Classification of the riparian vegetation along a 6-km reach of the Animas River, southwestern Colorado

Riparian ecosystems are important components of landscapes, particularly because of their role in biodiversity. A first step in using a ""coarse-filter"" approach to riparian biodiversity conservation is to determine the kinds of riparian ecosystems. These ecosystems vary substantially in plant species composition along a single river reach, as well as between rivers, and yet the river-reach scale has received little attention. We sampled the vascular plant composition of 67 contiguous patches of riparian vegetation along the reach of the Animas River, in southwestern Colorado's San Juan Mountains, that is relatively undisturbed by human land uses. Using cluster analysis and detrended correspondence analysis, we identified eight riparian community types along the reach. Using a new technique, we combined overstory size-class data and understory cover data to identify community types. The eight community types, which are in part the products of past floods, are spatially arranged along the reach in relation to variation in valley morphology, tributary location, and geomorphic landforms. These eight community types do not necessarily represent successional stages of a single potential vegetation type. This study at the river-reach scale suggests that sampling and analysis, as well as conservation, may need to be turned to the scale of patchiness produced by flood disturbances in the riverine landscape, since vegetation varies significantly at this scale.     

Riparian vegetation along the middle Snake River, Idaho: zonation, geographical trends, and historical changes

A baseline study was conducted on an 83-km free-flowing reach of the Snake River between Swan Falls Dam and the Idaho-Oregon border. The research had 2 components: (1) field characterization and inventory of existing riparian flora, vegetation, and environment (soils, topography, streamflow), and (2) determination and mapping, using a geographic information system, of historic changes in riparian vegetation based on a time series (1938-39, 1957, 1969, 1987) of aerial photographs. The flora was diverse, with 185 species of vascular plants identified, 63 of which were exotics. Vegetation was structured vertically along the riverbank gradient into lifeform-defined habitat types: emergent, riparian shrub-forb, tree, transitional grass-shrub, and upland. Riverbank seepage, probably of agricultural origin, blurred zonation patterns on some sites and added species to the overall flora. Upstream-downstream differences existed in the physical characteristics and vegetation of river subreaches. Coverage of riparian woodland, island riparian and total riparian vegetation, and area of islands increased since the 1930s, with the greatest changes in the 1969-1987 interval. Possible contributing facts were (1) significant declines in annual minimum flows since the 1950s, (2) decreases in peak flows following the completion of Swan Falls Dam and some upstream dams since the 1920s, (3) introduction and spread of exotic tree species ( Elaeagnus angustifolia and Tamarix spp.) and (4) possible effects of intensive agriculture on river sediment load and soil nutrients. The introduction and proliferation of purple loosestrife ( Lythrum salicaria ) could have considerable future effects on vegetation-channel dynamics in the middle Snake River.     

Macroinvertebrate community structure and related environmental variables in two forks of the Virgin River, Utah

Using multivariate analysis techniques, we analyzed variation in the structure of the benthic macroinvertebrate community in the North Fork and East Fork of the Virgin River in and above Zion National Park in concert with a number of naturally occurring and anthropogenic environmental variables. Correspondence analysis revealed that the community structure in the 2 streams was markedly different despite occurring in areas similar in topography, elevation, and terrestrial vegetation cover. Subsequent analyses of the 2 streams separately using canonical correspondence analysis to test for strength of association of environmental variables with variations in benthic macroinvertebrate community structure revealed that communities in the East Fork of the Virgin River were more associated with nitrogen-related variables influenced by agricultural and urban activities along the river and that communities in the North Fork were most strongly associated with naturally occurring physical variables.     

Small mammals within riparian habitats of a regulated and unregulated arid land river

In northwestern Colorado, flow regulation on the Green River has created a transitional plant community that features encroachment by upland vegetation into cottonwood ( Populus fremontii )-dominated, riparian forest on topographically high floodplain sites and reduced cottonwood regeneration on low floodplain sites. To assess how these changes might have affected small mammal distributions, in 1994 and 1995 we live-trapped during periods surrounding spring flooding at 3 sites: above and below the confluence of the regulated Green River and at the ecologically similar, but unregulated, Yampa River (reference site). More species were captured at the most regulated site along the Green River above its confluence with the Yampa River. Within sites, more species were captured in riparian habitats than adjacent upland habitats. Despite river regulation-induced habitat changes, we did not detect changes in species distributions within low and high floodplain habitat for Peromyscus maniculatus or Microtus montanus , but changes may have occurred for Dipodomys ordii . The total effect of regulation-induced habitat change on small mammal populations may not be fully revealed until current, mature cottonwood forests disappear and associated woody debris decomposes.     

