Impact of silvicultural management on the water balance in Calabrian pine stands

Forest systems notably affect the hydrological response of a basin, since they generally play a major role in ruling the soil water yield through processes of interception, infiltration and evapotranspiration. The main characteristics of forest cover (type, age, density, structure) are prone to be quite rapidly modified by both natural factors (e.g. chablis) or impact of man action, as wildfires, overgrazing, forest management. Several studies focused on the relationships between silvicultural practices and each single component of water balance. At this regard, general results agree that modifications in forest density and structure mainly involve increasing in flood discharge, changes in the proportion between surface and underground runoff, modifications through time in the different components of runoff. In the present paper, the effects of a thinning on the hydrological processes was investigated in a wooded, mountainous watershed, located in Sila Greca, Calabria, Southern Italy. The investigated area extends 1.39 km2 , 93% of which is covered by forest vegetation that is dominated by about 50-years-old, artificial and, secondarily, natural Calabrian pine (Pinus laricio Poiret) stands. Since 1986, the catchment is equipped with three mechanical, 20-minutes-span recording rain gauges, in addition to a runoff gauge. After seven years monitoring of the catchment hydrological behaviour, a low thinning was carried out in 1993, involving the removal of about 50% of plants number (about 30% of basal area). Interception losses are also monitored since 1994 in two 150 m2 pilot plots, respectively control and thinned. The silvicultural intervention involved just 8% decrease in interception loss, which is much less than proportional to the degree of the thinning. The global surface runoff remarkably increased, respectively before and after the thinning, but differences are notably smaller in autumn-winter as compared to the spring-summer period. At this regard, no statistically significant variation in monthly precipitations can be invoked in the time span 1986-1993 in respect to 1994-2000. Most likely, this particular situation is to be related especially to the modifications in transpiration losses, due to the reduction of tree density after the thinning. Then, a proper forest management can have a key role in water supply management, especially in the Mediterranean environment, where water deficit during the summer season represents a major problem in many areas of the Region.