Climate change is predicted to alter species ranges through changes in trait-environment matching. We used a free air climate warming experiment to test the potential for projected climate warming to alter tree function, species composition, and ecosystem processes at the boreal-temperate forest ecotone. The Boreal Forest Warming at an Ecotone in Danger (B4WarmED) experiment uses simultaneous plant (via infrared lamps) and soil (via buried cables) warming to elevate the temperature in 72, 3-m diameter plots at two sites in northern Minnesota, USA. Treatments include ambient and +1.7°C and +3.4°C warming. In addition, we have altered rainfall, reducing growing season precipitation by 40% by selectively removing rainfall events. Ten species, half of which are common temperate species and half boreal species, are planted into treated plots. We measured acclimation of photosynthesis and respiration, in situ net photosynthesis, leafing phenology and growth.
Results/Conclusions
Overall, we found that species shift phenology and acclimate photosynthesis and respiration under warmer growth conditions but differ in growth responses to warming. All species extended the growing season and shifted the thermal optimum of photosynthesis with warming. In addition, plants showed strong acclimation of respiration that eliminated 80% of the expected increase in leaf respiration of non-acclimated plants. Despite acclimation, in situ measurements of carbon gain indicate that species growing nearest their warm range limit (temperate species) exhibited reductions in net photosynthesis and growth, whereas species near their cold range limit (boreal) responded positively to warming. Species differences were in part related to response to soil drying. Overall, we found that stomatal conductance and photosynthesis decreased more sharply with declining soil moisture in warmed than ambient plants. Consequently, warming reduced photosynthesis on the driest two-thirds of days, and on average across the growing season. Species differed in the sensitivity to drying as well as stimulation by warming. Overall, temperate species were more stimulated by warming and less sensitive to soil moisture deficits than boreal species. Taken together our results support hypotheses of poleward range expansions of temperate species and retreat of boreal species with climate change.