Interacting effects of harvest gap size, competing vegetation, and deer browse pressure on the growth and survival of 15 tree species in northern hardwood ecosystems
Ecological theory suggests that varying harvest gap size may lead to the recruitment of a more diverse tree species cohort. In northern hardwood forests of the Great Lakes region, canopy gap size, competing vegetation, and herbivory by abundant Odocoileus virginianus populations could be strong bottlenecks limiting seedling recruitment within these gaps. Little is known about the relative magnitude of the interacting effects of these factors on tree seedling development in uneven-aged (i.e. gap) silvicultural systems and how these effects may vary among species. In this study we examined the impact of deer, gap size, and competing vegetation over five years on the growth and survival of planted and naturally established seedlings of 15 tree species in fenced/unfenced and weeded/unweeded plots across a range of gap sizes and undisturbed canopy locations (0-6600m2).
For 14 of 15 species, height growth responded to gap size, with 10 showing increases in height up to 40-50% of full light (i.e. multi-tree harvest gaps), followed by a decline up to 80% light. Shade intolerant Populus tremuloides and conifers Pinus strobus, Abies balsamea, and Picea glauca followed this initial pattern but showed saturation rather than a decline in height above 50% light. Survival decreased with increasing gap size for 5 of the 15 species. Deer herbivory negatively impacted height growth for 11 species, with only short statured Abies balsamea, Tsuga canadensis and Thuja occidentalis and unbrowsed P. glauca showing no effects. Deer negatively impacted the survival of 8 species, with P. glauca being the only species whose survival and growth was unaffected by deer. Survival, but not height, was strongly impacted by competing vegetation. For 11 of 15 species, the main effects of competing vegetation and/or its interaction with light (harvest gap size) were significant, and generally indicated greater negative impacts of competing vegetation in the largest harvest gaps where competing vegetation density was greatest. Results from the naturally established seedlings (Acer saccharum, Prunus pensylvanica, and Fraxinus americana) generally corroborate our findings for planted seedlings. Contrary to predictions, gap size did not positively influence seedling survival. Instead, survival was strongly negatively impacted by proximal competing vegetation (e.g. Rubus spp., Sambuccus spp.) for all three species and by deer herbivory for P. pensylvanica. Collectively, our results suggest that the proposed positive benefits of varying harvest gap size for promoting a diverse set of tree species may be negated by the impacts of deer herbivory and competing vegetation.