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

Abstract

The herbaceous layer accounts for the majority of plant biodiversity in eastern North American forests, encompassing substantial variation in life history strategy and function. One group of early season herbaceous understory species, colloquially referred to as "spring ephemeral wildflowers," are of particular ecological and cultural importance. Despite this, little is known about the prevalence and biogeographic patterns of the spring ephemeral strategy. Here, we used georeferenced and dated observations from the Global Biodiversity Information Facility (GBIF) to define the phenological strategies of 559 herbaceous, vascular, understory plant species in eastern North America from a composite species list encompassing 16 site-level species lists ranging from Georgia to southern Canada. Specifically, we estimated activity periods from regional observations (primarily consisting of citizen scientist iNaturalist observations) and classified species as ephemeral if they completed all aboveground activity (including leafing, flowering, fruiting, and senescence) prior to an estimated date of canopy closure derived from remote-sensed data. We then evaluated the richness of these species at the landscape scale using estimates of biogeographic and environmental drivers aggregated for 100 km x 100 km grid cells. Importantly, our spatially-explicit approach defines each species spring ephemerality along a continuous scale (which we call the Ephemerality Index, EI) based on the proportion of its range in which it senesces before canopy closure (with EI = 0 indicating a species that is never ephemeral and EI = 1 indicating a species that is always ephemeral). We found that 18.4% (103 species) of understory wildflowers exhibited spring ephemerality in at least part of their range, with only 3.4% of all species exhibiting ephemeral behavior in all parts of their range. Ephemeral species had higher overall richness and composed a higher proportion of understory biodiversity in low-elevation areas with intermediate spring temperatures and elevated spring precipitation. Spring ephemerals peaked in both absolute species richness and relative proportion at mid latitudes. These biogeographic patterns deserve further study in other regions of the world and to uncover mechanisms behind these patterns. Using our new metric, our results demonstrate that the spring ephemeral strategy is not a discrete category, but rather a continuum that can vary across species ranges.