General informations
Description
It is increasingly recognized that both the availability and the quality of water are strongly influenced by forests and that water resources in many regions are under growing threat from overuse, misuse and pollution. The relationship between forests and water is therefore a critical issue that must be accorded high priority. Forest management measures can increase water yield, regulate water flow, and reduce drought stress for a forest. By intercepting precipitation, evaporating moisture from vegetative surfaces, transpiring soil moisture, capturing fog water and maintaining soil infiltration, forests influence the amount of water available from groundwater, surface watercourses and water bodies. By maintaining or improving soil infiltration and soil water storage capacity, they influence the timing of water delivery. By minimizing erosion, they minimize impairment of water quality due to sedimentation. Forests can also protect water bodies and watercourses by trapping sediments and pollutants from other upslope land uses and activities. In addition, along streams, forests provide shade, thus reducing water temperature.The relationship between forests and water is complex and subject to myth and misinterpretation. Forests have been associated with improved water yields. As a general rule, however, in temperate climate conditions, forests consume more water than other land uses, increasing pressure on water resource security. In fact, an increasing number of studies have challenged the popular idea that more forests imply more and better water. To identify and correctly apply forest management to reduce water use is therefore a crucial aspect regarding water scarcity. Literature presents an equation to describe the water flow of a natural ecosystem: the water balance equation. The change in soil water storage for a given period is related to the amount of precipitations, rate of evapotranspiration (combination of the evaporation of the plants and the transpiration of the land surface to the atmosphere), surface runoff and deep drainage in a given period of time. The effects of forest management on the soil water storage are multiple and diverse. Measures that address existing forests include:<ul><li>reduced density of stand stocking;</li><li>shorter length of the cutting cycles;</li><li>planting hardwood species;</li><li>regeneration from seedlings rather than sprouts;</li><li>afforestation.</li></ul>This last measure, in particular near watercourses, brings benefits for the regulation of water flow and the maintenance of water quality, reducing the intensity of floods and the severity of droughts. Particularly relevant, in this context, are also practices such as harvesting, thinning and species mix choice. Canopy structure of mixed species plantations reduce transpiration, imposing less pressure on water if compared to mono-species plantations. By decreasing the number of trees in the stand, thinning may also be used to mitigate excessive forest water use. The positive impact of this measure may however be offset by increase water consumption due to increased growth of the remaining trees. Depending on the fraction of harvested land and on harvesting patterns, water yield usually increase after timber harvest. Different harvesting regimes may therefore impact differently on water resource security. Finally, shorter rotations decrease the period of time for which canopy is completely closed and may therefore also reduce water forest consumption. A relatively constant population of the stand by young trees may, however, counterbalance this effect. In addition, the use of fast growing species is usually more water intensive than slow growing species with higher rotations. While the use of forests has been increasingly advocated for a wide range of different objectives, the knowledge of the impacts of such programs on water scarcity still poses major research questions to the scientific community.