Does red alder (Alnus rubra) in upland riparianforests elevate macroinvertebrate and detritusexport from headwater streams to downstreamhabitats in southeastern Alaska?
Jack J. Piccolo and Mark S. Wipfli
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.
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.
[Traduit par la Rdaction] Piccolo and Wipfli 513
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,
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-
Can. J. Fish. Aquat. Sci.59: 503513 (2002) DOI: 10.1139/F02-019 2002 NRC Canada
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
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.
1Corresponding authors (e-mail: firstname.lastname@example.org, email@example.com).2Present address: University of Alaska Fairbanks, School of Fisheries and Ocean Sciences, Juneau Fisheries Center,11120 Glacier Highway, Juneau AK 99801, U.S.A.
3Present address: Pacific Northwest Research Station, U.S. Department of Agriculture Forest Service, 1133 N. Western Avenue,Wenatchee, WA 98801, U.S.A.
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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.
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.
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.
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.
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.
Study siteResearch was conducted in small (mean discharge
250m) export monthly,from April to August 1998, in 24 streams characterized byfour distinct riparian canopy types: old growth (OG); clear-cut (CC),
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-
mated, and total discharge was calculated using thiscorrection factor.
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
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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 typ