Regional-scale impacts of climate and environmental variability on tree carbon reserves

Background/Question/Methods

Carbon reserves serve as a critical buffer against stress in all organisms and are particularly important for the survival of long-lived organisms such as trees. Despite the importance of carbon reserves, in the form of nonstructural carbohydrates, for the growth, survival, and reproduction of trees there does not exist a general theory predicting how reserves should vary across space, time, and life-history traits. In this analysis we employ the Ecosystem Demography model, a physiologically-based stage and age structured terrestrial biosphere model, to explore carbon reserve dynamics in the temperate forests of the eastern U.S. and compare these predictions to a quantitative synthesis of literature data on nonstructural carbohydrates. Specifically we investigate: the seasonal pattern of reserves, the impact of evergreen vs deciduous phenology, the impact of tree size and canopy position, the impact of climate in terms of average temperature and precipitation, the impact of inter-annual variability in weather and extreme climate events such as droughts, the impact of disturbance in terms insect defoliation, the effect of successional status, responses to soil nutrients, and the impact of elevated CO2.

Results/Conclusions

ED predicts a strong seasonal pattern to carbohydrate dynamics, with a peak in the early fall and a spring low with much stronger seasonal carbohydrate dynamics for deciduous species than for evergreen species. Evergreen species produced an earlier low earlier in the spring than deciduous species, whose low corresponds to the leaf-flush period. Carbohydrate reserves consistently increased with relative canopy position, mean annual temperature, and mean annual precipitation. When examining inter-annual climate variability, temperature variability produced consistent variability while precipitation variability exhibited a more nonlinear response, with low variability in most years but a stronger response to dry years. Under persistent drought conditions most trees took 3-4 years to exhaust their reserves with early successional species showing greater sensitivity due to a higher respiration rate. Evergreen species were predicted to be more sensitive to defoliation than deciduous species. Plant reserves depended on nutrient availability, with larger reserves under N-limited conditions, and reserves increased under elevated CO2. There are a number of places where alternate allocation assumptions could be implemented in ED to represent competing assumptions about plant allocation priorities. Additional field data is required to test these hypotheses and, more importantly, to quantify the connection between reserves and demographic responses.