In cooperation with the National Park Service

Monitoring of Vegetation Response to Elk Population and 

Habitat Management in Rocky Mountain National Park, 

2008–14

By Linda C. Zeigenfuss and Therese L. Johnson

Open-File Report 2015–1216

U.S. Department of the Interior

U.S. Geological Survey
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 Johnson, T.L., 2015, Monitoring of vegetation response to elk population and habitat 

management in Rocky Mountain National Park, 2008–14: U.S. Geological Survey Open-File Report 2015–1216, 

44 p., http://dx.doi.org/10.3133/ofr20151216.

ISSN 2331-1258 (online)

Cover: Elk habitat improvement exclosures in Beaver Meadows, Rocky Mountain National Park, Colorado.
iii

Contents

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

Introduction................................................................................................................................................................. 2

Purpose and Scope .................................................................................................................................................... 3

Study Area.................................................................................................................................................................. 5

Methods...................................................................................................................................................................... 7

Vegetation Data Collection...................................................................................................................................... 9

Aspen ................................................................................................................................................................ 11

Upland............................................................................................................................................................... 11

Willow................................................................................................................................................................ 12

Elk Population and Climate Data........................................................................................................................... 13

Statistical Analysis ................................................................................................................................................ 13

Results...................................................................................................................................................................... 14

Aspen.................................................................................................................................................................... 14

Stand Structure ................................................................................................................................................. 14

Sapling Regeneration........................................................................................................................................ 18

Effects of Burning .............................................................................................................................................. 20

Upland................................................................................................................................................................... 24

Offtake............................................................................................................................................................... 24

Shrub Cover ...................................................................................................................................................... 27

Willow.................................................................................................................................................................... 28

Offtake............................................................................................................................................................... 28

Willow Height..................................................................................................................................................... 29

Shrub Cover ...................................................................................................................................................... 29

Effects of Burning .............................................................................................................................................. 31

Kawuneeche Valley........................................................................................................................................... 34

Discussion ................................................................................................................................................................ 35

Progress Toward Vegetation Goals ...................................................................................................................... 35

Aspen ................................................................................................................................................................ 35

Upland............................................................................................................................................................... 36

Willow................................................................................................................................................................ 36

Current State of Elk-Vegetation Management on Winter Range........................................................................... 38

Acknowledgments..................................................................................................................................................... 40

References Cited...................................................................................................................................................... 41

Appendix................................................................................................................................................................... 44

Figures

1. The elk winter range and Kawuneeche Valley of Rocky Mountain National Park, Colorado.......................... 6

2. Location of aspen and willow monitoring sites, existing elk exclosures, and the 2012 Fern Lake 

Fire on the Rocky Mountain National Park elk winter range, Colorado, as of January 2014.......................... 7

3. Distribution of aspen tree stem diameters in monitoring sites on core winter range of 

Rocky Mountain National Park, Colorado .................................................................................................... 15
iv

4. Distribution of aspen tree stem diameters in sampled sites on noncore winter range of 

Rocky Mountain National Park, Colorado .................................................................................................... 16

5. Distribution of aspen tree stem diameters in sampled sites in the Kawuneeche Valley of Rocky

Mountain National Park, Colorado............................................................................................................... 17

6. Comparison of two fenced aspen sites on the elk winter range of Rocky Mountain National Park, 

Colorado...................................................................................................................................................... 20

7. Tree stem distribution of aspen in sites burned by the 2012 Fern Lake Fire................................................ 21

8. Comparison of moderately burned aspen site on the elk winter range of Rocky Mountain National Park, 

Colorado...................................................................................................................................................... 22

9. New aspen site established in 2013 in area burned by 2012 fire, Rocky Mountain National Park, 

Colorado...................................................................................................................................................... 22

10. Patterns of upland herbaceous offtake relative to estimated park elk population size on the elk winter 

range of Rocky Mountain National Park, Colorado, 2007–14 ...................................................................... 25

11. Relationship between upland herbaceous offtake and growing season precipitation on the elk winter 

range of Rocky Mountain National Park, Colorado...................................................................................... 26

12. Mean shrub cover on upland sites on elk winter range in Rocky Mountain National Park, Colorado........... 27

13. Mean winter willow offtake relative to park elk population size on the elk winter range of Rocky Mountain 

National Park, Colorado, 2007–14............................................................................................................... 28

14. Comparison of unfenced willow site on the elk winter range of Rocky Mountain National Park, 

Colorado...................................................................................................................................................... 31

15. Comparison of unburned willow site on the elk winter range of Rocky Mountain National Park, 

Colorado...................................................................................................................................................... 31

16. Comparison of burned willow site on the elk winter range of Rocky Mountain National Park, Colorado...... 34

17. Comparison of heavily burned willow site on the elk winter range of Rocky Mountain National Park, 

Colorado...................................................................................................................................................... 34

18. Comparison of willow site in Beaver Meadows on the elk winter range of Rocky Mountain National Park, 

Colorado...................................................................................................................................................... 38

Tables

1. Vegetation types and their desired future conditions, indicator variables, and thresholds to be used in 

implementing the Rocky Mountain National Park elk and vegetation management plan............................... 4

2. Summary of data to be used in 5-yr analysis of EVMP monitoring in Rocky Mountain National Park ......... 10

3. Number of sites distributed between fencing treatments and areas burned in the Fern Lake fire on 

the elk winter range of Rocky Mountain National Park, Colorado, as of January 2014................................ 11

4. Least squares means of aspen sapling density at all aspen monitoring sites in Rocky Mountain National 

Park, Colorado............................................................................................................................................. 19

5. Least squares means of aspen sapling density at aspen monitoring sites at sites burned 

in the 2012 Fern Lake Fire compared to unburned sites in Rocky Mountain National Park, Colorado ........ 23

6. Annual upland herbaceous offtake on elk winter range in Rocky Mountain National Park, 

Colorado, 2007–14 ...................................................................................................................................... 24

7. Distribution of annual upland herbaceous offtake on elk winter range of Rocky Mountain 

National Park, Colorado............................................................................................................................... 26

8. Average and maximum willow height and percent willow cover on the elk winter range of Rocky Mountain 

National Park, Colorado, at baseline measurement (2008–9) and at first 5-yr sampling (2013).................. 30
v

9. Least squares means of average and maximum willow height and percent willow cover on burned sites 

compared to unburned sites on the elk winter range of Rocky Mountain National Park, Colorado, at 

baseline measurement and at first 5-yr sampling ........................................................................................ 33

10. Percent of aspen monitoring sites on elk winter range and Kawuneeche Valley of Rocky Mountain 

National Park, Colorado, that had a recruiting sapling cohort at baseline sampling and in 2013................. 35

1–1. Willow offtake using alternate stem-scaled diameter-difference calculation method ................................... 44

1–2. Least squares means of willow cover and heights....................................................................................... 44
vi

Conversion Factors

International System of Units to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 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 hectometer (hm2) 2.471 acre

square kilometer (km2) 247.1 acre

square meter (m2) 10.76 square foot (ft2) 

hectare (ha) 0.003861 square mile (mi2) 

square kilometer (km2) 0.3861 square mile (mi2)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as °F = (1.8 × °C) + 32.

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as °C = (°F – 32) / 1.8.

Datum

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

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

Abbreviations

dbh diameter at breast height

EVMP elk and vegetation management plan

R2 coefficient of determination

USGS U.S. Geological Survey

yr year
1

Monitoring of Vegetation Response to Elk Population and 

Habitat Management in Rocky Mountain National Park, 

2008–14

By Linda C. Zeigenfuss and Therese L. Johnson1

Abstract

Since 2008, Rocky Mountain National Park in Colorado has been implementing an elk and 

vegetation management plan with the goal of managing elk populations and their habitats to improve the 

condition of key vegetation communities on elk winter range. Management actions that have been taken 

thus far include small reductions in the elk herd through culling of animals and temporary fencing of 

large areas of willow and aspen habitat to protect them from elk browsing. As part of the park’s elk and 

vegetation management plan (EVMP), a monitoring program was established to assess effectiveness of 

management actions in achieving vegetation goals. We collected data to monitor offtake (consumption) 

of upland herbaceous plants and willow annually from 2008 to 2014 and to assess aspen stand structure 

and regeneration and willow cover and height in 2013, 5 years after plan implementation. Loss of many 

willow and a few aspen monitoring sites to a fire in late 2012 complicated data collection and 

interpretation of results but will provide opportunities to observe habitat recovery following fire and in 

the presence and absence of elk herbivory, which will offer important insights into the use of prescribed 

fire as an additional management tool in these habitats.

Increases in the number of small-diameter, tree-sized (stems greater than 2.5 meter height) aspen 

stems were observed but only inside fences that excluded ungulates. In unfenced areas, stand structure 

was stagnant, with many medium- and large-diameter (older) stems and no replacement of smalldiameter stems. By 2013, aspen saplings (stems less than or equal to 2.5 meter height) were recruiting

on 29 percent of sampled sites, an increase from 13 percent of sites at baseline, but this was mainly due 

to growth inside fences. Upland herbaceous offtake dropped below baseline levels (61 percent) on both 

core and noncore winter range in 2010–14. Less than 10 percent of the upland areas had intense 

herbivory (greater than 85 percent offtake), and less than 30 percent of the landscape had offtake greater 

than 70 percent after 2009. Offtake levels in 2013 and 2014 indicated an increase in grazing pressure on 

upland sites compared to 2010–12 levels, but this change may have been in response to loss of large 

patches of both herbaceous and woody forage in Moraine Park following the 2012 Fern Lake Fire. 

Winter willow offtake remained steady from 2009 to 2014, and although there were no substantial 

increases in offtake, there were also no consistent declines. Winter-range willow offtake was below the 

baseline level of 35 percent only in 2013 and 2014. Willow heights have stayed at or above baseline 

levels of 0.9 meter. Average heights of willow increased compared to baseline measures within fenced 

habitat on the core winter range and on noncore (all unfenced) winter range. Willow cover increased at 

least 75 percent compared to baseline within core winter-range fenced areas and roughly 25 percent in

noncore winter range. Overall, during the first 5 years of implementation, the EVMP at Rocky Mountain 

 1 Rocky Mountain National Park
2

National Park seems to be making steady progress toward the vegetation objectives set out by the 

EVMP. Habitat fencing has been the most effective means of improving aspen and willow habitat 

conditions. 

Introduction

Rocky Mountain National Park encompasses nearly 108,000 hectares (ha) of high-elevation 

forest, shrublands, meadows, and alpine tundra and rocklands in north-central Colorado. The park 

supports numerous species of wildlife, including several large ungulate species. The most abundant 

ungulate species in the park is Cervus elaphus (elk). In recent years, there has been growing concern 

about the condition of vegetation in the park and conflicts between elk and humans, both inside and 

outside the park. In particular, condition of Populus tremuloides (quaking aspen) and Salix spp. (willow)

habitats on the winter range of Rocky Mountain National Park have declined.

