With increasing needs for water and food security and rapid urbanisation resulting from a booming global population, forests maintenance and development face great competition with other priorities. While we are losing forest resources at a high rate globally, afforestation efforts are made in some regions to offset this adverse effect.

Afforestation is the preferred measure for policymakers to solve a range of ecological problems, as it is generally believed that forests help to improve soil health and quality, reverse land degradation, halt land desertification, and provide clean water through an intensified water cycle. Especially within the context of SDG 15 (life on land), does afforestation always have a positive effect? The answer is ‘no,’ at least not always for water provision. A study of the relationship between afforestation and water supply in several regions under different climate conditions shows afforestation resulted in a decline in water supply in all climates (see Table 1).

These outcomes contradict the common belief that afforestation increases water supply. A critical driving process of these results is evapotranspiration, which is a key hydrological process in the water cycle. When lands are converted from other land uses to forests, the partitioning of precipitation into green (part of water available for plant use) and blue (part of water available for human use) water fluxes changes.

Evapotranspiration is a major green water flux, while river discharge and groundwater are typical blue water fluxes. Tree growth can consume more water than other shorter vegetation. According to the mass balance principle, if more water is used by trees, less water will flow into rivers and lakes or recharge the groundwater that people can directly use. The direct impact on water supply is becoming an increasingly urgent issue for water resource managers and planners as demands for water continue to rise.

How then can a positive effect of afforestation on water supply be achieved? We need to consider three central aspects.

First, scale matters: afforestation can accelerate the water cycle and movement at the global level. For example, more than 2 square kilometers of forest expansion can increase the possibility of rainfall. Trees transport water to the air, and water vapour moves to another place which can be geographically far from the afforested location, such as to another country or even continent through wind-driven circulation. This increases the possibility of precipitation in another location and thus contributes to the water supply in that other region. This indicates that, at a global scale, afforestation can indeed bring benefits to the water cycle.

Second, species matters: we must choose suitable tree species for afforestation to have a positive effect on the local water supply. Depending on the species, plantation forests may have different structure and amounts of water consumption (Schwärzel et al. 2018). Tree water use also declines with age. Over time, runoff reduction trends can be reversed, and water supply can be improved. Evidence shows, for example, that water supply may start to improve after 30 years in the case of pines and 15 years in the case of eucalypts (Scott and Prinsloo 2008).

Third, site matters: we need to establish forests in the right place. In dryland, less water is available from precipitation. As tree cover increases, the amount of water available is likely to become less and less, which is critical for areas that already suffer from water shortage and have high demand and competition for water (Zhang and Schwärzel 2017). Compared to dry-prone climates, water is not a limiting factor in tropical climate due to high precipitation. The decline in water supply resulting from afforestation is likely less critical and might even be negligible.

At the global level, many afforestation efforts are ongoing. There are, among others, the Grain-for-Green Program (GFGP) in China, National Afforestation and Reforestation Plan (PNFR) in Ecuador, and National Afforestation Programme (NAP) in India. In Nigeria, afforestation is considered as a promising solution to achieve climate change mitigation and to solve water supply issues (Olorunfemi et al. 2016).

Nigeria recognises the importance of keeping their forests. The Nigerian savannah has a tropical climate with temperatures ranging from 16°C to 37°C and an annual rainfall about 1,220 milimeters. Afforestation in this region could induce more precipitation, and water availability could potentially increase in the long term. However, we should also recognise that there are risks for water supply in the short term.

Forests – when managed properly – can increase resilience of water supply to enable adaptation to global change. The International Union of Forest Research Organizations (IUFRO) recently published a global assessment report highlighting that all of the SDGs are either directly or indirectly related to water and the forest-water nexus. Scientific insights on the forest-water nexus are particularly important to manage and optimize the trade-offs. Afforestation does not always have positive effects on the water supply. The seasonal and annual water yield from forested areas largely depends on forest management, covering selection of tree species, forest structure, and density.

Understanding the interlinkages and feedback loops – as advocated by the UN University Institute for Integrated Management of Material Fluxes and of Resources (UNU-FLORES) via the Nexus Approach – will support more effective and integrated policymaking, and represents an important contribution towards achieving the SDGs.

References

Buendia, Cristina, Ramon J. Batalla, Sergi Sabater, Antoni Palau, and Rafael Marcé. 2016. “Runoff trends driven by climate and afforestation in a Pyrenean Basin.” Land degradation & development 27 (3): 823-838.

Fahey, Barry, Rick Jackson, and Lindsay Rowe. 1998. “Hydrological effects of afforestation and pasture improvement in montane grasslands, South Island, New Zealand.” Environmental Forest Science: 395-404. Springer, Dordrecht.

Huang, Mingbin, Lu Zhang, and Jacques Gallichand. 2003. “Runoff responses to afforestation in a watershed of the Loess Plateau, China.” Hydrological Processes 17 (13): 2599-2609.

Ochoa‐Tocachi, Boris F., Wouter Buytaert, Bert De Bièvre, Rolando Célleri, Patricio Crespo, Marcos Villacís, Carlos A. Llerena et al. 2016. “Impacts of land use on the hydrological response of tropical Andean catchments.” Hydrological Processes 30 (22): 4074-4089.

Olorunfemi, Felix, Mayowa Fasona, Grace Oloukoi, Peter Elias, and Vide Adedayo. 2016. “Natural Resource Use and Adaptation to Climate Change in the Nigerian Savanna.” DNC Policy Brief DNC2015/02, Edited by Hiroshan Hettiarachchi. Dresden: UNU-FLORES.

Schwärzel, Kai, Lulu Zhang, Andreas Strecker and Christian Podlasly. 2018. “Improved Water Consumption Estimates of Black Locust Plantations in China’s Loess Plateau.” Forests 9(4): 1-21.

Scott, David F., and F. W. Prinsloo. 2008. “Longer‐term effects of pine and eucalypt plantations on streamflow.” Water Resources Research 44(7).

Zaady, Eli, Moshe Shachak, and Yitzhak Moshe. 2001. “Ecological approach for afforestation in arid regions of the northern Negev Desert, Israel.” Deforestation, Environment and Sustainable Development, A Comparative Analysis, edited by: Vajpeyi, D: 219-238.

Zhang, Lulu and Schwärzel, Kai. 2017. “China’s Land Resources Dilemma: Problems, Outcomes, and Options for Sustainable Land Restoration.” Sustainability 9(12): 1-13.

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The article was written by Hakyung Lee and Lulu Zhang, Soil and Land-Use Management Unit, UN University Institute for Integrated Management of Material Fluxes and of Resources (UNU-FLORES).