PERSPECTIVES Large-scale and threaten the water supply for local and regional communities in Indonesia by altering streamflow conditions.

and climate. Fires often cause hydrophobic soils, with reduced soil infiltration and ac- celeration of surface runoff and soil erosion. In a recent national assessment of the con- tiguous United States, fires had the greatest increase in annual streamflow in semiarid regions, followed by warm temper- ate and humid continental climate regions, with insignificant responses in the subtropi- cal Southeast (6). The hydrological impact of insect infestation is likely less pronounced than those of other disturbances. Large-scale beetle outbreaks in the western United States and British Columbia, Canada, over recent de- cades were predicted to increase streamflow, with reduced evapotranspiration because of Downloaded from the death of infested (5). However, fur- ther evidence showed negligible impacts of beetle infestation on annual streamflow, ow- ing to increased evapotranspiration of surviv- ing trees and understory vegetation (7). Forestation can either reduce annual streamflow or increase it (4, 8). Zhang et al. http://science.sciencemag.org/ HYDROLOGY (4) found that 60% of the forestation water- sheds had annual streamflow reduced by 0.7 to 65.1% with 0.7 to 100% forest cover gain, Deforestation, forestation, whereas 30% of them (mostly small water- sheds) had annual streamflow increased by 7 to 167.7% with 12 to 100% forest cover gain. and water supply Variations in annual streamflow response to forestation are even greater than those A systematic approach helps to illuminate caused by deforestation, possibly owing to the complex forest-water nexus site conditions prior to forestation and on March 13, 2021 species selected. Planting with a single fast- growing exotic species can have greater re- By Mingfang Zhang1 and Xiaohua Wei2 concerns and debates (2, 3), particularly after duction in annual streamflow than with na- recent catastrophic fires in Australia and the tive species (8). Streamflow reductions after orests as natural reservoirs and filters western United States. However, hydrologi- forestation are more common in semiarid can store, release, and purify water cal consequences of forest changes are never and arid regions than in the humid subtrop- through their interactions with hydro- simple, and future research and watershed ics and tropics (4, 5). Large-scale reforesta- logical processes. For humans, a clean, management require a systematic approach tion programs in the semiarid Loess Plateau stable, and predictable water supply is that considers key contributing factors and a in China caused substantial streamflow re- one of the most valuable ecosystem ser- broad spectrum of response variables related ductions that consequently approached wa- Fvices provided by . Yet, globally, forests to hydrological services. ter resource limits (9). have undergone many changes driven by hu- Zhang et al. showed the consistent ten- Dry-season low flow is critical for water man activities (, , affor- dency of deforestation to increase annual supply, particularly in the face of more se- estation, agriculture, and urbanization) and streamflow (4). More than 80% of deforested vere droughts under climate change. Low- natural disturbances ( and insect watersheds had annual streamflow increases flow response to forest change can be posi- infestations). From 2010 to 2015, tropical for- ranging from 0.4 to 599.1%, mainly owing tive, neutral, or negative (5, 10). The variable ests declined by 5.5 million ha year–1, whereas to reduced evapotranspiration after 1.7 to low-flow responses are mainly attributed to temperate forests expanded by 2.2 million ha 100% forest cover loss (4). The large varia- low-flow generation processes, forest char- year–1 (1). The effects of both deforestation tions in the magnitude of changes depend acteristics (age, species, and regeneration), and forestation (reforestation and afforesta- on the scale, type, and severity of forest dis- practices (retention of riparian buf- tion) on water supply have generated serious turbance, climate, and watershed proper- fers, logging methods, and ), ties (4, 5). Larger-scale disturbance tends to changes in soil conditions, and choice of low- 1School of Resources and Environment, University of cause greater increase in annual streamflow. flow metrics (daily or 7-day minimum flow). Electronic Science and Technology of China, Chengdu Hydrological response to fire is similar to the Nevertheless, negative low-flow response is 611731, China. 2Department of Earth, Environmental and response to logging, but the severity of the commonly expected if soil water storage and Geographic Sciences, University of British Columbia (Okanagan), Kelowna, British Columbia, V1V 1V7, Canada. impact varies with climate, fuel accumula- infiltration capacities are impaired by forest

Email: [email protected] tion, fire intensity, overstory tree mortality, disturbances (soil compaction and erosion IMAGES IFANSASTI/GETTY ULET PHOTO:

