Uncompahgre Plateau Research Articles

Red-green-bleached redox interfaces in the proximal Permian Cutler red beds: implications for regional fluid alteration

Siliciclastic strata of the Colorado Plateau attract attention for their striking red, green, bleached, and variegated colors that potentially record both early depositional and later diagenetic events. We investigated the proximal-most strata of the Paradox Basin, from their onlap contact with the Precambrian basement of the Uncompahgre Plateau to the younger Cutler strata exposed within 10 km of the Uncompahgre Plateau to attempt to understand the significance of the striking colors that occur here. These strata preserve a complex geology associated with buried paleorelief and sediment-related permeability variations at a major basin-uplift interface. Strata exposed within ∼1.5 km of the onlap contact exhibit a pervasive drab color in contrast to the generally red colors that predominate farther from this front. In-between, strata commonly host variegated red/green/bleached intercalations. Thin-section petrography, SEM, XRD, Raman spectroscopy, Mössbauer spectroscopy, and whole-rock geochemistry of samples representing different color variations from demonstrate that water–rock interactions charged the rocks with Fe(II) that persists primarily in the phyllosilicate fraction. Color variations reflect grain-size differences that allowed the reduction of fluids from regional fault and basement/fill contacts to permeate coarser-grained Cutler sediments. Hematite and chlorite occur in both red and green sediments but are absent in the bleached sediments. Pervasive hematite in both red and green layers suggests that sediments were hematite-rich before later alteration. Chlorite and smectite are elevated in green samples and inversely correlated with biotite content. Green coloration is generally associated with 1) coarser grain sizes, 2) spatial association with basement contacts, 3) elevated smectite and/or chlorite, 4) less total Fe but greater Fe(II)/Fe(III) primarily in the phyllosilicate fraction, and 5) uranium enrichment. The bleached coloration reflects the removal of pigmentary Fe(III) oxide, while the green coloration is due to the removal of pigmentary hematite and the abundance of Fe(II)-bearing phyllosilicates. Abundant mixed-layer and swelling clays such as smectite, illite/smectite, and chlorite/smectite (including tosudite) dominate the mineralogy of the clay fraction. These results are consistent with other studies demonstrating fault-associated fluid alteration in the Paradox Basin region. However, the pervasive greening was not observed in many of these studies and appears to reflect the unique aspects of the paleovalley system and the importance of biotite alteration to Fe(II)-bearing phyllosilicates.

Seismic Reflection and Electrical Resistivity Imaging Support Pre‐Quaternary Glaciation in the Rocky Mountains (Unaweep Canyon, Colorado)

AbstractUnaweep Canyon (Uncompahgre Plateau, Colorado) represents an enigmatic landscape with a complex evolution. Interpretations for its origin have ranged from ancestral fluvial erosion in the late Cenozoic to glacial erosion in the Paleozoic, or some combination thereof, with significant implications for global climatic and large‐scale tectonic reconstructions. To address the conflicting interpretations, we acquired a high‐resolution seismic reflection profile to investigate the depth, structure, and sedimentary infill in the canyon. The data set is further complemented with an electrical resistivity survey. Integrated with other geophysical and geological data, the results show an overdeepened Precambrian basement with transverse U shape and support the hypothesis of a pre‐Quaternary glacial origin. Our data constitute the first detailed image of a buried pre‐Quaternary glacial valley in North America; if substantiated with core studies, these results have far‐reaching implications for our understanding of global ice houses as well as the tectonic conditions, enabling preservation of such systems.

Effects of Hunting on a Puma Population in Colorado

Effects of Hunting on a Puma Population in Colorado

Early Jurassic Batrachopus-rich track assemblages from interdune deposits in the Wingate Sandstone, Dolores Valley, Colorado, USA

