Headwater macroinvertebrate community response to riparian red alder (Alnus rubra Bong.) in southeast Alaska

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    Headwater macroinvertebrate community response to riparianred alder (Alnus rubra Bong.) in southeast AlaskaAuthor(s): R. K. Kimbirauskas, R. W. Merritt, M. S. Wipfli, and P. HennonSource: Pan-Pacific Entomologist, 84(3):220-237. 2008.Published By: Pacific Coast Entomological SocietyDOI: http://dx.doi.org/10.3956/2008-06.1URL: http://www.bioone.org/doi/full/10.3956/2008-06.1

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  • Headwater macroinvertebrate community response to riparian red alder(Alnus rubra Bong.) in southeast Alaska


    1Michigan State University, Department of Entomology, 243 Natural Science Building,

    East Lansing, MI 488242USGS Alaska Cooperative Fish and Wildlife Research Unit,

    Institute of Artic Biology, 209 Irving I Building, University of Alaska,

    Fairbanks, AK 99775-70203USDA Forest Service, Pacific Northwest Research Station,

    Juneau Forestry Sciences Laboratory 2770 Sherwood Lane, Suite 2A, Juneau,

    AK 99801-8545


    Headwater streams make up a significant part of the total channel length and

    fluvial networks of forested watersheds (Gomi et al. 2002, Benda et al. 2005), and are

    critical to the structure and functional stability of downstream habitats (Bilby and

    Likens 1980, Cummins et al. 1989, Naiman et al. 1992, Haigh et al. 1998). Headwater

    streams provide downstream ecosystems with water, sediments, wood debris,

    nutrients and food resources (Carline & Spotts 1998, Meyer & Wallace 2001, Wipfli

    and Gregovich 2002). Allochthanous inputs produced from riparian vegetation

    along headwater streams are particularly critical to the biocomplexity of

    downstream reaches and are likely to be the energy base of aquatic food webs in

    forested watersheds (Wipfli et al. 1997, Naiman et al. 2005). In southeast Alaska,

    headwater streams account for 70%90% of watershed channel systems (Swanston

    1967, Gomi et al. 2002) and the food generated from these headwaters has been

    shown to play a major role in regulating salmonid production (Wipfli 1997, Wipfli et

    al. 2002, Wipfli & Gregovich 2002). As much as half of the diets of salmonids in

    streams running through coastal forests in southeast Alaska has been shown to

    consist of terrestrial invertebrates falling into fluvial systems from streamside

    vegetation (Wipfli & Musselwhite 2004).

    Riparian zones are the interfaces between terrestrial and aquatic environments

    and represent one of the most dynamic habitats of forested watersheds (Gregory et

    al. 1991, Merritt & Lawson 1992). Disturbing riparian habitats alters the physical

    characteristics of adjacent streams and disrupts the energy flow in aquatic

    ecosystems (Alaback 1982, Bryant 1985, Bisson et al. 1987). Timber harvesting

    within riparian corridors, particularly clearcutting, significantly reduces wood inputs

    into streams altering channel morphology, habitat availability and refugia

    opportunities for terrestrial and aquatic organisms (Gomi et al. 2001, Benda et al.

    2005, Meyer & Wallace 2007). Opening or removing streamside canopies also

    reduces the amount of organic litter and terrestrial invertebrate food items that fall

    into streams (Wipfli 1997, Wipfli & Musselwhite 2004) and shifts the energy source

    of headwater streams from allochthanous inputs to autochthanous production

    (Cummins et al. 1989).

