Water Erosion Trends Under the Impact of Different Forest Fire Intensities in a Mediterranean Environment

摘要

In this work, the temporal evolution of the incidence of fire on water erosion in a typical Mediterranean forest environment has been evaluated. The effects of fire intensity on soil and its influence on water erosion have been studied on a permanent field station (La Concordia, Valencia, SPAIN) equipped with devices to study climatic, soil and water erosion parameters. This station has nine experimental plots (4 X 20 m) installed in a calcareous hillside representative of Mediterranean shrubland areas. Experimental burnings of two intensity levels (high and moderate) were carried out on two sets of three plots each. The remainder three plots were used as control. The impact of water erosion processes has been monitored for each rain event occurred before and after the experimental fire from April 1995 to December 2000. Runoff production and soil loss were measured in each rain event during this period in all plots. In the studied period 69 erosive rain events occurred, from them 55 produced sediment delivery. During the whole period, great differences have been observed between the plots not affected by fire and those correspondents to the fire treatments, with a maximum in 1999. In this year, the control plots reached average values of 97.32% in sediment production and 78.84% in runoff generation lower than the burned plots. The runoff and sediment production has been always higher in the plots affected by fire than in the control ones (mainly the treatment of high intensity) and this difference is maintained with time. Nevertheless, the differences between the plots that suffer high fire intensity and those that suffer a moderate one have decreased gradually with time, being the sediment production in the moderate intensity plots a 64.51% lower than in the high intensity treatment in 1996 (the year after the fire) and a 23.88% in the year 2000. Climate variability during this period has also influenced significantly the action of the erosive processes and soil response to them. The effects of a drought period since 1998, with changes in the distribution and characteristics of rains have influenced the erosion trends in the different fire intensity treatments. In this sense, the three years period immediately after the fire shows the highest soil losses in all the treatments reaching a total of 35.62 t · ha~(-1) meanwhile in the sequent years (1998-2000) was of 3.81 t · ha~(-1). Since the second half of 1999, the change in climate characteristics, mainly in the rain regime, has induced the slow recovery of the vegetation cover in the plots affected by fire and changes in the soil hydrological conditions that have allowed to a better response to water erosion reducing soil losses. However, this recovery has been not enough to reach, after six years, the level of the natural vegetation cover unaffected by fire. It clearly shows the fragility and difficulty of the Mediterranean ecosystems to recover under the impact of forest fires in a variable and drought prone climate.