Planted forests and water in perspective

Afforestation is increasingly considered as a land use activity that threatens water resources security. At the same time, it is advocated for a wide range of other water-related benefits. We review the contributions to this special issue and the wider literature, intended as a contribution towards a framework for predicting the impact on water resources and other water-related issues of afforestation in agricultural landscapes. Current evidence suggests that afforestation will typically reduce local average water yield as well as low flows. Downstream effects on water resources security and river ecosystem health are generally likely to be small unless the area afforested is large. In some cases, afforestation may increase groundwater recharge and low flows due to improved infiltration. Depending on the level of degradation of agricultural land, well designed, located and managed forestry can reduce the volume of sediment, nutrients and salt volumes transported into river systems, although not necessarily their concentrations. Afforestation is not likely to reduce major large-scale flooding or deep-seated land slides, but may reduce shallow land slides and local ‘flash’ floods. The influence of afforestation on global precipitation patterns is complex and land use change would need to occur on a large scale to have a discernible influence. Afforestation may influence precipitation patterns at local to regional scales by changing surface-atmosphere transfers of heat and moisture. Factors determining afforestation impacts on water resources security include: (i) plantation characteristics that affect forest hydrology (differences in tree physiology, plantation design and management, forestry operations), (ii) the benchmark (current agricultural use or historic native vegetation) against which changes are assessed and (iii) the water system configuration (scale of afforestation, timing of impacts, location in the system and internal system losses). Similar factors confound the impact of afforestation on other water-related issues.