DOI: 10.5061/dryad.q573n5tdx
title: Priority effects will impede range shifts of temperate tree species into the boreal forest

description:  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.
description: 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 m<sup>2</sup>) 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 &lt;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.
description: <b>Permanent Plots - Tree Species Codes</b>
 This .csv file provides the code names used in the data files for
 the tree seedling recruitment, overstorey, and their common English
 names.  <strong>abitibitrees, lebictrees,
 suttontrees</strong> These . csv files
 provide the overstorey (DBH&gt;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). 
 <strong>abitibiregen, lebicregen,
 suttonregen</strong> 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
 (1m<sup>2</sup>) (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). 
 <strong>abitibisubstrate, lebicsubstrate,
 suttonsubstrate</strong> These .csv files provide
 information on plot level substrate (% of plot coverage). The following
 information are included: recruitment plot location
 (1m<sup>2</sup>) (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, %).