Relative influence of wildfire on soil properties and erosion processes in different Mediterranean environments in NE Spain.

Abandonment of terraced soils and increased brushland cover has increased wildfire occurrence to almost an annual rate in the Cap de Creus Peninsula, NE Pyrenees Range, Province of Girona, Spain. A wildfire occurred in August 2000 and affected an area of 6760 ha of shrubs and cork trees, whereas still cultivated plots were only slightly affected. Five stations of erosion measurements, corresponding to five different environments (from present cultivation to late abandonment) were destroyed by the passage of fire, and were promptly replaced to allow to monitoring post-fire effects on soil erosion. Selected soil properties were determined monthly before the fire and during 6 months after the fire at a monthly rate. Runoff and sediment yield together with dissolved organic carbon (DOC) in runoff water and organic carbon losses in eroded sediments (EOC) were evaluated throughout 2000. The last stage of abandonment, stands of cork trees, had the highest soil stability. Nevertheless, evidence of unfavourable soil conditions was detected at the shrub stage, when Cistus monspeliensis cover was the dominant opportunistic plant. This stage was considered to be a critical threshold leading either to degradation or regeneration processes according to fire frequency. A drastic change in soil properties, erosion and nutrient depletion occurred after the fire in all the environments. Statistics enabled to state that environments differed significantly in main soil properties. By statistically comparing the measured variables between the environments before and after the fire, DOC was found to be the soil parameter showing the highest significance between environments. Absolute values of erosion were low with respect to other Mediterranean environments although the shallow nature of these soils might deserve special attention because of a comparatively higher risk of degradation.

Runoff erosion and nutrient depletion in five Mediterranean soils of NE Spain under different land use.

We investigated the influence of agricultural management and various plant covers related to the period of abandonment on soil properties, erosion and nutrient depletion in a typical Mediterranean area with sandy loam shallow soils. Cultivated soils (CS) with insufficient management, 5 year abandoned soils covered with meadow (A5), 25 year abandoned soils covered with dense scrubs (A25), 50 year abandoned soils covered with cork trees (A50) and soils in a 50 year pine reforested area (P50) were studied over a period of 6 months (May-October 1999). The soils were classified as Lithic Xerorthents. Both the differences in soil properties and response to rainfall events were mainly attributed to the different vegetation types and stages in land management. Principal components analysis (PCA) was performed on the results, by running the overall data determined after five rainfall events. The factors extracted by PCA of the samples by variables matrix represented the response of the environments to different rainfall intensities as a function of management or natural evolution after abandonment. CS environments showed the highest runoff and sediment yield as well as the highest amount of dissolved organic carbon (DOC) and nitrogen in runoff water. The sequence of abandonment (A5, A25 and A50) showed approximately the same runoff production, whereas eroded sediments (ES) and DOC were inversely correlated. Organic carbon in the ES and DOC in runoff water always increased with the period of abandonment, which accounted for consistent nutrient depletion. Nevertheless, the A50 environment (dominated by Quercus suber) showed the best soil properties, whilst the A25 environment with dense cover of Cistus monspeliensis and Calicotome espinosa seemed to cause a worsening effect on the soil's physical and chemical properties. This is probably because these environments are more severely damaged by wild fire occurrence. In terms of sediment yield, the P50 environment followed CS environment, indicating that reforestation followed by insufficient forest management may negatively affect both soil properties and response to the erosive action of rainfall.