Abstract:

Objective: Taking the seedlings of 10 main tree species in the Northeast Forest as the research object, this study explores the response differences of leaf functional traits to nitrogen addition. The aim is to clarify the adaptation strategies of different tree species to nitrogen deposition and provide a scientific basis for nitrogen deposition management in the forest ecosystem of this region. Method: In the biodiversity control experimental plot in Shulan, Jilin, a study was conducted to investigate the effects of nitrogen addition on leaf functional traits of 2-to 3-year-old saplings from 10 major tree species commonly found in northeastern forests, namely Betula platyphylla, Pinus koraiensis, Juglans mandshurica, Maackia amurensis, Phellodendron amurense, Quercus mongolica, Acer pictum subsp. mono, Fraxinus mandshurica , Tilia amurensis, and Picea asperata. The experiment was carried out from summer 2021 to 2022, where urea (CH₄N₂O) was applied to simulate nitrogen deposition. Three nitrogen deposition gradients were established with concentrations of 0 (CK), 50 (low nitrogen), and 100 kg·hm^?2a^?1 (high nitrogen). Leaf samples were collected after the last nitrogen application each year, and eight leaf functional traits (leaf area, specific leaf area, leaf dry matter content, leaf carbon content, leaf nitrogen content, leaf phosphorus content, leaf nitrogen-to-phosphorus ratio, and leaf carbon-to-nitrogen ratio) were measured and compared. Result: 1）The leaf functional traits of plants exhibited significant responses to different nitrogen addition treatments, with leaf nitrogen content, specific leaf area, and leaf phosphorus content significantly increasing under nitrogen addition, while leaf dry matter content, leaf nitrogen-to-phosphorus ratio, and leaf carbon-to-nitrogen ratio significantly decreased. There were also correlations among leaf functional traits; specific leaf area was significantly positively correlated with leaf area and leaf nitrogen content, but negatively correlated with leaf dry matter content. 2）The response of leaf functional traits to nitrogen addition varied among different tree species. For example, only the leaf area of B. platyphylla, P. koraiensis, and P. amurense showed significant responses to nitrogen addition, while other species showed no significant changes; however, all species exhibited significant responses in leaf phosphorus content to nitrogen addition.3）Different types of tree species adopted distinct survival strategies. Broadleaf species represented by B. platyphylla and J. mandshurica increased leaf nitrogen content and specific leaf area to enhance photosynthetic efficiency for rapid growth, whereas the coniferous species P. koraiensis , and P. asperata exhibited relatively slow growth while maintaining higher leaf dry matter content. Conclusion: This study demonstrates that nitrogen addition exerts differential effects on leaf functional traits across tree species. Under nitrogen enrichment, resource-acquisition-related functional traits, such as specific leaf area , were enhanced in seedlings of the region. Broadleaf species exhibited increased leaf nitrogen content and specific leaf area to improve resource acquisition, whereas conifer species adjusted leaf dry matter content to strengthen resource conservation strategies, thereby adapting to nitrogen-rich environments. These findings provide critical insights into the impacts of nitrogen deposition on forest ecosystems and offer practical guidance for forest management, such as selecting tree species with higher nitrogen-use efficiency. The results further contribute to theoretical understanding of nitrogen deposition effects on forest ecosystems and provide a scientific basis for ecological management practices addressing nitrogen deposition.

Key words: nitrogen deposition, functional traits, leaf economics spectrum, variation