THE INFLUENCE OF ASPECT ON RUNOFF AND SOIL LOSS IN A MEDITERRANEAN BURNT FOREST (SPAIN)

In order to evaluate the soil loss in a burnt forest and the role played by slope orientation in erosion, a study has been carried out in the Montserrat area (NE Spain) affected by a fire in summer 1986. Previous geomorphological surveys allowed us to propose the hypothesis that slope orientation acted as a macrofactor or integrating factor in erosion control. Aspect controls temperature, moisture and water supply and then vegetation and soil development. The evaluation was determined by monitoring two experimental plots 250 m apart developed on equal substrate and slope but one facing north and the other facing south. The plots, with a surface area of 200 m2 are equipped with collecting and sampling units (Multi-slot divisors) and storage tanks. The results (August 1987 - December 1988) confirm the contrast in runoff and soil losses according to slope orientation (2176 g/m2 south-facing plot and 352 g/m2 north-facing plot). This different erosive response is mainly due to: a) the state of the vegetation and soil development before the fire, b) the differences in vegetation recovery after fire. The north-facing slopes had a denser plant cover and thicker soil than the south-facing ones. After the fire the north-facing slopes were covered by a thicker blanket of ashes and burnt plant debris and, later, more vegetation giving the soil higher infiltration capacity and resistance. Moreover, the two major events displayed different behaviour according to slope orientation. The first of those events, due to the conditions on each slope (higher or lower protection and available erodible material) gave rise to a change in the threshold conditions: a) the rill formation on the south-facing plot enhanced the flow concentration and sediment transport (lowering of threshold) in the second event and b) on the north-facing one, the lack of rills and the decrease in erodible material caused by the first event hindered subsequent erosion (threshold rise). Finally, the progressive recovery of spontaneous vegetation caused erosion to drop, reaching low rates in 1988 (22 g/m2 and 2 g/m2 on each plot).