Temperature sensitivity of drought-induced tree mortality portends increased regional die-off under global-change-type drought

Abstract

Large-scale biogeographical shifts in vegetation are predicted in response to the altered precipitation and temperature regimes associated with global climate change. 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. Of particular concern is the potential for warmer temperatures to compound the effects of increasingly severe droughts by triggering widespread vegetation shifts via woody plant mortality. The sensitivity of tree mortality to temperature is dependent on which of 2 non-mutually-exclusive mechanisms predominates--temperature-sensitive carbon starvation in response to a period of protracted water stress or temperature-insensitive sudden hydraulic failure under extreme water stress (cavitation). Here we show that experimentally induced warmer temperatures (approximately 4 degrees C) shortened the time to drought-induced mortality in Pinus edulis (piñon shortened pine) trees by nearly a third, with temperature-dependent differences in cumulative respiration costs implicating carbon starvation as the primary mechanism of mortality. Extrapolating this temperature effect to the historic frequency of water deficit in the southwestern United States predicts a 5-fold increase in the frequency of regional-scale tree die-off events for this species due to temperature alone. Projected increases in drought frequency due to changes in precipitation and increases in stress from biotic agents (e.g., bark beetles) would further exacerbate mortality. Our results demonstrate the mechanism by which warmer temperatures have exacerbated recent regional die-off events and background mortality rates. Because of pervasive projected increases in temperature, our results portend widespread increases in the extent and frequency of vegetation die-off.

Fig. 1.

Water relations progression and death dates. (A) Predawn stem water potential (circles), death dates (triangles), and death date means (squares) of piñon pines during simulated drought under ambient (blue) and elevated (red) temperatures. Error bars are standard errors. (B) Water loss from a subset of trees in A.

Fig. 2.

Leaf carbon exchange progression. (A and B) Instantaneous midday net photosynthesis (A) and predawn respiration (B) of piñon pines during simulated drought under ambient (blue) and elevated (red) temperatures. Error bars are standard errors. (C) Cumulative time-integrated respiration costs of piñon pines during simulated drought under ambient (blue) and elevated (red) temperatures. Error bars are standard errors calculated by following standard methods for summation.

Fig. 3.

Drought frequency and die-off projections. (A) Duration and frequency of drought events from a 103-year record of regional climate. (B) A comparison of the frequency of a widespread die-off causing drought from the 103-year record (26.1-week regional drought, blue), and under a warmer-temperature, accelerated mortality scenario (18.7-week regional drought, red) for the Four Corners region.