Restoration of deforested drylands to combat global warming needs to be done and funded based on the impacts of forests on both the global carbon and water cycles Provisionally Accepted

Koen Kramer^{1, 2*} Douglas Sheil²

• ¹Land Life Company, Netherlands
• ²Wageningen University and Research, Netherlands

Drylands and forests
Drylands covered two-fifths of the Earth’s land surface in 2015 (1), with trees present on one-third of these areas (2). This area of drylands has expanded by almost 1% per year since 2015 because of large-scale drying and land degradation at low and middle latitudes (3). Thus, conservation and restoration of drylands are needed. Restoration is often performed with a focus on the carbon sequestration potential that can be funded by carbon markets (4). In contrast, the role of trees and forests in water cycling has been relatively neglected.
The recognition of tree cover to promote water security would allow to fund restoration of deforested drylands based on water as ecosystem service (5–7). One option would be to consider the price of water directly (8), which at the catchment scale with the concept of Green Water Credits (9). Another approach would be to develop credit for the value of water to the wider environment using multivariate measure such as those suggested for biodiversity (10–12). For these water valuation approaches, policies, financial instruments, and markets remain to be developed; this process that takes years to accept and apply. A simpler approach given existing global markets might be to convert the water benefits of forest water into some form of carbon equivalent (13–15).
In the following, we first outline different mechanisms and scales at which trees and forests interact with the water cycle. Second, we discuss the impacts of reforestation of drylands on temperatures. Finally, we argue that the selection of the right trees at the right place and at the right scale for restoration of deforested drylands to combat global warming needs to be based on the forests’ impacts on the carbon and water cycle and thus funded based on both.

Interactions between trees and forest with water cycling
Regionally and continentally, trees can enhance the atmospheric moisture content and stimulate cloud formation, leading to additional precipitation (13–15). However, many authors claim that trees invariably diminish the availability of useful water (16). Such views are outdated and misleading. While trees indeed use water, a range of related studies on local and regional effects show how having the right trees in the right places can enhance water availability (17).
At the local scale, many studies indicate that when compared to treeless lands, tree cover can enhance the groundwater recharge and the baseflow that sustains dry season stream flows in degraded regions with poor infiltration with seasonal heavy rainfall and deep soils. One well studied case involves an agroforestry landscape in Burkina Faso where measurements demonstrate that infiltration and groundwater recharge was several times more with partial tree cover (20-40%) than with a treeless landscape (18). This improvement of hydrological functioning depends on local rain, soil, and drainage conditions but in dry areas, an intermediate tree cover whilst at both lower and higher values of tree cover, these landscapes would store less water (19,20).
At regional and continental scales, tree cover is increasingly highlighted as shaping the atmospheric processes that influence and determine rainfall. Indeed, most rain on land derives from recycled rain returned from the land surface and trees dominate this process (15,21). Air that passes over forests captures more water and produces more rain than air that passes over sparse vegetation or even open water (22). The likelihood of rain, and the amount, are sensitive to atmospheric moisture, e.g., a 10% drop in relative humidity may reduce precipitation by over 50% (23). Thus, small changes in humidity can have a marked influence on rainfall (24,25). While the local impacts of forest loss vary, studies have shown how a decline in tree cover can cause a marked decline in rainfall in a wider region. The contrasting process with recovery of tree cover boosting regional rainfall is credible but often ignored in reforestation studies (15,17). By drawing on water accessible to deep roots and stored in large stems, trees can maintain transpiration when other vegetation cannot. Access to such moisture permits trees to develop new leaves and transpire even after a protracted dry season, a phenomenon well established in African drylands (26). Such green-up is associated with transpiration which contributes to the atmospheric moisture needed to trigger rains in monsoon climates. Vegetation is thus active in the processes that maintain the local climate.
Particles and compounds emitted by forests influence rainfall too. Under common atmospheric conditions water vapour remains a gas and neither freezes nor condenses without condensation nuclei. All else being equal, condensation occurs at lower vapour concentrations in air containing condensation nuclei than otherwise, hence these can exert a major influence on clouds and precipitation (27). Changes in the abundance, character or dynamics of these nuclei impact condensation, cloud dynamics and the water-cycle (28–30). Many key relationships are nonlinear. For example, increasing densities of condensation nuclei can increase or decrease both cloud cover and precipitation (28) and influence associated atmospheric behaviours (31,32). At a larger scale the feedback between tree cover and climate are also nonlinear and include tipping points where a dry region once sufficiently wet may become wetter, or a wet region that becomes dry, becomes drier, if a threshold has been crossed (3,33,34).
Context is important in all these processes. There is considerable variation in the infiltration rates found among soils across Africa for example, though in general there is a positive association with tree cover (35) Furthermore, while tree cover can fail to recover seriously compacted soils (36) there are also cases where even young forest fallows can reduce overland flow and erosion and improve infiltration on degraded soils (35). Similarly with the atmospheric relationships, there are nuances. For example, one recent study combined observations and theory to suggest that under drier conditions and early stages of ecological restoration, reforestation increased terrestrial precipitation recycling, while once a wetter climate is established additional vegetation also enhances moisture import 2023). This latter process likely determines much of the global response of the terrestrial water cycle to tree cover. Such findings suggest that many dryland regions of the Earth currently unable to support forest could do so through their various self-watering effects.

