Subalpine Bogs
Cuchara, Colorado

James S. Aber
Emporia State University

Introduction

The subalpine environment of mountains is the forested zone just below timberline. In the Culebra Range of the Sangre de Cristo Mountains in southern Colorado, this is generally in the elevation range of 10,000 feet (3000 m) to 12,000 feet (3600 m). Spruce and aspen are the primary trees with some Douglas fir and lodgepole pine. This zone experiences long, cold winters in which snow cover typically lasts from October until May; summers are short and cool with substantial monsoon rain in July and August. The relatively wet, cool environment is favorable for development of bogs in places with slow, blocked or enclosed drainage. Kettleholes in glacial moraines are ideal places for bogs.

The upper Cucharas Creek drainage basin has been investigated for the past several years by ESU faculty, students,1 and visiting scholars.2 Studies have focused on Pleistocene (ice age) and Holocene (recent/modern) environmental conditions, including the extent of glaciation, periglacial (frozen ground) phenomena, dendroclimatology, lichenometry (Vopata et al. 2006), and wetland conditions. The region in question includes a substantial complex of end and lateral moraines at Bear and Blue Lakes and extends upward to Trinchera Peak, spanning an elevation range from about 3000 m to over 4100 m.

Map of the upper Cucharas Creek and Culebra Range vicinity, south-central Colorado. Extent of Pinedale Glaciation is indicated by green shading. Bogs are located in kettleholes of an end moraine between Blue and Bear lakes: 1 - Cottongrass bog, 2 - Beaver bog. Adapted from Aber (2008, fig. 2).

The moraine complex at Blue and Bear lakes was deposited during a late stage of the Pinedale Glaciation, approximately 12,000 to 18,000 years ago (Richmond 1986; Wallace and Lindsey 1996; Armour et al. 2002). Three valley glaciers contributed bouldery sediment to construction of the moraine, which built up during a temporary stillstand, as the glaciers retreated upvalley. Buried blocks of stagnant ice later melted away leaving numerous closed depressions, known as kettleholes, many of which contain lakes or bogs today.

View from Cordova Pass westward to the Culebra Range. Typical spring snow cover depicted in May 2004. Glaciers in three valleys (1,2,3) descended and joined to build a massive moraine complex where Blue and Bear lakes are located.
View from crest of Culebra Range down to the east. Bogs are situated in kettleholes of the moraine complex at Blue and Bear lakes. 1 - Cottongrass bog, 2 - Beaver bog. Photo date Sept. 2005.

Bog descriptions

Two bogs described here are both situated in kettleholes within the moraine complex between Blue and Bear lakes (see map above). They are nearly identical in their overall physical settings in terms of climate, geology, and surrounding spruce-aspen forest vegetation. The substratum is composed of till rich in sandstone and lacking limestone. Both contain peat more than 2 m deep beneath the surface. Yet the two bogs are strikingly different in their morphology, vegetation, and surface water. This difference is ascribed primarily to the impact of beaver.

Panoramic view of Cottongrass bog looking southward; east to left, west to right. Picture is assembled from three images that were taken from a tall tree (~8 m high) on north side of bog. E. Volkova stands in water to left side. Images taken in late Aug. 2005.
Panoramic view of Beaver bog looking toward northwest; east to right, west to left. Picture is assembled from three images that were taken from a tall tree (~8 m high) on southeast side of bog. Images taken in late Aug. 2005.

Cottongrass bog

This informal name indicates the presence of cottongrass (Eriophorum angustifolium) growing around the periphery of the bog. The mire is elliptical, ~40 m x 50 m in dimension. The surface is largely open, and shallow water extends across the entire bog without any discernible flow. Vegetation is arranged in concentric bands, and the center of the bog has somewhat taller emergent vegetation and deeper water; some small spruce trees occupy the western margin. This bog has a nearly ideal arrangement of vegetation zones and water depths around its central point; the site has no evidence for beaver activity.

View looking southward over central and western margin of Cottongrass bog. Small trees and woody vegetation can be seen on the near margin. Image date late Aug. 2005.
Left: western margin of the bog with cottongrass in the foreground. Right: closeup view of cottongrass. Image date late Aug. 2005.
Closeup view of bog center showing higher stand of emergent aquatic vegetion. Image date late Aug. 2005.
Closeup view of moss growing beneath various grass and herbaceous plants. Moss along with roots of other plants is the primary constituent of the underlying peat. Image date late Aug. 2005.

Beaver bog

Named for the obvious evidence of past beaver activity, this bog has highly varied vegetation cover, along with numerous water channels, and several generations of beaver dams in different portions of the mire complex. The overall shape is elliptical, ~55 m x 75 m in size. The beaver dams are in a state of disrepair, and water level is currently ~ m below the crest of the main dam. Brushy vegetation occupies much of the mire, and older dead spruce remain standing toward the eastern side.

Panoramic view of the Beaver bog. Notice numerous channels cutting through the bog surface. Peat is exposed in walls and floors of these channels. Dead spruce trees to right stand on an old beaver dam.

View from northern margin of the bog toward the southwest. Notice brushy vegetation, which indicates a relatively low water table beneath the surface. Image date late Aug. 2005.
Soil pit in the brushy zone, near center of bog. A temperature data logger is buried ~ m deep. Notice peat piled behind the hole and lack of water at bottom of hole, indicating low water table. Image date early Aug. 2006.

A long-term soil temperature record was collected from early September 2005 until early August 2006. The temperature logger was buried ~ m deep in peat near the center of the bog. Beginning in late summer just below 10C (50F), temperature declined steady reaching a stable level just above freezing in January 2006. This stable temperature continued until May, after which temperature gradually increased, finally reaching ~10C again in late July 2006.

This region currently is included in plant hardiness zone 5, meaning that minimum winter air temperature is typically in the -10 to -20F range (-23 to -29C)--see hardiness zone map. However, the insulating effect of peat and snow cover prevented freezing at shallow depth in the bog throughout the long, cold winter. This temperature regime is characteristic of many bogs in cold climates and has important implications for year-round biologic activity and organic conditions in the shallow peat.

Soil temperature record for the Beaver bog between
Blue and Bear lakes, upper Cucharas Creek valley.

A peat core was collected from near the center of Beaver bog to determine peat thickness and date the age of the basal peat. The core was 2 m long, and the basal peat yielded a conventional radiocarbon age of 342040 years before present (BP) with a corrected calendar date of 3700 to 3630 years BP (Beta 2007). This suggests that peat accumulation begin nearly four millennia ago during the Neoglacial period of cooler, wetter climate in the late Holocene. Given a thickness of 2 m and maximum age of 3700 years, the average rate of peat accumulation is about two-thirds mm per year, which is typical for bogs in cold climates.

Peat core extracted from near the center of Beaver bog by students in field geology. Left: coring tool extended to 2 m length. Right: core laid out on plastic. Below: closeup view of the basal end of the core. Photos taken June 2007.


References

Notes

  1. ESU M.S. theses and research projects conducted by Obie Pepper, Jet Tilton, Steven Veatch, and Jon Vopata.
  2. Visting scholars: Volli Kalm, University of Tartu, Estonia; Elena Volkova, Tula State Pedagogical University, Russia.

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© Notice: Wetland Environments is presented for the use and benefit of students enrolled at Emporia State University. Any other use of text, imagery or curriculum materials is prohibited without permission of the instructor, J.S. Aber (2009).