Assistant Professor Montana State University Ecology Bozeman, Montana, United States
High-elevation five-needle pines are ecologically important, long-lived tree species that face grave threats due to global change. Warming and drying climates increase the physiological stress, susceptibility to pest and pathogens, and the already widespread mortality of these species. Insights into physiological mechanisms of seedling drought resistance may improve our understanding of and ability to predict mortality events. One tool that plants use to buffer against drought-induced mortality, is non-structural carbohydrates (NSCs), which are the primary products of photosynthesis used in regulating physiological responses to stress and include starch, sucrose, glucose, and fructose. To investigate the role of NSCs in seedling drought stress responses, we exposed greenhouse-grown 5-year-old whitebark pine (Pinus albicaulis), limber pine (P. flexilis), and Great Basin bristlecone pine (P. longaeva) to an experimental drought and monitored mortality, physiology, and NSC concentrations of leaves, stems, and roots before and throughout the drought treatment to establish a relationship between NSCs and survival. Our results revealed interspecific variation in response to drought, with P. albicaulis and P. flexilis exhibiting drought avoidance (as indicated by hydroscape area, HA = 0.65 and 0.28 MPa2 respectively), and P. longaeva exhibiting drought tolerance (HA = 1.96 MPa2). These values suggest that P. albicaulis and P. flexilis shut their stomata earlier during the drought, which may increase reliance on stored C but reduce risk of embolism, while P. longaeva kept stomata open to maintain assimilation of new C. These patterns were reflected in NSC concentrations. Foliar starch concentrations were lower in P. longaeva than P. albicaulis and P. flexilis when measured pretreatment (p = 0.05), but there were no differences between species at the end of the drought (p = 0.15), suggesting that the two drought-avoidant species utilized their starch stores during periods of zero net C assimilation. Our results also showed that initial starch concentrations were positively related to seedling survival across species. Using a logistic regression, we found a lethal threshold starting NSC value of 0.16 mg of foliar starch per gram of dry mass, with seedlings below that threshold value having a greater than 50% chance of mortality (LD50, Nagelkerke R2 = 0.34), across species. These data suggest that NSCs represent a valuable tool for predicting and monitoring forest mortality and future vegetation dynamics in the face of warming and drying climates.
