Survival and growth of native and alien woody seedlings in open and understory environments

Although plant invasion is often facilitated by disturbance, several non-native trees and shrubs have successfully invaded intact forest habitats in northeastern North America. To better understand invasive plant performance in intact versus disturbed forest habitats, we compared survival, relative height growth rate, aboveground biomass allocation, and leaf area of alien and native woody seedlings. In replicated understory versus open treatment plots at two sites we planted three pairs of native and alien species that appear ecologically similar: sugar maple ( Acer saccharum) and Norway maple ( Acer platanoides), arrow-wood ( Viburnum dentatum) and glossy buckthorn ( Rhamnus frangula), and silky dogwood ( Cornus amomum) and autumn olive ( Eleagnus umbellata). Seedlings were protected from deer browsing by open-topped cages made of chicken wire. Norway maple survival in open and understory environments was greater than that of sugar maple, and autumn olive survival in understory environments was greater than that of silky dogwood. The species differed in their responses to open versus understory environments in height growth, aboveground biomass and leaf area. The four shrub species grew faster in the open, with glossy buckthorn growing faster than the others, while the two tree species did not differ in growth between the environments. Leaf mass per unit leaf area was greater in the open than in the understory for all six species, but the difference did not vary among species. Arrow-wood, silky dogwood, and autumn olive decreased leaf area per unit biomass in the understory, but variation in survival among species was not related to maintenance of leaf area ratio. Among the six species, the increase in biomass from understory to open environments was negatively correlated with growing season survival in the understory. This apparent trade-off relationship applied equally to native and alien species, indicating that release from physiological constraints does not explain the success of the invasive species.