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.

A grid of 20 m x 20 m quadrats were established at each site to map overstorey canopy trees. Each site varied in plot size, where the total area of mapped grids ranged from 200-320 m in width and 200-1000 m in plot length. All trees within the study grid had their coordinates mapped using Leica Flexline© TS06 and TCR805 Total Stations. Each tree with a diameter at breast height (DBH) ≥10.0 cm was mapped, identified by species, given a health status (living, dead, declining, fallen) and its DBH (1.3 m) measured. In the late spring of 2015, four recruitment plots (1 m²) were established within each of the 20 x 20 m grids at a 5 x 5 m spacing (3,424 total recruitment plots across all three sites). Recruitment was measured as stem tallies by species over two years (2015 and 2016) for two recruitment stages: (i) first-year seedlings (referred to as stage 1 seedlings); identified by the presences of cotyledons, absence of terminal bud scars, suppleness of the stem, and number of leaves, (ii) older seedlings (referred to as stage 2 seedlings); which were not first year and were <30.0 cm in height. We visually estimated the percent coverage (to the nearest 5%, total summing to 100%) of the recruitment plots by the following substrates: grass, leaf, needles, moss, decayed logs, solid logs, and bole cover from overstorey/sapling trees. In addition to the substrates, we also assessed the cover of deciduous plant cover (non-canopy tree species) within the recruitment plot.

Permanent Plots - Tree Species Codes

This .csv file provides the code names used in the data files for the tree seedling recruitment, overstorey, and their common English names. 

abitibitrees, lebictrees, suttontrees

These . csv files provide the overstorey (DBH>10.0cm) tree information for the three permanent plots included in the study: Abitibi-Temiscaminque (abitibitrees), Le Bic (lebictrees), Sutton (suttontrees). The following information are included: tree identification (ID), tree species (SP, four-letter code), diameter at breast height (DBH, 1.3m), basal area (BA), macroplot location (20m x 20m) (X and Y, in meters). 

abitibiregen, lebicregen, suttonregen

These .csv files provide the tree recruitment (stages _1 and _2) count data for the three permanent plots included in the study: Abitibi-Temiscaminque (abitibiregen), Le Bic (lebicregen), Sutton (suttonregen). The following information are included: Year of collection (Year), recruitment plot location (1m²) (X and Y), seedling count data by stage (indicated by two-letter species code and seedling stage, e.g., BF_1, indicates the number of first year seedlings of balsam fir). 

abitibisubstrate, lebicsubstrate, suttonsubstrate

These .csv files provide information on plot level substrate (% of plot coverage). The following information are included: recruitment plot location (1m²) (X and Y), grass coverage (%), decidous plant coverage (%), deciduous leaf coverage (%), conifer needle coverage (%), moss coverage (%), solid log coverage (hard, %), decayed log coverage (soft, %), rock coverage (%), sapling or overstorey canopy tree coverage (sap, %).