The interaction between invasive tree Ailanthus altissima and native Robinia pseudoacacia in eastern deciduous forest

Background/Question/Methods

Native plant populations, communities and ecosystems are fundamentally impacted by interactions between native plants and non-indigenous invasive plants. These interactions are most likely to occur among plants of similar functional type. The presented research focuses on understanding the extent and mechanism of interaction between non-indigenous invader Ailanthus altissima and Robinia pseudoacacia, a functionally similar native to the Appalachian region. A field plantation was established at the Blandy Experimental Farm, Broce, VA, to test inter- and intraspecific competition. The hypotheses tested are: 1) A. altissima will be the winner in competition with R. pseudoacacia; 2) The presence of insects (Locust leaf miner and Ailanthus webworm) will increase the advantage of A. altissima over R. pseudoacacia. Height (H) and basal diameter (D) of all plants was measured at the beginning and at the end of this first growing season. Additional plants of both species were harvested from extra plots for dimension analysis to estimate first year biomass. In a linked field survey, trees, saplings and seedlings were mapped in four types of randomly chosen 20×50m plots around the Blandy Experimental Farm to address spatial distribution of these two species in early successional forest.

Results/Conclusions

The growth of A. altissima tended to decrease as its density increased in monocultures, but R. pseudoacacia growth had the reverse relationship (p=0.01). 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. Ailanthus altissima had no negative influence on R. pseudoacacia after one-year of growth. A. altissima had a greater increase of height, volume and biomass in sprayed plots compared with plants in unsprayed plots (p<0.01, p<0.05, p<0.05). Biomass and leaf area of R. pseudoacacia increased more in sprayed plots than in unsprayed plots (p<0.001). Although both species grew more in sprayed plots, neither species gained advantage over the other species in sprayed plots compared with unsprayed plots. We expected these trends will become more explicit in subsequent years of study and will further clarify the nature of the interaction between these two species. The two subject species were not significantly clustered in early successional plots. However, A. altissima seedlings were associated more with R. pseudoacacia trees than other dominant trees; and the presence of A. altissima increased the total coverage of invasive seedlings and saplings.