Whole-plant evidence for mechanisms of drought-induced mortality of Pinus flexilis seedlings at their lower-elevation limit

Background/Question/Methods

Rising temperatures associated with climate change are predicted to result in uphill shifts in subalpine forest ranges at both cold (upper) and warm (lower) edges, which occur via seedling recruitment.  While encroachment of tree seedlings into alpine zones has been shown to be constrained by carbon-acquisition (vs. carbon-processing) limitations, it is unknown what aspects of carbon balance limit seedling recruitment at the lower treeline, which is generally considered to be moisture–as compared to thermally–limited.  We measured and modeled carbon fluxes (photosynthetic inputs and respiration processes) in 1^st-year Pinus flexilis seedlings (a widespread subalpine species) in garden plots just beyond the warm edge of their natural range, and compared these to germination, dry-mass gain, and survival across two summers. We hypothesized that mortality in these seedlings would be associated with an imbalance of carbon demand vs. supply, similar to recent investigations into carbon starvation of adult trees under drought.

Results/Conclusions

Preliminary findings suggest that while area-based photosynthesis remained high until critical hydraulic thresholds were exceeded (e.g., water potentials < -2.5 MPa), increasing tissue density led to a decline in mass-based photosynthesis throughout the growing season.  Respiration patterns (both area- and mass-based) were similar to photosynthesis, although seasonal declines in photosynthesis outpaced those in respiration. While whole-plant non-structural carbohydrate concentrations were not exhausted, we observed a 90% loss in maximum hydraulic conductivity and a decline in carbon-flux balance (photosynthesis:respiration) to below 1.0 across the summer, and all seedlings died.  This suggests that dwindling carbon-flux balance, combined with disruptions in hydraulic transport, resulted in inadequate carbon supply for root growth particularly at a time when the soil drying-front surpassed root-extension depth.  Thus the large-scale mortality that we observed near the end of the summer was likely a result of imbalanced whole-plant carbon relations triggered by the hydraulic failure (i.e., mismatch of supply, need, and transport among tissue compartments), and was not due to limitations to whole-plant carbon-gain or -use directly. Because precipitation is expected to become more variable in many ecosystems, understanding the physiological limitations under seasonally dry conditions is important for predicting forest community and ecosystem stability under future climate scenarios.