Beta diversity dynamics across topographic gradients in the herbaceous layer of an old-growth deciduous forest

Background/Question/Methods

The herbaceous layer in eastern deciduous forests of North America has long been recognized for high levels of diversity and spatial variation in species composition, but the suite of potentially interacting factors that drive variations in herb diversity are not fully understood. Herbaceous species composition and productivity are known to vary with topography (i.e. slope, aspect, elevation), but how this spatial variation is linked to temporal dynamics in ecological communities is not well studied.  Our objective was to understand how spatial patterns of both alpha and beta diversity in the herbaceous layer of an old-growth forest vary over time.  Shannon-Wiener diversity, spatial species turnover, and temporal species turnover were examined with regard to three topographic gradients (aspect, slope, and elevation) over a single growing season. The herbaceous layer was sampled in April, June, and August of 2011 within 320 1m²plots stratified throughout an old-growth central Appalachian forest. Multiple regression models were used to test whether Shannon diversity and temporal species turnover (Bray-Curtis dissimilarity) were significantly related to slope, aspect, and elevation.  Distance-decay analysis was used to test whether significant spatial species turnover occurred across topographic gradients. 

Results/Conclusions

Mean seasonal species turnover was significantly greater (P < 0.001) between April and June (0.56 ± 0.02 SE) than between June and August (0.27 ± 0.02). Seasonal species turnover was generally not statistically related to topography; however, a weak linear relationship was found between aspect and turnover across the entire growing season (April to August; r² = 0.05, P < 0.05), suggesting greater species turnover occurred on north-facing slopes.  Diversity was linearly related to aspect and slope, but unimodally related to elevation, indicating steep, mid-elevation, and south-facing plots tended to be most diverse.  Spatial species turnover tended to be greater between plots with greater differences in aspect, slope, and elevation. Topographic variation contributes to the habitat heterogeneity that influences spatiotemporal patterns of herbaceous layer diversity.