Development of a Grazing Monitoring Program for Great 

Sand Dunes National Park, Colorado

By Linda C. Zeigenfuss and Kathryn A. Schoenecker

Open-File Report 2015–1136

U.S. Department of the Interior

U.S. Geological Survey
ii

U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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Suggested citation:

Zeigenfuss, L.C., and Schoenecker, K.A., 2015, Development of a grazing monitoring program for Great Sand 

Dunes National Park: U.S. Geological Survey, Open-File Report 2015–1136, 44 p., 

http://dx.doi.org/10.3133/ofr20151136.

ISSN 2331-1258 (online)

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secured from the copyright owner.
iii

Contents

Abstract ...................................................................................................................................................................... 1

Introduction................................................................................................................................................................. 1

Study Area.................................................................................................................................................................. 3

Location .................................................................................................................................................................. 3

Vegetation............................................................................................................................................................... 5

Hydrology................................................................................................................................................................ 8

Ungulate Populations and Grazing History ............................................................................................................. 8

Vegetation Communities and Existing Data.............................................................................................................. 10

Woody Riparian Communities............................................................................................................................... 10

Cottonwood ....................................................................................................................................................... 11

Willow................................................................................................................................................................ 15

Wetlands............................................................................................................................................................... 18

Salt Flat Wetlands ............................................................................................................................................. 21

Wet Meadows.................................................................................................................................................... 22

Riparian Wetlands ............................................................................................................................................. 23

Marshes............................................................................................................................................................. 23

Grazing Effects in Wetlands .............................................................................................................................. 24

Upland Shrub and Grassland................................................................................................................................ 27

Rabbitbrush....................................................................................................................................................... 28

Greasewood...................................................................................................................................................... 29

Herbaceous Stabilized Dunes ........................................................................................................................... 30

Piedmont Semi-Desert Grasslands ................................................................................................................... 30

Other Plant Communities of Interest ..................................................................................................................... 30

Alpine Shrub and Herbaceous........................................................................................................................... 30

Aspen ................................................................................................................................................................ 31

Design of an Ungulate-Vegetation Monitoring Program............................................................................................ 32

Selecting Monitoring Methods............................................................................................................................... 33

Sampling Design and Site Selection ..................................................................................................................... 34

Baseline Data Collection....................................................................................................................................... 35

Conclusion................................................................................................................................................................ 35

Acknowledgments..................................................................................................................................................... 36

References Cited...................................................................................................................................................... 36

Figures

1. Land ownership of the Great Sand Dunes region in the San Luis Valley, Colorado. Inset shows study

area in relation to landmarks in the state of Colorado.................................................................................... 4

2. Vegetation classifications of ungulate vegetation monitoring study area in Great Sand Dunes National 

Park, Colorado............................................................................................................................................... 6

3. Apparent changes in stream channel of lower Sand Creek, Great Sand Dunes National Park and 

Preserve, Colorado...................................................................................................................................... 14

4. Additional areas with a significant component of willow identified by 2011 U.S. Geological Survey 

(USGS) survey in lower Sand Creek compared to vegetation map classification (left) and overlaid 

on aerial photo (right)................................................................................................................................... 16
iv

5. Wetland vegetation map created by the National Park Service Rocky Mountain Vital Signs Monitoring 

Network (ROMN) correlated to vegetation types of the Salas and others (2011) Great Sand Dunes 

area vegetation map for the primary concentration of wetlands on and surrounding the Medano 

Ranch bison range....................................................................................................................................... 19

Tables

1. Vegetation types of Great Sand Dunes National Park, Colorado study area as defined by the park 

vegetation map (Salas and others, 2011), Rocky Mountain Vital Signs Monitoring Network (ROMN) 

wetland classifications, and pooled into potential monitoring categories ....................................................... 7

2. Wetland vegetation types created by the National Park Service Rocky Mountain Vital Signs Monitoring 

Network (ROMN) correlated to vegetation types of the Salas and others (2011) Great Sand Dunes area 

vegetation map and Schoenecker’s (2012) research study classification for the primary concentration of 

wetlands on and surrounding the Medano Ranch bison range.................................................................... 20

Abbreviations

CPW Colorado Parks and Wildlife

ESD Ecological Site Description

FWS U.S. Fish and Wildlife Service

GRSA Great Sand Dunes National Park and Preserve

GMP general management plan

NRCS Natural Resources Conservation Service

NPS National Park Service

NWR National Wildlife Refuge

ROMN Rocky Mountain Network of NPS Vital Signs Monitoring Program

TNC The Nature Conservancy

UMP ungulate management plan

USGS U.S. Geological Survey

WEI wetland ecological integrity
v

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

meter (m) 3.281 foot (ft.) 

kilometer (km) 0.6214 mile (mi)

meter (m) 1.094 yard (yd.) 

Area

square meter (m2) 0.0002471 acre 

hectare (ha) 2.471 acre

square kilometer (km2) 247.1 acre

square centimeter (cm2) 0.001076 square foot (ft2)

square meter (m2) 10.76 square foot (ft2) 

square centimeter (cm2) 0.1550 square inch (ft2) 

hectare (ha) 0.003861 square mile (mi2) 

square kilometer (km2) 0.3861 square mile (mi2)

Volume

cubic meter (m3) 35.31 cubic foot (ft3)

cubic meter (m3) 1.308 cubic yard (yd3) 

Mass

gram (g) 0.03527 ounce, avoirdupois (oz.)

kilogram (kg) 2.205 pound avoirdupois (lb.)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Horizontal coordinate information is referenced to North American Datum of 1983 (NAD 83)

Altitude, as used in this report, refers to distance above the vertical datum.
1

Development of a Grazing Monitoring Program for Great 

Sand Dunes National Park, Colorado

By Linda C. Zeigenfuss and Kathryn A. Schoenecker

Abstract

National parks in the United States face the difficult task of managing natural resources within 

park boundaries that are influenced to a large degree by historical land uses or by forces outside of the 

park’s protection and mandate. Among the many challenges faced by parks is management of wildlife 

populations that occupy larger landscapes than individual park units but that concentrate within park 

lands both seasonally and opportunistically. Great Sand Dunes National Park and Preserve in southcentral Colorado is currently developing an Ungulate Management Plan to address management of elk 

and bison populations within the park. Execution of the Ungulate Management Plan will require 

monitoring and assessment of habitat conditions in areas that appear sensitive to ungulate use or heavily

used by elk and bison. Several sources of information on the various habitats within the park and their 

use and response to foraging elk and bison exist from recent and on-going research in Great Sand Dunes 

National Park and Preserve as well as from studies in other regions of the Intermountain West. All of 

this data can be used to inform the planning process. This report provides background on vegetation 

types that make up the primary bison and elk ranges in Great Sand Dunes National Park and Preserve 

and on the potential effects of ungulate grazing and browsing in these specific vegetation communities 

(both locally and regionally). The report also provides a review of the elements necessary to develop a 

long-term monitoring program for Great Sand Dunes National Park and Preserve that addresses both the 

responses to ungulate herbivory seen in important habitats in the park and the amount and patterns of 

ungulate habitat use.

Introduction

In 2000, the U.S. Congress authorized the expansion of the former Great Sand Dunes National 

Monument by establishing Great Sand Dunes National Park and Preserve (GRSA) in its place, and 

establishing the new Baca National Wildlife Refuge (NWR) administered by the U.S. Fish and Wildlife 

Service (FWS). Great Sand Dunes National Park and Preserve covers 605 square kilometers (km2

) of 

land in south-central Colorado extending from the floor of the San Luis Valley to the crest of the Sangre 

de Cristo Mountains. The area encompassed by this designation protects a number of natural wonders 

and features including a unique ecosystem of natural sand dunes, the entire watershed of surface and 

groundwaters necessary to preserve and recharge the dunes and adjacent wetlands, and other valuable 

riparian and upland vegetation communities that support a host of associated wildlife and bird species.

Bison (Bison bison), elk (Cervus elaphus), mule deer (Odocoileus hemionus), bighorn sheep 

(Ovis canadensis), and pronghorn antelope (Antilocapra americana) were native to the area until about 
2

the turn of the 20th century, when bison were extirpated (Meany and Van Vuren, 1993). Elk and 

pronghorn survived in small numbers (Warren, 1910; Cary, 1911; Swift, 1945). Elk and pronghorn 

likely moved back into the area from surrounding populations to the north and south, as well as from 

nearby animal releases by the Colorado Division of Wildlife from the 1910s through the 1960s (Swift, 

1945; Colorado Division of Wildlife, 2008), and their populations increased over time. Mule deer 

populations have varied through time (Cary, 1911; Colorado Parks and Wildlife, 2014). Bison were 

reintroduced to a limited area of the landscape when a commercial bison herd was established in 1989 

on the private Medano and Zapata ranches adjacent to GRSA to the south and west (fig. 1). The 

Medano-Zapata Ranch and the bison herd are now owned by The Nature Conservancy (TNC) and 

managed by Ranchlands/Zapata Partners under a contract with TNC. Livestock, including sheep (Ovis 

aries) and cattle (Bos taurus), were grazed historically throughout the study area, especially in the sand 

sheet type within the National Park boundary.

In 2009, Great Sand Dunes National Park acquired 7,000 hectares (ha) of Medano Ranch land

that adjoins the park and was formerly managed by the State of Colorado (fig. 1). No fences separated 

these former state lands from other lands on the ranch; thus, bison grazing occurs on the National Park 

Service (NPS) lands within the Medano Ranch. Wild herds of elk, mule deer, and pronghorn also graze 

these lands to varying degrees. When the National Park was initially established, there were concerns 

about over-concentrations of a large and possibly growing elk population and the population’s effects on 

native plant communities. It was anticipated that elk might seek refuge from hunting in the newly 

expanded park unit and TNC lands where they might over-concentrate and affect unique vegetation 

communities (Schoenecker and others, 2006). In these sites of potential refugia, or at preferred loafing 

sites where elk may rest and seek shade, the combined foraging and trampling by elk and bison could 

accelerate a decline in density, regeneration, and(or) production of woody riparian shrubs and trees as 

well as affecting understory plants. Elk and bison may also exacerbate possible existing declines that are 

already occurring due to changes in hydrology, climatic, or dunal processes.

The State of Colorado manages the elk herd outside of NPS and FWS federal lands, whereas the

federal agencies are responsible for management of the elk herd on lands under their jurisdiction. Great 

Sand Dunes National Park is in the process of creating an ungulate management plan (UMP) that will 

cover management of elk and bison within the park and preserve over the next 20 years (yr). The 

establishment of a free-ranging herd of 400–1,000 bison on NPS lands has been proposed (U.S. 

Department of the Interior, 2014a, 2014b); however, it is unknown how a herd ranging over the larger 

landscape of NPS lands would affect this expanded range. The UMP will incorporate long-term 

monitoring of the effects of ungulate grazing on habitat condition and potentially population size and 

distribution of ungulate species.

Management decisions concerning bison and elk could have implications for the density and 

distribution of ungulates beyond those areas grazed by both species. Singer and Norland (1994) found 

that when not controlled by predators or human management, bison in Yellowstone National Park 

expanded their range, thus increasing their habitat breadth and habitat overlaps with other ungulate 

species (elk, mule deer, and pronghorn). These authors concluded that, while no negative effects on 

population sizes of other ungulates were observed in response to these increases in bison range, bison

habitat breadth and overlap with all other ungulates simultaneously increased bison diet overlap with elk 

and bighorns. This overlap affected intra- and inter-specific competition (Singer and Norland, 1994).

Keller (2011) found high diet and habitat overlap between elk and bison in the Black Hills of South 

Dakota.

The purpose of this Open-File Report is to provide background on vegetation types that make up 

the primary bison and elk ranges in GRSA and to describe the potential effects of continued elk and 
3

bison grazing in these specific vegetation communities (both locally and regionally) at under current 

population sizes and management, and with increases or decreases in the amount of grazing. This report 

will also provide information on the elements necessary to develop a long-term monitoring program for 

GRSA that addresses both the responses seen in important habitats in the park and the amount and 

patterns of ungulate habitat use. This will assist managers in the development of vegetation condition 

objectives, identification of potential indicators of ungulate effects on vegetation communities, and 

creation of grazing-monitoring protocols as the UMP is finalized. The anticipated result would be 

simultaneous implementation of monitoring and management. The basic elements included here are:

• description of the study area that would likely be encompassed in a GRSA grazing 

monitoring program; 

• identification of vegetation communities in this study area, their ecological value, current 

condition, and what is known about their use by bison and elk; 

• characterization of the response to grazing of these vegetation communities based on 

available literature and summarization of the available data specific to the study area; 

• identification of areas where available data may be lacking; 

• description of vegetation monitoring methods being used both within the study area and 

on adjacent lands, to provide a basis for compatibility and comparability between 

programs; and 

• identification of sources for developing monitoring methods and sampling design 

specific to a grazing monitoring plan for GRSA. 

Study Area

Location

Great Sand Dunes National Park and Preserve covers an area of 605 km2 entirely within the San 

Luis Valley of south-central Colorado. The Park extends from the base of the foothills of the Sangre de 

Cristo Mountains in the east approximately 20 kilometers (km) across the valley floor to the west 

between the community of Baca Grande to the north and Alamosa County Lane 6 to the south. The Park 

boundary includes lands on the Medano Ranch presently owned by The Nature Conservancy. The Baca 

National Wildlife Refuge bounds the park on the west. The Preserve extends eastward from the Park to 

the crest of the Sangre de Cristo Mountains (fig. 1). The study area considered here encompasses 

approximately 413 km2 of Great Sand Dunes National Park below 2,744 meters [m] (9,000 feet [ft]) 

elevation, and includes the dune field and surrounding lands on the valley floor, as well as roughly 65 

percent (10,482 ha) of the current fenced bison range (fig. 1). Some reference is made to higher 

elevation areas of the Preserve section of GRSA outside this primary study area because these areas 

contain vegetation types that are highly sensitive, are preferred elk habitat in other regions of the Rocky 

Mountains, and have been identified as areas of potential concern by GRSA managers. 

