Great Basin Naturalist 57(4), 1997, pp.315-326

 HOLOCENE VEGETATION AND HISTORIC GRAZING IMPACTS AT CAPITOL REEF NATIONAL PARK RECONSTRUCTED USING PACKRAT MIDDENS

 Kenneth L. Cole, Norman Henderson, and David S. Shafer

     ABSTRACT. - Mid- to Late Holocene vegetation change from a remote high desert site was reconstructed using plant macrofossils and pollen from nine packrat middens ranging from 0 to 5400 years in age. Presettlement middens consistently contained abundant macrofossils of plant species palatable to large herbivores that are now absent or reduced, such as winterfat (Ceratoides lanata) and ricegrass (Stipa hymenoides). Macrofossils and pollen of pinyon pine (Pinus edulis) , sagebrush (Artemisia spp.) and roundleaf buffaloberry (Sheperdia rotundifolia) were also recently reduced to their lowest levels for the 5400 year record. Conversely, species typical of overgrazed range, such as snakeweed (Gutterrezia sarothrae), viscid rabbitbrush (Chrysothamnus visidiflorus), and Russian thistle (Salsola sp.) were not recorded prior to the historic introduction of grazing animals. Pollen of Utah juniper (Juniperus osteosperma) also increased during the last 200 years. These records demonstrate that the most severe vegetation changes of the last 5400 years occurred during the last 200 years. The nature and timing of these changes suggest that they were primarily caused by nineteenth century open-land sheep and cattle ranching. The reduction of pinyon and sagebrush concurrent with the other grazing impacts suggest that the effects of cattle grazing at modern stocking levels may be a poor analog for the effects of intense sheep grazing during drought.

KEY WORDS: Holocene vegetation history, grazing impacts, packrat middens, fossil pollen, presettlement vegetation.

STUDY AREA

   The purpose of this study was to produce a Holocene vegetation history of Capitol Reef National Park, reconstructing past changes in vegetation and relating those changes to their most probable causes. Packrat midden chronologies were developed from several sites (Cole 1992), but only the most complete series from a single site, collected from the Hartnet Draw site, is reported here. Seven fossil and two modern middens were collected from Hartnet Draw in northern Capitol Reef National Park (38 15 N.; 111 20 W.; Figure 1). This site, at 1920 m elevation in Wayne County, Utah, was chosen because of its remote location, free from most anthropogenic disturbances other than grazing, and the abundant fossil packrat middens.

 Figure 1. Map showing site location.

     The site is underlain by the Salt Creek Member of the Morrison Formation which forms many overhangs protecting the fossil packrat middens. Today, the most abundant plant species are Utah juniper (Juniperus osteosperma), Bigelow sagebrush (Artemisia biglovii), big sagebrush (A. tridentata), snakeweed (Gutierrezia sarothrae), Torrey ephedra (Ephedra torreyana), viscid rabbitbrush (Chrysothamnus visidiflorus), and central pricklypear (Opuntia polyacantha) (Table 1). Low areas with thicker soil support a sparse growth of the grasses: ricegrass (Stipa hymenoides), sand dropseed (Sporobolus cryptandrus), and blue grama (Bouteloua gracilis). Plant taxonomy follows Welsh et al. (1987).

  Figure 2. Two images (255 k gif file) of the Hartnet Draw study site showing shelter where middens #’s 1 through 4 were collected.  Top image is reconstructed with presettlement vegetation as visualized from fossil data, bottom image shows the site in 1989.

     Mean annual precipitation at the site is close to the 18 cm yr^-1 recorded at Fruita, 15 km to the southwest at 1670 m elevation (Heil et al. 1993). The precipitation is bi-seasonal with peaks in winter and late summer. Temperatures extremes are great at this arid continental site. The mean January minimum temperature at Fruita is -8 C, and the mean July maximum is 33 C.

     GRAZING HISTORY -- There are no recorded historical disturbances to this area other than the introduction of exotic herbivores during the mid nineteenth century. Native large herbivores which may have been present in the study area during the last 5000 years include: bighorn sheep (Ovis canadensis), mule deer (Odocoileus hemionus), pronghorn (Antilocapra americana), and possibly bison (Bison bison) and elk (Cervus elaphus) (Van Gelder 1928; Mead et al. 1991). Native North American equids (horses and asses) became extinct before 10,000 yr B.P. (Mead and Meltzer 1984). Eurasian horses and asses (Equus spp.) were introduced to New Mexico by Spanish colonists as early as A.D. 1598 (Underhill 1971). By the late 1600's feral horses were reported in parts of the west, but it is not likely that they existed in the study area prior to the 1800's.

