Leaf Area Index Drives Soil Water Availability and Extreme Drought-Related Mortality under Elevated CO(2) in a Temperate Grassland Model System

The magnitude and frequency of climatic extremes, such as drought, are predicted to increase under future climate change conditions. However, little is known about how other factors such as CO(2) concentration will modify plant community responses to these extreme climatic events, even though such modifications are highly likely. We asked whether the response of grasslands to repeat extreme drought events is modified by elevated CO(2), and if so, what are the underlying mechanisms? We grew grassland mesocosms consisting of 10 co-occurring grass species common to the Cumberland Plain Woodland of western Sydney under ambient and elevated CO(2) and subjected them to repeated extreme drought treatments. The 10 species included a mix of C(3), C(4), native and exotic species. We hypothesized that a reduction in the stomatal conductance of the grasses under elevated CO(2) would be offset by increases in the leaf area index thus the retention of soil water and the consequent vulnerability of the grasses to extreme drought would not differ between the CO(2) treatments. Our results did not support this hypothesis: soil water content was significantly lower in the mesocosms grown under elevated CO(2) and extreme drought-related mortality of the grasses was greater. The C(4) and native grasses had significantly higher leaf area index under elevated CO(2) levels. This offset the reduction in the stomatal conductance of the exotic grasses as well as increased rainfall interception, resulting in reduced soil water content in the elevated CO(2) mesocosms. Our results suggest that projected increases in net primary productivity globally of grasslands in a high CO(2) world may be limited by reduced soil water availability in the future.