Harvest gap size and proportion of forest area in gaps influence species diversity of ground-layer vascular plants

Background/Question/Methods

Biodiversity conservation within managed forests depends, in part, on management practices that promote components of diversity, such as species richness. In temperate forests the ground-layer vegetation represents a major component of plant diversity. Canopy gaps increase available resources and the range of microsite conditions in the ground layer, facilitating species coexistence. Patterns of timber harvesting can be manipulated to vary individual gap size and proportion of forest area in gaps, influencing the range and distribution of understory conditions, and thus potentially ground-layer plant diversity. We tested this hypothesis with a well-replicated gap-size experiment in a second-growth northern hardwood forest. We evaluated ground-layer vascular plant species richness within and among gaps of differing size, 13 years after an initial harvest.  We also used a novel resampling approach to estimate how richness might vary in forests with differing gap sizes and proportions of forest area in gaps.

Results/Conclusions

Results from our field experiment showed that medium-sized gaps (20–30 m dia.) were most diverse, although all gap sizes (6–46 m dia.) had more species than uncut, closed forest understories. For all gap sizes, total species richness (aggregate) was largely composed of uncommon species that differed among individual gaps (β-diversity) rather than the common species that dominated individual gaps (α-diversity). In harvest scenarios created by resampling our empirical data, we found that gaps increased estimated species richness from closed forest and the magnitude of increase was associated with gap size and the proportion of forest in gaps. Overall, harvest scenarios with larger gaps resulted in higher and less variable species richness estimates than forests with smaller gaps. Intermediate proportions of forest in gaps had more diversity than those with high or low proportions in gaps. Our results support the concepts that harvest-created gaps increase forest diversity in previously-unmanaged, second-growth forests and that gap size and their proportion in a forest can be manipulated to attain biodiversity goals.