyramids, trace the nitrogen, phosphorus, carbon and water cycles in a forest, explain succession and climax theories with examples, and calculate NPP and NEP, decomposition constants, fire rotation, diversity indices, species-area changes, frequency, density, abundance, IVI and point-centred quarter densities. Field practice should include laying quadrats and transects, recording vegetation, collecting voucher specimens and interviewing community members ethically.
A forester five years into service uses ecology daily: reading a degraded forest's stage of retrogression and deciding whether natural regeneration can recover it, judging whether gaps are large enough for the desired species, planning fire and grazing control, protecting pollinators and seed dispersers, describing vegetation for working plans with quadrats and IVI, and working with communities whose traditional knowledge and sacred groves are part of the forest's management.
A senior forester with twenty-five years' experience applies ecological understanding to policy and planning: setting disturbance and fire management regimes for whole landscapes, designing protected area networks and corridors using species-area and connectivity principles, guiding restoration of degraded forests, integrating carbon and biodiversity goals into working plans, and negotiating access and benefit-sharing arrangements with communities under biodiversity law.
A researcher with long field experience studies forest ecosystem processes: carbon and nutrient budgets with eddy covariance and long-term plots, mechanisms of tree diversity such as Janzen-Connell effects, responses of forests to changing disturbance regimes and climate, the consequences of defaunation for regeneration, and the integration of traditional ecological knowledge with scientific monitoring. Such work informs management guidelines and international assessments.
This manual gives ecological principles with representative figures; productivity, biomass and decomposition values vary widely by site and method and are given only as approximate global summaries. Forest type classification and biogeography are in forest-types-biogeography; fire behaviour and management in forest-fire-science; watershed processes in forest-hydrology-watershed; wildlife ecology in wildlife-biology-ecology; and sampling designs in forest-inventory-sampling. Multivariate community analysis is mentioned only by name. Regional ethnobotanical inventories and specific medicinal uses are not listed, because they require species-by-species verification and raise questions of consent and benefit sharing.
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