Soil surface organic layers in Arctic Alaska: Spatial distribution, rates of formation, and microclimatic effects

Abstract

Organic layers of living and dead vegetation cover the ground surface in many permafrost landscapes and play important roles in ecosystem processes. These soil surface organic layers (SSOLs) store large amounts of carbon and buffer the underlying permafrost and its contained carbon from changes in aboveground climate. Understanding the dynamics of SSOLs is a prerequisite for predicting how permafrost and carbon stocks will respond to warming climate. Here we ask three questions about SSOLs in a representative area of the Arctic Foothills region of northern Alaska: (1) What environmental factors control the thickness of SSOLs and the carbon they store? (2) How long do SSOLs take to develop on newly stabilized point bars? (3) How do SSOLs affect temperature in the underlying ground? Results show that SSOL thickness and distribution correlate with elevation, drainage area, vegetation productivity, and incoming solar radiation. A multiple regression model based on these correlations can simulate spatial distribution of SSOLs and estimate the organic carbon stored there. SSOLs develop within a few decades after a new, sandy, geomorphic surface stabilizes but require 500-700 years to reach steady state thickness. Mature SSOLs lower the growing season temperature and mean annual temperature of the underlying mineral soil by 8 and 3°C, respectively. We suggest that the proximate effects of warming climate on permafrost landscapes now covered by SSOLs will occur indirectly via climate's effects on the frequency, extent, and severity of disturbances like fires and landslides that disrupt the SSOLs and interfere with their protection of the underlying permafrost.

Rainfall–runoff responses on Arctic hillslopes underlain by continuous permafrost, North Slope, Alaska, USA

“…In contrast, the hillslope locations store water in the soil column, at least temporarily, increasing the local hydraulic gradient towards the water track. This inference is supported by observations that the soil organic layer, which is an order of magnitude more hydraulically conductive than the mineral soil (Hinzman et al, ), is thicker in the water tracks than the nontrack hillslope at our sites and in many water tracks in this region (Baughman, ).…”

Climate‐driven ecological stability as a globally shared cause of Late Quaternary megafaunal extinctions: the Plaids and Stripes Hypothesis

“…Slow rates of soil development can delay primary succession by decades to centuries (Matthews, 1982). For instance, near the Klutlan Glacier in the Yukon, soil development was still proceeeding apace 250 years after deglaciation (Jacobson & Birks, 1980), and in Arctic Alaska, the development of a steady state, organic soil horizon requires over 500 years (Baughman et al, 2015). Even in temperate rainforests, soil development continues more than 5000 years after deglaciation (Klaar et al, 2015).…”

Life and extinction of megafauna in the ice-age Arctic

“…The resulting combination of increased moisture and colder temperature retards decomposition, reduces nutrient availability, and encourages more peat to form (35). Many regions in northern Eurasia and northwestern North America that supported mammoth steppe during the ice age are today blanketed by peat-rich plant communities (36) incapable of supporting large biomasses of grazing mammals.…”

“…A short-lived bloom of ruderal plant species provided a grazing bonanza during this transition period, when both soils and climate were relatively warm and moist. Megafauna populations crashed after widespread paludification occurred and moist acidic tundra vegetation became widespread, a process requiring ∼1,000 y after an interstadial began (35). Mammoth steppe consisting of sparse grass and forb vegetation was widespread during the colder, drier stadials, when megafauna existed at some intermediate level of abundance.…”

“…Today, grasses and forbs flourish at tundra sites where soils and vegetation have been disturbed (38,39) but only until paludification recurs. On the North Slope, 500-700 y are required for peat to accumulate to a steady-state thickness on a previously bare surface of a mineral soil (35). During the ice age, short-lived pulses of high-quality range occurring before either paludification became widespread or full glacial climate resumed may have created transitory bonanzas for megafaunal grazers, which resulted in short-lived peaks in the relative abundance of these animals on the landscape.…”