Replanting forests in the headwaters of catchments is a widely applied conservation strategy from Kenya to Colombia. Frequently, this afforestation is done in the belief that forest cover will lead to greater rainfall infiltration, thereby recharging aquifers and ensuring better stream flow during dry periods. But exactly what type of forest is planted appears to matter enormously.
Eneko Garmendia and colleagues looked at catchment flow in Spain, and found that foresting catchments with fast growing pine or eucalyptus species actually decreases stream flow compared with grazed pasture. This is because the evapotranspiration associated with these forests is greater than the additional water recharge. Forests composed of native species do slightly better, but still require careful management or many years before they are likely to increase the water available in the catchment.
The lesson: protect existing native forests first, because more trees doesn’t necessarily mean more water.
The study: Garmedia et al. 2012 Assessing the effect of alternative land uses in the provision of water resources: Evidence and policy implications from southern Europe. Land Use Policy 29:761– 770.
(Image: In California, Elder Creek flows into the South Fork of the Eel River at the Angelo Reserve, one of the largest intact old-growth douglas fir forests in the world. Source: Ian Shive.)