Characteristics of hummocks and hummocked wetlands in Colorado

Abstract

There is considerable uncertainty regarding mechanisms of hummock formation. The first step in assessing hummocks in Colorado was to compare the characteristics associated with hummocked and non-hummocked sites. To do this, site characteristics of hummocked and non-hummocked riparian areas and wetlands across Colorado were sampled. Two site characteristics were positively related, and three site characteristics were negatively related to hummock occurrence. Three groups of hummocked wetlands with distinct morphological, vegetative and climatic characteristics were identified. A finer-scale approach was then used to examine mechanisms of hummock formation. Four hummocked sites in north-central Colorado were selected for detailed research. Soil temperature regimes and presence of water in interspaces were evaluated to determine whether or not conditions described in the most widely accepted theories of hummock formation occurred. Hummock/interspace pairs were instrumented with soil temperature sensors and water level indicators. Results indicate that there were sufficient air freeze-thaw cycles to support hummock formation by needle ice. Conditions supporting differential frost heave through ice lensing were also documented. Numerous temperature differentials were detected between the hummock top, hummock base and interspace creating temperature gradients that could lead to hummock formation by differential frost heave. The final step was to evaluate soil and vegetation characteristics of hummocks and interspaces with respect to hummock formation theories based on differential frost heave and plant biomass accumulation. Soil cores were collected from hummocks and interspaces to evaluate soil horizon orientation and thickness of the surface organic horizon. Bulk density, vegetation cover and herbaceous biomass production were also determined. Bent soil horizons indicative of differential frost heave were observed in four hummock/interspace pairs. The organic horizon was thicker and bulk density was lower in the hummocks compared to interspaces but the amount of organic matter in the two positions was similar. Accumulation of plant biomass may lead to increased hummock height. The finer-scale study revealed evidence supporting multiple mechanisms of hummock formation and development. These mechanisms may form different hummock types which is consistent with the findings of the larger-scale study.