Persistence of high elevation fens in the Southern Rocky Mountains, on Grand Mesa, Colorado, U.S.A.

Abstract

Small headwater fens at high elevations exist in the dry climatic regime of western Colorado, despite increasing demands for water development since the 1800’s. Fens on Grand Mesa have accumulated plant material as peat for thousands of years due to cold temperatures and consistently saturated soils. The peatlands maintain unique plant communities, wildlife habitat, biodiversity, and carbon storage. We located and differentiated 88 fens from 15 wet meadows and 2 marshes on Grand Mesa. Field work included determining vegetation, soils, moisture regimes, and impacts from human activities. All fens were groundwater-supported systems that occurred in depressions and slopes within sedimentary landslide and volcanic glacial till landscapes. Fens occupied 400 ha or less than 1 % of the 46,845 ha research area and ranged in size from 1 to 46 ha. Peat water pH in undisturbed sites ranged from 4.3 to 7.1. Most fens had plant communities dominated by sedges (Carex) with an understory of brown mosses. Variation in vegetation was controlled by stand wetness, water table level, organic C, conductivity (EC), and temperature °C. Fen soils ranged from 13.6 to 44.1 % organic C with a mean of 30.3 %. Species diversity in fens was restricted by cold short growing seasons, stressful anaerobic conditions, and disturbance. Multivariate analysis was used to analyze relationships between vegetation, environmental, and impact variables. Stand wetness, water table level, OC, electrical conductivity (EC), and temperature were used to analyze vegetation variance in undisturbed fens, wet meadows, and marshes. Vegetation composition in impacted fens was influenced by flooding, sedimentation, stand wetness, water table level, OC, EC, and temperature. Hydrologically modified fens supported 58 plant species compared to 101 species in undisturbed fens. Analysis of historical 1936–2007 aerial photographs and condition scalars helped quantify impacts of human activities in fens as well as vegetation changes. Fourteen fens had evidence of peat subsidence, from organic soil collapse, blocks of peat in the margins, soil instability, and differences in surface peat height between the fen soil surface and the annually flooded soil surface. Of 374 ha of fens in the Grand Mesa study area, 294 ha (79 %) have been impacted by human activities such as ditching, drainage, flooding, or vehicular rutting. Many fens had little restoration potential due to severe hydrological and peat mass impacts, water rights, or the cost of restoration.

Appendices

Appendix 1

Historic aerial photographs of impacts in Kennecott Fen on Grand Mesa. Top left clockwise, 1936, 1956, 1978, 2011. The 1936 and 1956 photographs indicate that the primary peat mass is still intact. The 1978 photograph shows flooding (dark color) and the peat mass breaking apart. In the Earth Google (2011) photograph the peat mass has sunken to the bottom of the reservoir and any remaining plant communities are drowning.

Appendix 2

Descriptions of the 16 fen, marsh, wet meadow, and modified fen plant communities.

Semi-aquatic communities

• (1) Nuphar lutea ssp. polysepala—Potamogeton nodosus a non-peat forming floating aquatic plant community, occurred in fen pools (n = 10) and marshes (n = 1). Similar communities occur in Wyoming (Cooper and Andrus 1994; Heidel and Laursen 2003; Lemly 2007), Alaska (Viereck et al. 1992), Idaho, Montana, Oregon, Washington, California, and British Columbia (NatureServe 2008).

• (2) Eleocharis macrostachya—Potamogeton foliosus; a semi-aquatic non-peat-forming plant community, occurred primarily in the pools of basins that had been modified for reservoir use. Similar communities (A2) occur primarily in hydrologically modified fens on Grand Mesa and marshes with fluctuating water levels across North America (Kartesz 1999).

Large sedge communities

• (3) Carex vesicaria dominated a peat-forming plant community that was distributed widely across the study area, primarily in fens and wet meadow basins. Standing water occurred during measurement periods. Similar communities occur in Colorado (Carsey et al. 2003), Wyoming (Heidel and Laursen 2003; Heidel and Jones 2006), Montana, Idaho, Washington, Oregon, and California (NatureServe 2008).

• (4) Carex utriculata dominated a peat-forming plant community that was widespread in the study area. Carex utriculata dominated vegetation occurs widely in Alaska (Viereck et al. 1992), Colorado (Cooper 1990; Carsey et al. 2003), and Wyoming (Cooper and Andrus 1994; Heidel and Laursen 2003).

