Leading‐edge disequilibrium in alder and spruce populations across the forest–tundra ecotone

Abstract

The distribution and composition of Arctic vegetation are expected to shift with ongoing climate change. Global models generally predict northward shifts in high‐latitude ecotones, and analysis of remote sensing data shows widespread greening and changes in vegetation structure across the circumpolar Arctic. However, there are still uncertainties related to the timing of these shifts and variation among different plant functional types. In this paper, we investigate disequilibrium dynamics of green alder and white spruce in the Tuktoyaktuk Coastal Plain, NWT. We used high‐resolution air photographs captured in the 1970s and 2000s to quantify changes in the distribution and abundance of alder and spruce near their northern limits. We found increases in alder and spruce stem density over time, but no change in their range limits, indicating that both species are affected by leading‐edge disequilibrium. Low stand density and temperature limitation of reproduction along the northern margin likely contributed to observed disequilibrium in both species. We also observed the greatest change in species occupancy within a burned area, suggesting that the increased frequency of fire will play a significant role in the timing and magnitude of near‐term vegetation change.

Impacts of ecological succession and climate warming on permafrost aggradation in drained lake basins of the Tuktoyaktuk Coastlands, Northwest Territories, Canada

“…However, the rarity of these communities on younger sites, raises the possibility that the warmer climate, which has predominated since the 1950s, favors the development and persistence of shrub communities following drainage 100,101 . This is also consistent with a large body of evidence demonstrating that increasing regional air temperatures have driven shrub expansion in undisturbed tundra across the western Arctic 88,102–104 . More extensive sampling across a range of drained lake ages, coupled with ongoing monitoring, could help to resolve these uncertainties.…”

“…The landscape is predominantly hummocky uplands with mineral soils, 56 but organic deposits are also common in low low‐lying areas commonly associated with lacustrine basins 57,58 . Vegetation in the southern part of this area is characterized by the transition from open spruce woodlands to a landscape dominated by tundra 59 . Moving northward across this region, upright shrub tundra dominated by green alder, dwarf birch, and willows gradually shifts to graminoid and dwarf shrub‐dominated tundra 60,61 .…”

Biophysical Determinants of Shifting Tundra Vegetation Productivity in the Beaufort Delta Region of Canada

“…Average annual and summer temperatures across the Beaufort Delta region have increased by 3. 5°C and 1.9°C, respectively, between 1926(Travers-Smith and Lantz 2020 with largely homogenous warming across the region (Vincent and others 2015). Widespread increases in tundra productivity are likely a response to the direct and indirect effects of this warming.…”

“…Increasing temperatures in the Arctic (Arctic Monitoring and Assessment Programme [AMAP] 2004; Serreze and others 2009; Johannessen and others 2016; Davy and others 2018), while regionally variable in magnitude, are driving rapid changes to the structure and composition of tundra vegetation. Plot-based and fine-scale remote sensing studies have documented shifts in the dominant vegetation, with deciduous shrubs now proliferating in what was once lichen-and graminoid-dominated tundra (Elmendorf and others 2012;Ropars and Boudreau 2012;Lantz and others 2013;Moffat and others 2016;Travers-Smith and Lantz 2020). Vegetation productivity can also be measured at broad scales using multispectral satellite vegetation indices (Gao and others 2000) such as the Enhanced Vegetation Index (EVI) and the Normalized Difference Vegetation Index (NDVI).…”

A global framework for linking alpine‐treeline ecotone patterns to underlying processes

“…abrupt treelines turning diffuse and shifting upwards due to sudden tree establishment after passing a climatic threshold – Danby and Hik 2007a, Elliott 2012, Camarero et al 2015 – or after a change in land use – Vitali et al 2017). Thereby, revisits do not need to imply resampling, or even field visits, but could also be based on high‐resolution remote‐sensing (aerial or ground‐based) images, including images from the past (Danby and Hik 2007b, Lutz et al 2013, Greenwood et al 2014, Travers‐Smith and Lantz 2020).…”