Enhanced productivity and evapotranspiration dominated by woody plant encroachment-induced vegetation greening in boreal wetland ecosystems

Woody plant encroachment (WPE), a global phenomenon documented across various biomes and continents, has the potential to significantly impact ecosystem carbon and water cycling. However, the impacts of WPE on carbon and water cycling in wetland ecosystems of middle and high latitudes are still lacking. In this study, the interannual and seasonal impacts of WPE on gross primary production (GPP) and evapotranspiration (ET), as well as their underlying mechanisms, within boreal wetland ecosystems located in middle-high latitude regions, were examined using remote sensing datasets spanning the period 2001-2016. Our results demonstrated that WPE enhanced annual GPP, ET, Normalized Difference Vegetation Index (NDVI), and solar-induced chlorophyll fluorescence (SIF) in boreal wetlands with impacts increasing over time. The multi-year average GPP and ET in fully encroached wetland ecosystems were approximately 31% and 3% higher, respectively, compared to pure wetland ecosystems. Prominent increases in wetland GPP occurred during the early growing season, while an exacerbation of ET was observed during the peak growing season. The impacts of WPE on wetland GPP and ET were predominantly attributed to increased vegetation greenness followed by secondary contributions from climate change. Climate change not only directly influenced the responses of GPP and ET to WPE but also exerted indirect effects by regulating vegetation greenness and the degree of encroachment. Our findings offer valuable insights into the understanding of the interaction between WPE and climate change, highlighting the importance of considering WPE effects and their drivers for accurate predictions of carbon and water cycles, as well as atmosphere-biosphere feedbacks in boreal wetlands.