The importance of ecological constraints on the control of multi‐species treeline dynamics in eastern Nunavik, Québec

• Premise of the study: Treelines are temperature‐sensitive ecotones that should be able to expand in response to global warming; however, they are also controlled by ecological constraints. These constraints can create bottlenecks for tree regeneration, hindering treeline advances. Near Kangiqsualujjuaq (Nunavik, subarctic Québec), previous studies suggested successful recruitment of Larix laricina above the altitudinal treeline, while Picea mariana establishment remains scarce. We studied regeneration of both species to identify factors responsible for such contrasting responses. • Methods: We measured seeds and wings to evaluate species dispersal potential. We compared seed viability and tolerance to shrub leachates with germination trials. To evaluate seedbed preferences, we compared seedling occurrence on the different seedbeds with seedbed relative abundance in the field. • Key results: Seed germination was similar between L. laricina and P. mariana , whereas dispersal potential was higher for the latter. Germination of P. mariana seeds was more strongly inhibited by shrub leachates than were L. laricina seeds. In the field, we found only a few Picea seedlings, but numerous seedlings of Larix had established disproportionally on several seedbeds. While Betula glandulosa , mosses, and Vaccinium uliginosim impeded Larix establishment, numerous seedlings were found on lichens, mineral soil, and liverworts. The low occurrence of suitable seedbeds for Picea , mainly mineral soil, could explain the seedling scarcity of this species. • Conclusions: This study highlighted that allelopathy and unsuitable seedbeds could contribute to regeneration failure of P. mariana in eastern Nunavik and emphasizes the need to consider ecological preferences of species before predicting treeline expansion under a warmer climate.