Hydrological Impacts of Tropical Land Degradation, Natural Forest Regeneration and Reforestation:An Overview

Abstract:

Tropical landscapes are changing rapidly due to continued deforestation and shifting cultivation in some areas vs. large-scale land abandonment and natural regrowth in others. Advanced post-forest land degradation is shown to lead to major disruption in flow regime in many areas, with higher wet-season flows and reduced dry-season flows. Secondary forests now cover >50% of total tropical forest area and are the dominant forest type in most tropical countries. Yet, the associated hydrological impacts are far from clear. Throughfall tends to be lower compared to old-growth forest (OGF) during the first 10 years of regrowth but stabilizes after ~20 years while stemflow may be greatly enhanced at the early stage. Transpiration appears to be higher relative to OGF after 5 years but it is unclear for how long this situation applies. Soil infiltration capacity recovery requires at least 1–2 decades after abandonment, with corresponding drops in overland flow production but peak flows in headwater areas do not seem to vary systematically with age of regrowth while reported trends for baseflows are contradictory. Much has been made of the high water use of fast-growing tree plantations causing streams to dry up after foresting grass- or cropland but this widely held view is based exclusively on controlled experimental studies in which soil degradation was not prevalent. Evidence is on the increase that dry-season flows may indeed be boosted after planting trees on highly degraded land through improved infiltration that exceeds the new vegetation’s higher water use. A global modelling exercise of this ‘trade-off’ between changes in infiltration and tree water use indicates the greatest positive impact on dry-season flows in areas with high rainfall and initially advanced degradation. The hydrology of agro-forestry systems (which are likely to widely replace both OGF and regrowth in future as population pressure continues to increase) has remained greatly under-researched.