Ecological indicators and ecosystem function in large river-floodplain landscapes

Contacts

Monica G. Turner

Research Overview

The Wisconsin River Floodplain project aimed to identify landscape indicators that are well correlated with specific aspects of ecological function is a crucial research need requiring an integrated approach that combines landscape monitoring with field studies. Large river-floodplain systems are among the most diverse and dynamic landscapes in the US, providing many important societal values, but relatively little effort has been devoted to development and testing of landscape indicators for these systems.

We developed and tested ecological indicators for large river-floodplain landscapes along reaches of the Wisconsin River to determine which landscape metrics are most useful for monitoring population, community and ecosystem processes in large river-floodplain landscapes. Spatially extensive field sampling was combined with landscape analysis in nine reaches of the Wisconsin River sampling to quantify the ability of landscape indicators to predict ecological variables over broad scales. Landscape indicators were evaluated by their utility for detecting changes in the structure and function of the Wisconsin River floodplain landscape that were related to modification of the natural flow regime, historical land use, and current land-use patterns. Our field studies were concentrated in floodplain forest in nine 12 to 20-km reaches along the lower 400 km of the Wisconsin River.

Key Findings

• Analyses of current and historic landscape patterns in the floodplain of the Wisconsin River demonstrated that natural cover types (forest and wetland) were dominant and well connected, but that significant areas of the floodplain that are currently occupied by forest had been in nonforest cover earlier during the 20th century.
 
• Forest vegetation varied with flooding and past land cover, but current landscape pattern explained little variation. Indicators of physiography (e.g., geographic province) and flooding regime (e.g., relative elevation and distance from main channel) were consistently important in predicting occurrence, community composition, and abundance of trees in the Wisconsin River floodplain, although the direction of effects varied among species. Correspondence analysis revealed that flood-tolerant and intolerant species segregated along the primary axis, and late-successional species tended to segregate from flood-tolerant species along the secondary axis. Current landscape configuration was not usually important for species presence or abundance, except in forests that developed during recent decades.  However, land-cover history was important for tree species presence and for abundance of late-successional species.
 
• The presence and abundance of invasive shrubs were characterized by both habitat configuration and quality. Our models provided a much stronger prediction of species presence than species abundance. Although each species had similar life forms, models differed among species and seem to be driven by differences in flood tolerances.
 
• Analyses of the bird data have focused on the effects of landscape context of forest patches and prior land uses. Overall, typical measures of habitat fragmentation explained a relatively small portion of the variation in the structure and composition of the bird communities that we studied. These results may reflect the fact that riparian forests still occupy a relatively large portion of the floodplain and also tend to be naturally fragmented.  Distance from the confluence with the Mississippi River showed a strong negative correlation with species richness and abundance for several avian guilds, particularly woodpeckers, suggesting that forests along the Mississippi may serve as a population source for some species. Patterns of habitat use by individual bird species reinforced some of the conclusions drawn from guild- and community-based studies.  For instance, variables describing regional variation dominated models for most floodplain birds. Overall, these results suggest that at least some species respond in a fundamentally different way to landscape measures such as patch size or proximity to habitat edge in floodplains as opposed to upland habitats.  Although landscape measures were included in models for some species, the strongest gradient associated with the Wisconsin River apparently occurs at regional, rather than landscape scales.
 
• Denitrification was extremely variable within and among reaches; rates and coefficients of variation ranged from 0 to 65.14 ng N g-1 h-1 and 150 to 198%, respectively, and differences between reaches were not significant.  Strongest correlates with denitrification shifted between organic matter and moisture among reaches, and usually explained < 30% of observed variation.  All reaches had a small number of samples with high rates, and several samples with low or zero values.  These results suggest a fruitful avenue for future research is to examine landscape attributes that may be related to the frequency or probability of occurrence of sites with high denitrification rates (hotspots) rather than trying to identify patterns that consider the full range of variation within or among reaches.
 
• Levees constructed in the early 1900s in one of the reaches used in this study offered an outstanding opportunity for more intensive studies of the effects of river flow modification on ecological responses.  Vegetation studies conducted on the riverine and upland sides of the levee revealed compositional differences related to the levee, with flood-intolerant tree species having greater abundance upland of the levee (Gergel et al. in press). However, tree growth rates varied among species but did not differ significantly with position relative to the levee, suggesting that compositional differences may be an important driver of productivity. No significant differences in the amount of organic matter or coarse woody debris were observed outside the levee compared to areas inside the levee.  However, significantly higher microbial activities (dehydrogenase, ß-glucosidase, phosphatase) per g organic matter were found for soils inside the levee. These results suggest that modification of the hydrologic regime by levees may alter not only the structure of plant communities but also the organic matter dynamics of floodplain.