Late Triassic and Early Jurassic tetrapod track assemblages are abundant and quite well documented from the respective Chinle and Glen Canyon groups of the Dolores Valley region of western Colorado and adjacent regions, especially eastern Utah. However, gaps still exist in our knowledge of the spatial and temporal distribution of representative ichnogenera. The stratigraphic distribution of track assemblages in the cliff-forming Wingate Sandstone Formation, the lower unit of the Glen Canyon Group, underlying the Kayenta and Navajo formations, is difficult to determine because almost all Wingate assemblages are known only from fallen blocks, originating from precipitous, vertical cliffs, which yield both dune and interdune ichnofaunas. Among the characteristic Lower Jurassic biochron ichnogenera Grallator, Eubrontes, Otozoum, Anomoepus and Batrachopus, in the Dolores Valley only Grallator is known abundantly, while Otozoum, and Batrachopus were previously known only from single occurrences, and Eubrontes and Anomoepus have yet to be reported. We here report two large, Batrachopus-rich assemblages that give new insight into the interdune paleoenvironments frequented by the Batrachopus trackmaker, traditionally regarded as a basal crocdylomorph, and the theropodan trackmaker of Grallator. Both assemblages contain the best-preserved and most abundant Batrachopus currently known, locally in high densities. Where individual trackways are clearly visible they indicate that the trackmakers had variable gaits. A third new assemblage containing Batrachopus from Salt Creek is also described, as is a fourth new assemblage from the Uncompahgre Plateau which yields moderately large theropod tracks (cf. Eubrontes), but no Batrachopus. Although diagnostic Late Triassic biochron ichnogenera like Brachychirotherium occur in basal units of the Wingate Sandstone, Batrachopus which is considered an Early Jurassic biochron ichnotaxon, occurs within the formation. Thus, vertebrate ichnotaxa help place the Triassic–Jurassic boundary within the Wingate Formation, shed light on their spatio-temporal distribution, and underscore their utility for paleoecological interpretation, in a unit devoid of tetrapod body fossils.

Responses of Small Mammal Communities to Pinyon-Juniper Habitat Treatments

Small mammals should be considered in wildlife management decisions because they are an important component of ecosystems. We examined small mammal population abundances (N) on 4 habitat treatment types in Colorado pinyon pine (Pinus edulis)—Utah juniper (Juniperus osteosperma) woodlands on the Uncompahgre Plateau, Montrose, Colorado. We trapped small mammals in Sherman live traps on 4 habitat treatment types—including 2 types of management treatments (chaining, roller chop), sites with high drought-related mortality of pinyon pine, and sites of mature pinyon-juniper (PJ) woodland—on 3 mesas. We modeled detection probability of deer mice (Peromyscus maniculatus) and least chipmunks (Tamias minimus) as a function of 4 treatment types on 3 mesas during 2 trapping periods by using Huggins closed mark-recapture models. A variance components analysis was conducted to obtain abundance shrinkage estimates and to separate spatial process variance from the sampling variance to calculate the percent of variation in abundance estimates explained by treatment type, mesa, and trapping period. Abundance estimates were higher for deer mice and least chipmunks on chaining and roller chop treatment sites than on mature PJ woodland and pinyon pine natural mortality sites. The highest percent of variation in abundance estimates was explained by treatment type for deer mice (33.67%) and least chipmunks (61.45%) and secondarily by mesa for deer mice (21.64%). We calculated species diversity using Shannon's, Simpson's, Pielou's evenness, and species richness indices, and we used analysis of variance to test for significant differences (P < 0.05) in species composition. Species diversity and richness were higher on natural mortality sites than on chaining, roller chop, and mature woodland sites. Our results indicate that chaining and roller chop habitat treatments within PJ woodlands have a positive impact on deer mouse and least chipmunk abundances, whereas natural mortality and chaining treatments have a positive impact on small mammal composition.

Examining the Influence of Positionality in Evaluating Collaborative Progress in Natural Resource Management: Reflections of an Academic and a Practitioner

ABSTRACTIn this paper, we present a reflexive examination of how and why we, an academic and a practitioner, arrive at different evaluations of collaborative progress in natural resource management. We situate this examination in our long-standing involvement in designing, adaptively managing, and participating in the Uncompahgre Plateau collaborative forest restoration project in western Colorado, USA. Drawing on the concept of “positionality” in qualitative social science research, we disclose our respective motivations, assumptions, roles, and power relative to the collaborative process. The differences in evaluating collaborative progress stem from our respective professional positionality. For the academic, the guiding interest was to test theory and promote success for his applied research institute; for the practitioner, the motivation was to build trust to allow her field staff the flexibility to implement management actions and demonstrate effectiveness as an agency line officer. These epistemological differences draw attention to the importance of transdisciplinary approaches to producing knowledge from shared practice, starting with efforts to explicitly disclose and honor differing interests, assumptions, and frames of reference stemming from each party’s personal and professional biographies and institutional norms. This reflexivity is essential to advancing knowledge about collaboration in natural resource management.

