Windstorm disturbance triggers multiple species invasion in a Mediterranean forest

Plant invasion in forest ecosystems is a serious ecological and economic issue, deserving attention by researchers, managers and policy-makers worldwide. Many invasive plants have been reported as early successional species able to colonize disturbed areas following abrupt changes in microhabitat and resource availability. We investigated disturbance effects of a severe windstorm generated by a wet microburst (hail and rain at 160 mm h-1) remarkably affecting the canopy cover of an old-growth Quercus ilex urban forest in Southern Italy. This stand-replacing disturbance produced a mosaic of 103 gaps, 5.6 to 1632 m2 in size, over an area of 1.53 ha, uprooting 76% of the trees and decreasing thereby 85% of the standing above-ground dry biomass into the gaps. By intensive monitoring we compared above-and below-ground microclimate, soil moisture and mineral N availability in paired disturbed and control areas of the study forest. Within newly formed gaps we observed a seasonally consistent 70% higher content of nitrate nitrogen, 29% and 47% decreases of ammonia nitrogen in summer and autumn, respectively, and a higher moisture in topsoil, in addition to different above-and below-ground microclimatic conditions, with canopy cover mitigating extreme temperatures. One year after the windstorm, the microhabitat shift promoted the establishment in gaps of 15 native and 10 alien taxa previously absent in both disturbed and control plots. In such conditions, the rarefaction of the dominant Q. ilex canopy cover and the occurrence of empty niches prone to invasion could dramatically affect the local community structure and diversity. Our data indicate that stand-replacing windstorm can transiently transform the studied urban evergreen forest to an early allogenic successional community dominated, in the medium and large gaps, by annual and perennial non-native species. This is particularly relevant under a perspective of possible increasing frequency of windstorm events in the Mediterranean region in the near future.