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Relationships between Riparian Vegetation, Hydrology, Climate and Disturbance across the Western United States
Abstract and Figures
Flow regime, the magnitude, duration and timing of streamflow, controls the development of floodplain landforms on which riparian vegetation communities assemble. Streamflow scours and deposits sediment, structures floodplain soil moisture dynamics, and transports propagules. Flow regime interacts with environmental gradients like climate, land-use, and biomass-removing disturbance to shape riparian plant distributions across landscapes. These gradients select for groups of riparian plant species with traits that allow them to establish, grow, and reproduce on floodplains – riparian vegetation guilds. Here I ask, what governs the distributions of groups of similar riparian plant species across landscapes? To answer this question, I identify relationships between riparian vegetation guilds and communities and environmental gradients across the American West. In Chapter One, I discuss guild-based classification in the context of community ecology and streams. In Chapter Two, I identified five woody riparian vegetation guilds across the interior Columbia and upper Missouri River Basins, USA, based on species’ traits and morphological attributes. I modeled guild occurrence across environmental gradients, including climate, disturbance, channel form attributes that reflect hydrology, and relationships between guilds. I found guilds’ distributions were related to hydrology, disturbance, and competitive or complementary interactions (niche partitioning) between co-occurring guilds. In Chapter Three, I examine floodplain riparian vegetation across the American West, identifying how hydrology, climate, and floodplain alteration shape riparian vegetation communities and their guilds. I identified eight distinct plant communities ranging from high elevation mixed conifer forests to gallery cottonwood forests to Tamarisk-dominated novel shrublands. I aggregated woody species into four guilds based on their traits and morphological attributes: an evergreen tree guild, a mesoriparian shrub guild, a mesoriparian tree guild, and a drought and hydrologic disturbance tolerant shrub guild. Communities and guilds’ distributions were governed by climate directly, and indirectly as mediated through streamflow. In Chapter Four, I discuss the utility of guild-based assessments of riparian vegetation, current limitations to these approaches, and potential future applications of the riparian vegetation guild concept to floodplain conservation and management. The classification of vegetation into functional trait-based guilds provides a flexible, framework from which to understand riparian biogeography, complementing other models frameworks for riparian vegetation.
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Estudiar la vegetación ribereña permite comprender los patrones de diversidad florística a lo largo de la red fluvial y, dado el nivel de transformación que presentan, es necesario generar conocimiento que guíe su recuperación. Este trabajo caracterizó la vegetación arbórea ribereña remanente en 30 sitios ubicados en arroyos de la Reserva de la Biósfera Mariposa Monarca, a partir de la descripción de la estructura, la riqueza de especies y su contexto geográfico (elevación, precipitación, orden hidrológico y cubierta del suelo), e identificó las especies potencialmente invasoras. La elevación de los sitios mostró estar asociada con la precipitación, el orden hidrológico y la cubierta del suelo que es atravesada por los arroyos. Se registraron 56 especies, principalmente arbóreas, cuya densidad y altura se incrementó con la elevación. Entre las especies con mayor valor de importancia estuvieron Roldana angulifolia, Cestrum fulvescens, Ilex tolucana, Alnus acuminata, Buddleja cordata y Fraxinus udhei. Surgieron cuatro grupos fisonómicos en función del número de especies, géneros y familias, número de ramas, así como de número, altura y diámetro de los individuos. Se encontró un alto recambio de especies entre los sitios principalmente con los ubicados a mayor elevación. La presencia de especies potencialmente invasoras mostró estar asociada con la densidad de individuos, con el índice de diversidad de Shannon (H´) y con rasgos geográficos como la elevación y el orden hidrológico. Las riberas analizadas muestran intervención humana, siendo necesario distinguir los impactos de la alteración en el caudal de aquellos asociados al cambio de cubierta del suelo.
Ecological processes and distribution of biodiversity within river floodplains are driven by the hydrological regime of the area. The relationship between fish abundance/diversity and surface water quality at four impact points of the Sashadanga oxbow lake was examined based on hydrological connectivity (distance of lake sections from parent channel; density of buffer vegetation and hydrological phases of seasons). Fish sample populations (n=1440) belonging to 10 orders showed greater numerical abundance in the pre-and post-flooding phases. Oxbow sites proximal to the parent channel showed higher comprehensive pollution index (CPI), higher dominance and lower species evenness during the pre-monsoon (minor flooding) and vice versa during the monsoon and post-monsoon (greater flooding periods). Diversity descriptors using curve/asymptote based on Hill's numbers indicate significant site-specific fish diversity within the oxbow lake at different seasons. Discriminant analysis revealed that change in temperature, turbidity and concentration of inorganic nitrogen were significant predictors in differentiating the quality of the different sites of the lake. Principal component analysis (PCA) revealed that the relative abundance of each species order across sites was significantly influenced by changes in seasons and physicochemical parameters, and showed a weak association with the density of buffer vegetation at sites. Depending on the distance of oxbow areas to the parent channel, the combination of periodic flooding, riparian buffer and lake morphology define the degree of hydrological connectivity which in turn affects the composition and abundance of fish communities at different points of the lake.
