Forests provide excellent introductions to the foundational concepts of ecology, since they are among the most structurally and biologically diverse of the planet’s ecosystems. Forests are diverse, ranging from thorn-scrub woodlands along East Africa’s Indian Ocean coast to cold boreal woodlands of spruce and birch along the Arctic Circle to bottomland swamp fronting a blackwater river in the American South. Forests are built upon highly complex relationships between living and non-living components.
Abiotic and Biotic
Abiotic components are non-living parts of an ecosystem involved in its function. In a forest, they include such fundamentals as sunlight, moisture, nutrients, bedrock and topography. Biotic components are the living members of the system, from sky-scraping canopy trees to the intestinal microbes of a shrew. A defining trait of an ecosystem is the cycling of nutrients and energy between organisms and their environment, and it is these abiotic components that provide the foundation for interaction between members of ecological communities.
The cycling and recycling of elements within an ecosystem and the associated interdependence of its organisms are results of the inherently finite amount of matter on Earth. Energy flow and transformation allow countless species to utilize these foundational ingredients. Plants harvest essential elements -- such as water, carbon and nitrogen -- from the soil, using them to construct tissue and manufacture food in combination with sunlight. By consuming plants or plant-eaters, animals access the fruits of these chemical processes. The decomposition of dead plants and animals returns nutrients to the soil for future use in the ecosystem web.
An ecological disturbance is a discrete event that invokes a notable alteration of a given ecosystem. While some disturbances are biological -- outbreaks of insect pests, for example, or human logging -- many are abiotic. Windstorms, lightning-sparked wildfires, and volcanic eruptions are examples of abiotic disturbances. The inevitability of such dramatic events -- even where they are relatively uncommon -- ensures that most forests are a patchwork of successional stages: that is, within the broader framework of the forest ecosystem are distinct vegetation communities distinguished by species composition, age and structure. In some parts of the Yellowstone Plateau, for example, Engelmann spruce and subalpine fir, which are shade-tolerant, eventually overtop a canopy of lodgepole pine, which is not. However, summer wildfire, a native abiotic force in Yellowstone, commonly curtails the conversion of the forest to mixed spruce-fir by allowing lodgepole pine to quickly pioneer a burned tract. In many cases, in fact, lodgepole pine depends on fire for reproduction. This cycle guarantees it an enduring presence in the landscape.
The basic lay of the land exerts a huge influence on the biotic components of an ecosystem. Topography, in conjunction with climate, helps determine the distribution of moisture and influences local patterns of temperature and air movement. Within a single mile, trees on a ridgetop contend with dry, rocky soil, while those in an adjoining hollow are adapted to heavy, poorly drained conditions and even periods of standing water. A south-facing slope in the Northern Hemisphere receives more sunlight throughout the year than a north-facing one, so woods with southern exposures are commonly more open and dominated by drought-tolerant trees than those on the northern side of the divide.