title: 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.
authors.affiliation: Department of Environmental Science, Alaska Pacific University, Anchorage, AK, USA.
authors.forename: Roman J
authors.initials: RJ
authors.lastname: Dial
authors.affiliation: Department of Environmental Science, Alaska Pacific University, Anchorage, AK, USA.
authors.forename: T Scott
authors.initials: TS
authors.lastname: Smeltz
authors.affiliation: Environment and Natural Resources Institute, University of Alaska Anchorage, Anchorage, AK, USA.
authors.forename: Patrick F
authors.initials: PF
authors.lastname: Sullivan
authors.affiliation: Department of Environmental Science, Alaska Pacific University, Anchorage, AK, USA.
authors.forename: Christina L
authors.initials: CL
authors.lastname: Rinas
authors.affiliation: Department of Environmental Science, Alaska Pacific University, Anchorage, AK, USA.
authors.forename: Katriina
authors.initials: K
authors.lastname: Timm
authors.affiliation: Department of Environmental Science, Alaska Pacific University, Anchorage, AK, USA.
authors.forename: Jason E
authors.initials: JE
authors.lastname: Geck
authors.affiliation: Department of Environmental Science, Alaska Pacific University, Anchorage, AK, USA.
authors.forename: S Carl
authors.initials: SC
authors.lastname: Tobin
authors.affiliation: Department of Environmental Science, Alaska Pacific University, Anchorage, AK, USA.
authors.forename: Trevor S
authors.initials: TS
authors.lastname: Golden
authors.affiliation: Kenai National Wildlife Refuge, Soldotna, AK, USA.
authors.forename: Edward C
authors.initials: EC
authors.lastname: Berg
mesh_terms: D000413:Alaska; D000531:Altitude; D000818:Animals; D001499:Bayes Theorem; D057231:Climate Change; D017753:Ecosystem; D065928:Forests; D012621:Seasons; D065929:Tundra
keywords: Alaska; alpine shrubs; climate warming; elevational rise; lapse rate; range shift; shrubs; treeline