Restoring Riparian Zone Function

Transcription

1 Restoring Riparian Zone Function What is riparian function? Ecosystem function can be defined as all of the processes necessary to preserve and create goods or services valued by humans. Functioning riparian zones control erosion, purify water, stabilize banks, regulate water temperature, delay floodwaters, sequesters carbon, groundwater recharge, and provides plant and animal habitat. As a riparian zone is degraded, having been reduced in quality or value, these basic goods and services humans rely upon can be fundamentally altered. Degradation in urban riparian environments can take many forms. Altered hydrology, or how the water from the land is delivered to the stream, is the primary disturbance driving reductions in ecosystem function. Altering the vegetation community, soil health, and width of the riparian zone can also lead to losses in ecosystem function. Any change in plant composition will bring about shifts in ecosystem function. Dominant species exert the most influence, and thus the greatest functional changes will occur if the abundance of these species is altered (Richardson et al. 2007). Riparian zone restoration attempts to restore the natural process necessary to maintain a high level of ecosystem function. In general, the larger the riparian buffer the more ecosystem functions it can provide (Figure 1). Figure 1: Shows the various buffer widths associated with riparian zone function. Organic inputs into the stream are important sources of nutrients and habitat (width ft). Stream stabilization is maintained by riparian vegetation (width ft). Water quality is the ability of the vegetation to intercept runoff, retain sediments, remove pollutants, and promote groundwater recharge (width ft). Flood control is the ability for the floodplain to intercept water and reduce peak flows (width ft). Riparian habitat is the ability of the buffer to support diverse vegetation and provide food and shelter for riparian and aquatic wildlife (width ft). Riparian Zone 1

2 What are methods for restoring riparian zones? There are three generalized approaches to restoring a disturbed riparian environment: (1) rely completely on passive (spontaneous succession), (2) exclusively adopt active, technical reclamation measures, (3) or a combination of both passive and active techniques toward a target goal (Hobbs and Prach 2008). Passively restored sites exhibit robust biota better adapted to site conditions with increased natural value and wildlife habitat than do actively restored sites (Hobbs and Prach 2008). Passive restoration requires minimal management and is more cost effective than alternative methods. However, passive restoration is often the slower approach and is more dependent on adjacent site conditions. When relying on spontaneous succession the vegetation community of adjacent sites, an approximate 100 meter distance from the disturbed site, is critical for successful Figure 2: Relative preference of spontaneous restoration (Hobbs and Prach 2008). In succession and technical reclamation along the general, passive restoration that relies on productivity stress gradient (Hobbs and Prach 2008). spontaneous succession should be employed when environmental disturbance is not very extreme (Figure 2) and no negative results (erosion, water contamination, negative aesthetic perception, etc ) are foreseen (Hobbs and Prach 2008). When site productivity and stress are extremely high or low, active (technical reclamation) may be necessary (Figure 2). The persistence of undesirable functional states is an indication that the system may be stuck and will require active intervention to move it to a more desirable state (Hobbs and Prach 2008). What is plant succession? Plant succession is defined as the cumulative change in the species that occupy a given area over time (Barbour et al. 1987). Primary succession is the establishment of plants on land not previously vegetated (e.g. after a volcanic eruption) and is relatively rare. Secondary succession is the establishment of plant species on land that was previously vegetated; but, through some disturbance event, either natural (e.g. fire) or human (e.g. agriculture), the vegetation was removed. The entire progression of seral stages, from the pioneer community up to the climax community, is called succession or a sere (figure 3) 2 Riparian Zone

3 (Barbour et al. 1987). A pioneer community is one that is able to first occupy bare ground following a disturbance event and are generally annual/ weedy species. A climax or mature community has had a relatively stable composition of plant species whereby the dominant plant community has existed for several hundred years. Climax communities are not static; rather, the ecosystem is both resistant and resilient to disturbance events. Resistance, the ability of an ecosystem to avoid significant changes in function and species during a disturbance event, and resilience, the ability of an ecosystem to quickly regain function and species following a disturbance, are both necessary components of a sustainable ecosystem. Overall, the main goal of all ecological restorations is to improve ecosystem function to the point at which a system is sustainable, both resistant and resilient to future disturbance events. Time Figure 3: General plant succession from bare soil to mature hardwood forest following passive restoration. Early pioneer/ weedy species are replaced by hardwood species over time. Illustration by Jacques Graber, adopted from Department of Resources Recycling and Recovery of California Visual indicators of riparian zone function Plant cover and structural diversity: High cover and structural diversity of vegetation (groundcover, understory and upper canopy) indicates a productive plant community, high species diversity, adequate food resources and habitat for wildlife, and reduced flooding impacts along stream banks (Stacey et al. 2006). Structural diversity is often absent in riparian areas that have been heavily damaged by human activities and can result in native wildlife species being extirpated from the area (Stacey et al. 2006). A diverse vegetation community is essential for the functioning and sustainability of an ecosystem (Alberti 2005). Riparian Zone 3