Reforestation of drylands and impacts on local and global temperatures
Vegetation cover influences local and global temperatures in multiple ways. On a local scale, trees provide shade, moderating extreme temperatures. Beyond the direct effect of shade, moisture plays a key role in cooling the air. Globally, nearly half of the sun's energy absorbed by Earth goes towards water evaporation (38). Without sufficient water—for example in drylands—this energy simply becomes heat. This heating is opposed by vegetation: energy that goes into transpiration also not contributing to heat. In many dryland settings trees can access water unavailable to other vegetation and release it through transpiration, contributing to local cooling (39). What happens to the moisture and energy that rises over forest? While atmospheric water vapor enhances the local greenhouse effect, the increased moisture over extensive forests also transports heat energy higher, ultimately contributing to cooling not just locally but at larger scales too (41). Another crucial factor, but one still under active research, is the impact via cloud cover. Understanding how tree cover influences cloud formation across diverse global contexts remains a complex challenge (14,15). Combining these effects on local and global temperatures paints a complex picture, with uncertainties and ongoing debate, but also with first quantitative insights, e.g., an empirical space-for-time remote sensing study based on global data found that deforestation in arid zones leads to a 4.4ºC increase of the mean daytime air temperature compared to adjacent forested areas (42). On average, when scaling up this cooling effect, the same study found an additional contribution of deforestation to global warming that is about 18% of that due to CO2, and up to 42% for arid zones (42).

Combating global warming based on forest impacts on global carbon and water cycles
The effects of the capture of carbon by reforestation on the global climate are accepted and quantifiable: the more additional trees there are and the faster they grow, the more CO2 is sequestered, and the more climate change mitigation occurs during a reforestation project. However, the effects on hydrological functioning and thus on local and regional climates remain neglected. Thus, many reforestation activities focus exclusively on carbon capture–trees are considered to mitigate climate change solely by removing CO2 from the atmosphere (43–45). This selective focus and the conflicting reforestation methods associated with maximising CO2 capture and what is required for improving hydrological functioning, and the fact that many reforestation projects are in drylands has resulted in many failed projects where carbon capture targets are not met, landscapes are further desiccated, and biodiversity and livelihoods are impacted (46–52).
Bringing back water requires that impacts on hydrological functioning are considered in the restoration of deforested drylands and that this reforestation is feasible, that means financially and practically. This might be achieved by assessing the local impacts of reforestation via certified regional climate models if these models can capture all the key mechanisms and impacts with sufficient accuracy. The projected effects of water could then be expressed in terms of CO2 equivalents (CO2e) and made fundable by carbon offset markets. This allows optimising the species choice and tree cover by balancing CO2 capture with the effects on water and on cooling more generally. Restoration of deforested drylands can bring back water if species selection, scale, and suitability conditions are carefully considered.
Many deforested dryland regions that currently appear too arid to support tree cover could do so because tree cover would itself bolster the water cycle to support such tree cover. Restoring drylands to restore water opens new opportunities for greening vast regions and promises benefits both for capturing carbon and thus addressing climate change, and of improving the land and lives of those living there.