Call for caution regarding the efficacy of large-scale afforestation and its hydrological effects

• Published in: The Science of the total environment Vol. 950; p. 175299
• Main Authors: Douville, Hervé, Allan, Richard P., Arias, Paola A., Fisher, Rosie A.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 10-11-2024
• Subjects: Afforestation; Earth System Models; Evaluation; Hydrological impacts; Mitigation; Water limitations; Mitigation; Water limitations; Evaluation; Hydrological impacts; Afforestation; Earth System Models
• ISSN: 0048-9697; 1879-1026; 1879-1026
• DOI: 10.1016/j.scitotenv.2024.175299

Large-scale afforestation programmes are generally presented as effective ways of increasing the terrestrial carbon sink while preserving water availability and biodiversity. Yet, a meta-analysis of both numerical and observational studies suggests that further research is needed to support this view. The use of inappropriate concepts (e.g., the biotic pump theory), the poor simulation of key processes (e.g., tree mortality, water use efficiency), and the limited model ability to capture recent observed trends (e.g., increasing water vapour deficit, terrestrial carbon uptake) should all draw our attention to the limitations of available theories and Earth System Models. Observations, either based on remote sensing or on early afforestation initiatives, also suggest potential trade-offs between terrestrial carbon uptake and water availability. There is thus a need to better monitor and physically understand the observed fluctuations of the terrestrial water and carbon cycles to promote suitable nature-based mitigation pathways depending on pre-existing vegetation, scale, as well as baseline and future climates.
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•Indiscriminate tree planting may be both inefficient and harmful to water resources.•Inappropriate concepts or insufficiently evaluated Earth System Models may provide misleading policy recommendations.•Observations suggest potential trade-offs between terrestrial carbon uptake and water availability.•Further research efforts are needed to support more effective and sustainable tree restoration policies.