First-year seedlings and climate change : species-specific responses of 15 North American tree species

Species will respond individually to climate change and this poses a challenge for modeling climate-vegetation dynamics using broader taxonomic or biogeographical classifications. Additionally, responses to climate and environmental conditions may shift with ontogeny, further complicating efforts to understand the likely rates and directions of vegetation change. We measured emergence, leaf-out rate, growth, and survival of first-year seedlings in response to warming, precipitation regime shifts, and seedbed condition (leaf litter presence/absence). We grouped species into three levels of organization (species-specific, biome-level and broad taxonomic group) and hypothesized that most metrics of seedling performance would be best described by species-specific models, as even similar species may respond in vastly different ways to global change. Results showed that the species-specific model was the best fit for emergence and development rates, whereas growth and survival could be captured through broader groupings, with the broadleaf temperate group exhibiting the greatest growth and conifers the shortest survival times. The sign and magnitude of response to climate and seedbed condition varied with treatment combinations and metric of performance. For example, seedlings grew more in response to warming, but conditions too dry or too wet limited this positive response. Also, warmer temperatures generally increased emergence, development, and growth, but decreased survival, whereas leaf litter presence decreased emergence and slowed development, but increased survival. The results presented here are for first-year seedlings and in many cases the responses are different from other studies using older plants. Future research and climate vegetation modeling needs to assess performance at multiple development stages and determine where key bottleneck phases for population growth occur for individual species.