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Most research on forest dynamics has focused on species' light requirements as the major driver for successional change. However, soil resource availability may modify seedling responses to light and ultimately alter the course of succession. In the present study, we examined how seedlings in mixed conifer-broad-leaved forests in eastern North America differed in their growth and mortality responses to manipulated nitrogen avail- ability. We then incorporated these responses into an individual-based model of forest dynamics (SORTIE) to assess potential longer-term consequences of seedling responses to nitrogen for temperate forest community dynamics. We grew seedlings of six study species, both individually and in mixed-species competitive stands, in a common garden for two years. The earlier successional broad-leaved species (yellow birch and red maple) consis- tently showed the greatest increases in biomass in response to nitrogen addition, while the most late successional of the broad-leaved species (sugar maple) and all the coniferous species did not grow significantly larger with increased nitrogen. We found a significant correlation between species' early growth rate and nitrogen growth enhancement. For those species that underwent significant nitrogen-induced shifts in growth and/or mortality, we adjusted their parameters in the seedling/sapling growth and mortality submodels of SOR- TIE (covering up to 10 cm dbh). Simulations revealed that nitrogen effects on both seedling growth in high light and seedling mortality in low light (data from parallel experiment) changed overall forest structure and dynamics. Increased nitrogen led to: (1) further dom- inance of young forests by earlier successional species (yellow birch in particular), through its impacts on seedling high-light growth, and (2) even greater persistence of later suc- cessional species (predominantly hemlock) in older forests, through its impacts on seedling low-light mortality. These findings were robust to an uncertainty analysis that incorporated experimentally derived error into the seedling/sapling submodels. In contrast, the identity of the species replaced by yellow birch and hemlock was more sensitive to uncertainty in parameter values. We conclude that seedling physiological and demographic responses to increased nitrogen availability have the potential to scale up and influence successional dynamics in mixed temperate forests, provided these effects persist throughout seedling and sapling life stages. |
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