Thermal evolution and exhumation history of the Uncompahgre Plateau (northeastern Colorado Plateau), based on apatite fission track and (U-Th)-He thermochronology and zircon U-Pb dating

Over the past two decades, thermochronological studies have greatly increased our knowledge of the Cenozoic evolution of the Colorado Plateau (western United States). There has been particular interest in the southwestern part of the plateau, leading to debate regarding the timing of uplift and fluvial incision along the Colorado River system. We here combine apatite fission track (AFT) and apatite (U-Th)-He (AHe) analyses as well as zircon U-Pb dating to investigate the much less studied northeastern Colorado Plateau, particularly the Uncompahgre Plateau and the Unaweep Canyon, which has a very unusual drainage pattern in two opposite directions. We obtained 12 AFT ages from the Uncompahgre Plateau: 3 from the top of the basement of the plateau reveal Laramide ages (65–63 Ma), and 6 samples from the Unaweep Canyon (35–27 Ma) and 3 from the northeastern plateau margin (33–17 Ma) underwent complete thermal resetting in the late Eocene to Oligocene. Thermal history modeling of top basement samples reveals Late Cretaceous heating to temperatures of at least 90 °C, implying sedimentary burial to ∼3 km, followed by cooling throughout the latest Cretaceous to Eocene. However, AHe ages (38–31 Ma) indicate minor reheating to 40–80 °C for these samples in the late Eocene to Oligocene. Zircons from the La Sal Mountains laccolith gave an Oligocene U-Pb crystallization age of 29.1 ± 0.3 Ma. AFT ages from the laccolith range from 33 to 27 Ma, confirming rapid cooling of this shallow subvolcanic intrusion. This late Eocene to Oligocene magmatism caused thermal resetting of most of the AFT (except top basement) and AHe ages from the Uncompahgre Plateau, even though samples were collected as much as 60 km away from the intrusion. Canyon samples also underwent an increase in cooling rates in the past 5–10 m.y. This Miocene–Pliocene cooling event is interpreted as regional uplift of the Colorado Plateau associated with canyon incision.

Geology of Unaweep Canyon and its role in the drainage evolution of the northern Colorado Plateau

Unaweep Canyon (Colorado, USA) is a large, Precambrian-cored gorge that bisects the Uncompahgre Plateau of the northeastern Colorado Plateau, but has no through-flowing axial stream; it is drained by two underfit creeks (East and West Creek) that head at a divide within the canyon. The history of the canyon and its role in drainage evolution of the Colorado River system remain controversial. New mapping of both bedrock and Quaternary units as well as analyses of Quaternary deposits in and near the canyon shed light on its late Cenozoic history, and call into question whether the canyon was incised by a Cenozoic river, or merely exhumed by one. Gravels near the western mouth of Unaweep Canyon (Gateway, Colorado) exhibit a distinctive intermediate volcanic provenance recording the presence of an ancestral Gunnison River; the youngest gravels are dated to 1.46 ± 0.33 Ma. Previously documented coring within the canyon reveals a thick (locally >330 m) fill that includes a lacustrine succession (∼140 m thick), dated to 1.4–1.3 Ma, overlain by stacked paleosols and a thick (∼160 m) conglomeratic unit emplaced between 1.3 Ma and the present, in addition to a basal unit of possible late Paleozoic age. Lake formation reflects catastrophic mass wasting in western Unaweep Canyon that blocked the ancestral Gunnison River, causing partial backfilling of the canyon, and forcing the river to seek a lower elevation exit eastward by breaching the Mesozoic rim at the northeast end of Cactus Park (Mesa County, Colorado). Ultimately, the ancestral Gunnison River joined the lower elevation Colorado River near Grand Junction by 1.3 Ma, incising the East Creek of Unaweep Canyon during the overspilling event. Thermochronological data from Precambrian basement within Unaweep Canyon and Permian strata at the western mouth of the canyon indicate onset of incision in latest Miocene time (ca. 6–5 Ma), at a time-averaged rate of ∼210–275 m/m.y. Onset of canyon occupation and rapid incision by the ancestral Gunnison River coincided with the timing of integration of the lower Colorado River system to the Gulf of California. The synchroneity of this incision across the Colorado Plateau supports the inference of an ultimate tectonic or epeirorogenic driver for this widespread incision and ultimate drainage integration. Several aspects of this data set support the previously published hypothesis that the ancestral Gunnison River exhumed a paleovalley. New mapping at the western mouth of the canyon documents a paleovalley filled with Permian strata that leads into the modern Precambrian-hosted gorge of Unaweep Canyon. In addition, the ancestral Gunnison River paralleled the Uncompahgre Plateau before making a 90° turn to bisect the structural axis in a manner that opposes both the northwestward plunge of the uplift and the northeastward dip of its northern flank. The rate of incision of Unaweep Canyon exceeds regional time-averaged incision rates, consistent with removal of sedimentary fill rather than incision of crystalline basement. This hypothesis implies that very ancient landforms can influence drainage evolution in even tectonically active landscapes.

