The linkage of stomatal behavior with photosynthetic processes is critical to understanding water and carbon cycles under global change. The slope ( g1) of stomatal conductance ( gs) versus CO 2 assimilation ( Anet) serves as a proxy of the marginal water cost of carbon acquisition and the trade-off between carbon gain and water loss. Here we use g1 to assess species differences in the response of stomatal behavior to experimental climate change manipulations, asking whether generalizable patterns exist across species and climate contexts. A total of 17,727 Anet- gs measurements made in a long-term open-air experiment under ambient and +3.3°C warming, and ambient and ~40% summer rainfall reduction provided > 2,700 estimates of g1 across 21 boreal and temperate tree species. All species became more conservative in their water use (lower g1) in warming and/or reduced rainfall treatments because of lower soil moisture. In contrast to these phenotypic responses, species from warmer and drier habitats tended to have slightly higher g1 and to be the least sensitive to the decrease in soil water. Overall, both warming and rainfall reduction consistently made stomatal behavior more conservative in terms of water loss per unit carbon gain across 21 species and a decade of experimental observation.

The Spruce and Peatland Responses Under Changing Environments (SPRUCE) project is a warming and elevated carbon dioxide (CO2) experiment designed to test how peatland ecosystems will respond to climate change. Here, we report changes in the vascular plant community during the first five years of SPRUCE. We tracked species composition, diversity, and aboveground net primary production (ANPP) in chambers warmed at a wide range of temperatures (+0, +2.25, +4.5, +6.75, +9 ˚C), and two CO2 levels (~400 [ambient] and 900 parts per million). We observed an increase in aboveground vascular plant biomass accumulation, due primarily to an increase in shrub abundances. Overall species diversity decreased substantially, likely due in part to shading by a denser shrub canopy. These results indicate an overall increase in NPP with warming, but highlight the importance of interactions between direct (warming) and indirect (competition) effects in determining how boreal ecosystems will respond to climate change.