Priority effects will impede range shifts of temperate tree species into the boreal forest

 Temperate tree species are expected to expand their distribution into the boreal forest in response to climate change. However, it is becoming increasingly clear that many species will experience significant setbacks in capacity to migrate due to a series of unfavourable conditions impacting their recruitment success, and thus their ability to colonize new locations. We quantify the relative influence of a series of factors important for tree seedling recruitment at range margins: propagule dispersal, substrate favourability, and the influence of the local hetero-specific species canopy composition. We hypothesized that boreal trees are responsible for priority effects that influence the establishment of temperate tree species seedlings. To do so, we analyzed two recruitment stages (first-year seedlings and older seedlings) for seven tree species; Abies balsamea (ABBA), Acer rubrum (ACRU), Acer saccharum (ACSA), Betula papyrifera (BEPA), Betula alleghaniensis (BEAL), Populus tremuloides (POTR), and Fagus grandifolia (FAGR) commonly found within the temperate-boreal ecotone forests of northeastern North America. Overall, we found that boreal canopy trees influence the distribution of substrates, more specifically the occurrence of needle cover and decayed wood in recruitment plots. Interestingly, decayed wood was a poor substrate in almost all cases. This association between canopy and substrate led to highly unfavourable substrates that affected the seedling densities of all temperate tree species. In addition, we found that seedling dispersion was highly localized, where the mean dispersal distance of all trees occurred in close proximity of parent trees. Unfavourable substrates and limited MDD (mean dispersal distance) of trees due to resident boreal trees generate (strong) priority effects within the temperate-boreal ecotone. Together these conditions promise to cause significant lags in temperate tree species migration into the boreal forest in the future.