Climate change is expected to alter patterns of species co-occurrence, in both space and time. Species-specific shifts in reproductive phenology may produce changes in the assemblages of plant species in flower at any given time during the growing season. Temporal overlap in the flowering periods (co-flowering) of animal-pollinated species may be an important determinant of reproductive success, if competitive or facilitative interactions between plant species affect pollinator services. We used a unique 33-year dataset on flowering phenology in subalpine meadow communities to determine whether interannual variation in snowmelt date caused predictable changes in co-flowering patterns. We focussed on the flowering period of Lathyrus leucanthus (Fabaceae), a bee-pollinated species that flowers in early summer and that is well represented in the dataset.

Results/Conclusions

We found that the composition of the assemblage of plants with flowering periods overlapping that of L. leucanthus was significantly affected by variation in snowmelt date across years: L. leucanthus tended to flower alongside earlier-flowering species in years of early snowmelt, and with later-flowering species in years of late snowmelt. In particular, overlap with the flowering period of Lupinus argenteus (Fabaceae), with which L. leucanthus shares pollinators, was significantly reduced in years of early snowmelt. This association between timing of snowmelt and patterns of flowering overlap could not have been predicted simply by examining temporal trends in the dates of peak flowering of the dominant species in the community, as peak flowering dates have largely shifted in parallel with respect to snowmelt date. Interannual changes in the intensity and duration of flowering have likely contributed to the observed relationship. Although much of the year-to-year variation in flowering overlap remains unexplained, our finding of a measurable signal of climate variation suggests that future climate change may lead to altered competitive environments for these wildflower species.