The effects of harvest-created gaps on plant species diversity, composition, and abundance in a Maine oak–pine forest

Abstract

In forests where large, stand-replacing disturbances are infrequent, small-scale disturbances associated with the mortality and replacement of individual trees are a primary source of heterogeneity in forest composition and structure. The disturbances considered in this study were canopy gaps created by a partial harvest in the winter of 1987–1988 in an oak–pine ( Quercus–Pinus) forest that is part of a long-term ecosystem study at the Holt Research Forest in coastal Maine. This study examined the gap phase of the forest turnover cycle: the brief episodes of rapid change when processes determining the structure and composition of a forest community occur. The objective was to evaluate the effects of harvest-created gaps and soil moisture (as reflected by soil drainage classes) on woody and herbaceous species diversity, composition, and abundance relative to unharvested control areas. Because the vegetation was sampled in both 1993 and 1998, detection of short-term changes was possible. Harvest gaps had higher total plant species richness in the understory (vegetation <1 m tall) than controls, primarily due to more infrequently-occurring species. Average species richness varied by soil drainage class but not treatment, whereas the Shannon–Wiener diversity index was not affected by treatment, drainage class, or time. Species abundance was more evenly distributed (i.e., higher evenness) in controls than in gaps. Species unique to harvest gaps decreased in number and abundance over time. Detrended correspondence analysis (DCA) revealed that harvest gaps exhibited greater variability in species composition than controls, although many of the gaps and controls were similar in composition. Shifts in individual species abundance were evident with treatment, drainage, and time. Diversity measures calculated for sapling density showed a significant difference among drainage classes for species richness, but no significant difference between treatments for species richness, the Shannon–Wiener index, or the evenness index. Gap size was positively correlated with the total density of saplings, the density of several species, and species richness. There were significant differences between treatments for the density of all species tested in at least one of the four height classes. DCA of sapling data revealed some separation of harvest gaps and controls but there was considerable overlap. Axis 1 tended to separate early successional species in the harvest gaps from shade tolerant species in the controls. The understory plant community at the HRF appears to be resilient to the partial harvest that created a variety of gap sizes. Silvicultural objectives were met; the harvest gaps created sites for establishment of seedlings and sprouts of tree species, particularly early successional species.