Precipitation, primarily in the form of rainfall, averages 28 centimeters (cm) annually and falls 

mostly during monsoonal rains in July through September. Summers are warm with average daytime 

temperatures ranging from 26.5 oC to 29.5 oC on the valley floor. Winters are cold and relatively dry 

with average valley daytime temperatures ranging from –9.5 oC to 1.5oC. Elevations on the valley floor 

range from 2,285 m to 2,744 m.
4

Figure 1. Land ownership of the Great Sand Dunes region in the San Luis Valley, Colorado. Inset shows study 

area in relation to landmarks in the state of Colorado. (USFWS, U.S. Fish and Wildlife Service) 
5

Vegetation

The San Luis Valley is a high elevation desert and lies within the Great Basin desert zone 

(Barbour and Billings, 1988) and the Southern Rocky Mountains floristic zone (Daubenmire, 1943). 

West (1988) describes a saltbush (Atriplex canescens)-greasewood (Sarcobatus vermiculatus) 

vegetation type that dominates lowlands of the upper Rio Grande drainage. The vegetation is correlated 

with halomorphic (containing salt) soils, where vegetation composition and production is closely linked 

to soil characteristics (West, 1988). At the lowest elevations of the San Luis Valley, both rabbitbrush 

(Ericameria/Chrysothamnus spp.) and saltbush/greasewood vegetation types predominate. 

The valley landscape contains the highest sand dunes in North America (229 m) with associated 

vegetation. The sand dunes contain blowout grass (Redfieldia flexuosa) growing in low densities atop 

the most active dunes. Sand sheet surrounds the active dunes complex to the west, north, and south, and 

is dominated by greasewood (Sarcobatus vermiculatus) and rubber rabbitbrush (Ericameria nauseosa). 

Swales are flooded ephemerally during spring from snowmelt off the mountains, and support a dense 

ground cover of sedges (Carex spp.) and wet meadow grasses. Ephemeral wetlands form the western 

boundary of the dunes complex and are watered from natural springs and Sand Creek. A large and 

healthy population of the rare endemic, slender spider flower (Cleome multicaulis), occurs in 

ephemerally flooded meadows (Dixon, 1971). Cottonwoods (Populus spp.) and willows (primarily 

coyote willow, Salix exigua) grow atop some dunes, along Sand Creek, portions of Medano Creek, and 

near springs. Cottonwood stands occurring along Sand Creek are predominantly pure narrowleaf

cottonwood (Populus angustifolia), though some limited hybridization with other cottonwood species

has been found along Sand Creek (Phyllis Pineda Bovin, Biologist, Great Sand Dunes National Park and 

Preserve, oral commun., May 2015). Some narrowleaf cottonwoods along Sand Creek have been aged at 

more than 300 yr old and are among the oldest of this species that have been identified (David Meko, 

Tree-Ring Research Laboratory, University of Arizona, written commun. to Great Sand Dunes National 

Monument, 1986). This area is interspersed with wet meadows along creeks and mesic meadows (some 

formed by irrigation) away from riparian areas. Dominant graminoids include arctic rush (Juncus 

arcticus), needle-and-thread grass (Hesperostipa comata), Indian rice grass (Achnatherum hymenoides), 

sand dropseed (Sporobolus cryptandrus), saltgrass (Distichlis spicata), slender wheatgrass (Elymus 

trachycaulus), and beardless wildrye (Leymus triticoides). 

The current park vegetation map (hereafter, the Salas vegetation map) has 48 categories of 

vegetated lands plus an additional 8 non-vegetated, sparsely vegetated, or developed categories (Salas 

and others, 2011). However, this map covers 1,673 km2 of NPS, TNC, FWS, USFS, Bureau of Land 

Management, and private lands, but our study area encompasses only one-fourth of the vegetation map 

area. Sixteen of the vegetation types occur on the study area (table 1); however, this is many more 

vegetation types than would be practical to sample based on labor and funding constraints. Additionally, 

many of these vegetation types are not used to any great extent by either elk or bison, either for foraging 

or other uses (wallowing, resting, thermal cover, shading), and some of the vegetation types are very 

similar to one another. One type, Invasive Forbland, is not extensive in the study area (about 2.5 ha) and 

does not group neatly any with other types. Therefore, it has not been considered further in this analysis.

All vegetation types used hereafter are derived from the Salas vegetation map or the NPS Rocky 

Mountain Vital Signs Monitoring Network (ROMN) protocol for monitoring wetland ecological 

integrity [WEI] (Schweiger and others, in press). Unfortunately, a precise correlation between Salas’ 

vegetation types and ROMN’s wetland classifications is not possible, but the major alignments of types 

has been listed here (table 1) and will be described in detail in the following sections. 
6

Figure 2. Vegetation classifications of ungulate vegetation monitoring study area in Great Sand Dunes National 

Park, Colorado.
7

Table 1. Vegetation types of Great Sand Dunes National Park, Colorado study area as defined by the park vegetation map (Salas and others, 

2011), Rocky Mountain Vital Signs Monitoring Network (ROMN) wetland classifications, and pooled into potential monitoring categories. High use 

indicates that radiocollared elk and bison were observed in these habitats more than expected based on their availability on the landscape, moderate 

use indicates expected use was roughly equal to availability of the habitat, and low use indicates that elk and bison use was less than expected 

based on the amount of available habitat.

Potential Vegetation 

Monitoring Category

Vegetation Map “Formal” Classification ROMN Wetland Classification Elk use Bison 

use

Percent 

of total 

area

Willow Coyote Willow Temporarily Flooded Shrubland Alliances Low Low <1

Wetland

Emergent Marsh Alliances Riparian wetlands or marsh High High

7 

Interdunal Swale Wetland Alliances Wet meadows

Alluvial Flat Herbaceous Alliances High High San Luis Valley Mesic Meadow Alliances

Playa Alliances Salt flat wetlands Unknown Seasonal

Greasewood/Saltbush Fourwing Saltbush Shrubland Alliance

Greasewood Flat Shrubland and Steppe Alliances Low Low 7 

Greasewood Sand Deposit Shrubland and Steppe Alliances

Xeric grass Herbaceous Stabilized Dune and Sandsheet Alliances Moderate Moderate 11 Piedmont Semi-Desert Grassland Alliances

Cottonwood

Narrowleaf Cottonwood Sand Dune Woodland 

Association High Low 1 Narrowleaf Cottonwood Temporarily Flooded Woodland 

Alliance

Rabbitbrush Sandsheet Rabbitbrush Shrubland and Steppe Alliances Moderate Moderate 56 Alluvial Fan Rabbitbrush Shrubland and Steppe Alliances

Non-vegetated Non-vegetated types (water, barren sand dunes, wash) 18
8

Hydrology

The GRSA study area lies within the Closed Basin of the northern San Luis Valley: a closed 

hydrologic drainage system with no surface outlet. It is separated from the Rio Grande River by a low 

ridge. The Closed Basin lies over two aquifers, one shallow (unconfined aquifer) and one deep 

(confined aquifer; Rupert and Plummer, 2004a). These two aquifers are separated by an 8-ft thick 

impermeable layer of clay (Rupert and Plummer, 2004b). Five major streams (Deadman Creek, Pole 

Creek, Sand Creek, Little Spring Creek, and Big Spring Creek) traverse the study area from east to west. 

Medano Creek wraps around the east side of the dunes, flowing roughly south and west. Flow is 

seasonal for much of the length of Deadman, Pole, Medano, and Sand creeks once they reach the valley 

floor. In years when montane snowpack is low, these creeks may disappear underground at their lower 

reaches. Big Spring and Little Spring creeks are perennial and are still diverted for irrigation on the 

Medano Ranch. Most of the streams have been manipulated for irrigation at some point in the past 

century. Medano and Sand Creeks within GRSA infiltrate to recharge the shallow aquifer. Water in the 

shallow aquifer may be only a few years old, while water in the deep aquifer may be over 30,000 yr old 

(Rupert and Plummer, 2004b). 

Irrigated agriculture was instituted in the 1600s, but remained small in acreage until the late 

1800s. Most irrigation was limited to subirrigation practices until the 1960s, when it was no longer 

effective because increased withdrawals from the unconfined aquifer had lowered the water table

(Bexfield and Anderholm, 2010). As a result, there was an increase in the use of center pivot irrigation

(which draws from the confined aquifer) in the 1970s. 

The continued seasonal flow of Medano and Sand creeks is vital to the structure of the sand 

dunes (Wiegand, 1977; Madole and others, 2008). The flow of these streams rarely extends more than 

3.5 km from the mountain front; however, during peak spring runoff they may flow up to 8 km from the 

mountain front (Wiegand, 1977). During floods, sand is eroded from the dunes by the creeks and is 

carried out toward the sand sheet. After the creeks dry, this sand is blown back into the main dune mass.

Vegetation cover and surface water decrease and dune movement increases during droughts (Marín and 

others, 2005). Increased dune migration can lead to burial and death of herbaceous and woody plants.

Ungulate Populations and Grazing History

In 2013, an estimated population of 4,400 elk inhabited the 2,819 km2 game-management unit 

which includes GRSA (roughly 20 percent of the GMU) as well as public and private lands to the north 

and west of GRSA (Colorado Parks and Wildlife, 2013). Bighorn sheep (Ovis canadensis) are found in 

small numbers at high elevations of the Preserve section of GRSA (hereafter Preserve). Moose (Alces 

alces) were reintroduced to the San Juan mountains on the west side of the San Luis Valley in the early 

1990s (Colorado Parks and Wildlife, 2014, 

http://cpw.state.co.us/learn/Pages/LivingwithWildlifeMoose.aspx), and are reported to have been 

sighted on more than one occasion in the Deadman Creek drainage on the Rio Grande National Forest 

adjacent to GRSA (Phyllis Pineda Bovin, Great Sand Dunes National Park and Preserve, personal 

communication, October 2014). Because moose may browse on many sensitive woody plant 

communities in GRSA (willow, aspen, cottonwood), and there is evidence that they are expanding their 

range in the state (Colorado Parks and Wildlife, 2014), a potential influx of moose may need to be 

considered when managing ungulate use in GRSA in the future. Domestic sheep grazing in the San Luis 

Valley dates back to the early 1800s; cattle ranching then predominated from the late 1870s through 

recent decades (http://www.zranch.com). From 2005 to 2014 the herd size ranged between 1,500 and 

2,400 bison (Schoenecker, 2012) which roamed freely over 16,100 hectares (ha) of the Medano Ranch. 
9

The former Luis Maria Baca Ranch (hereafter “the Baca”), which makes up the northern part of the Park 

portion of GRSA and all of the Baca NWR (fig. 1), was actively grazed by 7,000–10,000 cattle through 

2004 (Ron Garcia, Refuge Manager, Baca National Wildlife Refuge, written commun., June 2009). The 

current bison herd was established in 1989 for commercial meat production. In 1999, TNC purchased 

the ranch and established it as a bison conservation herd, and eventually subcontracted management to a 

private rancher in 2005.

Ungulate populations in the valley are regulated by hunting, with the exception of bison which 

are gathered and culled annually to maintain population levels. Bison are restricted by fencing to a 

16,100 ha area on the Medano Ranch that lies about half within the boundary of GRSA and half within 

the boundary of the Baca NWR (fig 1). A joint U.S. Geological Survey (USGS)/NPS elk-bison study 

collected data on elk and bison movements over several years using both VHF and GPS radiocollars 

(Schoenecker, 2012). These data were analyzed to determine habitat selection of both elk and bison 

(Schoenecker and others, in press; Kate Schoenecker, Ecologist, USGS, Fort Collins Science Center, 

unpublished data), to predict bison density and use on lands outside the current bison range 

(Schoenecker and others, in press) and to estimate carrying capacity of this landscape under a variety of 

population scenarios (Wockner and others, 2014). This information was used to determine which 

vegetation types were incurring the greatest use by elk and bison and that might be considered the 

highest priority for sampling (table 1). 

The bison habitat-selection model (Schoenecker and others, in press) indicated that bison 

selected wet or mesic grassland habitats year-round and that willow-dominated areas were avoided. 

Cottonwood communities were also generally avoided except during fall. Playas were selected during 

summer, likely because of increased herbaceous production while seasonal ponds and wetlands were 

present, but avoided during other seasons.

The carrying capacity model presented by Wockner and others (2014) suggests that the 

landscape is currently being grazed by elk between the low threshold of 25 percent consumption of 

aboveground net primary production (ANPP) and mid-point of 28.5 percent consumption of ANPP. 

These authors concluded that the available forage for herbivores was not likely being maximally used

across the entire landscape. They did caution that hot spots of elk overconcentration or overgrazing may 

exist due to the uneven distribution of elk across the landscape. Likewise, increased use of a particular 

area by one ungulate species may cause other ungulate species to use areas they previously avoided. 

Wockner and others (2014) also modeled a potential 10 percent reduction in forage due to 

drought conditions induced by climate change. This change to the model parameters resulted in a

substantial reduction in elk carrying capacity, indicating the landscape would support 20 percent fewer 

elk compared to the current conditions with 2,000 bison, irrigation, haying, and livestock operations on 

the Baca NWR. These authors noted that other types of changes may also occur that are difficult to 

estimate, including changes in vegetation composition which could shift elk distribution and(or)

migration patterns. Their model results suggested that climate change has the potential to exert a much 

greater effect on elk carrying capacity than management activities pertaining to elk or bison (Wockner 

and others, 2014).

Elk use higher elevation areas off the valley floor, but studies of radiocollared elk (Kate

Schoenecker, unpublished data) indicate that the greatest concentration of this higher-elevation use 

occurs north of NPS lands. As a result, grazing in montane and subalpine zones off the valley floor has 

generally not been considered in the GRSA. However, alpine and aspen habitats in the Preserve have 

been identified in discussions with park staff as areas of potential concern because they are often 

targeted by concentrations of elk in other areas of the Rocky Mountains and may not be able to sustain 

intense elk herbivory over the long-term. Alpine areas, in particular, are highly sensitive to disturbance
10

and take long time periods to recover due to short growing seasons, low plant productivity, and 

environmental stresses, such as wind and extreme cold (Milchunas, 2006; Zeigenfuss, 2006). The 

effects of elk browsing in aspen communities have been widely documented (Seager and others, 2013).

