The interactions between invasive tree Ailanthus altissima and native Robinia pseudoacacia in eastern deciduous forest of the U.S

Background/Question/Methods

Native plant populations, communities and ecosystems are fundamentally impacted by interactions between native and invasive plants. Our research focuses on understanding the extent and mechanisms of interactions between non-indigenous invader Ailanthus altissima and its functionally similar native Robinia pseudoacacia in the Appalachian region. A plantation experiment was established at the Blandy Experimental Farm, VA, to test inter- and intra-specific competition. The hypotheses tested are: 1) A. altissima will outcompete R. pseudoacacia; 2) one mechanism behind the inhibition of R. pseudoacacia by A. altissima might be decrease in nodulation of R. pseudoacacia roots; 3) The presence of insects might increase the advantage of A. altissima over R. pseudoacacia. Height and basal diameter of all plants were measured at the beginning and end of each of two field seasons. Plants of both species were harvested for dimension analysis to estimate biomass.  A greenhouse experiment has been established to determine the influence of soil nitrogen availability on competition between these two species. In a linked field survey, trees and seedlings were mapped in 20x 50m early-successional forest plots to address spatial distribution of these two species and their influence on forest regeneration.  

Results/Conclusions

Both A. altissima and R. pseudoacacia showed a significant (p < 0.01) decrease in growth with increasing density of the monocultures. After one year, A. altissima grown with R. pseudoacacia tended to be larger than A. altissima in monoculture, suggesting that R. pseudoacacia facilitated the growth of A. altissima. At the same time A. altissima had no negative influence on R. pseudoacacia.  However, after the second year, R. pseudoacacia growth decreased as the proportion of coexisting A. altissima increased, suggesting an inhibition of A. altissima on R. pseudoacacia. Concurrently, there was no longer any sign of R. pseudoacacia facilitating A. altissima growth. After the second field season, R. pseudoacacia grew more in insecticide spray treatment plots compared to those in unsprayed plots, but insecticide treatment had little effect on the growth of A. altissima. The two subject species significantly clustered with themselves in early successional plots, but not with each other. However, A. altissima seedlings were associated more with R. pseudoacacia trees than other dominant trees, and the presence of A. altissima trees decreased the abundance of other tree seedlings and saplings.