Many years of research indicated that high elk densities in Rocky Mountain National Park were 

resulting in the complete loss of aspen clones or reduction of many aspen on core winter-range areas to 

a shrub-like state (Baker and others, 1997; Suzuki and others, 1999; Weisberg and Coughenour, 2003). 

Elk browsing was determined to be suppressing growth of or killing all young aspen (those less than 2.5 

meters [m] in height—also called “suckers” or “saplings”) on the core elk winter range and in some 

parts of the Kawuneeche Valley (Olmsted, 1979; Baker and others, 1997; Zeigenfuss and others, 2008). 

As a result, aspen stand regeneration on the elk winter range was limited with few or no suckers 

growing large enough to be recruited into the tree canopy in recent decades (Suzuki and others, 1999; 

Binkley, 2008), resulting in overmature, deteriorating aspen stands with no small- or medium-diameter

trees (Baker and others, 1997; Binkley, 2008). These stands would likely be permanently lost if the level 

of elk herbivory remained high, although it is difficult to predict when this would happen (National Park 

Service, 2007). Research indicated that management actions that enhance aspen recruitment could help 

sustain and preserve aspen stands in the park over the long term (Kaye and others, 2005). 

Elk were also suppressing the growth of willow plants, both in height and areal cover, and tall 

willow stands were being converted to short willow (Peinetti and others, 2002; Zeigenfuss and others, 

2002). Willow is the dominant woody shrub on almost all wet meadow or riparian areas in Rocky 

Mountain National Park. Heavy browsing inhibited the ability of many winter-range willows to 

reproduce because few willow plants on the primary elk winter range produced seed. Suitable seedling 

establishment sites were limited as well, and seedling survival was poor due to downcutting of stream 

channels and associated lowering of the water table as well as browsing (Gage and Cooper, 2005). Elk 

were consuming herbaceous plants at extremely high rates on the elk winter range and potentially 

resulting in the alteration of herbaceous plant communities on the elk range. Annual herbaceous 

consumption rates on willow and upland shrub communities on the elk winter range averaged 55 to 60

percent, in the 1990s (Singer and others, 2002).

In response to these concerns, Rocky Mountain National Park developed an elk and vegetation 

management plan (EVMP) to evaluate the effects of a range of alternatives for managing elk and 

vegetation in the park. The purpose of the EVMP is to guide management actions in the park during a 

20-year (yr) time period to reduce the impacts of elk on vegetation and restore, to the extent possible, 

the natural range of variability in the elk population and affected plant communities (National Park 

Service, 2007), which includes the following:

• prevention of loss of aspen clones within high elk-use areas; 

• restoration and maintenance of sustainable montane riparian willow as indicated by (1) increase 

of montane riparian willow cover within suitable willow habitat on the primary winter range; 
3

and (2) maintenance or improvement of the condition of riparian willow on the primary summer 

range; and

• reduction of the level of elk grazing on herbaceous vegetation.

The plan outlines the desired future condition for each of these identified vegetation types (table 1). The 

management alternative that was selected relies on a variety of conservation tools including temporary

fencing, nonlethal redistribution of elk, use of various vegetation-restoration techniques, and lethal 

reduction of elk (culling). 

Purpose and Scope

The EVMP incorporates the principles of adaptive management to assess the effectiveness of 

management actions. Use of adaptive management in the EVMP means that Rocky Mountain National 

Park managers will adjust management actions as needed to successfully achieve the EVMP’s 

objectives. Determination of whether vegetation objectives are being achieved requires monitoring and

evaluation of target vegetation communities. From 2006 to 2009, an EVMP monitoring program 

(Zeigenfuss and others, 2011) was established on the elk winter range of Rocky Mountain National Park

to monitor the effects of elk management actions (through both population management and habitat 

management) in key vegetation communities (aspen, willow, upland). This program called for 

monitoring of (1) vegetation offtake by herbivores in willow (shrub offtake) and upland (herbaceous 

offtake) communities through annual sampling of a subset of sites, (2) shrub height and cover in willow

every 5 yrs through resampling of all sites, and (3) stem density and stem size distribution in aspen 

every 5 yrs through resampling of all sites. Because of logistical constraints, all monitoring sites could 

not be established in the same year; however, the bulk of baseline sampling had taken place by 2008 and 

fence construction began in the fall of 2008, thus 2013 was the fifth year after plan implementation.

The purpose of this report is to provide an analysis of the first 5 yrs of monitoring data collected 

in association with the EVMP. This analysis will be used as a means to assess progress toward the 

vegetation goals. We examined the changes that have taken place during this first phase of elk and 

vegetation management at Rocky Mountain National Park. Additional factors that may be influencing 

the type and degree of vegetation change on the elk winter range are also addressed. The EVMP 

monitoring plan, data collection, and this report focus on evaluation of the elk winter range because that 

has been the focus of plan implementation. For the Kawuneeche Valley, which is primarily summer 

range, we provide baseline information for a limited number of willow sites established in 2012, 3 yrs of 

willow offtake sampling, and data from aspen sites 5 yrs after baseline data collection.
4

Table 1. Vegetation types and their desired future conditions, indicator variables, and thresholds to be used in implementing the Rocky Mountain 

National Park elk and vegetation management plan. [table taken from Zeigenfuss and others (2011); ~, about; <, less than; cm, centimeter; dbh, diameter at breast height; m, meter; ≤, less than or equal to; 

>, greater than]

Vegetation 

category Desired future condition Indicator Thresholds/objectives

Aspen At least 45 percent of aspen across the winter 

range regenerating. Distribution of stem dbh 

reflects many (~75 percent) small-diameter 

stems, some (~20 percent) mediumdiameter stems, and few (~5 percent) largediameter stems. 

Stem density by 

height and 

diameter class

Progressive increase in aspen recruitment above the baseline level 

of 13 percent (presence of stems <2 cm dbh reaching 1.5–2.5 m 

tall). Progressive shift in the distribution of stem sizes toward 

the desired future condition.

Riparian 

montane 

willow

At least 31 percent willow cover within 

suitable willow habitat across the winter 

range. Average willow height of at least 

1.1 m. 

Willow 

consumption

No net increase in annual willow offtake across the winter range 

above the baseline level of 35 percent. 

Cover Progressive increase in willow cover across the winter range 

above the baseline level of 21 percent.

Structure Progressive increase in willow height across the winter range 

above the baseline level of 0.9 m.

Upland 

herbaceous

Reduction of the level of elk grazing on 

herbaceous vegetation and maintenance of a 

diversity of grazing levels across the 

landscape.

Herbaceous 

consumption

No net increase in winter upland herbaceous offtake across the 

winter range above baseline levels of 47 percent, with ≤ 25

percent of sites with offtake > 70 percent and ≤ 10 percent of 

sites with offtake > 85 percent. 
5

Study Area

Most of the Rocky Mountain National Park EVMP monitoring sites are located within the elk

winter range on the east side of Rocky Mountain National Park (fig. 1; Zeigenfuss and others, 2011). 

This winter range encompasses approximately 10,000 ha in five major valleys in the upper montane 

zone along the park boundary near the town of Estes Park, Colorado. The elk winter range includes 

areas where elk concentrate during winter (Moraine Park, Beaver Meadows, Horseshoe Park) that are 

referred to as the “core” elk winter range and other areas that elk use to a lesser degree, referred to as the 

“noncore” winter range (National Park Service, 2007; fig. 1). Elevation in the area ranges from 2,400 to 

2,800 m. Valley bottom vegetation includes Carex spp. (sedges), grasses, and riparian shrubs (willow, 

Betula spp. [birch], Alnus incana [gray alder]). Slopes contain Pinus ponderosa (ponderosa pine)/shrub, 

mixed conifer (ponderosa pine–Pseudotsuga menziesii [Douglas-fir]), Pinus contorta (lodgepole pine), 

aspen, and upland grass/shrub (predominantly Purshia tridentata [antelope bitterbrush]) vegetation 

types. The Kawuneeche Valley is an area of about (~) 825 ha on the western boundary of the park 

running north to south along the headwaters of the Colorado River (fig. 1). Elevations in the 

Kawuneeche Valley range from 2,650 m to 2,800 m. The valley is dominated by wet meadows (sedge 

species, Calamagrostis canadensis (bluejoint reedgrass), Deschampsia cespitosa (tufted hairgrass), and 

riparian shrubs (willow, Betula glandulosa [resin birch], gray alder).

Elk were extirpated, or nearly so, from Rocky Mountain National Park by human exploitation in 

the late 1800s but were then reintroduced in 1913 and 1914. The population steadily increased until it 

reached an estimated 1,000 animals within the park boundaries in 1944 (Packard, 1947). Elk in the park 

were artificially reduced from 1943 to 1968, but starting in 1968, they were no longer controlled within 

the park’s boundaries, as a result of a change to a U.S. National Park Service management paradigm 

know as natural regulation (National Park Service, 2007). Elk steadily increased in the park after 1968 

and also in the town of Estes Park after the town area was recolonized in the late 1970s, reaching a 

population of roughly 1,000 animals on the park winter range and 2,000 in the Estes Valley by the late 

1990s (Lubow and others, 2002). Since 2001, this population has been declining and by 2008 numbered 

roughly 650 animals wintering in the park (N.T. Hobbs, Senior Research Scientist, Natural Resources 

Ecology Lab, Colorado State University, unpub. data, 2014). In addition to the population that winters 

in the park from November through March, the Rocky Mountain National Park winter range is used in 

the spring, summer, and fall by elk that migrate between summer range in the park and lower-elevation 

winter range in the Estes Valley and Front Range foothills (Bear 1989, Larkins 1997). In addition to elk,

mule deer (Odocoileus hemionus) use elk winter-range areas in the park year-round, tending to 

concentrate mainly in upland shrub communities. Moose (Alces alces) were introduced to North Park, 

Colo., northwest of Rocky Mountain National Park in the late 1970s and by 1980 had appeared in the 

Kawuneeche Valley. There is no recent population estimate of moose in the Rocky Mountain National 

Park; however, Dungan (2007) estimated approximately 100 moose inhabited the west side of the park

in the Colorado River drainage during summer 2003, with roughly one-fourth of that population 

residing in the Kawuneeche Valley. Moose have begun to appear on the elk winter range on the east 

side of Rocky Mountain National Park in the last decade, though numbers appear to be low. Both elk 

and moose inhabit the Kawuneeche Valley year-round but are most common in spring through fall.
6

Figure 1. The elk winter range and Kawuneeche Valley of Rocky Mountain National Park, Colorado.
7

From 2008 to 2013, the park constructed temporary elk-proof fences around patches of key 

aspen and willow habitat to facilitate vegetation restoration. By 2013, when data for this assessment 

were collected, approximately 68.5 ha of fenced willow areas and 17 ha of fenced aspen stands had been 

established on the core elk winter range (fig. 2). In 2011, the park installed an experimental fence 

around willow in the Kawuneeche Valley, excluding elk and moose from a 6.5-ha area. An additional 

1.4 ha of willow habitat and 1 ha of aspen habitat on the winter range, as well as 0.6 ha of Kawuneeche 

Valley willow, were already fenced, mostly in disjunct small patches, as part of earlier studies and 

restoration efforts.

