Microsite conditions in retrogressive thaw slumps may facilitate increased seedling recruitment in the Alaskan Low Arctic

Abstract

In Low Arctic tundra, thermal erosion of ice‐rich permafrost soils (thermokarst) has increased in frequency since the 1980s. Retrogressive thaw slumps (RTS) are thermokarst disturbances forming large open depressions on hillslopes through soil wasting and vegetation displacement. Tall (>0.5 m) deciduous shrubs have been observed in RTS a decade after disturbance. RTS may provide conditions suitable for seedling recruitment, which may contribute to Arctic shrub expansion. We quantified in situ seedling abundance, and size and viability of soil seedbanks in greenhouse trials for two RTS chronosequences near lakes on Alaska's North Slope. We hypothesized recent RTS provide microsites for greater recruitment than mature RTS or undisturbed tundra. We also hypothesized soil seedbanks demonstrate quantity–quality trade‐offs; younger seedbanks contain smaller numbers of mostly viable seed that decrease in viability as seed accumulates over time. We found five times as many seedlings in younger RTS as in older RTS, including birch and willow, and no seedlings in undisturbed tundra. Higher seedling counts were associated with bare soil, warmer soils, higher soil available nitrogen, and less plant cover. Seedbank viability was unrelated to size. Older seedbanks were larger at one chronosequence, with no difference in percent germination. At the other chronosequence, germination was lower from older seedbanks but seedbank size was not different. Seedbank germination was positively associated with in situ seedling abundance at one RTS chronosequence, suggesting postdisturbance revegetation from seedbanks. Thermal erosion may be important for recruitment in tundra by providing bare microsites that are warmer, more nutrient‐rich, and less vegetated than in undisturbed ground. Differences between two chronosequences in seedbank size, viability, and species composition suggest disturbance interacts with local conditions to form seedbanks. RTS may act as seedling nurseries to benefit many Arctic species as climate changes, particularly those that do not produce persistent seed.

Rapid Vegetation Succession and Coupled Permafrost Dynamics in Arctic Thaw Ponds in the Siberian Lowland Tundra

“…This indicates that recruitment of dwarf shrubs, both through sexual reproduction and clonally, occurs in thaw ponds. Similarly, Huebner and Bret‐Harte (2019) found increased shrub recruitment on bare soil in retrogressive thaw slumps compared to undisturbed tundra vegetation, which was attributed to reduced competition by surrounding vegetation and creation of suitable conditions for germination. However, whereas Huebner and Bret‐Harte (2019) found the highest recruitment densities on more recently disturbed sites (1–10 years since disturbance), we found the majority of recruitment on Sphagnum carpets in older, decreasing thaw ponds.…”

“…Similarly, Huebner and Bret‐Harte (2019) found increased shrub recruitment on bare soil in retrogressive thaw slumps compared to undisturbed tundra vegetation, which was attributed to reduced competition by surrounding vegetation and creation of suitable conditions for germination. However, whereas Huebner and Bret‐Harte (2019) found the highest recruitment densities on more recently disturbed sites (1–10 years since disturbance), we found the majority of recruitment on Sphagnum carpets in older, decreasing thaw ponds. This may be explained by the ability of Sphagnum species to grow above the water table, generating the moist but unsaturated substrate needed for recruitment of dwarf shrubs (Bell & Bliss, 1980; Huebner & Bret‐Harte, 2019).…”

“…However, whereas Huebner and Bret-Harte (2019) found the highest recruitment densities on more recently disturbed sites (1-10 years since disturbance), we found the majority of recruitment on Sphagnum carpets in older, decreasing thaw ponds. This may be explained by the ability of Sphagnum species to grow above the water table, generating the moist but unsaturated substrate needed for recruitment of dwarf shrubs (Bell & Bliss, 1980;Huebner & Bret-Harte, 2019). Frost heave may contribute substantially to the generation of unsaturated conditions (Seppälä, 1988).…”

Enhanced shrub growth in the Arctic increases habitat connectivity for browsing herbivores

“…Permafrost thaw occurring in many Arctic locations (Liljedahl et al, 2016) could trigger disturbances that either enhance or curtail shrub growth, depending on microsite characteristics; these feedbacks are also not considered in our model. Whereas feltleaf willows can capitalize on in situ nutrients in floodplains with well‐drained soil and higher soil pH (Swanson, 2015), areas with frequent tundra fires (Jones et al, 2015) and thermokarst and thaw slumps (Huebner & Bret‐Harte, 2019; Lantz, Kokelj, Gergel, & Henry, 2009) also release nutrients and provide favorable conditions for establishment and growth of deciduous shrubs. An improved understanding of tundra fire and permafrost disturbance would likely improve the accuracy of projected tall‐shrub distributions in tundra regions.…”

“…Permafrost thaw occurring in many Arctic locations (Liljedahl et al, 2016) could trigger disturbances that either enhance or curtail shrub growth, depending on microsite characteristics; these feedbacks are also not considered in our model. Whereas feltleaf willows can capitalize on in situ nutrients in floodplains with well-drained soil and higher soil pH (Swanson, 2015), areas with frequent tundra fires (Jones et al, 2015) and thermokarst and thaw slumps (Huebner & Bret-Harte, 2019;Lantz, Kokelj, Gergel, & Henry, 2009) Herbivory in the floodplain also can potentially reduce shrub height and retard expansion rate for palatable species (Bryant, 1987; Bryant, Joly, Chapin, DeAngelis, & Kielland, 2014; Kielland et al, 1997;Olofsson et al, 2009;Olofsson & Post, 2018), though our model does not include these effects. Feltleaf willow is the preferred forage species of moose, hares, and ptarmigan (L. lagopus, Lagopus muta) in our study area (Zhou et al, 2017), and other arctic herbivores in the region, including muskox (Ovibo moschatus) and caribou, also prefer willows over well-defended species such as alder (Bryant & Kuropat, 1980;Christie et al, 2015).…”