However, conditions in montane and subalpine forests, shrublands, and grasslands, related to elk and 

other ungulate herbivory, may change with changing animal distribution, population size, and 

management, or due to current levels of herbivory which may be high enough to accelerate or 

exacerbate vegetation decline under changing climate conditions. Therefore, managers should be aware 

of potential herbivory effects in these habitats and respond appropriately through increased monitoring 

and possibly through management actions if signs of habitat deterioration are observed in these nontarget habitats.

Vegetation Communities and Existing Data

The following sections describe the main vegetation types found on the valley floor (see table 1), 

as well as some higher elevation aspen and alpine types. Information specific to research findings in 

these vegetation types in GRSA has been included when available as well as information about ungulate 

effects from other locations throughout the Intermountain West.

Woody Riparian Communities

Woody riparian communities on the valley floor are made up primarily of stands of cottonwood 

and willow. While some of these communities in GRSA have understory and soil characteristics typical 

of wetlands, we address wetlands separately from these woody communities. Riparian communities 

typically support higher densities of insects, particularly butterflies, and may provide more critical 

habitat for some bat species (Naiman and others, 1993; Swystun and others, 2007) than other plant 

communities. Cottonwood communities are structurally diverse and have been found to support high 

densities and richness of breeding birds, including songbirds, raptors, and woodpeckers (Szaro, 1980; 

Knopf and others, 1988; Krueper and others, 2003; Brand and others, 2008). Similar to cottonwood 

communities, willow communities provide habitat for a variety of birds, bats, and insects (Scott and 

others, 2003) and the abundance and diversity of these supported species have been found to be affected 

by high levels of ungulate herbivory. Anderson (2007) found that willow stands near elk feedgrounds in 

Wyoming (where elk densities were very high and willow cover was low) had lower avian species 

richness and relative abundance than areas away from feedgrounds. Willow height was found to be the 

most important factor explaining songbird species richness and abundance in the Greater Yellowstone 

region (Olechnowski and Debinski, 2008).

Numerous studies have investigated the effects of elk herbivory on willow and cottonwood, 

particularly combined with drought or declining water tables, throughout the Rocky Mountains 

(Keigley, 1997; Singer and others, 1998; Zeigenfuss and others, 2002; Baker and others, 2005; Beschta, 

2003, 2005; Bilyeu and others, 2008; Johnston and others, 2011; Rose, 2012). While bison are 

considered to be primarily grazers, they have been found to browse willows and cottonwoods during 

summer in Yellowstone National Park at rates that reduce available forage to wintering elk (Painter and 

Ripple, 2012; Rose, 2012). However, initial studies on diet of elk and bison in GRSA suggest that bison 

browse very minimally and only seasonally in this arid ecosystem (Kate Schoenecker, Ecologist, USGS, 

unpublished data).
11

Cottonwood

Roughly 400 ha of narrowleaf cottonwood (P. angustifolia) are located within the GRSA 

boundary on the valley floor with 20 percent (roughly 80 ha) of these cottonwood stands lying within 

the bison range of the Medano Ranch. The GRSA General Management Plan (GMP; U.S. Department 

of the Interior, 2007) identified unhybridized narrowleaf cottonwood stands containing trees up to 340 

yr old among the key resources that contribute to the unusual species diversity of GRSA. The park 

vegetation map defines two types of cottonwood communities: Narrowleaf Cottonwood Sand Dune 

Woodlands and Narrowleaf Cottonwood Temporarily Flooded Woodlands. These two types are found 

side-by-side in the study area and have been combined for this discussion. Temporarily flooded 

woodland alliances are identified as having a moderate to dense herbaceous understory with a diverse 

complement of understory shrubs (these types are sometimes classified as wetlands by ROMN). Sand 

dune woodland alliances are found along sand dune ridges and have a sparse herbaceous understory and 

associated shrub species, and are mainly limited to rabbitbrush and saltbush (Salas and others, 2011). 

Large trees in cottonwood stands provide the only natural nesting and perching locations for raptors in a 

valley landscape that is shrub dominated. Dead trees and snags provide nesting and feeding habitats for 

woodpeckers. Half of all the bird species encountered in a 2005 avian inventory of GRSA, as well as the 

highest bird densities, were found in riparian habitats (Glenn Giroir, Rocky Mountain Bird Observatory, 

2005, unpublished data).

Cottonwoods are dependent on shallow groundwater that is linked to nearby stream water. When 

groundwater is depleted through dewatering of the associated river or stream via groundwater pumping,

irrigation, or drought, this can lead to drought-stress responses in cottonwood, including eventual 

mortality (Rood and others, 2003). Seedlings require moist soil for germination and establishment, but 

seedlings and saplings are not tolerant of anaerobic conditions (Amlin and Rood, 2002), so inundation 

periods must be brief. Growth and development of seedlings is correlated to soil moisture and light, and 

seedlings do not compete well in vegetated sites (Braatne and others, 1996), so bare ground is optimal

for seedling growth. Root growth and seedling establishment is greater in fine, silty sands than coarse, 

gravelly soils (Braatne and others, 1996). Gradual declines in the water table promote shoot elongation 

and root growth, but abrupt declines reduce growth and induce mortality (Amlin and Rood, 2002). This 

makes for a delicate balance for seedling establishment and continued sapling growth and recruitment. 

Seed rain, wet soils, and gradual water table decline during early growth are all vital to establishment 

and recruitment of cottonwood, and increased recruitment leads to stand expansion. A limited amount of 

expansion through root suckers from mature trees may be expected where the canopy allows light 

infiltration. Excessive browsing of regenerating stems can limit seedling and sucker survival, 

particularly when environmental conditions for growth are poor. Thus, any vegetation objectives tied to 

increasing cottonwood stands on the landscape should be carefully considered, as manipulation of 

ungulate populations alone is unlikely to result in cottonwood stand expansion. Creation or restoration 

of hydrologic conditions conducive to seedling establishment and growth, as well as removal of 

competing vegetation, would likely be necessary to provide conditions that would expand the current 

footprint of cottonwood stands on the landscape and may even be required to maintain existing stands.

Seven research exclosures were erected in cottonwood stands on GRSA or Medano Ranch lands

in 2005 as part of a USGS study investigating the effects of elk and bison herbivory in GRSA. 

Cottonwood tree size and density, sapling density and height, and ungulate browsing levels were 

monitored from 2005–2009 inside and outside of these research exclosures. Some of these exclosures 

continue to provide restoration areas for cottonwood saplings, but others had very little regeneration 

following 5 yr of ungulate exclusion. Based on this research, Schoenecker (2012) determined that

GRSA cottonwood communities were sensitive to grazing, because of recent long-term drought 
12

conditions and changes in hydrologic regimes supporting cottonwood stands. Schoenecker’s conclusion 

was based on evidence of height suppression and decreased sapling density in browsed cottonwoods, 

and a lack of short-term (5-yr) response in sapling density when browsing was eliminated. Winter 

herbaceous offtake in cottonwood understory was 31 percent on the bison range compared to 11 percent

in park areas with no bison. Mean annual leader use of cottonwood was 46 percent on the bison range

and 22 percent on surrounding lands browsed primarily by elk (Schoenecker, 2012). These levels are 

indicative of light to moderate intensity of grazing (Milchunas and Lauenroth, 1993; Lucas and others, 

2004).

Radiocollar studies demonstrated bison avoidance of cottonwood except during the fall 

(Schoenecker and others, in press), but elk preferred cottonwood areas during most seasons (Kate 

Schoenecker, Ecologist, USGS, Fort Collins Science Center, unpublished data). Herbaceous biomass is 

relatively low in cottonwood stands on lower Sand Creek where both ungulates were present, and bison, 

being primarily grazers, likely prefer the more productive meadows in the area. There is evidence of 

activity by both ungulates in cottonwood stands (tracks, scat, plant damage from thrashing), and elk 

were commonly observed in cottonwood sites (personal observation). Lack of telemetry evidence for

bison in cottonwood habitats could be due to daytime-only bison location data and nighttime use of 

these habitats that were not captured by the telemetry study. Data collected using remote cameras 

located in lower Sand Creek cottonwood and willow habitat indicate that bison travel through these 

habitat at night, particularly during summer and fall (Kate Schoenecker, Ecologist, U.S. Geological 

Survey, Fort Collins Science Center, unpublished data). Elk use these areas primarily during the 

daytime in all seasons, but less elk use of these areas was recorded by cameras from late January 

through May than June through December.

Analysis of the plant DNA found in fecal samples from bison revealed that bison consume some 

very small amounts of both willow and cottonwood (averaging 1.1 percent of their entire protein 

consumption during summer). Analysis of plant DNA found in elk fecal samples revealed a greater 

proportion of total dietary protein from willow and cottonwood than bison (up to 5 percent in summer).

A USGS study conducted in 2007–2012 (D.C. Andersen, Research Ecologist, U.S. Geological 

Survey, Fort Collins Science Center, unpublished data) found cottonwood seedlings to be rare or nonexistent along lower Sand Creek on the bison range, and while root suckers were present, they were not 

abundant. Cottonwood stands in this area have many dead mature trees, missing size classes, and an 

inverted age structure with many large diameter trees and fewer small diameter trees. Both willow and 

cottonwood areas on the Medano Ranch have limited aboveground water flow and occur almost 

exclusively along Sand Creek. 

Andersen’s study found that 34–82 percent of the variation in radial growth of lower Sand Creek 

cottonwood trees over the past several decades could be explained by streamflow and climate (D.C. 

Andersen, Research Ecologist, U.S. Geological Survey, Fort Collins Science Center, unpublished data).

While he found no clear evidence of an herbivory effect, he indicated that such an effect is likely 

masked or manifested elsewhere in the cottonwood stand (for example, sucker production rate). Along 

Deadman Creek, 20–66 percent of the variation in radial growth could be explained by climate.

While Schoenecker (2012) found that removal of both elk and bison browsing in cottonwood

resulted in increased sapling height, other variables such as sapling density and recruitment of 

cottonwood saplings into the canopy showed mixed results. Removing browse pressure led to increases 

in sapling density in elk and bison grazed areas, but not in areas with only elk. This may have been due

to a number of factors including heavy overstory canopy at some sites which blocked light penetration 

to the forest floor, a deep water table (at least at one of the four sites; D.C. Andersen, Research 

Ecologist, U.S. Geological Survey, Fort Collins Science Center, unpublished data), or drought 
13

conditions that limited seedling and sucker establishment. However, under existing browse pressure, 

sapling density decreased slightly from 2005–2009 (P=0.048; Schoenecker, 2012) in three of four sites

where only elk were present, but showed no significant change in areas with both elk and bison 

browsing. Several factors may contribute to observed lack of response to reduced browse pressure. 

Hydrological characteristics of individual sites may affect the speed with which cottonwoods respond to 

decreased browse pressure. Also, sites currently grazed only by elk had a recent history of intense cattle 

grazing which can cause legacy effects. These sites grazed only by elk also had a more closed canopy 

than sites where bison were present (bison may or may not be a causal factor in the open understory of 

cottonwood stands on the bison range). Only a limited area of cottonwood exists on the bison range, and 

bison and elk may concentrate more in these relatively rare stands compared to stands off the bison 

range. The lack of universal response to removal of browse pressure indicates that additional factors 

such as history of cattle grazing and water availability are influential. 

Gaps in the current knowledge of the park cottonwood stands include: historic extent and origin 

of cottonwood stands along Sand and Medano creeks, the extent and rate of death of overstory canopy 

trees, and seed production of these larger trees. It is currently unknown whether the current extent of 

cottonwood on this landscape is due to natural historic conditions or anthropogenic water 

manipulations. Previous land owners installed water diversions along lower Sand Creek that may have 

provided conditions for stand establishment or expansion, or moved water away from existing stands

leaving them as remnants with a poor supporting water regime. Anecdotal evidence and aerial photos 

appear to show an older channel located north of the current channel (fig. 3; David Cooper, Colorado 

State University, 2009, written commun.) on the other side of existing cottonwood stands. The 

conditions for seedling establishment for cottonwood may no longer occur at many existing stands due 

to changes in stream flow, drought, or insufficient seed rain. These factors limit regeneration to root 

suckers from existing mature trees. Anthropogenic factors (livestock grazing, water manipulation, 

construction) may also play a role in other areas were cottonwoods appear to be declining, including 

areas along Medano Creek near the Dunes parking lot, and very small (fewer than 10 trees) remnant 

stands on lower Big Spring and Deadman creeks.

A substantial amount of dead canopy exists in GRSA cottonwood stands which could be the 

result of the ongoing drought since approximately 2000. The average percent of dead canopy on trees 

with a diameter at breast height (dbh) greater than 50 cm was over 50 percent in areas with bison 

grazing (Kate Schoenecker and Linda Zeigenfuss, Ecologists, USGS, Fort Collins Science Center, 

unpublished data). Canopy dieback in cottonwoods has been observed as a response to drought stress 

over several seasons (Scott and others, 1999; Horton and others 2001a, b; Cooper and others, 2003). 

However, in stands with closed canopies, this mortality may provide a means for light infiltration to the 

understory where seedling establishment might occur, provided hydrologic conditions are adequate. The 

GRSA GMP (U.S. Department of the Interior, 2007) identifies a need to inventory and map cottonwood 

stands in the park. If a cottonwood inventory is eventually conducted, measures of canopy health and 

seed rain could be incorporated to identify areas that might be appropriate for restoration. 
14

Figure 3. Apparent changes in stream channel of lower Sand Creek, Great Sand Dunes National Park and 

Preserve, Colorado as identified by David Cooper, hydrologist, Colorado State University.

