Phenology, the timing of biological events, is driven by species-specific responses to environmental variation, both seasonal and atypical (e.g., due to climate change), and affects species’ fitness directly as well as indirectly via biotic interactions with competitors, antagonists, and mutualists. A fundamental challenge in ecology is to disentangle how species’ phenological responses to environmental variation affect biotic interactions and ultimately species’ fitness. Here, we experimentally manipulated the timing of snowmelt to investigate its effects on plant phenology and reproduction.
Specifically, 1m2 meadow plots at two elevations (1400m and 2200m) in the Swiss Alps were subject to one of three snow treatments: snow removal, snow addition, and control (no snow manipulation). We monitored budding, flowering, and fruiting phenology of marked plants in each plot for 12 plant species across both elevations. To assess how flowering phenology affects plant reproduction via interactions with pollinators, we quantified seed set of marked plants in each snowmelt treatment relative to that of caged (self-pollinated) plants. We hypothesize that earlier-flowering and higher-elevation species are more likely to exhibit phenological shifts and experience differential reproductive success under variable conditions (in snow removal and addition treatments) than are later-flowering or lower-elevation species.
Results/Conclusions
We report two key results. First, earlier-flowering plants did not significantly advance their phenologies, but did delay their phenologies in snow addition treatments at both elevations. Later-flowering plants did not exhibit any significant phenological shifts. Thus, in years with later snowmelt, delayed phenology of earlier-flowering species may force their phenologies to overlap more with later-flowering plants, perhaps leading to stronger plant-plant competition. Second, variation in the timing of snowmelt affects plant reproduction regardless of phenology or elevation. At low elevation, one early-flowering species had greater seed set in addition treatments and the other in removal treatments. Two later-flowering species were affected by snow removal, causing one species to have greater, and the other lower, seed set. At high elevation, both early-flowering species were affected by snow addition, causing one species to have greater, and the other lower, seed set. One later-flowering species had lower seed set in addition treatments and another in removal treatments. In general, early (late) snowmelt has stronger effects on reproduction in low (high) elevation communities. Taken together, these results demonstrate that variation in the timing of snowmelt has important consequences for plant phenology and reproduction, but that responses are species-specific and difficult to predict a priori.