EE-Unit-I Forest ecosystem

Ecosystem concept

Often forest ecosystems are studied in watersheds draining to a monitored stream: thestructure is then defined in vertical and horizontal dimensions. Usually the canopy of the tallesttrees forms the upper ecosystem boundary, and plants with the deepest roots form the lowerboundary. The horizontal structure is usually described by how individual trees, shrubs, herbs,and openings or gaps are distributed. Wildlife ecologists study the relation of stand andlandscape patterns to habitat conditions for animals.

Woody trees and shrubs are unique in their ability to extend their branches and foliage skywardand to capture carbon dioxide and most of the incoming photosynthetically active solarradiation. Some light is reflected back to the atmosphere and some passes through leaves tothe ground (infrared light). High rates of photosynthesis require lots of water, and many woodyplants have deep and extensive root systems that tap stored ground water between rainstorms. Root systems of most plants are greatly extended through a relation between plantsand fungi, called mycorrhizal symbiosis.

The biomass of a forest is defined here as the mass of living plants, normally expressed as dryweight per unit area. Biomass production is the rate at which biomass is accrued per unit areaover a fixed interval, usually one year. If the forest is used to grow timber crops, productionmeasures focus on the biomass or volume of commercial trees. Likewise, if wildlife populationsare the focus of management, managers may choose to measure biomass or numbers ofindividual animals. Ecologists interested in the general responses of forest ecosystems,however, try to measure net primary production (Npp), usually expressed as gross primaryproduction (Gpp) minus the respiration of autotrophs (Ra).

Another response commonly of interest is net ecosystem production (NEP), usually expressed as where Rh is respiration of heterotrophs.

Productivity is the change in production over multiple years. Monitoring productivity isespecially important in managed forests. Changes in forest productivity can be detected only over very long periods.

Forested ecosystems have great effect on the cycling of carbon, water, and nutrients, andthese effects are important in understanding long-term productivity. Cycling of carbon, oxygen,and hydrogen are dominated by photosynthesis, respiration, and decomposition, but they arealso affected by other processes. Forests control the hydrologic cycle in important ways.Photosynthesis requires much more water than is required in its products. Water is lost back tothe atmosphere (transpiration), and water on leaf and branch surfaces also evaporates underwarm and windy conditions. Water not taken up or evaporated flows into the soil and eventuallyappears in streams, rivers, and oceans where it can be reevaporated and moved back overland, completing the cycle.

Forest plants and animals alter soil characteristics, for example, by adding organic matter,which generally increases the rate at which water infiltrates and is retained. Nutrient elementscycle differently from water and from each other.

Elements such as phosphorus, calcium, and magnesium are released from primary minerals inrocks through chemical weathering. Elements incorporated into biomass are returned to the soilwith litterfall and root death; these elements become part of soil organic matter and aremineralized by decomposers or become a component of secondary minerals. All elements canleave ecosystems through erosion of particles and then be transported to the oceans anddeposited as sediment. Deeply buried sediments undergo intense pressure and heat thatreforms primary minerals. Volcanoes and plate tectonic movements eventually distribute thesenew minerals back to land.

Nitrogen is the most common gas in the atmosphere. Only certain bacteria can form a specialenzyme (nitrogenase) which breaks apart N₂ and combines with photosynthates to form aminoacids and proteins. In nature, free-living N₂-fixing microbes and a few plants that can harborN₂-fixing bacteria in root nodules play important roles controlling the long-term productivity offorests limited by nitrogen supply. Bacteria that convert ammonium ion (NH⁺₄) to NO^–₃(nitrifiers) and bacteria that convert NO^–₃ back to N₂ (denitrifiers) are important in nitrogen cycling as well.

Changes in the plant species of a forest over 10 to 100 years or more are referred to assuccession. Changes in forest structure are called stand development; changes in composition,structure, and function are called ecosystem development. Simplified models of succession and development have been created and largely abandoned because the inherent complexity of the interacting forces makes model predictions inaccurate.

Streams

One of the products of an undisturbed forest is water of high quality flowing in streams. Theecological integrity of the stream is a reflection of the forested watershed that it drains. Whenthe forest is disturbed (for example, by cutting or fire), the stream ecosystem will also bealtered. Forest streams are altered by any practices or chemical input that alter forestvegetation, by the introduction of exotic species, and by the construction of roads that increasesediment delivery to streams.

