Water availability affects seasonal CO 2 -induced photosynthetic enhancement in herbaceous species in a periodically dry woodland

Elevated atmospheric CO (eCO ) is expected to reduce the impacts of drought and increase photosynthetic rates via two key mechanisms: first, through decreased stomatal conductance (g ) and increased soil water content (V ) and second, through increased leaf internal CO (C ) and decreased stomatal limitations (S ). It is unclear if such findings from temperate grassland studies similarly pertain to warmer ecosystems with periodic water deficits. We tested these mechanisms in three important C herbaceous species in a periodically dry Eucalyptus woodland and investigated how eCO -induced photosynthetic enhancement varied with seasonal water availability, over a 3 year period. Leaf photosynthesis increased by 10%-50% with a 150 μmol mol increase in atmospheric CO across seasons. This eCO -induced increase in photosynthesis was a function of seasonal water availability, given by recent precipitation and mean daily V . The highest photosynthetic enhancement by eCO (>30%) was observed during the most water-limited period, for example, with V 30%) and decreased C under the ambient CO concentration (aCO ), with leaf photosynthesis strongly carboxylation-limited. The alleviation of S by eCO was facilitated by increasing C , thus yielding a larger photosynthetic enhancement during dry periods. We demonstrated that water availability, but not eCO , controls g and hence the magnitude of photosynthetic enhancement in the understory herbaceous plants. Thus, eCO has the potential to alter vegetation functioning in a periodically dry woodland understory through changes in stomatal limitation to photosynthesis, not by the "water-savings effect" usually invoked in grasslands.