Elevated CO2 mitigates the adverse effects of drought on daytime net ecosystem CO2 exchange and photosynthesis in a Florida scrub-oak ecosystem

Drought is a normal, recurrent feature of climate. In order to understand the potential effect of increasing atmospheric CO2 concentration (C-a) on ecosystems, it is essential to determine the combined effects of drought and elevated C-a (E-C) under field conditions. A severe drought occurred in Central Florida in 1998 when precipitation was 88 % less than the average between 1984 and 2002. We determined daytime net ecosystem CO2 exchange (NEE) before, during, and after the drought in the Florida scrub-oak ecosystem exposed to doubled C-a in open-top chamber since May 1996. We measured diurnal leaf net photosynthetic rate (P-N) of Quercus myrtfolia Willd, the dominant species, during and after the drought. Drought caused a midday depression in NEE and P-N at ambient CO2 concentration (AC) and EC. EC mitigated the midday depression in NEE by about 60 % compared to AC and the effect of EC on leaf P-N was similar to its effect on NEE. Growth in EC lowered the sensitivity of NEE to air vapor pressure deficit under drought. Thus EC would help the scrub-oak ecosystem to survive the consequences of the effects of rising atmospheric CO2 on climate change, including increased frequency of drought, while simultaneously sequestering more anthropogenic carbon.