Spatial pattern and interference in pi?on-juniper woodlands of northwest Colorado

The local spatial arrangement of the coniferous trees Pinus edulis and Juniperus osteosperma was mapped in two woodland stands and measured in two shrub-dominated stands in the semiarid Piceance Basin of northwest Colorado. In the woodlands, small trees were often clumped, while medium and large trees were either randomly or uniformly dispersed. Significant regressions were obtained between a tree?s basal area or canopy area and the area of its Dirichlet domain (the region closer to it than to any other tree). Both findings from the woodland stands accord with results obtained by other workers in other vegetation. Like earlier workers, we interpret these patterns to indicate density-dependent mortality and density-dependent depression of growth rates among the trees in the woodlands. In contrast, the trees in the shrub-dominated stands are located at random with respect to each other. However, they are strongly associated with shrub cover. Apparently tree seeds arrive in these stands primarily by long-distance dispersal, and the establishment of seedlings is more likely in the shade of shrubs.     

Rodent weights in modified pinyon-juniper woodlands of southwestern New Mexico

Changing habitat structure in pinyon ( Pinus edulis )—one-seed juniper ( Juniperus monosperma ) stands by (1) pushing trees down with a bulldozer and leaving them in place, (2) pushing, then piling and burning slash, or (3)thinning to a spacing of 6. 1 m and leaving slash did not affect weights of individuals of nine rodent species. Previous studies have shown that habitat modifications influence kinds of species and numbers of individuals, but changes in total rodent biomass are a function of sizes of different species occupying different habitats, not changes in weights of individuals.     

Infection of young Douglas-firs and spruces by dwarf mistletoes in the Southwest

Mistletoe infection of Douglas-fir and spruce seedlings increased as the mean dwarf mistletoe rating of the overstory, seedling density, and total age of seedlings increased. Percent of spruce seedlings infected increased more rapidly than for Douglas-fir as overstory dwarf mistletoe ratings increased. However, the intensity of infection as measured by the mean dwarf mistletoe rating of seedlings, increased at about the same rate for spruce and Douglas-fir. Percent infection of seedlings less than 20 years total age was higher for spruce than for Douglas-fir. Douglas-fir and spruce seedling mortality was from two to three times greater in heavily infested stands than in healthy stands.     

Fungi associated with soils collected beneath and between pinyon and juniper canopies in New Mexico

The soil fungal community beneath pinyon ( Pinus edulis Engelm.) and one-seeded juniper ( Juniperus monosperma [Engelm.] Sar.) tree canopies is described and compared with fungi from adjacent interspace soils dominated by blue grama ( Bouteloua gracilis [H. B. K.] Lag.). Significantly higher organic matter contents and fungal propagule levels were found in soils beneath pinyon and juniper trees than in interspace soils. Soils under pinyon and juniper trees contained similar chemical, physical, and biological properties and, consequently, many groups of fungi in common (64% of the species isolated were common to both). In contrast soil fungi in adjacent interspace soils were vastly different from those collected in soils beneath pinyon and juniper canopies (44% and 48% species in common, respectively). Soil fungi that were isolated more often from pinyon-juniper soils than from interspace soils included Absidia sspp., Beauvaria spp., Gliocladium spp., Mucor spp., Penicillium cyclopium , P. fasciculata , P. frequentans , P. restrictum , Thamnidium spp., and Trichoderma spp. Soil fungi that were isolated more often in interspace soils than in pinyon or juniper soils included Aspergillus alutaceus spp., A. fumigatus , some Fusarium spp., Penicillium luteum , and P. talaromyces .     

Microhabitat partitioning by two chipmunk species ( Tamias ) in western Colorado

We examined microhabitat use of sympatrically occurring Tamias minimus (least chipmunk) and T. rufus (Hopi chipmunk) in a pi?on-juniper/sagebrush vegetative community near Molina, western Colorado, from October 1994 to June 1999. This community is dominated by 2 major microhabitat types: shrub (sage; Artemisia spp.) and tree (pine and juniper; Pinus edulis and Juniperus scopulorum ). Small mammals were live-trapped, marked, and released throughout this study. When it was the most abundant Tamias species on the study plots (1994-1997), Tamias minimus captures were associated with trees. Tamias rufus also exhibited this association but was captured at very low abundances during this period. Tamias rufus abundance was much greater, on average, than that of T. minimus between 1998 and 1999. During this time T. minimus captures were not associated with trees, but T. rufus captures remained associated with trees. As has been previously reported for other Tamias species, the greater abundance of 1 of 2 coexisting congeners in select areas may play a role in the microhabitat use of these 2 chipmunk species.     

