Southwestern Rare and Endangered Plants: Proceedings of the Fourth Conference

Dune Communities of SE Colorado: Patterns of Rarity, Disjunction and Succession 1 2 1 2 3 T. KELSO *, N. BOWER , P. HALTEMAN , K. TENNEY , AND S. WEAVER *Author for correspondence 1 Dept of Biology, Colorado College, Colorado Springs, CO 80903 2 Dept of Chemistry, Colorado College, Colorado Springs, CO 80903 3 Dept of Geology, Colorado College, Colorado Springs, CO 80903 ABSTRACT. Dune communities occur across the western Great Plains and in isolated spots in eastern Colorado. They are biologically important due to their endemic nature, their rapid succession, and their ephemeral abundance in response to climate, grazing practices, and ranchland management. The abundance of these terrestrial islands has changed considerably over scales from tens to thousands of years. The Colorado dune communities have high conservation value due to their unusual biota and diminished presence. They also are of value as sentinel communities for more wide-scale biotic change in surrounding grasslands. These communities have not received detailed documentation of their interactive biotic and geological profiles in recent years. This study provides such a profile for an isolated dune complex in El Paso County, Colorado where we examine their plant species, vegetation patterns, and geochemical characteristics. Dune communities are threatened in part because of ranchland practices that seek to diminish their presence. We identify here areas of mutual interest and potential collaboration between ranchers and biologists that might serve to mitigate conflicts between conservation goals for a unique biota and the practical exigencies of ranchland management in semi-arid grasslands. INTRODUCTION drought and inappropriate agricultural Dune “blowouts” occur as craters practices. of unstable sand in dune fields around Signature vegetation patterns with the globe. Across arid grasslands of unique elements of the grasslands flora southeastern Colorado, New Mexico and (Rydberg, 1895; Pool, 1914; Rameley, the western Great Plains, dune extent has 1939) occur across blowouts on their varied over scales that range from slopes, crests, and crater bottoms. thousands of years to less than a decade, Although not species-rich, blowout responding to patterns of temperature, communities are of conservation concern precipitation and aeolian activity as well in Colorado (Colorado Natural Heritage as land use practices. Reports from Program, 1999; 2001) due to their nineteenth century explorers suggest that infrequency and the habitat endemism of dunefields were then considerably more the biota, in which some members are extensive than today (Muhs and rare. Others exemplify disjunctions in Holliday, 1995), although expansion distribution where substantial occurred during the 1930’s and 1950’s populations appear only in habitat (McGinnies and others, 1991) due to islands across an intervening matrix of more vegetated grasslands on stabilized 39 soils. From a ranching point of view, systems under climatic and grazing however, blowout communities are stress. problematic. They are prone to spreading Our study focused on a dune complex and thus diminish range already limited ca 50 km southeast of Colorado Springs in quality and quantity. For decades, Colorado in the Chico Basin region of El ranchers have expended considerable Paso County (Fig. 1). We held the effort to eradicate blowouts with following objectives: attempts at revegetation and sand control. The prospect of a continuing To document plant drought and its related economic stresses community stratification have understandably exacerbated these across the Chico Basin dunes concerns. and the dominant plant species that represent different As land management practices have stages of succession here. aimed at dune minimization conservation interest in these restricted To document the regional communities has risen (Colorado Natural dune flora with particular Heritage Program 2001) for several attention to the abundance and reasons. In addition to their unique ecology of rare species. biota, dune communities may be highly To provide a detailed sensitive indicators of short and topographic profile of the longterm climate change (Muhs and dune complex. Maat, 1993; Muhs and Holliday, 1995; Muhs and others, 1996; Muhs and To assess differences of soil others, 1999, Swinehart, 1998). Early texture on different dune faces studies on plant succession in dune that correlate with vegetation communities of Nebraska (Weaver, zonation. 