Does red alder ( Alnus rubra ) in upland riparian forests elevate macroinvertebrate and detritus export from headwater streams to downstream habitats in southeastern Alaska?

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<ul><li><p>Does red alder (Alnus rubra) in upland riparianforests elevate macroinvertebrate and detritusexport from headwater streams to downstreamhabitats in southeastern Alaska?</p><p>Jack J. Piccolo and Mark S. Wipfli</p><p>Abstract: We assessed the influence of riparian forest canopy type on macroinvertebrate and detritus export fromheadwater streams to downstream habitats in the Tongass National Forest, southeastern Alaska. Twenty-four fishlessheadwater streams were sampled monthly, from April to August 1998, across four riparian canopy types: old growth,clearcut, young-growth alder, and young-growth conifer. Young-growth alder sites exported significantly greater count(mean = 9.4 individualsm3 water, standard error (SE) = 3.7) and biomass (mean = 3.1 mg dry massm3 water, SE =1.2) densities of macroinvertebrates than did young-growth conifer sites (mean = 2.7 individualsm3 water, SE = 0.4,and mean = 1.0 mg dry massm3 water, SE = 0.2), enough prey to support up to four times more fish biomass ifdownstream habitat is suitable. We detected no significant differences in macroinvertebrate export between other can-opy types or in detritus export among different canopy types. Roughly 70% of the invertebrates were aquatic; the restwere terrestrial or could not be identified. Although we do not recommend clearcutting as a means of generating redalder, maintaining an alder component in previously harvested stands may offset other potentially negative effects oftimber harvest (such as sedimentation and loss of coarse woody debris) on downstream, salmonid-bearing food webs.</p><p>Rsum: Nous avons valu linfluence du type de couverture forestire de la rive sur lexportation de macroinvert-brs et de dtritus depuis des ruisseaux damont vers les habitats daval dans la fort nationale de Tongass dans le sud-est de lAlaska. Vingt-quatre ruisseaux damont dpourvus de poissons ont t chantillonns tous les mois, davril aot 1998, dans quatre types de couverture vgtale riparienne: fort mature, surface coupe blanc, jeune fortdaulnes et jeune fort de conifres. Les sites de fort jeune aulnes exportent significativement plus de macroinvert-brs, en nombre (moyenne = 9,4 individusm3 deau, erreur type (SE) = 3,7) et en biomasse (moyenne = 3,1 mg demasse schem3 deau, SE = 1,2) que les sites de fort jeune conifres (moyenne = 2,7 individusm3 deau, SE =0,4; moyenne = 1,0 mg de masse schem3 deau, SE = 0,2), soit assez de proies pour subvenir des biomasses depoissons quatre fois plus grandes, condition que les habitats daval soient convenables. Il ny a pas de diffrencessignificatives dcelables dexportation de macroinvertbrs entre les autres types forestiers, ni de diffrencesdexportation de dtritus entre les quatre types de couverture forestire. En gros, 70 % des invertbrs sont aquatiques;les autres sont terrestres ou impossibles identifier. Bien que nous ne recommandions pas la coupe blanc commemoyen de favoriser laulne rouge, le maintien dune composante daulnes dans les secteurs dj coups pourrait contre-balancer les effets potentiellement ngatifs de la coupe du bois, tels que la sdimentation et la perte de dbris ligneuxgrossiers, sur les rseaux alimentaires daval qui contiennent des salmonids.</p><p>[Traduit par la Rdaction] Piccolo and Wipfli 513</p><p>Introduction</p><p>Headwater habitats play a critical role in the structure andfunction of downstream ecosystems (Haigh et al. 1998). Theheadwater streams that drain these habitats transport water,</p><p>sediment, debris, and invertebrates (Cuffney and Wallace1988; Naiman et al. 1992; Wipfli and Gregovich 2002), in-fluencing downstream habitats and food webs. Physical andbiological attributes that may influence the productivity ofheadwater streams and the material available for export in-</p><p>Can. J. Fish. Aquat. Sci.59: 503513 (2002) DOI: 10.1139/F02-019 2002 NRC Canada</p><p>503</p><p>Received 5 July 2001. Accepted 12 February 2002. Published on the NRC Research Press Web site at http://cjfas.nrc.ca on6 April 2002.J16435</p><p>J.J. Piccolo1,2 and M.S. Wipfli. 1,3 Pacific Northwest Research Station, U.S. Department of Agriculture Forest Service,2770 Sherwood Lane, Suite 2A, Juneau, AK 99801-8545, U.S.A.</p><p>1Corresponding authors (e-mail: ftjjp1@uaf.edu, mwipfli@fs.fed.us).2Present address: University of Alaska Fairbanks, School of Fisheries and Ocean Sciences, Juneau Fisheries Center,11120 Glacier Highway, Juneau AK 99801, U.S.A.</p><p>3Present address: Pacific Northwest Research Station, U.S. Department of Agriculture Forest Service, 1133 N. Western Avenue,Wenatchee, WA 98801, U.S.A.</p><p>J:\cjfas\cjfas59\cjfas-03\F02-019.vpThursday, April 04, 2002 1:49:57 PM</p><p>Color profile: DisabledComposite Default screen</p></li><li><p>clude catchment size, geology, soil type, aspect, gradient,temperature, precipitation, and plant and animal communi-ties (Naiman et al. 1992). Natural or human-induced distur-bances can alter these attributes (Resh et al. 1988), therebyaltering productivity in both headwater streams and down-stream habitats.