990 5 MARCH 2021 • VOL 371 ISSUE 6533 sciencemag.org SCIENCE

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from logging, and soil water re- The complex influence of forests on water supply assessments often use limited pellency following severe fires), Forests in watersheds play a critical role in regulating downstream water supply hydrological variables (such as and their recovery through re- and associated ecosystem services. annual mean flows), which could forestation could take much lon- underestimate total hydrologi- ger, because of the difficulty in cal functions or even produce restoring damaged soils (10). Upstream forests misleading conclusions resulting Generally, climate, watershed regulate streamfows by from different or contrasting re- properties, forest characteristics, canopy interception, Deforestation and sponses of various flow variables. and their interactions are the evapotranspiration, and forestation can afect A systematic assessment of the major drivers for large variations soil infltration. river fows positively effects of deforestation and fores- or negatively. in hydrological responses to for- tation on water supply requires est change (2, 4). Zhou et al. as- multidisciplinary collaborations. sessed global land-cover effects The classic paired watershed ex- on annual streamflow, based on periment (PWE: one watershed a general theoretical framework as a control and the others as the (11). They found that hydrological treatment) (12), mainly designed sensitivity to land-cover change to assess streamflow response to was determined by watershed forest change, has limitations to properties (watershed size, slope, evaluate interactions and feed- configuration, and soil), climate back among water, forests, cli- Downloaded from (precipitation or potential evap- mate, and watershed properties. oration), and their interactions, Future PWEs should system- where land cover and watershed atically consider more variables properties jointly indicate water and processes (flow pathways, retention ability. Land cover or Downstream water storage and retention, and forest change can cause greater water supply hydrological sensitivity) with hydrological responses in drier various approaches (isotopic http://science.sciencemag.org/ watersheds or those with low water retention low-flow responses over three distinct time tracing, telemetering, and modeling). With capacity. Similarly, McDonnell et al. (12) rec- periods associated with the development of long-term in situ monitoring and growing re- ommended studying watershed storages and forest canopy leaf area index and correspond- mote-sensing data, the forest-water nexus at water movements in the vertical zone that in- ing evapotranspiration: consistent increase larger spatial scales should be explored using cludes forest canopy, soil, fresh bedrock, and in the first 5 to 10 years, variable responses advanced analytical tools (machine learning, the bottom of groundwater (13), to further re- (increase, no change, or decline) during the and coupled climatic-ecohydrological mod- veal the mechanisms for variable hydrologi- next 10 to 20 years, and substantial decline eling) within a systematic context. Future cal response to forest change. in some (16 out of 25) watersheds multiple assessment should also focus on watershed The feedback between forests and cli- decades later. However, no decline in low management tools such as payments for eco-

mate may also introduce complexity. flows was found in nine watersheds during system services, with the inclusion of more on March 13, 2021 Forests can supply atmospheric moisture the third period—likely dependent on similar representative water variables to support through evapotranspiration and poten- factors previously identified for variations in synergies or trade-offs between hydrological tially increase precipitation (precipitation low-flow response. The dynamic hydrologi- and other ecosystem services provided by for- recycling) locally and in downwind direc- cal responses suggest that long-term studies ests in a changing environment. j tions. Therefore, forest change affects not are critical for fully capturing possible trends only downstream river flow, but also pre- and variations in the effects of forest change REFERENCES AND NOTES 1. R. J. Keenan et al., For. Ecol. Manage. 352, 9 (2015). cipitation and water supply downwind (5). on water supply (5). 2. X. Wei et al., Glob. Change Biol. 24, 786 (2018). Lawrence and Vandecar revealed variable The consistencies and large variations over 3. K. D. Holl, P. H. S. Brancalion, Science 368, 580 (2020). rainfall responses to tropical deforestation space and time in streamflow responses to 4. M. Zhang et al., J. Hydrol. (Amst.) 546, 44 (2017). 5. I. F. Creed et al., in Forest and Water on a Changing across landscapes, depending on deforesta- forest change call for a systematic perspec- Planet: Vulnerability, Adaptation and Governance tion thresholds, such as reduced rainfall by tive to elucidate both explanatory (factors af- Opportunities. A Global Assessment Report, I. F. Creed, large-scale deforestation and increased rain- fecting hydrological functions) and response M. van Noordwijk, Eds. (International Union of Forest Research Organizations, 2018). fall by small clearings (14). The effects of for- (hydrological functions) variables in future 6. D. W. Hallema et al., Nat. Commun. 9, 1307 (2018). est change on precipitation are likely related studies (see the figure). In the systematic con- 7. K. M. Slinski, T. S. Hogue, A. T. Porter, J. E. McCray, to topography, prevailing wind, and climate, text, explanatory variables, including climate, Environ. Res. Lett. 11, 074010 (2016). because they affect moisture residence time, forest, watershed properties, and their inter- 8. S. Filoso, M. O. Bezerra, K. C. B. Weiss, M. A. Palmer, PLOS ONE 12, e0183210 (2017). moisture transportation, and precipitation actions and feedback across multiple spatial- 9. X. Feng et al., Nat. Clim. Chang. 6, 1019 (2016). generation. The lack of observational evi- temporal scales that jointly control stream- 10. A. A. Coble et al., Sci. Total Environ. 730, 138926 (2020). dence highlights the need for research on flow responses, should all be assessed. To 11. G. Zhou et al., Nat. Commun. 6, 5918 (2015). 12. J. McDonnell et al., Nat. Sustain. 1, 378 (2018). the feedback between climate and forest better clarify the response, a more complete 13. G. Grant, W. Dietrich, Water Resour. Res. 53, 2605 (2017). change at regional or continental scales. spectrum of hydrological variables, including 14. D. Lawrence, K. Vandecar, Nat. Clim. Chang. 5, 27 (2015). Time scale is important for understanding the magnitude, duration, timing, frequency, 15. N. L. Poff, J. K. H. Zimmerman, Freshw. Biol. 55, 194 (2010). SCIENCE these variations. Hydrological effects of forest and variability of flows, which collectively change can vary with time as forests regrow. determine river flow conditions, aquatic ACKNOWLEDGMENTS Coble et al. reviewed long-term responses of functions, and thus ecosystem services such This paper was supported by China National Science low flows to logging in 25 small catchments in as water supply, should be included in an Foundation (no. 31770759).

GRAPHIC: N. DESAI/ N. GRAPHIC: North America (10). They identified dynamic assessment (15). Nonetheless, water-supply 10.1126/science.abe7821

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