Divergent responses to climate change and disturbance drive recruitment patterns underlying latitudinal shifts of tree species

Climate change is expected to result in a reorganization of the continental distribution of tree species. Recent shifts in distribution patterns have been reported, but it is not always clear how climate change influences these patterns locally, especially in relation to other disturbances. We investigated latitudinal shifts of four ecologically important tree species between 1970 and 2014 within a study area that encompasses their northernmost range limit in northeastern North America (Quebec, Canada; ~761,000 km²). Changes in latitudinal limits were defined in relation to changes in tree saplings’ occurrence patterns within forest plots resampled over two time periods (1970–1977 and 2003–2014). By examining changes in the frequency of occurrences in different portions of the study area along a latitudinal gradient, we were able to identify spatially explicit patterns of loss or gain (sapling recruitment) resulting in the shifts observed. We then estimated the probability of observing a recruitment event in response to changes in climate, disturbance and their interaction, using a multimodel selection approach. Latitudinal limits of all four species shifted northward, but these shifts resulted from different patterns of plot occurrence changes, depending on the species and the location examined. Greater recruitment at northern locations than at southern ones drove shifts for Acer saccharum Marsh., Fagus grandifolia Ehrh. and Acer rubrum L., but less so for Betula alleghaniensis Britt. Climate variables indicating changes in early or late growing season conditions were most often selected in models. Warming tended to reduce recruitment probability in the south but increase it in the north, leading to divergent responses for a given species across the study area. Disturbance effects were generally less important than climate change effects, as was their interaction. Synthesis. Spatially explicit and divergent responses to climate change and disturbance drive recruitment patterns underlying latitudinal shifts of tree species. The importance of early‐ or late‐season climate variables points towards biological processes being affected at critical stages of the life cycle. Understanding the factors that influence species’ migration capacity in a changing climate is crucial to inform adaptive management and conservation practices.