Print PageUnderstanding the mechanisms behind tree mortality is increasingly important because climate change appears to be increasing drought severity and duration worldwide, with concomitant increases in mortality. Two mechanisms for drought-induced mortality have been suggested recently: hydraulic failure and/or carbon starvation. Hydraulic failure occurs when xylem tension exceeds a critical threshold associated with widespread cavitation and embolism that leaves the canopy isolated from available water in the soil. Carbon starvation, on the other hand, may result when stomata close to avoid xylem tensions leading to hydraulic failure, reducing photosynthesis and leaving the plants susceptible to carbon starvation as respiration consumes stored energy reserves. To quantify the physiological changes and weak links in the plant water and nutrient transport system leading to mortality via these two mechanisms we conducted a greenhouse study where pinon pine (Pinus edulis) trees were killed using two treatments: complete drought and complete darkness. Our hypothesis was that despite the isohydric tendency of these trees, complete drought would lead to mortality via hydraulic failure, while carbon starvation would be the cause of mortality in complete darkness.
Results/Conclusions
Our results show that 6-8 weeks after treatments were imposed, trees in both treatments lost their ability to photosynthesize even in saturating light. Trees in the dark treatment died after an additional 15 weeks, while survival of trees subjected to drought treatment varied from additional 4 weeks to 9 months. Supporting our hypothesis, non-structural carbohydrate content of needles decreased significantly towards death in the dark but not under drought. At death the needles of drought trees turned brown very quickly, starting from the tips of the branches inwards, and the xylem hydraulic conductance dropped to zero. In contrast, trees in the dark retained the needles produced the previous year and maintained xylem hydraulic conductance until the end. Interestingly, even at death, the needles of trees in dark treatment stayed green and the non-structural carbohydrate content of bark tissue remained relatively high. This suggests that during carbon starvation not all carbohydrate reserves were available for utilization. Our results indicate that both hydraulic failure and carbon starvation can occur even in isohydric species. Which one is more likely in natural conditions depends on the severity and duration of drought.
