Effects of invasion of Pinus virginiana on soil properties in serpentine barrens in southeastern Pennsylvania

The New Texas serpentine barrens is a rare, natural savanna in southeastern PA that is rapidly succeeding to forest, probably as a result of fire exclusion. We tested the hypothesis that the invasive tree, Pinus virginiana, is modifying soil conditions in a way that may promote conversion to forest. We analyzed soil (1) next to P. virginiana of different ages and sizes and at varying distances from these trees and (2) in early successional savanna sites versus late succession forest sites. Mineral soil depth and litter depth increased with size of and proximity to trees and from savanna to forest, suggesting that P. virginiana exerts strong effects on soil at the tree scale that lead to similar changes at the stand level. Individual P. virginiana also appear to increase organic matter and Ca:Mg ratio, a key indicator of serpentine infertility. These soil chemical trends, however, were not replicated at the stand level: forest soils actually had similar or lower nutrient and organic matter levels than early successional savanna and exhibited no difference in Ca:Mg ratio. Differences between soil development trends on the two scales may be related to slower decomposition rates in pine forest compared to grassland, although much more complex mechanisms may also be involved. Although P. virginiana does not, at the stand level, appear to ameliorate soil chemical imbalances endemic to serpentine soils, its positive influence on soil depth alone could promote further succession towards forests typical of non-serpentine sites in the region.