The influence of river regulation and land use on floodplain forest regeneration in the semi-arid Upper Colorado River Basin, USA

Department of Forest, Rangeland and Watershed Stewardship, Colorado State University, Fort Collins, Colorado, United States
River Research and Applications (Impact Factor: 2.03). 07/2007; 23(6):565 - 577. DOI: 10.1002/rra.1007


Flow regulation effects on floodplain forests in the semi-arid western United States are moderately well understood, whereas effects associated with changes in floodplain land use are poorly documented. We mapped land cover patterns from recent aerial photos and applied a classification scheme to mainstem alluvial floodplains in 10 subjectively selected 4th order hydrologic units (subbasins) in the Upper Colorado River Basin (UCRB) in order to document land use patterns (floodplain development) and assess their effects on Fremont cottonwood forest (CF) regeneration. Three of the mainstem rivers were unregulated, five were moderately regulated and two were highly regulated. We classified polygons as Undeveloped (with two categories, including CF) and Developed (with five categories). We ground-truthed 501 randomly selected polygons (4–28% of the floodplain area in each subbasin) to verify classification accuracy and to search for cottonwood regeneration, defined as stands established since regulation began or 1950, whichever is most recent. From 40% to 95% of the floodplain area remained undeveloped, but only 19–70% of the floodplain area was classified as forest. Regeneration occupied a mean of 5% (range 1–17%) of the floodplain. The likelihood of the presence of regeneration in a polygon was reduced 65% by development and independently in a complex manner by flow regulation. Our analyses indicate that floodplain forests may be in jeopardy on both regulated and unregulated rivers and that information on historical forest extent is needed to better understand their current status in the UCRB. Conservation efforts need to be coordinated at a regional level and address the potentially adverse affects of both flow regulation and floodplain development. Published in 2007 by John Wiley & Sons, Ltd.

