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- Copyright
- Copyright 2013 IOP Publishing Ltd
- eISSN
- 1748-9326
- DOI
- 10.1088/1748-9326/8/3/035017
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Abstract
The diversity of ecosystems across boreal landscapes, successional changes after disturbance and complicated permafrost histories, present enormous challenges for assessing how vegetation, water and soil carbon may respond to climate change in boreal regions. To address this complexity, we used a chronosequence approach to assess changes in vegetation composition, water storage and soil organic carbon (SOC) stocks along successional gradients within four landscapes: (1) rocky uplands on ice-poor hillside colluvium, (2) silty uplands on extremely ice-rich loess, (3) gravelly–sandy lowlands on ice-poor eolian sand and (4) peaty–silty lowlands on thick ice-rich peat deposits over reworked lowland loess. In rocky uplands, after fire permafrost thawed rapidly due to low ice contents, soils became well drained and SOC stocks decreased slightly. In silty uplands, after fire permafrost persisted, soils remained saturated and SOC decreased slightly. In gravelly–sandy lowlands where permafrost persisted in drier forest soils, loss of deeper permafrost around lakes has allowed recent widespread drainage of lakes that has exposed limnic material with high SOC to aerobic decomposition. In peaty–silty lowlands, 2–4 m of thaw settlement led to fragmented drainage patterns in isolated thermokarst bogs and flooding of soils, and surface soils accumulated new bog peat. We were not able to detect SOC changes in deeper soils, however, due to high variability. Complicated soil stratigraphy revealed that permafrost has repeatedly aggraded and degraded in all landscapes during the Holocene, although in silty uplands only the upper permafrost was affected. Overall, permafrost thaw has led to the reorganization of vegetation, water storage and flow paths, and patterns of SOC accumulation. However, changes have occurred over different timescales among landscapes: over decades in rocky uplands and gravelly–sandy lowlands in response to fire and lake drainage, over decades to centuries in Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. 1748-9326/13/035017C13$33.00 1 2013 IOP Publishing Ltd Printed in the UK Environ. Res. Lett. 8 (2013) 035017 M Torre Jorgenson et al peaty–silty lowlands with a legacy of complicated Holocene changes, and over centuries in silty uplands where ice-rich soil and ecological recovery protect permafrost. Keywords: landscape change, vegetation, water storage, soil carbon, permafrost, fire S Online supplementary data available from stacks.iop.org/ERL/8/035017/mmedia 1. Introduction to positive feedbacks on soil temperature and permafrost stability related to albedo effects from water impoundment, Permafrost degradation alters ecosystem structure and or negative feedbacks from vegetation succession and organic function through changes in microtopography associated with matter accumulation that promote surface stabilization and differential thaw settlement (Osterkamp et al 2009), surface permafrost recovery (Jorgenson et al 2006, 2010). The and subsurface hydrology (Walvoord et al 2012, Quinton magnitude of the feedback on soil temperatures can be et al 2009, Yoshikawa and Hinzman 2003), lake levels and twice as large as air temperatures and as a result, most of drainage (Jones et al 2011, Jepsen et al 2013), biogeochemical the permafrost in boreal Alaska is considered ecosystem- processing and export of carbon and nutrients (Striegl et al driven permafrost (Shur and Jorgenson 2007). In boreal 2005, Walvoord and Striegl 2007), and vegetation (Racine Alaska and Yukon, some permafrost has persisted through et al 1998, Jorgenson et al 2001). Permafrost degradation is many interglacial periods over hundreds of thousands of of global concern because even small losses of the estimated years (Froese et al 2008). 1400–1850 Pg of soil organic carbon (SOC) in high-latitude Surface topography, water storage, and hydrologic flow soils could affect trace gas concentrations of the atmosphere paths are immediately altered by permafrost thaw. Thawing of (Schuur et al 2008, McGuire et al 2009, Tarnocai et al permafrost in upland watersheds, even if ice contents are low, 2009, Grosse et al 2011). A substantial portion of SOC is can lead to shifting surface water flow to deeper groundwater in the upper permafrost and could become susceptible to flow paths and result in changes in seasonal discharge patterns decomposition after thaw (Harden et al 2012b). The rate (Walvoord and Striegl 2007, Lyon and Destouni 2010, Koch that permafrost thaws and the surface subsides, however, et al 2013a) and biogeochemical processing and export of is affected by the high variability in ground ice in soils dissolved organic carbon (Striegl et al 2005, O’Donnell et al (Kreig and Reger 1982, Jorgenson et al 2008, Kanevskiy 2012a, Koch et al 2013b). In lowland regions, changes in et al 2013), which in turn is the product of the history permafrost can lead to changes in lake extent from lateral of permafrost aggradation and degradation over periods of or subsurface drainage (Jones et al 2011, Yoshikawa and decades to hundreds of thousands of years (Shur 1988, Froese Hinzman 2003, Jepsen et al 2013). On slopes and lowlands, et al 2008). Thus, we would expect that vegetation, water differential permafrost thaw can lead to fragmentation of and SOC responses could be highly variable among the integrated networks into fill and spill flow paths, caused by diverse ecological and permafrost conditions across boreal impoundment of water in thermokarst pits and bogs (Quinton landscapes. et al 2009, Woo 2012). The diversity of boreal ecosystems, and their vegetation Carbon accumulation and distribution in permafrost- composition, is due to large gradients in climate, topography, affected soils depend on complicated interactions of climate, surficial materials, hydrologic regimes, soil properties, geomorphology, vegetation composition, microbial activity, permafrost, disturbance and time (Van Cleve et al 1983, and temperature and moisture as affected by permafrost Viereck 1992). These co-varying biophysical characteristics (Robinson and Moore 1999, Turetsky et al 2007, Schuur et al have been classified into 40–70 ecosystem types across boreal 2008, Wickland et al 2010, Johnson et al 2011, Harden et al Alaska, and have been aggregated into broader soil landscapes 2012b, Quillet et al 2013). Short-term flux studies have shown that have characteristic successional trends after disturbance substantial fluctuations in annual carbon budgets in tundra and (Jorgenson et al 1999, 2009). Both fire and permafrost are forest ecosystems and the high spatial and temporal variability dominant agents in affecting successional processes (Chapin in ground-based carbon budgets has led to substantial et al 2006). This diversity presents an enormous challenge uncertainty as to whether these ecosystems are acting as to assessing regional trends in ecological changes after long-term sources or sinks to the atmosphere (McGuire et al permafrost degradation. 