Influence of leaf phenology and site nitrogen on invasive species establishment in temperate deciduous forest understories

Invasion by nonnative species is considered one of the greatest threats to ecosystem structure and function worldwide. Human disturbance and landscape fragmentation create edge habitats that are often higher in resource (i.e., light and nutrient) availability and are noted for having a high degree of invasive species establishment and colonization. In contrast, interior forest understories, which are often light- and nutrient-limited, are typically less susceptible to invasion. We assessed invasive species presence in interior, intact, temperate forest understories in relation to canopy-induced environmental conditions. We hypothesized that nonnative invasive species presence would be greatest under those canopy tree species that allow higher understory light and/or soil nitrate levels as a result of inherent differences in canopy transmission, spring leaf phenology, and nitrogen fixation. Five native forest canopy types were used, Populus tremuloides (quaking aspen), Acer saccharum (sugar maple), Quercus spp./Carya spp. (oak/hickory), Fraxinus americana (white ash), evergreen species mix Pinus spp./Tsuga canadensis (pine/hemlock), along with one nonnative canopy species, Robinia pseudoacacia (black locust). Measures of understory light availability post leaf-expansion were significantly higher under quaking aspen than other canopies with the exception of black locust and white ash. Canopy bud break and leaf flush occurred first in quaking aspen stands followed by sugar maple, oak/hickory, white ash and black locust stands, respectively. As expected, soil analyses showed higher nitrate levels in the nitrogen-fixing black locust stands, but unexpectedly, also in the white ash stands. Under these two canopy types, invasive species cover and relative abundance were significantly higher suggesting the importance of greater resource availability in invasive plant success. As such, intact forest stands with greater light transmission, later phenology, and nitrate fixation act as islands of invasion in resource-limited forested landscapes.

► We assess invasive plant presence relative to canopy-induced understory conditions.
► Invasive presence was greater under canopies with higher resource availability.
► PAR, spring phenology, and soil nitrate significantly explained invasive presence.
► R. pseudoacacia may facilitate invasive plants into interior forest understories.