Interactive canopy nitrogen and water additions delay phenology in a warm-temperate forest

Plant phenology is a critical link between environmental change and ecosystem processes, representing a key indicator for evaluating plant growth and carbon sink dynamics under global change. However, the interactive effects of canopy-level nitrogen and water additions on temperate forest phenology remain poorly characterized.

This study aimed to elucidate the interactive effects of N and water addition on the phenology of dominant temperate forest species.

We investigated the effects of canopy N addition (CN), water addition (CW), and their combination (CNW) on the spring and autumn phenology of three functional groups (large trees, small trees, and shrubs) in a warm-temperate forest.

The plant responses were highly dependent on both functional group and species. CN significantly delayed bud swelling stage in large trees (0.83 ± 1.04 days) and budburst stage in shrubs (1.50 ± 0.38 days). In small trees, CW alone advanced bud swelling (12.50 ± 1.90 days), whereas CNW produced a more complex, antagonistic effect (N-water interaction p < 0.05). For large trees, N-water interaction synergistically delayed autumn coloration and leaf fall (p < 0.05). In contrast, CNW alone delayed most autumn phenophases in small trees, while advancing the peak and end of leaf coloration in shrubs (9.38 ± 3.48 and 10.88 ± 2.99 days), without a significant N-water interaction.

CN and CW did not directly regulate plant phenology in a simple manner. Instead, they altered habitat conditions and plant physiological status, leading to heterogeneous phenological shifts among functional groups, including inhibitory effects as well as phenological delays and advancements. These findings offer critical insights for predicting forest phenology and carbon cycling dynamics under global change.