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 (~10m/year) and largest in Western Eurasia (~100m/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.
Highlights
Biomes and the transition zones between them are expected to shift under the influence of climate change because of species’ range shifts (Parmesan & Yohe, 2003; Settele et al, 2014)
We address the question of advance of the circumarctic forest–tundra ecotone (FTE) over the 20th century by the use of data on its behaviour assembled from a large number of sites distributed around the circumarctic region
We investigate whether the phenomenon of FTE advance is uniformly distributed in the circumarctic region, and whether the presence or absence of advancing behaviour and advance rates are related to 20th century climate history
Summary
Biomes and the transition zones between them are expected to shift under the influence of climate change because of species’ range shifts (Parmesan & Yohe, 2003; Settele et al, 2014). We consider the transition zone between the boreal forest and the arctic tundra at regional and circumarctic levels This zone, the forest–tundra ecotone (henceforward FTE), is Earth's areally most extensive ecological boundary (Callaghan et al, 2002) and is expected to undergo substantial climate-mediated change during the present century (Larsen et al, 2014). Future CCVs depend on the assumed climate change scenario, but typical values are of order 103–104 m/year over the 21st century (Settele et al, 2014) These values underpin predictions by ENMs of northward forest advances of up to 7,000–20,000 m/year and upslope shifts of 2–6 m/year, and huge (11%–50%) displacement of arctic tundra by boreal forest (Callaghan et al, 2005; Pearson et al, 2013). We consider whether rates of advance are consistent with typical CCV values and discuss the implications of climate history associations for the behaviour of the FTE in the 21st century in light of projected climate change
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