     In the late 1800's introduced herbivore populations increased dramatically in southern Utah with the widespread increase in open-land grazing. Livestock grazing within and near Capitol Reef National Park has been documented since at least the 1870's A.D.(Frye 1995). The earliest detailed herbivore population estimates from the Capitol Reef area are from summer grazing permits issued for Powell (now part of Dixie) National Forest (Frye 1995). In 1909 the Forest Service issued permits for 67,000 sheep and 11,000 cattle. The animals that grazed in these high summer pastures presumably spent the winter in the lower adjacent areas of Capitol Reef National Park.

     A Bureau of Land Management survey described past use at the Hartnet Draw site:
"Prior to the passage of the Taylor Grazing act in 1934, large numbers of livestock were brought from Wayne, Seiver, and Emery Counties to winter on these lands. Many of the animals remained on the range year-long, resulting in the progressive destruction of soils and vegetation. Reports from stockman in the area indicate that many trespass horses used the area until about 1955. Prior to 1946 there were at least 163 cattle and 20 horses yearlong in this area" (Hartnet Allotment File, 1966).

     Currently, the area is grazed under an allotment issued by Capitol Reef National Park

     RECONSTRUCTING PAST VEGETATION-- Fossil packrat middens are valuable sources of paleoecological information in arid regions of the southwestern United States (Cole 1990; Betancourt et al. 1990, Thompson and Anderson, 1997). Plant fossils in packrat middens are often identifiable to the species level and they grew close to the midden location, most likely within 50 m. Because plant identification and location can be precisely known, this method has extremely high spatial and taxonomic resolution compared to other methods of reconstructing past vegetation.

     Studies comparing trees and shrubs at midden sites with plant specimens from modern middens typically report similarities exceeding 80% using a Sorensen's Index of Similarity (Cole and Webb 1985; Cole 1985; Spaulding et al. 1990; Frase and Sera 1993) especially when small macrofossils (< 2 mm) are identified using a 10 X microscope. Similarity with forbs and grasses have been reported to be lower (Frase and Sera 1993), but inventories of current forbs are usually incomplete due to seasonal and yearly variability in the forb flora, and identification of diverse forbs and grasses within midden assemblages is very challenging.

     The quantitative correlation between species abundance and midden specimens is complex (Spaulding et al. 1990). Plant species producing abundant, readily identifiable plant parts (eg. Atriplex leaves), or packrat food items (eg. juniper), or plants having deterrence values in protecting packrats from predators (eg. cactus spines), tend to occur in the highest numbers within middens. But, perennial species which are abundant near a midden are most often represented by high numbers of plant specimens, while less abundant species, or those further from the middens, are represented by fewer specimens. As a result, interpreting changes from midden assemblages requires experience with the macrofossil types (leaves, twigs, flowers, seeds) and abundances typically found for that species. This element of judgement is present in any retrospective study. For example, fossil pollen magnifies the presence of wind pollinated plants while insect pollinated species may not be represented at all. Phytolith studies only detect those species producing identifiable opal phytoliths. Historical writings record only those species of interest to the writers. Repeat photography is useful only for those species identifiable in photographs. Midden records can be viewed as representing something similar to a plant relevé (Mueller-Dombois and Ellenberg 1974) from the past. This relevé contains a detailed species list, but also more complex information on past species abundance comparable to coverage classes.

     Fossil pollen within the middens can also be analyzed (King and Van Devender 1977; Thompson 1985; Davis and Anderson 1988) emphasizing different types of vegetation and representing a larger source area than the plant macrofossils. Interpretation of the fossil pollen abundances, like the macrofossil abundances, requires caution and experience, as some species are better represented than others. Through the consideration of both the macrofossil and pollen records, a more comprehensive understanding of past environments can be achieved.

     Seven of the eight middens were collected within a radius of about 200 m from a small shelter (Figure 2). The eighth was found 1 km east of the shelter. Approximately 1 kg of each midden was separated from larger masses using a hammer and chisel and returned to the laboratory. The samples were then dissected producing horizontally stratified sub-samples typically measuring about 15 x 20 cm with a thickness of several centimeters. Weathering rinds and large rocks were removed from each sub-sample, yielding 300 to 600 g of the hardened midden material. This sample was then weighed and disaggregated in water. Two unconsolidated middens (Hartnet Draw #'s 3 and 4) were considered modern because of the presence of green leafy material, cow feces, and a peanut shell.

