Influence of bedrock and aspect on soils and plant distribution in the Holyoke Range, Massachusetts

and 24 pairs on the north side of the range. For each plot we measured slope, plot aspect, soil pH and soil nutrients, the importance value for trees, within-plot frequency for shrubs and recorded presence/absence for herbaceous taxa. Soil on basalt had a higher pH and was richer in nutrients, particularly exchangeable Ca and Mg than soil on arkose. Principal Components Analysis of plots using soil variables indicated that those associated with bedrock accounted for 51% of the variance while soil differences related to aspect accounted for 22%. The multi-response permutation procedure indicated significant differences in distribution in all vegetation layers with respect to both bedrock and aspect. Species richness on basalt was significantly higher for both trees and herbaceous plants, and was also higher on the south side of the range for herbaceous plants. Of the 89 species present in more than 5% of the plots, 32% had significantly different distributions with respect to bedrock alone, 21% differed significantly with respect to both bedrock type and aspect while only 9% differed significantly with aspect independent of bedrock. The largest differences were in the herbaceous layer. Canonical correspondence analysis using soil and site variables indicated that for all vegetation layers the strongest gradient was related to bedrock.