Plant age/size distributions in black sagebrush ( Artemisia nova ): effects on community structure

The demography of black sagebrush ( Artemisia nova Nelson) was investigated in the Buckskin Mountains of western Nevada to determine patterns of stand renewal in sagebrush communities currently free from wildfires. Biomass sampling was conducted to develop growth classes that reflected apparent age of the shrubs. The density of black sagebrush plants was twice that of basin big sagebrush ( A. tridentata ssp. tridentata Nutt.) in adjacent communities on contrasting soils (2.2 versus 1.1 plants per m 2 ). Black sagebrush accumulated only 75% as much woody biomass as big sagebrush. Regression equations were developed and tested for predicting total woody biomass, current annual growth (CAG), and leaf weight of black sagebrush plants. Apparent age classes were developed both for the black sagebrush plants and the sub-canopy mounds on which they grew. Discriminant analysis was used to test this classification system. Plant succession, apparently controlled by nitrate content of the surface soil, appeared to eliminate the successful establishment of black sagebrush seedlings on the mounds. After the shrubs die, the mounds eventually deflate. We propose that mounds reform around shrub seedlings, but because seedling establishment is so rare in these communities, this could not be verified.     

Facilitation of Yucca brevifolia recruitment by Mojave Desert shrubs

Patterns of recruitment for Yucca brevifolia (Joshua tree) were investigated on 3 elevational transects, 1000-2000 m, in the Spring and Sheep Mountain ranges of southern Nevada. Yucca brevifolia is distributed throughout a broad range of plant communities dominated by Larrea tridentata and Ambrosia dumosa at low elevations, Coleogyne ramosissima at middle elevations, and an Artemisia-Pinus-Juniperus community at upper elevations. The density of Y. brevifolia gradually increased from the lowest elevations, peaked at 1600 m, and remained at intermediate levels at high elevations until reaching an abrupt upper elevational limit at 2000 m. Open substrate dominated the study areas; however, a large majority of Y. brevifolia seedlings were found growing under the canopy of other woody shrubs. This pattern of recruitment did not vary by site or elevation. Thirty-five species of perennial shrubs were identified in the study areas, 16 of which were found in association with at least 1 Y. brevifolia seedling. However, 4 shrubs were found in a nurse plant relationship with Y. brevifolia above the frequency predicted by either their canopy area or numerical dominance. Seedlings exhibited significant variation in aspect, relative to the center of the nurse shrub. In Lee and Lucky Strike canyons, recruitment occurred predominantly on the east and west sides of nurse shrubs, indicating the importance of specific microhabitats. Local presence of specific perennial shrubs resulted in higher levels of recruitment, causing a distinct pattern of community development, resumably through amelioration of abiotic stresses.     

Does the presence of Wyethia mollis affect growth of Pinus jeffreyi seedlings?

Regeneration of Pinus jeffreyi in the Sierra Nevada is often limited on sites dominated by Wyethia mollis . Allelopathic chemicals and competition for soil moisture have been suggested as possible mechanisms for limiting regeneration. We tested the hypothesis that soil chemical and microbial properties from sites in different stages of succession influence seedling growth of Pinus jeffreyi . Soil was collected from an early-seral site dominated by Wyethia mollis , a mid-seral site dominated by the shrubs Arctostaphylos patula, Ceanothus prostratus, C. velutinus , and Purshia tridentata , and a late-seral site dominated by mature Pinus . These sites were compared for nutrient content, Pinus seedling growth capacity, and microbial population size. Soil (0-33 cm) from the early-seral site had the lowest C, microbial biomass, and fungal and bacterial populations. There were no consistent trends in soil nutrient content among sites. The early-seral site had the lowest soil Ca and Mg contents but also had a lower C/N ratio and more than twofold greater P content than either the mid- or late-seral site. Pinus seedling growth and foliar nutrient concentrations were compared at 3 harvest dates (220, 314, and 417 days after germination) in a greenhouse bioassay. The treatment design was a 3 × 2 factorial with soil from each of the 3 sites either with or without Pinus seedlings. Pots without seedlings were used as controls to assess the effects of seedlings on microbial biomass. Seedling growth in the early-seral soil was initially suppressed in comparison to growth in the mid-seral soil, but by the final harvest total seedling weight was similar between these 2 treatments. The most obvious treatment effect was a reduction in growth for seedlings planted in lateseral soil, probably due to a nutrient imbalance in the soil. Seedlings grown in late-seral soil had Fe and Al levels that were nearly twice those of seedlings grown in early- and mid-seral soils. Microbial biomass followed a temporal pattern similar to that found for seedling growth. Differences in microbial biomass between the early- and mid-seral soils, although initially large, were not detected by the final harvest. We interpret these results to indicate that allelopathy or soil nutrient deficiencies resulting from the presence of Wyethia are unlikely to be responsible for limited growth of Pinus seedlings in Wyethia -dominated stands.     

