Forest Communities

The Cumberland Plateau is an extremely varied landscape that consists of mountains, deeply incised stream valleys, broad floodplains, steep to rolling hills, and a tableland surface with minimal relief. Each of these land types is inhabited by different forest communities. Forest composition is largely a function of slope position, aspect, and form (Hinkle and others 1993) and disturbance history. On the Cumberland Plateau, true mixed mesophytic forests are restricted to protected mesic sites such as gorges and coves (Martin 1992). The entire Cumberland Plateau does not support true mixed mesophytic forests, and the forests on the Cumberland Plateau, while still a component of the Mixed Mesophytic Forest Region, support lower quality forests relative to the Cumberland Mountains because of extreme relief and nutrient-poor soils (Braun 1942). Best examples of true mixed mesophytic forests appear in the Cumberland Mountains (Braun 1942).

In general, forests of the Cumberland Plateau proper exhibit greater dominance of American beech and white oak and reduced dominance of yellow buckeye, white basswood, sugar maple, yellow-poplar, and northern red oak compared to the Cumberland Mountains (Muller 1982). Because of these variations within the Mixed Mesophytic Forest Region, Braun (1950) further subdivided the classifications into sections, subsections, and districts. The majority of the Cumberland Plateau in Tennessee was classified into the Cliff Section subdivision of the Cumberland and Allegheny Plateaus section (the Cliff Section extends from southern Kentucky to northern Alabama). Braun (1950) named this section for the rugged terrain or “bold cliffs” that characterize the Plateau through much of Tennessee and Braun (1950) subdivided the Cliff Section into Northern and Southern districts, with the Cumberland Plateau of Tennessee largely within the Southern district.

The complex topography and variations in substrates create a wide range of microclimatic and edaphic conditions on the Cumberland Plateau. However, the vegetation of the region can be classed into one of two broad categories: the flat to rolling plateau uplands and the plateau ravines and gorges (Hinkle 1989). The most spatially comprehensive study on Cumberland Plateau forest communities was conducted by Hinkle (1978) and summarized in Hinkle (1989). Hinkle documented 12 community types through the flat to rolling plateau uplands that ranged from stands dominated by red maple, river birch, and American holly on floodplain terraces to stands dominated by red maple, white oak, and blackgum on poorly drained swales to stands dominated by blackjack oak and Virginia pine on xeric ridge tops (Hinkle 1978; Hinkle 1989). Slope forests are generally dominated by mixed oak species with white oak being the most abundant followed by scarlet oak and black oak. Other common canopy dominants on the Plateau tabletop include shortleaf pine, post oak, chestnut oak, and mixed hickory species (Hinkle 1989). On upland sites, species composition is controlled by topography, factors related to soil water availability (such as depth to bedrock and surface stone cover), and disturbance history (Hinkle 1978). Aspect has little influence on the Plateau surface, but is significant on the slopes of higher ridges and mountains and on the steep escarpment slopes (Clatterbuck and others 2006).

Ravines and gorges occur throughout the Plateau but become more common near the eastern and western escarpments. In these areas, erosion of the caprock has created steep and often very deep gorges. These gorges were the inspiration for Braun (1950) to name this region the Cliff Section. Hinkle (1978) identified 12 community types in the ravines that ranged from communities dominated by chestnut oak, white oak, or black oak at upper slope positions to rich white oak, northern red oak, and sugar maple stands at middle and lower slope positions (Hinkle 1989). Hinkle (1978) noted that the common mixed mesophytic indicator species (yellow buckeye and white basswood) were present but not important in most of the sugar maple-white oak, northern red oak-sugar maple, and white oak-northern red oak communities sampled. Hinkle noted, however, that many stands seemed successional to mixed mesophytic types. Stands dominated by eastern hemlock are restricted to headwaters and along bedrock streams where they occur as ribbon forests (Hart and Shankman 2005). The only other common conifer in ravines is white pine, which usually occurs in association with eastern hemlock (Hinkle 1989). Factors related to soil moisture and nutrient availability control community composition in ravine communities (Hinkle 1978). Forest communities on the Cumberland Plateau can be classified as evergreen coves, mixed mesophytic, mixed hardwoods, or xeric pine-hardwoods.