Forest vegetation and boulder streams in the central Appalachian Valley and Ridge province, southwestern Virginia

The pattern and structure of forest vegetation in small boulder-floored str eamless hollows vary as a function of the type of surficial deposit and top ographic position on a scale of tens of meters. Data collected for a 50 m l ong belt transect across a 20 m wide boulder stream showed that tree specie s characteristic of mesic sites were most abundant near the center of the b oulder stream. In addition, the lowest values of absolute tree density were recorded near the center. Some tree species were found to display noticeab le differences in distribution. For example, black birch was much more abun dant on the boulder stream than along its margins, whereas chestnut oak, al though abundant at the margins, rarely occurred on the stream. Because boul der size reached a maximum in the lowest part of the hollow cross section ( i.e., the center), it was not possible to decide whether the main effect wa s the nature of the substrate or the topographic position (i.e., higher lev els of moisture in the lower part of the cross section and lower levels of moisture in the higher parts). To distinguish these effects, a second trans ect was established across one side of a much wider boulder stream that sho wed large lateral variation in boulder size. Data collected for this transe ct showed that for several tree species the main factor affecting distribut ion was topographic position (i.e., near to or distant from the lowest part of the hollow cross section). However, black birch showed an affinity for areas underlain by large boulders, regardless of their topographic position .