How do forests affect local climate and water flow in drylands?

The gradual transition from closed forests via scattered trees to open grassland is determined largely by water, because trees need more water than grasses. Climate change is forecast to reduce precipitation and increase temperatures in many places, resulting in less water being available to plants and a contraction of forests. But forests are also believed to ameliorate local climates, and conservationists have advocated planting forests to reduce the impact of climate change. A recent study in the FORGER project, by Csaba Mátyás and Ge Sun, of the University of West Hungary and the USDA Forest Service, concludes that this may not be a very good idea. Contrary to expectations, trees are likely to make things worse if planted in dryer areas.

Mátyás and Sun looked at case studies from China, Hungary and the eastern United States. In Hungary, forest cover on the Great Plain, which covers almost 830,000 ha in the east of the country, has increased from 5 percent to 26 percent over the past four decades as a result of tree planting. These new forests use about 620 mm of water a year through evaporation and transpiration, about 80 mm more than precipitation, extracting the balance from groundwater. Elsewhere, evapotranspiration exceeds rainfall by even greater amounts; in neighbouring areas an oak forest uses around 250 mm of additional water per year through evapotranspiration, while grassland uses just 85 mm per year more. So in drylands, forests often deplete the available groundwater, lower the groundwater table and do not contribute to recharge or runoff.

The evidence from the USA is somewhat different. Since the 1930s there have been many studies of paired watersheds to compare the effects of forest management on water quality and yield. The results are complex; humid areas with high precipitation show a greater absolute response, but in drier areas with low water availability the relative response may be greater. Clear-cutting a forest in the humid south-east can increase the stream runoff by 130–410 mm, or 15–40 percent per year, compared to an undisturbed control watershed. In the much drier area of northern Arizona, a similar operation increases the water in the streams only by 60 mm, which is, however, more than 40 percent of the runoff in the undisturbed control.

Records from China are neither as long nor as accurate, but they offer a very similar picture. New forests planted on the Loess Plateau are associated with decreased stream flows because the trees are actually using more of the available water than grasslands.

The overall conclusion is that planting forests in dry areas will reduce the amount of water available for other uses and ecosystems. Beyond water, there is also evidence that forests do not necessarily ameliorate local climates. In Hungary, temperatures have increased more in the eastern region, where forest cover has increased significantly. In Canada, however, the partial removal of aspen forests increased summertime temperatures, and there was less rain and a shorter growing season.

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