The carbon use efficiency of five deciduous tree seedlings in response to warming and altered precipitation in a northeastern old-field ecosystem

Background/Question/Methods

Proper understanding of how plant carbon exchange will respond to future climate change is essential if we are to correctly predict terrestrial carbon feedbacks to future climate change. The ratio of autotrophic respiration to photosynthesis (R/P) is directly related to plant carbon use efficiency (CUE): CUE=1-R/P. CUE describes the carbon assimilated by plants that is directly used for growth and development. Some studies, including those cited by many vegetation models, suggest that CUE is conserved over a range of growth conditions in many species; however, other studies show that plants decrease their CUE under stressful conditions, such as high temperature and drought, in order to allocate increased amounts of carbon to maintenance respiration. Here, we tested the hypothesis that CUE is conserved over a range of growth conditions using five species of deciduous tree seedlings at the Boston Area Climate Experiment (BACE). The BACE is a field-based, climate manipulation experiment in an old-field ecosystem in Waltham, MA with three levels of precipitation manipulation (ambient, -50%, and +50% ambient rainfall). Within each precipitation treatment ceramic heaters provide four warming treatments (up to +4^oC). We hypothesized that warming and drought would decrease leaf CUE due to an increase in respiratory demands. 

Results/Conclusions

We tested the effects of climate on plant carbon exchange and found that photosynthesis increased with increasing amounts of precipitation and under moderate warming, but decreased under high warming. However, these effects varied by species and differed throughout the season. Dark respiration was not affected by the warming treatments, but did decrease under drought. On average, the rate of dark respiration was 36% of the rate of photosynthesis and was not significantly affected by the warming treatments. However, this ratio decreased under elevated precipitation (increasing CUE), an effect most pronounced during early and late season measurement periods. The precipitation effect differed slightly between species with Betula populifolia and Ulmus Americana being most sensitive to changes in precipitation. These results indicate that CUE in deciduous tree seedlings is unaffected by warming, but sensitive to changes in precipitation. This signifies an enhanced ability to acclimate to changes in temperature compared to precipitation. This lack of ability to adapt to precipitation change could result in large changes in carbon storage by vegetation in a future, more variable climate. Model simulations using fixed CUE values may yield erroneous projections if precipitation effects are not taken into consideration.