Abstract:

Objective: This study targets eight dominant understory species in a subtropical moso bamboo forest, and investigates their leaf functional traits in response to simulated nitrogen deposition, aiming to elucidate the ecological adaptation strategies of understory plants to nitrogen deposition, and provide a scientific basis for nitrogen deposition management in regional moso bamboo forest ecosystems. Method: Two simulated nitrogen deposition modes was set up, one above the canopy and the other below the canopy with organic (a urea-glycine mixture) and inorganic (ammonium nitrate). There were a total of six treatment groups: canopy inorganic nitrogen addition (CIN), canopy organic nitrogen addition (CON), canopy control (CCK), understory inorganic nitrogen addition (UIN), understory organic nitrogen addition (UON), and understory control (UCK). The nitrogen addition amount for each treatment group was uniformly at a rate of 50 kg·hm^?2a^?1, while control plots received an equivalent volume of water. After three years, eight leaf functional traits (specific leaf area, leaf dry matter content, leaf thickness, plant height, and contents of total nitrogen, phosphorus, potassium, and N∶P ratio) were determined for four shrub species (Rubus buergeri, Rubus corchorifolius, Ardisia japonica, Wisteria sinensis) and four herbaceous species (Chelonopsis chekiangensis, Lophatherum gracile, Parathelypteris glanduligera, Carex spp.). Result: 1) Except for leaf dry matter content, leaf functional traits did not significantly respond to the simulated nitrogen deposition (above/below canopy) or nitrogen form (organic/inorganic). However, there were significant interspecific differences, particularly between herbs and shrubs, and between Fabaceae and non-Fabaceae shrubs. 2) Simulated nitrogen deposition significantly increased leaf nitrogen content in both herbs and shrubs. Herbs exhibited increased leaf potassium content and nitrogen-to-phosphorus ratio, but decreased leaf dry matter content. Shrub height significantly increased. Fabaceae shrubs showed increased plant height but decreased specific leaf area, while non-Fabaceae shrubs displayed increased leaf nitrogen content and nitrogen-to-phosphorus ratio. 3) Leaf dry matter content was negatively correlated with nitrogen content across all treatments. Leaf nitrogen content was positively correlated with phosphorus content only under CCK, CIN, and UCK treatments. Nitrogen-to-phosphorus ratio was positively correlated with nitrogen content only in CCK, CON, and UIN treatments. Specific leaf area was negatively correlated with leaf thickness solely under CON treatment, and leaf thickness was positively correlated with plant height under CCK and CIN treatments. Conclusion: This study has revealed differences in adaptation strategies to nitrogen deposition between herbaceous plants and shrubs, and between Fabaceae and non-Fabaceae shrubs in a moso bamboo forest. In response to nitrogen deposition, herbaceous plants not only enhance their resource-acquisition strategy through increasing leaf nitrogen content but also attenuate their resource-conservation strategy by reducing leaf dry matter content. Non-Fabaceae shrubs only increase resource-acquisition functional traits such as leaf nitrogen content, without adjusting resource-conservation traits. In contrast, Fabaceae shrubs adapt to nitrogen-enriched environments by maintaining leaf nitrogen-phosphorus stoichiometric balance.

Key words: nitrogen deposition, subtropical forests, moso bamboo, leaf functional traits, herbaceous plants, shrub plants, Fabaceae