Packrat middens and the Holocene palaeohistory of Colorado piñon (Pinus edulis) in western Colorado

AbstractAimOur aim was to determine the age and dispersal history of Colorado piñon (Pinus edulis) in western Colorado during the early to late Holocene using radiocarbon‐dated needles and nutshells from packrat (Neotoma spp.) middens.LocationThe Uncompahgre Plateau (UP) comprises more than 600,000 ha of public and private lands in west‐central Colorado, USA. Elevations within UP range from 1400 to 3140 m and it is characterized by numerous deep canyons and flat‐topped mesas. The Upper Gunnison Basin (UGB) encompasses a 243,000‐ha area enclosed within the southern Rocky Mountains in south‐western Colorado, USA. It spans 2200–4300 m in elevation, with no outlet apart from the Black Canyon of the Gunnison lower than 2650 m.MethodsMiddens were sampled by breaking off small sections of solidified deposits located in caves and crevices that were then disaggregated in water with repeated rinsing to separate all the plant remains. Single conifer needles and nutshells from 28 middens were radiocarbon dated to determine the age and timing of dispersal into western Colorado.ResultsThirty‐two midden samples were collected, of which 28 produced identifiable plant remains that were radiocarbon dated. Piñon needles and/or nutshells were identified and radiocarbon dated from 20 of these middens from seven localities in UP. The results indicated that piñon did not become established in UP until c. 6000 yr bp, which contrasts with the nearby UGB, where piñon charcoal has been identified from seven archaeological sites with dates ranging from 8000 to 3000 yr bp.Main conclusionsOur data are consistent with a model of late Pleistocene/early Holocene piñon migration into the Colorado Plateau from Arizona and New Mexico before becoming established in western Colorado. The timing of the piñon migration into south‐central Colorado along the Rio Grande corridor from New Mexico remains uncertain. While piñon–juniper woodlands currently dominate the lower elevations of UP, piñon had disappeared from UGB by c. 3000 yr bp and remains absent there today except for a few isolated trees.

Age structure of aspen forests on the Uncompahgre Plateau, Colorado

Aspen forests are one of the most dynamic forest types in western North America, responding to chronic factors of competition for resources, as well as episodes of intense herbivory, drought, and fires. The interactions of these driving factors lead to varying age structures of aspen across landscapes and through time. We characterized the age structure of aspen trees on the Uncompahgre Plateau in western Colorado, USA, to inform collaborative efforts of landscape-scale forest restoration. Over 1000 cores from 51 locations showed few aspen older than 140 years (&lt;0.5% for aspen numbers, &lt;2.5% of aspen basal area). Heavy recruitment in the late 1800s (following the last major fires) led to cohorts from 100 to 140 years of age that account for 15% of current aspen numbers and 40% of current aspen basal area. Perhaps the most important character of the current age structure is a relatively low number of aspen younger than 50 years; normal rates of tree survivorship in coming decades will lead to a substantial decline in aspen on the Plateau as these cohorts progress into older age classes. Patterns of aspen ages on the Uncompahgre Plateau differ substantially from those on the Kaibab Plateau and in Rocky Mountain National Park, owing the varying importance in space and time of driving factors. Landscape-scale increases in aspen regeneration (from major events such as fire) would be necessary to moderate the long-term decline in aspen on the Uncompahgre Plateau.

