DOI: 10.5061/dryad.s1rn8pk7d
title: Boreal aspen understory diversity along a continental gradient

description: This dataset contains vascular plant species cover and diversity indices
 (richness, evenness, and beta diversity) of understory communities and
 site-level environmental variables from 33 mature aspen (Populus
 tremuloides) stands across the Canadian boreal forest. The objective of
 the study was to evaluate the relative impacts of local and regional
 environmental conditions on vascular plant community composition and
 diversity in the boreal forest. We measured community composition and
 vegetation diversity indices of the vascular understory in 400 m2 plots
 and measured environmental variables at the local (drainage, nutrient and
 light availability, site heterogeneity) and regional (annual temperature
 and precipitation, fire weather index, landscape heterogeneity)
 scales. 
description: Sampling took place in 33 trembling aspen (<i>Populus
 tremuloides</i>) stands along a transcontinental gradient in
 precipitation and fire regimes in boreal Canada. We measured community
 composition and vegetation diversity indices of the vascular understory in
 400 m<sup>2</sup> plots and measured
 environmental variables at the local (drainage, nutrient and light
 availability, site heterogeneity) and regional (annual temperature and
 precipitation, fire weather index, landscape heterogeneity) scales.
 Specific details about data collection of individual variables are
 presented in the Usage Notes section. 
description: Associated paper: Crispo, Jean, Fenton, Leduc, and Bergeron.
 Factors explaining the composition and diversity of vascular plant
 understories along a transcontinental gradient in the Canadian boreal
 forest. Journal of Vegetation Science. Manuscript ID JVS-RA-05658.
 <em>Under review</em> Authors names: Maude
 Crispo<sup>1</sup>, Mélanie Jean<sup>2,3</sup>
 (ORCID: 0000-0002-7393-5566), Nicole J. Fenton<sup>2</sup>
 (ORCID: 0000-0002-3782-2361), Alain Leduc<sup>1</sup>, Yves
 Bergeron<sup>1,2</sup> (ORCID: 0000-0003-3707-3687)
 Author's institutional affiliations: 1
 Département des sciences biologiques, Université du Québec à
 Montréal 2 Institut de recherche sur les forêts,
 Université du Québec en Abitibi-Témiscamingue 3
 Département de biologie, Université de Moncton
 Corresponding author: Mélanie Jean
 (melanie.jean@umoncton.ca) <b>Usage
 information: </b> <strong>Species_cover
 sheet</strong> Vascular plant species cover data
 at each site. One 400 m2 circular plot was randomly positioned in each of
 the 33 selected stands. In each circular plot, 20 quadrats (1 m2) were
 arranged in a cross shape pointing at the four cardinal directions with
 five quadrats on each branch. A survey of the vascular understory
 vegetation was carried out inside each quadrat by identifying all
 herbaceous and woody species and defining their percent cover based on
 vertical projections. Percentages were visually assessed by two
 independent observers and rare species (&lt;1% cover) were assigned a
 cover of 0.5%. For each site, plant covers were summed up by species using
 the 20 quadrats (covers ranged from 0-2000). 
 <em>Site</em>: Site code that contains the Canadian
 province it was in (AB: Alberta, BC: British Columbia, MB: Manitoba, ON:
 Ontario, QC: Quebec, SK: Saskatchewan) and an alphanueric code. 
 <em>Columns</em>: Columns are alpharnumeric codes
 attributed to the species found in the surveys. Species names associated
 with the codes are in the Species_codes sheet. Unknown species that could
 not be identified have codes that start by "INC".
 <strong>Species_codes sheet</strong>
 <em>CODE</em>: Code used to identify the species in
 the Species_cover sheet.  <em>Genus</em> :
 Genus of the species.  <em>Species</em>:
 Species when known, otherwise the code "sp" was used to
 indicated that identification was done at the genus level only. 
 <em>Subspecies</em>: When possible. 
 <em>Variety</em>: When possible. 
