Forests as global public goods

 Forests: A positive force for global water availability
A recent study suggests that, since forests play a significant role in the regional and global supply of water vapour in the atmosphere, they should be thought of as global public goods and not viewed negatively in terms of water consumption.

Although it is generally accepted that forests increase carbon sequestration and promote biodiversity, their role in water availability is still hotly contested. From a demand perspective, forest cover is said to diminish the amount of water available at the local level for other uses, including agriculture, energy, industry and domestic consumption. A counter-argument from the supply perspective is that forest cover raises the amount of water available at the regional and global scales, by increasing the rate at which precipitation (rainfall and snow) is recycled.

Since the two schools of thought address very different geographical scales, the new study reviews both sides of the argument at both relevant scales: the small-scale catchment level (less than 1 km2 to about 10 km2) and larger regional and continental scales.

Although the water demand of trees can be estimated for small-scale catchments (less than 2 km2), basins of this size can collect precipitation from other areas. Thus, the effects of changes, such as deforestation, might only be apparent further away, making it difficult to link cause and effect.

At larger scales, evapotranspiration ('ET', evaporation through transpiration of water by vegetation) increases the availability of water vapour for transport in the atmosphere, in particular in continental interiors that are distant from oceans. In hot summer months, ET is a significant source of rainfall over land, contributing up to 48% of rainfall at the regional and continental scales and 16% at the local level. A number of studies have found a link between deforestation and lower regional precipitation. For example, deforestation in the Amazon and Cerrado regions of Brazil increases the average length of the dry season by one month.

With anticipated climate change, diminished recycling of water vapour in the atmosphere as a result of deforestation, particularly in dry areas, could increase the likelihood and extent of drought. Planting more forested areas in Europe, on the other hand, should increase precipitation at the regional level through greater recycling of water via ET. The type of vegetation significantly affects the rate of ET. Compared with other forms of land cover, forests and wetlands are almost twice as efficient at returning water to the atmosphere.

Overall, the researchers conclude that policymakers should consider forests as 'global public goods', in terms of their strong positive impact on regional and global precipitation, in addition to their water demand. This approach would affect the valuation of ecosystem services for water-pricing strategies, determining water footprints of products, drought mitigation strategies and afforestation policies.
 
Source: Ellison, D., Futter, M.N. and Bishop, K. (2012) On the forest cover–water yield debate: from demand- to supply-side thinking. Global Change Biology. 18: 806–820.

fuente: Science for Environment Policy