Is subarctic forest advance able to keep pace with climate change?

Abstract

Recent climate warming and scenarios for further warming have led to expectations of rapid movement of ecological boundaries. Here we focus on the circumarctic forest–tundra ecotone (FTE), which represents an important bioclimatic zone with feedbacks from forest advance and corresponding tundra disappearance (up to 50% loss predicted this century) driving widespread ecological and climatic changes. We address FTE advance and climate history relations over the 20th century, using FTE response data from 151 sites across the circumarctic area and site‐specific climate data. Specifically, we investigate spatial uniformity of FTE advance, statistical associations with 20th century climate trends, and whether advance rates match climate change velocities (CCVs). Study sites diverged into four regions (Eastern Canada; Central and Western Canada and Alaska; Siberia; and Western Eurasia) based on their climate history, although all were characterized by similar qualitative patterns of behaviour (with about half of the sites showing advancing behaviour). The main associations between climate trend variables and behaviour indicate the importance of precipitation rather than temperature for both qualitative and quantitative behaviours, and the importance of non‐growing season as well as growing season months. Poleward latitudinal advance rates differed significantly among regions, being smallest in Eastern Canada (~10 m/year) and largest in Western Eurasia (~100 m/year). These rates were 1–2 orders of magnitude smaller than expected if vegetation distribution remained in equilibrium with climate. The many biotic and abiotic factors influencing FTE behaviour make poleward advance rates matching predicted 21st century CCVs (~103–104 m/year) unlikely. The lack of empirical evidence for swift forest relocation and the discrepancy between CCV and FTE response contradict equilibrium model‐based assumptions and warrant caution when assessing global‐change‐related biotic and abiotic implications, including land–atmosphere feedbacks and carbon sequestration.

Mountain treelines climb slowly despite rapid climate warming

“…A recent subarctic treeline study mainly focused on the treeline sites located close to (> 57° N) or beyond 60° N ( c . 93%, Rees et al., 2020). Mountain treelines in subarctic regions contained both alpine and arctic treelines (Danby & Hik, 2007).…”

“…12° N, whereas all other sites are situated beyond 27° N. To enable comparisons of our findings with those from Rees et al. (2020), we divided our hemispheric dataset into two sub‐regions (subarctic region, north of 60° N and temperate region, south of 60° N) for regional scale analyses and inter‐comparisons.…”

“…Moreover, this study showed that diffuse treelines, characterized by a gradual decrease in tree density and height with elevation, were particularly sensitive to the temperature increase as compared with abrupt treelines that show a sharp decrease in tree density and height (Harsch et al., 2009). A recent study further highlighted the importance of precipitation in driving the shift of subarctic forest‐tundra ecotones (Rees et al., 2020). These results indicated that treeline shift rates and their climatic drivers may differ between subarctic and other ecoregions in the Northern Hemisphere.…”

“…Due to low solar angles and varied topography, most mountain treelines in subarctic regions contain characteristics of both alpine and latitudinal treelines (Danby & Hik, 2007). Moreover, most of the available treeline shift studies are located close to or beyond 60° N in the subarctic region (Rees et al., 2020). Hence, we chose 60° N as the threshold between the temperate and subarctic sub‐regions.…”

“…To minimize potential biases from non‐climatic drivers, we ensured that there were no large disturbances such as fire and logging in the surroundings of the selected treeline sites. Inspired by a growing body of literature suggesting that water availability and demand limit trees even towards their uppermost distribution edge (Biondi, 2001; Rees et al., 2020; Sigdel et al., 2018), we tested the hypothesis that precipitation mediates the sensitivity of treeline shift rates to climate warming at hemispheric scales.…”