Figure 2. Location of aspen and willow monitoring sites, existing elk exclosures, and the 2012 Fern Lake Fire on 

the Rocky Mountain National Park elk winter range, Colorado, as of January 2014.

Methods

The EVMP monitoring program was developed using a sampling design that stratified 

vegetation communities into elk winter-range zones of fenced core winter range, unfenced (open to elk 

browsing and grazing) core winter range, and noncore winter range (all unfenced). This design was 

intended to allow assessment of vegetation response to fencing and detection of shifts in habitat use 

from core to noncore areas (Zeigenfuss and others, 2011).
8

Twenty sites per elk winter-range stratum (core and noncore winter range) were determined to 

be adequate to assess the level of change indicated for both fenced and browsed aspen in the EVMP 

(Zeigenfuss and others, 2011). A sample size of 35 sites for willow monitoring in each elk winter-range 

zone was needed to adequately measure browsed willow (Zeigenfuss and others, 2011). Nearly one-half 

the area of core winter-range willow habitat was planned for fencing, and due to the large sizes of these 

fenced enclosures, several monitoring sites were located in each fenced area; therefore, the sample size 

in fenced core winter-range willow was limited to 20 sites. A sample size of 25 sites per elk winterrange stratum was determined to be adequate to predict mean values of upland herbaceous offtake 

within 20 percent of the true mean 90 percent of the time. 

Zeigenfuss and others (2011) used a stratified, random sampling design to select potential 

monitoring sites. Using ArcGIS® 9.2 software program, Zeigenfuss and others (2011) selected the 

vegetation communities of interest and clipped them to the winter-range boundary. Those vegetation 

polygons that fell within the core winter range were further separated from the entire winter range to 

create maps of core and noncore winter-range vegetation communities. Maps of potential fence 

locations were used to clip vegetation polygons from the core winter-range map. The final results were 

three maps each for aspen and willow types (noncore winter range (all unfenced); fenced, core winter 

range; and unfenced core winter range) and two maps for upland vegetation types (core and noncore 

winter range). Zeigenfuss and others (2011) generated random points in each vegetation type and elk 

winter-range zone. A minimum distance of 250 m between upland sites was imposed to allow 

movement of grazing cages over a large enough area to prevent site overlap while also preventing 

resampling within the same site over a short timeframe. Since willow and aspen monitoring plots were 

fixed in size, no minimum distance between plots was used, but plot boundaries could not overlap each 

other or be in close proximity to fences. Not all sites that appeared appropriate on the map were 

determined to be suitable upon site visit, in which case the site was replaced with another randomly 

selected site.

The temporal sampling design varied across metrics and community type. Offtake indicators 

were measured annually due to the high degree of interannual variability related to factors such as 

weather and ungulate population size. To reduce costs related to sampling all plots every year, a splitpanel, serially-alternating design with consecutive year revisits was employed (Urquhart and Kincaid, 

1999). The 20-yr monitoring period (2009–28) was divided into five panels, such that each site is visited 

for offtake measures twice within each 5-yr period. The total number of sites in each vegetation type 

and elk winter-range zone was divided by five to determine the number of sites assigned to any given 

panel. 

The social nature of an ungulate that moves and feeds in herds (like elk) can result in large 

spatial heterogeneity in offtake on the landscape (with some locations being more heavily grazed due to 

number of animals concentrated in a group) in addition to heterogeneity in offtake resulting from the 

spatial distribution of desirable forage species. Therefore, the temporal aspects of this design were also 

balanced with the spatial distribution of sites by visually grouping the random sites into spatial 

“neighborhoods” of five sites (one for each panel) and randomly assigning them to one of the five 

panels. The “neighborhood” was defined as a group of sites that were spatially close together and linked 

by topography as much as possible. 

Variables such as willow cover and structure, upland shrub cover, and aspen stem density tend to 

change more slowly so these were measured only every 5 yrs. All monitoring sites were revisited in 

2013 to collect measurements associated with these variables.
9

Vegetation Data Collection

Field sampling methods followed protocols detailed in the park’s EVMP monitoring plan 

(Zeigenfuss and others, 2011), but the protocols are briefly summarized here to provide a context for the 

results presented in this report. We collected data at the established plots according to the measurement 

timetables established in the protocols (table 2). Baseline data were collected 2006–9 (Zeigenfuss and 

others, 2011). Park staff were responsible for collecting all monitoring data as well as establishing and 

collecting baseline data for sites that were added after 2009. Annual visits to sample offtake from a 

subsample of willow and upland sites began in 2007 (upland) and 2009 (willow) and continued through 

2014. Target sample sizes (table 2) were not always achieved due to a variety of circumstances 

including weather, inability to relocate sites, labor shortages, and oversight. Data collection dates varied 

due to snow cover and availability of personnel. Willow offtake measures were typically collected 

during the months of May and June; however, in 2010, data was collected on some sites as late as midJuly. All other variables (table 2) were measured during the year of site establishment and again during 

June through September 2013.
10

Table 2. Summary of data to be used in 5-yr analysis of EVMP monitoring in Rocky Mountain National Park.

[yr, year; m, meter; cm, centimeter]

Vegetation 

type

Measured 

variable

Major comparisons Sampling 

interval

Original target 

sample size/interval

Units

Willow Percent 

offtake

Winter range Unfenced core and noncore 

winter range

All yrs 2008–14

yr 1 to yr 6

Annually with

one-half of 

sampled sites 

measured in 

consecutive 

years

14 core

14 noncore

(all unfenced)

Current year production 

removed from 0-100

percent

Kawuneeche Valley yrs 2012–14 Annually 4 unfenced

Willow cover 

and height

Winter range Fenced and unfenced core winter 

range

Unfenced core and noncore 

winter range

yr 1 and yr 5

Burned and unburned

Sampled once 

every 5 yrs

35 core, unfenced 

20 core, fenced

35 noncore (all 

unfenced)

Cover ( percent) from 

0–100 percent

Willow height (m)—

maximum and 

average

Cover and height of 

Kawuneeche Valley Baseline Annually 4 fenced willow

4 unfenced

Aspen Aspen density 

and height

Winter range Fenced and unfenced core winter 

range

Unfenced core and noncore 

winter range

yr 1 and yr 5

Burned and unburned

Sampled once 

every 5 yrs

20 core, unfenced

20 core, fenced

20 noncore (all 

unfenced)

Stem size and density 

of aspen

Sapling height (cm)

Kawuneeche Valley yr 1 and yr 5 8 (all unfenced)

Upland Percent offtake Winter range Unfenced core and noncore 

winter range

All yrs 2008–14

Annually with

one-half of 

sampled sites 

measured in 

consecutive 

years

10 core (all 

unfenced)

10 noncore (all 

unfenced)

Percent standing crop 

removed from 0–100

percent

Shrub cover Winter range Unfenced core and noncore 

winter range

yr 1 and yr 5

Sampled once 

every 5 yrs

25 core (all 

unfenced)

23 noncore (all 

unfenced)

Cover ( percent) from 

0–100 percent

Cover by species 
11

A fire in December 2012 burned through Moraine Park affecting 21 core willow monitoring

sites, 9 core aspen sites, and 1 noncore aspen site (fig. 2). Of these sites, 17 willow (9 fenced, 8

unfenced) and 4 aspen (3 fenced, 1 unfenced) core sites were burned severely enough to require 

replacement. The burned plots will be maintained, however, to measure comparative growth inside and 

outside elk exclusion fences. New sites were established and measured in 2013, which led to changes in 

the sample sizes and design from the original monitoring plan (table 3). A few other site replacements 

were made as well due to fence proximity issues after fence construction. An additional year of offtake 

data collected in 2014 was available by the time this report was finished and is included in this report as 

well.

Eight willow monitoring sites were established in the Kawuneeche Valley in 2012 in connection 

with the construction of an experimental exclosure in the fall of 2011. Baseline data were collected at 

these sites in 2012–14.

Table 3. Number of sites distributed between fencing treatments and areas burned in the Fern Lake Fire on the 

elk winter range of Rocky Mountain National Park, Colorado, as of January 2014.

[na, not applicable]

Vegetation Type Total 

sites

FencedunburnedUnfencedunburnedFencedburnedUnfencedburned

Aspen

Core winter range 47 11 29* 3 4

Noncore winter range 21 na 20 na 1

Willow

Core winter range 85 23 34 10 18

Noncore winter range 34 na 34 na na

Upland

Core winter range 25 na 25 na na

Noncore winter range 23 na 23 na na

*Some of these sites may potentially be fenced in the future.

Aspen

At each aspen monitoring site, all live and dead aspen stems greater than 2.5 m in height falling 

within a 5-m x 5-m square plot were tallied according to stem-size class. Each stem-size class covered a 

2-centimeter (cm) range of diameter at breast height (dbh) from 0 to 2 cm dbh to greater than 34 cm 

dbh. Based on these stem-size classes, stems were then tallied into three diameter classes: smalldiameter (0–10 cm dbh) stems, medium-diameter (10–20 cm dbh) stems, and large-diameter (greater 

than 20 cm dbh) trees. Aspen stems less than or equal to 2.5 m in height were tallied by height class (0–

50 cm, 51–100 cm, 101–150 cm, 151–200 cm, 201–250 cm). 

Upland

Upland offtake data were collected in April and May during 2009–14. Herbaceous offtake was 

measured using movable 1-square-meter (m2) grazing cages to protect plants from grazers during the 

grazing season (in this case, winter) and then clipping all herbaceous vegetation from 0.25-m

2 plots 

protected by the grazing cage and equally sized paired plots outside the cage to determine percentage of 

offtake (Bonham, 1989). The cages were moved to a new location within each site at the beginning of 

each sampling period to protect a new plot from grazing. Winter offtake was determined by comparing 

biomass at the end of winter inside the cage to biomass remaining in the uncaged, grazed plot using 

equation 1.
12

( )





 



 − = ×

i

i o

w B

B B O 100

(1)

where

Ow is the percent of end of growing season standing crop that is used overwinter,

Bi is the amount of biomass in caged plot at the end of winter, and 

Bo is the amount of biomass in paired uncaged plot at the end of winter.

