Purpose
Frost or hydrologic heaving occurs when soil pores contain free water conducive to the development of segregated ice lenses or crystals and when temperatures drop below freezing. Expansion when water changes from a liquid to a solid state and
continued growth of ice crystals or lenses over time can expand the soil surface upward. This is a natural process that can be aggravated by impacts that either seal parts of the surface, which restricts water infiltration between plants, or reduces pore space by compaction between plants. Excessive removal of vegetation acting as thermal cover can also exaggerate the effects of freezing. Over time, vegetated hummocks of increasing elevation develop between the sealed or compacted inter- spaces. Riparian wetland vegetation on the hummocks may be diminished or replaced by upland vegetation as the surface becomes elevated above the water table. Root shearing becomes a problem and interspace areas are exposed to increased erosional forces. Slope wetlands may experience a higher ground water discharge from the site as hummocks increase. The intent of this item is to deter- mine whether frost or hydrologic heaving is occurring, and if so, whether it is occur- ring at a normal or aggravated rate.
Examples
Before answering item 14, the ID team has to determine that frost or hydrologic heaving can occur on the site. Many riparian areas will not experience this attribute/process. For frost or hydrologic heaving to occur, the right amount of moisture, soil conditions, and freezing temperatures must be present to allow water droplets in the soil to form ice crystals. For areas that do not meet these require- ments, including most of the riparian-wetlands that occur in the desert Southwest, the answer to item 14 would be “N/A.”
When this process does occur, the ID team judges if the rate of frost heaving is normal or aggravated. If the configuration of frost heaves is normal relative to height and density of mounds, the answer to item 14 is “yes.” If the mounds are significantly higher than normal or more frequent, then the answer is “no.” Appendix C provides an example of a lacustrine wetland in Colorado that shows both. The frost heaving on the left is more column like, with more frequent and often narrower hummocks. Most hummocks on the left are also slightly higher. Also note the higher proportion of non- vegetated interspace. The answer to item 14 would be “no” for the area on the left and “yes” for the area on the right, which is considered to be “natural.”
The example provides an easy comparison because of the fenceline contrast across the same valley bottom with presumably the same soil and hydrology. Comparisons of different locations should be assessed on sites with similar soil, moisture, and ambient temperature.
Appendix C provides another example of excessive hydrologic heaving on a wet meadow wetland in Idaho.
Supporting Science/Quantitative Methodologies
Frost heaving doesn’t typically occur in clean sands and gravels, but does occur as the silt and nonplastic clay content in the soil increases. The proper moisture con- tent and freezing temperatures are also necessary for frost heaving to occur (Hough 1957).
The National Soils Handbook (USDA 1983) describes the basic processes and engi- neering significance of frost heaving. Empirical evidence indicates that severity of the frost action can be aggravated through management practices such as improper livestock grazing. However, there is little additional literature on the precise mecha- nisms leading to that result or quantification of ecological consequences in different settings. The hummock topography in wet meadows is different than other frost heave situations. Raised portions of natural frost boils are higher on the barren por- tion of the heave than vegetated surroundings (Fahey 1974), whereas the raised por- tion of meadow hummocks are vegetated. The compacted area in vehicle tracks seems to have greater frost heave and subsequent subsidence (Gatto 1997), whereas the probable compacted area between the hummocks appears to have less severe heaving. Differential frost heave (Fowler and Noon 1997) may have some bearing on the differences observed, but the relationship has not been described for the situa- tion here (to our knowledge). The National Range and Pasture Handbook (USDA NRCS 1998c) also describes frost heaving of forage plants, but it does not describe the hummock topography. It is possible that downward and outward expansion in the interspace is squeezing the soil and plant tussocks up like squeezing toothpaste out of a tube, but that is speculation by the authors. Our suggestion is that this process is sufficiently common to warrant additional research on management impacts in natural settings subject to frost heave.
Measurement of the hummock topography for comparison purposes can be accom- plished using a one-dimensional soil roughness analysis described by Grossman and Pringle (1987). One dimensional soil roughness is the variation in ground surface height along a line. In the procedure used, the distance is measured at regular inter- vals to the ground surface from a leveled rod. Heights are then corrected for the ground surface slope and the standard deviation of the corrected height is calculated. The procedure can be adjusted to obtain roughness at different scales.
Item 15: Favorable microsite condition (i.e., woody material,