Print PageUncertainty remains about biotic responses to climate change and their underlying mechanisms, especially regarding the direction, magnitude, and ubiquity of interactive effects of various aspects of climate change (Luo et al. 2008, 2010). B4Warmed is a manipulative experiment and mechanistic study addressing the potential for projected climate warming to alter tree species composition at the boreal-temperate forest ecotone through effects on two critical life history stages – germinant establishment, and juvenile growth and survival. Specifically, our study addressed the hypothesis that warming would enhance the growth and survival of seedlings of temperate species at the cold edge of their range but reduce growth and survival of seedlings of boreal species at the warm edge of their range. To test this hypothesis, we established 48 3-m diameter plots in both open and closed canopy conditions established an experiment at two sites in northern Minnesota, with three levels of simultaneous plant (via infrared lamps) and soil (via buried cables) warming (ambient, +1.8 °C, and +3.6 °C). Across all plots we planted 10,000 seedlings and 100,000 seeds of 11 temperate and boreal tree species. We have completed two years of experimental warming with comprehensive measurements focusing on tree phenology, photosynthesis, respiration, growth, and on soil processes such as carbon dioxide flux and net nitrogen mineralization rates.
Results/Conclusions
Initial results include: Phenology- Warming enhanced growing season length via both earlier leafing and later senescence, and extension of the growing season across years varied from 15 to 34 days. Photosynthesis and Respiration-.Warming had positive impacts overall on light-saturated net photosynthesis (Asat) when soils had ample moisture, but negative effects with increasing drought (significant interaction between warming and soil water status). A total of 758 photosynthetic temperature-response curves show a significant shift in the temperature optima of photosynthesis (of ≈1.7 °C on average for plants in the +3.6 °C treatment) at both sites and in both habitats. Seedling Growth and Survival- There was high survivorship of planted seedlings across all treatments but seedlings sprouted from planted seeds had significantly lower survival in warmed treatments. These results suggest the potential for negative impacts of warming on regeneration processes. For all planted seedlings, non-linear responses in growth were common with +1.8 °C often showing enhancement while +3.6 °C differed little from ambient or was less than ambient
