The impact of biotic interactions on seedling recruitment during tree range expansion

Background/Question/Methods

Climate change is expected to initiate a process of poleward range expansion for many plant species and range expansion rates will be determined by the success of individuals establishing beyond their range margins. However, these migrant individuals will be establishing in communities which are novel for their species, and biotic interactions with indigenous herbivores and pathogens are likely to impact seedling recruitment. If biotic interactions are strong, they could contribute to the success or failure of a species to migrate.

To investigate the role of biotic interactions in range expansion we conducted a transplant experiment and quantified the interactions between migrant plants and existing communities. In this study we address the following questions: 1) Do migrant seedlings experience less herbivory and disease in their new ranges than native seedlings? 2) Are migrant species exposed to less herbivory and disease than in their native range? 3) How important is this to plant growth and survival?

Ten species of woody plants were planted within and beyond their current distributional ranges and foliar herbivory, pathogen activity, growth, and survival were monitored for three years. Seedlings were also planted across light and moisture gradients at each site to separate the effects of environmental covariates that could also affect establishment.

Results/Conclusions

Herbivory rates at higher latitudinal sites were higher for many species, but were especially high for some migrant species including Carya glabra and Robinia psuedoacacia. Herbivory had significant effects on seedling survival, and although the results are highly species specific, we show examples of how biotic resistance is preventing the establishment of certain lower latitude species in the higher latitude communities. In contrast, pathogen activity was often significantly lower for plants occurring beyond their current range; migrant seedlings had lower percentages of their leaf area affected by foliar pathogen activity over the growing season, compared to indigenous plants.

Our results establish that interactions with herbivores and pathogens differ within and beyond range edges for certain species and confirm the importance of these interactions to seedling growth and survival. In doing so, our study shows that, at least in the short term, release from pathogens could give some migrant species an advantage over native competitors whereas native herbivores could help prevent some potential migrants from establishing beyond their original ranges. Overall, we demonstrate that biotic interactions will influence plant range expansion and highlight the importance of including biotic interactions in predictions of short-term plant range shifts.