Experimental advancement of snowmelt influences flowering phenology and pollinator visitation in an alpine ecosystem

Abstract

Abstract Climate change is altering interactions among species, including plants and pollinators. In alpine ecosystems, where snowmelt timing is a key driver of phenology, earlier snowmelt may generate shifts in plant and pollinator phenology that vary across the landscape, potentially disrupting interactions. Here we ask how experimentally induced changes in snowmelt timing in a topographically heterogeneous alpine-subalpine landscape impact flowering, insect pollinator visitation, and the pathways connecting key predictors of plant-pollinator interaction. Snowmelt was advanced via the application of black sand on top of snow in manipulated plots, which were paired with control plots. For each forb species, we documented flowering onset and counted flowers throughout the season. We also performed pollinator observations to measure visitation rates. We found that plants flowered earlier in advanced snowmelt plots, with the largest advances in later-flowering species, but flowering duration and visitation rate did not differ between advanced snowmelt and control plots. Using piecewise structural equation models, we assessed the interactive effects of topography on snowmelt timing, flowering phenology, floral abundance, and pollinator visitation. We found that all of these factors interacted to predict visitation rate in control plots. However, in plots with experimentally advanced snowmelt, none of these predictors explained a significant amount of the variation in visitation rate, indicating that different predictors are needed to understand the processes that shape pollinator visitation to flowers under novel climate conditions. Our findings demonstrate that climate change-induced early snowmelt may fundamentally change the relationships between abiotic and biotic drivers of plant-pollinator interactions in alpine environments.