CO₂ and land-use effects on Australian vegetation over the last two centuries

Coincident with major changes in land use and management in Australia over the past 200 years, there has been a 20% increase in the concentration of carbon dioxide ([CO₂]) in the atmosphere. We investigate the way in which these two factors have modified the natural vegetation at the continental scale. We describe landscapes in terms of the abundance of three leaf functional types, 'turgor' (T), 'mesic' (M) and 'sclerophyll' (S). We have previously shown that continental-scale estimates of the TMS composition can be made from climatic and satellite data. In this study, we extend those results by estimating the TMS composition for the 'natural' vegetation (i.e. the present vegetation if there had been no change in land use and management). By comparing those estimates with the existing vegetation, we show that changes in land use have resulted in a large increase in the abundance of T leaves (seasonally green leaves of annual and ephemeral herbaceous plants) and there is a corresponding decline in plants with M and S leaves (evergreen trees, shrubs and perennial grasses). We then use a water use efficiency model to estimate the natural vegetation cover 200 years ago when the [CO₂] was about 280 pmol mol^-1. According to the model, 200 years ago the seasonally green (T) vegetation cover was similar to that at the present (excluding cultural vegetation cover), but there was lower evergreen cover. The increase in evergreen cover with increasing [CO₂] could have been expressed as an increase in trees, shrubs or grasses having M and S leaf forms, depending on soil moisture and nutrient availability. According to our model, while 'woody vegetation thickening' may have been exacerbated by the increase in [CO₂], other changes associated with European settlement are also important.