Aftermath of a stream capture: Cactus Park lake spillover and the origin of East Creek, Uncompahgre Plateau, western Colorado

Newly discovered lacustrine strata suggest that the most significant episode of stream capture in the upper Colorado River system (western USA), namely the abandonment of Unaweep Canyon, probably involved a combination of headward erosion and lake spillover. The abandonment of Unaweep Canyon occurred in two stages. The first stage was marked by the capture of the Colorado River, after which time the Gunnison River continued to incise. Continued incision by the Gunnison River created a wall of rock on the east side of the Gunnison River valley in Cactus Park and left the abandoned Colorado River bed well above the Gunnison River. The second stage involved two blockages, one created by the thick fill within Unaweep Canyon and one at the south end of Cactus Park, which led to the creation of a lake within Cactus Park. The lake level rose until it flooded the abandoned Colorado River bed and spilled over at the point where the Colorado River had been captured earlier, during the first stage of abandonment. Present-day East Creek was created by re-incision of the abandoned Colorado River course, which explains why the course of East Creek has a northeastward trend that is completely anomalous with respect to all other tributaries draining this area of the Uncompahgre Plateau. The rapid incision created a large quantity of debris that deflected the Gunnison River eastward at the mouth of East Creek. The evidence suggests that the abandonment of Cactus Park and Unaweep Canyon by the Gunnison River and the creation and destruction of Cactus Park lake all likely occurred ca. 800 ka and shortly thereafter. The Unaweep Canyon classic example of stream piracy illustrates how piracy alone can dramatically influence landscape development even in the absence of significant tectonic and climatic influences.

Fire History, Woodland Structure, and Mortality in a Piñon-Juniper Woodland in the Colorado National Monument

The Colorado National Monument (COLM), on the northeastern edge of the Uncompahgre Plateau, supports a persistent Piñon (Pinus edulis Engelm.) — juniper (Juniperus osteosperma (Torr.) Little) woodland, which has not been disturbed by large stand-replacing fires since modern fire records began. We examined the fire history of large (> 100 ha) stand-replacing fires, documented tree population structures, and characterized tree density, quadratic mean diameter (QMD), relative composition, and cumulative mortality using 431 -0.1-haplots distributed over 1600 ha of the Monument. We found no evidence of large stand-replacing fires (charred wood or truncated stand structures) in the study area. Stand ages inferred from size structures suggest that large stand-replacing fires have been absent for possibly a millennia. Tree population structures show a more stable stand structure for juniper; Piñon pine population structures show a more recent and sustained regeneration pulse. Cumulative mortality of Piñon pines was 18%, peaking at 47% in trees 20 – 24.5 cm diameter. Spatial patterns of juniper density, QMD, and mortality were more homogeneous than those of Piñon pine. Results suggest temporal dynamics and spatial patterns of the COLM woodland are more influenced by drought and small fires (< 10 ha) than large fires (> 100 ha). This study provides important baseline data for changes that may be brought about by climate change in coming decades. It also stresses the importance of controlling cheatgrass (Bromus tectorum) and other invasive species to increase resistance of these persistent Piñon-juniper woodlands to future fires.

Estimating above ground tree biomass for the Uncompahgre Plateau in Western Colorado using NAIP imagery and a series of textural and probabilistic metrics

Estimating above ground tree biomass for the Uncompahgre Plateau in Western Colorado using NAIP imagery and a series of textural and probabilistic metrics

Effect of Enhanced Nutrition on Mule Deer Population Rate of Change

Abstract Concerns over declining mule deer (Odocoileus hemionus) populations during the 1990s prompted research efforts to identify and understand key limiting factors of deer. Similar to past deer declines, a top priority of state wildlife agencies was to evaluate the relative importance of habitat and predation. We therefore evaluated the effect of enhanced nutrition of deer during winter and spring on fecundity and survival rates using a life table response experiment involving free-ranging mule deer on the Uncompahgre Plateau in southwest Colorado, USA. The treatment represented an instantaneous increase in nutritional carrying capacity of a pinyon (Pinus edulis)−Utah juniper (Juniperus osteosperma) winter range and was intended to simulate optimum habitat quality. Prior studies on the Uncompahgre Plateau indicated predation and disease were the most common proximate causes of deer mortality. By manipulating nutrition and leaving natural predation unaltered, we determined whether habitat quality was u...

