Abstract
Summary Determining physiological mechanisms and thresholds for climate‐driven tree die‐off could help improve global predictions of future terrestrial carbon sinks. We directly tested for the lethal threshold in hydraulic failure – an inability to move water due to drought‐induced xylem embolism – in a pine sapling experiment.In a glasshouse experiment, we exposed loblolly pine (Pinus taeda) saplings (n = 83) to drought‐induced water stress ranging from mild to lethal. Before rewatering to relieve drought stress, we measured native hydraulic conductivity and foliar color change. We monitored all measured individuals for survival or mortality.We found a lethal threshold at 80% loss of hydraulic conductivity – a point of hydraulic failure beyond which it is more likely trees will die, than survive, and describe mortality risk across all levels of water stress. Foliar color changes lagged behind hydraulic failure – best predicting when trees had been dead for some time, rather than when they were dying.Our direct measurement of native conductivity, while monitoring the same individuals for survival or mortality, quantifies a continuous probability of mortality risk from hydraulic failure. Predicting tree die‐off events and understanding the mechanism involved requires knowledge not only of when trees are dead, but when they begin dying – having passed the point of no return.
Highlights
The Earth is undergoing rapid shifts in ecosystem structure and composition due to an unprecedented rate of warming accompanied by increased variability in precipitation driven by anthropogenic climate change (IPCC, 2014)
Euclidean color distance between initial canopy foliar color and canopy foliar color at rewatering was significantly higher for trees that died than for those that either survived drought or did not experience drought (ANOVA, F2,80 = 4.737, P = 0.0114, post-hoc Fisher’s least significant difference (LSD), Fig. 3d)
Observations from active xylem stains of stems from recovered trees revealed that embolism remained in tissues which experienced the drought treatment, but that surviving trees had active xylem near the vascular cambium when drought was relieved, which remained functional during our stains (Fig. 4)
Summary
The Earth is undergoing rapid shifts in ecosystem structure and composition due to an unprecedented rate of warming accompanied by increased variability in precipitation driven by anthropogenic climate change (IPCC, 2014). The fate of feedbacks in carbon exchange between forests and the atmosphere under a changing climate remains one of the largest uncertainties in projecting future climate change (Friedlingstein et al, 2006, 2014; Friend et al, 2014). Despite the importance of forest dieoff, predicting when and where it will occur in response to climate remains a challenge for vegetation modeling. Development of process-based models that simulate the physiological mechanisms of stress and mortality may be the best solution for prediction of rapid, nonlinear tree mortality events under future climate scenarios that are not analogous to current climate conditions (Allen et al, 2010; McDowell et al, 2011)
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