Spring ephemeral Erythronium umbilicatum may not be vulnerable to phenological mismatch with overstory trees

The defining life history strategy of spring ephemeral wildflowers is their avoidance of shading by trees during the brief, high-light period before canopy leaf out. Studies suggest that spring ephemerals will experience increased light competition because canopy leaf out is more sensitive to warming than is the phenology of spring ephemerals. However, it remains unclear how longer durations of shade will alter the population dynamics of spring ephemerals and whether all populations are at risk.

We experimentally shaded Erythronium umbilicatum for one to six additional weeks before canopy leaf out to test for immediate and lagged effects of early shading on the timing of senescence and the probability of survival and flowering. To predict the potential for earlier shading, we combined long-term time series of spring air temperature, remotely sensed tree leaf out, and E. umbilicatum flowering phenology in North Carolina, United States.

Early shading did not alter E. umbilicatum until the following year, when more-shaded plants senesced later. Year-to-year survival did not change, and the probability of flowering was reduced only when plants experienced extremely early shading. Moreover, E. umbilicatum phenology was more sensitive than tree leaf out to warming temperatures. We project that, under climate warming, E. umbilicatum is unlikely to experience shortened periods of high light.

Our findings show that a plant species' defining life history strategy does not necessarily predict their sensitivity to phenological mismatches. This incongruity complicates, but also underscores the importance of identifying the most vulnerable species and directing our research efforts accordingly.