DOI: 10.5061/dryad.0cfxpnw0p
title: Permafrost thaw in boreal peatlands is rapidly altering forest community composition

description: Boreal peatlands are frequently underlain by permafrost, which is thawing
 rapidly. A common ecological response to thaw is the conversion of raised
 forested plateaus to treeless wetlands, but unexplained spatial variation
 in responses, combined with a lack of stand-level data, make it difficult
 to predict future trajectories of boreal forest composition and
 structure. We sought to characterize patterns and identify
 drivers of forest structure, composition, mortality, and recruitment in a
 boreal peatland experiencing permafrost thaw. To do this, we established a
 large (10 ha) permanent forest plot (completed in 2014), located in the
 Northwest Territories, Canada, that includes 40,584 mapped and measured
 trees. In 2018, we conducted a comprehensive mortality and recruitment
 recensus. We also measured frost table depth, soil moisture, soil
 humification, and organic layer thickness within the plot between 2012 and
 2018, and used habitat association tests to link these variables to forest
 characteristics and dynamics. Forest composition and structure
 varied markedly throughout the plot and were strongly governed by patterns
 in permafrost presence and organic layer thickness. Overall, there was a
 net loss of trees from the plot at a rate of 0.7% yr-1. Mortality of black
 spruce, the dominant tree species, was more than double that of
 recruitment and was strongly associated with permafrost thaw. In contrast,
 recruitment of larch was over four times greater than mortality, and
 occurred primarily in low-lying, permafrost-free wetlands with mineral
 soil near the surface. The trends in tree demography and
 underlying drivers suggest that spruce-dominated permafrost plateaus will
 be converted into larch-dominated wetlands as permafrost thaw progresses
 in boreal peatlands, particularly in areas where mineral soil is near the
 surface. In the longer term, thaw could increase the hydrologic
 connectivity of the landscape, resulting in widespread drainage and
 re-vegetation by spruce, but we did not find evidence that this is
 occurring yet. Given the increasing rates of permafrost thaw, and positive
 feedbacks between thaw and forest change, we predict that larch abundance
 will continue to increase in boreal peatlands over the coming decades,
 leading to shifts in ecosystem function, wildlife habitat, albedo, and
 snow dynamics.
description: These two datasets (“baltzer_jecol_scotty_tree_gricell_data.csv”,
 and “baltzer_jecol_env_post_data.csv”) contain the forest census data and
 raw environmental data we collected in the Scotty Creek ForestGEO plot (a
 9.6 ha permanent forest dynamics plot located in the Northwest
 Territories, Canada) and used in the manuscript titled, "Permafrost
 thaw in  boreal peatlands is rapidly altering forest community
 composition" (accepted for publication in Journal of Ecology on
 November 13, 2020).  A schematic diagram of the plot is
 shown in “scotty_plot_layout.pdf”. We used the southernmost 120 m of the
 plot (depicted by dark grey shading in the schematic diagram) for all
 analyses we performed for the paper. This portion of the plot measures 800
 m east-west by 120 m north-south for a total of 96,000 m2 or 9.6 ha and is
 divided into 240 20 m by 20 m “grid cells” to enable systematic
 inventorying of trees. The four corners of each of these grid cells are
 marked with a total of 287 aluminum posts, each of which is labelled with
 a number indicating the “row” it is in (beginning with 1 for the
 southernmost edge of the plot and ending with 7 for the northernmost
 edge), as well as a letter indicating the “column” it is in (beginning
 with A for the westernmost edge of the plot and ending with AO for the
 easternmost edge). The grid cell name always matches the name of the post
 situated at its southwest corner (e.g. the grid cell located at the
 southwest corner of the plot is labelled “A1” and is demarcated by the
 posts “A1”, “B1”, “A2”, and “B2”). The plot contains
 over 40,000 trees with a diameter at breast height (i.e. a stem diameter
 at 1.3 m above the ground) greater than or equal to 1 cm. We initially
 tagged, identified (to species or genus), measured, and mapped each of
 these stems in 2013/2014, and then revisited each stem in 2018 to
 determine whether it was still alive or had died in the intervening years.
 We also tagged, identified (to species or genus), measured, and mapped any
 new stems that had reached the necessary size and height for inclusion in
 the plot (diameter at breast height greater than or equal to 1 cm) in the
 intervening years. We summarized the resulting tree demographic
 data (mortality and recruitment of the three dominant tree species over
 the five-year census interval) at a 20m by 20m scale. In other words, we
 divided the plot into 240 20 m by 20 m grid cells and tallied
 deaths/recruits in each grid cell. We used these grid cell summaries for
 all analyses we performed in the manuscript. Thus, this is how we present
 the demographic data in the .csv file entitled,
 "baltzer_jecol_scotty_tree_gricell_data.csv".
 In addition, we collected various abiotic variables at 20 m
 intervals throughout the entire plot (i.e. at 287 equally spaced points
 that form the corners of each 20 m by 20 m grid cell). We summarized major
 gradients in these variables using mixed principal components analysis. We
 then extracted the resulting first two principal components and averaged
 the four values associated with the corners of each 20 m by 20 m grid cell
 to come up with a grid cell average value for principal component 1 and 2
 (i.e. we took the average of each principal component at each of the four
 corners of each grid cell). We subsequently used these grid cell averages
 of principal components 1 and 2 to explain spatial variation in forest
 demographic dynamics. The averages of the first two principal components
 in each grid cell are included
 in "baltzer_jecol_scotty_tree_gricell_data.csv", while the raw
 measurements abiotic variables (at 287 points spaced 20 m apart) we used
 to perform the principal components analysis are included in
 "baltzer_jecol_env_post_data.csv".
description: Each data file comes has an associated metadata sheet
 ("baltzer_jecol_scotty_tree_gridcell_data_README.txt" and
 "baltzer_jecol_env_post_data_README.txt").