Positive fire-grass feedback in Mediterranean Basin woodlands

Fires can mediate switches between alternative vegetation types which may b e more flammable and thus reinforce fire spread. We tested if there is a po sitive feedback between the expansion of the tussock grass Ampelodesmos mau ritanica (hereafter Ampelodesmos) and fire hazard in Mediterranean Basin co mmunities and its relation to tree cover decline. The effect of fire on Amp elodesmos population structure was analysed by surveying stands burned at d ifferent fire frequencies. The effect of vegetation dominated by Ampelodesm os on fire behaviour compared to other species coexisting in the community was predicted by the Rothermel fire propagation model BEHAVE. There was a n egative correlation between pine cover and percentage of Ampelodesmos plant s. Ampelodesmos mortality after fire is very low. Recently burned stands ha d a higher proportion of reproductive plants and higher seedling density th an unburned stands. The high temperatures reached during fire may kill seed s, the higher seedling recruitment results from fast resprouting and higher seed production of burned plants compared to unburned plants 1 year after fire. Simulations with the BEHAVE fire model predict that Ampelodesmos incr eases fire intensity and spread because of its high accumulation of fuel lo ad and standing dead material. The results suggest that there is a positive relationship between Ampelodesmos abundance and fire regime which increase s the invasive potential of this grass and the fire risk of the community w here it dominates. (C) 2001 Elsevier Science B.V. All rights reserved.