Upland shrub species, cover, and height were measured for each shrub whose canopy intercepted 

a permanently marked 30-m transect line through the site. These measurements were taken from all 

upland sites at baseline and again in 2013.

Willow

Willow cover and average heights were determined by measuring all willows falling within a 4-

m x 4-m square plot (macroplot) at each site as well as those that intercepted a 5.7-m transect line (line 

intercept) that bisected the square plot from east to west. A more intensive and more accurate method 

(macroplot) and a less intensive and less accurate method (line-intercept) were used to provide a means 

for scaling down monitoring effort in the future if funding for data collection was reduced. Both 

methods were used to collect data at baseline and, in 2013, to provide an appropriate basis for 

comparison if the future need arises to scale back sampling to the less intensive measures. Because the 

macroplot method gives a better representation of willow cover, data from this method are reported in 

the “Results” and “Discussion” sections. The line-intercept results are included in the appendix (tables

1–1 and 1–2). Similarly, two methods of measuring ungulate browsing on willow were incorporated 

into the willow-monitoring protocols. The “stem-scaled diameter difference method” (also called the 

“DD2 method” in Bilyeu and others, 2007) calculates offtake as the difference between diameter at bud 

scar and at browse point scaled by the number of shoots browsed on the entire stem (see Zeigenfuss and 

others, 2011). The “production-weighted diameter difference method” (DD3) recommended by Bilyeu 

and others (2007), which accounts for browser preference for more productive shoots, was also used to 

estimate the percentage of biomass removed based upon shoot size of browsed and unbrowsed shoots 

(equation 2).

















−

− ×



 





× + ×

× =

b t

p t

D D

D D

b B u U

b B DD3

(2)

where b is the number of browsed shoots on the stem,

u is the number of unbrowsed shoots,

Dp is shoot diameter at the point of browsing,

Dt is the average diameter of unbrowsed shoot tips,

Db is the diameter at the base of the shoot,

B is average pre-browse mass of browsed shoots, and

U is the average mass of unbrowsed shoots. 

For clarity of discussion, the DD3 results are used in the “Results” and “Discussion” sections of this 

report, but the DD2 results have been included in the appendix (table 1–1).

The EVMP included the possibility of taking actions that would encourage elk migration to 

summer ranges, so summer offtake was anticipated to be minimal. The DD2 and DD3 methods 

described above estimate total annual willow offtake but were being used as analogous to winter offtake 

only because a minimal amount of summer willow offtake was anticipated. From the outset of the 
13

EVMP monitoring program, field crews observed summer browsing on willow and presence of elk on 

the winter range during the summer. At this time, no management actions have been taken to encourage 

movement of elk from the winter range during the summer; therefore, a rapid survey was conducted in 

August 2014 to document percent of willow shoots browsed and to determine whether additional 

monitoring of summer browsing may be needed.

Elk Population and Climate Data

A combination of annual aerial and ground surveys was used annually to estimate the winter 

population size of elk using low elevation range in the park. Aerial survey data were corrected based on 

sightability factors and a Bayesian population model was used to estimate trends in elk population size. 

Precipitation data were acquired from the National Climate Data Center (National Oceanic and 

Atmospheric Administration, 2014) for COOP weather station #52761 in Estes Park, Colo., Snow depth 

data were acquired from the Bear Lake Snotel Site #322 (Natural Resources Conservation Service, 

2014) located 5 kilometers southwest of the core elk winter range at an elevation of 2,900 m. These data 

were used to examine the relationship of weather and elk population to patterns in annual offtake data.

Statistical Analysis

The original analysis of baseline data involved making comparisons between core and noncore 

winter-range areas and presenting the descriptive characteristics of each type (table 2). We modified this 

structure to include the new burned sites. Statistical tests were carried out using SAS version 9.2 (SAS 

Institute, Cary, North Carolina). All percentage data were converted to proportions and then transformed 

using a logit transformation (Warton and Hui, 2011) prior to statistical testing. We tested for differences 

between baseline and 2013 values for all willow variables, except offtake, using mixed effects 

regression models (PROC MIXED, PROC GLIMMIX, SAS Institute, Cary, N.C.). Willow and upland 

offtake were analyzed using linear and quadratic regression models. Baseline upland herbaceous offtake 

estimates were originally derived using a method that included negative values (Zeigenfuss and others,

2011). Upon recent consultation with a statistician, however, we decided to remove all negative values 

from the data because they do not allow for the required data transformations and because such values 

are nonsensical (in other words, there is no opportunity for plant regrowth following grazing in winter, 

which is the only situation that could logically result in greater production outside the grazing cages 

compared to inside the cages and thus a negative offtake value). Changing the offtake calculation 

method allowed for easier comparison of trends and interannual differences but required recalculation of

baseline thresholds of distribution of offtake across the landscape. Offtake data were transformed using 

a logit transformation and were then analyzed for differences using a mixed model.

We used weighted averages based on the amount of aspen, willow, and upland area available in 

the core and noncore winter range to report winter-range-wide estimates of cover, offtake, heights, and 

aspen regeneration. The weighted averages were adjusted in 2013 to account for mean values and area 

of the winter range that was fenced and burned as well.
14

Results

Aspen

Stand Structure

Unfenced, core winter-range aspen stands maintained stable size (age) structure of tree-size 

(greater than 2.5 m height) stems from 2008 to 2013 (fig. 3). At baseline, 3 of 29 sites had a smalldiameter (0–10 cm dbh) component, and by 2013 only two sites had small-diameter trees because all 

small-diameter trees at one site had moved into the medium-diameter (10–20 cm dbh) class. At many 

sites, there was a steady progression into the large-diameter (greater than 20 cm dbh) tree class with no 

concurrent shift of saplings and small-diameter trees into the small- and medium-diameter tree classes. 

In these unfenced stands, lack of recruitment of regenerating trees into the stand canopy continues while 

the stand as a whole continues to age with some older, larger trees eventually dying.

Notable change was observed, however, within fenced aspen stands on the core winter range 

(fig. 3). An increase in the percent of tree-sized stems in small-diameter classes was seen in 8 of 10 

fenced sites that were measured at baseline. Only one of these aspen sites had a small-diameter 

component at baseline sampling. In addition, all of these stands had a substantial proportion of stems 

that were less than or equal to 2 cm dbh. At baseline sampling, this 0–2-cm class was so rarely 

encountered (only 1 stand of the original 65 winter-range sites) that stems of this size were not tallied at 

baseline, only noted. 

Overall, little change was observed in the size/age structure of noncore winter-range aspen 

stands between baseline and 2013 (fig. 4); however, some exceptions were found. One site (site number 

2) that had no tree-size stems but did have a cohort of saplings (stems less than or equal to 2.5 m height) 

at baseline, now has a small-diameter class of trees. This site is located in the Hidden Valley area. Only 

two other sites had any small-diameter trees in 2013 and these small-diameter trees existed in both sites 

at baseline; however, the percentage of stems in the small-diameter class has dropped at one site number 

23) since establishment and no very small trees (0–2 cm dbh) were observed, which indicated a lack of 

ongoing recruitment of saplings in this stand. Another two sites (numbers 6 and 10) lost their only living 

tree-size stems. In one site (site number 21), measurement error in determining whether a single tree 

was in or out of the site resulted in a situation where no tree-size stems were present at baseline, but a 

large-diameter tree class was present in 2013 (fig. 4). 
15

Figure 3. Distribution of aspen tree (height greater than 2.5 meters) stem diameters in monitoring sites on core 

winter range of Rocky Mountain National Park, Colorado. A, baseline measurement (2007/2008 for most sites). 

B, first measurement period (2013).
16

Figure 4. Distribution of aspen tree (height greater than 2.5 meters) stem diameters in sampled sites on noncore 

winter range of Rocky Mountain National Park, Colorado. A, baseline measurement (2007/2008 for most sites). 

B, first measurement period (2013). The 0–2-centimeter class at baseline was estimated.
17

Almost no change in stand structure was observed in the Kawuneeche Valley since site 

establishment in 2007 (no aspen are fenced in the Kawuneeche Valley). Only one of the eight sites here 

had a small-diameter (0–10 cm dbh) class in 2013 and none of these small-diameter stems were in the 

very smallest (0–2 cm dbh) class that would indicate that saplings are recruiting into the stand. In two 

other sites, medium-diameter trees moved into the older age/larger size class (fig. 5).

Figure 5. Distribution of aspen tree (height greater than 2.5 meters) stem diameters in sampled sites in the 

Kawuneeche Valley of Rocky Mountain National Park, Colorado. A, baseline measurement (2007/2008 for most 

sites). B, first measurement period (2013).
18

Sapling Regeneration

Short sapling (less than or equal to 1.5 m height) density was not different between 2013 and 

baseline for noncore winter range sites (table 4, p-value = 0.265). There was a nearly significant (pvalue = 0.073) increase in these short saplings on unfenced core winter-range aspen sites from baseline 

to 2013, whereas density of short saplings decreased on fenced core winter-range aspen sites (p-value = 

0.047, table 4); however, the 2013 sapling densities include several sites that were burned in the Fern 

Lake Fire and had an extremely high amount of aspen suckering post-fire. 

On the core winter range, density of tall aspen saplings (stems 1.5–2.5 m in height) was greater 

in 2013 than 2008 on sites that were fenced (table 4, p-value = 0.003). Eighty percent of the monitoring 

sites that had been protected by fences had high density (greater than 1,000 stems per acre) of tall (1.5–

2.5 m height) saplings (table 4, fig. 6) by 2013. On the noncore winter range, there was little change in 

either short or tall sapling density since the baseline sample (p-value greater than or equal to [≥] 0.21, 

table 4). 

At baseline there was no difference in tall aspen sapling density between the core and noncore 

sites (p-value = 0.240), but by 2013, density of tall aspen saplings was greater on fenced sites compared 

to unfenced sites, both in the core and noncore winter range sites (p-value less than [<] 0.001, fig. 6). 

Although noncore winter-range sites had the highest densities of tall saplings at baseline, by 2013 

fenced core winter-range sites had significantly greater density of tall saplings than noncore winterrange sites (p-value < 0.001). Tall saplings remained nonexistent on Kawuneeche Valley sites during

the time since site establishment (table 4).
19

Table 4. Least squares means of aspen sapling (plants with height less than or equal to 2.5 meters) density at all aspen monitoring sites in Rocky 

Mountain National Park, Colorado. Baseline data collected 2006–9, with most of the sites collected in 2007 or 2008. P-values report differences of 

least squares means of log-transformed density between baseline and 2013 samples. In 2013, this sample includes four sites burned by the Fern 

Lake Fire. 

