Impacts of forest restoration on water yield: A systematic review

<div><p>Background</p><p>Enhancing water provision services is a common target in forest restoration projects worldwide due to growing concerns over freshwater scarcity. However, whether or not forest cover expansion or restoration can improve water provision services is still unclear and highly disputed.</p><p>Purpose</p><p>The goal of this review is to provide a balanced and impartial assessment of the impacts of forest restoration and forest cover expansion on water yields as informed by the scientific literature. Potential sources of bias on the results of papers published are also examined.</p><p>Data sources</p><p>English, Spanish and Portuguese peer-review articles in Agricola, CAB Abstracts, ISI Web of Science, JSTOR, Google Scholar, and SciELO. Databases were searched through 2015.</p><p>Search terms</p><p>Intervention terms included forest restoration, regeneration/regrowth, forest second-growth, forestation/afforestation, and forestry. Target terms included water yield/quantity, streamflow, discharge, channel runoff, and annual flow.</p><p>Study selection and eligibility criteria</p><p>Articles were pre-selected based on key words in the title, abstract or text. Eligible articles addressed relevant interventions and targets and included quantitative information.</p><p>Results</p><p>Most studies reported decreases in water yields following the intervention, while other hydrological benefits have been observed. However, relatively few studies focused specifically on forest restoration, especially with native species, and/or on projects done at large spatial or temporal scales. Information is especially limited for the humid tropics and subtropics.</p><p>Conclusions and implications of key findings</p><p>While most studies reported a decrease in water yields, meta-analyses from a sub-set of studies suggest the potential influence of temporal and/or spatial scales on the outcomes of forest cover expansion or restoration projects. Given the many other benefits of forest restoration, improving our understanding of when and why forest restoration can lead to recovery of water yields is crucial to help improve positive outcomes and prevent unintended consequences. Our study identifies the critical types of studies and associated measurements needed.</p></div

Water yield by type of intervention.

<p>Percent distribution of direct metrics of water yield responses according to intervention type group.</p

Water yield responses measured indirectly.

<p>Percent distribution of study cases with a positive, negative, or neutral effect of forest cover restoration or expansion on indirect water yield metrics, including (a) flooding frequency or magnitude of peak flows (<i>n</i> = 43), (b) groundwater level or recharge (<i>n</i> = 15), and (c) infiltration capacity or soil infiltration (<i>n</i> = 18).</p

Meta-analyses of water yield response versus temporal and spatial scale of studies.

<p>Scatter plots with percentage change (decline) in water yield versus period of data collection (a) and catchment size in km² (b). The data included are from a subset of study cases selected for meta-analyses, and the x-axis are log10 scaled. Spearman correlation results are indicated for both datasets.</p

Distribution of studies by spatial and temporal scale categories.

<p>Percent distribution of study cases according to catchment area (a) and time scale of water yield dataset (b). The case studies included reported on water yield results from direct measurements.</p

Studies with field and model water yield data.

<p>Percent distribution of study cases with data collected with different methods (a), and relative abundance of studies with field or model data according to spatial (b) and temporal (c) scales.</p

Geographic location of study cases and their water yield outcomes.

<p>Global distribution of study cases providing data on changes in water yield following forest restoration or forest cover expansion. The pie charts indicate the distribution of water yield responses reported in the studies from the different regions. Red represents a negative response, green a positive response, and purple mixed results. Neutral response is represented by light blue. Source for the world map is the US National Park Service (Natural Earth physical map; <a href="https://www.arcgis.com/home/item.html?id=c4ec722a1cd34cf0a23904aadf8923a0" target="_blank">https://www.arcgis.com/home/item.html?id=c4ec722a1cd34cf0a23904aadf8923a0</a>).</p

Water yield responses measured directly.

<p>Percent distribution of results in study cases that directly measured changes in annual water yield/channel runoff (<i>n</i> = 308) (a), and baseflow (<i>n</i> = 53) (b).</p

Parameters and tags used in the systematic classification of papers or case studies selected (in order of relevance).

<p>Parameters 11 and 12 were used to select a subset of case studies to address the secondary objective.</p

PRISMA flowchart.

<p>PRISMA flowchart.</p