    Recent interest has been directed towards the ecological role of Red alder (Alnus

    rubra Bong.) in second-growth forests in the Pacific Northwest and their effect on

    THE PAN-PACIFIC ENTOMOLOGIST84(3):220237, (2008)

  • macroinvertebrate communities and associated food webs (e.g., Wipfli 1997, Wipfli

    & Gregovich 2002, Allan et al. 2003, Haggerty et al. 2004, Richardson et al. 2004,

    Hernandez et al. 2005, LeSage et al. 2005, Wilzbach et al. 2005). It has been

    suggested that the presence of Red alder in regenerating, second-growth forests in

    southeast Alaska may help to mitigate lost food subsides following timber harvest

    (Wipfli 1997, Wipfli & Gregovich 2002, Wilzbach et al. 2005). Red alder is an

    aggressive pioneer species in the Pacific Northwest which colonizes disturbed soils

    and dominates riparian corridors for up to 80 years following timber harvest

    (Newton et al. 1968, Harrington 1990, Newton & Cole 1994). Red alder produces

    greater biomass and more rapidly biologically processed organic litter to riparian

    and aquatic habitats than do conifers. Headwater corridors within young stands of

    Red alder have been shown to produce significantly more terrestrial and benthic

    macroinvertebrates than streams flowing through young conifer stands (Piccolo &

    Wipfli 2002, LeSage et al. 2005).

    Our objective was to assess the ecological role of red alder in benthic

    macroinvertebrate community structure and function in selected headwater streams

    of regenerating forests in southeast Alaska. We specifically compared and contrasted

    macroinvertebrate density, biomass, and diversity associated with red alder and

    conifer wood in headwater streams over a range of riparian red alder stands. The

    hypotheses tested were: 1) macroinvertebrate density, biomass, and diversity would

    be greater on red alder than on conifer wood; and 2) these same metrics would

    increase as the presence of riparian red alder increased. In addition to the role of

    alder in and along streams, we were interested in potential effects that the stages of

    wood decay might have on benthic macroinvertebrate communities. We tested the

    null hypothesis that there was no difference in macroinvertebrate density, biomass

    and diversity on pieces of wood in early stages of decay versus wood in more

    advanced stages of decay.


    Stand Selection and Study Sites. Research was conducted on Prince of Wales

    Island, southeast Alaska (132u679W, 55u499N). This region contains more extensivetracts of virgin, old growth forest than anywhere else in the United States (USDA

    1997), and offers exceptional opportunities to study ecological interactions among

    red alder, site conditions and stream communities. Study sites were in the Maybeso

    Creek and Harris Creek watersheds (46 km2 and 108 km2, respectively) on the

    eastern end of Prince of Wales Island (Fig. 1). This area was heavily logged in the

    1950s, and in the 1960s local and federal governments established the Maybeso

    Experimental Forest to study the impacts of commercial logging on wood

    production, wildlife, and fisheries (Wipfli et al. 2002, 2003). The varying amounts

    of riparian alder and standing old growth conifer within the Maybeso Experimental

    Forest made this an ideal site to complete our research objectives. Sampling sites for

    this project were limited to fishless headwater streams within forested stands of 5

    10 ha with elevations less than 150 m. The primary criterion for sample site selection

    was riparian tree species composition to provide a range of 053% red alder.

    Aerial photographs, U.S. Forest Service district files, and field visits were used to

    select 13 headwater streams for this study. To reduce variability, all streams were 3rd

    order or less, had channel slopes between 1028 degrees, and bankfull widths between

    0.73.4 m (Wipfli et al. 2002). Stream flow in 2002 ranged from 1116 L?s21 (Table 1).


  • Sampling was restricted to 250300 m reaches within each stream. Riparian

    understory vegetation and tree species composition was measured within thedesignated sampling area at each stream as described by Wipfli et al. (2002).

    Salmonberry (Rubus spectabilis Pursh.), Devils Club (Oplopanax horridum Miquel.),

    Skunkcabbage (Lysichitum americanum Hutten & S. J.), ferns, and mosses dominated

    the riparian understory. Western hemlock (Tsuga heterophylla (Raf.) Sarg.), Alaska

    yellow cedar (Chamaecyparis nootkatensis (D. Don) Spach.), and Sitka spruce

    (Piceasitchensis (Bong.) Carriere.) were common coniferous species, and the dominant

    hardwood was red alder. The streams selected were in stands with mixed alder-conifer

    vegetation that represented a range of riparian red alder from 053% (Table 1).Wood Collection and Experimental Design. To survey the macroinvertebrates

    associated with wood, pieces of naturally occurring alder and conifer (appro


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