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

It is often presumed that the establishment of woody plants into grass-dominated freshwater marshes is influenced by environmental changes such as alterations in water regimes and nutrient availability. However, it has not been rigorously tested through empirical studies. We conducted a three-year common garden experiment in a freshwater marsh to investigate the separate and combined effects of water (flooded vs. drained) and nitrogen levels (ambient vs. added) on germination, seedling survival, growth of seven woody plant species, and their potential competitive interactions with the resident vegetation. Drainage in the first year significantly improved both seed germination and seedling survival of woody plants, and these survival benefits persisted over the following two years. The structural equation model revealed that drainage indirectly facilitated seedling survival by reducing light asymmetry. In contrast, continuous flooding over three years resulted in complete seedling mortality. However, after three years of drainage, the addition of nitrogen significantly increased the leaf count, maximum height, and stem diameter of the surviving woody plants. Our study demonstrates 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 accelerating their encroachment into freshwater marshes.