Maintaining the cottonwood research exclosures could provide further information on long-term 

effects of ungulate removal, potentially provide seed source if young trees are able to grow and 

eventually recruit into the canopy, and possibly be used as protected areas for restoration experiments.

However, these exclosures require a commitment to maintain as they are subject to compromise from 

falling trees, sand blowouts that allow ungulate access under the fence, and dune buildup that allows 

animals to walk over the top of the fence. Without a regular commitment to monitoring and repairing 

these exclosures, their usefulness for research and monitoring is limited. But even when compromised, 

enclosures may still be useful for providing some protection to plants within, because usually only a 

small section of fence is affected and ungulates still need to “find” the access route. Park managers will 

need to weigh the costs and benefits of continued maintenance of cottonwood research exclosures.

Narrowleaf cottonwood forests are declining in the western U.S. and riparian forests are

considered threatened (Noss and others, 1995). If the park determines that cottonwood stands will be a 

management priority, it is possible that maintenance of stands may require hydrologic manipulations 

and(or) elimination of browsing by fencing out ungulates until regenerating saplings reach height 

beyond the reach of browsers. In light of current persistent drought, it may not be possible at all. 
15

Expansion of existing stands or establishment of new stands would require hydrologic manipulations, 

protection, and planting of either seeds or cuttings. If park managers decide to manage and protect 

cottonwoods, management actions could be assessed by monitoring of: (1) the distribution of 

cottonwood basal area in different stem size classes; (2) browse intensity, height, and density of saplings 

less than 2.5 m in height; and (3) seed rain, and seedling establishment and growth, but monitoring 

hydrologic conditions (wells, soil moisture, local precipitation) would also be necessary to separate the 

effects of ungulates from hydrology.

Willow

Coyote willow (Salix exigua) is the predominant willow type within the sand sheet area of 

GRSA. These willow areas are defined by the park vegetation classification map as “riparian shrubland 

dominated” or “dominated by coyote willow with other willows (S. ligulifolia, S. lucida ssp. caudata) 

codominant.” Scattered trees (such as cottonwood—less than 10 percent cover) may be present and the 

understory ranges from barren to abundant forb and graminoid species (Salas and others, 2011). A total 

of 16 ha of coyote willow are identified in the study area by the vegetation map and a little over half of 

this area falls inside the bison fence. 

Similar to cottonwoods, water availability is a major factor in the decline of willow communities 

at GRSA (Douglas Anderson, Research Ecologist [emeritus], U.S. Geological Survey, unpubl. data). 

Recruitment of coyote willow requires gradual recession of water depth to promote root elongation and 

shoot growth. Willow appears to be even more vulnerable than cottonwood to abrupt water-table 

declines, but can tolerate anaerobic conditions for a longer time (Amlin and Rood, 2002). Wurster and 

others (2003) identified climatic variation as well as channel incision along Sand Creek as the major 

factors in decline of interdunal wetlands at GRSA. Authors found that the Sand Creek channel had 

narrowed 30–80 percent between 1937 and 1995 (Wurster and others, 2003). 

Many areas with a significant willow component are not well represented on the park vegetation 

map. This may be due to the rarity of willow on the landscape, combined with the fact that coyote 

willow is often mixed with other shrub species, such as rabbitbrush and greasewood. A rough mapping 

of willow communities along lower Sand Creek and Medano Creek was attempted by USGS researchers 

(Linda Zeigenfuss, Kate Schoenecker, Sarah Beck, and Tracy Mask, USGS Fort Collins Science Center) 

in summer 2011. Many stands of small stature coyote willow mixed with rabbitbrush and greasewood 

were identified along lower Sand Creek (fig. 4). There was evidence of heavy browsing on most willow 

in Sand Creek and signs (tracks and scat) indicated extensive elk and bison use of the area. No bison 

were observed in the area during field visits, but several groups of bull elk were bedded, feeding, and 

moving through lower Sand Creek at the time of the survey. Along Medano Creek, many patches of 

willow were located even though none were identified there by the Salas vegetation map. Medano Creek 

willow appeared healthier (fewer dead stems, lower percentage of dead canopy) compared to Sand 

Creek willow, and appeared to be subject to lower intensity browsing. However, no quantitative 

measures were conducted to determine whether Medano Creek willow were, in fact, less decadent with 

larger, healthier crowns, and had less biomass removed by browsers. Radiocollared elk were rarely 

observed in the Medano Creek area during the 2005–2009 USGS/NPS study, probably due to high 

levels of human activity in the Dune Recreation Area (Kate Schoenecker, Ecologist, USGS, Fort Collins 

Science Center, unpublished data). It is also possible that Medano Creek willow benefit from a more 

favorable water regime, particularly in terms of precipitation, due to their close proximity to the 

mountain front.
16

Figure 4. Additional areas with a significant component of willow identified by 2011 U.S. Geological Survey 

(USGS) survey in lower Sand Creek compared to vegetation map classification (left) and overlaid on aerial photo 

(right). Areas identified by the vegetation map as coyote willow are accurately represented.

Schoenecker (2012) compared four willow sites on and four sites off the bison range. However, 

the sites off the bison range were located on the Baca NWR and there was limited comparability 

between the areas because the Baca NWR sites supported a different suite of willow species, several 

were in wet meadows or along irrigation channels, and Baca willow were subject to recent livestock 

grazing and irrigation. 

Mean annual woody offtake of willow was 23 percent on Medano Ranch lands used by both elk 

and bison (Schoenecker, 2012). Although willow species differed in areas only browsed by elk, average 

annual woody offtake of willow in these areas measured in 2006–2008 was 18 percent (Schoenecker, 

2012). The level of offtake on the bison range willow stands was comparable to areas intensively grazed 

primarily by elk in the Rocky Mountains (roughly 22 percent; Singer and others, 1998, 2002), while 

offtake on willow sites on the Baca was lower. However, the majority of willow plants on the Baca sites

were species other than S. exigua and half the sites were along an irrigation ditch which had water 

actively diverted to it (Schoenecker, 2012). Schoenecker (2012) determined that coyote willows in 

GRSA responded to removal of ungulate herbivory. Heights of unbrowsed coyote willow were nearly 
17

twice as tall as browsed willows after 4 yr of removal of ungulate browsing (P = 0.05). Absolute willow 

canopy area more than doubled in size after 4 yr of protection compared to initial condition, and

unbrowsed willow canopy area was 40 percent larger than willow canopy area subject to ungulate 

herbivory outside the exclosures, though this difference was not statistically significant (P=0.43), likely 

due to high variability in willow canopy sizes in sites outside the exclosures. The 4-yr timeframe may 

have been too short to observe a significant response between types given drought conditions during 

much of the study.

Analysis of plant DNA found in fecal samples from bison revealed that bison consume very 

small amounts of willow (up to nearly 1 percent of total protein consumption). Analysis of plant DNA 

found in elk fecal samples revealed a greater proportion of total dietary protein from willow and 

cottonwood than for bison. Willow made up a greater proportion of elk dietary protein than cottonwood 

in summer and winter (K. Schoenecker, unpublished data). 

As with cottonwood, the relationship of existing willow stands to their historic footprint on the 

landscape is difficult to determine. Friedman and others (2006) found that S. exigua occurred mostly on 

surfaces with an inundation recurrence interval of less than 2.2 yr, but because of its poor ability to 

compete for light and moisture, it did not persist on older and drier sites. Cooper and others (2006) 

found that greasewood and rabbitbrush invaded playa areas that were no longer subject to frequent 

inundation following installation of a large irrigation project (Closed Basin Canal) and subsequent 

ongoing water table drawdown. The mix of willow with rabbitbrush and greasewood in lower Sand 

Creek may indicate that these areas have been drying or that they are inundated less often than in the 

past, leading these communities to transition from willow to more xeric shrubs. Wurster and others 

(2003) concluded that a drier period from 1950 to 1980, along with stream channel incision, led to the 

disappearance of many interdunal wetlands in the area. However, it is also possible that these mixed 

communities developed during a period of several years of higher water tables. In this case, the willow 

might have invaded areas of rabbitbrush when water tables were high and are now reverting back to the 

previous rabbitbrush dominated condition. Examination of aerial photos or other historic photos, if they 

exist for these areas, may be able to provide some information on which vegetation type predominated 

70–80 yr ago. Analysis of pollen in sediment deposits of Head and San Luis Lakes, which are adjacent 

to the study area to the west, indicate a cooler and wetter period with arboreal pollen dominance 

between the mid-1400s and the late 1700s, followed by a drier period from the mid-1800s to the mid1900s dominated by greasewood and chenopod pollen (Jodry and Stanford, 1996). 

Given the small areal coverage and uncertain ecological development of S. exigua communities 

in the GRSA ecosystem, managers may need to make a value judgment as to the importance of willow

(for example, as bird and insect habitat, or providing stabilization to dunes) on this landscape. It is 

unclear the degree to which hydrologic changes are limiting willow growth in GRSA. If hydrologic and 

climatic conditions are ideal, reducing browse pressure might preserve the current condition in willow 

plant density, canopy cover, and volume, and in areal extent of willow habitats. However, increases in 

plant cover or height might require protection from browsing while plants recover, and then require

exposure to reduced levels of herbivory. As with cottonwoods, expansion of willow would likely 

require hydrologic manipulations, plant protection from herbivory, and seeding or planting of cuttings to 

establish new plants. Heavily browsed willow produce fewer catkins and less seed rain than unbrowsed 

willow (Kay, 1995; Christie and others, 2014), so seed sources outside the experimental exclosures may 

be limited and would need to be evaluated. Due to the importance of water to cottonwood and willow

communities, monitoring of hydrologic variables, such as water table depth, streamflow, and local scale 

precipitation, will be necessary for managers to determine responses to ungulate management as they 

relate to hydrologic conditions.
18

If park managers desire to assess recovery of these communities in response to ungulate 

management, monitoring of browse intensity, plant heights, shrub species composition, and shrub 

canopy cover could be used to determine progress toward recovery in as little as 5 yr, but only under 

favorable conditions (average or above-average precipitation; no substantial disease or insect outbreaks, 

fires, floods, or other major disturbance). In this semi-arid environment, measurable change may take 

longer. The existing willow exclosures built in 2005 might be useful as an ungrazed comparison to 

provide insights into environmental conditions that may confound expected plant responses to ungulate 

management (drought, disease, changing water availability). However, these exclosures would have to 

be consistently maintained and the issue of dune buildup along fence lines, which can create conditions 

conducive to ungulate access to exclosures, would have to be addressed and may ultimately limit their 

usefulness. 

The survival of both cottonwood and willow communities is intricately tied to the water table 

(Amlin and Rood, 2002; Rood and others, 2003; Friedman and others, 2006; Caplan and others, 2012). 

Therefore, the influence of drought, drawdown of the unconfined aquifer from irrigation elsewhere in 

the San Luis Valley, and channel incision on Sand Creek and Deadman Creek (and possibly Medano, 

Cottonwood, Pole, and Short creeks) cannot be easily separated from any ungulate management actions.

Monitoring of variables related to water availability, such as depth to water table, streamflow, and 

snowpack and precipitation, will be needed as covariates in any analyses of responses of cottonwood 

and willow communities to ungulate management actions.

Wetlands

Wetlands are areas where water is present either at or near the surface of the soil either 

seasonally or year-round, including during the growing season. In these communities, water is the 

dominant feature that determines soil development and animal and plant species composition (Cowardin 

and others, 1979). Wetlands present one of the more challenging vegetation types to identify using the 

vegetation map. Only two vegetation types are identified by the Salas vegetation map as “wetland” in 

the valley floor GRSA study area—Emergent Marsh Alliances and Interdunal Swale Wetland Alliances. 

However, the vegetation types identified as San Luis Valley Mesic Meadow Alliances and Playa 

Alliances also fit well within the description of wetlands. Together, these vegetation types total 2,834 ha 

on the study area with 95 percent of these wetlands (2,681 ha) on the bison range. However, ROMN 

WEI monitoring program has determined that the current park vegetation map is too coarse to 

accurately identify many wetland areas in the park (E.W. Schweiger, Ecologist, Rocky Mountain 

Network, Vital Signs Monitoring Program, National Park Service, written commun., March 2015). The

Rocky Mountain Network WEI program identifies four wetland types in the GRSA study area: Salt Flat 

Wetlands, Wet Meadows, Marshes, and Riparian Wetlands and they have developed a map that 

provides preliminary categorization of wetlands (fig. 5) However, this map has only been verified at 

sites that have been selected for WEI monitoring, and corrections based on these site visits have not 

been applied to this map. The ROMN wetland map identifies a total of 3,064 ha of wetlands in the study 

area with 2,635 ha (86 percent) within the bison range. The current ROMN WEI wetland map could be 

updated using the information already gathered as part of the WEI sampling for identifying potential

wetland monitoring sites as part of a future grazing monitoring plan. Because vegetation categories in

the Salas vegetation map do not correlate exactly to the ROMN wetland map (fig. 5, table 2), we used 

the ROMN classifications as the main wetland subcategories described below, then referenced to the

greatest extent possible correlations to the Salas vegetation map and the Schoenecker (2012) study.
19

Figure 5. Wetland vegetation map created by the National Park Service Rocky Mountain Vital Signs Monitoring 

Network (ROMN) correlated to vegetation types of the Salas and others (2011) Great Sand Dunes area vegetation 

map for the primary concentration of wetlands on and surrounding the Medano Ranch bison range.
20

Table 2. Wetland vegetation types created by the National Park Service Rocky Mountain Vital Signs Monitoring 

Network (ROMN) correlated to vegetation types of the Salas and others (2011) Great Sand Dunes area vegetation 

map and Schoenecker’s (2012) research study classification for the primary concentration of wetlands on and 

surrounding the Medano Ranch bison range. Bold denotes primary Salas class contributing to ROMN class.

