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

R. K. KIMBIRAUSKAS1, R. W. MERRITT

1, M. S. WIPFLI2, & P. HENNON

3

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

INTRODUCTION

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):220–237, (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.

MATERIALS AND METHODS

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

Island, southeast Alaska (132u679W, 55u499N). This region contains more extensive

tracts 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 0–53% 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 10–28 degrees, and bankfull widths between

0.7–3.4 m (Wipfli et al. 2002). Stream flow in 2002 ranged from 1–116 L?s21 (Table 1).

2008 KIMBIRAUSKAS ET AL.: WOOD MACROINVERTEBRATE COLONIZATION 221

Sampling was restricted to 250–300 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 0–53% (Table 1).Wood Collection and Experimental Design. To survey the macroinvertebrates

associated with wood, pieces of naturally occurring alder and conifer (approx. 5 3

20 cm) representing early and late decay classes were randomly collected from 13

Figure 1. Study sites within Maybeso Creek and Harris River drainages on Prince of WalesIsland, SE Alaska (132u679W, 55u499N).

222 THE PAN-PACIFIC ENTOMOLOGIST Vol. 84(3)

headwater streams during May and July of 2001 and 2002. A piece of wood was

considered late decay if a thumbnail could easily penetrate the surface—all others were

considered early decay. In addition to hand-collecting macroinvertebrates from naturally

occurring wood, we also collected macroinvertebrates from wood that had been secured

to substrate holders and experimentally added to streams as described below.

Wood added to streams was initially collected from standing snags within the

Maybeso Experimental Forest. Boles from both alder and conifer were divided into

early and late decay classes based on color, weight, and presence or absence of bark

and branches. Boles had uniform diameters ranging from 3–5 cm and lengths of 1–

6 m. The longer pieces were later cut into 20 cm sections. The middle section and end

pieces from each bole were dried, weighed, and water displacement was used to test

for consistency across the density gradient and to confirm the decay class. Boles that

showed irregularities in density and sections containing knots were excluded from

the study. Wood pieces representing four treatments: early decay alder (EA), late

decay alder (LA), early decay conifer (EC), and late decay conifer (LC) were then

randomly arranged and secured to plastic mesh substrate holders. A total of nine

substrate holders were placed in each of six streams in a randomized complete block

design. The sub-plot factor was wood type and consisted of the treatments: EA, LA,

EC, and LC. The whole-plot factor was riparian alder-conifer species composition

and represented a range of riparian red alder from 1.5–53%. Substrate holders were

placed in a variety of in-stream habitats; however, placement was often restricted to

shallow pools due to unusually low water levels in the year 2000. All holders were

deployed on 7–14 July, 2000, and remained in the streams for either 1 or 2 yrs. Three

substrate holders from each site were randomly selected and collected in early July of

2001 and 2002.

Macroinvertebrate Sampling and Analysis. Macroinvertebrates were sampled from

both added wood substrates and naturally occurring woody debris. Individual pieces

Table 1. Physical and biological characteristics of headwater streams and adjacent riparianforest on Prince of Wales Island, Alaska, 2000.

StreamChannel

Slope1 (deg)Bankfull

Width2 (m)Stream

Flow3 (L?s-1)3Basal Area4

(percent red alder)

Lost Bob 19.5 0.7 7.4 0.0Upper Good Example 13.6 0.9 3.0 1.5Cedar 2 18.8 1.7 4.5 3.8Upper Morning 14.5 1.1 4.7 3.9Creature Creek 15.2 1.1 36.0 10.0Gomi 26.1 1.0 5.5 25.0Mile 22 18.3 1.2 1.5 29.2Big Spruce 26.3 0.9 0.9 31.5Cedar 1 19.3 1.6 16.0 35.6Broken Bridge West 22.5 1.5 3.9 38.7Cotton 27.2* 3.4 116.2 43.2Broken Bridge East 10.7* 2.2 24 47.3Brushy 15.5 0.9 1.4 53.3

1 Channel slope gradient of entire stream; * measured on 25% of stream length only.2 Bankfull width – the dominant channel forming flow (Dunne and Leopold 1978).3 Mean stream flow at transition zone.4 Percent red alder as a proportion of total stand basal area.


