The role of leaf litter and edaphic conditions related to land-use history in facilitating invasion of the forest understory by Oriental bittersweet (Celastrus orbiculatus)

Background/Question/Methods

Land-use history is known to increase invasibility of many successional forests.  However, little is known about how land-use legacies might actually facilitate invasion by non-native plants.  A seeding experiment was conducted at the Bent Creek Experimental Forest in western North Carolina, USA, to compare germination, seedling survival, and growth rates of Oriental bittersweet (Celastrus orbiculatus Thunb.) in stands dominated by tulip poplar (Liriodendron tulipifera L.) that had been cultivated and abandoned a century earlier, and in nearby stands dominated by oaks (Quercus spp.) that had never been cultivated.  Bittersweet seeds were sown in plots on the forest floor in which leaf litter quantity (no litter, low litter mass, and high litter mass) were manipulated.  In plots with leaf litter, litter type (from tulip poplar stands or from oak stands) was also manipulated.  Seeds were also sown in pots in which soils were translocated between the oak and tulip poplar stands. The aim of this study was to determine the mechanisms underlying the increased invasion rates observed in successional stands in the study region. 

Results/Conclusions

Both germination and survival in plots was higher in tulip poplar stands than in oak stands.  Germination was also higher in plots with low litter mass than in those with no litter or high litter mass.  First-year seedling survival and survival after two years was also greatest in plots with intermediate litter mass. Differences in germination and survival among the pots were minimal, suggesting that soil type and site conditions unrelated to the forest floor were less important than litter conditions for C. orbiculatus establishment.  Two-year seedling biomass in the plots was not significantly different between stand types or among litter treatments, however seedlings grown in pots with soil from tulip poplar stands had greater biomass than those grown in soil from oak stands.  Overall, the results from this study suggest that while richer soils found in the successional tulip poplar stands might facilitate more rapid growth under ideal conditions, litter quantity plays a more important role in explaining the differential invasion success in the successional forest stands.