COS 110-1 - Earlier Flowering in Understory Spring Ephemerals Lags Behind Forest Leaf-out

Abstract: Many species are shifting their phenology due to anthropogenic climate change. However, species within communities often shift their phenology at different rates and even in opposite directions. These phenological mismatches can fundamentally alter species interactions, including competition, facilitation, or predation. For example, spring ephemeral wildflowers in the forest understory sequester the majority of their carbon pre-overstory leaf out, but we know little of their phenological shifts relative to overstory leaf out. In this study, we asked are spring ephemerals shifting their phenology in response to climate change? If so, are they able to track the same rate and direction of shifts in the timing of overstory leaf out? To test for phenological shifts of spring ephemerals, we downloaded occurrence data of 20 spring ephemeral species native to Eastern North America from the Global Biodiversity Information Facility (GBIF), utilizing collection or observation date as a proxy for their phenology. Because spring ephemerals have a short temporal window during which they emerge, flower, and senesce, we used observations with and without flowers present to estimate spring ephemeral phenological persistence. We then compared these data to two metrics of overstory leaf out, greenup (i.e., ~15% leaf out), and maturity (i.e., ~90% leaf out), provided within the MODIS/Terra+Aqua Land Cover Dynamics dataset and derived from satellite imagery. We then assessed whether phenological change over time differs between spring ephemerals versus overstory leaf out. All 20 spring ephemeral species have advanced their phenology over time, with an average rate of 0.3 (± 0.01) days earlier per year. However, overstory greenup has also shifted earlier by 1.5 days per year and overstory maturity has shifted by 1.4 days per year. These results show overstory leaf out is advancing at a rate five times faster than understory spring ephemerals. If this results in the overstory closing before spring ephemerals can emerge, spring ephemerals will have ever shorter periods to sequester carbon, potentially impacting fitness and population persistence. Given their important role as early food sources for many pollinators and insects, this phenological mismatch could have profoundly negative impacts on forest communities. If this phenological mismatch is shown to have sustained negative impacts on spring ephemeral populations, active management such as midstory thinning within susceptible populations may be needed to allow these species to perpetuate on the landscape.