Forests and water: silviculture, afforestation and the control of forest water use

Forests cover over one third of Italy; forest evapo-transpiration has therefore a primary role in the water cycle and in determining the availability of water for alternative uses. The possibility of modulating carefully the water balance of forest ecosystem would pave the way for the management of water resources at the local and landscape level. In the present work, we review available evidence on the effects on the water cycle of both afforestation and the silvicultural management of existing forests. Afforestation is known to have a dramatic effect on site evapotranspiration and runoff, but the magnitude of the effect largely depends on local environmental conditions. The management of existing forests has more modest local effects, but their extent suggests a greater role in the regulation of the water cycle. Forest thinnings result in an increase in runoff, albeit of short duration. Ecosystem evapo- transpiration is more strongly reduced by forest utilisation, to an extent which varies with management type. An increase in rotation length could result the most effective and long-lasting measure for the sustainable increase in water availability for alternative uses; contrasting evidence is provided in the literature, however, as the age-related decline in tree transpiration could be counter-balanced by negative feed-backs at the ecosystem level. New studies are required for the assessment of the effects of age on ecosystem water yield.