2009). Fire contributes to this uncertainty by burning portions Ground ice content in permafrost ranges widely, from of the surface organic layers, depending on fire severity, negligible in weathered bedrock and eolian sand to occupying and accelerating permafrost thaw (Harden et al 2006, 2012b, 80–90% of the top 30 m of permafrost in extremely Hinzman et al 2003, O’Donnell et al 2009, 2011a). ice-rich loess deposited during the Pleistocene (Kreig and To evaluate how the variations among vegetation, Reger 1982, Kanevskiy et al 2011, 2013). Differential hydrology, soils, and permafrost affect changes in SOC, we: thawing of permafrost can lead to a large diversity of (1) classified and aggregated the ecological characteristics thermokarst terrain depending on surficial materials, ground of diverse successional stages along chronosequences of ice morphology and volume (Jorgenson and Osterkamp 2005). boreal ecosystem development after disturbance from fire and Changing topography, hydrology and nutrient availability lead permafrost thaw into broad soil landscapes; (2) compared 2 Environ. Res. Lett. 8 (2013) 035017 M Torre Jorgenson et al Figure 2. Landsat satellite images of the four soil landscapes showing: rocky uplands along Taylor Highway with stable surfaces subject to frequent fires; silty uplands near Hess Creek with mostly stable surfaces with occasional deep thermokarst lakes; gravelly–sandy lowlands on the Yukon Flats with widespread lake drainage associated with thawing permafrost; and peaty–silty Figure 1. Study sites in relation to surficial geology and major soil lowlands on the Innoko Flats with widespread thermokarst. landscapes. Surficial materials are grouped into rock uplands Sampling locations shown as white dots. (browns), silty uplands (yellow), gravelly–sandy lowlands (orange), and peaty–silty lowlands (greens). Other landscapes, such as riverine (red) and glaciated (blue), were not included in this study. hierarchical system were assigned to surficial geology classes Boundaries of permafrost zones (gray) are based on Jorgenson et al (2008). Study areas include Innoko (I), Koyukuk (K), Hess Creek mapped by Karlstrom (1964). We established study areas (H), Nome Creek (N), Twelve-mile Lake (M), and Taylor in each one of these landscapes in remote areas across Highway (T). the Interior (figures 1 and 2). Within each study area, we established a chronosequence of successional stages that varied with time since disturbance. We tried to establish permafrost characteristics across landscapes; (3) assessed at least three replicate plots (5 m radius) in each of the changes in water storage and used the results to develop 3–6 successional stages at each study area, with each plot conceptual models of hydrologic reorganization caused by (site) located in a separate disturbance patch, but number of thawing; and (4) compared SOC stocks among successional replicates varied depending on availability of successional stages and across landscapes. This field study was part of a stages (see supplemental materials table S1 available at broader effort to assess effects of permafrost degradation on stacks.iop.org/ERL/8/035017/mmedia). In upland areas, we landscape evolution (Shur et al 2012), land cover changes (Lu used maps of historic fire perimeters to identify stands that and Zhuang 2011), soil carbon dynamics (O’Donnell et al varied with time since burn and thaw, and plots were widely 2011b, 2011a, 2012c, Harden et al 2012a, 2012b), laboratory distributed along the road system and not grouped along incubations and trace gas fluxes (Wickland et al 2010, surveyed transects. For the Hess Creek (N65.79, W149.49) Johnston et al 2012), water and aqueous carbon fluxes and and Taylor Highway (N63.46, W142.49) upland study areas, permafrost hydrology (Koch et al 2013a, 2013b), and thermal chronosequences included a young (0–10 years, after 2003, and trace gas modeling (Jiang et al 2012, Tang and Zhuang 2004 fires), intermediate (10–40 years, after 1990, 1993 2011, Lu and Zhuang 2012). and 1986 fires), old (40–100 years, 1964 fire) and mature forests with permafrost as reference (burns > 100 years 2. Methods ago), while at Nome Creek (N65.35 W146.92) there were only young burns and old forests. In lowland areas, plots The study was designed to assess changes in vegetation, were established along surveyed transects so that ground- and hydrology, and soil after permafrost degradation across water-surface elevations could be measured to assess effects a range of soil landscapes and successional stages that of thermokarst. In gravelly–sandy lowlands at Twelve-Mile dominate the boreal forest ecosystems in central Alaska. Lake (N66.45 W145.56), we had the unusual situation of We stratified the landscapes into four soil landscapes recently drained lakes after permafrost thaw, so we grouped that differentiated physiography and soil textures using late-successional mixed and needleleaf forests into old forests, the hierarchical ecological land classification approach of and grouped recently exposed lake sediments into recently Jorgenson et al (1999, 2009). Soil landscapes from this drained margins (10–40 year old shrub communities), and 3 Environ. Res. Lett. 8 (2013) 035017 M Torre Jorgenson et al herbaceous vegetation and barren areas in the lower centers carbonates. For alkaline soils with carbonates, soils were (<10 years old) into recently drained centers. In peaty–silty acid fumigated before analysis for total carbon. Radiocarbon lowlands, we used satellite imagery to identify regions dating was done by accelerator mass spectrometry at UC Irvine or Lawrence Livermore National Laboratory of extensive thermokarst bog and fen development. For and resulting C data are described in O’Donnell et al the Koyukuk (N65.19 W156.64) and Innoko Flats (N63.57 (2011a, 2012a, 2012b, 2012c) and Johnston et al (2012). W157.73), we classified landforms and vegetation stages as Data analysis involved: creating a relational database young bogs (10–40 years), intermediate bogs (30–100 years), to link site, stratigraphy, physical, and chemical data; gap old bogs (300–1700 years), and old forest with permafrost filling procedures to estimate properties for missing values; (>100 years) as a reference. We also sampled dry margins calculation of carbon densities for 1459 soil layers, and carbon on the permafrost plateaus to assess the effects of drying via and water stocks for 95 profiles (82 profiles >1 m, 64 >2 m); lateral drainage of suprapermafrost groundwater. and data summaries by successional stage and soil landscape. Field sampling at the six study areas was conducted We assigned soil