     Pollen samples were taken from the wash water after several days of soaking and the pollen was separated using standard methods (Faegri and Iversen 1975). The macrofossils of one modern midden, Hartnet Draw # 4, could not be analyzed because the midden had been burned charring much of the plant debris. But, the pollen content of this midden was not destroyed by the fire. Packrat debris piles are often burned in rangeland shelters, most likely to eliminate the rats who occupy a pleasant shelter.

     Vegetable debris, fecal pellets, and rocks were sieved from the dissolved middens with a 1 mm sieve and the resulting matrix was dried and weighed, producing 100 to 200 g of washed midden matrix. The dried matrix was sorted by hand under a 10 X dissecting microscope. Packrat fecal pellets and rocks were removed and weighed. Identifiable plant macrofossils, vertebrate bones, and insect fossils were identified, counted, labeled, and stored in plastic vials.

     Six to 13 g of packrat fecal pellets were submitted to radiocarbon laboratories for dating. Hartnet Draw # 5 was dated at 3615 ± 70 yr B.P. using a single Pinus needle after the initial pellet sample yielded an impossible result of 142% modern carbon. Some type of sample contamination with artificial carbon isotopes or sample mislabeling is suspected as it is impossible to contaminate an old sample with enough modern natural carbon isotopes to yield such a high number. Calendar year ranges for the radiocarbon ages were calculated using the calibration program of Stuiver and Reimer (1993).

     Data on midden contents were quantified by number, weight, percent of identified specimens, and log₁₀ of macrofossil concentration in midden matrix. In order to compensate for variability between middens, midden matrix weights were adjusted by subtracting the weight of the rocks and pellets from the dried washed matrix weight before calculation of the concentration as suggested in Betancourt et al. (1990). Using log₁₀ of macrofossil concentration calculates a number similar to the semi-quantitative abundance scale used by several other authors, but has the advantage of being quantitative.

     We used the program CONISS (Grimm 1987) to conduct a stratigraphically constrained cluster analysis using a square root transformation and Edwards and Cavalli-Sforza's chord distance as a dissimilarity coefficient. The square root transformation makes the skewed distributions of abundant species more closely conform to normal distributions. Plant taxa occurring in only a single midden sample were deleted from the analysis to eliminate false positive correlations due to shared absences. These deletions cause any differences between the modern and fossil middens to be understated.

RESULTS

      MIDDEN AGE -- The nine middens ranged in age from modern to 5450 yr B.P. (Table 2). Hartnet Draw # 3 contained 137% modern carbon ("modern" is defined as A.D. 1950) and thus post-dates atmospheric testing of nuclear weapons. Seven middens dated to presettlement times.

    PLANT MACROFOSSILS -- All of the middens contain abundant macrofossils of Utah juniper (Juniperus osteosperma), saltbush (Atriplex spp.), cliff rose (Cowania mexicana), and prickly pear (Opuntia sp.), which are all abundant at the site today (Table 1; Figure 3). The presettlement middens also contain pinyon pine (Pinus edulis), winterfat (Ceratoides lanata), sagebrush (Artemisia sp.) and ricegrass (Stipa hymenoides) which are absent from the one modern midden analyzed for macrofossils (Hartnet # 3). Winterfat was not observed during the field work and pinyon pine was rare in the area. The rarity or absence of sagebrush and ricegrass from the modern midden suggests that they are less common now than prior to settlement. Similarly, globe mallow (Sphaeralcea sp.), needlegrass (Stipa sp.), blue gramma (Bouteloua gracilis), dropseed (Sporabolis cryptandrus), and roundleaf buffaloberry (Sheperdia rotundifolia) are common in the presettlement middens but absent from the one modern midden.

Figure 3. Plant macrofossils from Hartnet Draw packrat middens.

     In contrast, Hartnet # 3, the modern midden, is the only midden containing viscid rabbitbrush (Chrysothamnus visidiflorus), greasewood (Sarcobatus vermiculatus), and Russian thistle (Salsola sp.) macrofossils. Only the two most recent middens (#3 and #2) contain snakeweed (Gutterrezia sarothrae). Rabbitbrush, snakeweed, and Russian thistle are frequent at the site today. The absence of these species from the presettlement middens indicates that these species were formerly absent, or so infrequent as to not be represented.

     The cluster analysis (right side, Figure 3) demonstrates the difference between the modern macrofossil assemblage and the other assemblages. It is the primary branch in the dendrogram even though the single occurrences of rabbitbrush, greasewood, and Russian thistle in the modern midden were disregarded in the analysis.

