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.

Presence of rapidly degrading permafrost plateaus in south-central Alaska

“…GPR survey data confirmed the presence of a continuous talik at the Browns Lake site (Fig. 10); however, we were unable to image the base of the permafrost using solely GPR, as similarly described by Lewkowicz et al (2011). Based on visual interpretation of the permafrost peat core acquired at the PF-BL-6 site in February 2016, the permafrost deposit consists mainly of ice-rich frozen peat with well-developed organic-matrix porphyritic and microlenticular cryostructures and some lay-ered ice lenses, and belt-like and suspended cryostructures (Figs.…”

“…Mapping forested-plateau features and their change over time is a common method for detection of permafrost thaw in boreal wetlands. The land cover change associated with conversion of a forested permafrost plateau to a lake or herbaceous wetland (i.e., bog or fen) is readily detectable in high-resolution remotely sensed imagery (Thie, 1974;Camill and Clark, 1998;Osterkamp et al, 2000;Jorgenson et al, 2001Jorgenson et al, , 2008aPayette et al, 2004;Quinton et al, 2011;Lara et al, 2016).…”

“…In general, permafrost in Alaska has warmed between 0.3 and 6.0 • C since ground temperature measurements began between the 1950 and 1980s (Lachenbruch and Marshall, 1986;Romanovsky and Osterkamp, 1995;Osterkamp, 2007). Warming and thawing of near-surface permafrost may lead to widespread terrain instability in ice-rich permafrost in the Arctic (Jorgenson et al, 2006;Lantz and Kokelj, 2008;Gooseff et al, 2009;Jones et al, 2015;Liljedahl et al, 2016) and the subArctic Jorgenson and Osterkamp, 2005;Lara et al, 2016). Such land surface changes can impact vegetation, hydrology, aquatic ecosystems, and soil-carbon dynamics (Grosse et al, 2011;Jorgenson et al, 2013;Kokelj et al, 2015;O'Donnell et al, 2011;Schuur et al, 2008;Vonk et al, 2015).…”

“…Such land surface changes can impact vegetation, hydrology, aquatic ecosystems, and soil-carbon dynamics (Grosse et al, 2011;Jorgenson et al, 2013;Kokelj et al, 2015;O'Donnell et al, 2011;Schuur et al, 2008;Vonk et al, 2015). For example, in boreal peatlands, thaw of ice-rich permafrost often converts forested permafrost plateaus into lake and wetland bog and fen complexes (Camill, 1999;Jorgenson et al, 2001;Payette et al, 2004;Kuhry, 2008, 2011;Quinton et al, 2011;Jorgenson et al, 2012;Kanevskiy et al, 2014;Swindles et al, 2015;Lara et al, 2016). Furthermore, the transition from permafrost peatlands to thawed or only seasonally frozen peatlands can have a positive or a negative feedback on regional and global carbon cycles depending on permafrost conditions and differential effects of thaw on net primary productivity and heterotrophic respiration (Turetsky et al, 2007;Swindles et al, 2015), as well as on the degree of loss of the former deep permafrost carbon pool .…”

In situ nuclear magnetic resonance response of permafrost and active layer soil in boreal and tundra ecosystems

“…These forests compose at least half of all the land area underlain by permafrost, and degradation of ice-rich permafrost can have severe consequences on these ecosystems, resulting in the conversion of forestland into bogs or fens in lowland environments Lara et al, 2016). Similar water impoundment can occur with subsidence in tundra ecosystems.…”