In montane meadows, the ground is only free from snow for a few months each year. Within this brief window, wildflower species must complete their entire annual cycles of growth and reproduction. There is evidence that long-term climate warming is causing phenological shifts in flowering—specifically, earlier snowmelt drives earlier flowering. Comparatively little is known about whether climate has additional effects, beyond triggering an earlier onset of reproduction, on subsequent phenological stages. For instance, is the timing of the transition between flowering and fruiting influenced by climate factors (e.g. snow melt, growing degree days, and soil moisture)? To what extent is there predictable interspecies variation in the sensitivity of the flower-fruit transition time to climate? We collected six years of data on 48 wildflower species growing in 70 plots along an elevational gradient in Mount Rainier National Park (1480m-1900m) and spanning three microtopography locations (ridge, slope and cove). Plots were visited weekly during each growing season and the proportional abundances of six phenophases (e.g. flowers, fruit ripe seeds) were recorded. We used a hierarchical Bayesian framework to simultaneously extract parameters of all phenological curves in the same model, and then analyzed the relationships between climate variables and flower-fruit transition time.
Our findings confirm that earlier snowmelt dates are tightly correlated with earlier flowering across species. However, the time it takes for plants to transition between flowering and fruiting varies extensively between species and years, and cannot be predicted completely either by species identity or the timing of flowering in a given year. Instead, we find support for the hypothesis that climate has additional influences on flower-fruit transition time. The particular climate drivers involved and the direction of these impacts are species-specific. For example, Lupinus arcticus tends to transition measurably more quickly from the flowering phase to the fruiting phase when snow disappears earlier in the year—a surprising result that may lead to an increased possibility of phenological mismatches between plants and frugivores. This research suggests that climate warming will affect the phenology of fruiting as well as flowering, and thus impact: i) the insects, birds, and mammals that feed on fruits and seeds; and ii) the timing and success of seed dispersal for individual plants. We therefore contribute to the growing evidence that climate-driven phenological shifts may have both fitness effects on individual species and cascading effects on interacting species at multiple trophic levels.