Invited talk at Binghamton Symposium on Connectivity in Geomorphology.
We assessed the climate-driven changes in ecologically relevant flow regimes expected to occur by the year 2100 in streams across the conterminous United States. We used long-term daily flow measurements from 601 gauged streams whose watersheds were in relatively natural condition to characterize spatial variation in 16 flow regime variables selected for their ecological importance. Principal component analysis of these 16 variables produced five uncorrelated factors that described patterns of spatial covariation in flow regimes. These five factors were associated with low flow, magnitude, flashiness, timing, and constancy characteristics of the daily flow regime. We applied hierarchical clustering to the five flow factors to classify the 601 streams into three coarses and eight more finely resolved flow regime classes. We then developed a random forest model that used watershed and climate attributes to predict the probabilities that streams belonged to each of the eight finely resolved flow regime classes. The model had a prediction accuracy (per cent correct classification) of 75%. We used the random forest model with downscaled climate (precipitation and temperature) projections to predict site-specific changes in flow regime classes expected by 2100. Thirty-three per cent of the 601 sites were predicted to change to a different flow regime class by 2100. Snow-fed streams in the western USA were predicted to be less likely to change regimes, whereas both small, perennial, rain-fed streams and intermittent streams in the central and eastern USA were predicted to be most likely to change regime.
Stream classification provides a means to understand the diversity and distribution of channels and floodplains that occur across a landscape while identifying links between geomorphic form and process. Accordingly, stream classification is frequently employed as a watershed planning, management, and restoration tool. At the same time, there has been intense debate and criticism of particular frameworks, on the grounds that these frameworks classify stream reaches based largely on their physical form, rather than direct measurements of their component hydrogeomorphic processes. Despite this debate surrounding stream classifications, and their ongoing use in watershed management, direct comparisons of channel classification frameworks are rare. Here we implement four stream classification frameworks and explore the degree to which each make inferences about hydrogeomorphic process from channel form within the Middle Fork John Day Basin, a watershed of high conservation interest within the Columbia River Basin, U.S.A. We compare the results of the River Styles Framework, Natural Channel Classification, Rosgen Classification System, and a channel form-based statistical classification at 33 field-monitored sites. We found that the four frameworks consistently classified reach types into similar groups based on each reach or segment?s dominant hydrogeomorphic elements. Where classified channel types diverged, differences could be attributed to the (a) spatial scale of input data used, (b) the requisite metrics and their order in completing a framework?s decision tree and/or, (c) whether the framework attempts to classify current or historic channel form. Divergence in framework agreement was also observed at reaches where channel planform was decoupled from valley setting. Overall, the relative agreement between frameworks indicates that criticism of individual classifications for their use of form in grouping stream channels may be overstated. These form-based criticisms may also ignore the geomorphic tenet that channel form reflects formative hydrogeomorphic processes across a given landscape.
Indicator species are species that are used as ecological indicators of community or habitat types, environmental conditions, or environmental changes. In order to determine indicator species, the characteristic to be predicted is represented in the form of a classifi cation of the sites, which is compared to the patterns of distribution of the species found at the sites. Indicator species analysis should take into account the fact that species have diff erent niche breadths: if a species is related to the conditions prevailing in two or more groups of sites, an indicator species analysis undertaken on individual groups of sites may fail to reveal this association. In this paper, we suggest improving indicator species analysis by considering all possible combinations of groups of sites and selecting the combination for which the species can be best used as indicator. When using a correlation index, such as the point-biserial correlation, the method yields the combination where the diff erence between the observed and expected abundance/frequency of the species is the largest. When an indicator value index (IndVal) is used, the method provides the set of site-groups that best matches the observed distribution pattern of the species. We illustrate the advantages of the method in three diff erent examples. Consideration of combinations of groups of sites provides an extra fl exibility to qualitatively model the habitat preferences of the species of interest. Th e method also allows users to cross multiple classifi cations of the same sites, increasing the amount of information resulting from the analysis. When applied to community types, it allows one to distinguish those species that characterize individual types from those that characterize the relationships between them. Th is distinction is useful to determine the number of types that maximizes the number of indicator species.
Hough-Snee, N. and D.D. Cooper. 2011. The effect of perigynia removal on the germination of awl-fruit sedge, Carex stipata Muhl. Ex Willd (Cyperaceae). Native Plants Journal 12(1): 41-43.
Hough-Snee, N., A.L. Long and R. Pond. 2012. Passive soil manipulation influences the successional trajectories of forest communities at a denuded former campsite. Ecological Restoration 30(1): 9-12.