Bird use of riparian vegetation along the Truckee River, Califonia and Nevada

The Truckee River in California and Nevada is subject to diverse water regimes and a corresponding variety of flow rates. Original riparian vegetation has been altered by these variable flow rates and by a variety of human uses resulting in loss of native riparian vegetation from its historic extent. We conducted bird surveys along the Truckee River during spring 193 to (1) determine relationships between birds and the present vegetation; (2) determine the importance of different vegetation types to sensitive bird species that have declined recently in the western United States due to competition from exotic plant species, cowbird ( Molothrus ater ) parasitism, reduction in nesting habitat, or other unidentified reasons; and (3) establish a monitoring program and collect baseline data for future comparisons. The most frequently detected bird species throughout the study was the Brown-headed Cowbird. The greatest number of bird species (98 of 116) was found in the native mixed willow ( Salix spp.) riparian scrub vegetation type. We recommend protecting the remaining native riparian vegetation types for bird habitat along the Truckee River.     

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.     

Response of tree ring holocellulose δ 13 C to moisutre availability in Populus fremontii at perennial and intermittent stream reaches

We measured δ 13 C of tree ring holocellulose to assess intra- and interannual variation in integrated leaf gas exchange responses of Frémont cottonwood ( Populus fremontii ) to monsoonal moisture inputs in southeastern Arizona. We predicted that δ 13 C of trees growing along drought-susceptible intermittent reaches of this semiarid river system would be more responsive to monsoonal moisture inputs than trees found along perennial reaches, where groundwater is consistently available. We sampled stem xylem cores from 7 trees, each at an intermittent and perennial reach of the San Pedro River near Tombstone, Arizona. We identified and subdivided individual rings from 1990 to 2000. δ 13 C of holocellulose from these subdivisions was compared with precipitation amount, atmospheric vapor pressure deficit ( D a ), and 90% exceedence flows ( Q 90 ) calculated from seasonal flow duration data. δ 13 C values were higher at the intermittent reach than at the perennial reach. Furthermore, annual ring δ 13 C values at the perennial reach were not correlated with stream flow, precipitation, or D a . δ 13 C values for trees at the intermittent reach were negatively correlated with monsoon stream flow, precipitation, or D a . δ 13 C values for trees at the intermittent reach were negatively correlated with monsoon season (1 July-15 September) Q 90 ( r 2 = 0.50, P = 0.015) and positively correlated with D a ( r 2 = 0.45, P = 0.03). Shifts in δ 13 C between the inner- and outer-third of the annual ring were used as a measure of intra-annual variation. These shifts were correlated with monsoon season D a ( r 2 = 0.57, P = 0.01) and Q 90 ( r 2 = 0.59, P = 0.005) for trees growing along the intermittent reach. Intra- and interannual variation in integrated photosynthetic response exists at the population-scale for these native, riparian forests. Changes in monsoonal precipitation and stream flow may differentially alter photosynthetic gas exchange of P. fremontii and function of these riparian ecosystems.     

Razorback sucker movements and habitat use in the San Juan River inflow, Lake Powell, Utah, 1995-1997

Seventeen subadult, hatchery-reared razorback suckers ( Xyrauchen texanus ; ( ￣x = 456 mm total length) were implanted with sonic transmitters and tracked for 23 months in the lower 89.6 km of the San Juan River (San Juan arm of Lake Powell, Utah). Fish were released at 2 sites, and 9 made extensive up- and downstream movements ( ￣x = 47.8 km; contact was lost with 4, and 4 others presumably died or lost their transmitters). The San Juan arm is primarily inundated canyon; however, most fish contacts occurred in shallow coves and shoreline with thick stands of flooded salt cedar in the upper inflow area. Eight fish frequented the Piute Farms river/lake mixing zone, and at least 4 moved upstream into the San Juan River. Seven fish were found in 2 aggregations in spring (3 fish in Neskahi Bay in 1996 and 4 fish just downstream of Piute Farms in 1997), and these may have been associated with spawning activity. Continued presence of razorback suckers in the Piute Farms area and lower San Juan River suggests the San Juan inflow to Lake Powell could be used as an alternate stocking site for reintroduction efforts.     

