ROOT BIOMASS IN RELATION TO CHANNEL MORPHOLOGY OF HEADWATER STREAMS

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  • JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATIONVOL. 37, NO. 6 AMERICAN WATER RESOURCES ASSOCIATION DECEMBER 2001

    ROOT BIOMASS IN RELATION TO CHANNELMORPHOLOGY OF HEAD WATER STREAMS'

    Zachary 0. Toledo and J. Boone Kauffman2

    ABSTRACT: Intact riparian zones are the product of an incrediblycomplex multitude of linkages between the geomorphic, hydrologic,and bioti features of the ecosystem. Land-use activities that severor alter these linkages result in ecosystem degradation. We exam-ined the relationship between riparian vegetation and channel mor-phology by sampling species composition and herbaceous rootbiomass in incised (down-cut and widened) versus unincised(intact) sections of unconstrained reaches in three headwaterstreams in northeastern Oregon. Incision resulted in a composition-al shift from wetland-obligate plant species to those adapted todrier environments. Root biomass was approximately two timesgreater in unincised sections than incised sections and decreasedwith depth more rapidly in incised sections than in unincised sec-tions. Thtal root biomass ranged from 2,153 g m2 to 4,759 g m2 inunincised sections and from 1,107 g m-2 to 2,215 g m-2 in incisedsections. In unincised sections less than 50 percent of the total rootbiomass was found in the top 10 cm, with approximately 20 percentin successive 10-cm depth increments. In contrast, incised sectionshad greater than 60 percent of the total root biomass in the top 10cm, approximately 15 percent in the 10 to 20 cm depth, less than 15percent in the 20 to 30 cm depth, and less than 10 percent in the 30to 40 cm depth. This distribution of root biomass suggests a positivefeedback between vegetation and channel incision: as incision pro-gresses, there is a loss of hydrologic connectivity, which causes ashift to a drier vegetation assemblage and decreased root structure,resulting in a reduced erosive resistance capacity in the lower zoneof the streambank, thereby allowing further incision and widening.(KEY TERMS: riparian ecosystems; erosion sedimentation; rootbiomass; root distribution; channel morphology; incision.)

    INTRODUCTION

    Riparian areas are zones of direct interactionbetween terrestrial and aquatic ecosystems (Gregoryet al., 1991). While riparian areas comprise only 1 to 2percent of the land area in arid systems (Kauffman

    and Krueger, 1984), they are disproportionately sig-nificant in terms of biological production and diversity(Gregory et al., 1991; Naiman and Descamps, 1997;Patten 1998, Kauffman et al., 2001). Riparian areasare valuable to society through their multitude ofecosystem functions and processes, such as floodabatement, habitat for migratory birds and aquaticspecies (Naiman et al., 1993), maintenance of regionalbiodiversity (Naiman and Descamps, 1997), andwater quality control and nutrient cycling (Green andKauffman, 1989). Riparian areas have received con-siderable attention by scientists and managers (John-son et al., 1985; Abell, 1989; Clary et al., 1992;Tellman et al., 1993; Feller, 1998; Koehler andThomas, 2000; Wigington and Beschta, 2000) becauseof concerns centering on their widespread degradation[National Resource Council (NRC), 1992; Beschta,19971.

    Riparian areas throughout the western UnitedStates have been altered or degraded through land-use activities including hydrologic alterations(Dominick and O'Neill, 1998), beaver removal(Naiman et al., 1988), and livestock grazing (Fleischn-er, 1994; Dwire et al., 1999; Kauffman and Pyke,2001). Like many western landscapes, eastern Oregonriparian areas have been affected by additional land-use activities, including mining, logging, splash dams,and road building (McIntosh et al., 1994).

    Riparian-stream degradation occurs when hydro-logic, geomorphic, or biotic processes are disruptedsuch that interactions or linkages between these fea-tures are disrupted (Figure 1). For example, channelincision can sever linkages between floodplains andstreams, which then alters the biotic communities

    'Paper No. 01046 of the Journal of the American Water Resources Association. Discussions are open until August 1, 2002.-

    'Respectively, Fisheries Biologist, Mason, Bruce and Girard, Inc., 707 SW. Washington Street, Suite 1300, Portland, Oregon 97205; andProfessor, Department of Fisheries and Wildlife, Oregon State University, 104 Nash Hall, Corvallis, Oregon 97331 (E-MaillToledo: ztoledo@masonbruce.com).

    JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 1653 JAWRA

  • Toledo and Kauffman

    (Junk et al., 1989; Robertson et al., 2001). Incisionmay be expected when there is an imbalance betweenerosive and resistive forces acting on the bank andbed material (Schumm, 1999). Shifts in the forces act-ing on the bank can be caused by a change in thehydrology, geomorphology, or vegetation, which influ-ence each other in a positive feedback response (Fig-ure 1; Kauffman et al., 1997). The positive feedbackresponse can be described as follows: channel mor-phology influences the water availability to the ripari-an area (floodplain) (Leopold and Maddock, 1953),water availability affects plant species compositionthat comprises the riparian communities (Hupp andOsterkamp, 1996; Otting, 1998; Chapin et al., 2000),and both aboveground and belowground vegetationcomponents affect the channel morphology by influ-encing both erosive and resistive forces (Hickin, 1984;Thorne, 1990; Hupp, 1999).

    Figure 1. Linkages Between Geomorphology; Hydrology,and the Biota. Altering one of these features will result

    in a positive feedback response to the others(modified from Kauffman et al., 1997).

    Vegetation has been shown to strongly influencechannel morphology (Gregory and Gurnell 1988).Hickin and Nanson (1984) found that unvegetatedchannels could exhibit double the lateral channelmigration rates of vegetated channels. Smith (1976)found that bank sediment with a 5 cm root mat and aroot volume of 16 to 18 percent had 20,000 times theresistance of equivalent bank sediment without vege-tation or roots. Clifton (1989) reported that whenWickiup Creek (in central Oregon) was allowed to re-vegetate following livestock exclusion, the channelgained 60 cm of sediment within ten years and the

    channel cross-sectional area had decreased by 94 per-cent after 50 years.

    We hypothesized that there are four potentialresponses of root biomass to a decrease in water avail-ability associated with channel incision (Figure 2).There can be: (a) a lower total root biomass but with asimilar distribution within the soil horizons; (b) alower total root biomass and an increased rate of losswith depth; (c) no change in total root biomass or dis-tribution; or (d) an increased level of total rootbiomass. The second response, (b), would have thegreatest potential to affect channel morphology anderosion potential because there would be an overalldecrease in root biomass, particularly at greaterdepths. The objective of this study was to examine therelationships between riparian vegetation, rootbiomass, and channel morphology in incised andunincised stream channels.

    STUDY SITES

    The study reaches are located in the Blue Moun-tains of northeastern Oregon. During preliminaryreconnaissance, we examined 21 streams and selectedthree that met our criteria: an unconstrained alluvialchannel; a low level of recent human impact (i.e.,enhancement structures, livestock grazing, etc.); andthe presence of a hydrologic knickpoint (i.e., an areawhere an abrupt change of elevation and slope gradi-ent occurs (Brooks et al., 1997) that separates anupstream, unincised section from a downstream,incised section. Starting points of the sections werechosen with regards to tributary junctions, location ofknickpoints, and changes in valley form. Incised sec-tions were determined using previously collectedwater table data and/or physical parameters such asbank height and active channel width. Incised andunincised sections were similar in floodplain geomor-phology. The causes of channel incision likely includedroads, mining, and grazing.

    Crane Creek (445308"N, 11823'SO'W; elevation1,680 m) is a third-order tributary to the North ForkJohn Day River. The knickpoint was a road culvert;however, it was not determined whether the culvertinitiated or halted the headcut (knickpoint). Theunincised section was approximately 100 m upstreamfrom the incised section.

    Little Fly Creek (4503'45"N, 118'30'15'W; eleva-tion 1,460 m) is a third-order tributary to the GrandeRonde River. The knickpoint was also a culvert, whichhas halted the upstream migration of the headcut(Figure 3). The incised section was approximately200 m downstream of the unincised section.

    JAWRA 1 654 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION

  • Root Biomass in Relation to Channel Morphology of Headwater Streams

    Root Biomass

    /'______(a

    Root Biomass

    .

    bRoot Biomass

    7. /- (

    c

    Root Biomass

    D 7?( d

    Figure 2. Four Hypothesized Responses of Root Biomass to Channel Incision: (a) Less Overall Root Biomass ThanUnincised Stream, Same Rate of Loss With Depth; (b) Less Overall Root Biomass Than Unincised Stream, and anIncreased Rate of Loss With Depth; (c) No Change From an Unincised Stream; and (d) Increased Level of Biomass.

    1977

    Little Fly Creek

    1999Figure 3. The Incised Section of Little Fly Creek in 1977 (left) and 1999 (right). The man (circled) was standing

    at the headcut in 1977. The tree (circled) to the left of both photos provides reference and scale.Note the change in channel width and bank height - an obvious disconnect from the floodplain.

    JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 1655 JAWRA

  • Thiedo and Kauffman

    Squaw Creek (4507'44N, 1183223"W; elevation1,370 m) is a second-order tributary to the GrandeRonde River and has been the subject of a previ

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