Reply to Sala: Temperature sensitivity in drought-induced tree mortality hastens the need to further resolve a physiological model of death

Abstract

Our recent study (1) of pinon pine (Pinus edulis) response to change in climate, on which Sala (2) comments, documented that drought-induced mortality was temperature-sensitive. In addition, we showed that time to tree mortality was predicted by leaf-level cumulative respiration for ambient and warmer treatments. Notably, our study experimentally assessed temperature sensitivity of drought mortality by tracking individual physiological responses throughout the death process. Ambient and warmer treatments did not differ in water balance in such a manner as to drive differences in mortality, yet higher respiration rates under warmer temperatures were associated with earlier death of individual trees. Two related studies provide additional support implicating carbon starvation via respiration during protracted water stress. First, modeling of physiological responses indicated that even short droughts drove leaf water potential of pinon pine—a drought-avoiding, isohydric species—quickly below its zero-carbon assimilation point (3). Second, long-term observational measurements of predawn water potential of pinon pine documented that trees could survive shorter but not longer periods of water stress below their zero-carbon assimilation point (4).