adapted from Woody Plants of the Blue Ridge by Ron Lance. Used by permission.
Circumboreal species occur around the globe in boreal communities, which are generally limited to northern latitudes or at successively higher elevations as one journeys toward the equator. In the Blue Ridge, boreal zones are found at higher elevations, from about 4000' to the 6684' limit of our highest peak.
Middle elevations lie within 2500 to 3800' elevation, and lower elevations in the Blue Ridge include elevations of 1000 to 2500'. Foothill, intermountain hill, valley and bottomland forest communities are all found in lower elevations.
Bogs and cove forest communities may be found at any elevation, but species composition varies greatly with the elevation and soil condition.
The acidity of soils (measured by pH) greatly influences mountain vegetation. Where pH is low (acidic), ericaceous understory plants and canopies of oak. birch, and conifers are characteristic. Such acidic sites may be seen on steep slopes, high elevations, acidic coves, and in most bogs.
Where acidity decreases (pH rises), the soil becomes "sweeter" or "rich" (these being common language terms used by settlers) and supports more diverse vegetation. The soil may become circumneutral or basic when pH rises from 6.0 to 7.0, such as in some rich coves, or in soils overlying rock substrates which act to raise pH. Some forms of granite, hornblend gneiss, or calcareous rock like limestone tend to affect overlying soils in this manner. Calcareous soils and limestone outcroppings are much more common on mountain provinces and escarpments west of the Blue Ridge.
Classifications of soil moisture levels range from xeric (dry) to hydric (wet). Mesic sites exhibit a relatively stable intermediate moisture regime, and these moist areas often produce some of our most attractive forests. Soil moisture levels are affected by the depth and physical structure of the soil, by the degree and duration of direct sunlight during the day (exposure and aspect of topography), and by the steepness and drainage rates of the topography.
Plants tolerant of shade are able to grow below the canopy for a large part of, or all of their life cycle. Understory species remain tolerant of shade through maturity. Intolerant species need direct sunlight and do not reach maturity if shaded by other species. These by necessity tend to be canopy species.