were pulled from the stream and transferred to a 19 liter bucket for processing. Each

piece of wood was rinsed with a hand-pumped portable pressurized sprayer and the

contents of the bucket were strained through a 250-mm sieve. Invertebrates were

washed into 250-ml WhirlpaksH with 80-percent ethanol and returned to the lab for

sorting under a dissecting microscope. Macroinvertebrates were picked from each

sample and identified to the lowest taxon possible using Merritt and Cummins

(1996a). Measurements of the total length were made for each identified specimen

and length-weight regressions were used to estimate individual taxa biomass (Smock

1980, Hodar 1996, Benke et al. 1999). Wood surface area was estimated from length

and diameter using the equation for the surface area of a cylinder, and

macroinvertebrate biomass m2 and number m22 were calculated for each piece of

wood. Diversity was measured using the Shannon-Weiner diversity index (Hauer &

Resh 1996). Functional feeding groups were assigned to taxa using Merritt and

Cummins (1996b) and ratios were calculated to help assess trophic groupings.

Results from the dependant variables of invertebrate density, biomass m2, and

diversity were tested for normality, log (x + 1) transformed when necessary, and

analyzed for significance using SAS (SAS Institute 1996). Multiple T-tests and

ANOVA were generated contrasting the dependant variables with percent riparian

alder, wood type (conifer or red alder), and decay class (early or late decay). Percent

riparian alder was grouped into three bins of low (0–9%, n54), medium (10–35%,

n55) and high alder (36–53%, n54) and Bonferroni adjustments were made for the

analysis (Winer et al. 1991). Pairwise comparisons were generated for all dependant

variable results with a significant ANOVA (p,0.05) (Sokal & Rolf 1969) using

Tukey’s Studentized Range (HSD) post-hoc test. All graphs and tables are presented

using nontransformed data.

RESULTS

Macroinvertebrates Community and Functional Feeding Group Composition. Over

10, 000 macroinvertebrates from 14 different orders were collected from wood

samples. At least 54 insect taxa were identified, but of those taxa a relatively small

number comprised most of the total count and biomass densities (Table 2). Total

chironomid taxa comprised 56% of the total relative abundance, and chironomids in

the subfamily Orthocladiinae accounted for 70% of all chironomids (Fig. 2). Nearly

half of the total of other aquatic insect biomass consisted of only two genera from

two different orders—the mayfly Cinygma (Heptageniidae), and the caddisfly

Rhyacophila (Rhyacophilidae). Functionally, collector-gatherers (Chironomidae,

Baetidae, Leptophlebiidae) were the most abundant group collected (71%), followed

by predators (Rhyacophilidae, Chloroperlidae, Empididae), shredders (Nemouridae,

Leuctridae), scrapers (Heptageniidae), and collector-filterers (Simuliidae) (Fig. 3).

Overall, functional feeding group colonization of alder and conifer was not

significantly different. A slight increase in scrapers on alder (79%) compared to

conifer (59%) was observed in May 2001, however, by July 2001 the relative

abundance of scrapers on alder was similar (58%). During the same period in 2001,

the relative abundance of predators on alder increased from 6% in May to 36% in

July. This was primarily due to an increase in numbers of the caddisfly Rhyacophila

on pieces of alder wood. The relative abundance of predators and Rhyacophila were

consistent on conifer wood throughout 2001 and 2002 (34%). The amount of

riparian alder did appear to influence the distribution of some feeding groups and


individual taxa. Over the entire sampling period, collector-gatherer relative biomass

increased from 9% to 20%, as the percentage of riparian alder increased from low (0–

9%) to high (36–53%). The relative biomass of collector-filterers, specifically

Prosimulium (Simuliidae), also increased with an increase in alder surface area for

attachment. The relative biomass of scrapers, on the other hand, decreased from 71%

to 56% as the percentage of riparian alder increased, and shredder biomass also

decreased as the amount of riparian alder increased. The caddisfly Rhyacophila was

positively correlated with an increasing presence of riparian alder; however total

predator distributions decreased as riparian alder increased (Fig. 4).