     POLLEN SAMPLES -- The results of the pollen analysis (Figure 4) are similar to the those from the plant macrofossils. The presettlement middens contained much more pine and sagebrush pollen than the two modern samples (#3 and #4). Similarly, the percentages of grass and buffaloberry pollen were generally higher in the presettlement middens. In contrast, only the two modern middens contained pollen of the exotic Russian thistle and high amounts of juniper pollen. Like the macrofossil cluster analysis, the pollen cluster analysis (right side of Figure 4) showed that the modern samples are very different from all of the presettlement middens.

Figure 4. Fossil pollen from Hartnet Draw packrat middens. 

     Figure 2 shows the site as it is now and a reconstructed image of how the site may have looked prior to settlement. The reconstructed image shows the greater coverage of grasses, winterfat, sagebrush, and pinyon inferred from the macrofossils and pollen found in the packrat middens. Although the vegetation probably fluctuated continuously throughout the late Holocene, this midden record suggests that previous changes were minor compared to the changes of the last two hundred years. Sites of similar appearance to the reconstructed image are now present on ungrazed terraces which are inaccessible to large herbivores along Halls Creek, 90 km to the south (Heil et al., 1993). The presettlement plant community was likely more like the Pinyon-Juniper-Grass Community described by Heil et al. (1993) than the juniper-shrub community present at the site today.

     It is clear from the results of macrofossil and pollen analyses, reinforced by the two cluster analyses (Figures 3 and 4), that the modern plant contents are dramatically different from those of the presettlement middens. Furthermore, the presettlement middens are more similar to each other than to either of the modern middens. This suggests that the change in vegetation during the last 200 years was far greater in magnitude than changes during the previous 5000 years. Hypothetical causes of this vegetation change should account for both the timing of the change and the specific taxa which increased or decreased.

     DROUGHT HISTORY-- The reductions in winterfat, pinyon pine, sagebrush, and ricegrass, and increases in juniper, rabbitbrush, and snakeweed might be attributed to droughts during the nineteenth or twentieth centuries. But, an analysis of past drought frequency for southeastern Utah (Figure 5) using four hundred years of tree-ring data compiled by Fritts (1991) suggests that the droughts of the nineteenth century were not unusually severe when compared to the seventeenth century. Severe droughts, defined here as years with less than 165 mm of annual precipitation reconstructed for Moab, Utah, occurred nine times in the seventeenth century, four times in the eighteenth century, and seven times in the nineteenth century. The five driest years reconstructed from the tree-ring record were: A.D. 1667 (132 mm), 1684 (142 mm), 1668 (143 mm), 1879 (147 mm ), and 1861 (150 mm). Exceptionally dry successive years occurred between 1624–1626, 1666–1670, 1684–1685, 1728–1729, 1822–1823, 1879–1880, and 1899–1900.

Figure 5. Reconstruction of the last 400 years of precipitation at Moab Utah from tree-ring data generated by a program distributed with by Fritts (1991).

     Less is known about the climatic variability in this region over the previous 5000 years (prior to this tree-ring record), but it seems unlikely that any climatic event of the last 200 years was sufficient to cause a change with no precedent during the previous 5000. Drought may have precipitated some of the dramatic vegetation changes of the last 200 years, but did not set the stage for them. This would require an event unprecedented during the previous 5000 years.

     FIRE HISTORY -- Some of the changes recorded in the middens could have been caused by changes in fire regime. The increase in juniper could result from a decrease in fire frequency caused by the elimination of the grassy fuels by grazing. But this does little to explain the shift from palatable to non-palatable species or the reductions of pinyon, sagebrush, and buffaloberry just at the time that fire frequency would decrease. Fires of unprecedented severity could have been set during the settlement era, but this hypothesis is without any support in the data. Studies of tree fire scars or sedimentary charcoal would have to be conducted to test this possibility.

     GRAZING IMPACTS -- Impacts from introduced herbivores, especially large sheep herds in the late nineteenth and early twentieth centuries, are the most likely cause of the recent radical vegetation changes. The introduction of sheep, goats, cattle, and horses was without precedent during the previous 5000 years. Overall, the vegetation has shifted from palatable forage toward less palatable forage. Specifically, palatable grasses, winterfat, and buffaloberry decreased, while the less palatable species, rabbitbrush, snakeweed, and greasewood increased. Rabbitbrush and greasewood are poor forage, while snakeweed is typically an invader or increaser on overgrazed range (Benson and Darrow 1981; Heil et al. 1993; Cronquist et al. 1994).

     Other studies conducted on grazing at Capitol Reef support this conclusion. Heil et al. (1993), in a survey of the vegetation of Capitol Reef National Park, suggest that: "Some of the most preferred plant species (for grazers), e.g. Ceratoides lanata and Stipa comata, may have been locally extirpated by grazing". This packrat midden record demonstrates that for the Hartnet Draw site, this was the case.

