Non-structural carbohydrates influence water relations in Pinus ponderosa

Worldwide forest mortality events associated with climate change are of increasing concern and could have profound consequences on global carbon cycles. There is an increasing need to assess the risk of forest mortality due to climate change. However, model predictions of how forests will respond to drought require specific knowledge of the physiological mechanisms underlying drought-induced mortality. Two main physiological mechanisms have been proposed to explain tree mortality under drought. First, hydraulic failure occurs when the water column in the xylem (vascular tissue) becomes under excessive tension and breaks, thus interrupting water transport. Second, Carbon starvation occurs when plants prevent excessive water loss by closing stomata (tiny pores on leaves), a strategy that also 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. I conducted an experiment to test whether ponderosa pine (Pinus ponderosa) seedlings require minimum NSC thresholds to survive and whether these thresholds change with drought. Initially NSCs were artificially lowered with one to six week increments of shade. For each dark treatment plants were then brought back to light and divided into two groups, well-watered and drought stressed. Survival and health were monitored. The strongest decrease in NSC occurred in the first week. Preliminary results indicate that mortality of well-watered and drought stressed plants occurred when whole plant NSC decreased to below 40% of that in control plants. Plants who had higher NSC concentrations tended to take longer to die under drought. These data suggest that plants must keep their stored NSC above certain thresholds to survive under drought