Earlier snowmelt and reduced summer precipitation alter floral traits important to pollination

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John M. Powers, Heather M. Briggs, Xinyu Li and Diane R. Campbell, Ecology and Evolutionary Biology, University of California, Irvine, Irvine, CA, John M. Powers, Heather M. Briggs, Rachel G. Dickson, Xinyu Li and Diane R. Campbell, Rocky Mountain Biological Laboratory, Crested Butte, CO

Background/Question/Methods
Climate change can cause changes in expression of organismal traits that influence fitness. In flowering plants, floral traits can respond to drought, and that phenotypic plasticity has the potential to affect pollination and plant reproductive success. Global climate change is leading to earlier snowmelt in snow-dominated ecosystems as well as affecting precipitation during the growing season, but the effects of snowmelt timing on floral morphology and rewards remain unknown. We conducted crossed manipulations of spring snowmelt timing (early vs. control) and summer monsoon precipitation (addition, control, and reduction) that mimicked recent natural variation, and examined plastic responses in floral traits of the subalpine herb Ipomopsis aggregata over three years in the Rocky Mountains. We tested whether increased summer precipitation compensated for earlier snowmelt, and if plasticity was driven by changes in soil moisture and/or leaf gas exchange. We also predicted the effects of plastic trait changes on pollinator visitation rates, pollination success, and seed production using prior studies on I. aggregata.
Results/Conclusions
Lower summer precipitation decreased corolla length, style length, corolla width, sepal width, and nectar production, and increased nectar concentration. Earlier snowmelt (taking into account natural and experimental variation) decreased these traits and inflorescence height. The effect of reduced summer precipitation was stronger in earlier snowmelt years for corolla length and sepal width. Trait reductions were explained by drier soil during the flowering period, but this effect was only partially explained by how drier soils affected plant water stress, as measured by leaf gas exchange. The largest predicted effect of drier soil on relative fitness components via plasticity was a decrease in male fitness caused by reduced pollinator rewards (nectar production). Early snowmelt and reduced precipitation are strong drivers of phenotypic plasticity, and both should be considered when predicting effects of climate change on plant traits in snow-dominated ecosystems.