Between 16 and 7 thousand years before present (kaBP), the distributions of North American woody plant genera shifted considerably, corresponding to rapid increases in temperature and almost complete retreat of continental ice sheets. These shifts varied widely in rate and direction for reasons that remain unclear. Familiar plant traits such as seed mass and shade tolerance have not satisfactorily explained this variation, suggesting that other factors are at play. Mycorrhizal symbioses have long been recognized as potential moderators of plant responses to climate change, because they typically enhance establishment, nutrient uptake, growth, and resilience of plant hosts. However, evidence of their roles in biogeographic processes, such as distribution expansion in response to climate change, remains scarce.
We tested whether variation in the post-glacial distribution responses of 23 plant genera was related to the type of mycorrhizal association (arbuscular or “AM”; ectomycorrhizal or “EM”) and, for the EM host genera (N = 13), host promiscuity (an index representing the diversity of EM fungi with which a host genus associates).
We used fossil pollen observations to estimate average rates of post-glacial biotic velocity at leading (northern) and trailing (southern) boundaries of core distributions.
We applied an information-theoretic approach to identify plausible models for explaining biotic velocities, and combined this with model averaging to identify predictors that best accounted for the observed responses. Additional explanatory variables included climate velocity, seed mass, height at maturity, shade tolerance, and minimum temperature tolerance.
Results/Conclusions
Climate velocity, height at maturity, and mycorrhizal type together accounted for 40% of the variation in contraction rates at trailing boundaries of core distributions. Remarkably, 75% of the variation in northern rates of expansion among EM hosts was explained by positive effects of host promiscuity (accounting for 46% alone) and minimum temperature tolerance, with a negative effect of seed mass and no discernable effect of climate velocity.
Our findings highlight key roles for symbiont-facilitated interactions in forest ecosystem responses to climate change, and suggest that efforts to facilitate forest adaptation should incorporate roles for mycorrhizal mutualisms. More generally, our results reinforce calls to explicitly consider biotic interactions in addition to climate-related variables as drivers of geographical range shifts, but also caution that their respective influences can vary geographically, differing for example between leading and trailing boundaries of shifting distributions