Print PageClimate change may alter forest composition by differentially affecting the responses of tree species to drought, which limits primary production of most terrestrial ecosystems for part or most of the year. However, the interactive effect of rising [CO2] and temperature on tree responses to drought is a key unresolved issue regarding the effects of climate change on forests, and few studies have examined the effects of past increases in [CO2] or projected increases in temperature on drought sensitivity of trees. The objective of our study was to examine interactive effects of temperature (ambient, ambient + 4°C) and [CO2] (290, 400 and 650 μL L-1) on drought sensitivity of Eucalyptus saligna and E. sideroxylon seedlings. We hypothesized that rising [CO2] would reduce drought sensitivity, that increasing temperature would offset the effect of rising [CO2] on drought sensitivity, and that rising [CO2] and temperature would have a larger effect on the drought sensitivity of E. sideroxylon compared with E. saligna. Seedlings were grown in a glasshouse for 135 days prior to drought initiation. Drought was imposed by reducing soil water in controlled steps. Plants were rewatered when seedlings were visibly wilted and stomatal conductance in mid-morning was < 50 mmol m-2 s-1.
Results/Conclusions
Increasing [CO2] was associated with higher light-saturated net photosynthetic rates at the onset of the drought and delayed reductions in net photosynthetic rates as the drought progressed. In contrast, elevated temperature was associated with faster reductions in net photosynthetic rates as the drought progressed. Differences in net photosynthetic rates among treatments and over time were closely associated with variation in stomatal conductance, and rising [CO2] was associated with a reduction in the stomatal conductance required to maintain light-saturated net photosynthetic rates. E. sideroxylon maintained positive net photosynthetic rates at lower leaf water potential and soil water content compared with E. saligna, and this did not vary among treatments. Photosynthetic recovery from drought was nearly complete (90% of pre-drought rates) after two days following rewatering, and this did not vary between species or treatments. Our results suggest that the effects of rising [CO2] and elevated temperature on sensitivity to drought were largely regulated by effects on stomatal conductance. Further, our results suggest that slower-growing E. sideroxylon was less sensitive to drought than faster-growing E. saligna.
