Will biotic interactions determine temperate tree range expansion success?

Background/Question/Methods

Tree range expansion in response to climate change has been widely predicted.  Range expansion will depend on whether individuals can successfully colonize areas beyond their range edges, where they will interact with new suites of herbivores and pathogens.  If biotic interactions vary across range margins and have strong effects on plant demography they could contribute to the success or failure of a species to track climate change.

To investigate the potential for biotic interactions to influence 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 five years. Seedlings were planted across light and moisture gradients at each site to separate the effects of environmental covariates that could also affect establishment.  

Results/Conclusions

Many species planted beyond their range edges had higher survival rates than when planted within their range (including Carya glabra, Liriodendron tulipifera, and Robinia psuedoacacia).  Although some species, such as Liriodendron tulipifera, experienced reduced herbivory beyond their range edge, herbivory tended to be specific to the forest type.  Overall, the effects of foliar herbivory on survival were not high enough to explain net differences in survival between existing and potential ranges. 

Foliar pathogen activity was significantly lower beyond range margins for Carya glabra and Quercus alba.  Although foliar pathogens had significant effects on survival for some species, they were not strong enough to account for the large observed differences in survival.  Overall, our results show that herbivores and foliar pathogens can have important effects on the survival of individual seedlings, but they are unlikely to occur at rates high enough to determine colonization success of range expanding temperate tree species.