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Permafrost thaw in boreal peatlands is rapidly altering forest community composition

Citation

Baltzer, Jennifer; Patankar, Rajit; Dearborn, Katherine; Wallace, Cory A. (2021), Permafrost thaw in boreal peatlands is rapidly altering forest community composition, Dryad, Dataset, https://doi.org/10.5061/dryad.0cfxpnw0p

Abstract

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.

Methods

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".

Usage Notes

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").

Funding

Natural Sciences and Engineering Research Council of Canada

Global Water Futures

Northern Water Futures

Canada Foundation for Innovation

Canada Foundation for Climate and Atmospheric Sciences

Smithsonian Centre for Tropical Forest Science (ForestGEO)

Smithsonian Institution

Northern Water Futures

Smithsonian Centre for Tropical Forest Science (ForestGEO)