The landscape indicators tested in this study explained relatively little of the variability in the forest community structure, the bird community, and soil indicators including denitrification.  We hypothesize that this may be due, in part, to the relatively high spatial connectivity of natural habitat types that characterize the Wisconsin River floodplain combined with our focus on sampling within the forest habitat.  Landscape indicators may be more important in floodplains in which natural habitats are greatly reduced and fragmented, as has been demonstrated in other landscapes.  Furthermore, ecological responses within the river channel (e.g., nitrogen concentrations) may be more sensitive to the occurrence and distribution of gaps in natural habitats that would be reflected in landscape indicators.  However, landscape indicators were very useful in exploring alternative land-acquisition scenarios in which relative increases in habitat connectivity for a given increase in habitat extent could be compared.

Acknowledgements

Funding provided by Environmental Protection Agency STAR Program

Selected Publications (updated April 2008)

Bürgi, M. and M. G. Turner. 2002. Factors and processes shaping land cover and land cover changes along the Wisconsin River, USA. Ecosystems 5:184-201.

Dixon, M. D. 2003. Effects of flow pattern on riparian seedling recruitment on sandbars in the Wisconsin River, Wisconsin, USA. Wetlands 23:125-139.

Dixon, M. D., M. G. Turner and C. Jin. 2002. Distribution of riparian tree seedlings on Wisconsin River sandbars: controls at different spatial scales. Ecological Monographs 72:465-485.

Dixon, M. D. and M. G. Turner. 2006. Simulated recruitment of riparian trees and shrubs under natural and regulated flow regimes on the Wisconsin River, USA. River Research and Applications 22: 1057-1083.

Forshay, K.J. and E.H. Stanley. 2005. Rapid nitrate loss and denitrification in a temperate river floodplain. Biogeochemistry 75:43-64.

Freeman, R. E., E. H. Stanley and M. G. Turner. 2003. Analysis and conservation implications of landscape change in the Wisconsin River floodplain, USA. Ecological Applications 13:416-431.

Freeman, R.E. and R.O. Ray. 2001. Landscape ecology practice by small scale river conservation groups. Landscape and Urban Planning 56:171-184.

Gergel, S.E. 2002. Cumulative impact of levees and dams on the duration of temporary floodplain ponds: a terrain model approach for assessing multiple disturbances at broad scales. Ecological Applications 12:1740-1754.

Gergel, S.E., S.R. Carpenter, and E.H. Stanley. 2006. Do dams and levees impact nitrogen cycling? Simulating the effects of flood alterations on floodplain denitrification. Global Change Biology: 11:1352-1367.

Gergel, S. E., M. D. Dixon and M. G. Turner. 2002. Consequences of human-altered floods: levees, floods and floodplain forests along the Wisconsin River. Ecological Applications 12:1755-1770.

Gergel, S. E., M. G. Turner, J. R. Miller, J. M. Melack and E. H. Stanley. 2002. Landscape indicators of human impacts to river-floodplain systems. Aquatic Sciences 64:118-128.

Kang, H., E.H. Stanley, and S.S. Park. 2003. A sensitive method for the measurement of ammonium in soil extract and water using an OPA compound. Communications in Soil Science and Plant Analysis 34:2193-2201.

Kang, H. and E.H. Stanley. 2004. Effects of levees on soil microbial activities in a large river floodplain. River Research and Management 20:1-7.

Miller, J. R., M. D. Dixon, and M. G. Turner. 2004. Response of avian communities in large-river floodplains to environmental variation at multiple scales. Ecological Applications 14:1394-1410.

Miller, J. R., M. G. Turner, E. H. Stanley, L. C. Dent and E. A. H. Smithwick. 2004. Spatial extrapolation: the science of predicting ecological patterns and processes. BioScience 54:310-320.

Predick, K. I. and M. G. Turner. 2008. Landscape configuration and flood frequency influence invasive shrubs in floodplain forests of the Wisconsin River (USA). Journal of Ecology (In press).

Predick, K. I., S. E. Gergel and M. G. Turner. Effect of flood regime on tree growth in the floodplain and surrounding uplands of the Wisconsin River. River Research and Applications (In press).

Turner, M. G., S. E. Gergel, M. D. Dixon and J. R. Miller. 2004. Distribution and abundance of trees in floodplain forests of the Wisconsin River: environmental influences at different scales. Journal of Vegetation Science 15:729-738.

Turner, M. G., E. H. Stanley, M. Bürgi and D. J. Mladenoff. 2008. Changes in the Wisconsin River and its floodplain. In: D. M. Waller and T. P. Rooney, editors. The vanishing present: ecological change in Wisconsin. University of Chicago Press (In press).