Heterogeneous terrain provides microclimates (small-scale variations in the conditions experienced by plants), which have important impacts on plant flowering phenology. These microclimates are not only produced by topography, but also structure-forming species. Shrubs, for example, can induce buffered thermal conditions and trap snow, which are two important axes of variation for plant flowering phenology. Importantly, shrub microclimates may be modified by topography and thus, as shrubs continue to expand into the alpine, the interaction between these microclimatic drivers and their effects on phenology will be critical to understand. In our study, we asked how shrubs interact with aspect to drive flowering phenology and floral resources (number of flowers) across alpine plant communities. To answer these questions, we established 108 plots in the alpine tundra of Niwot Ridge on the Front Range of Colorado. Our plots were split evenly between north- and south-facing aspects, with half placed on the westward (leeward) side of a shrub, and the other half paired outside of the influence of any shrubs, about one meter away. We monitored the number of open flowers present for 21 common alpine species weekly throughout the duration of the 2019 flowering season.
Results/Conclusions
Shrubs delayed community-level peak flowering by about one week and significantly decreased the average community floral resources. Aspect had no effect on the timing of peak flowering, though the south-facing aspect significantly decreased floral resources. Previous work in these plots demonstrated that shrubs alter soil temperatures only on the south-facing aspect. Hence, the lack of a main effect of aspect and an interaction between shrub presence and aspect on community flowering phenology suggest that temperature was not a driving force behind the shrub-induced phenological changes. Instead, it may be a delay in the start of the growing season resulting from later snowmelt out on the leeward side of shrubs. Our results suggest that shrubs could negatively impact plant population dynamics through phenological mismatch between plant species and their pollinators as well as diminished floral resources.