Challenging predictions for the impact of elevated CO2 on eucalyptus woodlands

Author: Teresa E. Gimeno is an Ikerbasque Research Fellow at The Basque Centre for Climate Change

One of the expected effects of rising atmospheric CO₂ concentration in the atmosphere is an increase in plant water use efficiency. This is due to the expected enhancement of photosynthesis under future atmospheric CO₂ concentrations, together with a partial closure of plant stomata. Stomata are the pores on the leaf surface that open and close to allow plants to exchange carbon for water. These pores are sensitive to both internal plant signals and external environmental conditions, including increased atmospheric CO₂ concentration.

Under elevated CO₂, photosynthesis (the ability of plants to take up carbon) should increase while the stomatal aperture should be reduced, thus increasing plant water use efficiency at the leaf level. For water limited ecosystems, increased plant water use efficiency should translate into reduced ecosystem water use and increased availability of water for runoff and groundwater recharge and ultimately increased water availability for human consumption. Indeed, some retrospective analyses ascribed observed increases in river runoff across the world to greater plant water use efficiency under elevated CO₂. Nevertheless, these studies addressed how the vegetation has responded to past changes in the atmosphere during the XXth century (e.g. ¹), but these trends might not necessary apply to predict the response to future increases in CO₂ concentration in the XXI century.

In addition, these predictions strongly rely on experimental manipulations at the leaf or single plant level, while there is limited evidence from whole ecosystem studies and for forests this is only available for non-water limited regions ². Thus it is particularly challenging for current dynamic global vegetation models (DGVM) to provide realistic predictions of the contribution of the vegetation to mitigate CO₂ emissions from anthropogenic fossil fuel burning, in water-limited regions ³. This is the case in many regions across the Mediterranean and temperate zones and the vast majority of the Australian continent.

Australia is the driest permanently inhabited continent and its native ecosystems all across its territory are periodically subjected to sustained periods of limited water availability. In order to predict freshwater availability in the face of climate and global change, we need to be able to estimate the impact of future atmospheric conditions on water use for Australian ecosystems. Quantifying the effect of elevated CO₂ at the whole ecosystem level is challenging, particularly for forested ecosystems and one of the few opportunities we have to do so is in the so-called FACE experiment (Free Air CO₂ Enrichment). The EucFACE experiment, located in one of the few remnant patches of native Cumberland plain forest in Western Sydney (Australia), is the only one of its kind established on a water-limited and native woodland.

In this study ⁴, we increased the CO₂ concentration in the atmosphere to mimic that of the atmosphere in the year 2050 in a native Eucalyptus woodland. We expected to find a reduction in the amount of water transpired by the trees ⁵, which should have eventually led to an increase in the water available for runoff and groundwater recharge, but this was not the case.

We quantified the amount of water transpired by the vegetation and lost through evaporation to estimate total ecosystem water use, under ambient and elevated CO₂. We expected to find an increase in soil water storage, due to a reduction in vegetation water use. Instead we found that there was no effective water savings by the vegetation.

Our study comprised a transition period between el Niño and la Niña cycles, thus including a broad range of water-availability and climatic conditions: from no-water limitation to extremely dry and hot weather, such as the period preceding a catastrophic series of wild fires in the area (the 2013 New South Wales bushfires). Still, we found that ecosystem water use did not decrease under elevated CO₂, regardless of the prevailing climatic conditions.

In our paper we argue that marginal temporary reductions in stomatal aperture at the leaf level do not contribute to drive effective water-savings, when the ecosystem is already water-limited. However, in contrast to previous studies ⁶, at the EucFACE site, vegetation structural changes, mainly increased leaf area index under elevated CO₂ ⁷, did not underlay the offset of the expected water-savings under elevated CO₂. Our results challenge predictions from dynamic global vegetation models and suggest that the vegetation-climatic feedback might not to be able to mitigate climate change as much as we expected.