properties to layers with missing data from: during late summer over a five-year period. At each (1) layers above or below the layer if horizon designation area, we sampled topography, hydrology, soil stratigraphy, and soil texture were the same (affecting 190 layers); or and vegetation. For topography in lowland areas, we (2) mean characteristics calculated for 45 combinations of measured ground-surface elevations every 1–2 m along genetic horizons, simplified textures and peat types (affecting 300–500 m transects using an autolevel. Water-surface 339 layers). We also split and duplicated layers that crossed elevations and water-table depth (above/below ground 1 m increments so data could be summarized by depth surface) were measured where water was present, and thaw increments across all sites. To allow comparison of SOC depths were measured with a metal probe at 1–2 m intervals across all sites, we extended deep layers (29 short for <0:5 m, on permafrost plateaus and at 5 m intervals in thermokarst 24 long for 1 m) in rocky soils assuming that similar bogs. In upland areas, where plots were widely scattered properties for gravelly or channery materials continued at in differing fire patches, elevations were measured with a depth. SOC density for each layer and SOC stocks for each recreational grade GPS by averaging WAAS-differentially- profile were calculated following methods by O’Donnell et al corrected positions (2–5 m accuracy). For vegetation cover, (2011a, 2012a, 2012b, 2012c). After summarizing soil and we visually estimated the per cent cover of each vascular environmental characteristics by plot and using the plot as species and dominant nonvascular species within the 5 m the sample unit, we tested for effects of successional stage (2 m for small pits) radius plots. Soil stratigraphy descriptions on water storage and SOC stocks within soil landscapes and soil sampling was done at one location at the center using one-way analysis of variance (ANOVA) and considered of the plot, using a variety of methods depending on soil overall effects significant if p < 0:05. Post hoc tests for conditions. For rocky soils, we used a shovel to excavate a comparing differences among means were done with the 0.5–1 m deep pit in unfrozen soils and used a jack hammer Tukey HSD test. Because of low sample sizes and concern for to excavate to 1 m depth in frozen soils. In unfrozen normality, medians also were tested with the nonparametric loamy and peaty soils, we extracted 30 cm diameter soil Kruskal–Wallis ANOVA to confirm results. Data are reported plugs of surface soils with a shovel for subsampling with in O’Donnell et al (2012c, 2013). knives or small corers, and used coring tubes for deeper soils down to 2–3 m. For frozen fine-grained soils, we used a 3. Results and discussion SIPRE (Snow, Ice, and Permafrost Research Establishment) corer (7.5 cm diam.) and cored to 3–5 m depths. Soil 3.1. Landscape heterogeneity and vegetation stratigraphy was described according to USDA field methods. Cryostructures were described according to French and Shur Field sampling encompassed a large diversity of boreal (2010). Peat types were differentiated by dominant plant ecosystems due to differences in topography, surficial macrofossils identifiable in the field with a hand lens. geology, hydrology, soils, permafrost, vegetation, and dis- Volumetric soil samples were obtained at 20-cm-depth turbance. The ecological classification of landscape com- increments in the center of the profile, with additional ponents from 82 sites across six study areas (figure 1) targeted samples taken from thin, distinctive horizons. For resulted in 16 ecosystem types grouped within four soil age determinations, organic-rich soil samples were taken from landscapes: rocky uplands, silty uplands, gravelly–sandy most soil profiles at distinctive breaks in peat stratigraphy. In lowlands and peaty–silty lowlands (figure 2; see also sup- the field, determinations were made for wet weights, and soil plemental tables S1 and S2 available at stacks.iop.org/ERL/ pH and electrical conductivity (EC) were measured in thawed 8/035017/mmedia). These ecotypes represent assemblages liquids or a saturated paste with a portable meter. of co-varying vegetation–soil–permafrost conditions that are Soils were analyzed for a variety of physical, biological, linked through disturbance regimes. Our six upland and ten and chemical properties. For soil moisture and bulk density, lowland–lacustrine ecotypes were similar to those previously soils were oven dried at 60 C (organic) or 105 C (mineral). described for interior Alaska (Jorgenson et al 1999). The Total carbon and nitrogen contents were measured with a dominant ecological characteristics of each soil landscape are LECO CN analyzer. For acidic (pH < 6:8) soils, which were described below (for more detail see supplemental figure S1 obtained from most sites, organic carbon was assumed to and tables S3–S5 available at stacks.iop.org/ERL/8/035017/ be equal to the total carbon values because of the lack of mmedia). 4 Environ. Res. Lett. 8 (2013) 035017 M Torre Jorgenson et al Rocky uplands were associated with residual soil and were somewhat poorly drained, saturated near the permafrost hillside colluvium, which cover 32% of the area in the table, and extremely acidic (pH 2.8–4.1). Vegetation was boreal portion of the discontinuous permafrost zone (figure 1), dominated by an open canopy of black spruce (P. mariana), although permafrost-affected terrain is probably about half ericaceous shrubs, herbs (Rubus chamaemorus), sphagnum to two-thirds of that area. Old forests had soils with thick (S. fuscum), feathermosses and lichens (Cladina rangiferina, C. arbuscula). Willows and alder were notably absent. (25–43 cm) organic horizons underlain by cryoturbated Young and intermediate bogs were dominated by sedges organic-mineral mixtures and permanently frozen rocky soils, (Eriophorum russeolum) and sphagnum (S. riparium, S. and were somewhat poorly drained, saturated at depth, jensnii, S. lindbergii, S. rubellum). Old bogs had dwarf and extremely acidic (pH 3.3–4.6 at 10 cm). Vegetation ericaceous shrubs (Andromeda polifolia, Oxycoccus micro- was dominated by an open canopy of black spruce (Picea carpus), sedges (Carex rotundata, Eriophorum russeolum) mariana), low willow (Salix planifolia), ericaceous shrubs and sphagnum (S. balticum, S. flexuosum, S. fuscum). Fens (Ledum groenlandicum, Vaccinium uliginosum, V. vitis- had forbs (Menyanthes trifoliata), sedges (E. russeolum, idaea) and mosses (Hylocomium splendens, Pleurozium C. chordorrhiza, C. limosa, C. rotundata), sphagnum (S. schreberi, Sphagnum fuscum). After fire, young burns fallax, S. papillosum) and brown mosses (Warnstorfia fluitans, were dominated by tall and low shrubs (S. glauca, Betula nana, Vaccinium uliginosum), forbs (Epilobium Drepanocladus spp.). angustifolium), grasses (Calamagrostis