Artificial light at night advances reproductive phenology and reduces reproductive capacity of a wild plant

With the rapid increase in artificial light at night (ALAN) during the last century, a growing proportion of the world’s ecosystems have been exposed to ALAN. While the effects of low-intensity ALAN on morphology and phenology are well known in laboratory and greenhouse studies, little is known about these effects of ALAN on wild plants. Moreover, how these morphological and phenological responses affect the fitness of wild plants is unclear. Using the annual herbaceous plant, Elsholtzia densa, as a model species, we performed a field manipulation experiment and tested the responses of plant morphology, reproductive phenology, and reproductive capacity to ALAN. We found that ALAN significantly increased the specific leaf area but decreased the number of secondary branches. It also advanced the onset of budding, blooming, fruiting, and seed maturity from 3.4 to 6.2 d. In addition, ALAN significantly increased the length of the top inflorescences of the species but decreased the percentage of the individuals with undeveloped top inflorescences. Consequently, ALAN reduced the number of fruity inflorescences. Our results demonstrated that low-intensity ALAN, even with a relatively short period of exposure (i.e., one growing season), could cause passive responses in morphological traits of the wild plant, resulting in a decline in its reproductive capacity. In addition, the advanced reproductive phenology (e.g., flowering and blooming) induced by ALAN may cause plant-pollinator mismatches and threaten species coexistence at the community level.