As temperature is a common regulator of temperate plant phenology, future increases in global temperatures are likely to cause shifts in the timing of plant phenophases such as flowering and senescence, with potential feedbacks on species interactions and carbon cycling. We used a 3‐year field warming study in a temperate grassland to investigate the effects of two levels of warming (+ ~1.5°C and + ~3°C) on the phenology of budding, flowering onset, and peak flowering of ten perennial plant species at both individual and population scales. We also examined the effect of warming on green‐up and senescence by measuring normalized difference vegetation index (NDVI) for one year. Populations and individuals of Amorpha canescens, Dalea purpurea (Petalostemum purpureum), and Andropogon gerardii flowered five to eight days earlier under high warming. For seven species, interannual variability in flowering time equaled or exceeded experimental warming treatment effects in any given year. Responses to warming were not consistent among years for several species, especially Liatris aspera. Warming led to higher NDVI values in the spring, indicating that warming accelerated spring biomass growth but did not significantly affect senescence. These results suggest that the community flowering profile may be altered under warming, potentially affecting pollinator, trophic and competitive interactions, and indicate that the timing to peak biomass may be accelerated, possibly affecting ecosystem carbon cycling.

Abstract. Increases in global temperature are likely to have effects on the nitrogen cycle, including those mediated through effects on legumes, which have a role in the N cycle by fixing N 2 . These effects may alter plant functioning and community structure, especially in N-limited ecosystems. We manipulated temperature and plant diversity in the field to investigate the effects of elevated temperature on aboveground biomass, shoot N concentration ([N]), and reliance on N 2 fixation of four prairie legumes (Amorpha canescens Pursh., Dalea purpurea Vent., Lespedeza capitata Michx. and Lupinus perennis L.) planted in plots of varying species numbers. We monitored the effect of warming on soil microclimate and net N mineralisation rates, as these variables may mediate the effect of warming on legumes. Warming decreased soil moisture and increased soil temperature, but had no effect on net N mineralisation. Warming increased the aboveground biomass of D. purpurea and L. perennis, but decreased shoot [N] for all species in one year. Though the data were not optimal for quantifying N 2 fixation using stable isotopes, they suggest that warming did not affect the reliance on N 2 fixation. Species diversity did not have strong effects on the response to warming. These results suggest that legume-mediated effects of temperature on N cycling will arise from changes in biomass and tissue chemistry, not N 2 fixation. We observed strong interannual variation between a wet and dry year for N mineralisation, shoot [N] and reliance on N 2 fixation, suggesting that these may be more responsive to precipitation changes than elevated temperature.

Elevated temperature enhanced seedling growth of some species, while inhibiting nodulation in another. Temperature-induced shifts in legume composition or nitrogen dynamics may be another potential mechanism through which climate change affects unmanaged ecosystems.