Non-climatic constraints on species' elevational range expansion under climate change

Background/Question/Methods

Species’ range limits are expected to shift northward and upward in elevation with increased temperatures as climatic constraints on species’ establishment and survival lessen. However, little attention has been paid to non-climatic factors influencing range expansion. Mountains in northeastern North America often exhibit a transition with increasing elevation from sugar-maple dominated deciduous forests characteristic of milder climates to coniferous forests distinctive to northern and subarctic ecosystems. With increasing summer temperatures and growing season length, one might expect an upward shift in the elevational range limit of sugar maple. To disentangle climatic versus substrate constraints on the distribution of sugar maple, we implemented reciprocal transplant experiments of both seed and substrate (i.e., soil) across different elevations along an elevational gradient in southern Québec, Canada. We used mixed models to compare sugar maple emergence, survival, and growth both along the gradient and between different soil sources.

Results/Conclusions

Sugar maple distributions in temperate montane forests appear to be limited by multiple, interacting biotic and abiotic factors. Sugar maple recruitment decreased with increasing elevation, indicating that the current distribution of sugar maple is not only limited by seed availability. Sugar maple recruitment was reduced on soil transplanted from beyond its range to within its range, where climatic conditions are currently favourable for regeneration. Surprisingly, recruitment via seed planted beyond the elevational range limit was also severely limited by seed predation. Of the seedlings that were able to establish beyond the range limit, one third exhibited symptoms of antagonistic fungal infection prior to desiccation. Substrate properties, seed predation, and soil biota communities all appear to constrain sugar maple range expansion beyond its current elevational limit. These biotic and abiotic non-climatic factors can thus be expected to substantially slow climate-driven elevational range expansion. This research demonstrates that direct effects of climate on biogeographical patterns may often been countered by the effects of non-climatic factors via edaphic properties and species interactions.