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 (Ϸ4°C) shortened the time to droughtinduced mortality in Pinus edulis (piñ on 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.biosphere-atmosphere feedbacks ͉ drought impacts ͉ global-change ecology ͉ Pinus edulis ͉ carbon starvation

Drought and shade deplete nonstructural carbohydrate reserves in seedlings of five temperate tree species

“…2006; Adams et al. 2009). Thus, long‐term studies that link NSC dynamics with carbon sources and sinks as well as mortality events are needed.…”

Water and climate: Recognize anthropogenic drought

“…More than 12 million trees have died (see go.nature. com/vrgp1e), with cascading impacts on amphibians, birds and mammals 3 . Streams and wetlands are drying up, including the American River hatcheries of steelhead and Chinook salmon.…”

Ecosystem CO2/H2O fluxes are explained by hydraulically limited gas exchange during tree mortality from spruce bark beetles

“…Though the primary physiological response to disturbance might be obvious on the plant scale, the ultimate forest ecosystem response can be complex and difficult to predict [Gough et al, 2013]. Forest ecosystems with one or two dominant trees are an ideal setting to test whether tree level physiology can be used to predict ecosystem fluxes during a mortality event, a major assumption of many ecological investigations [Adams et al, 2009;Anderegg et al, 2012]. In this study, we investigate disturbance in a subalpine forest to determine if known mechanisms of hydraulic failure and subsequent impacts on carbon uptake from individual trees can explain changes at the ecosystem scale.…”