[s.e., standard error; n, number sites sampled; m, meter; na, not applicable]

Winter range zone

Aspen sapling density (stems/acre)

Short (stems ≤ 1.5 m height) Tall (stems 1.5–2.5 m height)

Baseline 2013 P-value1 Baseline 2013 P-value1

(mean ± s.e.) n (mean ± s.e.) n (mean ± s. e.) n (mean ± s.e.) n

Core winter range, unfenced 6,667 ± 2,483 32 8,317 ± 2,481 33 0.073 5 ± 8 32 20 ± 8 33 0.179

Core winter range, fenced 7,269 ± 1,600 13 4,664 ± 1,550 14 0.047 25 ± 513 13 1,851 ± 495 14 0.003

Noncore winter range 1,902 ± 438 20 2,329 ± 433 21 0.265 131 ± 93 20 162 ± 93 21 0.210

Kawuneeche Valley 1,640 ± 478 8 1,984 ± 478 8 0.783 0 8 0 8 na

1

Reported p-value is for log-transformed sapling density variable.
20

Figure 6. Comparison of two fenced aspen sites on the elk winter range of Rocky Mountain National Park, 

Colorado. Colored arrows indicate landmarks for reference. A, B, baseline measurement prior to fencing. 

C, D, after 4 years of protection from ungulate browsing (2013).

Effects of Burning

Although the Fern Lake Fire caused mortality of some small-diameter aspen trees in the burned 

sites (mortality of one-half or more of measured stems less than 10 cm dbh, fig. 7), burning seemed to 

have a positive effect on sapling regeneration. Extensive suckering was observed on nearly all the 

burned sites. The seven core winter-range sites measured in 2013 that were burned had high density of 

short (less than or equal to 1.5 m height) saplings (table 5, figs. 8 and 9). Three of the four newly

established burned sites had over 25,000 suckers per ha, indicating the potential for good recruitment in 

the next few years as these suckers continue to grow. All three of these sites with high sucker densities 

were located outside of fences. One noncore site that was burned had almost no suckering. This site was 

in a moist area where abundant growth of grasses and forbs may have outcompeted new aspen saplings. 

Tall aspen saplings (1.5 to 2.5 m height) had the highest density in unburned, fenced core winter-range 

sites compared to all other sites (p-value < 0.001). The fire may have removed this size on burned sites 
21

inside fences and many new suckers likely did not have enough time to reach this taller height in one 

season after burning.

Figure 7. Tree (height greater than 2.5 meters) stem distribution of aspen in sites burned by the 2012 Fern Lake 

Fire. Four of the seven sites displayed were established after the fire to assess effects of burning on aspen sites. 

No trees with 0–2-centimeter diameter at breast height [dbh] stems were found. A, at baseline. B, in 2013. 
22

Figure 8. Comparison of moderately burned aspen site on the elk winter range of Rocky Mountain National Park, 

Colorado. Colored arrows indicate landmarks for reference. A, baseline measurement prior to fencing and burning.

B, after 4 years of protection from ungulate browsing in the first season post fire (2013).

Figure 9. New aspen site established in 2013 in area burned by 2012 fire, Rocky Mountain National Park, 

Colorado. This site had extremely dense regeneration and was unprotected by fences.
23

Table 5. Least squares means of aspen sapling (plants with height less than or equal to 2.5 meters) density at aspen monitoring sites at sites 

burned in the 2012 Fern Lake Fire compared to unburned sites in Rocky Mountain National Park, Colorado. Baseline data collected 2006–9, with 

most of the sites collected in 2007 or 2008.

[s.e., standard error; n, number sites sampled; m, meter; na, not applicable]

Winter range zone

Aspen sapling density (stems/acre)

Short (stems ≤ 1.5 m height) Tall (stems 1.5–2.5 m height)

Baseline 2013 P-value

(test between 

sample dates)1

Baseline 2013 P-value

(test between 

sample dates) (mean ± s.e.) n (mean ± s.e.) n (mean ± s.e.) n (mean ± s.e.) n 1

Core winter range, unfenced

Burned 324 1 35,505 ± 17,386 4 na 162 1 40 ± 40 4 na

Unburned 3,275 ± 709 31 4,736 ± 723 29 0.135 0 ± 6 31 17 ± 7 29 0.068

Core winter range, fenced

Burned 5,506 ± 607 3 7,881 ± 607 3 0.061 0 ± 0 3 0 ± 0 3 na

Unburned 7,899 ± 2,051 10 3,798 ± 1,983 11 0.013 32 ± 618 10 2,355 ± 589 11 0.004

Noncore winter range

Burned 1,296 1 162 1 na 0 1 162 1 na

Unburned 1,930 ± 452 19 2,437 ± 448 20 0.214 138 ± 98 19 162 ± 97 20 0.358

1

Reported p-value is for log-transformed sapling density variable.
24

Upland

Offtake

Winter offtake levels on upland herbaceous plants were highly variable from 2007 to 2014, 

declining from 2009–11, but then increased from 2012 through 2014 (fig. 10). Overall, upland offtake 

dropped from an average 61.8 ± 3.8 (least squares mean ± standard error) percent in 2007–9 to 44.5 ± 

2.9 percent in 2010–14 (p-value = 0.012). Prior to 2013, the overall trend was a similar and slightly 

significant downward trend in both core (coefficient of determination [R2] = 0.584, p-value = 0.077) and 

noncore (R2 = 0.672, p-value = 0.046) winter-range sites. Both offtake estimation methods yielded 

similar results with comparable patterns and trends in offtake levels. Although the offtake values were 

different for core compared to noncore sites, similar patterns in offtake trends were observed on both 

core and noncore winter range, except in 2009 and 2014, (table 6). Few direct correspondences could be 

determined between elk population size in the park and upland offtake (fig. 10). One interesting pattern 

in upland offtake involves an apparent shift in offtake from the core winter range to the noncore winter 

range. By 2014, offtake on the noncore winter range was much greater than core winter range, though 

this difference was not significant (p-value = 0.126).

Table 6. Annual upland herbaceous offtake on elk winter range in Rocky Mountain National Park, Colorado, 

2007–14. Original 2007–8 data were estimated using a method that included offtake values of -100 to 100. Current 

analysis removed all negative values from the dataset. * denotes difference between core and noncore winter 

range offtake in that year (p-value less than 0.05).

[yr, year; s.e., standard error; n, number sites sampled]

Sample yr

Entire winter range Core winter range Noncore winter range

Percent offtake 

(mean ± s.e.) n

Percent offtake

(mean ± s.e.) n

Percent offtake 

(mean ± s.e.) n

Baseline (2007–8) Original 47.1 ± 4.3 75 52.4 ± 3.9 44 40.3 ± 5.9 31

Revised 61.3 ± 1.8 43 61.3 ± 2.6 45 60.0 ± 3.2 30

2009 63.7 ± 4.5 17 72.1 ± 3.8* 10 47.3 ± 6.8* 7

2010 45.9 ± 4.8 18 49.6 ± 6.5 9 40.8 ± 7.4 9

2011 32.6 ± 4.0 18 31.9 ± 5.3 9 33.4 ± 6.5 9

2012 42.4 ± 6.3 17 41.5 ± 6.5 10 44.4 ± 13.5 7

2013 52.8 ± 5.6 15 51.1 ±7.4 10 57.3 ± 8.2 5

2014 48.8 ± 5.4 15 41.4 ± 5.6 8 60.5 ± 8.8 7
25

Figure 10. Patterns of upland herbaceous offtake relative to estimated park elk population size on the elk winter 

range of Rocky Mountain National Park, Colorado, 2007–14. Population estimates from N.T. Hobbs, Senior 

Research Ecologist, Natural Resources Ecology Lab, Colorado State University (unpub. data, 2014). 

We also looked for patterns in offtake related to snow depth recorded at the Bear Lake Snotel 

site and April–July precipitation as recorded at the Estes Park weather station. Most upland sites are 

located on south- and west-facing slopes that remain snow free or have low snow accumulations for 

most of the winter and, therefore, might be subject to grazing when other parts of the winter range are 

snow-covered. Precipitation is very important to production on these dry upland sites.

No obvious connection was noted between snowpack and offtake, and offtake was not correlated 

with maximum snow depth at the Bear Lake Snotel site; however, offtake on the entire winter range was 

lower in years with higher April–July precipitation (p-value = 0.001, R2 = 0.86, fig. 11). Offtake rates 

were significantly greater in core winter range in 2009 compared to noncore winter range (p-value = 

0.006), but not in any other year. A prescribed burn conducted in the Upper Beaver Meadows and Deer 

Ridge areas in fall of 2008 burned approximately 208 ha, and this may have reduced available upland 

forage on burned areas during winter 2008–9, resulting in higher concentration of elk use on other 

upland areas of the core winter range.
26

Figure 11. Relationship between upland herbaceous offtake and growing season precipitation (April–July) on the 

elk winter range of Rocky Mountain National Park, Colorado.

Distribution of offtake across the landscape (core and noncore sites combined) varied from 2008 

through 2014 (table 7). We used the revised offtake calculation method to estimate baseline offtake and 

then revised distribution objectives to align them with this new method. The revised objectives are (1) 

no net increase in winter upland herbaceous offtake across the winter range above baseline levels of 61 

percent, (2) with less than or equal to 30 percent of sites with offtake greater than 70 percent and (3) less 

than or equal to 10 percent of sites with offtake greater than 85 percent. Much less upland winter range 

was grazed intensively (greater than 85 percent offtake) from 2010 to 2014 than at baseline or 2009.

Table 7. Distribution of annual upland herbaceous offtake on elk winter range of Rocky Mountain National Park, 

Colorado. 

[Gray shading highlight indicates years when offtake targets for high levels of browsing set in the Elk-Vegetation 

Management Plan (National Park Service, 2007) were met.]

Offtake level Annual percent of sites within offtake levels

(all negative offtake values removed)

Baseline 2009 2010 2011 2012 2013 2014

0–50 percent 26 24 56 78 65 40 53

50–70 percent 44 29 28 22 18 40 40

70–85 percent 19 41 17 0 12 20 0

>85 percent 11 6 0 0 6 0 7
27

Shrub Cover

Mean shrub cover on upland monitoring sites did not change between baseline sampling in 2007 

and 2013. Core winter-range sites had mean shrub cover of 9.4 ± 2.9 percent (mean ± standard error) 

and noncore sites had a mean shrub cover of 21.1 ± 4.2 percent (fig. 12) in 2013. Noncore upland 

winter-range sites had greater shrub cover both at baseline and in 2013 compared to core winter-range 

sites. Using transformed data variables, this difference was not significant at the baseline sampling but 

was significant by 2013 (p-value = 0.046), which may reflect removal of shrubs by prescribed burns 

conducted on the core winter range during the monitoring period, rather than any effect of herbivory.

