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.
The Coweeta Forests Aren't the Same as Two Centuries Ago
Forests with large, old trees may give an impression of an unchanging system that seem to be stable for decades and centuries. Some temperate forests may fit this image, but most are quite dynamic. If we could visit a forest before and after 50 years of changes occurred, we would likely find that many of the small trees had died (perhaps replaced by others), along with some of the medium‐ and large‐size trees. The overall size of the forest, in terms of height or mass of wood in living trees, may have increased, but typically this increase in the size of larger trees comes in part at the expense of smaller trees that died.
Forests also change more rapidly, as a result of rapid events that alter the typical year‐to‐year progression of changes. The forests at Coweeta experienced massive changes in the past two centuries (Figure 1.8) as a result of direct human impacts and unintended, indirect impacts.
The most noticeable change in the forests in the Coweeta Basin is the loss of the formerly dominant tree species, American chestnut. Long‐lived, large chestnut trees were the most notable part of the forest in 1900. About half the trees in the forest were chestnuts, and chestnuts comprised about half of the forest biomass. An exotic fungal disease from Asia, chestnut blight, killed almost all the mature chestnuts in forests of eastern North America within a few decades. Not all the mature trees were killed outright, as the fungus creates a canker on the stem that topples the tree. Surviving root systems continue to send up hopeful shoots, but these also form cankers when the stems are few meters tall.
What did the demise of chestnut mean for the forest? Given that competition is so important in the interactions among trees, the loss of chestnut led to a dramatic increase in the biomass of other species, particularly oaks, red maple, and tulip poplar. These species responded not by increasing the number of trees in the forest, but with accelerated growth of the already‐present stems.
FIGURE 1.8 Forest composition in the Coweeta Basin in 1935 and in 1990. The total density of trees (left) declined from 3000 trees ha−1 to 1200 trees ha −1, while basal area (right) increased slightly from 27 to 28 m2 ha−1. The decline in tree density is a common feature of growing forests; as dominant trees increase in size, many smaller trees die. However, the trend in this forest was largely influenced by the drastic decline of chestnut. This formerly dominant species was decimated by the exotic chestnut blight disease.
Source: Data from Elliott 2008.
Another major event reshaped parts of the forests of Coweeta after the vegetation survey in 1990 summarized in Figure 1.8. Before this time, eastern hemlock was a major tree species in wetter locations, such as coves and valley bottoms. The hemlock woolly adelgid is an exotic, invasive insect that has killed most of the eastern hemlocks across much of eastern
