Wildflower phenological escape differs by continent and spring temperature

Abstract

Temperate understory plant species are at risk from climate change and anthropogenic threats that include increased deer herbivory, habitat loss, pollinator declines and mismatch, and nutrient pollution. Recent work suggests that spring ephemeral wildflowers may be at additional risk due to phenological mismatch with deciduous canopy trees. The study of this dynamic, commonly referred to as “phenological escape”, and its sensitivity to spring temperature is limited to eastern North America. Here, we use herbarium specimens to show that phenological sensitivity to spring temperature is remarkably conserved for understory wildflowers across North America, Europe, and Asia, but that canopy trees in North America are significantly more sensitive to spring temperature compared to in Asia and Europe. We predict that advancing tree phenology will lead to decreasing spring light windows in North America while spring light windows will be maintained or even increase in Asia and Europe in response to projected climate warming.

Generating ecological insights from historical data

“…To test the generality of this mismatch between tree and wildflower phenology, we worked with colleagues to examine thousands of herbarium specimens (Figure 1c) collected over the past 100 years from across eastern North America, East Asia, and Europe. We found that the tree-wildflower mismatch is occurring across eastern North America (Figure 1b) (Miller et al 2023) but is absent from East Asia or Europe, where trees are less responsive to temperature (Lee et al 2022).…”

“…(d) Using herbarium specimens, the window of spring sunlight available to woodland wildflowers is predicted to decrease by 2100, with the strongest effects in the north (from Lee et al . 2022).…”

“…2023) but is absent from East Asia or Europe, where trees are less responsive to temperature (Lee et al . 2022).…”

Evaluating the definition and distribution of spring ephemeral wildflowers in eastern North America

“…Because we estimated wildflower active period length from observations made only over a recent seven-year-long period (2015-2021), we find it likely that our results reflect higher overlap between wildflower and tree phenology than historically occurred, and therefore that our results are likely biased in favor of conservative ephemerality estimates. Furthermore, because access to spring light is expected to be reduced even more in the future (Lee et al 2022), it is possible that some ephemeral species will no longer be defined as such under warmer future spring conditions. This seven-year-long window also relied on non-standardized surveying from volunteers on the platform iNaturalist, where the sampling effort is not uniform across species' ranges leading to discrepancies among observations spatially which limits the representation across ranges (Di Cecco et al 2021).…”

“…Another important caveat in the interpretation of our results is that both understory and overstory phenology is shifting in response to climate. For example, several recent studies provided evidence that canopy tree phenology is shifting earlier in the year at a faster rate than understory wildflower phenology is shifting (Heberling et al 2019, Lee et al 2022, Miller et al 2022; but see Alecrim et al 2023), a pattern which has continued for at least the past 120 years and which is projected to continue into the future. Because we estimated wildflower active period length from observations made only over a recent seven-year-long period (2015-2021), we find it likely that our results reflect higher overlap between wildflower and tree phenology than historically occurred, and therefore that our results are likely biased in favor of conservative ephemerality estimates.…”