Differential Canopy Leaf Flushing and Site Nitrogen Status Facilitate 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. In the temperate forests of the northeastern United States invasive plant species frequently establish and colonize edge environments; however, they are not confined to these areas. In this study, I hypothesized that nonnative species presence would be greatest under those canopy tree species that allow higher understory light levels because of phenological differences in spring leafing as well as canopy transmission and site-nutrient availability in these understories. Six canopy types were used: Populus tremuloides (quaking aspen), Acer saccharum (sugar maple), Quercus spp./Carya spp. (oak/hickory), Fraxinus americana (white ash), Robinia pseudoacacia (black locust), and Pinus spp./Tsuga canadensis (pine/hemlock). 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. Significant differences in phenology were: black locust ≥ white ash ≥ oak/hickory > sugar maple > quaking aspen > pine/hemlock. As expected, soil analyses showed higher nitrate levels in the nitrogen-fixing black locust stands, but unexpectedly, also for the white ash stands. Invasive species cover and relative abundance were significantly higher under black locust and white ash canopies suggesting the importance of higher resource availability (i.e., later phenology, greater light transmission, and nitrogen fixation) in invasive plant success. Collectively, late leaf phenology, relatively high understory light levels, and higher site nitrogen might allow these stands to facilitate nonnative species success and to act as islands of invasion in resource-poor forested landscapes.
invasive, temperate forest, phenology, light transmission, Robinia
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