Experimental manipulations of fertile islands and nurse plant effects in the Mojave Desert, USA

In a mixed desert shrub community we removed and added shrub canopies to examine above- and belowground influences of 3 species of shrubs on islands of soil fertility and the survival of transplanted Ambrosia dumosa seedlings. Soils sampled under shrubs in the wet season had higher pH, water content, organic matter, and both total and mineralizable nitrogen than soils in adjacent open areas, confirming a widely established pattern in arid lands. However, we also found species differences in soil parameters. Soils under Coleogyne ramosissima had highest pH, soils under A. dumosa had highest water content and nitrogen mineralization rates, and soils under Larrea tridentata had lowest water content. Soils sampled under shrubs in the dry season, 7 months after experimental shrub removal, maintained higher organic matter and total and mineralizable nitrogen content than adjacent open soils, but pH and water were altered by shrub manipulations. Species differences persisted only in soil water levels ( A. dumosa soils were driest). Over a 1-year period, transplanted A. dumosa seedlings had highest survivorship in shrub removal and open treatments and died most rapidly under control shrubs of all 3 species, suggesting that shrubs had a strong negative effect on seedling survival, even in the presence of higher organic matter, nutrients, and (initially) higher water content of fertile islands. Our results suggest that nurse plants and islands of soil fertility have the potential to facilitate growth of other species by nutrient additions, but that the net effect of nurse plants can be negative due to shading and/or root competition.     

Variability among five riparian cottonwood ( Populus fremontii Wats.) populations: an examination of size, density, and spatial distribution

Various abiotic and biotic factors are known to affect tree size, including age, genetics, and environment. Knowledge of size variation within natural riparian tree populations has both ecological and restorative importance. We determined tree sizes, basal area densities, and spatial distributions of 5 Populus fremontii Wats. populations within the Rio Grande watershed in New Mexico. At each site 10 randomly spaced plots, perpendicular to the river and extending from the river to the end of the forest, were established. Diameter at breast height (DBH) and distance to the river were determined for 1803 trees within the 5 populations, and stand cover (measured as basal area [BA] per hectare) was determined for each population. Significant variation in tree size and basal area density existed among sites. Mean DBH per site ranged from 11.7 to 58.4 cm and differed significantly ( P P 2 ha -1 . Spatial distribution of trees in relation to the river also differed among sites. Mean distance from the river ranged from 50 to 353 m and differed significantly ( P P. fremontii populations may be influenced by differences in water availability across a site; trees farthest from water sources may experience greater water stress and, therefore, growth limitation. Increasing BA cover with increasing tree size indicated no real thinning of mature trees within a population. Recruitment and establishment of cottonwood seedlings and saplings was evident only at sites with newly formed floodplains. For these 5 populations tree size appeared to be affected by environmental factors.     

Ruffed Grouse ( Bonasa umbellus ) drumming log and habitat use in Grand Teton National Park, Wyoming

We described 15 Ruffed Grouse ( Bonasa umbellus ) drumming logs and adjacent habitat within Grand Teton National Park, Wyoming. Drumming logs and adjacent habitat differed from 30 random non-drumming sites. Drumming logs had fewer limbs (8; P = 0.003) and a smaller percentage of bark remaining (12%; P = 0.0001). These logs were in advanced stages of decay but were still firm to the touch. Additionally, drumming logs were found close to clearings but in areas with increased amounts of undergrowth and mature trees. Adjacent habitat analysis (0.04-ha circular plot centered on logs) indicated drumming locations had significantly greater average canopy height, more vegetative cover consisting of conifer and total canopy cover, and more vertical foliage between 0.3 m and 3.0 m in height. Adjacent habitat was in advanced stages of maturity as indicated by significant numbers of both large-diameter logs and largediameter lodgepole pine ( Pinus contorta ) and quaking aspen ( Populus tremuloides ) snags. Tree species dominating the canopy and subcanopy were large-diameter Engelmann spruce ( Picea engelmannii ), lodgepole pine, and quaking aspen. Subalpine fir ( Abies lasiocarpa ) and quaking aspen saplings were more numerous at used sites. Ruffed Grouse drummed in coniferous areas within close proximity of quaking aspen.     

