Variable influence of photosynthetic thermal acclimation on future carbon uptake in Australian wooded ecosystems under climate change

Climate change will impact gross primary productivity (GPP), net primary productivity (NPP), and carbon storage in wooded ecosystems. The extent of change will be influenced by thermal acclimation of photosynthesis – the ability of plants to adjust net photosynthetic rates in response to growth temperatures – yet regional differences in acclimation effects among wooded ecosystems are currently unknown.  We examined the effects of changing climate on 17 Australian wooded ecosystems with and without the effects of thermal acclimation of C₃ photosynthesis. Ecosystems were drawn from five ecoregions (tropical savanna, tropical forest, Mediterranean woodlands, temperate woodlands, and temperate forests) that span Australia's climatic range. We used the CABLE-POP land surface model adapted with thermal acclimation functions and forced with HadGEM2-ES climate projections from RCP8.5. For each site and ecoregion, we examined a) effects of climate change on GPP, NPP, and live tree carbon storage; and b) impacts of thermal acclimation of photosynthesis on simulated changes. Between the end of the historical (1976–2005) and projected (2070–2099) periods, simulated annual carbon uptake increased in the majority of ecosystems by 26.1 to 63.3% for GPP and 15 to 61.5% for NPP.  Thermal acclimation of photosynthesis further increased GPP and NPP in tropical savannas by 27.2% and 22.4% and by 11% and 10.1% in tropical forests with positive effects concentrated in the wet season (tropical savannas) and the warmer months (tropical forests). We predicted minimal effects of thermal acclimation of photosynthesis on GPP, NPP and carbon storage in Mediterranean woodlands, temperate woodlands and temperate forests. Overall, positive effects were strongly enhanced by increasing CO₂ concentrations under RCP8.5. We conclude that the direct effects of climate change will enhance carbon uptake and storage in Australian wooded ecosystems (likely due to CO₂ enrichment) and that benefits of thermal acclimation of photosynthesis will be restricted to tropical ecoregions.