Global Tree Restoration and its Impact on Regional Water Availability

By Laura ThomsonMay 18 2022

The United Nations reported the Decade on Environmental Restoration in June 2021 to prevent, slow, and reverse ecosystem degradation worldwide. Large-scale tree restoration is critical for combating climate change, biodiversity enhancement, and ecosystem services protection. This article looks at a recent study published in Nature Geoscience, which details shifts in regional water availability caused by global tree restoration.

Image Credit: GoncharukMaks/Shutterstock.com

It is projected that an extra 900 million hectares of tree cover might exist on Earth’s surface under current climate conditions without intruding on agriculture or metropolitan areas. Several global and regional projects to enhance tree cover have been launched in the last decade, and this will play an essential role in influencing global land use in the coming decades.

Despite these continuous efforts and assertions that ecosystem restoration benefits all of the Sustainable Development Goals, the effect of tree planting on the water cycle and the availability of water remains unknown. As a result, the potential effects of ecosystem restoration on water supply downstream and downwind are frequently underestimated.

Radiation balance, infiltration and soil water storage, evaporation, streamflow, and precipitation are all affected by the increase in tree cover. Local influences of forest cover on streamflow have historically been studied primarily utilizing a paired catchment technique.

Researchers calculate how large-scale forest restoration affects the availability of water and streamflow in the idealized study.

They assess the influence of a recent estimate of the worldwide tree-restoration potential (Figure 1a) on evaporation, precipitation, and streamflow fluxes.

In addition, they also tested the multimodel streamflow against big river-basin run-off under current climate and forest-cover conditions (Figure 1b).

Figure 1. Impacts of forest restoration on water fluxes and water availability. a, The tree-restoration potential: the percentage area of each pixel that is suitable for tree restoration. b, Model ensemble mean versus observed streamflow (Q) measurements for 19 validated river basins. The error bars for Q Budyko indicate the standard deviation over the six Budyko models. The error bars for Q validation indicate a 20% error. The river basins are Amazon (AM), Brahmaputra (BR), Colorado (COL), Congo (CON), Danube (DA), Ganges (GA), La Plata (LA), Mackenzie (MA), Mekong (ME), Mississippi (MI), Murray–Darling (MU), Niger (NIG), Nile (NIL), Orinoco (OR), Rhine (RH), Volga (VO), Yangtze (YA), Yenisei (YE) and Zambezi (ZA). c–f, The absolute annual change in water fluxes after tree restoration: change in evaporation (c), precipitation (d), water availability without evaporation recycling (e) and water availability with evaporation recycling (f). Note that e is the inverse of c: without the feedback of evaporation recycling, the local increase in evaporation equals the local decrease in water availability. g, The histogram shows the distribution of the global changes in water availability without and with evaporation recycling; 89% (without recycling) and 91% (with recycling) of the data fall within the displayed range of –20 mm yr^–1 to +10 mm yr^–1. All maps display the 0.1° mean value, except for c, which displays the 0.5° mean value. Image Credit: Hoek van Dijke, et al., 2022

Researchers combine the direct and indirect effects of tree restoration at the river-basin size for 21 significant river basins to assess the implications of tree restoration at the river-basin scale. Improved evaporation reduces streamflow in all river basins (up to 9%), although streamflow loss can be close to nil when increased precipitation overcomes increased evaporation (Figure 2).

Figure 2. Impacts of global tree restoration on hydrological fluxes in selected river basins. a,b, Mean wetness index (ratio of precipitation to potential evaporation; a) and tree-restoration potential (b) for each river basin. c, The change in streamflow (ΔQ) with evaporation recycling. The dots indicate the mean river-basin change in streamflow, and the bars indicate one standard deviation (the variation over the six Budyko models). d, The change in evaporation (ΔE, in green) and precipitation (ΔP, in blue). The green bar indicates the increase in evaporation without evaporation recycling, and the green line indicates the increase in evaporation with evaporation recycling (the increase in evaporation when taking into account the increased precipitation). The river basins are sorted from the lowest to the highest decrease in streamflow. The river-basin boundaries are taken from reference 31. Image Credit: Hoek van Dijke, et al., 2022

Since arid environments may only support tree growth if there is sufficient groundwater convergence, many river basins with a poor wetness index (water-limited basins) have a low tree-restoration capability.