Distribution of the eastern woodrat ( Neotoma floridana campestris ) in southern Nebraska

The eastern woodrat ( Neotoma floridana ) occurs throughout eastern and central parts of the United States. In Nebraska, 3 of 9 subspecies inhabit the state, including N. f. campestris , N. f. attwateri , and N. f. baileyi . We determined distributional limits of N. f. campestris along 2 river systems in southern Nebraska. As observed with other mammalian species in the Great Plains, we suspected that the distribution of woodrats likely had expanded, reflecting continued regulation of rivers and the concomitant increase in forests along them. We documented N. f. campestris at 7 sites in 5 counties, including a recent (since the 1960s) eastward expansion along the Republican River. We observed little movement along the Platte River. The greatest concentration of houses constructed by woodrats occurred in a shelterbelt near the Republican River; otherwise, abundances of houses tended to be greater along the Platte River. We suspect that the distribution of woodrats will continue to change in Nebraska unless breaks exist or are established in riparian forests along the Platte and Republican rivers.     

A comparison of riparian condition and aquatic invertebrate community indices in central Nevada

The importance of maintaining healthy riparian communities to sustain natural stream processes and function is well documented. Land management agencies in the West are currently developing methods to assess and monitor riparian community condition to adapt land use practices that would better protect rangeland ecosystems. To determine whether these methods also provide an indication of abiotic and biotic stream condition, we compared the classification system of riparian communities developed by the U.S. Forest Service (USFS) to physical parameters of stream condition and to aquatic invertebrate community assemblages. Thirty-three sites in 19 different streams of the Toiyabe Range in central Nevada were measured for water quality, substrate characteristics, and fish abundance and diversity. We sampled aquatic invertebrates and calculated community indices based on environmental tolerance levels, taxonomic diversity, and abundance of sensitive taxa. USFS personnel classified these sites by dominant riparian plant community type (meadow, willow, or aspen) and ecological status (low, moderate, or high) using plant abundance data, rooting depth, and soil infiltration to determine similarities to potential natural communities. Riparian condition indices as well as community diversity were significantly correlated to proportions of fine and small-diameter substrate in streambeds. Accumulation of silt was significantly related to plant community type, with meadow sites expressing highest proportions. Further examinations indicated that 2 of 6 invertebrate community indices were significantly related to ecological status, with highest diversity levels occurring mainly in willow- and aspen-dominated sites in moderate ecological condition. Nevertheless, we show that several other environmental variables, including substrate characteristics, dissolved oxygen, water temperature, and species richness of fish communities, were more strongly and consistently related to invertebrate assemblage patterns. Our results demonstrate that information on aquatic invertebrates and stream condition could augment the existing riparian classification system and provide useful monitoring tools to more thoroughly examine ecosystem health in rangelands.     

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.     

Woody riparian vegetation response to different alluvial water table regimes

Woody riparian vegetation in western North American riparian ecosystems is commonly dependent on alluvial groundwater. Various natural and anthropogenic mechanisms can cause groundwater declines that stress riparian vegetation, but little quantitative information exists on the nature of plant response to different magnitudes, rates, and durations of groundwater decline. We observed groundwater dynamics and the response of Populus fremontii , Salix gooddingii , and Tamarix ramosissima saplings at 3 sites between 1995 and 1997 along the Bill Williams River, Arizona. At a site where the lowest observed groundwater level in 1996 (-1.97 m) was 1.11 m lower than that in 1995 (-0.86 m), 92-100% of Populus and Salix saplings died, whereas 0-13% of Tamarix stems died. A site with greater absolute water table depths in 1996 (-2.55 m), but less change from the 1995 condition (0.55 m), showed less Populus and Salix mortality and increased basal area. Excavations of sapling roots suggest that root distribution is related to groundwater history. Therefore, a decline in water table relative to the condition under which roots developed may strand plant roots where they cannot obtain sufficient moisture. Plant response is likely mediated by other factors such as soil texture and stratigraphy, availability of precipitation-derived soil moisture, physiological and morphological adaptations to water stress, and tree age. An understanding of the relationships between water table declines and plant response may enable land and water managers to avoid activities that are likely to stress desirable riparian vegetation.     

