Shrubline but not treeline advance matches climate velocity in montane ecosystems of south‐central Alaska

Abstract

Tall shrubs and trees are advancing into many tundra and wetland ecosystems but at a rate that often falls short of that predicted due to climate change. For forest, tall shrub, and tundra ecosystems in two pristine mountain ranges of Alaska, we apply a Bayesian, error-propagated calculation of expected elevational rise (climate velocity), observed rise (biotic velocity), and their difference (biotic inertia). We show a sensitive dependence of climate velocity on lapse rate and derive biotic velocity as a rigid elevational shift. Ecosystem presence identified from recent and historic orthophotos ~50 years apart was regressed on elevation. Biotic velocity was estimated as the difference between critical point elevations of recent and historic logistic fits divided by time between imagery. For both mountain ranges, the 95% highest posterior density of climate velocity enclosed the posterior distributions of all biotic velocities. In the Kenai Mountains, mean tall shrub and climate velocities were both 2.8 m y(-1). In the better sampled Chugach Mountains, mean tundra retreat was 1.2 m y(-1) and climate velocity 1.3 m y(-1). In each mountain range, the posterior mode of tall woody vegetation velocity (the complement of tundra) matched climate velocity better than either forest or tall shrub alone, suggesting competitive compensation can be important. Forest velocity was consistently low at 0.1-1.1 m y(-1), indicating treeline is advancing slowly. We hypothesize that the high biotic inertia of forest ecosystems in south-central Alaska may be due to competition with tall shrubs and/or more complex climate controls on the elevational limits of trees than tall shrubs. Among tall shrubs, those that disperse farthest had lowest inertia. Finally, the rapid upward advance of woody vegetation may be contributing to regional declines in Dall's sheep (Ovis dalli), a poorly dispersing alpine specialist herbivore with substantial biotic inertia due to dispersal reluctance.

Thermal segregation drives patterns of alder and willow expansion in a montane ecosystem subject to climate warming

“…In North America and Eurasia, tall‐shrubs have rapidly increased in abundance while extending their geographical range both northward in latitude (Naito & Cairns ) and upward in elevation (Dial et al . , ; Hallinger, Manthe & Wilmking ; Rundqvist et al . ; Stöckli et al .…”

When Winners Become Losers: Predicted Nonlinear Responses of Arctic Birds to Increasing Woody Vegetation

“…Significant climate-driven changes have already been documented in arctic regions, such as longer growing seasons, thawing permafrost, increased vegetation productivity, and changes in the frequency, intensity, and geographic scale of fires [7–10]. Numerous studies have indicated that deciduous shrubs are becoming increasingly dominant in arctic regions [11–13]. …”

“…Finally, the shrubline may advance northward or upslope in response to climatic changes, allowing colonization by shrub-nesting birds into areas previously out of a species’ range or elevational limit [11,17]. Infilling, upslope movement, expansion along drainages, and increases in shrub height have been noted across a number of tundra ecosystems [11,13]. As shrubs become more dominant in tundra ecosystems, we expect that bird species will demonstrate variable levels of tolerance to changes.…”

Ecotone response to climatic variability depends on stress gradient interactions

“…de Dios et al showed how a temporal switch in interaction could be a factor in ecotone response to climate change [20]. Rates of response to climate change may also be tied to species interactions [21]. Moreover, treeline dynamics have been shown to respond to interannual climatic variability [22].…”