Managing for diversity: harvest gap size drives complex light, vegetation, and deer herbivory impacts on tree seedlings

Abstract

AbstractMany managed northern hardwood forests are characterized by low‐diversity tree regeneration. Small harvest gaps, competition from shrub–herb vegetation, and browsing by white‐tailed deer (Odocoileus virginianus) contribute to this pattern, but we know little about how these factors interact. With a stand‐scale experiment, we examined the effects of gap size (0–3234 m2), vegetation (weeded:unweeded), and deer (fenced:unfenced) on seedling growth and survival for 18 tree species. With increasing gap size and light, shrub–herb vegetation density increased, while deer browsing on seedlings in unweeded plots decreased. Fenced:weeded seedlings of all species increased in height up to 35–45% light, with optimal growth in large‐group selection and patch cut harvest gaps. Height growth rank order among tree species changed between gap sizes, but growth varied little in small, low‐light gaps. Instead, a low‐light survival (i.e., shade tolerance) vs. high‐light growth tradeoff we observed is likely more important for species sorting of gap sizes. Shrub–herb vegetation decreased seedling survival and growth, especially in larger harvest gaps, shifting gap size optima to smaller gaps, but had little effect on growth/survival rank order among species. In contrast, deer had strong impacts on growth rank order, especially in larger gaps where species differences in growth potential were trumped by differences in deer browsing pressure responses. However, contrary to their consistently negative main effects, vegetation and deer had two positive interacting effects: dense shrub–herb vegetation in large gaps protected seedlings of faster‐growing species from browsing and deer browsing of shrub–herb vegetation modestly increased light and growth of short, suppressed, browsing‐avoided species. In summary, harvest gap size‐mediated light availability, shrub–herb vegetation, and deer herbivory had strong interacting effects on tree seedling interspecific performance ranks and intraspecific optimal gap sizes. For management, a broad range of harvest gap sizes and rapid establishment of tree regeneration (naturally or planted) to minimize shrub–herb competition should increase tree diversity in forests with few deer. However, with deer browsing pressure, a more limited set of lesser‐browsed species are likely to recruit successfully regardless of gap size, except in large patch cut gaps, where recruitment of faster‐growing, shade‐intolerant species is possible.