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

Pathare, Varsha S., Crous, Kristine Y., Cooke, Julia, Creek, Danielle, Ghannoum, Oula, Ellsworth, David S.
Global change biology 2017 v.23 no.12 pp. 5164-5178
C3 plants, Eucalyptus, carbon dioxide, carbon dioxide enrichment, drought, ecosystems, grasslands, herbaceous plants, leaves, photosynthesis, soil water, soil water content, stomatal conductance, understory, woodlands
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 <0.07 in this sandy surface soil. Under eCO₂ 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.