Vertebrates

Forest animals are the consumers in forest ecosystems. They influence the flow of energy andcycling of nutrients through systems, as well as the structure and composition of forests,through their feeding behavior and the disturbances that they create. In turn, their abundanceand diversity is influenced by the structure and composition of the forest and the intensity,frequency, size, and pattern of disturbances that occur in forests. Forest vertebrates make upless than 1% of the biomass in most forests, yet they can play important functional roles inforest systems.

Invertebrates

Invertebrates are major components of forest ecosystems, affecting virtually all forestprocesses and uses. Many species are recognized as important pollinators and seeddispersers that ensure plant reproduction. Even so-called pests may be instrumental inmaintaining ecosystem processes critical to soil fertility, plant productivity, and forest health.

Invertebrates affect forests primarily through the processes of herbivory and decomposition.They are also involved in the regulation of plant growth, survival, and reproduction; forestdiversity; and nutrient cycling. Typically, invertebrate effects on ecosystem structure andfunction are modest compared to the more conspicuous effects of plants and fungi. However,invertebrates can have effects disproportionate to their numbers or biomass.

Changes in population size also affect the ecological roles of invertebrates. For example, smallpopulations of invertebrates that feed on plants may maintain low rates of foliage turnover andnutrient cycling, with little effect on plant growth or survival, whereas large populations candefoliate entire trees, alter forest structure, and contribute a large amount of plant material andnutrients to the forest floor. Different life stages also may represent different roles. Immaturebutterflies and moths are defoliators, whereas the adults often are important pollinators.

Microorganisms

Microorganisms, including bacteria, fungi, and protists, are the most numerous and the mostdiverse of the life forms that make up any forest ecosystem. The structure and functioning offorests are dependent on microbial interactions. Four processes are particularly important:nitrogen fixation, decomposition and nutrient cycling, pathogenesis, and mutualistic symbiosis.

Nitrogen fixation is crucial to forest function. While atmospheric nitrogen is abundant, it isunavailable to trees or other plants unless fixed, that is, converted to ammonia (NH₄), by eithersymbiotic or free-living soil bacteria.

Most microorganisms are saprophytic decomposers, gaining carbon from the dead remains ofother plants or animals. In the process of their growth and death, they release nutrients fromthe forest litter, making them available once again for the growth of plants. Their roles incarbon, nitrogen, and phosphorus cycling are particularly important. Fungi are generally mostimportant in acid soils beneath conifer forests, while bacteria are more important in soils with ahigher pH. Bacteria often are the last scavengers in the food web and in turn serve as food to ahost of microarthropods.

Microorganisms reduce the mass of forest litter and, in the process, contribute significantly tothe structure and fertility of soils as the organic residue is incorporated.

Some bacteria and many fungi are plant pathogens, obtaining their nutrients from living plants.Some are opportunists, successful as saprophytes, but capable of killing weakened orwounded plant tissues. Others require a living host, often preferring the most vigorous trees inthe forest. Pathogenic fungi usually specialize on roots or stems or leaves, on one species orgenus of trees.

Pathogenic fungi are important parts of all natural forest ecosystems. The forest trees evolvedwith the fungi, and have effective means of defense and escape, reducing the frequency ofinfection and slowing the rates of tissue death and tree mortality. However, trees are killed, andthe composition and structure of the forest is shaped in large part by pathogens.

Pathogens remove weak or poorly adapted organisms from the forest, thus maintaining thefitness of the population. Decay fungi that kill parts of trees or rot the heartwood of living treescreate an essential habitat for cavity-nesting birds and the other animals dependent on hollowtrees.

By killing trees, pathogens create light gaps in the forest canopy. The size and rate of light gapformation and the relative susceptibility of the tree species present on the site determine theecological consequences of mortality. Forest succession is often advanced as shade-toleranttrees are released in small gaps. Gaps allow the growth of herbaceous plants in the island oflight, creating habitat and food diversity for animals within the forest. In many forests,pathogens are the most important gap formers and the principal determinants of structure and succession in the long intervals between stand-replacing disturbances such as wildfires orhurricanes.

The fungus roots of trees, and indeed most plants, represent an intimate physical andphysiological association of particular fungi and their hosts. Mycorrhizae are the products oflong coevolution between fungus and plant, resulting in mutual dependency. Mycorrhizae areparticularly important to trees because they enhance the uptake of phosphorus from soils.Mycorrhizal fungi greatly extend the absorptive surface of roots through the network of externalhyphae.