Tim Cater
Originally published by the Society for Ecological Restoration—Northwest Chapter, November 2021.
Intact blocks of live vegetation and the soil in which they are rooted (i.e., sod) provide a viable option for revegetating and rehabilitating disturbed lands in northern ecosystems. Sod harvested from tundra donor sites has been used successfully to revegetate and rehabilitate wetlands in arctic Alaska. To the best of our knowledge, the first time that sod was harvested to rehabilitate a disturbed wetland site on the North Slope of Alaska was in September 1999 when ABR, Inc.—Environmental Research & Services planted sod in ~300 sq. feet of arctic wetlands. Sod was harvested during summer at a mine site where the use of explosives resulted in scattered blocks of sod called "flyrock."
A distinctive feature of the tundra sodding technique is that it originated from Iñupiaq peoples’ use of sod blocks to construct traditional sod houses and to insulate entryways to ice cellars. The same characteristics that make sod useful for traditional purposes also make it useful for rehabilitating disturbed wetlands. Most importantly, the bulk mass of sod insulates the ground surface, which mitigates thawing of permafrost by immediately helping to re-establish a stable thermal regime. Preserving permafrost prevents the collapse of the ground surface which can otherwise occur due to the loss of soil volume as ice melts.
In some areas, ice makes up 50% or more of the volume of the frozen soil. This subsidence due to thawing, commonly referred to as thermokarst, often leads to permanent flooding with water too deep for most rooted plants to establish. Thus, maintaining a stable ground surface is a prerequisite for successful rehabilitation of many disturbed sites in permafrost terrain. Using other rehabilitation techniques, such as seeding, would require a minimum of several decades to develop a surface organic mat capable of providing effective thermal insulation.
Another major benefit of the sod treatment for tundra rehabilitation is that it immediately provides a diverse community of indigenous plant species that is similar to nearby tundra wetlands. This approach effectively bypasses the long period of time (decades, and probably much longer) that is otherwise needed to develop a productive, self-sustaining community of tundra plants. Given favorable conditions, the transplanted vegetation can be expected to remain productive indefinitely.
The characteristics of sod vary on micro and macro scales, which results in variable sod quality at a donor site. In general, soil characteristics and species composition typical of wet tundra are preferred over those typically found in moist and dry tundra. The thick organic mat typical of wet tundra allows for the harvesting of 30-40 cm-thick pieces. Thick sod is preferred because plants should experience less transplant shock with rooting systems contained in a larger volume of soil. Also, fast-growing rhizomatous perennial graminoids are typically dominant in wet tundra. Preferred species include the grass Dupontia fisheri (Fisher’s tundragrass) and the hydrophytic sedges Carex aquatilis (water sedge), Eriophorum angustifolium (tall cottongrass), and E. scheuchzeri (white cottongrass).
In contrast, the organic mat in drier tundra is usually thinner, the rooting systems are less cohesive, and vegetation typically includes evergreen and deciduous shrubs, which do not survive transplanting as well. Lichens and mosses also are not preferred because they typically die off after several years.
Sod develops over many growing seasons when a new layer of plant litter is produced and previous additions of litter begin to decompose, adding to the increasingly thick layer of organic matter that accumulates at the ground surface. The process of sod development is slow in arctic ecosystems because cold temperatures and low oxygen levels in soil limit microbial activity, especially when the soil pore space is flooded.
Harvesting sod may require a permit from the U.S. Army Corps of Engineers, a State of Alaska Material Sales Contract issued by the Alaska Department of Natural Resources, or a harvest permit from the Bureau of Land Management, depending on local land management. Donor sites have typically been areas that were slated for development of gravel mines (Figures 2–3), but other donor sites could include vehicle parking lots or trenches excavated for the burial of power cables or telecommunications lines. In these situations, harvesting sod should be considered a best management practice, as it results in beneficial use of a valuable resource that would otherwise be wasted. Harvesting sod from areas not slated for development is not a viable option because it does not result in a net benefit to the environment.
Tundra sod can be harvested and transplanted using heavy equipment and/or hand labor. Maximizing the use of heavy equipment minimizes costs for most projects and improves worker safety. Both techniques produce immediate results, with the treated areas resembling undisturbed tundra. Innovations continue to improve the efficiency of this technique. In particular, the techniques developed in summer 2020 by Red Mountain Consulting appeared to be more efficient than all previous attempts. This approach included using a standard excavator fitted with a 4-ft wide frost bucket modified with plates welded to the bucket’s sides. This equipment allowed the extraction of thick, intact pieces of sod that could be loaded directly onto standard 4-ft × 4-ft pallets, which could then be transported with a loader.
In a typical tundra sod harvesting operation at a gravel mine, the live vegetation and rooting zone (i.e., tundra sod) must be separated from the rest of the “overburden”, a term that refers to all of the material overlying the gravel. The 30–40-cm thick pieces of sod generally make up a small fraction of the overburden, which often is a 3–10 meter-deep mixture of silt, sand, and gravel. The density of tundra sod is similar to that of water (1 g/cm3), which means that relatively small pieces must be used if a person must carry blocks to the transplant site by hand. In practice, sod often includes substantial amounts of mineral soil, making it heavier and more difficult for a person to handle.