</p><p>Stone and Wallace (1998) identify clearcut logging as along-term disturbance that alters stream ecosystems. Theirliterature review identifies the following effects of clear-cutting on stream macroinvertebrate communities: changesin stream temperature, stream flow, primary production, andcommunity structure. The short-term effects of clearcuttingmay include reduced allochthonous input and increased auto-chthonous production, which alter macroinvertebrate com-munities and, consequently, detritus export. The long-termeffects of clearcutting on macroinvertebrate export fromheadwater streams to downstream food webs, however, havenot been addressed.</p><p>In the temperate rainforest of southeastern Alaska, clear-cutting changes terrestrial and aquatic productivity (Alaback1982; Duncan and Brusven 1985) and the energy flow fromterrestrial to aquatic habitats (Wipfli 1997). Canopy removalincreases the amount of light reaching the forest floor, in-creasing the productivity of understory plants (Alaback1982) and streams (Duncan and Brusven 1985; Hetrick et al.1998). Following clearcutting, forest succession often in-cludes the development of red alder (Alnus rubra) stands inthose places where there has been enough surface distur-bance to expose mineral soils (Newton and Cole 1994). Theestablishment of alder may increase understory plant diver-sity (Deal 1997) and abundance (Hanley and Barnard 1998),and alder-dominated riparian canopies may also providemore terrestrial invertebrates as prey for juvenile salmonidsthan do conifer-dominated canopies (Wipfli 1997). Althoughthe productivity of terrestrial habitat declines as a dense co-niferous canopy develops in later successional stages(Alaback 1982), little information exists on the influence ofriparian forest succession on headwater streams and associ-ated export of material.</p><p>Forest management plans for southeastern Alaska includetimber harvesting in forested headwaters (U.S. Departmentof Agriculture (USDA) Forest Service 1997). These headwa-ters contain many small, fishless streams (Swanston 1967),classified by the USDA Forest Service as classes III and IV.Riparian buffer protection during logging operations variesalong these streams; most class III streams receive somebuffer protection, but few class VI streams do (USDA ForestService 1997). Many of these streams drain into salmonid-rearing habitats, and it is important to understand how tim-ber harvest and forest regeneration in upland forests influ-ence downstream food webs, including the flow of energy(invertebrates and detritus) to these habitats.</p><p>Our objective was to measure the effects of different ripar-ian forest canopy types on macroinvertebrate (aquatic and ter-restrial) and organic detritus export from headwater streams todownstream habitats. We tested the null hypothesis that dur-ing the salmonid growing season (AprilAugust), headwaterstreams with old growth, clearcut, young-growth alder, andyoung-growth conifer riparian canopies do not differ in theamount (number and mass) of macroinvertebrates and detritusthey export.</p><p>Methods</p><p>Study siteResearch was conducted in small (mean discharge</p><p>250m) export monthly,from April to August 1998, in 24 streams characterized byfour distinct riparian canopy types: old growth (OG); clear-cut (CC), </p></li><li><p>for data analysis. Discharge was measured by using a stop-watch to count the number of seconds it took for water fromthe dam outflow pipe to fill a known-volume container.Mean discharge for each stream during each sampling periodwas calculated by averaging measurements taken at 0, 48,and 96 h (3 replicates at each time). During occasional highflows, discharge sometimes exceeded pipe capacity and wewere able to sample only a portion of the stream flow. Inthese instances, discharge through the pipe was measured asdescribed above, the percentage of total streamflow flowingthrough the pipe (typically 90% or greater) was visually esti-</p><p>mated, and total discharge was calculated using thiscorrection factor.</p><p>Samples were sorted under a 7 dissecting microscope.Macroinvertebrates were identified to lowest reliable taxon,measured to the nearest millimetre (total length excludingantennae and cerci), and enumerated. Unusually large sam-ples were subsampled. We used a subsampler that had alarge, funnel-shaped container to which we added 16 L ofwater containing the sample and through which a continu-ous, vigorous stream of air was injected from the bottom toensure uniform mixing and distribution of the sample. Part</p><p> 2002 NRC Canada</p><p>Piccolo and Wipfli 505</p><p>Fig. 1. Map of Alaska, U.S.A., with inset showing the Harris and Maybeso drainages (13267W, 5549N) on eastern Prince of WalesIsland. The drainage basin boundaries are represented by the dotted line. Both drainages empty into salt water in Twelvemile Arm(shaded area). Study streams are identified by canopy type: old growth (OG), clearcut (CC), young-growth alder (YA), and young-growth conifer (YC).