Structure and function of C3and C4Chihuahuan Desert plant communities. Standing crop and leaf area index

During the past 150 years, native C 3 shrubs have invaded and dominated extensive areas of former C 4 grasslands in deserts of the south-western United States. This vegetation shift has caused large changes in several aspects of the structure and function of these plant communities. To examine structural changes, we measured the standing crop of green herbaceous plants and live shrubs, leaf area index (LAI), and canopy cover and shrub density in grass and shrub plant communities that exist on sandy and clay soils in the Chihuahuan Desert. Standing crop ranged from 800 to 1800 kg ha −1 for grass communities and from 2800 to 3500 kg ha −1 for shrub communities. The LAI in grass communities was typically 0·1 to 0·2 and in shrub communities was 0·3 to 0·4. Averaged over the 2 years, the greatest LAI was in the shrub community on a clay soil that also had a herbaceous understory. Shrub canopy cover varied from 17% ( Flourensia cernua , tarbush) to 33% ( Prosopis glandulosa , honey mesquite). On sandy soils, this vegetation shift has resulted in a replacement of herbaceous vegetation with shrubs and a large reduction in species diversity, while on clay soils, due to greater soil fertility and soil water availability, there was only a slight decrease in herbaceous vegetation and little change in species diversity. This vegetation shift of desert grasslands has dramatically changed plant community structure.