||In 2009, we began to address the potential for climate warming to alter tree species composition at the boreal-temperate forest ecotone through effects on two 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 an experiment at two sites in northern Minnesota just inside the southern limit of boreal forest: Cloquet, a more southern site, and Ely, a site 150 km further north. At each site, we established 48 3-m diameter plots in open and closed canopy conditions, with three levels of plant (via infrared lamps) and soil (via buried cables) warming (ambient, +1.8 °C, and +3.6 °C), and an additional ambient treatment without buried soil heating cables. Across all plots we planted 11,616 seedlings and ≈200,000 seeds of 11 temperate and boreal tree species. We completed three years of experimental warming with comprehensive measurements of plant performance such as phenology, photosynthesis, respiration, growth, and survival and of soil processes such as CO2 flux and net nitrogen mineralization. Major findings to date include (i) evidence (from establishment via seed, growth of planted seedlings, and leaf net CO2 exchange) that warming adversely impacts several boreal species while having positive impacts on several temperate species, (ii) all species show acclimation of phenology and the temperature response of photosynthesis and respiration, and (iii) physiological responses to warming are influenced by the magnitude of soil water deficit caused by variation in ambient rainfall. In 2012 we are beginning a second phase of the study that focuses on understanding the sensitivity of warming effects to soil moisture variability by implementing a treatment that reduces ambient rainfall in a subset of plots. Our objectives are (i) to study how soil moisture modulates the patterns and mechanisms of warming effects on diverse physiological, organismal, and ecosystem processes by incorporating three years of rainfall manipulation into the ongoing manipulation of plant and soil temperature, while (ii) elucidating the long-term (six-year) response of these same processes to continued warming. This second stage maintains the original goals while focusing on new knowledge that can help to advance synthetic understanding and quantitative modeling of carbon fluxes under a changing climate.