We use a spatially explicit landscape model to investigate the potential role of rainfall onshrub–grass transitions in the Jornada Basin of southern New Mexico during the pastcentury. In long-term simulations (1915-1998) along a 2700m transect running from a drylake bed to the foothills of a small mountain, we test two hypotheses: (i) that wetterwinters and drier summers may have facilitated shrub encroachment in grasslands, and (ii) that increases in large precipitation events may have increased soil water recharge atdeeper layers, thus favoring shrub establishment and growth. Our model simulationsgenerally support the hypothesis that wetter winters and drier summers may haveplayed a key role, but we are unable to reproduce the major shifts from grass-to shrubdominationthat occurred in this landscape during the early part of the 1900s;furthermore, the positive shrub response to wetter winters and drier summers was onlyrealized subsequent to the drought of 1951-1956, which was a relatively short 'windowof opportunity' for increased shrub establishment and growth. Our simulations alsogenerally support the hypothesis that an increase in the number of large precipitationevents may also have favored shrub establishment and growth, although these resultsare equivocal, depending upon what constitutes a large' event and the timing of suchevents. We found complex interactions among (i) the amount/seasonality of rainfall, (ii)its redistribution in the landscape via run-on and runoff, (iii) the depth of the soil waterrecharge, and (iv) subsequent water availability for the growth and reproduction ofshrubs vs. herbaceous plants at various landscape positions. Our results suggest thatonly a mechanistic understanding of these interactions, plus the role of domestic cattlegrazing, will enable us to elucidate fully the relative importance of biotic vs. abioticfactors in vegetation dynamics in this semiarid landscape.