Responses of plant phenology to nitrogen addition: a meta-analysis

Phenology is one of the most sensitive processes of plant in response to global change. Anthropogenic activities have considerably increased nitrogen (N) deposition, which significant affects plant phenology. Although numerous individual studies have been conducted, it remains controversial how N addition affects phenological stages, and a comprehensive understanding of how plant phenology responds to external N inputs remains elusive. To reconcile the differences, we conducted a meta-analysis of 117 species to examine the responses of plant phenology to N addition in terrestrial ecosystems, and assessed variations in their responses in relation to ecosystem types, functional groups, and environmental conditions. Our results showed that plant phenology changed significantly after N addition, and phenology time delayed and phenology duration shortened significantly across all biomes except fruiting duration, but varied with biome types. The phenology change in cropland was more dramatical than in grassland after N addition, even in opposite directions. The response of phenological stages to N addition was consistent in two pollination types except the flowering time, the flowering time had no change in anemophilous but significantly delayed in entomophilous. In addition, the response of phenology to N addition was discrepancy among functional groups, the phenology time advanced and duration shortened in sedge, while phenology time delayed and duration shortened in other groups, and the phenology change in legume was larger than grass and forbs. We also found that environmental factors had little effects on the response of plant phenology to N addition, but significant correlation was found between the response ratios of different phenological stages. Our study suggested that phenology was sensitive to N deposition at many phenological stages, and changes in phenology may be smaller with community biodiversity increasing at ecosystem level.