Changes in the timing of snow cover have the potential to reshape key components of polar, alpine and subalpine ecosystems. We examined how year-to-year variation in the timing of snow cover affects plant-insect interactions using both an 8-year observational study and an experimental manipulation. We documented these patterns in a subalpine system consisting of a host plant (Ligusticum porteri), ab aphid herbivore (Aphis asclepiadis) and the arthropods with which these two species interact. We censused host plants (N=100) in ten replicate population near Gothic, CO each week from June through August. We used a model-selection approach to determine how environmental variables (snow melt timing and temperature) predicted the abundance and phenology of aphids, mutualist ants and host plants. We experimentally manipulated snow cover and soil moisture on plots of host plants to parse how snow cover timing effects arthropod abundance through plant phenology versus soil moisture. In May of 2018, we doubled snow cover of plots (n=3), and in June through July, we added water to additional plot (n=3) and monitored ambient plots (n=3). We compared soil moisture, host plant phenology and arthropod abundance among these treatments using linear mixed effects models.
Results/Conclusions
Based on eight years of monitoring, we found that snow melt timing significantly influenced the abundance and phenology of aphids, but temperature better predicted these responses in their mutualist ants (Temperature X Snow melt date X Species: z=-3.27, P<0.01). Years of later snow melt date increased colonization of host plants by aphids while also delaying their phenology. While temperature was a poor predictor of aphid colonization, warmer temperatures consistently reduced ant abundance on host plants. These results suggest the potential for a phenological mismatch between aphids and their mutualist ants driven by differential responses to two key climate cues, snow melt timing and temperature. In the experimental study, we found both snow and water addition to increase soil moisture relative to ambient conditions (Treatment X Date: LR χ2=36.52, P < 0.01). However, water addition delayed host plant senescence to a greater degree than did snow addition (Treatment: LR χ2=15.10, P < 0.01). Overall rates of aphid colonization across host plants were low (19 out of 92 host plants), but these colonies tended to occur on host plants in water addition plots. We conclude that changes in the timing of snow cover affect aphid herbivore abundance by altering host plant phenology.