4 Figure 4: High cover and structural diversity of groundcover understory, and canopy vegetation is an indication of high species diversity, increased sediment retention, adequate wildlife habitat and overall improved ecosystem function. Wildflowers, grasses and other groundcover plants grow densely without canopy cover, thinning out as the canopy develops. Hardwood demography and ecological succession: Size and age class distribution of the dominant tree species indicates recruitment success and disturbance intervals. Missing age classes is often a result of disruptions to natural ecosystem processes and can result in successional changes and species loss (Stacey et al. 2006). Dominant species exert the most influence, and thus the greatest functional changes will occur if the abundance of these species is altered (Richardson et al. 2007). The understory (sapling) community reflects a habitats current ecological condition; while overstory (tree) communities are reminders of past environmental condition (Woosley et al. 2005). Understanding if the understory composition is significantly different from the overstory can be an indication of disturbance that could potentially alter succession and change ecological function. Figure 5: A healthy functioning riparian zone will contain all age classes of the dominant hardwood tree species. Seedlings are defined as 12 or less having sprouted within the last year, saplings are > 12 in 4 Riparian Zone

5 height but have yet to reach half its mature height and lack a full defined canopy, mature trees are approaching their maximum height and display a full developed canopy, and snags are dead trees with little to no vegetation and reduced canopy coverage often resulting from broken limbs. Illustration by Heidi Snell (Stacey et al. 2006). Riparian zone width: A wide riparian buffer has been shown to filter pollutants, control erosion, prevent flooding, and provide habitat and nutrient inputs into the stream (Fischer and Fischenich 2000). Generally, the larger the riparian zone buffer the greater the ecosystem function provided by that system. Understanding the relationship between the dominant tree species and riparian buffer width can help to identify potential ecosystem function. A riparian buffer width equal to the average height of the dominant tree species is able to provide adequate root strength (i.e. streambank stability), maximum leaf litter accumulation, close to 100% stream shading, and course material inputs (FEMAT 1993). As the buffer width increases to 3 times the average height of the dominant tree species, riparian forests are able to maintain microclimate function (FEMAT 1993). A forest system able to maintain its own microclimate can directly influence ecological processes such as plant regeneration and growth, soil respiration (nutrient cycling), wildlife habitat, species distribution, and disturbances (i.e. fire, drought, heat) (Chen et al. 1999). Figure 6: Site Potential Tree Height (SPTH) is defined as the average maximum height to which a dominant tree will grow if left undisturbed. Dominant trees possess the highest percent coverage (shadow cast by the vegetation layer) of the study plot. Riparian buffers of 1 SPTH provide bank stability, organic matter inputs, and stream shading. As the riparian buffer width approaches 3 SPTH microclimate function is sustained, resulting in improved plant regeneration and growth, nutrient cycling, wildlife habitat and buffering of environmental disturbances. Riparian Zone 5

6 Bird abundance and diversity: Bird abundance and diversity generally increases with canopy cover and can be an indication of improved forest conditions. Woodland and riparian dependent species have been shown to respond to changes in local vegetation composition (Oneal and Rotenberry 2009). Landscape managers can maximize native bird diversity and abundance by minimizing soil disturbances (compaction, erosion, moisture, etc ) and maintaining high plant cover and structural diversity within riparian zones. Monitoring for improved species diversity and abundance (bird activity or number of nest sites) is a clear indication of improved ecosystem function. Figure 5: Photo taken from Willowbrook Restoration site on Boggy Creek showing increased bird habitat with the presence of new nesting sites and increased bird activity. 6 Riparian Zone

7 Works Cited Alberti M The effects of Urban Patterns on Ecosystem Function. International Regional Science Review 28(2) Barbour, M. G., Burk, J. H., and Pitts, W. D Terrestrial Plant Ecology: Second Edition. Benjamin/Cummings Publishing Company Inc. Menolo Park, California Chen, J., Saunders, S. C., Crow, T. R., Naiman, R. J., Brosofske, K. D., Mroz, G. D., Brookshire, B. L., and Franklin, J. F Microclimate in Forest Ecosystem and Landscape Ecology. BioScience, 49(4): FEMAT, Forest ecosystem management: an ecological, economic, and social assessment, U.S. Government Printing Office, Washington, DC. Fischer, R.A. and Fischenich, J.C Design recommendations for riparian corridors and vegetated buffer strips. U.S. Army Engineer Research and Development Center, Environmental Laboratory. Vicksburg, MS.Mayer, P. M., Reynolds Jr., S. K., and Hobbs, R. J., and Prach, K Spontaneous Succession versus Technical Reclamation in the Restoration of Disturbed Sites. Restoration Ecology, Vol. 16, no. 3, pp Richardson, M. D., Holmes, P. M., Esler, K. J., Galatowitsch, S. M., Stromberg, J. C., Kirkman, S. P., Pysek, P., and Hobbs, R. J Riparian vegetation: degradation, alien plant invasion, and restoration prospects. Diversity and Distributions, 13: Stacey, P. B., Jones, A., Catlin, J. C., Duff, D. A., Stevens, L. E., and Gourley, C User's Guide for the Rapid Assessment of the Functional Condition of Stream Riparian Ecosystems in the American Southwest. Wild Utah Project. Tongway D. J., and Ludwig, J. A Restoring Disturbed Landscapes: Putting principles into practice. Society for Ecological Restoration International. Island Press, Washington DC. Riparian Zone 7