Figure 12. Mean shrub cover on upland sites on elk winter range in Rocky Mountain National Park, Colorado. 
28

Willow

Offtake

Willow offtake was highly variable from 2009 to 2014 on the core winter range and the winter 

range as a whole but remained fairly consistent on the noncore winter range from 2009 to 2011 and then 

began to decline (fig. 13). Offtake was significantly lower throughout the winter range in 2013 (p-value

= 0.045) but increased again in 2014. Although offtake on the noncore winter range seemed to track the 

decline in elk populations, a similar pattern was not evident for the core winter range (fig. 13). Willow 

offtake was higher on the core winter range than the noncore winter range throughout the 5 yrs

evaluated (p-value = 0.004). Patterns in offtake for the entire winter range paralleled those on the core 

winter range because most of the winter-range willow areas are on the core winter range (fig. 13).

Willow offtake was not correlated to elk population size, snow depth, total water year precipitation, or 

growing season (April–July) precipitation. Patterns in offtake were similar using the DD2 offtake 

estimation method (appendix, table 1–1).

Figure 13. Mean winter willow offtake relative to park elk population size (divided by 10) on the elk winter range of 

Rocky Mountain National Park, Colorado, 2007–14. Population estimates from N.T. Hobbs, Senior Research 

Ecologist, Natural Resources Ecology Lab, Colorado State University (unpub. data, 2014).
29

Willow Height

Heights of unfenced core winter-range willows did not change significantly during 2008–13

(p-value = 0.304), but willow heights did increase on fenced areas of the core winter range as well as on 

the noncore winter range (p-value ≤ 0.009, table 8). Average heights of fenced willows were roughly 

30 cm taller in 2013 than 2008. The same was true for maximum willow heights with maximum heights 

of fenced willows increasing roughly 40 cm (table 8). Height gains were similar in the noncore winter 

range. 

Noncore willows were taller than core willows in 2008 and 2013, even after 5 yrs of fencing 

(p-value < 0.001). In 2008, core winter-range willows that were eventually fenced had lower maximum 

heights than willows on sites that were not fenced in the core winter range, though this difference was 

not significant (p-value = 0.009). It is not surprising that fenced sites had lower baseline heights because 

the willow sites selected for fencing were those with the poorest willow condition. Similar statistical 

results were observed using both the macroplot and the line intercept method, although the actual size 

estimates were often quite different (see appendix, table 1–2).

Shrub Cover

Between 2008 and 2013, the percentage of original winter-range willow sites (those not affected 

by the Fern Lake Fire) that had living willow plants dropped slightly from 94.5 percent to 93 percent. 

No significant change in willow cover was observed in the unfenced, core winter range between the 

baseline samples and 2013 (p-value = 0.181; fig. 14; table 8), but willow cover in fenced, core winterrange sites had a nearly significant (p-value = 0.081) increase and cover of noncore winter-range willow

increased significantly (p-value = 0.009; fig. 15; table 8). 

At baseline, there was greater cover in the noncore winter-range willow sites compared to sites 

destined for fencing (p-value = 0.034). By 2013, noncore winter-range sites had greater willow cover 

than both fenced and unfenced, core winter-range sites (p-value ≤ 0.008).

The line-intercept method resulted in similar statistical results compared to the macroplot

method of measurement, and the line-intercept method did not pick up the difference in cover for fenced 

willows. Sample size was not sufficient to adequately test differences between core winter-range sites in 

Horseshoe Park, Moraine Park, and Beaver Meadows because this was not the original intent of the 

monitoring; however, Horseshoe Park had substantially greater percent willow cover (30 ± 5) in 2008 

than sites in Moraine Park (8 ± 2) or Beaver Meadows (8 ± 3), fairly similar to noncore sites (35 ± 5). 

By 2013, willow cover in Beaver Meadows and Moraine Park (including those sites burned in the Fern 

Lake Fire) had increased to 10 ± 3 percent and 18 ± 4 percent, respectively, but were still much lower 

than Horseshoe Park (41 ± 5 percent) and noncore winter-range sites (44 ± 6 percent). 
30

Table 8. Average and maximum willow height and percent willow cover on the elk winter range of Rocky Mountain National Park, Colorado, at 

baseline measurement (2008–9) and at first 5-yr sampling (2013). Estimates are for the macroplot methods described in Zeigenfuss and others 

(2011) and include sites that were burned in 2013. Least squares means and standard errors are presented for individual winter range categories, 

and weighted means and related standard errors based on proportion of winter range in each category are presented for the entire winter range 

(bottom row).

[cm, centimeter; s.e., standard error; yr, year; na, not applicable]

Winter range zone Willow height (cm) Maximum willow height (cm) Willow cover ( percent)

Baseline 

(mean ± s.e.)

2013

(mean ± s.e.)

P-value 

(yr)

Baseline 

(mean ± s.e.)

2013

(mean ± s.e.)

P-value 

(yr)

Baseline 

(mean ± s.e.)

2013 

(mean ± s.e.)

P-value1 

(yr)

Core winter range,

unfenced 92 ± 14 102 ± 14 0.304 196 ± 24 168 ± 21 0.131 21 ± 4 21 ± 3 0.181

Core winter range,

fenced 65 ± 6 98 ± 9 <0.001 99 ± 14 140 ± 13 0.004 16 ± 5 28 ± 5 0.081

Noncore winter 

range

160 ± 20 185 ± 20 0.009 256 ± 27 300 ± 27 0.001 35 ± 5 44 ± 6 0.009

Entire winter range 

(weighted 

average)

79 ± 7 102 ± 9 na 157 ± 14 177 ± 14 na 20 ± 2 24 ± 3 na

1

Reported p-value is for transformed willow cover variable.
31

Figure 14. Comparison of unfenced willow site on the elk winter range of Rocky Mountain National Park, 

Colorado. A, baseline measurement (2009). B, 4 years after baseline measurement (2013).

Figure 15. Comparison of unburned willow site on the elk winter range of Rocky Mountain National Park, 

Colorado. Colored arrows indicate reference landmarks. A, one growing season after being fenced (baseline 

measurement). B, after 5 years of protection from ungulate browsing (2013). 

Effects of Burning

The Fern Lake Fire moderately to severely burned 81 percent of the willow sites in Moraine 

Park and 30 percent of all sites on the core winter range. Although willow cover on individual burned 

sites may have decreased substantially, burned sites had much lower cover (p-value = 0.002) from the 

outset because they were all in Moraine Park, which generally had lower cover than winter range in 

Horseshoe Park. Fenced, burned sites had particularly low cover at the outset because sites selected for 

fencing were those in the worst condition. It is possible that there was willow growth inside the fences 

prior to the 2012 fire, but such growth would have likely been lost to the fire. As a result, willow cover 

on these sites did not change significantly between baseline and 2013 (table 9). 

In 2013, willow cover was lower at both fenced and unfenced, burned sites, than all unburned 

winter-range willow sites (p-value ≤ 0.038). No differences in average willow height were evident at 
32

baseline between plots that were eventually burned in 2013 (p-value ≥ 0.172) in the core winter range, 

but all core winter-range willow sites had shorter willows than noncore sites (p-value ≤ 0.003); 

however, after the fire in 2013, all burned sites, whether fenced or not, had shorter willows than 

unburned sites (p-value ≤ 0.032).

Burning had mixed effects on willow growth in this first season post-fire, with many sites 

having complete consumption of older plants, no regeneration of new plants (K. Kaczynski, 

Postdoctoral Research Associate, Department of Forest and Rangeland Stewardship, Colorado State 

University, written commun., June 2014), and low resprouting of older plants (figs. 16 and 17). Sites 

within fences seem to have burned more completely than those outside of fences in many cases (fig. 17), 

possibly due to accumulation of herbaceous litter inside fences.
33

Table 9. Least squares means of average and maximum willow height and percent willow cover on burned sites compared to unburned sites on 

the elk winter range of Rocky Mountain National Park, Colorado, at baseline measurement (2008 for most sites) and at first 5-yr sampling (2013). 

Estimates are for macroplot methods described in Zeigenfuss and others (2011). Least squares means and standard errors are presented for 

individual winter range categories, and weighted means and related standard errors based on proportion of winter range in each category are 

presented for the entire winter range (bottom row).

[cm, centimeter; s.e., standard error; yr, year]

Winter range zone Willow height (cm) Maximum willow height (cm) Willow cover ( percent)

Baseline

(mean ± s.e.)

2013

(mean ± s.e.) 

P-value 

(yr)

Baseline

(mean ± s.e.)

2013

(mean ± s.e.)

P-value 

(yr)

Baseline

(mean ± s.e.)

2013

(mean ± s.e.)

P-value 

(yr)1

Core winter range, unfenced

Burned 67 ± 14 42 ± 9 0.137 91 ± 18 56 ± 12 0.122 15 ± 6 12 ± 6 0.242

Unburned 119 ± 19 118 ± 19 0.922 242 ± 30 226 ± 27 0.350 25 ± 5 26 ± 4 0.767

Core winter range, fenced

Burned 46 ± 13 46 ± 13 0.993 53 ± 14 57 ± 13 0.852 2 ± 2 5 ± 2 0.797

Unburned 71 ± 10 121 ± 9 <0.001 121 ± 15 177 ± 13 0.001 21 ± 6 38 ± 5 <0.001

Noncore winter range 

(all unburned) 160 ± 20 185 ± 20 0.009 256 ± 27 300 ± 27 0.001 35 ± 5 44 ± 6 0.009

1

Reported p-value is for transformed willow cover variable.
34

Figure 16. Comparison of burned willow site on the elk winter range of Rocky Mountain National Park, Colorado. 

Colored arrows indicate reference landmarks. A, baseline measurement (2009). B, 4 years after baseline and 

following one season of post-fire growth (2013). 

Figure 17. Comparison of heavily burned willow site on the elk winter range of Rocky Mountain National Park, 

Colorado. Colored arrows indicate reference landmarks. A, prior to fencing and burning (baseline measurement). B, 

after 4 years of protection from ungulate browsing and first season post-fire (2013). No regrowth of willow was 

indicated in this site in 2013. 

