Phenology across ontogeny in woody native and invasive plants in New England
The annual timing of leafing, flowering, and fruiting of invasive plants relative to natives may explain some of their success. Species-level differences may also help predict responses to climate change. Earlier springtime leafing of invasives may lead to an increase in resource acquisition, and by extension may bolster population growth. Later leaf coloration and drop effectively extends the growing season. Phenological separation between species may vary by life stage, such that mature plants exhibit different patterns than seedlings. If phenological separation of species is sensitive to ontogeny, the benefit may depend on the stage of invasion, such that phenological novelty of seedlings aids establishing populations and mature plant phenological separation better facilitates spread. We used a combination of experimental plantings and wild populations over two years to estimate the phenological separation in leafing events in two pairs of native and invasive ecologically analogous species in southern New England. Our study species include Berberis thunbergii (Japanese barberry, invasive shrub), Lindera benzoin (spicebush, native shrub), Celastrus orbiculatus (Oriental bittersweet, invasive vine), and Vitus labrusca (fox grape, native vine).
Our results show that invasive species (B. thunbergii and C. orbiculatus) display a significantly longer growing season than their native analogs (L. benzoin and V. labrusca) as seedlings and mature plants. The shrubs had an average difference of 28 days in their growing season length and the vines differed by about 13 days. Earlier springtime leafing of invasives drove the pattern in growing season length; autumn leaf coloration and drop was highly variable within and across species. Across all species in the study, mature plants generally had greater species-level phenological separation than seedlings. However, the differences in leafing phenology within native and invasive species pairs specifically (i.e., B. thunbergii vs. L. benzoin and C. orbiculatus vs. V. labrusca) were not significantly different across ontonogenic stages. For example, B. thunbergii had a growing season that was 30 days longer than L. benzoin as a seedling and 26 days longer as a mature plant. We did not find a significant effect of growing season length on seedling growth, which suggests that phenological differences at the seedling stage are likely poor predictors of invasion success. Linking phenology to demography in mature plants may provide additional insight into current and future distributions.