Drainage and nitrogen enrichment facilitate the encroachment of woody plants at various developmental stages in freshwater marshes

Woody plant encroachment into graminoid‐dominated freshwater marshes is a widespread phenomenon globally, yet the mechanisms driving it are not well understood. It is often presumed that the establishment of woody plants—encompassing seed germination, seedling survival, and seedling growth—is influenced by environmental changes such as alterations in water regimes and nutrient availability. However, this hypothesis has not been rigorously tested through empirical studies. In this study, we conducted a 3‐year common garden experiment in a freshwater marsh in northeastern China to investigate the separate and combined effects of water and nitrogen levels on woody plant establishment in a graminoid‐dominated vegetation. The experiment involved manipulating water levels (flooded vs. drained) and nitrogen concentrations (ambient vs. added) in a fully crossed factorial design to examine their individual and combined effects. We sowed seeds of seven woody species under each treatment combination and tracked their germination, seedling survival, growth and potential competitive interactions with the resident vegetation. This approach allowed us to assess the direct effects of water and nitrogen on woody plant establishment, as well as any indirect effects mediated by changes in the resident plant community. The results show that drainage in the first year significantly improved both seed germination and seedling survival of woody plants, and these survival benefits persisted over the following years. The structural equation model revealed that drainage indirectly facilitated seedling survival by reducing light asymmetry. In contrast, continuous flooding over 3 years resulted in complete seedling mortality. However, after 3 years of drainage, the addition of nitrogen significantly increased the leaf count, maximum height, and stem diameter of the surviving woody plants. Synthesis. This multispecies introduction experiment has demonstrated that water regime is the primary factor driving the establishment of woody plant seedlings in freshwater marshes. As seedlings mature, nutrient supplementation can accelerate their development, potentially facilitating their encroachment into freshwater marshes. By demonstrating the distinct and varying impacts of water and nitrogen treatments across different stages of woody plant establishment, our study offers valuable insights into the complex interplay of environmental factors shaping this phenomenon.