Large-scale shifts in vegetation are expected in response to altered precipitation and temperatures associated with global change.  Dryland regions of the world face the potential loss of overstory plants from a combination of warmer temperatures and increases in the frequency and severity of drought.  Such vegetation shifts have profound ecological impacts and are an important climate-ecosystem feedback through their alteration of carbon, water, and energy exchanges of the land-surface. However, we cannot currently predict drought-induced tree mortality with confidence, nor do we know its temperature sensitivity.  Here we highlight recent findings of our research program to address this issue.  We tested the hypothesis that warmer drought kills trees faster and investigated the physiological mechanism of mortality, using the environmental controls of the Biosphere 2 facility.  We placed transplanted piñon pines (Pinus edulis) in two areas of the glasshouse, one with temperatures close to ambient for the species, the other elevated by 4.3°C over ambient.  We simulated severe drought on half of the trees in each area by restricting all precipitation or irrigation until they died. We also report on progress of complementary field study in which trees have been transplanted to a warmer area and discuss implications of tree die-off.

Results/Conclusions

We show that drought-induced tree mortality is highly sensitive to temperature:  First, drought trees in the elevated temperature treatment died faster than those in the ambient treatment. Second, measurements of pre-dawn stem water potential showed no difference between temperature treatments, but warmer drought treatment trees respired carbon faster than ambient-temperature drought trees, revealing carbon starvation as the mechanism by which piñon pines died from drought.  Because a shorter drought is sufficient to cause tree die-off with warmer temperatures and shorter droughts are more frequent, a large increase in the frequency of widespread tree die-off can be predicted from temperature increases alone, without including expected, but difficult to predict, shifts in precipitation regimes.  The ongoing field study provides a bridge from the Biosphere 2 study to field conditions.  Our results are significant in that they reveal the mechanism behind recent increases in background rates of coniferous tree mortality as well as exacerbated tree die-off in response to severe drought.