title: Thermokarst rates intensify due to climate change and forest fragmentation in an Alaskan boreal forest lowland.
abstract: Lowland boreal forest ecosystems in Alaska are dominated by wetlands comprised of a complex mosaic of fens, collapse-scar bogs, low shrub/scrub, and forests growing on elevated ice-rich permafrost soils. Thermokarst has affected the lowlands of the Tanana Flats in central Alaska for centuries, as thawing permafrost collapses forests that transition to wetlands. Located within the discontinuous permafrost zone, this region has significantly warmed over the past half-century, and much of these carbon-rich permafrost soils are now within ~0.5 °C of thawing. Increased permafrost thaw in lowland boreal forests in response to warming may have consequences for the climate system. This study evaluates the trajectories and potential drivers of 60 years of forest change in a landscape subjected to permafrost thaw in unburned dominant forest types (paper birch and black spruce) associated with location on elevated permafrost plateau and across multiple time periods (1949, 1978, 1986, 1998, and 2009) using historical and contemporary aerial and satellite images for change detection. We developed (i) a deterministic statistical model to evaluate the potential climatic controls on forest change using gradient boosting and regression tree analysis, and (ii) a 30 × 30 m land cover map of the Tanana Flats to estimate the potential landscape-level losses of forest area due to thermokarst from 1949 to 2009. Over the 60-year period, we observed a nonlinear loss of birch forests and a relatively continuous gain of spruce forest associated with thermokarst and forest succession, while gradient boosting/regression tree models identify precipitation and forest fragmentation as the primary factors controlling birch and spruce forest change, respectively. Between 1950 and 2009, landscape-level analysis estimates a transition of ~15 km² or ~7% of birch forests to wetlands, where the greatest change followed warm periods. This work highlights that the vulnerability and resilience of lowland ice-rich permafrost ecosystems to climate changes depend on forest type.
authors.affiliation: Institute of Arctic Biology, University of Alaska, Fairbanks, AK, 99775, USA.
authors.forename: Mark J
authors.initials: MJ
authors.lastname: Lara
authors.affiliation: Institute of Arctic Biology, University of Alaska, Fairbanks, AK, 99775, USA.
authors.forename: Hélène
authors.initials: H
authors.lastname: Genet
authors.affiliation: U.S. Geological Survey, Alaska Cooperative Fish and Wildlife Research Unit, University of Alaska Fairbanks, Fairbanks, AK, 99775, USA.
authors.forename: Anthony D
authors.initials: AD
authors.lastname: McGuire
authors.affiliation: Institute of Arctic Biology, University of Alaska, Fairbanks, AK, 99775, USA.
authors.forename: Eugénie S
authors.initials: ES
authors.lastname: Euskirchen
authors.affiliation: Institute of Arctic Biology, University of Alaska, Fairbanks, AK, 99775, USA.
authors.forename: Yujin
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: Institute of Arctic Biology, University of Alaska, Fairbanks, AK, 99775, USA.
authors.forename: Dana R N
authors.initials: DR
authors.lastname: Brown
authors.affiliation: Alaska Ecoscience, Fairbanks, AK, 99775, USA.
authors.forename: Mark T
authors.initials: MT
authors.lastname: Jorgenson
authors.affiliation: Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK, 99775, USA.
authors.forename: Vladimir
authors.initials: V
authors.lastname: Romanovsky
authors.affiliation: Scenarios Network for Alaska and Arctic Planning, International Arctic Research Center, University of Alaska Fairbanks, Fairbanks, AK, 99775, USA.
authors.forename: Amy
authors.initials: A
authors.lastname: Breen
authors.affiliation: International Arctic Research Center, University of Alaska Fairbanks, Fairbanks, Alaska, 99775, USA.
authors.forename: William R
authors.initials: WR
authors.lastname: Bolton
mesh_terms: D000413:Alaska; D029662:Betula; D057231:Climate Change; D065928:Forests; D065946:Permafrost; D010781:Photography; D028222:Picea; D011891:Rain; D058998:Remote Sensing Technology; D013696:Temperature; D053833:Wetlands
keywords: Tanana Flats; birch; boreal; climate change; interior Alaska; permafrost; spruce; succession; thermokarst; wetlands