 <strong>Diversity_indices sheet</strong>
 <em>Site</em>: Site code that contains the Canadian
 province it was in (AB: Alberta, BC: British Columbia, MB: Manitoba, ON:
 Ontario, QC: Quebec, SK: Saskatchewan) and an alphanueric code. 
 <em>Richness</em>: Species richness encountered at a
 given site was calculated by counting the species inside and outside the
 quadrats within the 400 m2 plot. 
 <em>BetaDiversity</em> :The total non-directional
 beta diversity within each site, or intrasite species heterogeneity, was
 determined by the total variance observed in species abundance data across
 the 20 quadrats at a site following Legendre and De Cáceres (2013) with
 the beta.div function in the adespatial package (Dray et al.,
 2017).  <em>Evar</em>: Evenness was
 measured using the Evar index (Smith and Wilson, 1996), which is
 independent from richness and is not sensitive to rare species. 
 <strong>Environmental_variables
 sheet</strong> <em>Sites</em>: Site
 code that contains the Canadian province it was in (AB: Alberta, BC:
 British Columbia, MB: Manitoba, ON: Ontario, QC: Quebec, SK: Saskatchewan)
 and an alphanueric code.  <em>LAI</em>: We
 calculated the leaf area index (LAI; inversely proportional to light
 availability) by processing hemispherical photographs taken at a height of
 1 m in the centre of each plot and 7.5 m from the centre at the four
 cardinal points with Gap Light Analyzer 2.0 (Frazer et al., 1999).
 Photographs were taken after moving shrubs (˂4 m) out of the
 way.  <em>HET</em>: A local heterogeneity
 index was calculated to represent the diversity of ecological niches at
 each site. This index characterized, in a single relative numerical value,
 dead wood or rock presence on the ground, microtopography, and presence of
 windfalls and windbreaks creating canopy gaps and altering the physical
 structure of the site (Appendix S1). Individually, these components had
 highly skewed distributions and seemed insufficient to fully represent
 ecological niche diversity, while the synthetic local heterogeneity index
 variable had a normal distribution. See Maude Crispo's Masters thesis
 (Université du Québec à Montréal) for more details on this
 variable.  <em>DRAIN</em>: At each site, a
 drainage class ranging between 0 (excessive drainage) and 6 (very poor
 drainage) was established based on soil characteristics (Saucier et al.,
 1994).  <em>lnCEC</em>: Cation exchange
 capacity (CEC) was determined by the Mehlich-3 method on mineral soil
 samples taken 15 cm below the organic layer, at the four cardinal points
 of the plots. This variable was transformed with the natural log to
 improve linearity in our models (CEC; see below). 
 <em>sqFWI</em>: Fire susceptibility was quantified by
 the forest fire weather index (FWI), calculated from six components taking
 into account effects of fuel water content, wind, and weather conditions
 on fire behaviour, provides an estimate of forest fire occurrence risk
 (Wotton, 2009). FWI was square root transformed to respect assumptions of
 normality and homoscedasticity. Fire susceptibility was averaged over 30
 years by the BioSIM 9 software (Régnière and Saint-Amant, 2008) by
 interpolating data from the closest weather stations to each site
 (1981-2010; Environment Canada, 2013). 
 <em>DD</em>: Annual sum of degree days (&gt; 0º
 C) at the site averaged over 30 years by the BioSIM 9 software (Régnière
 and Saint-Amant, 2008) by interpolating data from the closest weather
 stations to each site (1981-2010; Environment Canada, 2013).
 <em>LH</em>: Landscape heterogeneity was calculated
 as the proportion of forest cover within a 1 km radius around the site
 (see Crispo [2016] for calculation and rationale details), thus accounting
 for the presence of species from open or disturbed environments, such as
 agricultural land, roads, or clearcuts.
 <em>RAIN</em>: Annual precipitation as rain (mm) at
 the site,  averaged over 30 years by the BioSIM 9 software (Régnière and
 Saint-Amant, 2008) by interpolating data from the closest weather stations
 to each site (1981-2010; Environment Canada, 2013). 
 <em>Latitude</em>: Site latitude in degree
 decimal.  <em>Longitude</em>: Site latitude
 in degree decimal.