Abstract: Climate change is significantly altering plant reproductive phenology, including in flowering and fruiting times, which affect reproductive success and community structure. However, the mechanisms through which phenological changes influence reproductive outcomes are not yet fully understood. Specifically, it remains unclear whether such responses are symmetric under both warming and cooling phases that are commonly embedded within broader climate change scenarios, and whether species' temporal niches mediate their responses to temperature changes. Fifteen years after initiating a reciprocal transplant experiment in an alpine grassland (since 2007), we quantified the reproductive traits of six dominant species representing two flowering functional groups. This allowed us to examine how these species respond to long-term warming and cooling across different temporal niches. We found that cooling increased seed mass and seed number in early-spring flowering (ESF) species but reduced seed number in mid-summer flowering (MSF) species. In contrast, reproductive success showed no significant response to warming, indicating an asymmetric response to temperature change. Differences in temporal niches between flowering functional groups led to contrasting relationships between flowering time and reproductive success. Specifically, in ESF species, flowering time was positively associated with seed mass, whereas in MSF species, it was negatively associated with seed number. Our findings demonstrate that warming and cooling exert asymmetric effects on plant reproductive success, and that temporal niche differentiation plays a crucial role in determining plant reproductive response to temperature changes. Ignoring the effects of cooling may lead to incomplete understanding and underestimation of ecological risks associated with future climate change.

Key words: Reciprocal transplant experiment, flowering functional groups, reproductive phenology, reproductive effort, reproductive success