Anomalous cold in the Pangaean tropics: COMMENT

[Soreghan et al. (2008)][1] present the hypothesis that glaciation existed to near sea level in the tropical region of Pangaea in late Paleozoic time. Their most important evidence for this comes from Unaweep Canyon, a wind gap that crosses the Uncompahgre Plateau, (Colorado, United States). The

An Exhumed Late Paleozoic Canyon in the Rocky Mountains: A Discussion

Previous articleNext article No AccessAn Exhumed Late Paleozoic Canyon in the Rocky Mountains: A DiscussionWilliam C. HoodWilliam C. Hood515 Dove Court, Grand Junction, Colorado 81507, U.S.A.(e-mail: [email protected]) Search for more articles by this author 515 Dove Court, Grand Junction, Colorado 81507, U.S.A.(e-mail: [email protected])PDFPDF PLUSFull Text Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Journal of Geology Volume 117, Number 2March 2009 Article DOIhttps://doi.org/10.1086/595789 Views: 34Total views on this site Citations: 7Citations are reported from Crossref HistoryReceived 15 August 2008Accepted 20 October 2008 © 2009 by The University of Chicago. All rights reserved.PDF download Crossref reports the following articles citing this article:Anna Patterson, Michael Behm, Werner Chwatal, Adrian Flores‐Orozco, Yichuan Wang, Gerilyn S. Soreghan Seismic Reflection and Electrical Resistivity Imaging Support Pre‐Quaternary Glaciation in the Rocky Mountains (Unaweep Canyon, Colorado), Geophysical Research Letters 48, no.1818 (Sep 2021).https://doi.org/10.1029/2021GL094706Christian Rønnevik, Anna K. Ksienzyk, Haakon Fossen, Joachim Jacobs Thermal evolution and exhumation history of the Uncompahgre Plateau (northeastern Colorado Plateau), based on apatite fission track and (U-Th)-He thermochronology and zircon U-Pb dating, Geosphere 13, no.22 (Mar 2017): 518–537.https://doi.org/10.1130/GES01415.1Gerilyn S. Soreghan, Dustin E. Sweet, Stuart N. Thomson, Sara A. Kaplan, Kristen R. Marra, Greg Balco, Thaddeus M. Eccles Geology of Unaweep Canyon and its role in the drainage evolution of the northern Colorado Plateau, Geosphere 11, no.22 (Apr 2015): 320–341.https://doi.org/10.1130/GES01112.1William C. Hood, Andres Aslan, Charles Betton Aftermath of a stream capture: Cactus Park lake spillover and the origin of East Creek, Uncompahgre Plateau, western Colorado, Geosphere 10, no.33 (May 2014): 447–461.https://doi.org/10.1130/GES00970.1William C. Hood, Rex D. Cole, Andres Aslan Anomalous cold in the Pangaean tropics: COMMENT, Geology 37, no.66 (Jun 2009): e192–e192.https://doi.org/10.1130/G30035C.1Gerilyn S. Soreghan, Michael J. Soreghan, Christopher J. Poulsen, Roger A. Young, Cortland F. Eble, Dustin E. Sweet, Oswaldo C. Davogustto Anomalous cold in the Pangaean tropics: REPLY, Geology 37, no.66 (Jun 2009): e193–e194.https://doi.org/10.1130/G30148Y.1 G. S. Soreghan , D. E. Sweet , K. R. Marra , C. F. Eble , M. J. Soreghan , R. D. Elmore , S. A. Kaplan , and M. D. Blum An Exhumed Late Paleozoic Canyon in the Rocky Mountains: A Reply, The Journal of Geology 117, no.22 (Jul 2015): 215–220.https://doi.org/10.1086/595788

Malignant Catarrhal Fever Associated with Ovine Herpesvirus-2 in Free-ranging Mule Deer in Colorado

Malignant catarrhal fever (MCF) was diagnosed in four free-ranging mule deer (Odocoileus hemionus) in January and February of 2003. Diagnosis was based on typical histologic lesions of lymphocytic vasculitis and PCR identification of ovine herpesvirus-2 (OHV-2) viral genetic sequences in formalin-fixed tissues. The animals were from the Uncompahgre Plateau of southwestern Colorado. Deer from these herds occasionally resided in close proximity to domestic sheep (Ovis aries), the reservoir host of OHV-2, in agricultural valleys adjacent to their winter range. These cases indicate that fatal OHV-2 associated MCF can occur in free-ranging mule deer exposed to domestic sheep that overlap their range.