Natural history and invasion of Russian olive along eastern Montana rivers

Russian olive and tamarisk are introduced woody plants invading western North American riparian communities. Beavers can play an important role in structuring these communities by removing the dominant cottonwood trees. Our study explored the way in which beavers interact with cottonwood, Russian olive, and tamarisk along 4 rivers on the Great Plains of eastern Montana. We sampled cottonwood stands that supported populations of 1 or both exotic species, recording beaver damage and density in addition to size and age of cottonwood, Russian olive, and tamarisk. In stands where beaver had been present, they felled an average of 80% of cottonwood trees while rarely using Russian olive or tamarisk. Beaver foraging was apparent in nearly 90% of stands within 50 m of the river channel but only 21% of stands farther away, creating a sunny corridor along the river channel that may increase the invasive potential of Russian olive and tamarisk. Growth rates of both Russian olive and tamarisk were substantially higher where beavers had reduced the cottonwood canopy cover. Managers wishing to reintroduce beavers should consider the potential effect on invasive exotic plants.     

Beavers indirectly enhance the growth of Russian olive and tamarisk along eastern Montana rivers

Russian olive and tamarisk are introduced woody plants invading western North American riparian communities. Beavers can play an important role in structuring these communities by removing the dominant cottonwood trees. Our study explored the way in which beavers interact with cottonwood, Russian olive, and tamarisk along 4 rivers on the Great Plains of eastern Montana. We sampled cottonwood stands that supported populations of 1 or both exotic species, recording beaver damage and density in addition to size and age of cottonwood, Russian olive, and tamarisk. In stands where beaver had been present, they felled an average of 80% of cottonwood trees while rarely using Russian olive or tamarisk. Beaver foraging was apparent in nearly 90% of stands within 50 m of the river channel but only 21% of stands farther away, creating a sunny corridor along the river channel that may increase the invasive potential of Russian olive and tamarisk. Growth rates of both Russian olive and tamarisk were substantially higher where beavers had reduced the cottonwood canopy cover. Managers wishing to reintroduce beavers should consider the potential effect on invasive exotic plants.     

Tundra vegetation of three cirque basins in the northern San Juan Mountains, Colorado

The vegetation of three alpine cirque basins in the northern San Juan Mountains of southwestern Colorado was inventoried and analyzed for the degree of specificity shown by vascular plant communities for certain types of habitats identified as representative of the basins. A total of 197 vascular plant species representing 31 families was inventoried. Growth forms of all species were noted and a growth form spectrum for all of the communities was derived. The caespitose monocot and erect dicot growth forms are the most important growth forms among the community dominants. The most common growth form among all species is the rosette dicot.           

Red Butte Canyon Research Natural Area: history, flora, geology, climate, and ecology

Red Butte Canyon is a protected, near pristine canyon entering Salt Lake Valley, Utah. It contains a well-developed riparian zone and a perennial stream; hillside vegetation ranges from grasslands on the lower limits to Douglas-fir and aspen stands at the upper elevations. In this paper we describe the history of human impact, natural history aspects of climate, geology, and ecology, and faunal and floral information for key species in the canyon. The role and importance of Research Natural Areas is discussed, particularly with respect to the need to protect Red Butte Canyon:one of the few remaining undisturbed riparian ecosystems in the Intermountain West.     

