Facilitative interactions of two co-occurring invasive trees in the Southeastern U.S

Background/Question/Methods

To better understand mechanisms of successful invasions, we conducted an outdoor pot experiment that examined nodule production and growth of a leguminous invasive tree species Alibizia julibrissin grown with the allelopath producing Ailanthus altissima. We specifically examined the effect of neighbor status on the nodule production and biomass accumulation of Albizia (growth with Ailanthus, intraspecifically, and solitary). We measured stem height and diameter repeatedly for the duration of this study and used total dry weight for an assessment of the competitive ability of Ailanthus growing with a co-occurring nitrogen fixing invasive.  We hypothesized that Albizia would produce more nodules in the presence of Ailanthus compared to when grown alone or with con-specifics. We also hypothesized that Ailanthus, when grown with Albizia would have greater biomass, stem height, and diameter due to increased nitrogen fixation by Albizia. We used a fully factorial design and randomly assigned four planting combinations within three blocks and each combination was replicated six times. We used general linear MANOVA’s and individual GLM ANOVA’s for biomass analyses, lnRR for competitive ability, and repeated measures multivariate analysis of variance for stem height and diameter . Nodule production was analyzed using a poisson generalized linear model (GLM).

Results/Conclusions

MANOVA results showed significant differences among species and neighbor status for root, shoot, total biomass and root/shoot ratio. Ailanthus allocated more resources to shoot and total biomass, contrarily, Albizia allocated more biomass to root production. Albizia produced significantly more nodules when Ailanthus was the neighbor and produced equal yet insignificantly fewer nodules when grown with another Albizia or when grown by itself. Neighbor identity did not influence competitive ability (lnRR). There were significant differences for height and diameter between Ailanthus and Albizia, however neighbor status had no influence. Our results indicate that indeed there appears to be a positive interaction in regards to increased nodule production when Albizia is grown with Ailanthus. Interestingly, our lnRR results did not indicate a facilitative interaction, however there was an increase in competition when Albizia was grown with Ailanthus.  These findings are consistent with other studies suggesting that positive interactions between invasive species may facilitate subsequent invasions, intensifying the impact of invasive species.