(<, less than) 

ROMN Wetland 

Classification

Salas Vegetation Map Classification Schoenecker 

Classification

Percent Contribution 

of Salas Class to 

ROMN Class 

Marsh Emergent Marsh Alliances 75

San Luis Valley Mesic Meadow Alliances 

/Emergent Marsh Alliances mix

9

San Luis Valley Mesic Meadow Alliances 10

Playa Alliances <1

Water 6

Greasewood Alliances <1

Riparian Narrowleaf Cottonwood Sand Dune 

Woodland Association

Narrowleaf Cottonwood Temporarily 

Flooded Woodland Alliance

Cottonwood 83

Emergent Marsh Alliances Wet meadow 13

Coyote Willow Temporarily Flooded 

Shrubland Alliances

Willow 3

San Luis Valley Mesic Meadow Alliances <1

Salt Flat 

Wetlands

Alluvial Flat Herbaceous Alliances 39

San Luis Valley Mesic Meadow Alliances 23

Playa Alliances 20

San Luis Valley Mesic Meadow Alliances /

Playa Alliances mix

6

Alluvial Flat Herbaceous Alliances / San 

Luis Valley Mesic Meadow Alliances 

mix

5

Alluvial Flat Herbaceous Alliances / Playa 

Alliances mix

3

Emergent Marsh <1

Greasewood Flat Shrubland and Steppe 

Alliances

Greasewood Sand Deposit Shrubland and 

Steppe Alliances

3

Sandsheet Rabbitbrush 1

Greasewood Alliances /San Luis Valley 

Mesic Meadow Alliances

<1

Wet Meadow San Luis Valley Mesic Meadow Alliances Mesic meadow 85

Alluvial Flat Herbaceous Alliances 7

San Luis Valley Mesic Meadow Alliances 

/Alluvial Flat Herbaceous Alliances

3

San Luis Valley Mesic Meadow Alliances/ 

Greasewood Alliances

1

San Luis Valley Mesic Meadow Alliances 

/Emergent Marsh Alliances

1

Interdunal Swale Wetland Alliances 2

Emergent Marsh Alliances <1

Sandsheet Rabbitbrush Shrubland and 

Steppe Alliances / Greasewood Alliances

<1

Other <1
21

Salt Flat Wetlands

Salt flats are identified by the ROMN WEI protocols (Schweiger and others, in press) as a 

widespread wetland type at low elevations forming in closed basins west of the dune field with heavy

textured soils or where evaporation from a high water table promotes the accumulation of salts. 

Productivity is generally low, and species composition is limited to salt-tolerant species including

saltgrass, greasewood, and saltbush. There is considerable variability in hydrologic characteristics;

water levels fluctuate widely both seasonally and among years, with extended periods of deep 

inundation alternating with periods where surface water may be completely absent (Cooper and Severn, 

1992). Salt flats may also be influenced by groundwater, through direct discharge in discrete springs or 

through capillary movement of water from seasonally high water tables. Both high-water periods and

multi-year droughts can lead to high plant mortality. 

Nearly all the areas identified as Playa Alliances on the Salas and others (2011) vegetation map 

are included in ROMN’s salt flat wetlands. Because they seasonally hold water and may therefore have 

green vegetation later into the summer when herbaceous vegetation in surrounding areas has already 

senesced, they may provide local hotspots for grazing by both bison and elk. In other landscapes with 

playas where livestock grazing is present, livestock often congregate in playas because of early growth 

of plants, increased plant production, and favorable microclimate during harsh weather (Haukos and 

Smith, 2003). This can subject playa vegetation to reductions through consumption and trampling by 

livestock. In general, playa lakes are important to waterfowl and other migrating birds, providing food, 

cover, and resting habitat. Bison do appear to slightly favor these playa areas during summer 

(Schoenecker and others, in press); however, we have little information on elk use of these areas. 

Although the San Luis Valley does not receive as many migrant shorebirds as other areas such as the 

Great Basin and Playa Lakes regions, playas within the San Luis Valley still provide important 

migration habitat for many shorebird species (U.S. Department of the Interior, 2014a).

During dry years many of these playas remain dry, but they still have high productivity. When 

water is available, playas fill during spring runoff and thunderstorms, and then dry slowly over the 

summer. This drying and wetting cycle provides for the nutrient cycling conditions ideal for the 

production of invertebrates, a valuable food resource for numerous vertebrate species (U.S. Department 

of the Interior, 2005). When precipitation is above average, these wetlands are some of the most 

productive in the area (Cooper and Severn, 1992). Saltgrass pans are important habitat for several rare 

species of small mammals and the sandhill skipper (Polites sabuleti ministig), a rare butterfly subspecies 

(U.S. Department of the Interior, 2005). 

Two other Salas and others (2011) vegetation types are heavily represented in the Salt Flat 

Wetlands—Alluvial Flat Herbaceous Alliances and San Luis Valley Mesic Meadow Alliances (see 

description in following section). These types, along with Playa Alliances, make up 94 percent of the 

Salt Flat wetlands type used by ROMN (table 2). The Salas map describes San Luis Valley Mesic 

Meadows as having a moderately dense to dense herbaceous layer dominated by native species such as 

saltgrass, sedges (Carex pellita, C. praegracilis), foxtail barley (Hordeum jubatum), arctic rush,

beardless wildrye, scratchgrass (Muhlenbergia asperifolia), spike muhly (M. wrightii), western 

wheatgrass (Pascopyrum smithii), muttongrass (Poa fendleriana), Nuttall’s alkaligrass (Puccinellia 

nuttalliana), alkali sacaton (Sporobolus airoides), silverweed (Argentina anserina), slender 

spiderflower (Cleome multicaulis), Rocky Mountain iris (Iris missouriensis), and woolly cinquefoil 

(Potentilla hippiana). Alluvial Flat Herbaceous Alliances are saltgrass-dominated patches within 

greasewood shrublands that may transition into San Luis Valley Mesic Meadows.
22

Wet Meadows

The ROMN WEI protocols (Schweiger and others, in press) identify wet meadows as areas that 

typically exhibit seasonally saturated soils, but lack the large seasonal and interannual water table 

fluctuations characteristic of marshes. Wet meadow soils are mineral, but have significantly more 

organic matter than soils in the surrounding upland. Mineral soils often have lower water-holding 

capacity than peat and mineral substrates, and may drain and dry more readily than peaty soils, but also 

have higher levels of nutrient availability. Wet meadow species include arctic rush, alkali sacaton, and 

sedges. Shrubs (commonly Salix) and trees may be present, but canopy cover is usually low. Conditions 

such as moist, unsaturated mineral soils, and high levels of nutrient availability in wet meadows often 

support high cover and diversity of exotic species (Stohlgren and others, 1999). Introduced perennial 

sod-forming grasses such as creeping bentgrass (Agrostis stolonifera) and redtop (A. gigantean) are 

common dominants, particularly at lower elevations and in wet meadows adjacent to riparian areas. 

Pasture grasses, such as Kentucky bluegrass (Poa pratensis) are also present in meadows affected by 

livestock and historic homesteading. 

Wet meadows frequently occur in stream valleys as part of larger wetland complexes, although 

unlike riparian wetlands, wet meadows are not subject to high-velocity surface flows or sediment 

deposition from fluvial processes. Wet meadows typically occur in sites where soils are seasonally 

saturated; however, unlike fens or marshes, perennially high water tables or seasonal flooding does not 

typically occur. High water levels are common early in summer, but water tables in wet meadows may 

drop to 1 m or more below the soil surface in late summer. Precipitation and groundwater flows are 

likely the most important contributors to hydrologic regimes of wet meadows. Local and regional 

aquifers contribute groundwater by creating shallow water tables along aquifer flow paths or 

discharging directly to the soil surface. This is especially true during spring snowmelt when aquifers are 

swollen, and spring melt water may be the primary source of seasonal inundation for wet meadows in 

dry areas. Direct surface water inputs to wet meadows are generally limited to rainfall and sheet flow 

following extreme precipitation or snowmelt events and may have a greater effect in middle to late 

summer when groundwater flows are diminished (from Schweiger and others, in press).

Most areas identified as San Luis Valley Mesic Meadows on the Salas vegetation map fall 

within the ROMN wet meadow classification (fig. 5). Also included under the ROMN wet meadow 

classification are saltgrass-dominated areas identified on the Salas vegetation map as Alluvial Flat 

Herbaceous Alliances, as well as Interdunal Swale Wetland Alliances. Interdunal wetlands are nonriparian wetlands that form in arid sand dune environments at locations where the regional water table 

intersects the ground surface (Wurster and others, 2003). The vegetation map defines interdunal 

wetlands as groundwater-fed wetland vegetation that occurs in distinctive interdunal swales (deflation 

areas between active dunes) with shallow water tables not visibly connected to other surface water. 

These sites occasionally have stands of coyote willow that persist during sand burial as dune 

encroachment occurs (Salas and others, 2011). However, some areas within these interdunal wetlands, 

particularly around Elk Springs, fall under the ROMN classification for marshes.

Wet meadows are typically associated with springs and are naturally subirrigated, but their 

footprint has also been expanded by historic and ongoing (by Ranchlands/Zapata Partners) irrigation. 

The GRSA General Management Plan (U.S. Department of the Interior, 2007) proposes cessation of 

irrigation and potential restoration of natural hydrologic regime on the Medano Ranch, if and when,

TNC ceases agricultural uses of the Medano ranch. When the NPS assumed management of new lands 

on the former Baca and current Medano ranches, there were several existing free-flowing wells that 

were not in compliance with Colorado water law, and these have been capped in recent years in order to 

meet compliance. Currently, irrigation takes place on private inholdings (TNC lands managed by 
23

Ranchlands/Zapata Partners) within the park boundary. Currently, the NPS Water Resources Division is

preparing an analysis on the possible effects of cessation of irrigation and water diversions on the 

Medano Ranch (Phyllis Pineda-Bovin, Great Sand Dunes National Park and Preserve, written commun., 

April 2015).

Riparian Wetlands

Riparian wetlands are diverse in terms of ecological functioning, hydrology, and vegetation 

composition. Because of the linear and interconnected nature of riparian environments, gradients in 

important environmental variables (water table depth, elevation, soil characteristics, and flood 

disturbance) and resulting vegetation patterns vary both laterally and longitudinally (Friedman and 

others, 2006; Schweiger and others, in press). Channel migration and episodic floods along meandering 

rivers with broad floodplains result in pulses of woody plant establishment creating patches of different 

age cohorts (Shafroth and others, 1998; Cooper and others, 2003). The principal characteristic unifying 

riparian wetlands is the presence of unidirectional moving water and a connection, at least part of the 

time, to lotic or lentic surficial hydrology. Surface flows within riparian wetlands are important as they 

have the potential to erode and transport sediment, with the potential energy contained in flowing water 

a key variable influencing ecosystem structure and function. The frequency, magnitude, and energy of 

floods affect all ecological processes in riparian wetlands from nutrient cycling to plant establishment 

(Schweiger and others, in press).

The vegetation of riparian wetlands is highly variable. Along larger mid-elevation montane 

streams, and lower in the foothill valleys, riparian wetland is dominated by cottonwoods, with shrubs 

such as red-osier dogwood (Cornus sericea) and coyote willow and diverse herbaceous canopies.

Surface water and floods are critical to the healthy functioning of riparian areas. Water tables along 

intermittent and ephemeral streams often drop well below stream channels, limiting the distribution of 

hydrophytes (except deep-rooted phreatophytes like cottonwoods, tamarisk, or upland species tolerant 

of low soil moisture) (Goodwin and others, 1997). Many headwater streams receive perennial 

groundwater inputs from side slopes, maintaining high base flows. Stream and riparian wetlands in 

GRSA are functionally unique owing to the high porosity of underlying sand (Wurster and others, 

2003).

The Salas vegetation map does not have a riparian wetland category, and several vegetation 

types are classified in the ROMN wetland map as riparian wetlands—Emergent Marsh, Narrowleaf 

Cottonwood Temporarily Flooded Woodland Alliances, Narrowleaf Cottonwood Sand Dune Woodland 

Association, and Coyote Willow Temporarily Flooded Shrubland Alliances. Because cottonwood and 

willow are addressed separately in this document, their inclusion in the riparian wetlands category is 

redundant and would not reflect their unique nature as riparian areas with a woody shrub or tree 

overstory. However, those areas along Big and Little Spring Creeks that have a highly productive 

herbaceous layer (see description of Emergent Marsh in the section on wet meadows), would be 

appropriate for sampling of riparian wetlands. All of the sites that Schoenecker (2012) termed “wet 

meadows” fall in this riparian wetlands category.

Marshes

Marshes are depressional wetlands characterized by inundation on a relatively frequent basis, 

often with deep standing water and large fluctuations in species composition commonly occurring over 

relatively short time scales (Schweiger and others, in press). The vegetation of marshes is often limited 

to species that are capable of rooting in mucky, saturated soil and can tolerate submersion and 

occasional dry periods and include monotypic freshwater communities dominated by hydrophytes such 
24

as cattail (Typha latifolia), common threesquare (Schoenoplectus pungens), and mare’s-tail (Hippuris

vulgaris), and in saline playa environments halophytes such as bulrush (Scirpus nevadensis) and 

saltgrass. Marsh hydrology can be extremely variable and frequently includes both prolonged periods of 

inundation as well as extended dry periods, particularly during dry years (Schweiger and others, in 

press). 

Most areas identified on the Salas vegetation map as Emergent Marsh are also classified as 

marsh by ROMN (fig. 5) with the exception of those areas along the upper reaches of Big Spring and 

Little Spring creeks that are identified as riparian wetlands by ROMN. However, ROMN has only eight 

WEI sample sites in marsh and only half of these sites align with the Salas and others (2011) emergent 

marsh type.

Grazing Effects in Wetlands

The effects of grazing on herbaceous communities have been found to be variable based on the 

metric used, the evolutionary history of grazing, precipitation, relative annual net primary productivity, 

season of grazing, and grazing intensity (Milchunas and Lauenroth, 1993). Gass and Binkley (2011) 

found that intensive elk grazing was correlated with drier more compacted soils, soil nutrient loss, and 

reduced plant biodiversity in montane wetlands in Rocky Mountain National Park in Colorado. 

