Afforestation and reforestation are promoted as natural climate solutions to mitigate climate change. China, since the 1990s, has implemented large-scale afforestation programs, leading to a significant increase in forest area. While these programs have yielded benefits such as reduced soil erosion and improved flood mitigation, concerns have arisen regarding their impact on water resources and wetlands. The reduction in runoff and soil moisture caused by increased evapotranspiration from extensive tree planting poses a threat to wetlands and their ecosystem services. China's ambitious plan to further expand forest coverage by 2035 necessitates a comprehensive understanding of the hydrological and ecological consequences of these programs, particularly regarding the trade-off between tree planting and wetland conservation. This study aims to quantify the impacts of past and future tree planting programs on wetland areas in China, resolving the uncertainty surrounding this trade-off.

The literature review highlights the effectiveness of afforestation and reforestation in carbon sequestration and climate change mitigation. It mentions China's large-scale afforestation programs, such as the Three-North Shelterbelt Program, the Natural Forest Conservation Program, and the Grain for Green Program. Previous studies have shown that these programs have reduced soil erosion, dust storms, and desertification, and improved flood mitigation. However, other studies have indicated that large-scale afforestation can negatively impact hydrological processes, leading to reduced runoff, soil moisture, and potentially wetland loss. The review underscores the need for a better understanding of the trade-off between tree planting and wetland conservation, particularly given China's ambitious near-term tree planting plans.

To assess the impact of tree planting on wetlands, the researchers combine satellite-based inundation data (GIEMS-2) and a process-based land surface model (ORCHIDEE-Hillslope). GIEMS-2 provides data on surface water extent, including wetlands, while ORCHIDEE-Hillslope simulates hydrological processes, partitioning water into evapotranspiration and runoff. Historical simulations (2000-2016) were conducted using ORCHIDEE-Hillslope forced by climate data and land-cover maps derived from China's National Forest Inventory. The contribution of forest change to wetland change is isolated through factorial simulations (with and without forest change). For future projections (2017-2035), simulations employed land-cover maps based on China's near-term tree planting plan. A conceptual water-balance model (Budyko framework) is also used to estimate the impact of afforestation on water delivery to wetlands. The model estimates subgrid wetland fraction using TOPMODEL, calibrated with satellite-based wetland products (RFW and GIEMS-2). The study also considers different climate zones based on the ratio of potential evapotranspiration to precipitation (PET/P) to assess the sensitivity of wetlands to afforestation across various climatic conditions. The impact of tree planting on protected wetland basins is analyzed using the List of Protected Wetlands in China and the HydroBASINS database. Finally, near-future simulations incorporate climate projections from ISIMIP3b under different shared socioeconomic pathways (SSPs) to assess the combined effects of climate change and tree planting on wetland areas.

The study's key findings include: 1. Historical tree planting (2000-2016) in China resulted in a net wetland loss of 1300-1500 km² (0.3-0.4% of total wetland area), with the dry northern and western regions experiencing more significant losses. 2. The analysis using the Budyko framework shows that afforestation leads to a decrease in runoff, with a stronger impact in mesic and dry regions. 3. ORCHIDEE-Hillslope simulations confirm the decrease in runoff and soil moisture due to afforestation, particularly in drier regions. 4. The sensitivity analysis reveals that wetland loss per unit area of afforestation is higher in dry compared to wet climate zones. 5. Protected wetlands in China, mostly located in dry northern China, experienced a higher rate of wetland loss due to afforestation than unprotected areas. 6. Near-term tree planting (2017-2035), following the national 15-year ecological plan, is projected to cause an additional loss of approximately 1300 km² of wetlands, concentrated in dry regions. 7. Scenario analysis shows that planting trees in drier areas leads to disproportionately larger wetland loss than planting in wetter regions. 8. Even considering near-future climate change projections from ISIMIP3b, the negative impact of tree planting on wetlands remains significant, highlighting that the wetland loss due to tree planting cannot be easily offset by potential increases in precipitation.

The findings highlight the trade-off between the benefits of large-scale afforestation for carbon sequestration and the risks to wetland ecosystems. The study demonstrates that the impact of tree planting on wetlands varies significantly across climate zones, with dry regions being particularly vulnerable. The disproportionate impact on protected wetlands underscores the need for careful planning and spatial optimization of afforestation activities. The results challenge the assumption that increased forest cover automatically leads to positive environmental outcomes and emphasize the importance of considering the potential negative consequences on other valuable ecosystems. The study's findings are relevant to other arid and semi-arid regions globally facing similar challenges in balancing afforestation goals with wetland conservation.

This study reveals a significant trade-off between afforestation and wetland conservation in China. Historical and projected afforestation leads to substantial wetland loss, particularly in dry regions where protected areas are concentrated. Spatial optimization of tree planting is crucial to mitigate these negative impacts. Future research should focus on refining land-atmosphere feedback mechanisms and incorporating species-specific and management-related factors into modeling efforts. Further research should also explore the ecological trade-offs between wetlands and forests, and address the feasibility of integrating spatial optimization into policy implementation.

The study acknowledges limitations such as the lack of considering land-atmosphere feedback from afforestation and uncertainties in coupled climate model simulations. The model's representation of vegetation types and forest management practices might also influence the results. The study doesn't explicitly address all ecological trade-offs beyond hydrological impacts, which would require further investigations.