Читать книгу Forest Ecology - Dan Binkley - Страница 25

The tulip poplar tree is enmeshed in a complex ecological system (Figure 1.5). The tree provides habitat for an intricate community of insects and other arthropods. Each kilogram of leaves supports a total arthropod community of about 1 g (Schowalter and Crossley 1988), so the total leaf mass of the crown of 25 kg would support about 25 g of arthropods. Some of these invertebrates feed on the tree, eating leaves (or the insides of leaves), sucking sap, and boring into the wood of branches and the stem. Occasionally the populations of tree‐feeding insects might increase to the point where much of the forest canopy is eaten; in most cases trees survive defoliation by native insect herbivores and form new canopies in the same season. Forests have other arthropods that feed on the species that feed on trees, forming complex food webs that include small mammals, a dozen or more species of birds, and even fish in streams and ponds that feed on arthropods from within the forests.

FIGURE 1.5 The dominant tulip poplar tree in the center of this springtime photo is part of a complex ecological system that includes other tulip poplars, other trees from more than a dozen species, several dozen species of understory plants, hundreds of species of arthropods and other invertebrates, and a soil that is itself a complex system with a level of biodiversity that dwarfs the diversity of the rest of the forest.

Does the tree benefit from neighbors, or is competition for resources the major effect of neighbors? Competition between trees is very important in all forests, but some possibilities exist for interactions between trees that actually benefit neighbors. One example is having a nitrogen‐fixing black locust tree as a neighbor. The tulip poplar would compete with the locust for light, water, and other nutrients, but it might benefit from the enrichment of the soil N supply by the locust. Dozens of species of plants in the understory also compete with overstory trees for soil water and nutrients.

The diversity of plant species may be impressive, but this diversity is overshadowed by the diversity of invertebrates. Each square meter of soil contains about 60 large invertebrates with a total mass of about 1 g (Seastedt and Crossley 1988). The number of small invertebrates would be on the order of 10 000 individuals (from hundreds of species) in each square meter; most of these feed on soil fungi.

FIGURE 1.6 Although this looks like a topographic map of the Coweeta Basin, the colors actually represent the amount of water available for use by trees (hot colors are droughty sites, cool colors are wetter sites), and for draining into streams. Higher elevations receive more rainfall (and snow) than lower elevations, but water also flows downslope through soils, enriching lower parts of landscapes.

Source: Map provided by D.L. Urban.

Each kg of the upper mineral soil contains about 1 or 2 g of fungi, bacteria, and Archaea (Wright and Coleman 2000). The microorganisms are responsible for the majority of the processing of dead plant materials, returning carbon dioxide to the atmosphere, releasing inorganic nutrients into the soil, and altering soil structure and aggregation in ways that protect some organic matter from decomposition for decades, centuries, and even millennia. The small size of the soil microorganisms is matched by an almost unimaginable diversity of “species” or taxonomic units (as the concept of species does not apply well to many microbes). A 10 m by 10 m patch of soil likely contains more than 1000 species (or taxonomic units) of Archaea, another 1000 species of fungi, more than 10 000 species of bacteria, and 10 000 varieties of viruses (Fierer et al. 2007). This biocomplexity remains a largely unexplored frontier in the ecology of forests.

No two locations in the Coweeta Basin have exactly the same forest structure and composition, because local details (such as small variations in soils, or legacy of historical events) always shape local forests. Some broad forest patterns do repeat across the landscapes, as a result of patterns in topography. Precipitation increases by about 5% with each 100 m increase in elevation, rising from about 1500 mm yr⁻¹ at 700 m elevation to more than 2200 mm yr⁻¹ at 1500 m. Local topography modifies this elevational pattern, as wind flow near ridges can lead to 30% less precipitation falling below the ridgelines than would be expected based on elevation alone (Swift et al. 1988). The water available for use by trees (and flow into streams) depends heavily on local topography. Forests on ridgelines receive water from precipitation, and lose water through evaporation, transpiration by plants, and seepage downhill. Forests lower on the landscape receive water not only as precipitation, but also as water draining from higher slopes. Although more rain falls at higher elevations at Coweeta, some forests at lower elevations have access to more water because of this downhill flow (Figure 1.6).

Temperature also changes with elevation, falling by about 0.5 °C for every 100 m gain in elevation; moist air shows less temperature change with elevation than dry air. The landscape pattern in temperature is also strongly influenced by slope and aspect; the amount of incoming sunlight can vary by more than a factor of two from south‐facing slopes to north‐facing slopes, generating temperature differences of several degrees. Steep slopes receive more light than flat areas if the aspect points toward the sun, or less light if the aspect faces away from the sun.

These patterns in soil water, sunlight, and temperature lead to predictable patterns in forest structure and composition. Concave slopes (coves) have abundant supplies of water and deep soils, with large forests dominated by tulip poplar, black birch, and eastern hemlock. Dry ridges and convex slopes have smaller forests of oaks and pitch pine. Uniform slopes at lower elevations have mixed‐deciduous forests dominated by white and red oaks, hickories, and nitrogen‐fixing black locust. Uniform slopes at higher elevations are typically dominated by northern hardwood forests, with sugar maple, red oak, and beech.

Differences in species with elevation and topography also lead to differences in forest diversity and size. Lower elevation forests in the Coweeta Basin average about 18 tree species in a hectare, with diversity declining to about 14 tree species ha⁻¹ at upper elevations (Figure 1.7). Diversity shows no trend with topography, as concave locations (coves) have about the same number of species ha⁻¹ as convex (ridge) locations. The largest forests occur at middle elevations, and in concave locations.

FIGURE 1.7 Forest patterns commonly vary with elevation and with local topography. The number of tree species occurring in a hectare at Coweeta declines slightly with increasing elevation (upper left), whereas tree diversity shows no pattern among concave (cove) locations through to convex (ridge) locations (upper right). The basal area of trees tends to be highest at middle elevations (lower left), and in concave slope locations.

Source: Data from Elliott 2008.