Phenological mismatch: Invasive shrub impact on wildflower survival

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Jessica Poteet and Sara Kuebbing, Biological Sciences, University of Pittsburgh, Pittsburgh, PA, Mason Heberling, Biology, Syracuse University, Syracuse, NY, Richard Primack, Biology, Boston University, Boston, MA

Background/Question/Methods
The phenology of forest plants is changing rapidly due to the introduction of nonnative species and climate change. Invasive shrubs exhibit novel phenology, frequently leafing out earlier in spring than native shrubs. Furthermore, climate change advances the timing of leaf out of forest plants but may do so unevenly across species. These phenomena can cause phenological mismatch between shrubs and wildflowers that rely on early spring light for much of their carbon budget. We analyze how these concurrent light regime changes are affecting native wildflower fitness and survival.
We monitored the phenology and chlorophyll concentration of four wildflower species and the soil and light environment under one native and three nonnative shrubs in Barking Slopes Conservation Area in southwestern Pennsylvania.
We hypothesize that phenological mismatch will cause nonnative shrubs to have a greater impact on their wildflowers’ fitness and survival than wildflowers under native shrubs. We expect that early canopy closure will cause wildflowers with earlier phenologies (i.e. spring ephemerals) to exhibit lower chlorophyll content than species that persist into the summer.
Results/Conclusions
Light environments varied by shrub species in the early spring. The native shrub, Lindera benzoin, canopy allowed 1.25 times more light through to the herbaceous layer than the nonnative shrubs in early spring (April to May). From May to the end of the fall, however, there was no difference between light penetration to the herbaceous layer under native compared to nonnative shrubs.
Throughout their relative growing seasons, early spring ephemerals like wild leek (Allium tricoccum) had 8.5% lower leaf chlorophyll concentrations under nonnative shrubs than under the native shrub. Conversely, goldenseal (Hydrastis canadensis), which grows throughout the summer and even into the fall, had similar chlorophyll concentrations throughout the growing season under both native and nonnative shrubs. There was no significant difference in soil moisture or temperature below native and nonnative shrubs.
Overall, our data suggests that invasions of nonnative shrub have a negative effect on spring ephemeral wildflower fitness which could contribute to decreasing wildflower diversity in forests across the northeastern United States as invasive shrubs become more and more ubiquitous within our forests.