The roles of hydraulic and carbon stress in a widespread climate-induced forest die-off

Abstract

Forest ecosystems store approximately 45% of the carbon found in terrestrial ecosystems, but they are sensitive to climate-induced dieback. Forest die-off constitutes a large uncertainty in projections of climate impacts on terrestrial ecosystems, climate-ecosystem interactions, 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. We report here a direct and in situ study of the mechanisms underlying recent widespread and climate-induced trembling aspen (Populus tremuloides) forest mortality in western North America. We find substantial evidence of hydraulic failure of roots and branches linked to landscape patterns of canopy and root mortality in this species. On the contrary, we find no evidence that drought stress led to depletion of carbohydrate reserves. Our results illuminate proximate mechanisms underpinning recent aspen forest mortality and provide guidance for understanding and projecting forest die-offs under climate change.

Fig. 1.

Precipitation and temperature deviations from 1900 to 1999 average, especially the severe drought of 2000 to 2004, at 51 aspen field sites across Colorado. (Top) Annual rainfall anomaly (blue) during 1900 to 2009 and 5-y average (black). (Middle) Summer temperature anomaly (maroon) during 1900 to 2009 and 5-y average (black). (Bottom) Annual Palmer Drought Severity Index (red) and 5-y average from 1900 to 2009 (black).

Fig. 2.

(A) Percent dry-mass (mean ± SE) of glucose (blue), sucrose (green), and starch (red) in root tissues across elevation, aspect, and stand health. Treatments are low elevation/north-facing (LE-N), high elevation/north-facing (HE-N), high elevation/south-facing (HE-S), low elevation/south-facing (LE-S), and SAD stands. (B) Percent dry mass of starch in five tissues from two isolated plots of mature aspen ramets after experimental drought (control, green; drought, red). (C) Percent dry mass of starch in seven clones on a gradient of stand mortality (healthy ramets in healthy area, blue; healthy ramets in SAD area, green; SAD ramets in SAD area, red).

Fig. 3.

Midday xylem pressure (mean ± SE) for induced-drought experiment in mature aspen ramets (Upper) and potted trees (Lower) over the experiment (black, drought; gray, control). Asterisks indicate statistically significant differences (P < 0.05).

Fig. 4.

(A) Percent loss of conductance of branches in mature healthy (Healthy), healthy in SAD areas (Healthy-SAD), and SAD ramets. (B) Pair-wise difference in percent loss of conductance (i.e. treatment minus baseline) in branches of control potted trees (Cont Branch) and drought potted trees (Drt Branch) after experiment. (C) Percent loss of conductance in roots of control trees (Cont Root) and drought trees (Drt Root) after experiment in potted trees. Asterisks indicate statistically significant differences (P < 0.05).