• (5) Carex saxatilis—Drepanocladus aduncus was the most widespread peat-forming plant community in fens (n = 54) and wet meadows (n = 9) in the study area. Similar communities occur in Colorado (Carsey et al. 2003), Alaska (Aiken et al. 1999), western Canada (Gignac et al. 2004), Utah, Montana, and Washington (Natureserve 2008).

Floating mat and bryophyte communities

• (6) Menyanthes trifoliata dominated a peat-forming plant community found only in seasonal to perennial pools in basins and small depressions. It is rare in the study area and occurred only in undisturbed sites. Similar communities occur in Alaska (Dachnowski-Stokes 1941; Viereck et al. 1992).

• (7) Carex limosa—Sphagnum teres—Calliergon cordifolium is a peat-forming plant community that occurred only in basins with consistent soil saturation and mean water table depth of 6 cm. Similar communities are found in Wyoming (Cooper and Andrus 1994; Heidel and Laursen 2003; Heidel and Jones 2006; Lemly 2007), Montana (Cooper and Jones 2004), Colorado (Johnson and Steingraeber 2003), Minnesota (Glaser et al. 1981), California (Wolf and Cooper 2015), and Canada (Gignac et al. 2004).

• (8) Calliergon stramineum—Carex limosa—Menyanthes trifoliata is a peat-forming plant community found in basins with open water.

• (9) Sphagnum teres—Calliergon cordifolium—Carex canescens, a peat-forming plant community found in basin fens with floating peat mats. This community is similar to C. canescens communities in Wyoming (Cooper and Andrus 1994; Heidel and Laursen 2003) and Canada (Gignac et al. 2004).

Small sedge communities

• (10) Carex aquatilis—Drepanocladus aduncus is the second most widespread peat-forming plant community found in fens (n = 37) and wet meadows (n = 3). Stands occur along wetter margins of fens or as the dominant community in wet meadows. Similar communities are common in Colorado (Johnston 1987; Cooper 1990; Carsey et al. 2003), Wyoming (Cooper and Andrus 1994; Heidel and Laursen 2003; Heidel and Jones 2006), Montana (Cooper and Jones 2004), and Canada (Gignac et al. 2004).

• (11) Carex simulata—Drepanocladus aduncus, a peat producing community in fens (n = 16) and wet meadows (n = 2) occurred on gently sloping hillsides and small depressions. These stands had higher EC and high canopy cover of D. aduncus. Similar communities occur in mid to high elevations in Colorado (Cooper 1990; Carsey et al. 2003; Rocchio 2004), Wyoming (Heidel and Laursen 2003; Lemly 2007), Montana (Cooper and Jones 2004), California (Wolf and Cooper 2015), Nevada, Utah, Oregon, and Idaho (NatureServe 2008).

• (12) Eleocharis quinqueflora—Drepanocladus aduncus—Carex simulata, a widespread peat-forming plant community found in fens (n = 24) and wet meadows (n = 7) on slopes and in depressions. Similar communities occur across the western United States (NatureServe 2008), including Colorado (Cooper 1990; Carsey et al. 2003), Wyoming (Heidel and Laursen 2003; Lemly 2007), and Montana (Cooper and Jones 2004).

• (13) Carex illota—Aulacomnium palustre—Pedicularis groenlandica, a peat-forming community found along the margins of seven basin fens and one wet meadow. Similar communities occur in Wyoming (Cooper and Andrus 1994), California (Wolf and Cooper 2015), Colorado (Carsey et al. 2003), and Oregon (NatureServe 2008).

• (14) Eleocharis acicularis—Hippuris vulgaris, a non-peat forming plant community found only in hydrologically modified fens with flooding, drainage, fluctuating water tables, and bare peat. Similar communities occur in Colorado, Wyoming, and California and typically has little plant diversity (NatureServe 2008).

Shrub and Forest communities

• (15) Salix planifolia—Calamagrostis canadensis occurred on the margins of fens and wet meadows. This type is common on Grand Mesa and in Colorado (Cooper 1990; Carsey et al. 2003; Johnson and Steingraeber 2003), Alaska (Viereck et al. 1992), Utah, and Wyoming (NatureServe 2008).

• (16) Picea engelmannii—Salix planifolia—Climacium dendroides occurred on the margins of fens and wet meadows. Of 13 stands, 11 were fens. Johnston (1987) classified a Picea engelmannii community in Colorado which is possibly related to a similar community (F1) on Grand Mesa.