spp.) and colonizing Overall, there were large shifts in vegetation structure mosses (Ceratodon purpureus, Polytrichum juniperinum). and composition after permafrost thaw and/or fire across all Intermediate burns were similar to young burns, but with landscapes. In rocky and silty uplands, the shift was mainly in higher shrub and grass cover and minor cover of paper the structure and in the relative dominance of the species after birch (Betula papyrifera). Old burns were similar to old fire, with most species being present at all successional stages forest, except they maintained higher shrub cover and lacked and only a small number of species constrained to either sphagnum. young burns or old forests. In the gravelly-sand lowlands, the Silty uplands were associated with eolian loess deposits, exposure of new sediments in drained-lake basins provided which cover 5% of the region, but are inadequately habitat for numerous early colonizing species not found in old mapped. Old forests had soils with thick (25–33 cm) organic forests. In peaty–silty lowlands, thermokarst bogs and fens horizons underlain by cryoturbated organic-mineral mixtures provided new habitat for numerous species not found in old and permanently frozen silty soils, and were somewhat poorly forests, although by the old bog stage nearly half its species also occurred in old forests. These changes in vegetation after drained, saturated at depth, and circumneutral (pH 5.7–6.7). disturbance are important to soil carbon dynamics because Vegetation was similar to that of rocky uplands, except tall differences in species composition, plant growth rates, and alder (Alnus crispa), willows (S. bebbiana, S. arbusculoides) tissue chemistry have the potential to affect aboveground and feathermosses were more abundant and sphagnum was biomass accumulation and soil decomposition rates (Turetsky rare. 2004, Quillet et al 2013). Compositional shifts involving the Gravelly-sandy lowlands were associated with eolian peat mosses (Sphagnum) appear particularly important for sands, which typically had a thin loess cap (10–150 cm) facilitating permafrost formation in upland forests and altering and were underlain by fluvial gravel at depths of 4–5 m; decomposition dynamics of lowland vegetation. this landscape covers 4% of region. Old forests had soils with thin (7–16 cm) organic horizons underlain by permanently frozen sandy soils, and were well drained, lacked 3.2. Permafrost water in the active layer and alkaline (pH 8–8.6) at depth. Vegetation was dominated by white spruce (Picea glauca) In our study, all four landscapes had permafrost and unfrozen and aspen (Populus tremuloides), tall willows (S. bebbiana, S. soils where permafrost had recently thawed, except silty glauca S. scouleriana), low shrubs (Shepherdia canadensis), uplands where all sites had retained permafrost even after herbs (Geocaulon lividum, Festuca altaica) and mosses (H. fire. Permafrost characteristics ranged from ice-poor sands to splendens). Margins of the recently drained-lake basins were ice-rich silts (figure 3). Moisture contents in permafrost varied dominated by tall willows (S. bebbiana, S. alaxensis, S. two-fold among soil landscapes (figure 4(a)). Differences hastata) and herbs (E. arvense, Solidago multiradiata, Aster in ground ice volumes were associated with varying spp.). Centers of the drained basins were partially vegetated morphologies and permafrost histories described below. with herbs (Chenopodium album, Erigeron acris), grasses In rocky uplands, ground ice near the surface was (Beckmannia syzigachne, Agrostis scabra, Hordeum jubatum) characteristic of quasi-syngenetic permafrost (Shur 1988), and sedges (Carex saxatilis, Juncus spp.). which aggrades in response to thick moss accumulation and Peaty-silty lowlands in our study areas were associated thinning of the active layer. Frozen soils were dominated by with reworked lacustrine and eolian silt, although this braided, lenticular and porphyritic cryostructures. Occurrence landscape also includes retransported colluvium, fluvial aban- of buried charcoal fragments and subducted organic masses doned/terrace, and glaciolacustrine deposits, which together in turbated organic-mineral mixtures indicate that permafrost cover 27% of the region. Old forests on permafrost has undergone repeated episodes of degradation and plateaus had thick peat underlain by limnic sediments and aggradation, primarily associated with repeat fires during the cryoturbated organic-mineral mixtures and clean silt. Soils late Holocene. 5 Environ. Res. Lett. 8 (2013) 035017 M Torre Jorgenson et al Figure 4. Mean (SD) soil moisture (a) for unfrozen and frozen soils (all successional stages combined) and water-table depths; (b) for undisturbed old forests and disturbed sites (post-fire) within rocky uplands (RU), silty uplands (SU), gravelly–sandy lowlands (GSL), and peaty–silty lowlands (PSL). Numbers represent sample size for individual samples for moisture and for sites for water table. extremely ice-rich Pleistocene-age silt (yedoma) below 2–4 m, epigenetic ice-poor permafrost formed during downward freezing of previously thawed soil between 1.5 and 4 m, and quasi-syngenetic ice-rich permafrost formed during thinning of the active layer in response to ecological succession. Cryostructures and radiocarbon dating indicate the yedoma formed during the late Pleistocene and has undergone repeated deep (2–3 m) thawing episodes caused by fire, but the main body of yedoma has remained remarkably resilient. We interpret the yedoma in the Hess Creek region to be similar to the yedoma exposed at the Itkillik bluff in northern Alaska, which has large, deep syngenetic ice wedges that occupy 60–80% of the volume of the upper 30 m (Kanevskiy et al 2011). Loess deposits are widespread in Alaska (Muhs and Budahn 2006, Kanevskiy et al 2011), are not well mapped, and many areas have thin loess caps that are insufficient to map as loess but which still can affect soil processes. The gravelly–sandy lowlands in the Yukon Flats were formed by alluvial gravel and sand covered by a wind-blown sand sheet near the end of the Pleistocene (Kennedy et al 2010). Due to the sandy material, the permafrost was ice-poor, dominated by pore ice and lenticular cryostructures with occasional thin lenses, and there was no topographic or stratigraphic evidence of thermokarst. The abundance of drained-lake basins formed in depressions in the undulating sand sheet, many of which now have recently exposed limnic sediments, makes this an unusual permafrost-affected landscape. Airborne electromagnetic resistivity in the region found permafrost surrounding the basins but absent under the lakes (Minsley et al 2012), and hydrologic analyses indicate Figure 3. Representative profiles for permafrost-affected soils for four soil landscapes. permafrost degradation is likely contributing to the loss of water (Jepsen