Untangling the forest cover-water yield debate – researchers call for long-term investments in landscapes

With support from IUCN’s forest programme, a collaborative research team led by Associate Research Professor Solange Filoso from the University of Maryland’s Center for Environmental Science (UMCES) examined whether forest landscape restoration (FLR) positively or negatively impacts surface water yields. Their conclusion is straightforward – both our project duration and research questions need to be longer and broader.

Photo: Copyright Craig Beatty

The team searched for scientific literature on the effects of forest cover expansion on water yield, whittling down their list from 666 papers to 167 for the final analysis. As each paper was evidenced by more than one case study, a total of 308 individual case studies were evaluated. It is pertinent to note that papers were based on forest restoration as well as on other types of forest cover expansion such as forestry and reforestation, a factor that was kept in mind while analysing the data. Studies spanned Asia, the Middle East, Oceania, North, South, and Central America, Africa and Europe covering mild temperate oceanic and humid subtropical climates.

The two main objectives of the study were to: (1) assess the state-of-the-science about the impacts of forest cover expansion and restoration on water yields and provide a quantitative analysis of the information available to determine if hydrologic outcomes from forest restoration differ from other types of forest cover expansion and (2) determine if the scientific information available allows us to draw broad conclusions across geographies, climate, and scale of restoration (spatial and temporal).

The review results substantiated the commonly held notion that expansion of forest cover does not result in an increase of annual water yield, the caveat being that the studies available in the literature to date are mostly based on relatively young forests growing in relatively small watersheds, and focused on forest stands composed of exotic tree species. Information from the tropics and sub-tropics are nearly non-existent; when available, studies in these regions typically focus on the impacts of forestry instead of the application of the forest landscape restoration approach.

Another key aspect of the review was the analysis of data that could be used as indicators of the recovery of key hydrological processes that sustain water yields in surface water bodies. The majority of studies with data available showed a positive result of forest restoration on soil infiltration capacity and groundwater levels, for instance, suggesting that eventually the positive effects of restoration on water yields may become clear.  Therefore, rather than considering the immediate impact of restoration on water yields, the focus should be on the recovery of important hydrological processes. Given that such processes can take decades to recover, it is important for both restoration projects and research to be designed accordingly, taking into consideration that the size of restored areas is also critical – if the forest cover expansion of a catchment area is relatively small, it is likely to result in a small change in water yield. This further emphasises the need for large-scale landscape restoration programmes.

The authors conclude by calling for more investment in long-term, large-scale research studies that consider native species and control sites to ensure that forest landscape restoration interventions lead to the recovery of all the important ecosystem services that forests can provide.

You can read the complete paper here.

The research team, comprising Solange Filoso, Maira Ometto Bezerra, Katherine C. B. Weiss and Margaret A. Palmer, representing the Chesapeake Biological Laboratory, National Socio-Environmental Synthesis Center and Department of Entomology at the University of Maryland, was supported in part by IUCN through the KNOWFOR programme, funded by UK Aid from the UK government.

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