Invasive exotic plant species may gain an advantage over native species in deciduous forest understories by extending photosynthetic activity into pre- and post-canopy periods of high resource availability. However, this potential advantage is mediated by overstory species composition, which determines understory light availability through overstory phenology and transmittance. We explored the role of community-level phenological interactions in the success of invasive exotic species in deciduous forest understories using a set of common garden experiments located at the University of Connecticut. Three understory sites, each containing four invasive and four native species, were replicated under three overstory types - sugar maple (Acer saccharum), red oak (Quercus rubra), and white ash (Fraxinus americana). Comparisons of success between invasive and native species were based on measures of annual relative growth and diagnostic gas exchange taken across two growing seasons. Additionally, we developed a stochastic model of photosynthesis based on responses to seasonal trends in air temperature and light availability to examine leaf-level carbon gain under deciduous broadleaf overstories.
Understory invasive exotic species generally exhibited greater phenological extension, flushing leaves, on average, 13 days earlier and abscising leaves 9 days later than native species. Understory phenological extension and light availability accounted for a significant amount of variation in relative annual growth. Consequently, the invasive species advantage was most pronounced under ash canopies, which flushed leaves later in the spring, abscised leaves earlier in the fall, and allowed greater light availability during periods of intact canopy relative to other overstory types. Model simulations of annual carbon gain support our hypothesis that invasive species foliage, occupying a broader temporal niche, assimilate more carbon than native species foliage. Importantly, these results also indicate that an extension of temporal niche prior to overstory leaf flush, rather than after overstory abscission, contributes most to the invasive exotic species advantage. This was in part due to temporal changes in light availability and photosynthetic capacity. These findings elucidate the importance of community-level phenological interactions for understory resource availability and growth success. Future phenological shifts, exacerbated by climate change, will likely have important implications for the susceptibility of temperate forest understories to non-native plant invasions.