Understanding the relationship between temperature and genome size (GS) is crucial for predicting plant performance under global warming. Smaller-GS species are hypothesized to be favored in warmer climates, while larger-GS species may thrive in cooler environments.

To test this hypothesis, we analyzed data from temperate grasslands spanning four decades (1982–2021), examining species composition, phenology, and productivity in response to rising temperatures.

With rising temperatures, larger-GS species exhibited earlier phenological events but slower growth rates, leading to a decline in biomass production. By contrast, smaller-GS species, though initiating growth later, demonstrated accelerated growth rates, resulting in increased biomass. This dynamic has driven a shift in community structure, with smaller-GS species becoming increasingly dominant. Phylogenetically informed models revealed a negative interaction between GS and temperature on species' biomass, underscoring the constraint of larger genomes under warming conditions. This is reflected in a community-wide reduction in GS over time, despite relatively stable overall productivity.

Our research highlights the pivotal role of GS in mediating plant community responses to climate warming, offering insight into the distinct growth strategies employed by species with different GS. Our findings underscore the importance of considering GS in predicting plant community dynamics in response to ongoing global climate change.