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    • "In combination , these indirect effects can have profound influences on vegetation communities that go beyond direct land use conversion from natural to anthropogenic vegetation types (Pedroli and Borger 1990; Schoorl and Veldkamp 2001). Across the western United States there are numerous examples of ecological systems that have undergone extreme and sometimes irreversible change due to the combination of human activities following agricultural establishment and settlement (Patten 1998; Stromberg 2001; Rood et al. 2003; Elmore, Mustard, et al. 2006; Northcott, Andersen, and Cooper 2007). "
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    ABSTRACT: In arid regions of the world, the conversion of native vegetation to agriculture requires the construction of an irrigation infrastructure that can include networks of ditches, reservoirs, flood control modifications, and supplemental groundwater pumping. The infrastructure required for agricultural development has cumulative and indirect effects, which alter native plant communities, in parallel with the direct effects of land use conversion to irrigated crops. Our study quantified historical land cover change over a 150-year period for the Walker River Basin of Nevada and California by comparing direct and indirect impacts of irrigated agriculture at the scale of a 10,217 km(2) watershed. We used General Land Office survey notes to reconstruct land cover at the time of settlement (1860-1910) and compared the settlement-era distribution of land cover to the current distribution. Direct conversion of natural vegetation to agricultural land uses accounted for 59 percent of total land cover change. Changes among nonagricultural vegetation included shifts from more mesic types to more xeric types and shifts from herbaceous wet meadow vegetation to woody phreatophytes, suggesting a progressive xerification. The area of meadow and wetland has experienced the most dramatic decline, with a loss of 95 percent of its former area. Our results also show Fremont cottonwood, a key riparian tree species in this region, is an order of magnitude more widely distributed within the watershed today than at the time of settlement. In contrast, areas that had riparian gallery forest at the time of settlement have seen a decline in the size and number of forest patches.
    Annals of the Association of American Geographers 05/2012; 102(3-3):531-548. DOI:10.1080/00045608.2011.641479 · 2.09 Impact Factor
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    • "Many studies have been conducted in flooded bottomland forests of the mesic Southeastern U.S. (Conner et al., 1997; Mitsch et al., 1991; Megonigal et al., 1997; Burke et al., 1999; Anderson and Mitsch, 2008). Studies in arid and semi-arid climates tend to focus on drought and/or water regulation impacts to riparian communities (e.g., Scott et al., 1999; Shafroth et al., 2002; Northcott et al., 2007), and dendroecological studies generally consider growth of individual trees (e.g., Stromberg and Patten, 1990). However, whole-stand productivity studies in the wetland forests of droughtprone areas are less common. "
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    ABSTRACT: In forested wetlands, hydrology exerts complex and sometimes compensatory influences on tree growth. This is particularly true in semi-arid ecosystems, where water can be both a limiting resource and a stressor. To better understand these relationships, we studied hydrologic and edaphic controls on the density, growth, tree architecture and overall productivity of forested wetlands dominated by the tree species Alnus glutinosa and Salix atrocinerea in Southern Europe. We sampled 49 plots set within 21 stands in the Atlantic coastal zone of the Iberian Peninsula, and quantified woody composition, size structure (diameter and height), and radial growth using dendrochronology. Plots were grouped into three saturation classes to compare tree growth characteristics (tree density, degree of sprouting, live basal area and productivity) across levels of saturation. We used Principal Component Analysis (PCA) to create integrated explanatory factors of hydrology, soil nutrient status and soil texture for use in linear mixed models to predict stand characteristics. Increased site saturation favoured a shift in species dominance from Alnus to Salix and resulted in a higher degree of multi-stemmed tree architecture (‘shrubbiness’), particularly for Alnus. Radial growth was negatively correlated with long-term soil saturation; however, annual productivity on a per-tree basis varied by species. Alnus growth and tree density were negatively correlated with waterlogging and fine-textured soils, possibly due to anaerobiosis in the rooting zone. In contrast, Salix growth was more influenced by nutrient limitation. Overall site productivity as measured by annual basal area increment decreased with prolonged saturation. In summary, soil saturation appears to act as a chronic stressor for tree species in this ecosystem. However, these species persist and maintain a dominant canopy position in the most permanently flooded patches through increased sprouting, albeit at a reduced rate of overall biomass accumulation relative to well-drained sites. The diversity in functional responses among wetland forest species has important implications for the conservation and management of these ecosystems. The sustainable management of these ecosystems is directly tied to their vulnerability to changing hydrological conditions. Non-equilibrium modifications to the hydrologic regime from land use and climate change, which are particularly severe in semi-arid regions, may further decrease productivity, integrity and resilience in these stress-adapted communities.
    Forest Ecology and Management 04/2010; 259(10-259):2015-2025. DOI:10.1016/j.foreco.2010.02.012 · 2.66 Impact Factor
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    ABSTRACT: Summary1. Hydropower is often presented as a clean and renewable energy source that is environmentally preferable to fossil fuels or nuclear power. Hydropower production, however, fundamentally transforms rivers and their ecosystems by fragmenting channels and altering river flows. These changes reduce flow velocity and the number of rapids, and reduce or alter wetland, floodplain and delta ecosystems. Dams disrupt dispersal of riverine organisms and sediment dynamics and may alter riverine biodiversity composition and abundance. Freshwater ecosystems now belong among the world’s most threatened ecosystems.2. Water managers are beginning to recognise the need to combine demands for social and economic development with the protection of the resource base on which socio-economic benefits rely. Environmental flows can help to balance ecosystem and human needs for water, both when constructing new dams and in re-licensing existing dams.3. We briefly review the impacts of hydropower generation on freshwater ecosystems by discussing different types of dams and development, and by providing examples from Sweden of how environmental effects of hydropower production could be mitigated. Special emphasis is given to flow regulation through re-operation of dams.4. Regulated rivers in Sweden were developed with little consideration of ecological effects, with most dams lacking migration pathways or minimum flow releases. There is thus a substantial potential for improvement of ecological conditions, such as naturalisation of flow regimes and reestablishment of connectivity, in regulated river reaches but technical hurdles imply major challenges for rehabilitation and mitigation. Most regulated rivers consist of cascades of consecutive reservoirs and impoundments, further constraining possible actions to improve ecological conditions.5. Most environmental mitigation measures require flow modifications to serve ecosystems, implying reduced power production. An important challenge for river management is to identify situations where measures involving relatively small production losses can have major ecological advantages.6. Climate change during the 21st century is expected to increase runoff in northern and central Sweden and make the annual hydrograph more similar to variation in electricity demand, i.e. a lower spring flood and increased run-off during winter months. This could provide opportunities for operating dams and power stations to the benefit of riverine ecosystems. On the other hand, demands to produce hydropower are likely to increase as fossil fuels are phased out, leading to increased pressures on free-flowing rivers and aquatic ecosystems.
    Freshwater Biology 12/2009; 55(1):49 - 67. DOI:10.1111/j.1365-2427.2009.02241.x · 2.74 Impact Factor
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