The identification of early-warning signals of critical transitions represents a crucial issue for semi-arid ecosystems which are strongly exposed to desertification risks. Previous studies in this field suggested that the observation of vegetation patchiness and, in particular, changes in the patch size distributions, could provide early indicators of desertification transitions. Through numerical simulations based on a cellular automaton model, we have investigated the time fluctuation properties of several quantities characterizing the vegetation patterns of semiarid ecosystems under different conditions. At increasing value of a mortality parameter measuring the strength of external stresses, we have found different and earlier transition indicators, related to the time fluctuations of the biggest cluster size.
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