On the carbon dynamics of Australian subalpine grasslands

2017-03-02T23:49:55Z (GMT) by McHugh, Ian David
Grasslands in the alpine and subalpine tracts of the Australian southeast occupy a small area but store substantial quantities of soil carbon, which is likely to be affected as climate change shifts the balance between processes of ecosystem carbon import and export. In order to investigate the effects of differences in climate on carbon balance in subalpine grassland ecosystems, eddy covariance instrumentation was established at two climatologically contrasting sites during 2007/08: a cooler, wetter site that sustains winter snow cover (Dargo), and a warmer, drier site that does not (Nimmo). Both were substantial carbon sinks in 2007 and 2008; 93.0 and 111.4gC m ⁻² a ⁻¹ for Dargo, respectively, and 203.8 and 322.0gC m ⁻² a ⁻¹ for Nimmo. Differences in annual carbon uptake were mainly contributed by photosynthetic rather than respiratory fluxes, largely in association with temperature differences, which had two main effects. First, the retention of snow (and associated cessation of photosynthesis) reduced winter gross primary production (GPP) at Dargo. Second, since both sites showed effectively the same temperature-normalised bulk canopy photosynthetic response, GPP was generally lower at the cooler Dargo. Both sites were also subject to drought (severe regional drought conditions during 2006 continued into early 2007), resulting in the observed lower carbon uptake in 2007. The cross-year NEE difference was reduced at Dargo because increased GPP during the wetter summer of 2008 (also observed at Nimmo) was offset by decreased GPP during the cooler winter – and associated longer, later-lying snow season - of 2008. Since the co-occurrence of warm, dry or cool, wet conditions are characteristic of the dominant mode of regional climate variability, ENSO, this suggests that snow cover may moderate the interannual amplitude of NEE variability in these ecosystems. Superimposed on this, however, are expected longer-term changes in carbon storage. Consistent with globally observed soil carbon / climate relationships, soil carbon storage was almost 50% higher at Dargo (19.4kgC m ⁻² ) relative to Nimmo (13.4kgC m ⁻² ). On the basis of such relationships, warming and drying would be expected to result in reductions in soil carbon, particularly at Dargo, where the cool, moist conditions presently limit decomposition and thus maximise storage of labile soil carbon. While losses of soil carbon may be partially offset by the effects of warming on plant growth, both structural limitations on carbon sequestration in grassland vegetation and the slow expected rate of succession towards woody functional plant groups suggest that net carbon losses are likely in coming decades.