Evaporation and precipitation alter very little in these desert river basins. Considering the indirect benefit of evaporation recycling, streamflow will drop in most of the world’s major river basins after forest restoration.

This research reveals that large-scale tree restoration will alter regional water availability. Since water supply from evaporation recycling is insufficient to compensate for evaporation loss, the hot locations for forest restoration may see a significant decline in water availability (Figure 3a).

However, due to the lateral flow of evaporated water into the region, locations with minimal tree-restoration potential may witness an increase in water availability. This demonstrates the need to properly strategize forest restoration projects.

Figure 3. Implications of the percentage restored area and the implications for water scarcity. (a) Water availability could decrease in ‘hot spots for tree restoration’. The lower, middle, and upper boundaries of the boxes display the 25th, 50th, and 75^th percentiles of the data (the interquartile range), respectively. The lines extend to a maximum of 1.5 times the interquartile range. (b) Water availability will decrease in several regions that currently face water scarcity: the color indicates whether local water availability will decrease (orange) or increase (blue) following the studied tree-restoration scenario. The cross-hatched regions currently face freshwater scarcity for at least three months per year, and the bold hatched regions face freshwater scarcity for at least nine months per year. Note that we quantified the change in water availability on an annual timescale. The sign of change could vary seasonally, and water availability could, for example, decrease every year but remain equal, or even increase, in the dry season. Image Credit: Hoek van Dijke, et al., 2022

Another significant conclusion is that this tree-restoration scenario could reduce annual water availability even further in certain locations that are already experiencing water constraints (Figure 3b).

Trees have been shown to improve soil porosity and organic carbon levels. As a result, tree restoration, particularly in treeless areas, improves infiltration and soil water storage capacity while reducing overland flow. Soils might store more water after tree regeneration, which would be partially mediated during the dry season.

Results and Discussion

The reported conclusions are based on cutting-edge data and methods, although various feedbacks and processes are left out of the calculations. The possible effects of enhanced tree cover on atmospheric circulation and land-atmosphere interactions are not considered in the methodology.

Localized tree restoration could improve convergence, cloud cover, and precipitation, as well as change the direction and distance traveled by atmospheric moisture.

Furthermore, the influence of tree restoration under current climate conditions is proposed in this research. However, global warming and forest regeneration will alter temperature and precipitation patterns, which are not taken into account in current calculations. By 2050, higher temperatures could diminish the worldwide tree-restoration capability by 25%.

Methodology

Researchers evaluate how a realistic large-scale tree restoration scenario will affect evaporation (E), precipitation (P), and streamflow fluxes (Q). There are four steps in the process: Before restoration, researchers employ Budyko models to determine E and Q. They estimate E and Q after restoration without evaporation recycling using Budyko models. Then, they evaluate P after restoration using the UTrack dataset and the rise in E is estimated in steps 1 and 2. To compute E and Q after restoration using evaporation recycling, they use Budyko models.

Conclusion

The impacts of worldwide large-scale forest regeneration on water fluxes, particularly evaporation recycling, were calculated in this research. The findings reveal that tree restoration affects the availability of water and streamflow on a local and continental scale and that trends of raising or lowering water availability are complicated over the world.

Increased precipitation following tree restoration may boost local water availability; nonetheless, researchers discover a global net loss in water availability across continents. Other data-driven or model-based investigations of the effects of land-cover change on water flows have reached similar conclusions.

Researchers emphasize the importance of considering the hydrological implications in future tree-restoration plans. To protect the water supply locally and remotely, or to improve the water supply in water-scarce locations, smart tree-restoration measures are required.

Journal Reference:

Hoek van Dijke, A. J., Herold, M., Mallick, K., Benedict, I., Machwitz, M., Schlerf, M., Teuling, A. J. (2022) Shifts in regional water availability due to global tree restoration. Nature Geoscience, 15(5), pp. 363-368. Available Online: https://www.nature.com/articles/s41561-022-00935-0.

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