Satellite observations document trends consistent with a boreal forest biome shift

Abstract

The boreal forest biome is a major component of Earth's biosphere and climate system that is projected to shift northward due to continued climate change over the coming century. Indicators of a biome shift will likely first be evident along the climatic margins of the boreal forest and include changes in vegetation productivity, mortality, and recruitment, as well as overall vegetation greenness. However, the extent to which a biome shift is already underway remains unclear because of the local nature of most field studies, sparsity of systematic ground‐based ecological monitoring, and reliance on coarse resolution satellite observations. Here, we evaluated early indicators of a boreal forest biome shift using four decades of moderate resolution (30 m) satellite observations and biogeoclimatic spatial datasets. Specifically, we quantified interannual trends in annual maximum vegetation greenness using an ensemble of vegetation indices derived from Landsat observations at 100,000 sample sites in areas without signs of recent disturbance. We found vegetation greenness increased (greened) at 38 [29, 42] % and 22 [15, 26] % of sample sites from 1985 to 2019 and 2000 to 2019, whereas vegetation greenness decreased (browned) at 13 [9, 15] % and 15 [13, 19] % of sample sites during these respective periods [95% Monte Carlo confidence intervals]. Greening was thus 3.0 [2.6, 3.5] and 1.5 [0.8, 2.0] times more common than browning and primarily occurred in cold sparsely treed areas with high soil nitrogen and moderate summer warming. Conversely, browning primarily occurred in the climatically warmest margins of both the boreal forest biome and major forest types (e.g., evergreen conifer forests), especially in densely treed areas where summers became warmer and drier. These macroecological trends reflect underlying shifts in vegetation productivity, mortality, and recruitment that are consistent with early stages of a boreal biome shift.

Sufficient conditions for rapid range expansion of a boreal conifer

“…The most rapid modern expansion of a boreal tree into Arctic tundra so far reported is for mountain birch (Betula pubescens), a small deciduous-broadleaf tree advancing across Fennoscandia in northwest Eurasia at rates similar to those seen for white spruce here 5 . Arctic-wide greening trends from high-resolution satellite imagery during 1985-2019 also suggest a global boreal biome shift northwards as temperatures continue to rise 49 . Over time, warming-both directly 16 and indirectly [34][35][36][37] -will support higher-latitude tree establishment, growth, reproduction and dispersal, particularly with increased southerly winds, greater snowfall and increases in nutrient availability, all induced by warming.…”

Letter to the editor on “Satellite observations document trends consistent with a boreal biome shift”

“…LETTER TO THE EDITOR they should be used in concert with ground truth and high-resolution imagery because unsupervised classifications can result in large errors (Timoney & Mamet, 2020) and because the mosaic of zonal tree and upland tundra vegetation can occur in patches that are smaller than imagery pixel sizes (McLeod, 2001;Timoney et al, 1993). I concur with Berner and Goetz (2022) when they note that improved monitoring will be important over the coming decades given climatic projections of increasing extreme events that could bring rapid and unanticipated changes to the boreal forest.…”

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Berner and Goetz (2022) reported the results of a study of changes in vegetation greenness using 30 m pixel Landsat satellite imagery comprising a total of 90 km 2 across a circumboreal 8.4 million km 2 . Prior to analyses, the authors removed a total of 6.7 million km 2 (44.4% of the 15.1 million km 2 study region) by masking out unvegetated, human footprint, and recently disturbed areas.

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Response to letter to the editor on “Satellite observations document trends consistent with a boreal biome shift”

“…In his Letter to the Editor, Timoney (2022) suggests that Berner and Goetz's (2022) remote sensing assessment of an emerging boreal forest biome shift is “problematic” due to the sample size, exclusion of recently burned areas, and lack of ground validation data, as well as inconsistent with a recent study focused on western and central Canada (Timoney & Mamet, 2020). Here, we address these concerns.…”

“…To improve clarity, a 9 × 9 pixel majority filter was applied to the disturbance data. Adapted with permission from Berner and Goetz (2022). (e.g., Tyukavina et al, 2015;Wulder et al, 2012).…”