Seasonal Variation in Understory Light Near a Gap Edge and its Association with Conifer Seedling Survival in a Southern New England Forest

摘要

Forest edge-generated gradients in understory light are known to influence the survival of tree seedlings in North American deciduous forests, but the seasonality of that influence has not been investigated for the forests of southern New England. We examined the winter versus summer difference in photosynthetically active radiation (PAR) transmitted through a southern New England forest canopy in the vicinity of a large experimental canopy gap and investigated if this difference influenced the survival of Pinus strobus L. (White Pine) and Tsuga canadensis (L.) Carr. (Eastern Hemlock) seedlings along a gap-forest gradient using Kaplan-Meier survival curves and Cox proportional hazards models. The mean and the spatial distribution of winter understory PAR transmittance (% T) across the gap-forest gradient were significantly different from those of summer % T. Distance from gap edge into the forest and variability in % T during the winter months were associated with the survival of White Pine seedlings, but not with that of Eastern Hemlock seedlings, which had a constant survival probability throughout the gap-forest gradient. Summer % T was not associated with the survival of seedlings of either species. Experimentally, we found that digital fisheye canopy photos did not accurately capture the range in understory light level along a gap-forest gradient. Thus they are a poor proxy for direct PAR transmittance measurements and should be used with caution in southern New England forests, which are highly fragmented and contain many edges. Forest management actions for the conservation of White Pine should take into account winter understory light regimes rather than summer regimes.