Rainfall seasonality determines annual/perennial grass balance in vegetation of Mediterranean Iberian

Much recent attention has been focused on the invasion and dominance of annual grass species in areas thought to have been historically dominated by perennial life forms. Explanations of this phenomenon in the literature have focused on two mechanisms favoring annuals: ruderal strategy associated with disturbance, and stress escaping associated with dry sites or deserts. Here I present evidence from vegetation surveys at 50 sites across a 1,200 km band of the Iberian Peninsula—a source region for many invasive annuals—showing that relative annual versus perennial grass composition is not well correlated with degree of disturbance or average annual precipitation. However, annual dominance is strongly and significantly linked to the seasonality of precipitation, in particular the relative intensity of summer drought. Disturbance was significantly associated with annual grass dominance in Iberia, but with much less explanatory power than summer drought intensity. Slope, aspect, and soil parent material were not significantly correlated with annual versus perennial grass dominance. These results suggest that subtle differences in rainfall seasonality largely drive grass composition in herbaceous Mediterranean vegetation. Furthermore, the patterns of annual grass invasion observed in the world’s other Mediterranean climate regions may be associated with similar climatic drivers.