et al 2013). In peaty–silty lowlands, permafrost was ice-rich and In silty uplands, permafrost ranged from ice-poor dominated by porphyritic cryostructure in organic soils, to extremely ice-rich and was dominated by pore ice, micro-braided or micro-lenticular cryostructures in interme- lenticular, micro-braided and micro-ataxitic cryostructures. diate horizons, and layered and reticulate cryostructures in The cryostructures indicated three types of permafrost, the underlying silt. Cryostructures and radiocarbon dating 6 Environ. Res. Lett. 8 (2013) 035017 M Torre Jorgenson et al indicate permafrost has had a complex history since the ‘thaw sinks’, a unique type of landscape change related to late Pleistocene that include initiation as extremely ice-rich permafrost degradation (Jorgenson and Osterkamp 2005). syngenetic permafrost (yedoma), degradation in thaw lakes, In peaty–silty lowlands, the peatlands have complicated reaggradation of permafrost after lake drainage, and renewed permafrost histories associated with yedoma degradation, degradation during the last thousand years. A few profiles and subsequent repeated episodes of epigenetic formation of ice-rich permafrost and degradation that creates highly from permafrost plateaus had buried sedge–sphagnum bog patchy landscapes with permafrost plateaus and thermokarst peat, which indicates small thermokarst pits have formed bogs in differing stages of development. Thus, permafrost and stabilized during the last few hundred years in isolated degradation happens at different rates: over decades in rocky patches. uplands and gravelly–sandy lowlands in response to fire and Permafrost occurrence and thaw depths after disturbance water impoundments; over decades to centuries in peaty–silty varied among soil landscapes. In rocky uplands, shallow lowlands where thaw is dominated by lateral heat fluxes at (within 1 m) permafrost usually was present in old forests the margins of permafrost plateaus; and over centuries to (7 of 7 sites frozen, plus 2 undetermined) and mostly absent millennia for extremely ice-rich silty uplands where high in burned areas (2=12 frozen). In silty uplands, permafrost ice contents, soil thermal properties, and ecological recovery was always present before (4=4) and after fire (7=7), with delays thawing. Over centuries of climate warming, however, mean thaw depths similar between old forests (55 12:9 cm) silty uplands have the potential to undergo 20–30 m of and burned sites (61 11:2 cm). In gravelly–sandy lowlands, collapse after thawing, which could result in flat, wet basins permafrost was nearly always present in old forests (4=5) that provide good conditions for peat accumulation (Jones and nearly always absent in drained-lake basins (1=10). In et al 2012). A critical component of this transformation will be peaty–silty lowlands, permafrost was always present (7=7) the redistribution of silt from upland to lowland environments in old forests, while permafrost was infrequent (2=6) below through both thaw slumping and fluvial transport of sediment. shallow taliks (<3 m) in intermediate bogs, and always absent (6=6, observed depths 1.6–3.3 m) in old bogs. When comparing differences in mean thaw depths for old forests 3.3. Water among landscapes, thaw depths were similar among rocky uplands (46 10:4 cm), silty uplands (55 12:9 cm) The hydrologic responses to thawing were very different and peaty–silty lowlands (56 8:3 cm), but much deeper among landscapes. Immediately upon thawing of the upper in gravelly–sandy lowlands (138 14:1 cm). Thus, all permafrost, surface hydrology is altered by changes in landscapes partly or completely lost permafrost after fire or subsurface permeability and microtopography associated with lateral expansion of pond thaw bulbs, except silty uplands. thaw settlement. Mean moisture contents in permafrost soils We attribute the persistence of permafrost after fire in were two- to four-fold higher than in unfrozen soils situated in silty uplands to high moisture contents that affect thermal the active layer or in soils where permafrost had thawed, due properties, slow thawing of ice-rich soil with high latent heat to accumulation of segregated ice (figure 4(a)). This frozen contents, and vegetation recovery at the surface. zone with high ice contents provides an impermeable barrier Overall, permafrost history, as it affects ground ice to subsurface drainage. characteristics, was a fundamental driver of ecological Water-table depths varied widely among upland and changes across all landscapes. In rocky uplands, permafrost lowland landscapes after disturbance from fire, permafrost thaws rapidly after fire due to low ice contents and highly thaw, and drainage (figure 4(b)). In rocky uplands, mean permeable material, allowing vertical subsurface drainage, water-table depths were near the bottom of the active layer but the minor thaw settlement has little effect on surface ( 40 cm) in old forests and increased to below the depths topography. Cryoturbated soils with deeply subducted organic of our observations (1 m) at most sites following fire and masses indicate that soils have undergone repeated episodes permafrost thaw. Mean water storage (water-depth equivalent) of thaw during the Holocene. In contrast, silty uplands for 0–2 m depths in old forests (0.7 m) were two-fold higher with their extremely ice-rich permafrost formed during the than in young to old burns (0.3–0.4 m) (figure 5). In silty late Pleistocene (Kanevskiy et al 2011), were resilient to uplands, mean water depths ( 45.3 to 47.2 cm) and storage fire, albeit with a deepening active layer and thawed zone, (1.2–1.3 m) were virtually the same before and after fire and soils stay saturated at depth. These landscapes have because soils remained near saturation due to underlying been found to be highly resilient even over large climatic frozen silt. Gravelly-sandy lowlands had intermediate water fluctuations (Froese et al 2008), although increased fire depths ( 60.8 cm) and low storage (0.5 m) in old forests with severity or frequency may increase vulnerability of ice-rich permafrost, compared to deeper water depths ( 138.3 cm) silt to thaw (O’Donnell et al 2011b). While permafrost and intermediate storage (0.7–1.1 m) in the unfrozen drained was persistent at our sampling sites, we did observe some basins. At Twelve-Mile Lake, our time-series analysis of surface polygonization, thermokarst mounds, and surface historical airphotos found that water levels had dropped3 m water channelization that provide evidence for larger erosional from 1978 to 2007 and were 4 m below the former lake changes that can occur as this landscape evolves over the margin, a large unidirectional drop indicative of subsurface longer term. Gravelly-sandy lowlands have ice-poor soils that drainage. In peaty–silty lowlands, mean water depths were behave similar to rocky uplands, except the loss of permafrost