Kawuneeche Valley

Willow offtake at the Kawuneeche Valley willow sites was compared across the 3 yrs the valley

was sampled. Baseline (2012) willow offtake was 30 ± 3 percent (mean ± standard error) and remained 

nearly constant in 2013 (28 ± 4 percent) and 2014 (29 ± 5 percent). The eight Kawuneeche Valley sites 

also had low willow cover (17 ± 5 percent), mean height (50 ± 4 cm), and maximum height (98 ± 12 

cm) at their baseline measure in 2012. These sites were all established in relation to one exclosure site, 

so they are not representative of the entire Kawuneeche Valley but only of the location of the exclosure.
35

Discussion

Progress Toward Vegetation Goals

Aspen

Objectives.—Progressive increase in aspen regeneration above the baseline level of 13 percent to at least 

45 percent of winter range stands (presence of stems less than 2 cm dbh reaching 1.5–2.5 m tall). 

Progressive shift in the distribution of stem sizes toward the desired future condition of 75 percent 

small-diameter stems, 20 percent medium-diameter stems, and 5 percent large-diameter stems.

In 2013, there were signs of a progressive shift toward the desired future condition of a more 

natural distribution of stem sizes to reflect recruitment of younger (smaller diameter) trees inside fences 

that excluded ungulates (figs. 3 and 4). In unfenced areas, however, on both core and noncore winter 

range, stand structure was stagnant, with more trees moving into larger (older) size classes, and no 

replacement of small-diameter trees. 

At baseline, 13 percent of sampled aspen sites on the winter range had a recruiting sapling class 

(had at least 400 stems per ha that were 1.5–2.5-m height). By 2013, 29 percent of sampled aspen sites 

had measurable sapling recruitment (table 10). Although an increase was noted across all the winter 

range (both core and noncore), the most substantial increase was inside fenced areas of the core winter 

range (tables 4 and 10, fig. 6). The increase in tall saplings inside fences was accompanied by a decrease 

in the number of short (less than 1.5-m height) saplings, likely due to movement of these short saplings 

into taller classes. Competition for space and light resources also likely contributed to a decrease in 

density in smaller size classes as density of larger size stems increased. In unfenced sites of the core 

winter range, these shorter (0–1.5-m height) saplings had a nearly significant (p-value=0.073, table 4) 

increase, but much of this increase is attributable to the high level of suckering on the four burned sites.

In 2013, there was still no recruiting sapling cohort in the Kawuneeche Valley stands. No fences 

were constructed in Kawuneeche Valley aspen. It is unknown whether the lack of sapling recruitment is 

related to the combined effects of moose and elk browsing in the Kawuneeche Valley or other factors 

such as disease or water stress. Signs of moose presence (scat, tracks, or browsing) were observed in 80 

percent of the Kawuneeche Valley sites, whereas less than 2 percent of winter-range sites had signs of 

moose presence.

Table 10. Percent of aspen monitoring sites on elk winter range and Kawuneeche Valley of Rocky Mountain 

National Park, Colorado, that had a recruiting sapling cohort (at least 400 stems per hectare of height between 1.5 

and 2.5 meters) at baseline sampling and in 2013. Entire winter range numbers are weighted by the percent area in 

each location.

Location Sample date

Baseline 2013

Entire winter range 13 percent 29 percent

Core winter range total 7 percent 32 percent

Core, fenced 64 percent

Core, unfenced 12 percent

Noncore winter range total 15 percent 28 percent

Kawuneeche Valley 0 percent 0 percent
36

It was likely too soon to observe a positive effect of the December 2012 Fern Lake fire on tall

saplings (those 1.5–2.5-m height); however, short sapling (stems with height less than or equal to 1.5 m) 

density on some burned plots on the core winter range exceeded 35,000 stems/acre (table 5, fig. 9). In 

burned aspen stands, particularly those protected by fences, the large amount of regeneration in the first 

post-burn year may lead to increased small-diameter trees within the next sampling interval, but high 

levels of browsing in unfenced, burned areas could also prevent these saplings from being recruited into 

the stand. Overall, there has been progress toward the aspen vegetation goals between 2008 and 2013, 

but this progress is primarily in fenced areas on the winter range.

Upland

Objectives.—No net increase in winter upland herbaceous offtake across the winter range above baseline 

levels of 61 percent by the revised calculation method, with less than or equal to 30 percent of sites with 

offtake greater than 70 percent and less than or equal to 10 percent of sites with offtake greater than 85 

percent.

Herbaceous offtake in upland sites increased above the baseline level on the core winter range in 

2009, but dropped below baseline for all other years on both core and noncore winter range. A spike in 

2013 was likely in response to loss of large patches of both herbaceous and woody forage in Moraine 

Park following the 2012 fire and associated shift in habitat use to adjacent upland areas. 

After 2009, all years met the objectives for distribution of offtake across the landscape such that 

not more than 30 percent of the landscape was heavily grazed (greater than 70 percent offtake) and not 

more than 10 percent of the landscape was very intensively grazed (greater than 85 percent offtake). The 

stability of shrub cover on upland sites indicates no apparent major shifts from herbaceous communities 

toward shrub communities due to grazing. Reduction and redistribution of herbaceous offtake in line 

with EVMP objectives were achieved during recent years.

Willow

Objectives.—No net increase in annual willow offtake across the winter range above the baseline level 

of 35 percent. Progressive increase in mean willow height across the winter range above the baseline 

level of 0.9 m to at least 1.1 m. Progressive increase in willow cover across the winter range above the 

baseline level of 21 percent to at least 31 percent.

Winter willow offtake remained steady during the past 5 yrs, and although there were no 

substantial increases in offtake, there were also no consistent declines. On the core winter-range, willow 

offtake was below the baseline level of 35 percent only in 2013, and this decline was likely due to the 

loss of forage in Moraine Park due to the Fern Lake Fire and associated shift in habitat use out of 

Moraine Park in winter 2012–13. Offtake did not increase on the noncore winter range as a result of 

fencing large portions of willow habitat (fig. 13), but pressure on core winter-range habitats has not 

lessened overall either. It is possible that winter willow offtake has actually decreased during 2008–14, 

but that an increase in summer browsing has masked this change because the spring measurement 

method currently being used to monitor offtake only provides data on annual browse offtake and cannot 

distinguish season (winter as compared to summer) of browsing. A 2014 survey of summer browsing on 

the winter range indicated an average of 14.5 percent of twigs were browsed by late summer on the core 

winter range (Moraine Park, Horseshoe Park, and Beaver Meadows), whereas the noncore winter range 

was only very lightly browsed (about 3.7 percent of twigs browsed). These data indicate that summer 

browse levels are much greater than anticipated and may need to be addressed at some point in the 

future. It should be noted that percent offtake of current year growth would likely be lower when 

accounting for removal of biomass per twig, but summer browsing may have greater physiological 
37

effects on willows than winter browsing. Danell and others (1994) observed increased shoot size, leaf 

size, and leaf nutrient concentrations following winter browsing, but decreases in most of these 

variables following summer browsing of birch by moose. 

By all metrics, willow heights have stayed at or above baseline levels of 0.9 m. Although there 

has been a slight increase in average willow heights above baseline measures during these first 5 yrs in 

the core winter range, this increase was due to height increases within fenced habitat (table 8). No 

willow height increases were observed in unfenced core winter-range areas, but noncore winter-range 

willows had a slight height increase, which may be indicative that lower populations of elk in the park 

during the past 5 yrs have also contributed to observed height increases on the winter range as a whole. 

The increased willow heights on the noncore and fenced core winter range have resulted in a 29-percent 

increase in average willow height on the entire winter range to 1.1 ± 0.1 m (mean ± standard error); 

therefore, the stated objective of increasing willow heights on the winter range by 10 percent has been 

achieved in this 5-yr span. These heights (1.1 m), however, are still much shorter than those being 

observed on the taller, less browsed, noncore winter range (1.6–1.8 m [160–185 cm], table 8). Knowing 

that this objective was achievable in a 5-yr timeframe, the park may want to adapt their stated objectives 

to try to continue 10 percent height growth each year until conditions on the entire winter range are 

closer to those seen on the noncore winter range at baseline.

The loss of both fenced and unfenced willows to the Fern Lake Fire in Moraine Park in 2012 

may have masked height gains made prior to 2013 within these sites because burned willows were 

shorter in 2013 than unburned willows regardless of whether they were fenced (tables 8 and 9). Willows 

in the sites that burned were shorter at baseline than willows in sites that did not burn, however, so this 

effect is not completely attributable to the fire. Presuming that willows do recover in burned areas (or at 

least in fenced areas within the burn), it is likely that willow heights within fences will increase by the 

2018 sampling based on the increases seen in fenced areas that were not burned.

As with willow heights, willow cover showed a 75-percent increase within the fenced areas 

since baseline measurement (table 8). The burned sites, on average, had much lower cover (about 5 

percent cover) than unburned winter-range sites (roughly 30 percent cover, tables 8 and 9). Over the 

entire winter range, willow cover increased roughly 20 percent between 2008 and 2013 reaching an 

average of 24 ± 3 (± standard error) percent cover by 2013, which is considerable progress toward the 

objective of 31 percent willow cover.

Again, the loss of much willow to the Fern Lake Fire likely influenced the overall results for the 

core winter range and the recovery of willow in burned areas is still to be determined. Continued 

monitoring of the burned sites will be necessary to determine the longer-term impacts of the fire on 

these willow communities. It would be ideal to be able to compare willow in the three major valleys 

(Moraine Park, Horseshoe Park, and Beaver Meadows) of the core winter range, but the original 

sampling design did not provide for such comparisons and it would require a substantial effort and 

establishment of numerous additional sites to provide for such comparisons.

It should be noted that in some sites, where there is a potential for both willow and aspen, 

growth of one species may be limited by the other (fig. 18). In such cases, increases in one of the target 

species may not be observed due to increasing dominance of the other and should be evaluated on a case 

by case basis to determine if the sites are now better suited to be included in another sampling stratum.
38

Figure 18. Comparison of willow site in Beaver Meadows on the elk winter range of Rocky Mountain National 

Park, Colorado. Heavy aspen regeneration that has taken place in this site. A, prior to fencing (baseline 

measurement). B, after 4 yrs of protection from ungulate browsing (2013). 

Current State of Elk-Vegetation Management on Winter Range

Management actions that have been employed since 2008 in Rocky Mountain National Park

include temporary fencing of critical habitat to exclude elk and elk culling to maintain a target 

population size. After 5 yrs of these actions, there is progress toward the vegetation objectives of the 

EVMP, but this largely seems to be in response to fencing. Willow cover and heights have increased 

inside fences as has aspen regeneration. Many fences had been in place less than 5 yrs at the time of the 

2013 sampling and, therefore, it is anticipated that increases in willow height and cover and aspen 

sapling density will continue into the 2018 sampling. Herbaceous offtake has decreased in some years, 

but this response seems to be correlated to growing-season precipitation rather than lower elk numbers. 

