Variations in soil moisture availability affect nectar production of three subalpine plant species in the Family Ranunculaceae
In the southwestern United States, climate change may decrease water availability and increase surface and air temperatures. Climatic variations, including soil moisture availability, could have adverse effects on nectar production and pollinator behavior. Nectar rewards provide energy for pollinators and facilitate pollen transfer. Thus, survival and fitness of pollinator and plant communities can be a function of nectar production. Lower moisture availability may affect nectar production of subalpine plants, which could impact pollinators that rely on these species throughout the season (e.g., Bombus spp., hummingbirds, hawk moths). During the summer of 2013 at the Rocky Mountain Biological Laboratory, Gothic, Colorado, I investigated the effects of soil moisture availability on nectar production (i.e., nectar volume, percent sugar concentration) and floral display characteristics (i.e., nectar spur length) in three subalpine species – Delphinium nutallianum, Aquilegia coerulea, and Delphinium barbeyi. Additionally, I investigated how nectar volume varied across time and how such variations could be explained by patterns in precipitation and mean temperature. The research consisted of a pairing methodology, in which one plant of each pair was watered while the other plant served as a control. The study on D. nutallianum consisted of 45 pairs; A. coerulea, 15 pairs; D. barbeyi, 30 pairs.
For D. nutallianum, A. coerulea, and D. barbeyi, watering significantly increased mean nectar spur length and nectar volume, indicating that drier conditions could damage floral characteristics and limit nectar production. For D. nutallianum and A. coerulea, nectar volume was positively associated with spur length; for D. barbeyi, no strong correlations existed. Average sugar concentration was significantly lower for watered A. coerulea. A negative association existed between nectar volume and sugar concentration for A. coerulea, indicating that A. coerulea, with limited soil moisture availability, may produce less diluted nectar. In D. barbeyi, watering significantly increased mean sugar concentration and the increase in percent sugar concentration was positively associated with nectar volume. Some pollinators prefer species with high nectar volume (i.e., hummingbirds); other pollinators are influenced by sugar concentration (i.e., Bombus spp.). Subsequently, certain pollinators may or may not benefit from variations in soil moisture availability. Environmental factors – specifically precipitation and mean temperature – appear to influence nectar volume. After periods of naturally occurring precipitation events, mean nectar volume for control and water treatments increased. A decrease in mean temperature appears to increase mean nectar volume; whereas, an increase in mean temperature appears to decrease mean nectar volume.