Worldwide forest mortality events associated with climate change are of increasing concern and could have profound consequences on global carbon cycles. Therefore, there is an increasing need to assess the risk of forest mortality due to climate change. However, modeling how forests will respond to drought requires specific knowledge of the physiological mechanisms underlying drought-induced mortality. Two main mechanisms have been proposed to explain tree mortality due to drought: hydraulic failure and carbon starvation. Hydraulic failure occurs when excessive tension in the water column of the xylem (vascular tissue) breaks, thus interrupting water transport. Carbon starvation occurs when plants prevent excessive water loss by closing stomata (tiny pores on leaves), a strategy that consequently limits photosynthesis (carbohydrate supply) and depletes stored non-structural carbohydrates (NSC). Increasing evidence suggests that hydraulic failure and carbon starvation are intimately interdependent and that plant hydraulic function depends on stored NSC. If so, plants must maintain NSC reserves above certain thresholds to maintain hydraulic function and survive.
This project tested whether ponderosa pine (Pinus ponderosa) seedlings demonstrate minimum NSC survival thresholds and whether these thresholds change with drought. Initially NSC was lowered with varying shade treatments, which was verified by an NSC harvest. For each shade treatment, plants were then brought back to the light and divided into two groups, well watered and drought stressed. Survival and health were monitored.
Results/Conclusions
As expected, shade treatment decreased NSC concentrations in roots, leaves, and stems, with lowest concentrations in seedlings subjected to the longest shade treatment. Although plants were well watered when brought back to the light, depletion of NSC was significantly correlated to a decrease of their xylem tension, indicating impaired hydraulic function. Seedling mortality in well-watered plants increased significantly when NSC concentration fell below 40% of that in control plants. Seedling mortality in drought stressed plants was generally higher than that in well-watered plants and plants with higher root NSC concentrations tended to take longer to die under drought. These results indicate that NSCs influence hydraulic function, and that plants must maintain NSC concentrations above certain thresholds to survive, even under well-watered conditions. My results are the first to demonstrate such clear effect of NSC on plant hydraulics and mortality. This research suggests that accurate predictions of mortality require incorporation of plant hydraulics, NSC storage, and their respective interaction.