Stable carbon isotopes in tree rings indicate improved water use efficiency and drought responses of a tropical dry forest tree species

Abstract

Understanding the responses of tropical trees to increasing [CO2] and climate change is important as tropical forests play an important role in carbon and hydrological cycles. We used stable carbon isotopes (δ13C) in tree rings to study the physiological responses of a tropical dry forest tree species in southern Mexico, Mimosa acantholoba to changes in atmospheric [CO2] and variation in climate. Based on annual records of tree ring δ13C, we calculated intrinsic water use efficiency (W i) and intercellular [CO2] (c i). Our results showed that trees responded strongly to the increase in atmospheric [CO2] over the last four decades; W i increased dramatically by 40%, while c i remained largely constant. The maintenance of a constant c i indicates that photosynthetic rates are unlikely to have increased in response to higher [CO2], and that improvements in W i are probably due to a reduction in stomatal conductance. This may have large consequences for the hydrological cycle. Inter-annual variation in c i was strongly correlated with total annual rainfall (r = 0.70), and not influenced by temperature, solar radiation or cloud cover. Our results show that δ13C in tree rings of tropical dry forest trees may be a powerful tool to evaluate long-term responses of trees to increasing [CO2] and to variation in climate.