     Additional research at Capitol Reef National Park indicates that grazing has caused, and may still be causing, changes to the natural habitat. An analysis of plant phytoliths in buried soil horizons shows a reduction of palatable grass species over the last several hundred years (Fisher et al. 1995). An analysis of riparian areas indicated that dramatic changes had occurred prior to the Taylor Grazing Act of 1934. Forage plants were heavily used, and in many instances cover was entirely removed. Recent grazing has perpetuated this removal or reduction of species and inhibited potential recovery (Barth and McCullough 1988). In a lightly grazed area palatable shrubs and grasses have increased significantly.

     Dramatic declines in pinyon, sagebrush, and buffaloberry may also have been caused by the grazing history, but the effects on these species are less well understood. These declines are also present in fourteen additional middens from sites elsewhere in Capitol Reef National Park such as in along Hall’s Creek (Murray 1989; Cole 1992).

     Pinyon-juniper woodlands have been reported to have increased during the historic period, especially when comparative photographic techniques are used (West et al. 1975; Tausch et al. 1981). This increase in pinyon-juniper woodlands is thought to be caused by reduced competition from grasses and forbs which were eliminated by grazing and by consequent reductions in fire frequency. But, studies discriminating between pinyon and juniper do not portray identical histories for both species. Pine and sagebrush both decline while juniper dramatically increases during the settlement period at Peck’s Lake, Arizona (Davis, 1987). A study of permanent plots in a presently ungrazed part of Pine Valley, Utah, demonstrated a significant decrease in juniper and a significant increase in pinyon between 1933 and 1989 (Yorks, et al. 1994). A study of tree age structure on a presently ungrazed site in the Needle Range in southwestern Utah found that during the nineteenth century many surviving juniper and few pinyon were established. By 1915 the situation had reversed with far more surviving pinyon becoming established in this century (Tausch and West 1988). These results demonstrate that pinyon and juniper respond differently to changing regimes of grazing, fire, or climate. The observation that heavy grazing causes an expansion of pinyon-juniper woodland (West et al. 1975) does not equate to the expansion of both species in all habitats.

     These results suggest that pinyon may be recovering now at some sites from a late nineteenth century/early twentieth century decline caused by grazing impacts. This recovery has not yet occurred at Hartnet Draw. This hypothesis has support despite the lack of recent observations of pine removal by grazing cattle. Knowledge of the effects of cattle grazing at present stocking levels forms an inadequate basis for judging the effects of an overstocked sheep range during the droughts of the late nineteenth century. Although cattle will consume some pine when it is available (Pfister and Adams 1993), sheep readily consume pine needles and strip pine bark even in the absence of drought conditions (Anderson et al. 1985). Sheep accomplished the near complete elimination of the Bishop pine forest ( Pinus muricata) on Santa Cruz Island, California, where they were not fenced out (Hobbs, 1980).

     Sagebrush populations may have a similar history despite observation of increases in sagebrush caused by the removal of their grass competitors (Young et al. 1978). Although sagebrush may be increasing on land presently grazed by cattle, this is not an appropriate analog for intense nineteenth century sheep grazing. Sagebrush is consumed by sheep during droughts. During the late nineteenth century, sheep severely reduced the populations of California sagebrush (Artemisia californica) on Santa Rosa Island, California, after first consuming the grass (Cole and Liu 1994).

     MAGNITUDE OF CHANGE - The recent vegetation changes recorded at Capitol Reef National Park are unique when compared to natural changes of the last 5000 years. These results echo those of Davis et al. (1977) , who found the vegetation change caused by domestic livestock reflected in fossil pollen at Wildcat Lake, Washington, to be the greater than any other event of the last 1000 years.

     It is also possible that both climate and grazing combined to produce the dramatic vegetation shifts of the last two hundred years. There is little doubt that the most severe grazing damage occurs when high populations of herbivores compete for food during a severe drought. The droughts of 1879-1880 and 1899-1900 probably exacerbated the damage caused by the high herbivore populations. More severe earlier droughts, such as those during the seventeenth century, did not cause such changes because the large introduced herbivores were absent.

ACKNOWLEDGEMENTS

     We were assisted in the field by Penny Hoge, Dan Huff, Rick Harris, Bill Romme, and John Spence. Debra Maddox, Debra Daugherty, and Betsy Jernigan assisted with the sorting and counting of plant macrofossils. Lyn Murray completed the analysis of some pollen samples. Robyn Flakne assisted in copy editing and suggestions on the manuscript were contributed by Walter Loope and an anonymous reviewer. This project was funded by the National Park Service and the National Biological Service.

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