near the bottom of the active layer in old forests ( 52.9 cm), around lakes allows subsurface drainage, which creates but near the surface in young to old thermokarst bogs 7 Environ. Res. Lett. 8 (2013) 035017 M Torre Jorgenson et al indicates some water can drain off the surface through pipes formed in the active layer or thawed through the ice-wedge network (Koch et al 2013a, Carey and Woo 2002). The lack of water impoundment on the sloping surface reduces the positive feedback of warming associated with the heat gain in shallow water bodies (Jorgenson et al 2010) and facilitates permafrost resilience. Gravelly-sandy lowlands have a dramatically different response, in that permafrost loss has allowed widespread, recent drainage of lakes on the sand sheet on the Yukon Flats (Jepsen et al 2013), as well as near the Kobuk sand dunes. Initially, surface water is impounded on the undulating sand sheet by permafrost that aggraded before the ponds formed or by permafrost that formed an impermeable curtain around the lakes. With climate warming or large fires, the permafrost table lowers, or pipes develop through the permafrost, allowing the lakes to drain slowly. This exposes organic-rich limnic sediments to aerobic decomposition, depending on lake-level lowering in relation to the elevation of the regional water table. Our observations Figure 5. Mean water storage by 0–1 and 0–2 m depths intervals of dead standing trees and buried terrestrial sedge peat for various successional stages (Yng D young, Int D intermediate, suggest that previous episodes of impoundment and drainage DL D drained lake) within four soil landscapes. SD bars are for have occurred in relation to permafrost dynamics. Finally, combined 0–2 intervals and numbers are for sample sizes. Means in peaty–silty lowlands, minor perturbations of the active with the same letter are not significantly different (p > 0:05), no contrast when overall ANOVA results not significant. layer from fire or climate warming lead to differential thaw settlement, fragmented drainage patterns, impoundment of water in isolated pits and bogs, and change in soil ( 1.0 cm). In contrast, mean water storage was slightly conditions from somewhat well-drained peat with a thick higher in the old forests (1.3 m) compared to the young aerobic layer at the surface to permanently saturated soils to old bogs (1.0–1.1 m) due to the high ice content in the near the surface. Over centuries, these depressions evolve permafrost. At Innoko and Koyukuk Flats, collapse of the from isolated depressions to bogs and lakes, and ultimately thawed surface led to differences in water-table elevations in to connected fen systems, where the water level is controlled the plateaus and bogs that varied by 1.6 and 4.0 m, for the by the elevations of the outlet gaps. Subsurface drainage respective study areas. Soil moisture profiles from volumetric through the underlying compacted silt is negligible. Further core samples reveal trends of increasing soil moisture with expansion of bogs leads to integrated surface and subsurface depth in the active layer and highly variable moisture at depth connections and development of linear fen systems with in frozen soils associated with varying cryostructure (for more surface water movement (Jorgenson et al 2001, Quinton et al detail see supplemental figure S2 available at stacks.iop.org/ 2009). While hydrologic reorganization can be dramatic at ERL/8/035017/mmedia). Overall, water depths, water storage local and landscape scales, they have only a small effect and moisture profiles indicate loss of moisture to subsurface on regional stream hydrology that involves a shift towards drainage after fire in rocky uplands and in gravelly–sandy increasing base flow discharge during winter (Walvoord and lowlands after lake drainage, little moisture change after Striegl 2007). fire in silty uplands where permafrost is preserved, and increased moisture at the surface due to surface collapse and 3.4. Soil carbon impoundment in peaty–silty lowlands. Based on changes in topography, surface water and SOC stocks for the top 2 m varied ten-fold across all groundwater depths, and soil moisture contents, we developed sites, from a young burn in rocky uplands (9.7 kg m ) conceptual models of how water storage and hydrologic to an old forest in a peaty–silty lowlands (108.8 kg m ). flow paths are reorganized upon permafrost thaw for each When comparing differences among landscapes for only landscape (figure 6). In rocky uplands, permafrost thaw undisturbed old forests, mean SOC in rocky uplands, silty results in loss of the permafrost aquitard and allows water to infiltrate rocky soils and drain to the groundwater system. uplands, and gravelly–sandy lowlands were remarkably Redirection of water into the groundwater system alters similar (28.9–30.5 m kg m ), in contrast to much higher stream hydrology by lowering summer discharge during and variable stocks in peaty–silty lowlands (91.4 kg m ) storm events and increasing winter discharge (Walvoord and (figure 7). Below we compare SOC in the top 2 m, which Striegl 2007). In silty uplands, persistence of permafrost allows comparisons within a consistent depth, and then for maintains saturated soils and flow paths dominated by surface various soil materials (texture and/or peat types) among and suprapermafrost flow. Observations of occasional sink successional stages within each landscape to evaluate changes holes on the upper slopes and seeps at the toe slopes in surface and deep soil materials over time. 8 Environ. Res. Lett. 8 (2013) 035017 M Torre Jorgenson et al Figure 6. Topographic profiles illustrating relationships among topography, hydrology, soils, permafrost and vegetation within four boreal landscapes. When comparing mean SOC in the top 2 m among accumulation of lower density sphagnum peat at the surface successional stages within soil landscapes, the landscapes had in the bogs. very different patterns in relation to disturbance associated When comparing differences in mean SOC stocks for with permafrost thaw and/or fire (figure 7). For rocky uplands, various soil materials among successional stages within SOC in old forests with permafrost (28.9 kg m ) was nearly landscapes, again there were large differences in patterns 50% higher than in young to old burns without permafrost among landscapes (figure 8). In rocky uplands before (15.6–21.8 kg m ), and when all burn stages were combined disturbance, 32% of mean SOC in the top 2 m was in surface (20.3 kg m ) the difference with old forests was significant organics (9.5 kg m ), 50% in turbated organic-mineral (p D 0:05). For silty uplands, SOC in old forests with horizons (14.4 kg m ) indicative of a history of repeated permafrost (30.5 kg m ) was similar to that of young to old permafrost thaw, and 17% in underlying rocky colluvium burns (26.6–45.8 kg m ) where permafrost was still present (7.6 kg