Twenty-year change in aspen dominance in pure aspen and mixed aspen/conifer stands on the Uncompahgre Plateau, Colorado, USA

Reports of decreasing quaking aspen ( Populus tremuloides) cover in forests of the western USA have caused concern about the long-term persistence of aspen on landscape scales. We assessed changes in overstory aspen dominance on the Uncompahgre Plateau in western Colorado over a 20 year period. We measured stand density, species composition and regeneration in 53 undisturbed, mature pure aspen, pure conifer, and mixed aspen/conifer stands originally inventoried between 1979 and 1983. Ages of overstory and understory trees were used to evaluate long-term change in regeneration and overstory development. While pure aspen stands occupy 16% of the study area, mixed aspen and conifer stands cover 62% of the forested landscape on the Uncompahgre Plateau. Pure aspen stands were self-thinning, but stable over the twenty-year study period, with high amounts of regeneration and without conifer invasion. Mixed stands of aspen and conifer had undergone significant change. In aspen dominated mixed species stands, conifer basal area increased from 10 to 23 m 2 ha −1 in the last 20 years, while aspen basal area decreased. In conifer dominated mixed species stands, conifer basal area increased from 18 to 24 m 2 ha −1. Most overstory aspen in pure aspen stands were between 80 and 120 years old. Substantial aspen suckering was occurring, but all suckers were <20 years old, indicating lack of current growth into the overstory. Aspen suckering was occurring in mixed species stands, but again, most suckers were <20 years old, and few overstory trees were <100 years old. In contrast, understory and overstory conifers spanned ages from <20 to over 250 years old. Aspen dominance is decreasing in the forested communities of the study area. Pure stands are likely to persist without decline for a considerable time. Mixed stands are likely to continue to experience a decrease in overstory aspen canopy dominance. These changes are probably within the historic range of variability, but restoration of aspen canopy cover consistent with an early- to mid-seral landscape condition would require disturbances such as fire or cutting to create canopy gaps to permit growth of suckers into the overstory of mixed species stands.

LONG-TERM TRENDS IN MULE DEER PREGNANCY AND FETAL RATES IN COLORADO

Mule deer (Odocoileus hemionus) fawn:doe (f:d) ratios in December have declined by 0.015 fawns/doe/year from 1972 through 1995 in Colorado, USA. To determine whether lower pregnancy or fetal production was the cause of low December f:d ratios, we estimated mule deer pregnancy rates in the Poudre River drainage during January 1998 and pregnancy and fetal rates on the Uncompahgre Plateau during February 1999. Twenty-seven (93.1%) of 29 yearling and adult does in the Poudre River drainage were detected pregnant with pregnancy-specific protein-B (PSPB). This proportion did not differ (P > 0.2) from the proportion of yearling and adult does that were pregnant when collected by various methods in previous studies in Colorado. Thirty-six (90%) of 40 adult does were detected pregnant with PSPB, and 37 (93%) of these 40 does on the Uncompahgre Plateau were detected pregnant with ultrasound. Proportion of adult does detected pregnant with ultrasound did not differ (P > 0.07) from pregnancy rate of adult does collected by various methods in previous studies. Average number of fetuses per doe for all adult does (pregnant and nonpregnant) on the Uncompahgre Plateau (n = 40, x̄ = 1.70, SE = 0.109) was not less (P > 0.06) than for adult does collected by various methods in previous studies. We concluded that failure to breed or maintain pregnancy through at least early January or early February does not appear to be the cause of low f:d ratios observed in Colorado.

NEONATAL MULE DEER FAWN SURVIVAL IN WEST-CENTRAL COLORADO

Abstract Declining mule deer (Odocoileus hemionus) populations resulting from apparent low recruitment brought management and political focus on neonatal fawn survival. We captured mule deer fawns on the Uncompahgre Plateau (5,957 km2) in west-central Colorado, USA, at a mean age of 3 days (range = newborn to 6 days), and we radiomarked them with mortality-sensing, drop-off radiocollars. We radiomarked 230 fawns with samples of 50 in 1999, 88 in 2000, and 92 in 2001. Designated neonate survival period was from capture to 14 December. Survival was different among years (χ22 = 6.160, P = 0.046) with 3-year mean survival of 0.501. Cause-specific mortality ordered from highest to lowest was sick/starve, coyote, unknown, trauma, bear, and feline. Neither all predation combined (coyote, bear, feline; P = 0.379) nor coyote predation alone (P > 0.989) differed among years. By 31 July, 74% of the sick/starve mortality and 75% of the predation mortality had taken place, with 76% of mortality from all sources occurr...