Factors controlling structural and floristic variation of riparian zones in a mountainous landscape of the western United States

We examined landscape patterns in the physical conditions and vegetative composition of montane riparian zones to identify their most important sources of variation. Information on plant species cover and on physical characteristics that occur at coarse, medium, and fine scales was collected for 144 riparian plots located throughout the Lake Tahoe Basin, which straddles the California-Nevada border in the western United States. Constrained and unconstrained ordination analyses were used to identify the most important correlates of physical form and plant species composition. Through multivariate analysis of environmental variables (principal components analysis), vegetation data (detrended correspondence analysis), and the combined relationship between the environmental and vegetation data (canonical correspondence analysis), we consistently found that the greatest variation occurred along a gradient of decreasing valley width, decreasing stream sinuosity, and increasing stream slope. Although surface characteristics reflected a 2nd important source of variation in physical conditions, plant species distribution was not strongly correlated with riparian surface conditions. Strong correlations among physical variables that occur at different scales, such as between valley form and geofluvial surface and between geofluvial surface and surface conditions, support the use of a physically based hierarchical framework for organizing riparian zones within the landscape. Such a hierarchical framework would be useful for interpreting patterns in riparian structure and process at different scales and could be applied to riparian zones in other mountain landscapes of the western United States and elsewhere. Moreover, our finding that riparian plant species composition is most strongly correlated with environmental variables that occur at coarse to moderate scales, most of which can be derived from existing data, supports the idea that modeling montane riparian community distribution using topographic and remotely sensed data could be useful; however, a large degree of species variation, unexplained by the variables we collected, indicates that other variables, perhaps disturbance regime, should be included in such a venture. Nomenclature: Hickman (1993).     

Biodiversity and the Three Gorges Reservoir: a troubled marriage

The Three Gorges region, located on the upper reaches of the Yangtze River in central China, is considered one of the most important biodiversity hotspots in China. It harbours more than 6000 plant species, over 500 terrestrial vertebrates, and about 160 species of fish. In addition to this taxonomic richness, the region is home to many endemic and threatened taxa, which may be the result of its role as a glacial refuge. The construction of the Three Gorges Dam may have serious consequences for this biodiversity. The flooding of more than 600 km² together with massive population resettlement will involve the loss and/or fragmentation of many terrestrial habitats in the reservoir area, affecting both plant and animal species. Furthermore, negative effects on downstream terrestrial biodiversity are expected due to the regression of floodplains and riparian wetlands after the dam is completed. The alteration of river flow caused by the dam will also have a severe impact on freshwater biodiversity throughout the whole Yangtze River system, especially on migratory fish but also on other aquatic life, such as the emblematic Yangtze River dolphin. The conservation measures implemented by the authorities, although valuable, may well be insufficient to mitigate the negative effects of the dam on biodiversity.     

Spring denitrification rates in soils of four eastside Sierra Nevada plant communities

Denitrification rates in soils of four subalpine plant communities in the Sierra Nevada were determined by the acetylene blockage method. The study area included riparian, meadow, forest, and barren sites. Data were collected during dawn-to-dusk measurements in April 1987. Soil atmosphere samples were analyzed for N 2 O content using gas chromatography. Generally, temporal variability in denitrification rate within each plant community was insubstantial. Denitrification rate and soil temperature were found to be significantly correlated only in the riparian and barren sites. Of the four communities, the riparian site was found to have the lowest rate of denitrification overall. However, differences among sites in denitrification rate could not be conclusively attributed to variation in soil temperature, moisture, organic matter, total C and N, C:N ratio, NO 3 - N, or pH.     

Bird community composition in relation to habitat and season in Betatakin Canyon, Navajo National Monument, Arizona

Bird species density, diversity, and species richness in relation to habitat and seasonal variations were studied in the Betatakin Canyon area of Navajo National Monument, Arizona. The two most prominent habitat types are a riparian forest deep in the canyon bottom and a mature pinyon-juniper woodland on the mesas and slopes above the canyon. One hundred thirty-five species of birds were encountered during the study and recorded by season and habitat. The avifauna assemblages demonstrate definite habitat selection into groups associated with the riparian and pinyon-juniper woodland communities. Diversity, density, and species richness were greatest in the riparian habitat during the spring and summer months. The differences in bird community composition were greater between seasons than between habitats. Pinyon pine and Gambel oak were highly selected as perch sites in the pinyon-juniper and riparian areas, respectively.