m ) (figure 8). The active layer had 51% of the mean at depth, and when burn stages were combined (36.2 kg m ) SOC. For surface organic horizons (moss, Oi, Oe, Oa), mean the difference with old forests was not significant (p D SOC was significantly two-fold higher in old forests (9:5 0:99). For sandy lowlands, where permafrost thaw has caused 2 2 5:5 kg m ) compared to burns combined (4:0 0:9 kg m ). widespread draining of lakes, SOC of sandy soils in old 2 For organic-mineral horizons, mean SOC was significantly forests (30.2 kg m ) was similar to upland soils, but there three-fold higher in old forests (14:4 9:8 kg m ) compared was large SOC in the recently exposed limnic sediments in to burns combined (5:28:1 kg m ). We attribute the change the center of the drained lakes (62.6. kg m ) and smaller of SOC in the surface organics to fire combustion, but the SOC along the sandy margins (21.1 kg m ). For peaty–silty differences in SOC in organic-mineral horizons were more lowlands (Innoko and Koyukuk combined), SOC in old forests likely due to sampling variability rather than decomposition, (91.4 kg m ) was two-fold higher than in young to old thermokarst bogs (44.6–57.1 kg m ) and the differences because 8 of the 11 burned sites were too young to have much in SOC were significant (p D 0:03), primarily due to the deep decomposition. 9 Environ. Res. Lett. 8 (2013) 035017 M Torre Jorgenson et al Figure 8. Mean soil organic carbon stocks by soil material for Figure 7. Mean soil organic carbon stocks by 0–1 and 0–2 m various successional stages (Yng D young, Int D intermediate, depths intervals for various successional stages (Yng D young, DL D drained lake) within four soil landscapes. Totals are for Int D intermediate, DL D drained lake) within four soil 0–2 m interval, except peaty–silty lowlands, which are for soils of landscapes. SD bars are for combined 0–2 intervals and numbers are variable depth above a common limnic horizon created after for sample sizes. Means with the same letter are not significantly thaw-lake drainage. Patterns represent soil materials of differing different (p > 0:05), no contrast when overall ANOVA results not origin. Organic materials were differentiated as woody forest or significant. shrub peat, graminoid peat from meadows, fen and bog peat dominated by various lifeforms, and limnic material formed in lakes. Mineral soils were differentiated by geomorphic deposit. Within silty uplands, 28% of mean SOC in the top Org-Min layers represent turbated mixtures caused by thermokarst. 2 m was in surface organics (8.6 kg m ), 29% in Numbers represent sample sizes. turbated organic-mineral horizons (8.7 kg m ), and 43% in underlying silty loess (13.2 kg m ) in old forests. The of permafrost thaw, bog succession and permafrost recovery active layer had 56% of the mean SOC. When comparing associated with evolution of thaw-lake plains in the Koyukuk surface organics among successional stages, mean SOC was and Innoko Flats. The soils reflected a general successional significantly two-fold higher in old forests (8:6 2:8 kg m ) trend that included: reworked silt and mixed organic-mineral compared to burns combined (4:0 1:8 kg m ), where 5 of sediments from the initial collapse of yedoma; limnic 7 sites were young burns. We attribute the change in surface sediments deposited in the deep water of thaw lakes; organics to fire combustion. For turbated organic-mineral followed by herb marsh, sedge fen, shrub and forest peat horizons, mean SOC was similar in old forests (8:7:4 2 2 formed during paludification and permafrost aggradation; 6:7 kg m ) compared to burns (9:2 8:9 kg m ). and finally to sedge–sphagnum bog and ericaceous bog Within sandy lowlands, the old forests had distinctly peat associated with recent thermokarst. In this landscape, different soil materials from the recently drained basins. In where our coring obtained a maximum depth of 5.2 m, old forests, 10% of mean SOC in the top 2 m was in we were most interested in organics that have accumulated surface organics (2.9 kg m ), 10% in organic-mineral soils 2 2 since drainage of the thaw lakes, as marked by a limnic (2.9 kg m ), 51% in silty loess (15.6 kg m ) and 29% horizon that served as a subsurface datum for assessing the the underlying sands (8.8 kg m ). In contrast, most of the effects of recent thermokarst on SOC. In old forests with SOC in the drained basins was in limnic sediments, with the centers (43.3 kg m ) having much higher SOC than the permafrost, 71% of mean SOC was in sphagnum and woody 2 2 margins (14.7 kg m ). We did not sample recent burns in peat (61.5 kg m ) associated with forests, 16% in sedge this landscape. fen peat (14.0 kg m ) mostly formed below the forest peat, Within peaty–silty lowlands, the peat had complicated and 5% in sedge–sphagnum peat (4.4 kg m ) indicative stratigraphy, indicating a rich history of repeated episodes of previous episodes of thermokarst in some profiles. When 10 Environ. Res. Lett. 8 (2013) 035017 M Torre Jorgenson et al layer to slowly become incorporated into frozen mineral–soil horizons, which reduces losses via decomposition (O’Donnell et al 2011a). In both uplands, fires will likely remain a dominant control in plant succession, primary productivity, and soil carbon dynamics (Chapin et al 2006). In rocky uplands, however, loss of permafrost after fire leads to drier aerobic soils that are conducive to long-term SOC loss, and permafrost may not be able to reestablish with a warming climate. In contrast, resilience of permafrost and maintenance of saturated soils in the silty uplands in the near term should lead to cyclical changes in SOC after fire. Over a longer term, permafrost degradation of ice-rich silty uplands should lead to SOC responses that are more divergent between wetting thermokarst depressions and drying remnant mounds and Figure 9. Exponential relationship between soil organic stock and hills. water storage for the top 0–2 m for all soil profiles (n D 64) across Lowlands had very different patterns. Gravelly-sandy four soil landscapes. Arrows represent likely directions in carbon landscapes had the unusual process of lake drainage that stocks as water regimes change after permafrost thaw; dashed lines exposed large SOC stocks to aerobic conditions, although indicate greater long-term uncertainty. the areal extent of this terrain is small. In the surrounding old forests, SOC will likely respond similarly to rocky uplands after permafrost loss under a warming climate. In comparing successional stages, mean SOC (all peat types peaty–silty lowlands, permafrost collapse and colonization combined) was highest in the old forest on the permafrost 2 2 by early successional hydrophilic sphagnum species under plateaus (86.9 kg m ), lowest in young (38.7 kg m ) extremely acidic conditions creates conditions for significant bogs, intermediate in intermediate bogs (47.3 kg m ) soon new