The Fern Lake Fire, which burned much of Moraine Park in December 2012, added a dimension 

of complexity to the EVMP monitoring program but also has provided opportunities to examine the 

response of aspen and willow to burning. The use of prescribed fire as a potential management tool was 

included in the EVMP (National Park Service, 2007) and although the 2012 fire was neither prescribed 

nor intended as a habitat management tool, the presence of monitoring plots within burned and 

unburned areas will provide further insights into using prescribed fire as a tool in these vegetation types. 

Similar to other studies (Kay and Bartos, 2000; Kay, 2001; Smith and others, 2001), aspen 

recruitment increased on core winter range primarily within elk exclosure fences. The high degree of 

suckering in burned aspen stands may potentially lead to rapid recruitment of young aspen into the 

canopy within fences, but this result will likely not be seen outside the fences. Romme and others 

(1995) found high densities of sprouting aspen on burned sites for up to 2 yrs following fire in 

Yellowstone National Park, but by the end of the third year, sprout density in burned and unburned 

stands was similar; however, in areas where ungulate density was less than one animal per square 

kilometer, sucker densities were observed to remain high for several years following burning (Durham 

and Marlow, 2010). To determine aspen responses to both fire and herbivory, additional burned sites 

were added to the monitoring program in 2013. Annual monitoring of these sites for at least the next 5 
39

yrs will provide information on the short-term effects of fire on aspen regeneration and recruitment in 

the presence and absence of elk herbivory. Keeping these sites in the 5-yr rotation of site measurements 

for the duration of the EVMP will provide information on long-term effects of fire and ungulate 

herbivory.

Herbaceous offtake did not respond linearly to reductions in elk population size on the winter 

range. There may be several contributing factors to this lack of response. There was a strong relation 

between upland herbaceous offtake and growing season precipitation. Plant primary production is 

strongly influenced by the amount and distribution of precipitation (Sala and others, 1988). In wet 

meadows of Rocky Mountain National Park winter range, herbaceous plants rely more heavily on 

precipitation water than groundwater (Alstad and others, 1999), and this is likely true to an even greater 

degree in these dry upland sites where water tables are farther from the surface. Furthermore, increased 

precipitation can mediate the effects of grazing on aboveground net primary production (Augustine and 

McNaughton, 2006), whereas drought can exacerbate the effects of grazers on aboveground net primary 

production and plant cover (Fahnestock, 1998; Fahnestock and Detling, 1999; Zeigenfuss and others, 

2014). Consumption of plant biomass from a more productive plant will result in a lower percentage of 

biomass removed (percent offtake) than consumption of the same amount of biomass from a less 

productive plant. In this way, offtake (percent of biomass consumed) could actually decrease while 

utilization (mass of plant biomass consumed) stays the same. 

Also, the elk population estimate only accounts for elk that winter in the park. A larger 

population of elk spend time on the winter range in spring and fall as they migrate from higher-elevation 

summer range in the park to lower-elevation winter range in the Estes Valley and foothills at the edge of 

the Colorado plains (Ben Kraft, Wildlife Biologist, Colorado Parks and Wildlife, oral commun., August 

2014). These elk contribute to the offtake on the winter range and the timing and duration of their 

residence on the winter range varies based on weather conditions; therefore, if the number of elk 

wintering on the winter range decreased, but the number of elk using the winter range during spring and 

fall migration increased or the length of time spent on the winter range during migration increased, it 

could essentially cancel out the effects of a smaller wintering herd on overall offtake. Based on a visual 

assessment of upland areas in 2014, high variability in annual offtake, and lack of a strong correlation 

between offtake and elk population size, a recommendation was made by a committee of grazing and 

wildlife ecologists in September 2014 to discontinue collection of upland herbaceous offtake data. 

Increases in average willow height and cover on the winter range within fenced sites indicate 

that fencing has been successful in allowing willow growth; however, the loss of sites to the Fern Lake 

Fire makes it difficult to discern how much change in willow growth may have taken place within 

fences in Moraine Park. Few studies of fire effects on willow in the Rocky Mountains are available for 

comparison, probably because intense fires are less likely to burn the lush vegetation associated with 

riparian areas where large patches of willow are found. Norland and others (1996) determined that 

resprouting burned willows in Yellowstone National Park had higher protein levels and greater 

digestibility, longer and heavier shoots, and greater leaf surface area than unburned willows; however, 

their height did not increase, likely due to intense browsing on these highly nutritious plants. It is 

possible that resprouting willows in Rocky Mountain National Park may also be subject to high levels 

of herbivory due to increased nutritional value of burned plants. Current studies of Moraine Park willow 

have determined that 45 percent of studied individuals had resprouted following the fire, but that those 

burned plants subject to browsing had 64 percent less biomass than unbrowsed, burned willows 

(Kaczynski and Cooper, 2015).

At some point, recovery inside the fences will likely plateau as available resources (space, light, 

water) are used. The density of living willow is low in some areas, including fenced areas, so growth of 
40

existing willow may not be enough to meet goals for willow cover. Some of these areas also lack 

suitable sites for willow establishment due to competition with herbaceous species, so willow may need 

to be planted to reach goals. In addition, if willow outside exclosed areas do not recover, even with 

reduced numbers of wintering elk, it may also be necessary to fence more willow, which may be 

particularly true in Moraine Park where recovery of willow from the Fern Lake Fire is compounded by 

intense browsing of new growth in burned areas. Annual offtake sampling of burned willow sites will 

provide information on response of offtake rates to burning. Annual monitoring of willow height and 

cover on burned plots, both fenced and unfenced, for the next 5 yrs, coupled with the data from the 

Kaczynski and Cooper (2015) study, will provide information on the short-term effects of fire on these 

willow communities. Continuing to monitor these sites as part of the regular monitoring program 

rotation after 2018 will provide data on the long-term responses of willow communities to fire. 

Meaningful reduction of willow offtake seems to be the most difficult of the vegetation goals to 

achieve. As in upland sites, willow offtake is tied not only to overall number of elk on the winter range 

but their distribution and the duration of their residence on the winter range, which in turn can be driven 

by other factors such as temperature, annual and seasonal precipitation, snowfall, and forage 

productivity and availability. However, weather conditions may cause summer offtake to be substantial 

in some years (for example, late snows might keep elk on the winter range into the calving season and 

then for a post-parturition period extending into the summer, or drought may cause elk to abandon 

summer range earlier because of low forage availability). Alternatively, portions of the elk herd may 

alter their behavior over time, which could lead to decreased migration to the summer range, despite 

management actions intended to encourage seasonal migration. If more elk begin to summer on areas 

that have traditionally been winter range, park managers may feel the need to begin to evaluate summer 

willow offtake annually. Kaczynski and Cooper (2015) observed summer offtake on willows in Moraine 

Park in 2013, and data collected in summer 2014 indicated that elk are browsing in summer throughout 

the core winter range.

In conclusion, the elk-vegetation management program at Rocky Mountain National Park seems

to be making slow but steady progress toward the vegetation goals set out by the EVMP. At this first 

evaluation point, it seems highly likely that plan objectives can be met by the end of the EVMP 

timeframe in 2028, mainly through increases in plant density, height, and cover within the fences; 

however, it might require additional fencing of willow and aspen to achieve these objectives, as well as 

planting and protecting willow in some locations. The EVMP called for fencing of as much as 105 ha 

(260 acres) of willow and 64 ha (160 acres) of aspen on the winter range (National Park Service, 2007). 

At the time of this analysis (2014), roughly 26 percent of willow on the entire winter range was fenced, 

all of it on the core winter range. Sixty-five percent of the potential amount of willow area that could be 

fenced under the EVMP has already been fenced. Ten percent of all aspen on the winter range were 

fenced by 2014, representing 25 percent of the potential amount of aspen that might be fenced under the 

EVMP. A substantial amount of willow and aspen could still be fenced if progress toward vegetation 

objectives remains limited outside fenced areas. The dedication of the Rocky Mountain National Park

resources management staff to the collection of monitoring data has greatly helped to compensate for 

the difficulties presented by nature and provides the opportunity to potentially learn from these events.

Acknowledgments

We appreciate the dedication and assistance of Apryle Craig and Gary Miller in supervising 

protocol testing and refinement and data collection and collation. We also appreciate the hard work of 

the field crews that participated in data collection and processing during 2009–14: Ben Anderson, Eli 

Ertl, Jen Gebo, Jillian Gericke, Alix Jensen, Patrick McNulty, Harold Manrique, Cassandra Montoya, 
41

Sara Moore, Nick Morris, Josh Phillips, Dave Pinigis, Jim Westfall, and Craig Wilks. Ron Thomas 

provided geographic information system support. Dan Binkley, Paul Griffin, and Kristin Kaczynski 

provided insightful reviews of the manuscript. Brian Cade provided much needed statistical advice. 

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ISSN 2331-1258 (online)

http://dx.doi.org/10.3133/ofr20151216
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Appendix 

Table 1–1. Willow offtake using alternate stem-scaled diameter-difference (DD2) calculation method (Bilyeu and others, 2007).

Year

Percent offtake (mean ± standard error)

Core winter range Noncore winter range Entire winter range (weighted average)

2009 34.4 ± 2.9 18.0 ± 3.2 32.3 ± 2.2

2010 43.4 ± 4.2 22.2 ± 6.3 40.2 ± 3.6

2011 34.5 ± 7.7 20.9 ± 3.1 30.5 ± 4.4

2012 44.5 ± 4.8 19.1 ± 2.6 37.2 ± 3.6

2013 26.1 ± 6.6 11.6 ± 2.8 19.9 ± 3.7

2014 33.4 ± 3.6 19.4 ± 5.4 30.5 ± 3.0

Table 1–2. Least squares means of willow cover and heights based using line intercept method in Zeigenfuss and others (2011).

[cm, centimeter; s.e., standard error; yr, year]

Winter range zone

Willow height (cm) Maximum willow height (cm) Willow cover ( percent)

Baseline 

(mean ± 

s.e.)

2013

(mean ± 

s.e.)

P-value 

(yr)

Baseline 

(mean ± s.e.)

2013

(mean ± s.e.)

P-value 

(yr)

Baseline 

(mean ± s.e.)

2013 

(mean ± s.e.)

P-value 

(yr)

Core winter range,

unfenced 85 ± 18 89 ± 18 0.810 121 ± 25 116 ± 25 0.841 20 ± 4 22 ± 4 0.460

Core winter range,

fenced 47 ± 12 83 ± 12 <0.001 60 ± 14 98 ± 14 <0.001 17 ± 5 24 ± 5 0.213

Noncore winter 

range

143 ± 23 200 ± 23 0.001 185 ± 28 239 ± 28 0.002 29 ± 5 37 ± 5 0.008