Colorado's rare flora

Distribution, status, habitat, elevational range, original literature citations, synonymy, and specimens deposited at Brigham Young University (BRY), University of Colorado (COLO), Colorado State University (CS),Denver Botanical Garden (KHD), Kansas State University (KSC), University of Kansas (KANU), Missouri Botanical Garden (MO), and the Rocky Mountain Herbarium (RM) are given for 79 species of rare Colorado plants. Species federally listed by the U.S. Fish and Wildlife Service under the Endangered Species Act, candidates for listing, and species that qualify as federal candidates because of limited range, small populations, or known threats are included. Suggested changes in status are offered where appropriate. Maps and tables indicating the Colorado range of these species are provided.     

Persistence of subalpine forest--meadow ecotones in the Gunnison Basin, Colorado

Forests of the southern Rocky Mountains are punctuated by persistent meadows called parks that are dominated by grasses and forbs. In an attempt to elucidate the maintenance of subalpine parks in the Gunnison Basin, Colorado, soil texture and tree morphology differences along 60-m transects spanning the forest-park ecotone were studied in 6 representative parks. Seedling survivorship, percent seed germination, and soil moisture available to plants were also studied along one of the transects in Willow Park. Soil analyses revealed 40% more silt and significantly less sand and clay in all 6 parks ( P P = 0.011), with 54% more water in forest than in park soils. Measures of growth rate obtained from tree height, dbh, and age were significantly higher nearer the ecotone ( P P = 0.002). These results suggest that stress of mature Engelmann spruce ( Picea engelmannii ) and lodgepole pine ( Pinus contorta ) is an unlikely explanation of park maintenance. P. engelmanii percent seed germination and seedling survivorship were significantly higher in the forest than in the park ( P < 0.001). This may be largely due to the more severe seedling microclimate observed in the park. Results indicate that limited seedling establishment is primarily responsible for maintenance of subalpine parks in the Gunnison Basin.     

Effects of Douglas-fir beetle (Coleoptera: Scolytidae) infestations on forest overstory and understory conditions in western Wyoming

Douglas-fir beetle ( Dendroctonus pseudotsugae Hopk.) infestations frequently result from disturbance events that create large volumes of weakened Douglas-fir trees, Pseudotsuga menziesii (Mirb.) Franco. Previous research has focused on determining susceptibility of forest stands to Douglas-fir beetle and predicting the amount of tree mortality from Douglas-fir beetle infestations following disturbance events. Little work has been done on consequent changes in the forest overstory and understory. In the early 1990s, populations of Douglas-fir beetle increased in fire-scorched trees, subsequently infesting undamaged neighboring stands in the Rocky Mountains of western Wyoming, USA. In 1999 transect sampling and 25 pairs of previously infested and uninfested plots were used to quantify changes in forest stand conditions and ensuing responses in the understory caused by Douglas-fir beetle infestations. Significant effects of the Douglas-fir beetle infestation comprised 3 general categories: (1) overstory effects: basal area was reduced by 40%-70%, average tree diameter decreased by 8%-40%, and the Douglas-fir component of the overstory decreased by more than 12%; (2) regeneration effects: conifer seedling regeneration increased nearly fourfold in infested plots and 90% of the regeneration was Douglas-fir; (3) understory effects: understory vegetation (forbs, grass, and shrubs) had a threefold increase in infested compared with uninfested plots. In addition, basal area of Douglas-fir killed by the Douglas-fir beetle was significantly correlated with initial Douglas-fir basal area and percentage of Douglas-fir, but not with stand density index, tree diameter, or trees per hectare. Significant inverse relationships also were found between post-infestation basal area and abundance of forbs, grass, and shrubs, and understory height. Thus, we found that Douglas-fir beetle infestations cause significant short-term effects in both the overstory and understory and contribute to an altered mosaic in forest structure.     

