The Floor of South Park

The floor of South Park supports four principal natural environments:

1. Grasslands on dry terraces, slopes, and hills with deep water tables.

2. Streams and their floodplains.

3. Wetlands fed by groundwater.

4. Salt flats.

The floor has been highly modified over the past hundred years by agricultural water diversion, flood irrigation, the clearing of riparian vegetation, and the replacement of indigenous bison with cattle. The last wild Colorado bison was killed in the northeastern portion of South Park early in the twentieth century.

Dry terraces, slopes, and hills throughout South Park support vegetation dominated by grass and sedge (figure 3.9). Colorado's other intermountain parks and basins, North Park, Middle Park, and the San Luis Valley, have shrub-dominated vegetation, with big sagebrush (Seriphidium tridentatum) and greasewood (Sarcobatus vermiculatus) being the most abundant species. However, these species are rare in South Park. The dominance of grasses most likely results from the annual precipitation peak in summer being coincident with the growing season. This coincidence means that plants use the water during the season in which it falls, and soil water reserves fail to accumulate, as they do when the annual precipitation peak occurs in the winter. In regions such as the Great Basin, where precipitation peaks in the winter, melting snow in the spring promotes a short-term abundance of water and deep recharge in the soil. The water therefore is most available to deep-rooting shrubs. The late summer monsoon-driven precipitation peak allows the warm-season grass blue grama to dominate. It is the most abundant grass on the shortgrass prairies of the western Great Plains from Alberta to Texas and is also widespread in the Great Basin. This species provided excellent forage for native and introduced ungulates.

On the level terraces and slopes the sedge Carex stenophylla ssp. eleocharis and fringed sage dominate rich steppe communities that have an abundance of ground lichens, particularly Xanthoparmelia chlorochroa. The two vascular plant species are common not only in montane and northern Great Plains grassland sites in North America, but also on the steppes of central and eastern Asia (Hulten, 1968; Korotkov et al., 1991). The similarity of this South Park steppe community to steppe communities in central Asia is striking. C. stenophylla ssp. eleocharis is synonymous with C. duriuscula, a steppe dominant in eastern Siberia (Yurtsev, 1963).

Steeper hills with gravel soils support a different steppe community dominated by the bunchgrasses Arizona fescue and mountain muhly. The bunchgrasses provide a sharp contrast to the more turf-forming shortgrasses and sedges, like blue grama grass. Arizona fescue is a dominant montane grass in the southwestern United States and Mexico, reaching its northern limit in Colorado in the area of South Park. Its distribution may be closely tied to monsoon rains.

Floodplains through South Park have historically been vegetated by thickets of mountain, plainleaf, and barrenground willow. However, most shrubs have by now been removed from riparian areas to permit cultivation of floodplains for grazing and mowing.

In a few areas the floodplains are too saline for willows and instead are dominated by salt marsh and salt flat plants. Salt

FIGURE 3.10 Salt flats southeast of Antero Reservoir. The dark, low plant is glasswort, and the bunchgrass is alkali grass. An unvegetated salt pan is seen at the middle right, with free salts on the soil surface.

marshes have seasonally standing water, whereas salt flats have high water tables but never standing water. The capillary fringe brings groundwater into the upper soil profile, where it evaporates, causing salts to accumulate. Salt-dominated ecosystems are widespread in the southern portion of South Park, particularly around Antero Reservoir (figure 3.10). This area has provided abundant salt licks for native mammals for millennia. Salt flats are largely barren yet support populations of the salt specialists alkali grass, saltgrass, glasswort, and sea-blight (Ungar, 1974). These sites also support an eastern Siberian salt flat specialist, the mustard Thellungiella salsuginosa.

Wetter saline marshes that periodically or regularly have standing water are strikingly similar to coastal salt marshes, dominated by three-square bulrush, alkali bulrush, arrow-grass, and sea milkwort.

Where groundwater discharges to the ground surface, the soils remain perennially saturated. Wetland plants grow over these springs, forming highly productive clones. The leaves, roots, and stems of these plants decompose only partly in the waterlogged and oxygen-poor soils, and over thousands of years an organic soil forms, called peat or muck. Most peat-lands in the northern and central portions of South Park are fed by calcareous water draining from the Mosquito Range and are the only calcareous peatlands, known as extreme rich fens, south of Jackson Hole, Wyoming. (A fen is a peatland supported by groundwater.) South Park's peatlands contain a number of unique plant communities, and probably have more rare and disjunct populations of plants than any other ecosystem type in Colorado (Cooper, 1996; Cooper and Sanderson, 1997). Two main community complexes occur in South Park's peatlands: water tracks and hummock-hollow complexes. Where springs discharge large volumes of ground-water to the surface, sheet flows of water develop, which are termed water tracks. Water tracks support communities dominated by spikerush and arrowgrasses. The other major community complex is created where peat hummocks up to 40 cm tall have developed from the long-term growth of tussock-forming sedges, such as elk sedges and Porter's feathergrass (figure 3.11). Kobresia simpliciuscula is a common circumpolar plant species of high-latitude and high-mountain regions. Its abundance in South Park is a striking example of the local abundance of a widely disjunct northern montane species (figure 3.12). Ptilagrostis porteri is the only North American species of feathergrass. It is closely allied with a large group of feathergrasses that characterize steppe and peat-lands in central Asia (figure 3.13). Its geographic relationships are typical of a group of species that reflect the striking similarity of the floras of South Park and central Asia.

The peat hummocks provide habitat for many of the rare plants occurring in the calcareous fens, including Greenland primrose (Primula egaliksensis), small bulrush (Trichophorum pumilum), and myrtle leaf willow (Salix myrtillifolia). These are all widely disjunct from the main ranges of their species farther north.

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