1965, 1968; Weaver and Albertson, To provide a botanical and 1954) and northern Colorado (Ramaley, geological baseline for 1939) described species-specific phases comparison with other dune that reflected interactive aspects of complexes in southeastern vegetation, climate and soil stabilization. Colorado and long term In light of increasing concern about monitoring. global warming and ecological impacts of climate change on grassland To provide a vegetation-based communities, the rapid succession on system of assessment for dune dune complexes in response to seasonal succession to assist local moisture and wind (McGinnies and ranchers in understanding the others, 1991) suggests the potential to dynamics of local dune use dunes as sentinel communities for communities. other more subtle changes in To suggest strategies that surrounding grasslands. Monitoring might balance conservation of small-scale community changes could dune biota with the provide an early opportunity to detect management needs of wider scale changes and implement ranchers. management strategies for grassland 40 northeastern Colorado by ca 270 km, although a few minor blowouts occur within 20 km. This dune system consists of three blowout complexes; our primary study site was the largest and most northerly of them, covering about four km2 with several connected parabolic dunes. Open craters occur primarily on the north and northwest faces, the direction of prevailing winds in the FIGURE 1. Location of Chico Basin dune system in El Paso County. winter. Like other parabolic dune systems, these show METHODS long trailing edges in the windward direction (Pye and Tsoar, Study Site. The dominant vegetation 1990) with dune movement to the south. of the Chico Basin region contains a Aerial photographic records shows complex of sand-sage and shortgrass progressive dune stabilization and prairies underlain by deep aeolian sands steadily increasing vegetative cover (Natural Resources Conservation across the regional sand sheet from 1955 Service, 2004). The area has been to the present; where significant recognized by the Nature Conservancy blowouts were numerous across the as a significant conservation region for landscape in the 1950’s, the Chico southeastern grasslands (The Nature complex is the only one remaining now. Conservancy, 1998); it contains a number of potential conservation sites Vegetation Sampling. We visited the with high biotic values for the county site regularly from May to October 2003 (Colorado Natural Heritage Program, for botanical surveys; preliminary 2001) . One of those sites is the dune surveys had been conducted on other complex. Given a history of intensive dunes and blowouts in the area in 1999, grazing, most of the grasslands have 2000, and 2001. Vegetation been impacted from their historical stratification and dominant species composition and the dunes are across different dune faces were surrounded by dense stands of sand-sage assessed throughout the growing season with a few interspersed native grasses. with voucher specimens deposited in the Current grazing practices implemented Colorado College Herbarium (COCO). over the past four years restrict cattle Overall conditions with respect to access to the dunes. The open sand rainfall and temperature in 2003 were complexes here occur in a three-dune consistent with 30-year averages after cluster over a regional sand sheet of ca the exceptional drought year of 2002 1000 ha overall; the dunes are isolated (Table 1). from other large complexes in 41 TABLE 1. Summer temperature and precipitation for RESULTS Pueblo, CO. Data from www.crh.noaa/gov/pub/climate indicate short and Soils. The results of the soil particle longterm means for the months of June-August size fractionation are presented in Fig. 3. Year Daily Total Precipitation The samples collected from the most Mean o F (inches) exposed locations (6, 8) were generally 1999 75.3 5.03 on the windward side of the dunes, or in 2000 74.2 4.88 saddles between ridges. These locations 2001 74.2 5.80 2002 76.5 1.57 are the most barren of vegetation and 2003 75.0 5.21 have the largest mean particle sizes. mean 1971-2000 5.64 Samples from the leeward side of the Soil Sampling and Topographic dunes have on average a finer grain size Mapping. A baseline profile of the dune and a more bimodal distribution of grain using a Trimble GPS unit was conducted sizes. These sites (2, 9) exhibit more in October, 2003 and soil samples plant cover than that found on the corresponding to different dune aspects windward sites. The third group (1, 5, 7) were collected and plotted at that time. exhibits the greatest plant cover with Soil analyses were performed in the samples taken from enclosed bowls and laboratory at Colorado College in the leeward sites. The final group of winter of 2003-2004. Physical samples (4, 10, 11) is from locations on fractionation was achieved with a series the south and northeast edge of the of sieves (Tyler #10, 25, 40, 50, 60, 70, complex (4 and 10, respectively) where 120, 140, and 230). The

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