The role of non-structural carbohydrates in tree mortality

Background/Question/Methods

Climate-induced stresses, including drought and increased temperature, are believed to be the primary cause of recent tree mortality events worldwide. The specific physiological mechanisms causing tree mortality are poorly understood, limiting models of vegetation dynamics needed to predict future responses of the carbon cycle to changing climate. Carbon starvation is one of the leading mechanisms hypothesized to cause tree mortality. Stomatal closure in the early stages of drought is thought to prevent significant carbon fixation, resulting in failure to maintain metabolism or fend off biotic agents due to prolonged negative carbon balance.  The occurrence of carbon starvation, however, is still under debate.

In order to better understand the possible role of carbon starvation in tree mortality, we asked “What happens to carbohydrate levels at death?” We addressed this question in two studies. In the first, we exposed piñon pine trees (Pinus edulis L) to drought and shade treatments (zero light) in a greenhouse setting. In the second, we evaluated the effects of ecosystem scale precipitation manipulations (drought and irrigation) on carbohydrate pools from mature piñon and juniper (Juniperus monosperma) trees.  Leaf and twig samples for carbohydrate analysis were collected regularly over the course of both experiments and after death. 

Results/Conclusions

In both studies, non-structural carbohydrate concentrations were significantly diminished by treatments that reduced photosynthesis.  Carbohydrate values did not decline to zero prior to or at mortality, suggesting that carbohydrate content need not be zero for plants to die. We also show that non-structural carbohydrate content at death is predictive of survival time in both studies.  Shade trees appeared to use all available sugars, except bark glucose and fructose, until ~1% dry weight remained. Greenhouse drought trees depleted starch reserves in all tissues, but had remaining transportable sugars (glucose, fructose, sucrose) which decreased in concentration the longer the trees lived. Conversely, droughted P. edulis in the field maintained ~50% of original starch content while depleting sucrose stores. These conflicting results from the two drought treatments are likely due to mortality of the field based P. edulis from bark beetle attack prior to outright metabolic failure. Our results emphasize the importance of distinguishing between total and utilizable carbohydrates, as well as between greenhouse and ecosystem-scale experiments, when defining mechanisms of mortality.