The effects of slope aspect on the spatial patterning of a mixed mesophytic old-growth forest

Background/Question/Methods

Understanding the spatial patterning of forest communities at a variety of scales is an important first step in determining the processes that shape those patterns. While studies of forest community pattern have been extensively studied across the globe, studies in temperate deciduous old-growth forests have been underexplored. Here, we describe woody stem-mapped forest plots from Dysart Woods, an old-growth mixed mesophytic forest in southeastern Ohio. Four 50 × 70 m (0.35 ha) plots were set up at Dysart Woods, and all stems ≥ 2.5 cm DBH were recorded and mapped on an X, Y coordinate system. To evaluate the potential influence of microclimate on the composition and spatial patterning of woody vegetation, two of the plots were established on northeast-facing slopes and two plots were placed on southwest-facing slopes. For analysis, Ripley’s K function was used. This function uses the distances between all points in a process to determine whether they are clumped, overdispersed, or randomly arranged (CSR). For this study, the function was used to quantify the spatial arrangement of trees and saplings (both together and separately) in the study plots. The spatstat package in the R programming environment was used for all analyses.

Results/Conclusions

Results from these plots suggest that there are marked differences in composition and spatial patterning between northeast- and southwest-facing slopes (designated N- and S-plots, respectively). Generally, the S-plots exhibited greater levels of aggregation than the N-plots. Stems for all species and all size classes conformed to CSR in the N-plots, but were aggregated in the S-plots. As seen in other studies, spatial patterns in the S-plots differed between size classes, with the smaller size class (DBH < 10cm) being more aggregated, and the larger size class (≥ 10 cm DBH) being randomly distributed. While small individuals in the N-plots did not aggregate, individuals ≥ 10 cm DBH were uniformly distributed. Differences were also observed among species. Beech (Fagus grandifolia) was highly aggregated on the N-plots and less so on the S-plots. Sugar maple (Acer saccharum) was aggregated on the N-plots but was random on the S-plots. These patterns were correlated with higher species diversity on the S-plots. Collectively, our results demonstrate that both slope aspect and diameter class have an important influence on the composition and spatial patterning of Eastern deciduous forest communities.