Macroinvertebrate Density, Biomass and Diversity. Overall, streams with medium

and high percent riparian alder supported greater mean density and biomass of

macroinvertebrates than did streams with low riparian alder, however only density

was significantly different (p.0.05)(Fig. 5). The macroinvertebrate diversity

collected from added wood was also slightly greater in streams with more riparian

alder, however, these differences were not significant and the diversity of

macroinvertebrates collected from natural wood showed no differences with lower

or higher amounts of riparian alder.

Wood type and decay class had some effect on benthic macroinvertebrate density,

biomass, and diversity. Biomass was significantly greater on individual pieces of

conifer than on pieces of in-stream alder (p,0.01; Fig. 6). Both density and diversity

of macroinvertebrates collected from natural and added wood substrates were

greater on alder than on conifer, but not significantly different. Overall, pieces of

wood in later stages of decay supported greater density, biomass and diversity than

did wood in more recent stages of decay, however, only diversity of macroinver-

tebrates associated with natural wood in later stages of decay was significantly

different (p,0.05).

DISCUSSION

Our results showed that the presence of riparian alder along southeast Alaskan

headwater streams appeared to influence stream benthic macroinvertebrate densities

associated with wood substrates. A number of related watershed studies in southeast

Alaskan and other areas of the Pacific Northwest also support the importance of

alder to macroinvertebrates. In a companion study with this one, Lesage et al. (2005)

compared macroinvertebrates colonizing terrestrial litter and wood along the same

streams and showed that invertebrate density, taxa richness, and biomass increased

when the percentage of riparian alder increased. Hernandez et al. (2005), also

working in the same watershed as in the current study, found that streams with

alder-dominated, young-growth riparian vegetation supported higher densities of

benthic macroinvertebrates on gravel, cobble, and wood substrates.

Anderson et al.’s (1978) study in two Oregon watersheds attributed higher in-

stream macroinvertebrate densities to increased organic inputs from riparian alder,

and Culp & Davies (1985) found higher macroinvertebrate abundance associated

with alder leaf litter in streams of British Columbia, Canada. Wipfli & Gregovich

(2002) showed that terrestrial insect abundance collected in stream drift from

southeast Alaskan headwater streams significantly increased as the percent basal

area of alder increased along the riparian zone corridor. A subsequent study by

Wipfli & Musselwhite (2004) further documented an increase in 24-hr macroinver-

tebrate drift with the presence of riparian alder.


Table 2. Checklist of taxa collected and the percent relative density and biomass for each taxoncollected. Totals are combined over the entire study period.

Taxon

Percent relative abundance

Density Biomass

CollembolaEntomobryiidae 0.7 0.1Hypogastruridae ,0.1 ,0.1Sminthuridae 0.1 ,0.1

ColeopteraDytiscidae

Liodessus ,0.1 ,0.1Elmidae

Narpus ,0.1 ,0.1Hydrophilidae

Ametor 0.1 0.5Ptilodactylidae

Araeopidius ,0.1 0.1Diptera

CeratopogonidaeCeratopogon ,0.1 ,0.1

CeratopogonidaeProbezzia 0.2 ,0.1

ChaoboridaeEucorethra ,0.1 ,0.1

Chironomidae 1.2 0.2Chironominae 4.2 1.1Diamesinae 0.1 ,0.1Orthocladiinae 39.5 1.3Tanypodinae 2.2 0.6Tanytarsini 10.8 2.2

DixidaeDixa 0.7 ,0.1

EmpididaeChelifera 3.5 0.4Clinocera ,0.1 ,0.1Oregeton ,0.1 ,0.1

PsychodidaePericoma ,0.1 ,0.1

Pyralidae ,0.1 ,0.1Sciaridae ,0.1 ,0.1Simuliidae

Prosimulium 0.9 3.0Tipulidae

Dicranota 0.3 0.1Hexatoma 0.1 ,0.1Limonia ,0.1 ,0.1Molophilus ,0.1 ,0.1

EphemeropteraAmeletidae

Ameletus ,0.1 ,0.1Baetidae

Baetis 6.7 7.3Ephemerellidae

Drunella 0.8 1.6Heptageniidae

Cinygma 4.1 37.6


Our study showed that with the increasing presence of alder there was an increase

