Water availability affects seasonal CO₂-induced photosynthetic enhancement in herbaceous species in a periodically dry woodland

Elevated atmospheric CO2 (eCO2) is expected to reduce the impacts of drought and increase photosynthetic rates via two key mechanisms: first, through decreased stomatal conductance (gs) and increased soil water content (VSWC) and second, through increased leaf internal CO2 (Ci) and decreased stomatal limitations (Slim>). 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 C3 herbaceous species in a periodically dry Eucalyptus woodland and investigated how eCO2-induced photosynthetic enhancement varied with seasonal water availability, over a 3 year period. Leaf photosynthesis increased by 10%–50% with a 150 μmol mol-1 increase in atmospheric CO2 across seasons. This eCO2-induced increase in photosynthesis was a function of seasonal water availability, given by recent precipitation and mean daily VSWC. The highest photosynthetic enhancement by eCO2 (>30%) was observed during the most water-limited period, for example, with VSWC 2 there was neither a significant decrease in gs in the three herbaceous species, nor increases in VSWC, indicating no “water-savings effect” of eCO2. Periods of low VSWC showed lower gs (less than ≈ 0.12 mol m-2 s-1), higher relative Slim (>30%) and decreased Ci under the ambient CO2 concentration (aCO2), with leaf photosynthesis strongly carboxylation-limited. The alleviation of Slim by eCO2 was facilitated by increasing Ci, thus yielding a larger photosynthetic enhancement during dry periods. We demonstrated that water availability, but not eCO2, controls gs and hence the magnitude of photosynthetic enhancement in the understory herbaceous plants. Thus, eCO2 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