Banded chenopod shrublands of arid Australia: modelling responses to interannual rainfall variability with cellular automata

Cellular modelling is used to investigate the behaviour of well-developed banded chenopod shrublands in western N.S.W. Australia, where annual rainfall exhibits high year-to-year variability. Rainfall records for the past 100 years show that the study region has a mean annual rainfall of about 215 mm/a and coefficient of variability of annual totals of 40–50%. Synthetic sequences of yearly rainfalls matching these characteristics are used in modelling. It is found that interannual rainfall variability like that of the modern environment does not inhibit the development of vegetation bands. However, band spacing is increased, while band regularity, and the long-term mean plant cover, are significantly diminished. Rainfall fluctuating about a mean of 215 mm/a results in an 8% diminution in plant cover, compared to an unvarying rainfall of the same average amount. Vegetation bands widen by downslope expansion in wet intervals and isolated band fragments may also link across the slope. In dry intervals, the lower margin retreats upslope and bands may re-fragment. No net upslope band migration is seen in the model during wet episodes, and this is consistent with historical aerial photograph evidence of band fixity in western N.S.W.