</p><p>J:\cjfas\cjfas59\cjfas-03\F02-019.vpThursday, April 04, 2002 1:49:59 PM</p><p>Color profile: DisabledComposite Default screen</p></li><li><p>of the sample (either 8 L or 4 L) wastaken with a smallerknown-volume container, and this portion was analyzed. Re-sults were then multiplied by two or four to obtain a full-sample estimate. Macroinvertebrate biomass (mg dry mass)was estimated for individual invertebrates by using eitherpublished taxa-specific lengthweight regression equations(e.g., Rogers et al. 1977; Sample et al. 1993; Burgherr andMeyer 1997) or our own equations. This was done for a pre-vious study (Wipfli and Gregovich 2002) by drying a sampleof known-length invertebrates of a given taxa (at 60C for24 h), weighing individuals (nearest 0.1 mg), and developinga lengthweight regression (Smock 1980). Detritus was dried(at 60C for 48 h), weighed, burned to ash (at 500C for 5 h),and reweighed to determine ash-free dry mass (AFDM), theamount of organic, nonmineral material in the sample.</p><p>Physical habitat and riparian canopy measurementsStream channel gradient, active channel width, canopy</p><p>coverage, and understory vegetation type and density weresurveyed along a 300-m reach of each stream directly up-stream from the sample stations. Gradient was measuredwith a hand-held clinometer and canopy coverage was esti-mated every 20 m along the stream using a circular viewingtube (10-cm inside-diameter 30-cm long). To estimate can-opy coverage and composition, the observer stood in thecenter of the stream channel, looked through the verticallyheld tube, and recorded the percentage of alder, conifer, andopen canopy. Foliage density (scale: 0 = none, 1 = sparse,2 = low, 3 = moderate, 5 = high) and relative abundance (topfive plant species in order of abundance) of understoryshrubs were estimated visually for a 2-m-wide band alongeach stream bank (Table 1).</p><p>Statistical analysisA completely randomized design was used incorporating</p><p>canopy type (OG, YA, YC, CC) as the main factor, and 24streams were sampled each month (AprilAugust). We useda split-plot analysis of variance (ANOVA) (SAS 1990) totest for significant effects of canopy type, month, and can-opy type month interaction ( &lt; 0.1). We used a split-plot</p><p>analysis because we were interested in the main effect ofcanopy type over the entire growing season (five monthscombined) and because we did not expect any carry-over ef-fects between months (Winer et al. 1991). To further iden-tify what differences might be driving the results of the maineffects test, statistical comparisons between individual can-opy types were tested using four a priori contrasts: OG vs.CC, OG vs. YA, OG vs. YC, and YA vs. YC. These con-trasts were chosen so that we could detect differences be-tween logged and unlogged areas (OG vs. CC, OG vs. YA,OG vs. YC) and differences between canopy types that com-monly regenerate after logging (YA vs. YC). Alpha for thecontrasts was set-wise at 0.025 (the comparison-wise errordivided by a Bonferroni correction factor of four) becausethe four contrasts were not orthogonal (we did not performall possible contrasts with the four canopy types) (SAS1990). We did not contrast individual months, because wewere interested in the effects of riparian canopy across theentire 5-month period. Response variables were total countdensity (number of macroinvertebratesm3 water), total bio-mass density (mg dry mass of macroinvertebratesm3 water),and detritus density (mg AFDMm3 water). All responsevariables were logarithmic transformed (ln(x + 0.1)) to meetANOVA assumptions of normally distributed residuals andequal variances among groups.</p><p>Results</p><p>Total count density differed significantly by canopy type(p &lt; 0.1; Table 2), and YA sites exported significantly moremacroinvertebrates than did YC sites (p &lt; 0.025; Fig. 2a, Ta-ble 2). There were no significant differences between othercontrasted canopy types (OG vs. CC, YA, YC). Total bio-mass density did not differ significantly among canopy typesin the overall test (p &gt; 0.1; Table 2), but YA sites exportedsignificantly more total biomass density than did YC sites(p &lt; 0.025; Fig. 2b, Table 2). There were no significant dif-ferences between other canopy types contrasted (OG vs. CC,YA, YC).</p><p> 2002 NRC Canada</p><p>506 Can. J. Fish. Aquat. Sci. Vol. 59, 2002</p><p>Canopy type</p><p>Parameter OGa CC YA YCMean</p><p>Stream reach length (m) 250 (260) 237 273 204Channel width (m) 1.8 (2.7) 1.2 1.9 1.4Gradient (%) 21.0 (20.4) 22.9 21.7 22.2Discharge (Ls1) 0.4 (3.5) 0.7 1.2 0.9Stream temperature (C) 7.7 (7.5) 8.5 9.0 8.2% open canopy 32 (36) 100 14 26% alder canopy </p></li><li><p>Both total count and biomass density differed significantlyby month, and both displayed a significant canopy type month interaction (p &lt; 0.1; Table 2). Total count densitypeaked in June and dropped off in July and August, althoughtemporal patterns varied among canopy types (Fig. 3a). To-tal biomass density peaked in June as well and dropped offsharply by August (Fig. 3b).</p><p>Detritus export did not vary significantly by treatment(p &gt; 0.1; Table 2), and no significant differences were found</p><p>between the canopy types contrasted (p &gt; 0...</p></li></ul>

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