Variation in species richness and species pool size across a pH gradient in forests of the southern Blue Ridge Mountains

Pärtel (Ecology 83: 2361–2366, 2002) andEwald (Folia Geobot. 38: 357–366, 2003) suggest that the relationship between local species density and soil pH is determined by regional species pool size, which in turn reflects the relative abundance of soil types during the evolutionary history of the flora. Ewald observed that calcareous sites in Central Europe have higher species density and larger species pools than acidic sites, and argues that this is the consequence of a Pleistocene bottleneck for acidophiles.The flora of the southern Blue Ridge Mountains USA has always been associated with primarily acidic soils. We used vegetation and soil data from 3328 100 m² southern Blue Ridge forest plots to examine the generality of the Pärtel-Ewald hypothesis. The Blue Ridge flora with less than 20% of species confined to sites above pH 4.7 contrasts dramatically with that of Europe. However, regional species pool size increases with pH. Genus- and family-level pools increase with pH, suggesting an ancient origin for this pattern. Mean species density is also strongly positively correlated with soil pH. Thus, both regional species pool size and plot species density of southern Blue Ridge forests fail to conform to predictions derived from the work of Pärtel and Ewald.The increase in species pool with increase in pH exhibited by southern Blue Ridge forests appears to reflect broad species distributions and tolerance for high pH conditions among species that grow predominantly under acid conditions. We conclude that richness on higher pH sites is a consequence of generally more favorable conditions for plant growth and/or establishment. Ewald may be correct in asserting that the abundance of high pH specialists in Central Europe is an historical artifact, but our data contradict the assertion of Pärtel and Ewald that evolution of a flora in a primarily low pH environment will necessarily translate into a negative correlation between pH and species density in local vegetation.