Recent Growth and Expansion of Birch Shrubs Across a Low Arctic Landscape in Continental Canada: Are These Responses More a Consequence of the Severely Declining Caribou Herd than of Climate Warming?

Abstract

The recent widespread expansion of deciduous shrubs across much of the Arctic has been largely attributed to climate warming. This study investigated decadal growth rates of dwarf birch (Betula glandulosa) across a low Arctic landscape in the continental interior of Canada. Detailed birch cover (100 m 2 replicate plots) and individual shrub stature measurement datasets for five representative habitat-types were compared between 2006 and 2016, and evaluated in relation to environmental characteristics. Furthermore, dendrochronologically-based annual growth rates were assessed in relation to the 20-year climate record. Birch height, lateral dimensions, and patch groundcover all increased 20-25% relative to 2006 values, but these increases were similar among the habitat-types. Together, the limited evidence of recent warming at this site, the absence of significant habitat-type growth rate differences, and the lack of correlation between annual climate and stem secondary growth strongly suggest that climate change was not the principal cause. Instead, we propose that release from caribou impacts following the recent severe herd decline may explain the net shrub growth. Individual shrub growth rates were correlated with soil nutrient availability, but the latter was highly variable, suggesting that growth rates are primarily determined by fine-scale rather than habitat-scale spatial heterogeneity in nutrient supply. Together, our results demonstrate that birch growth has been enhanced across a variety of habitat-types in the Daring Lake landscape over the decade since 2006, and suggest that the recent severe caribou herd declines may be at least as

Responses of low Arctic tundra plant species to experimental manipulations: Differences between abiotic and biotic factors and between short- and long-term effects

“…However, the Bathurst herd has experienced dramatic declines over the past several decades, from an estimated 470,000 in 1986 to 32,000 in 2009 and most recently to 8,200 in 2018 (Adamczewski et al 2019). Birch shrub expansion within some landscape patches and birch encroachment across caribou trail pathways illustrated by repeat photography indicate evidence of this caribou herd decline at Daring Lake (Andruko, Danby, and Grogan 2020). Personal observations at our study site also confirmed that only small groups of caribou were regularly observed from 2004 to 2009 (Zamin 2013), and just a few caribou passed through our research valley each summer from 2015 to 2017 (Q. Gu, personal observation).…”

Remotely sensed trends in vegetation productivity and phenology during population decline of the Bathurst caribou (Rangifer tarandus groenlandicus) herd

“…However, the loss of herbivory from the landscape undoubtedly also affected vegetation productivity, and it is challenging to infer causal mechanisms of either landscape change or population change from these trends alone (Andruko et al 2020). Determining the nature and magnitude of the relationship between these two phenomena will require spatial-temporal examination of caribou movement and habitat occupation in relation to the observed changes in productivity and phenology (e.g.…”

“…As such, the increases in maximum and time integrated EVI we observed could be occurring partly in response to, and not entirely because of, the widespread decline in caribou. This is the hypothesis advanced by Andruko et al (2020), who posit that the expansion of deciduous shrub cover observed in plots on the summer and fall range of the Bathurst herd from 2006-2016 was too great to be explained by the climate warming measured during the same period.…”

“…They concluded that a warmer climate related to reduced sea ice has increased tundra plant biomass, paradoxically leading to a decline in caribou populations because of an expansion of less palatable erect deciduous shrubs. Andruko et al (2020) found that cover and growth of dwarf birch increased in several different habitat types on the summer range of the Bathurst herd. It is a dominant species of erect shrub across much of the area beyond treeline but it is not a preferred food of caribou as it contains more phenols and less available protein than willows (Salix sp.…”

Recent greening driven by species-specific shrub growth characteristics in Nunatsiavut, Labrador, Canada

“…For example, increases in shrub cover have been shown to influence the composition of tundra vegetation (Fraser et al 2014, Myers-Smith et al 2019 and to alter biogeochemical feedbacks (Christiansen et al 2018, Wilcox et al 2019. Shrubification can also impede travel across tundra landscapes and can change the distribution and availability of culturally significant plants (e.g., berry plants; Cuerrier et al 2015;Anderson et al 2018; and wildlife (Andruko et al 2020). Investigations of the ecological processes that cause spatial variability in shrubification are important for predicting where future changes may occur in Arctic and Subarctic environments.…”