Juvenile Tree Survivorship as a Component of Shade Tolerance

With a view toward understanding species‐specific differences in juvenile tree mortality and the community‐level implications of these differences, we characterized juvenile survivorship of 10 dominant tree species of oak transition‐northern hardwood forests using species‐specific mathematical models. The mortality models predict a sapling's probability of dying as a function of its recent growth history. These models and species‐specific growth functions (published elsewhere), characterize a species' shade tolerance. Combined growth and mortality models express a sapling's probability of mortality as a function of light availability. We describe the statistical bases and the field methods used to calibrate the mortality models. We examined inter‐ and intraspecific variation in juvenile mortality across three sites: Great Mountain Forest (low pH, nutrient poor soils) in northwestern Connecticut, a calcareous bedrock region (neutral pH, nutrient rich soils) also in northwestern Connecticut, and a site in central‐western Michigan (low pH, nutrient poor soils). Interspecific differences in juvenile mortality have profound effects on community dynamics and composition; the importance of these effects is demonstrated through a spatially explicit simulator of forest dynamics (SORTIE). The 10 species we examined occupy a continuum of survivorship levels at 1% of full sun. There was surprisingly little intraspecific variation in mortality functions for sugar maple, American beech, eastern hemlock, and white ash between the Great Mountain and Michigan sites. However, there was a striking increase in survivorship for sugar maple in the calcareous site. Differences in survivorship among the sites are correlated with soil pH and presumably nutrient availability. Growth rates in high‐light and low‐light survivorship are inversely correlated across species; as level of shade tolerance increases, a species grows more slowly in high light and exhibits increased survivorship under low light. Our results indicate that interspecific differences in sapling mortality are critical components of forest community dynamics.