Seedling size and survival for Chrysothamnus nauseosus

Seedling size and survival in relation to summer drought were examined for Chrysothamnus nauseosus growing under field and greenhouse conditions. In the field, summer survival rates were less than 2% annually for the three years monitored. The effect of initial seedling height on subsequent survival was examined in both the field and greenhouse by grouping seedlings into live and dead categories on each census date and comparing initial heights for seedlings in these categories. For a majority of the census dates, the initial height of surviving seedlings was greater than the initial height of those that subsequently died (significant differences ranged from 1 to 8 mm), indicating that seedlings that were taller at the initiation of the drought period had a higher probability of survival. In the greenhouse, taller seedlings had greater shoot and root biomass and rooting depth. Seedlings that are larger (i.e., taller and have greater aboveground biomass) in late spring appear to have a higher probability of surviving summer drought due to greater rooting depth and hence increased access to moisture in deeper soil layers. Seed availability and safe sites for germination were probably not limiting since large numbers of seedlings successfully germinated in a patchy pattern during the study period. Seedling size and probability of survival were not related to either seedling density or the distance to nearest seedling neighbor. Survival through summer drought appears to be the main limitation to seedling recruitment in this population.     

Changes in riparian vegetation along the Colorado River and Rio Grande, Colorado

Changes in vegetation including area occupied, canopy cover, and maturity class of cottonwoods ( Populus spp.) within lower-elevation zones of the Colorado River and Rio Grande in Colorado were monitored over 25- and 27-year intervals, respectively, using photo-interpretative methods. Estimated loss of cottonwoods along the Colorado River was 2ha/km (-17.5%), and remaining stands had become more open and older. Cottonwoods along the Rio Grande increased 1.6 ha/km (9.3%) with minor canopy cover and maturity class changes. Area occupied by shrubs and river channel changed little along the Colorado River, but declined along the Rio Grande. Loss of hay meadow occurred along both rivers, whereas developed land increased along the Colorado River and farmland increased along the Rio Grande. Wildlife habitats along the Colorado deteriorated much more rapidly than those along the Rio Grande during monitored intervals.     

New records and range extensions of species of Dipogon (Hymenoptera, Pompilidae) in Colorado

Four species of Dipogon are recorded from Colorado for the first time. These are hurdi Evans, pulchripennis (Cresson), graenicheri Banks, and brevis (Cresson). All are inhabitants of wooded areas and are believed to nest in cavities in trees. Notes are presented on habitat and prey of these 4 species and of the recently described species kiowa Evans.     

Elevational changes in woody vegetation along three streams in Washington County, Utah

Patterns of change in the woody vegetation (trees and shrubs) of the riparian communities of three streams draining the slopes of the Pine Valley Mountains in Washington County, Utah, were examined. Thirty-nine study sites were established adjacent to the streams along an elevational gradient where vegetation, plant population, and species data were taken. Vegetation and species patterns varied with respect to elevation and geographical location. Vegetation of the high- and low-elevation areas was distinctly different, while that of the mid-elevations was transitional to both. Geographical patterns showed a north-south transition in vegetation between the Mojave and Great Basin deserts. Size-class distribution curves for 11 tree species showed varying degrees of survival with respect to age and location. Root sprouting was highly important in the reproductive effort of six of the trees. Species distribution patterns followed transriparian and intrariparian gradients within the riparian community.     

Natural variability of vegetation, soils, and physiography in the bristlecone pine forest of the Rocky Mountains

Pinus aristata Engelm. forest vegetation in Colorado was studied to determine vegetation composition and the relationship between vegetation and environment. Species percent cover, tree size class, and environmental variables were recorded for 49 plots. Previously collected data for 4 plots form New Mexico were included. Environmental variables included latitude, longitude, elevation, slope, aspect, topographic position, parent material, percent rock cover, mean rock size, litter depth, estimated plot age class, and evidence of anthropogenic disturbance. Soils were analyzed for texture, depth, and percent carbon and nitrogen. Pinus aristata foliage was analyzed for percent nitrogen and phosphorus. Direct and indirect gradient analyses (CANOCO) were used to determine environmental factors associated with community composition. Vegetation in P. aristata forests is influenced primarily by elevation and soil pH. Substrate, soil texture, topographic position, and geographic location are secondary factors. Six plant associations were identified using cluster analysis (listed in an elevational sequence from low to high): Pinus aristata/ Festuca arizonica Vasey, Pinus aristata / Festuca thurberi Vasey, Pinus aristata / Juniperus communis L., Pinus aristata / Vaccinium myrtilus L., Pinus aristata / Ribes montigenum McClatchie, and Pinus aristata / Trifolium dasyphyllum Torr. & Gray.