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

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 (delta C-13) 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 delta C-13, 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 delta C-13 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.