Whereas grazing has been found to accelerate weed invasions (Hobbs and Huenneke, 1992; 

Fleischner, 1994), other studies have found that grazing increases local species richness and reduces 

exotics on a local scale in some systems (Frank, 2005). Stohlgren and others (1999) found that grazing 

had little effect on native species richness or on the accelerated spread of exotic plants at landscape 

levels (though effects at local levels were variable), and that soil characteristics, climate, and 

disturbances (fire, flooding, insect and disease outbreaks) likely have a greater effect on plant species 

diversity than grazing does. In this landscape, pollen in sediment deposits indicates an increase in the 

exotic plant Russian thistle (Salsola spp.) coincident with increases in the dung fungus, Sporomiella,

associated with the introduction of cattle and sheep grazing in the late 1800s (Jodry and Stanford, 1996).

Anecdotal evidence also indicates increases in Canadian thistle (Cirsium arvense) inside the exclosures 

used by Schoenecker (2012) where habitat has now been released from herbivory pressure (C. Pague, 

TNC, written communication, 2015). 

Wetland areas were favored by both elk and bison (Schoenecker and others, in press; Kate 

Schoenecker, Ecologist, USGS, Fort Collins Science Center, unpublished data). The estimated 

herbaceous offtake in wet meadow wetlands (Schoenecker [2012] called these mesic meadows) was 

43.9 percent in the summer and 63.7 percent in the winter (measured in 2006 and 2008; annual 

herbaceous offtake data not available). In streamside riparian wetlands, herbaceous offtake in areas

grazed by elk and bison averaged 45 percent of production in summer and 45 percent of standing crop in 

winter, or roughly 70 percent removal of total herbaceous production during the year (Schoenecker, 

2012). Herbaceous offtake was lower in riparian wetlands off the bison range with 31 percent 

herbaceous offtake in the summer and 17 percent during the winter for an estimated total of 43 percent 

removal of total herbaceous production during the year (Schoenecker, 2012). Yet all streamside riparian 

wetlands had higher nitrogen yield and greater plant production in grazed grasses than grasses protected 

from grazing for 4 yr, suggesting these riparian wetlands were resilient to the high level of herbivory 

they received during 2005–2009.

Herbaceous offtake levels (greater than 70 percent annually) on riparian wetlands on the bison 

range indicate a very heavy level of ungulate use compared with data from grasslands around the world 

(Milchunas and Lauenroth, 1993). Between 2008 and present, the Medano Ranch bison herd size 

increased to a maximum of approximately 2,500 animals and then decreased to roughly 1,700 
25

(Schoenecker, 2012). Ranchlands/Zapata Partners monitor five transects in wet meadows every 3 yr

(most recently in 2012) and calculate a grazing response index (GRI, Reed and others, 1999). They 

estimated ungulate use on wet meadows ranged from 5 percent to 90 percent depending on location and 

year (Zapata Partners, 2013, unpublished data). GRI was –2 at all sites in 2012, which was lower than 

when sampled in 2009, likely due to lower precipitation and higher use at some sites (Zapata Partners, 

unpublished data). The GRI combines frequency, intensity, and season of grazing plus annual 

precipitation to determine an index value from –4 to +4 where positive numbers are indicative of better 

range condition.

Schoenecker (2012) found these areas to be highly resilient to changes in grazing levels during 

2005–2009 as measured by vegetation productivity (which was greater with grazing) and no observed 

shifts in species composition occurred after 4 yr of exclosure treatment. However, the 4-yr timeframe of 

her treatments may have been too brief to detect a treatment effect of grazing exclusion on species 

composition. Similarly, Przeszlowska (2006) found increased carbon (C) and nitrogen (N) 

mineralization rates in riparian wetlands grazed by bison and elk compared to riparian wetlands grazed 

only by elk, which can lead to increased soil fertility and higher forage quality in these areas.

The strong affinity of grazers for wetlands makes these communities susceptible to invasion of 

non-native plant species. Variables that will likely provide the most information about grazing effects 

on wetland communities include measures of percent bare ground, measures of mean invasiveness and 

mean conservatism scores (Herrick and others, 2009a, 2009b; Schweiger and others, in press). Mean 

conservatism scores assign values from 0–10 to each species present: low scores for exotic and ruderals, 

intermediate scores for competitors and matrix species, and high scores for native species that are highly 

dependent on remnants of habitat (Rooney and Rogers, 2002; Rocchio, 2007). An assessment of floristic 

quality of wet meadows of the Medano Ranch determined that the area was less disturbed than nearby 

Zapata Ranch, which has been subjected to higher rates of livestock grazing and which is generally drier 

(Elin Franzen, Colorado State University student, 2008, unpubl. undergraduate honors thesis). However, 

available data indicate that mean C (coefficient of conservatism) scores on low elevation wetlands in 

GRSA were generally less than 5, indicating that these communities have undergone a moderate degree 

of disturbance, likely due to historic grazing and irrigation practices (E.W. Schweiger, Ecologist, U.S. 

National Park Service, Inventory and Monitoring Program, Rocky Mountain Vital Signs Monitoring 

Network, oral commun.). 

Data from pilot studies by ROMN suggest that wetlands in the study area may be degraded 

(using a multimetric index based on numerous variables related to invasive and native plant cover, soil 

conditions, ground cover, and others) with up to half the salt flat and wet meadow wetlands on the sand 

sheet in a non-reference state. There appears to be an association between reduced wetland condition 

and ungulate habitat use, but more work is needed (E.W. Schweiger, Ecologist, U.S. National Park 

Service, Inventory and Monitoring Program, Rocky Mountain Vital Signs Monitoring Network, oral 

commun.). 

Although GRSA riparian wetlands appear resilient to year-round grazing by ungulates in some 

studies (vegetation productivity, nitrogen yield, species diversity, N mineralization rates, Schoenecker 

[2012]), other preliminary work suggests some substantial degradation may occur in wetland vegetation 

condition or ecological integrity with higher levels of ungulate habitat use (Schweiger and others, in 

press), while effects on other variables are unknown (for example, soil compaction). Increases in the 

number of grazers or changes in environmental conditions (drought, warming temperatures) may 

eventually lead to negative effects in these apparently grazing-adapted communities. These communities 

also contain plant species that are rare in this landscape, such as slender spiderflower and canyon bog 

orchid (Platanthera sparsiflora var. ensifolia) and which are a high priority for grazing monitoring. The 
26

greatest considerations in wetland communities are likely to be invasion of non-native plant species and 

soil compaction and erosion due to trampling in areas where grazers concentrate. Also of potential 

importance in these areas is habitat for ground-nesting waterbirds, particularly in the wetlands around 

Dollar and Cotton Lakes. Grazers can have both direct (trampling and disturbance) and indirect (effects 

on habitat, forage, and predation) effects on ground-nesting birds. Habitat characteristics such as 

vegetation cover and height can be influenced by removal of plant material and trampling of plants, 

leading to shorter and sparser vegetation cover (Powers and Glimp, 1997) that may make them less 

suitable for some species.

While Schoenecker’s conclusions of grazing resilience appear contradictory to ROMN’s

observations of wetland degradation, it is important to note several differences between these studies. 

Schoenecker’s research was conducted in 2005–2008, during a time when the bison population on the 

Medano Ranch was approximately 1,200–1,500 animals and in the early years of an extended drought 

which started around the year 2000. Her research was aimed at determining grazing effects on plant 

productivity and species composition in specific areas that were present both on and off the bison range 

and did not include any salt flat wetlands. A suite of factors including vegetation community 

composition, soil characteristics, water chemistry, and groundwater depth and dynamics was measured 

by the ROMN WEI program. The ROMN data were collected during a single year (2010) after the bison 

population had increased to 2,500 animals in 2009 and when the cumulative effects of a decade of 

drought conditions were likely to be having a significant effect. Additionally, the ROMN sample set 

included a much larger and more diverse sample of sites, including several vegetation types on the sand 

sheet identified by the Salas vegetation map as greasewood, playa, and rabbitbrush, whereas the 

Schoenecker (2012) study reported results strictly from streamside riparian wetlands. These differences

in timing and scope limit the comparability of the two studies. 

The number of interdunal wetlands at GRSA declined more than 80 percent from 1936 to 1979, 

then doubled again by 1995 for an overall loss of 52 percent of all interdunal wetlands from 1936 to 

1995 (Hammond, 1998). However, the loss of interdunal wetlands was related to changes in the water 

table due to channel incision and lowered average stream flow in Sand Creek (Wurster and others, 

2003). These authors concluded that many interdunal wetlands were ephemeral over a time-scale of 

several decades. Existence and persistence of interdunal wetlands are dependent on hydrology and longterm climate patterns that may have as much as a 20-yr lag effect. As such, inclusion of interdunal 

wetlands in a grazing monitoring plan should be carefully considered and sample designs and data 

interpretation should be mindful that the existence of these particular wetlands is highly dependent on 

factors beyond the scope of ungulate management. Ungulate herbivory may accelerate the loss of 

vegetation in interdunal wetlands, and they have been found to be highly preferred by ungulates. In a 

2006 survey of previously mapped (Hammond, 1998) interdunal wetlands, Schoenecker (2012) found 

that at 6 of 19 interdunal wetland sites where willow were present, 70 percent of twigs of interdunal 

willow plants were browsed. 

Park managers will have to carefully define what constitutes a wetland for the purposes of the 

ungulate management plan, which wetland types to include, and consider which aspects of wetland 

community integrity are important in terms of determining grazing effects. At a minimum, the most 

widespread wetland types, salt flats and wet meadows should be included in sampling. ROMN has 

already established many sites for monitoring wetland ecological integrity (WEI) in these lower 

elevation wetlands at GRSA. The monitoring conducted by the ROMN WEI program provides a

rigorous sample of wetlands and GRSA managers might use this program (Schweiger and others, in 

press) in part to implement wetland grazing monitoring as well. When and where possible, use of sites 

that are part of the ROMN WEI sampling would prevent duplication of work, expand data collection, 
27

and potentially save resources for both programs. Some of the measures necessary to determine grazing 

effects on wetlands in GRSA might be already included in sampling being conducted by ROMN.

Additional grazing-relevant measures, especially of explicit ungulate use of sites, would need to be 

added. Sharing crews for establishment and monitoring of sites could be mutually beneficial and prevent 

added non-natural disturbance to sites through visitation by separate crews. Sample design 

considerations will entail adding sites to appropriately distribute an adequate number of sites based on 

future ungulate presence on the landscape as a result of management decisions related to the UMP.

The protocols and plan for ROMN WEI monitoring are currently being finalized (Mike Britten, 

Program Manager, U.S. National Park Service Inventory and Monitoring Program, Rocky Mountain 

Vital Signs Monitoring Network, written commun., August 2014). There are eight exclosures in GRSA 

that were established by the USGS/NPS study in 2005 in this wetland vegetation type. Continued 

monitoring within these exclosures may be useful to ROMN to capture the characteristics of ungrazed 

wetlands as part of developing a gradient of disturbance and response (E.W. Schweiger, Ecologist, U.S. 

National Park Service, Inventory and Monitoring Program, Rocky Mountain Vital Signs Monitoring 

Network, February 2014, unpublished presentation). For this reason, park managers would be wise to 

discuss with ROMN ecologists the usefulness of continued maintenance of the wetland exclosures.

If, and when, TNC ceases agricultural uses of the Medano Ranch and these lands are transferred 

to NPS management, the GRSA GMP (U.S. Department of the Interior, 2007) proposes cessation of 

irrigation in order to restore natural hydrologic flows. However, before surface irrigation is discontinued 

on the Medano Ranch, a study will be conducted to better understand how this action might affect 

wetlands, groundwater supplies, federal water rights, and the Closed Basin Project. If this action were to 

occur, it is expected that long-term, moderate to major effects from cessation of agricultural irrigation 

and water diversions would occur (U.S. Department of the Interior, 2007). Geographic changes in 

vegetation communities resulting from hydrologic modification could influence the distribution of 

ungulates on the landscape as animals disperse to or concentrate in their preferred habitats. These 

potential changes will be important in selecting sites for monitoring effects of ungulate herbivory or 

interpreting the results of monitoring as hydrologic changes manifest on the landscape.

Given the amount of direct and indirect human influence on many GRSA wetlands from decades

of livestock grazing and water manipulation for irrigation, coupled with potential future hydrologic 

modification to support water-law compliance, determining how to include this type in grazing 

monitoring will be complicated. Measures of species composition and abundance, amount of bare 

ground, gaps between plants, and soil characteristics (bulk density and infiltration) would all be useful 

metrics to determine the extent and severity of any grazing or habitat management actions. 

Upland Shrub and Grassland

Rubber rabbitbrush (Ericameria nauseosa) is generally considered to be a grazing-resilient 

species and can be an important browse species for pronghorn and deer, and to a lesser extent, elk

(Tirmenstein, 1999; U.S. Department of Agriculture, 2014). It is likely used somewhat by elk, but may 

be a more primary component of pronghorn and deer diets (Tirmenstein, 1999; Boccadori and others, 

2008). Greasewood is generally considered grazing resistant and can be an important browse species for 

pronghorn and deer, and to a lesser extent, elk (Anderson, 2004; U.S. Department of Agriculture, 2014). 

Bison do not typically browse woody plants to any great extent, and diet studies reveal little to no use of 

either shrub species by bison (Singer and Norland, 1994; Kate Schoenecker, Research Ecologist, USGS, 

2014, unpublished data), though bison can cause damage to woody species through horning and rubbing 

(Coppedge and Shaw, 1997).
28

The Natural Resources Conservation Service Ecological Site Description (ESD) for the entire 

San Luis Valley addresses many important aspects of upland shrub and grassland communities. In 

particular, it notes that during drought, greasewood had diminished vigor and increased mortality and 

that warm season bunchgrasses declined, but persisted (Diller and Villa, 2013). The ESD notes that on 

sites where rabbitbrush and greasewood are codominant, mis-management through overgrazing may 

lead to greasewood dominance, whereas non-use and drought can lead to rabbitbrush dominance. 

