Species diversity and small-scale disturbance in an old-growth temperate forest: A consideration of gap partitioning concepts

The importance of spatial partitioning of gap resources in the maintenance of tree species diversity was studied in an Appalachian old-growth forest. We established three I-ha plots, subdivided into contiguous 10 x 10 m subplots, in a mesic cove forest with a disturbance regime characterized by small canopy gaps (<0.1 ha). For tests of partitioning along the gradient from gap interior to the shaded understory, canopy cover of each subplot was classified as gap, gap border, or non-gap. For tests of gap-size partitioning, gap and gap border subplots were classed by gap size. Species-accumulation curves for the regeneration stratum (stems >1.37 m tall and less than or equal to 10 cm DBH) based on: (1) species per unit area, and (2) species per individual were plotted for each of the canopy cover classes and for all subplots combined. The species-area curve for gaps was steeper than the all-subplots curve that represented the entire gradient from gap centers to the shaded understory. By contrast, all of the species-individuals curves were similar. Sorrenson's indices showed that only gap subplots, particularly those associated with large gaps, had highly self-similar species composition. Furthermore, the relative abundance of most species was similar between gaps and the shaded understory. However, intolerant species often had higher relative abundance in gaps, especially in gaps greater than or equal to 0.04 ha in size. We concluded that gap partitioning hypotheses applied to some intolerants, but not to tolerants. The density hypothesis, a null model attributing species richness levels to stem density levels, largely explained the richness patterns in our study forest.