accumulation of bog peat and increased methane after collapse, followed by increasing SOC in old bogs emissions (Johnston et al 2012). Peaty-silty lowlands in our (59.6 kg m ). Differences in SOC, however, were not region, as well as in Canada (Robinson and Moore 1999, significant (pD 0:17) due to high variability. When examining Turetsky 2004), have accumulated large SOC stocks through the accumulation of new bog peat formed after collapse, repeated thawing episodes over the Holocene. While we found SOC was low in young (1.5 kg m ) and intermediate bogs a significant increase in new bog peat, the magnitude of (2.7 kg m ), followed by a significant (p < 0:01) increase decomposition of old peat after thaw remains uncertain. In in old bogs (38.1 kg m ). SOC in our deepest (4.0–5.2 m) our study areas on the Koyukuk and Innoko Flats, as well cores in old forests, including underlying limnic and rework as the Tanana Flats (Jorgenson et al 2001), nearly half or loess soil, ranged from 152 to 228 kg m . more of the permafrost has already degraded during the last SOC for the top 2 m was strongly (p < 0:0001) related 1000 years. Thus, much of the change has already occurred, to total water storage across all sites (figure 9). There was diminishing the implications of future permafrost thaw on strong clustering by landscape, with rocky uplands having decomposition of old carbon once frozen in permafrost lower water storage and SOC, silty uplands with intermediate in boreal regions. Assessment of the overall fate of SOC values, and peaty–silty lowlands with high water storage and associated with permafrost dynamics in boreal regions given SOC. In gravelly–sandy lowlands, values clustered in two these divergent patterns will continue to be a challenge with groups associated with old forests and drained-lake basins. additional changes associated with potential increased fire SOC variability increased with increasing water storage, severity and long-term landscape evolution through physical particularly for peaty–silty lowlands, where we attribute the erosion in uplands and expanding flow networks in lowlands. large range in SOC to the low bulk density of peat in thermokarst bogs compared to the more compact peat in the old forests. 4. Conclusion Overall, large changes in vegetation and water after permafrost degradation and/or fire were affected by very Using a chronosequence approach to assessing changes different processes among the soil landscapes, and that SOC over time, we found the response of vegetation, water and also responds in divergent patterns (figure 9). In rocky soil organic carbon to permafrost degradation and/or fire and silty uplands, there were significant small decreases differed across successional stages within rocky uplands, (4.6–5.5 kg m ) in SOC in the surface organics after fire, but silty uplands, gravelly–sandy lowlands, and peaty–silty we were not able to detect changes during later successional landscapes that comprise much of boreal Alaska. We found stages due inability to access old burns to sample. In both large changes in vegetation composition and hydrologic uplands, the turbated organic-mineral layers at depth indicated patterns, and small significant changes in carbon in surface that substantial carbon has persisted through previous thaw organic soils after disturbance, but we were unable to cycles. The quasi-syngenetic aggradation of permafrost detect changes in deeper carbon due to high variability. associated with ecosystem-driven permafrost (Shur and Permafrost degradation occurred at different rates: over Jorgenson 2007), allows SOC accumulated in the active decades in rocky uplands and gravelly–sandy lowlands in 11 Environ. Res. Lett. 8 (2013) 035017 M Torre Jorgenson et al response to fire and water impoundments correspondingly, Jiang Y, Zhuang Q and O’Donnell A J 2012 Modeling thermal dynamics of active layer soils and near-surface permafrost over decades to centuries in peaty–silty lowlands with a using a fully coupled water and heat transport model legacy of complicated Holocene changes, and over centuries J. Geophys. Res. 117 D11110 to millennia for extremely ice-rich silty uplands where Johnson K D et al 2011 Soil carbon distribution in Alaska in soil properties and ecological recovery delays thawing. Our relation to soil-forming factors Geoderma 167/168 71–84 soils evidence indicates permafrost has been degrading and Johnston C E, Ewing S A, Stoy P C, Harden J W and Jorgenson M T 2012 The effect of permafrost thaw on methane emissions in a reforming throughout the Holocene in all landscapes, but Western Alaska peatland chronosequence Proc. 10th Int. Conf. in silty uplands degradation has been limited to the upper on Permafrost (Salekhard) ed K Hinkel, pp 241–2 permafrost, although the extremely high ice content makes Jones B M, Grosse G, Arp C D, Jones M C, Walter Anthony K M this landscape vulnerable to large-scale transformations. The and Romanovsky V E 2011 Modern thermokarst lake highly patchy disturbance dynamics, divergent ecological dynamics in the continuous permafrost zone, northern Seward Peninsula, Alaska J. Geophys. Res.—Biogeosci. 116 G00M3 recovery patterns over differing timescales, and substantial Jones M C, Grosse G, Jones B M and Walter A K 2012 Peat permafrost loss that has already occurred during the last accumulation in drained thermokarst lake basins in continuous, millennium, may combine to moderate overall changes in soil ice-rich permafrost, northern Seward Peninsula, Alaska carbon stocks and trace gas emissions over broader boreal J. Geophys. Res. 117 G00M7 regions. Jorgenson M T and Osterkamp T E 2005 Response of boreal ecosystems to varying modes of permafrost degradation Can. J. Forest Res. 35 2100–11 Acknowledgments Jorgenson M T, Racine C H, Walters J C and Osterkamp T E 2001 Permafrost degradation and ecological changes associated with Funding was provided by NSF (EAR 0630319) and USGS a warming climate in central Alaska Clim. Change 48 551–79 Jorgenson M T, Romanovsky V, Harden J, Shur Y, O’Donnell J, with logistical support by FWS. We thank Trish Miller, Schuur E A G, Kanevskiy M and Marchenko S 2010 Kate Beatty and Mark Winterstein of ABR, and Pedro Resilience and vulnerability of permafrost to climate change Rodriguez for help with field sampling. Olga Afonina and Can. J. Forest Res. 40 1219–36 Mikhail Zhurbenko identified moss and lichen voucher Jorgenson M T, Roth J E, Miller P F, Macander M J, Duffy M S, specimens. Additional thanks to Karin Bodony, FWS and Wells A F, Frost G V and Pullman E R 2009 An Ecological Land Survey and Landcover Map of the Arctic Network Jim Webster, Webster Flying Service, for logistical support. (NPS/ARCN/NRTR—2009/270) (Anchorage, AK: National Comments from two anonymous reviewers were very helpful. 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Journal
Environmental Research Letters – IOP Publishing
Published: Sep 1, 2013