Rabbitbrush tends to be groundwater dependent, whereas greasewood uses groundwater during dry 

periods and precipitation after heavy monsoonal rainfall (Kray and others, 2012). Understory grasses 

use soil water that is precipitation-recharged. Because both rabbitbrush and greasewood have the ability 

to use groundwater, they are more likely to be affected by water table declines than by lower levels of 

growing season precipitation, which may allow them to persist in the face of short-term drought 

conditions. However, if long-term drought leads to lowered water table, mortality of both species is 

possible. Both shrub species tend to be grazing resilient in terms of biological integrity, soil stability, 

and watershed function, but an eroded state can develop under recurrent early-season grazing that is 

exacerbated by extended drought (Diller and Villa, 2013).

Upland shrub and grassland communities make up a large component of the GRSA landscape,

and it is important to be aware of their condition. It may be impractical to attempt to intensively monitor 

these communities because of their relative resilience to elk and bison herbivory and given the 

likelihood of limited resources for monitoring. A particular concern in the rabbitbrush and greasewood 

areas with a loose sand substrate, and for herbaceous stabilized dunes, is that grazing and drought can

combine to reduce understory herbaceous plants which are holding loose sand in place. Rondeau and 

others (2013) found that grass cover decreased significantly during drought, as did rabbitbrush density, 

though rabbitbrush cover increased. Rabbitbrush is a deep-rooted species, which is likely key to its 

ability to thrive during and following drought. Rabbitbrush may also facilitate survival of understory 

herbaceous species, particularly when precipitation is below average, by modifying the microclimate 

(dampening temperature fluctuations, raising humidity, decreasing wind speed, reducing irradiance, and 

increasing soil moisture) under plant canopies (Peek and Forseth, 2003). However, when understory

herbaceous plants decrease, sand movement may occur and eventually affect rabbitbrush as well 

through burial of shrubs in the dune or exposure of roots. Mangan and others (2004) used modeling to 

investigate the effects of drought and grazing on vegetation and dune stability in the Nebraska Sand 

Hills and found that grazing alone had little effect on vegetation, but, combined with drought, led to 

biomass decreases. Even low-intensity grazing coupled with drought was projected to affect both aboveand below-ground plant biomass. Their modeling indicated that declines in plant biomass brought on by 

drought and exacerbated by grazing could lead to sand dune mobilization, but that even sparse 

vegetation cover (10 percent ground cover) could be sufficient to stabilize dunes and prevent sand 

movement (Valdez, 1990; Mangan and others, 2004).

Rabbitbrush

Rabbitbrush communities are the most abundant habitat type on the valley floor in the study area 

and comprise 23,135 ha (56 percent) of the study area, including 5,281 ha on the bison range. Most of 

the rabbitbrush on the valley floor is identified on the vegetation map as Sandsheet Rabbitbrush 

Shrubland and Steppe Alliances that are defined as areas of open to moderately dense shrub steppe or 

shrublands dominated by rabbitbrush and occurring widely on the stabilized sand sheet and sand ramp. 

Winterfat (Krascheninnikovia lanata) and greasewood may be present with low cover. Substrate is 

deep, loose sand. Herbaceous species include Indian ricegrass, needle-and-thread grass, sand muhly 

(Muhlenbergia pungens), or alkali sacaton (Salas and others, 2011). Also included in this type are 
29

Alluvial Fan Rabbitbrush Shrubland and Steppe Alliances: some small patches of rabbitbrushdominated shrublands restricted to alluvial fans located on coarse textured gravelly or cobbly loams or 

sandy loams mainly to the north of the dunefield.

Rabbitbrush communities were measured in one season and had lower summer herbaceous 

offtake (39 percent on the bison range, 18 percent off the bison range) than wet meadow and riparian

habitats (Schoenecker, 2012) These herbaceous offtake rates were comparable to rates found in other 

Rocky Mountain uplands heavily grazed by bison and elk (Frank and McNaughton, 1992). Winter use

rates and browse rates on rabbitbrush shrubs are unknown. Both elk and bison use rabbitbrush 

communities roughly equal to or less than expected based on their availability on the landscape 

(Schoenecker and others, in press; Kate Schoenecker, Ecologist, USGS, Fort Collins Science Center, 

unpublished data).

Greasewood

Greasewood communities are also prevalent in the study area, totaling 3,036 ha (approximately 

7 percent) of the study area including 1,846 ha on the bison range. The vegetation map (Salas and 

others, 2011) identifies two greasewood-dominated community types: Greasewood Sand Deposit 

Shrubland and Steppe Alliances as well as Greasewood Flat Shrubland and Steppe Alliance.

Greasewood communities are described as areas of open to moderately dense greasewood-dominated 

shrubland or steppe either on dunes and sand sheet or alluvial flats on the valley floor. Scattered 

rabbitbrush (E. nauseosa or Chrysothamnus greenei) are often present and may codominate. Herbaceous 

layer is sparse to moderate with Indian ricegrass, goosefoot (Chenopodium spp)., saltgrass, arctic rush, 

beardless wildrye, scratchgrass, western wheatgrass, alkali cordgrass (Spartina gracilis), and kochia

(Bassia americana) present. This community type typically occurs with a relatively shallow water table. 

Substrate is typically alkaline and saline. The main difference between Greasewood Sand Deposit and 

Greasewood Flat alliances is whether the substrate is loose, deep sand or cemented with carbonates 

(sabkha). Indian ricegrass appears to be a good indicator of greasewood-stabilized sand sheet type

(Salas and others, 2011). Our greasewood classification also includes small patches (less than 1 ha), 

which are identified as Fourwing Saltbush Shrubland Alliance which has similar characteristics to the 

greasewood community types.

Herbivory measurements in greasewood were also made in a single season, but indicated high 

summer herbaceous offtake in greasewood on the bison range (72 percent). Offtake rates outside the 

bison range were 18 percent, but offtake variance was very high in greasewood communities

(Schoenecker, 2012). The offtake rate on the bison range appeared to be very high compared to other 

published values (Frank and McNaughton, 1992). Bison and elk used greasewood habitats less than 

would be expected based on their availability (Schoenecker and others, in press).

The ROMN monitors upland habitats in other parks and may include upland grasslands and 

shrublands in the future at GRSA. ROMN shrubland-monitoring protocols currently have only a 

minimal indicator of wildlife-herbivory included in assessments of natural disturbance. Adding 

measures designed to quantify grazing- and browsing-specific effects in these plant communities could 

be an efficient and economical means of following trends in these communities. Cooper and others 

(2006) found that greasewood and rabbitbrush invaded playas that were no longer subject to frequent 

inundation due to installation of a large irrigation project (Closed Basin Canal) and subsequent water 

table drawdown.
30

Herbaceous Stabilized Dunes

Herbaceous stabilized dunes are described by Salas and others (2011) as vegetated dunes and 

sand sheets or patches (vegetated blowouts) within a shrubland or steppe, characterized by sparse to 

moderate (5–45 percent cover) herbaceous layer typically dominated by scurfpea (Psoralidium 

lanceolatum) and(or) blowout grass and Indian ricegrass, needle-and-thread grass, or sand muhly. These 

communities have deep, loose sand that is not cemented by carbonates (sabkha), or, if sabkha is present, 

a shallow sand lens covers more than half the ground surface. Herbaceous stabilized dunes cover nine

percent (3,668 ha) of the study area, including roughly 305 ha located on the bison range. Bison appear 

to use these habitats at a slightly greater rate than would be expected based on their availability

(Schoenecker and others, in press), but elk showed little or no preference (use was less than or equal to 

expected based on availability of habitat) for these types (Kate Schoenecker, Ecologist, USGS, 

unpublished data). No measures of offtake by ungulates or effects of grazing on productivity in these 

communities are available for GRSA.

Piedmont Semi-Desert Grasslands

This type covers roughly 2 percent (881 ha) of the study area, mostly along the eastern edge of 

the study area where the foothills meet the valley floor (fig. 2), and none of this type lies within the 

bison range. A particularly large area of these dry grasslands occurs between the Colorado Highway 150 

and Medano Creek, southeast of the dune field. Salas and others (2011) describe these areas as dry 

grasslands occurring on rocky alluvial fans above the sandsheet on the valley floor and in openings 

within pinyon-juniper woodlands. Characteristic species include blue grama (Bouteloua gracilis), 

needle-and-thread, with scattered rabbitbrush and prickly-pear cacti (Opuntia spp.) and sometimes low 

cover of mountain muhly (Muhlenbergia montana) and fescues (Festuca spp.). No measures of ungulate 

offtake were measured in this type, but grazing effects are likely to be similar to other semi-arid 

grasslands of the Intermountain West. This type is not found on the bison range.

Other Plant Communities of Interest

Although this grazing monitoring assessment focuses on plant communities found on the valley 

floor of Great Sand Dunes National Park, GRSA resource managers have identified two other 

vegetation types of concern regarding ungulate (primarily elk) herbivory effects—alpine habitats and 

aspen (Populus tremuloides). There is not an abundance of data specific to GRSA regarding elk 

herbivory in these habitats, but research has been conducted within similar habitats in the Rocky 

Mountains. 

Alpine Shrub and Herbaceous

Alpine vegetation includes those shrub and herbaceous communities identified on the park 

vegetation map as Alpine Turf Alliances, Alpine Fell-Field Alliances, Alpine-Upper Subalpine 

Herbaceous Wetland Alliances, and Alpine Willow (Spruce) Shrubland Alliances. Alpine turf 

vegetation is found on gentle to moderate slopes, flat ridges, valleys, and basins where the soil has 

become stabilized by vegetation. Sites are windswept with a thin snowpack that melts early in spring

and are characterized by a dense cover of low-growing, perennial graminoids and forbs (Salas and 

others, 2011). Alpine fell-fields include areas of wind-blasted alpine vegetation occurring from 3,476–

4,076 m in elevation with moderate to high cover of gravel and bare ground and sparse to moderate 

cover of herbaceous species dominated by cushion plants. Alpine-subalpine wetlands are found in low 

areas and slopes below snowfields and seeps and support a dense layer of herbaceous growth dominated 
31

by alpine or subalpine wetland species. Alpine shrub types include isolated patches of alpine willows 

(Salix planifolia and S. brachycarpa) found in mesic upland sites (Salas and others, 2011). Alpine shrub 

and herbaceous communities cover 1,524 ha of the 16,854 ha Preserve portion of GRSA.

Elk herbivory has been found to decrease cover and height of alpine willows (S. brachycarpa, S. 

planifolia) in other areas of the Colorado Rocky Mountains (Zeigenfuss, 2006). Long-term grazing can 

alter carbon dioxide and nitrogen dynamics in alpine grasslands. Grazed alpine grasslands have been 

found to have higher soil carbon and nitrogen concentrations, but lower plant biomass, lower grass leaf 

N concentrations, and lower early and late summer N mineralization rates compared to ungrazed sites 

(Welker and others, 2004). This could have implications for plant production and cover under high 

levels of herbivory in alpine areas. Grazing has also been found to decrease seed production and delay 

flowering in alpine forbs (Galen, 1990; Freeman and others, 2003).

Analysis of alpine willow patches on GRSA lands and adjacent U.S. Forest Service (USFS)

lands found low levels of browsing and relatively healthy willows in areas near well-used recreational 

trails in GRSA, as compared to USFS areas, where recreational activity was low and adjacent livestock 

grazing was present (Zeigenfuss, Schoenecker, and van Amburg, 2011). Alpine willows were shorter 

and had lower cover in areas with higher levels of browsing. 

The Rocky Mountain Network monitors native and exotic vegetation species (cover, presence, 

phenophase), soils (condition and temperature), and evidence of browsing (presence and frequency of 

browsing, scat, and trampling) at four alpine peak sites in GRSA on a 5-yr schedule (Ashton and others, 

2010b). The four sites range from two which were receiving little or no browse pressure in 2008 to one 

site with moderate amounts of natural disturbance which was attributed in part to native herbivory 

(Ashton and others, 2010a). However, a park biologist working on this team observed elk on nearby 

alpine areas of the park not included in their sampling (Phyllis Pineda Bovin, Biologist, Great Sand 

Dunes National Park and Preserve, October 2014, oral commun.). This observation may indicate a need 

to survey many more alpine areas in GRSA to obtain some general knowledge of elk and other ungulate 

distributions and herbivory effects on these communities. Alternatively, data collected through the 

ROMN monitoring, as well as anecdotal evidence along trails, can be used to determine when ungulate 

use in high-elevation alpine habitats may be increasing to a level which necessitates grazing monitoring.

Aspen

Roughly 1,656 ha of aspen-dominated stands exist in the Preserve area of GRSA, as well as 

1,545 ha of mixed aspen-conifer stands. The aspen-dominated stands are identified on the park 

vegetation map as Aspen Forest Alliances. Mixed aspen-conifer types are identified as Subalpine FirAspen Forest Alliance, Ponderosa Pine-Aspen Forest Alliance, Aspen-Limber Pine Forest Alliance, and 

Aspen-Douglas Fir Upland Forest Alliances. Small patches or scattered aspen can be found in many 

other conifer-dominated and deciduous riparian forest types as well. Elevation ranges from 2,507–3,555 

m on a variety of substrates, but sites tend to be relatively mesic. 

The status of aspen forests across western North America has been debated by ecologists over 

the past half century. Some researchers have identified declines that could lead to aspen disappearance 

on many western landscapes (Kay, 1997); others point to persisting or increasing stands (Romme and 

others, 2001; Manier and Laven, 2002) or argue that declines are apparent only at local, but not broader, 

scales (Suzuki and others, 1999; Barnett and Stohlgren, 2001). In the Rocky Mountains, mature aspen 

stands are susceptible to drought mortality. Combined with continuous, high herbivory of sprouts and 

young saplings, drought mortality can lead to stand collapse (Worrall and others, 2010; Zegler and 

others, 2012). The effects of intense herbivory on aspen stands in the Rocky Mountains are welldocumented (Romme and others, 1995; Baker and others, 1997; Suzuki and others, 1999; Dieni and 
32

others, 2001; Binkley and others, 2006; Binkley, 2008; Seager and others, 2013). Elk browse intensively 

on abundant aspen suckers following disturbances, particularly fire (Romme and others, 1995; Bailey 

and Whitham, 2002); elk browsing often suppresses heights of seedlings, suckers, and saplings, and 

ultimately prevents stand recruitment and recovery (Bartos and Mueggler, 1981).