in macroinvertebrate biomass associated with naturally occurring wood debris in

streams. This increase in biomass may have been the result of greater nutrient

availability. Streams with alder in the riparian zone deliver more litter to

macroinvertebrate communities than streams with more conifer (Stone & Wallace

1998). Alder litter is a better food source for herbivorous invertebrates than conifer

litter (Irons et al. 1988, Webster & Benfield 1986), and alder leaves are processed

more quickly than conifer needles (Sedell et al. 1975, Cummins et al. 1989, Lesage et

al. 2005). Piccolo and Wipfli (2002) identified alder in forested riparian zones as a

critical habitat for terrestrial invertebrates that fall prey to fish. Field studies have

demonstrated that over half of the prey biomass ingested by juvenile salmonids in

some southeast Alaskan streams was drifting terrestrial invertebrates originating

from upstream riparian vegetation (Wipfli 1997, Wipfli et al. 2002). In southeast

Taxon

Percent relative abundance

Density Biomass

Cinygmula 0.3 19.3Epeorus 1.8 0.4Ironodes ,0.1 1.0Rhithrogena ,0.1 ,0.1Paraleptophlebia 6.4 2.6

PlecopteraChloroperlidae

Suwalia 0.1 0.1Sweltsa 0.9 1.0

LeuctridaeDespaxia 1.3 0.9

NemouridaeZapada 7.0 1.7

TrichopteraBrachycentridae

Micrasema 1.3 0.2Goeridae

Goeracea 0.2 0.2Hydropsychidae

Arctopsyche 0.1 0.7Limnephilidae

Allomyia ,0.1 ,0.1Chyranda ,0.1 0.5Cryptochia 0.2 0.1Ecclisiomya ,0.1 ,0.1Moselyana ,0.1 ,0.1Psychaglypha 0.2 1.5

PhilopotamidaeDolophiloides 0.2 0.1Wormalidia ,0.1 ,0.1

RyacophilidaeRhyacophila 3.4 13.6

UenoidaeNeophylax 0.2 0.3

54 total taxa 100 100

Table 2. Continued.


Figure 2. Macroinvertebrates were collected from the surfaces of wood pieces representing fourtreatments: early decay alder (EA), late decay alder (LA), early decay conifer (EC) and late decayconifer (LC). Pieces were randomly arranged and secured to plastic substrate holders.

Figure 3. Total relative abundance of the top five families collected from wood surfaces between2001 and 2002. Chironomidae was the most abundant family collected and the bar graph representsthe distribution of midge subfamilies.


Alaska, where freshwater ecosystems appear to be nutrient limited (Ashley & Slaney

1997, Wipfli & Gregovich 2002), prey biomass generated from riparian alder

vegetation and its associated litter may improve stream productivity.

Our findings failed to show a strong relationship between macroinvertebrate

diversity and an increasing presence of live standing alder in riparian habitats on

either added wood substrates or naturally collected wood in headwater streams.

Previous studies in the Pacific Northwest have shown that stream benthic

macroinvertebrate diversity was greater in old-growth than in either recently

clearcut or second-growth forest riparian habitats (Newbold et al. 1980, Anderson

1992). However, on Prince of Wales Island, Hernandez et al. (2005) found that the

diversity of stream macroinvertebrates was significantly greater in alder dominated

second-growth than in old-growth watersheds and suggested that the greater

diversity was due to changes in food resources and the availability of more labile

allochthonous inputs generated from deciduous alder vegetation. Lesage et al. (2005)

found that terrestrial invertebrate richness and diversity was significantly greater in

riparian habitats where alder was present and concluded that increasing the presence

of alder may contribute to increased production of higher tropic levels. Although we

did not find a difference in macroinvertebrate diversity with increasing alder, the

mean biomass of the trichopteran Rhyacophila (Rhyacophilidae)— the most

common predator collected—did increase with the presence of riparian alder,

suggesting that this presence may contribute to an increased production of some

higher trophic levels in aquatic organisms.

