Forest Structure and Function

In ecological terms, forests can be structurally simple and uniform or highly varied and complex. The structure of a forest and its position in the larger landscape directly affect its ecological functions, including how well it works as habitat for different species.

Some forest stands consist of a single layer of overstory trees forming a canopy with few trees in the understory (ponderosa or long-leaf pine), but more commonly they are comprised of multiple types, species, ages and sizes of trees. Snags and logs of various sizes and states of decay, along with shrubs and herbaceous vegetation, all add to the complexity and habitat value of a forest stand. As a general rule (subject, of course, to many exceptions), greater complexity within a forest stand will provide a greater variety of habitats and thus support more diverse wildlife.

The size or extent of a stand, along with its landscape setting (context), further influence its habitat value. No matter how rich and complex a forest stand may be, if it's small and surrounded by recent clearcuts, agricultural land or residential development, it will provide poorer quality habitat for species associated with complex old forest habitat than a similar stand surrounded by extensive mature and old-growth forests. Habitat degradation of this sort can be a result in part of "edge effects" that allow wind and light to modify microclimates or allow competitors or predators associated with more open habitats to have detrimental effects on forest residents. Habitat value can also be diminished by isolation if surrounding environments prevent population or genetic replenishment by movement of wildlife from other forested areas.

Natural forest landscapes are often a "mosaic" of stands of varying sizes and ages, which adds variety and complexity at another scale, again generally supporting a greater diversity of wildlife than, for instance, an extensive industrial forest of few tree species in a limited range of ages.

These structural characteristics of forests in turn relate to how the forest functions, that is, how effectively does it convert sunlight, carbon dioxide, and water to wood and other living matter (biomass)? How does it influence runoff of water and streamflow? How well are nutrients cycled and soils built and maintained? What habitats does it provide, and how do the wildlife it supports in turn influence forest structure, which in turn affects forest function ?. . . One can quickly gain an appreciation for ecologist Frank Egler's observation that ecosystems are not only more complex than we think, they are more complex than we can think.

In this section. . .

Forest Biodiversity
Forests, in all their variety, provide benefits to biodiversity and management opportunities and challenges that are different from other ecosystem types.
Forest Structure and Function
Natural forests are a mosaic of stands of varying sizes and ages which supports a great diversity of wildlife.
Disturbance Regimes
Disturbances are events and processes that kill or remove substantial portions of trees in a forest: wind, flood, landslides, insect and disease outbreaks, and fire.
Little Things that Run the World
Forests depend on the rich variety of tiny living things residing in or on the soil of the forest.