Little information appears to exist on the state of aspen stands in GRSA. The USFS conducts 

aerial surveys annually to assess forest health on National Forest lands of Colorado and these surveys 

include the GRSA preserve (U.S. Department of Agriculture, Forest Service, Insect and Pest Detection

Aerial Survey, http://www.fs.usda.gov/detail/r2/forest-grasslandhealth/?cid=stelprd3827262). The 

USFS 2014 survey identified roughly 52 ha of aspen stands in the Preserve that had low levels of 

defoliation (less than 50 percent) caused by an unknown agent, but defoliation in other stands on the

nearby Rio Grande National Forest has been attributed to two insects: large aspen tortrix (Choristoneura 

conflictana) and western tent caterpillar (Malacosoma californicum; U.S. Forest Service, Region 2, 

Aerial Detection Survey Highlights for Colorado, 

http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprd3828662.pdf). A baseline survey of aspen 

stands in GRSA would be required to determine the current condition of stands and define the range of 

stand conditions for aspen on this landscape. At that point, it might be determined whether to include 

aspen as part of a grazing monitoring plan. In particular, the role of fire combined with herbivory should 

be considered when assessing potential effects from ungulates in aspen stands. GRSA has experienced

several large fires over the past several years or decades. 

Design of an Ungulate-Vegetation Monitoring Program

A first step in designing any monitoring program is to define the goals and objectives of the 

monitoring program. Therefore, these goals and objectives must be laid out in the forthcoming GRSA 

ungulate management plan. A goal is a brief statement of the overall purpose of the program (U.S. 

Department of the Interior, 2012). Objectives are more specific statements which provide additional 

detail about the desired outcome of monitoring and include the means by which to measure the success 

of ungulate management actions (U.S. Department of the Interior, 2012). Once the goals and objectives 

of the monitoring program have been determined, the steps of designing a monitoring plan that will 

meet these goals and objectives may commence.

Goals and objectives for the UMP will ultimately direct the goals and objectives set out in the

ungulate-vegetation monitoring plan. Ideally, the UMP will identify indicator variables and threshold 

levels for those variables which indicate the need for management action. The goals and objectives of 

the ungulate-vegetation monitoring plan will be shaped from these broad UMP goals and objectives. For 

example, the monitoring plan might have a goal of assessing management actions of the UMP to 

determine progress toward UMP goals and objectives. As a result, objectives of the ungulate-vegetation 

monitoring plan to support these goals might include:

• Monitor status and trends in target vegetation types (for example, cottonwoods or wetlands) for 

indicator variables identified in the UMP;

• Monitor status, trends, and distribution in intensity of herbivory in targeted vegetation types;

• Identify baseline conditions and use these to set minimum thresholds for condition in targeted 

vegetation types;

• Monitor elk and bison population size and distribution.

These are just an example of the types of objectives park staff might include in a monitoring plan but 

are not meant as comprehensive, nor a priority list of objectives. Similar objectives might also include 

monitoring effects of grazing on rare or threatened plant or animal species, dune stability, or others. 
33

Ultimately, those priorities set out in the UMP will be the appropriate guide for designing the 

monitoring plan.

A wealth of guidance exists for designing monitoring programs (Karr, 1987; Davis, 1993; 

Oakley and others, 2003; Fancy and others, 2009), but the guidance provided by the National Park 

Service’s Inventory and Monitoring Program (U.S. Department of the Interior, 2012) is ideal because it 

uses the same approach as ROMN’s Vital Signs Monitoring Program within GRSA and other Rocky 

Mountain National Parks. Integration of UMP monitoring with existing monitoring programs will help 

expand the data collection, avoid duplication of effort and unnecessary resource damage, and conserve 

financial and labor resources. Once the goals and objectives have been defined, a rough budget should 

also be determined before further development of the monitoring program. Considerations of costs

should be incorporated into designing UMP monitoring and include data collection, data management 

and quality assurance, training, data analysis, scientific oversight, and reporting (Caughlan and Oakley, 

2001). Vegetation types and metrics to be sampled will have to be carefully considered to ensure they

provide key information to determine whether management objectives are being met within limited 

budget frameworks. 

As noted in previously, UMP monitoring must also carefully include the important 

environmental and anthropogenic stressors or drivers that influence vegetation condition in this arid 

environment. Without including these covariates, it will be difficult to isolate the role of ungulate 

disturbance. Because increased use of a particular area by one ungulate species may cause other 

ungulate species to exploit areas they previously avoided, some level of monitoring vegetation types 

that currently receive less ungulate use may be necessary to determine landscape-level effects of 

management actions aimed at redistributing elk and bison herbivory. ROMN monitoring is a good 

example of a program that attempts to do this by including measures of hydrology, soil and water 

chemistry, and various disturbance gradients with estimation of vegetation structure and response. 

Creation of a monitoring program that will be executed consistently and completely will maximize

benefits in terms of providing scientifically credible data on which to base management decisions. 

Underfunded programs may have to reduce or even eliminate monitoring in some years, which may 

undermine the ability to detect trends.

Selecting Monitoring Methods

After determining goals, objectives, and budget, GRSA management and key collaborators will 

need to decide which vegetation communities and indicators will be monitored. Some of these 

indicators may already be highlighted in the ungulate management plan, but others may be identified 

during the process of creating a monitoring program. Review of existing monitoring programs being 

considered, in development, or in use on NPS and neighboring lands in the San Luis Valley will 

streamline the development of a grazing monitoring program. 

All federal, state, and TNC lands within or adjacent to GRSA are engaged in various stages of 

vegetation monitoring. One of these programs is specifically aimed at monitoring range condition and 

the effects of bison grazing. Ranchlands/Zapata Partners has been monitoring their bison pasture lands 

on the Medano Ranch using the Land EKG® method (Orchard and Mehus, 2001) since 2007. This 

method has many good points but is generally aimed at balancing ecosystem health with livestock 

production and may not be appropriate for monitoring grazing of wild ungulates. The Bureau of Land 

Management and the U.S. Department of Agriculture have developed guidance specifically for

monitoring grazing on western rangelands (Pellant and others, 2005; Herrick and others, 2009 a, 2009b), 

and many of these methods are also referenced in the Land EKG and ROMN Vital Signs Monitoring 

programs.
34

Methods currently being used by ROMN’s Vital Signs Monitoring for wetland ecological 

integrity (Schweiger and others, in press) and vegetation composition structure and soils in grasslands, 

shrublands, and woodlands (Manier and others, 2011) would be ideal for incorporation by GRSA 

managers into their grazing monitoring plan. These programs already collect vegetation and soils data 

on many variables important to assessing grazing effects, such as soil compaction, soil water content, 

soil erosion, vegetation cover and composition, substrate cover, invasive plants, browse measures, 

sapling and shrub height, shrub cover, and tree diameters. Where possible, monitoring sites already 

being measured by ROMN could be used to reduce the number of sites needed to be installed for 

grazing monitoring by the park. Another monitoring program that may prove useful is Rocky Mountain 

National Park’s elk and vegetation monitoring program that is designed to measure response in aspen 

and willow communities to elk population and habitat management activities (Zeigenfuss, Johnson, and 

Wiebe, 2011). Some metrics in this program may be directly applicable to GRSA and can be added to 

the ROMN WEI site structure with relative ease.

Recently, the Baca NWR began monitoring woody browse following a method proposed by 

retired USGS ecologist Richard Keigley and co-authors (Keigley, R.B., Warren, Jeffrey, and King, 

W.J., 2009, A multi-refuge program to evaluate the effect of ungulate browsing on habitat, U.S. 

Geological Survey, unpublished report) which is also being considered for use at several other national 

wildlife refuges in the region. These methods are also being considered by the Rio Grande National 

Forest (Randy Ghormley, Rio Grande National Forest, August 2012, oral commun.). However, these 

methods are not appropriate for use with coyote willow, and will likely have limited usefulness for 

GRSA. 

Sampling Design and Site Selection

The current park vegetation map (Salas and others, 2011) is a good starting point for delineating 

vegetation that is common, in large patches, or easily distinguished (cottonwood for example), but it 

may not be suitable for site selection for all vegetation types. Rare vegetation types, such as willow, are 

often under-represented. Wetland areas are misclassified, or the classifications do not include the detail 

required to delineate wetland areas properly. The sampling design must allow for site level adjustment. 

For example, if willow is included among the vegetation types to be monitored for grazing effects, a

detailed inventory of willow communities along all creeks in the study area will be necessary to allow 

for the best monitoring of change in the overall extent and health of willow in the study area. If wetlands 

are to be monitored, the ROMN wetland classification should also be incorporated into the vegetation 

map used sampling design. A reclassified vegetation map will need to be developed when final 

vegetation classes for monitoring are determined since many of the current classifications would likely

be combined for the purpose of monitoring ungulate herbivory effects.

Sample sizes should provide an appropriate level of power (1-β) to detect a certain degree of 

change in a measured variable over time with a certain degree of confidence (1-α). Assessing small 

amounts of change with a great deal of power and confidence typically requires larger sample sizes and 

will have associated costs to match. As a result, sample sizes often reflect a tradeoff between cost and 

the ability to detect trends in measured variables over time. The selected level of power and confidence 

to detect trends may be dependent upon the financial and labor resources available for carrying out a 

large-scale monitoring program, or fewer vegetation types may need to be monitored in order to achieve 

a high level of power and confidence to detect change. Therefore, careful selection of vegetation 

communities and indicator variables to be monitored is required. Sample size can be determined in a 

number of ways: using existing data from the study area, using data from other areas or studies, using 

published tables, or using formulas to calculate sample size. In this case, data for a subset of indicator 
35

variables exists from the Schoenecker (2012) study and ongoing work by ROMN will provide good data 

to parameterize power models.

A good sampling design requires appropriate, unbiased distribution of sampling sites throughout 

the landscape and within target populations to be sampled. Stevens and Olsen (2004) developed the 

generalized random tessellation sampling (GRTS) method which produces a selection of sample sites 

that is spatially balanced over the extent of the study area (including any subdivisions—for example, to 

separately measure areas with and without bison grazing), while still maintaining randomization. 

Designs employing GRTS are created using a combination of ArcGIS and R programming (Kincaid,

2012). ROMN wetland monitoring sites were mostly selected using GRTS design with wet meadow and 

salt flat wetlands on the sand sheet as the target population. For the UMP design, the valley floor 

regions within the GRSA boundary could be extracted from the reclassified vegetation map and these 

areas could be split into those currently grazed by bison and those grazed primarily by elk. The most 

efficient UMP design would integrate or expand upon the ROMN wetland design. A temporal or revisit 

design is also important for efficient and meaningful data collection and interpretation. While repeat 

sampling is important to detecting change in variables over time, sampling too often within a timeframe 

that is too short for detectable change to occur wastes time and money (for example, measuring changes 

in vegetation species composition on an annual basis). However, some measured attributes may be 

variable over short time periods, and capturing this variability over shorter intervals may provide insight 

into other processes which influence the measured variables (for example, the influence of growing 

season precipitation on plant productivity, or phenological differences between cool-season and warmseason vegetation). There are many different temporal revisit designs that should be considered 

(Urquhart and Kincaid, 1999; Larsen and others, 2001) with the final approach balancing cost and 

temporal patterns in the core variables being measured.

Baseline Data Collection

Baseline data measures should be collected at the time of site establishment; however, some 

measured variables are seasonally dependent, in which case all sites of a particular vegetation type 

should be revisited during the same time period to collect data. Habitat and vegetation conditions have 

likely changed since the conclusion of both the USGS ungulate herbivory study (Schoenecker, 2012) 

and the initial data collection by ROMN. Drought has continued for many of the intervening years and 

bison and elk populations have varied throughout this period. Use of baseline data collected at the outset 

of the monitoring program would be most appropriate for setting or adjusting any thresholds identified 

for triggering ungulate management actions. An inventory of cottonwood stands should be conducted if 

the park desires to monitor loss of overstory cottonwood canopy on a landscape scale. It may be 

possible to use remotely sensed data to conduct this inventory, and, if aerial photography or satellite 

data with fine enough resolution are available over recent decades, this could provide insight into trends 

in canopy health over time. An analysis of aerial photography as far back as data is available would also 

allow park managers to determine if any expansion or loss of cottonwood stands over the last 50–70 yr

on lower Sand Creek or Deadman Creek was in response to human alterations of the hydrology of these 

creeks. 

Conclusion

Development of a grazing monitoring program for Great Sand Dunes National Park and Preserve 

will involve evaluation and determination of vegetation communities of interest, selection of goals for 

vegetation condition, selection of objectives to assess success of ungulate management activities, and 
36

design of a monitoring program that will effectively measure progress toward those goals. The grazing 

monitoring program would ideally be used to adapt ungulate management actions to continue toward 

achievement of vegetation condition goals.

Acknowledgments

We appreciate the support and cooperation of Fred Bunch, Phyllis Bovin, and Andrew Valdez of 

GRSA. We thank Duke Phillips and Jeff Gossage of Zapata Partners/Ranchlands, and Mike Britten and 

Billy Schweiger of ROMN for providing data and information on their work in GRSA. Sarah Beck, 

Tracy Mask, and Sarah Garza assisted with field work. Mark Sturm (NPS) and Phyllis Bovin provided 

comments on early drafts of this work. Billy Schweiger (ROMN) and Therese Johnson (Rocky 

Mountain National Park) provided insightful reviews of the manuscript. This work was funded by the

USGS-NPS Park-Oriented Biological Support (POBS) Program.

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