COS 154-2 - Spring ephemeral phenology: Species-level classification and biogeographical distribution

Abstract: Herbaceous understory wildflower species compose a considerable proportion (up to 80%) of total biodiversity in North American temperate deciduous forests. This group of plants is taxonomically and functionally diverse and contributes important ecosystem functions ranging from promoting nutrient cycling in spring to serving as an important resource for pollinators throughout the growing season. Recently, increased attention has been placed on a subset of these species, colloquially referred to as “spring ephemeral” wildflowers. These early blooming herbs are characterized by their brief window of activity in early spring, prior to canopy trees leafing out and throwing the understory into shade. Most of these species only have functional leaves for several weeks each year. However, relatively little research has so far been done to quantitatively define species with this widespread phenological strategy and to describe their distribution across the landscape. Here, we combined information from large citizen science and remote-sensed datasets to quantify the variation and prevalence of forest herb species with this phenological strategy and to identify environmental constraints to their distribution across eastern North America.



Our results indicate a surprisingly high richness of spring ephemerals in eastern North American forests, with approximately 20% of all understory wildflowers defined as ephemeral in at least some part of their distributions. Approximately 10% of all forest understory herb species were defined as ephemeral across the entirety of their North American distribution. Further, we found important relationships between environmental drivers and the distribution of ephemeral wildflowers across the landscape. Areas with a higher proportion of ephemeral wildflower species were characterized by intermediate spring temperatures (i.e., a hump-shaped relationship that peaked around 12 °C), high precipitation in late spring, and low elevations. Highest concentrations of spring ephemeral wildflowers were concentrated in southern Appalachia. Our findings provide a novel framework for defining and discussing ephemerality of this important group of plants. Importantly, we contribute novel insight about the partial ephemerality of many species, suggesting that this strategy can vary across a species’ range. Lastly, we provide information that, when combined with recent research showing a strong vulnerability to climate change for these species, will be critical for their future conservation in eastern North America.