Widespread forest die-off: Observational and experimental tests of carbon starvation and water stress

Background/Question/Methods - 

Forest ecosystems store approximately 45% of the carbon found in terrestrial ecosystems, but they are sensitive to drought and climate-induced dieback. Widespread forest die-off constitutes a major uncertainty in climate change impacts on terrestrial ecosystems and carbon cycle feedbacks. Current understanding of the physiological mechanisms mediating climate-induced forest mortality limits the ability to model or project these threshold events under climate scenarios. We report here the first individual to regional scale direct tests of two broad physiological hypotheses underlying a recent trembling aspen (Populus tremuloides) forest mortality. We combine observational tests of standing gradients in hydraulic conductance and carbohydrate reserves with experimental drought manipulations on potted and mature trees to examine the roles of carbon starvation and hydraulic failure due to water stress in this forest die-off.

Results/Conclusions -

We find little evidence of carbohydrate-mediated mortality and substantial support for hydraulic failure of roots and branches linked directly to observed landscape patterns of canopy and root mortality in this species. Illuminating mechanisms underpinning forest mortality provides significant potential for understanding and projecting non-linear mortality events and the future of forest ecosystems under climate change.