Macroinvertebrate colonization of wood surfaces over the entire study showed

that mean biomass was significantly greater on conifer compared to alder wood

debris. The greater biomass on conifer wood in streams was primarily due the

presence of two taxa: the heptageniid mayfly, Cinygma, and the rhyacophilid

caddisfly, Rhyacophila. Mayflies in the genus Cinygma are largely restricted to wood

substrates (Anderson et al. 1978), and were observed in their study to scrape wood

surfaces—presumably feeding on periphyton and detritus. The caddisfly Rhyaco-

phila, one of the larger invertebrates collected, was often found associated with

protective interstitial spaces on wood surfaces and may have been more common on

Figure 4. Percent relative abundance of functional feeding groups collected from the surfaces ofwood substrates between 2001 and 2002.


conifer due to its cracking nature during decomposition (Wipfli et al. 2003). Both

mean density and diversity of macroinvertebrates was greater on alder; however, the

differences were not statistically significant due to large variation between samples.

Contributing to the variation in mean macroinvertebrate densities between coniferand alder were higher counts of midge taxa on some individual pieces of alder.

Because chironomids tend to be ubiquitous, it is difficult to identify the specific role

that alder played in their colonization, but the presence of xylophagous midges in the

Figure 5. Functional feeding group relative biomass in streams with low (0–9%, n54), medium(10–35%, n54), and high (36–53%, n55) percentages of riparian red alder. Relative biomass iscombined total over the two year sampling period.


subfamily Orthocladiinae and large numbers of pupae and tubes of midges in the

subfamily Tanytarsani, suggested that the wood surfaces are being used as a food

resource, refuge, and attachment site.

Overall, this study indicated that macroinvertebrate colonization of wood surfaces

appears to be more closely associated with stage of decay than wood type.

Figure 6. Mean macroinvertebrate density (A) and biomass (B) in streams with low (0–9%,n54), medium (10–35%, n54), and high (36–53%, n55) percentages of riparian red alder. Meanswith * are significantly different (p,0.05).


Macroinvertebrate density, biomass and diversity was greater on wood in advanced

stages of decay. Dudley & Anderson (1982) suggested that wood as a food resource

becomes available to more taxa as surface decay progresses, and Golladay &

Webster (1988) found that different functional feeding groups using wood were

directly associated with the decay of wood surfaces. There was evidence of feeding on

wood in our study, however most of the macroinvertebrates colonizing wood were

probably using this habitat for resting attachment, refuge, and oviposition. Wood in

later stages of decay provided more area of this habitat for colonization and evidence

that the decay of wood surfaces may be a greater contributor to macroinvertebrate

colonization than wood type was found in the results of our experiments with added

wood substrates. Although two decay classes of conifer and alder were used as added

wood, the surfaces of all wood pieces used were smooth and very similar structurally.

The similarities in wood surfaces may explain why there were no differences in

colonization between wood types. The rhyacophilid caddisflies were commonly

collected within cracks and crevices of natural wood which suggested that decay

influenced colonization patterns. Anderson et al. (1978) described several taxa from

streams in the Pacific Northwest that used the protective niches on wood surfaces for

oviposition, as a nursery for early instars, molting, pupation, emergence, and as a

source of prey. McLachlan (1970) found that mayflies more readily colonized

submerged wood with bark previously damaged compared to newer pieces with solid

bark. High densities of baetid mayflies were collected from wood surfaces in our

study, and both conifer and alder wood classified as late decay supported greater

mean densities. Whether the decay process actually conditions wood surfaces for

feeding or creates more interstitial spaces for refuge was difficult to ascertain,

however, it appeared that wood with greater surface decay and more available

interstitial spaces was more attractive to benthic macroinvertebrates (Aumen et al.

1983).

Changes in macroinvertebrate functional feeding groups shifted in relation to

riparian alder vegetation. Collector-gatherer communities, dominated by chirono-

mid midges and the baetid mayfly (Baetis sp.), gradually increased as the percentage

of riparian alder increased. Increases in collector-gatherer groups have been

associated with increased loads of fine particulate organic matter (Cummins 1973,

Cuffney & Wallace 1989, Richardson & Neil 1991), and inputs from riparian alder

may contribute to this nutrient load. Scraper populations, on the other hand,

decreased with more riparian alder. In the light-limited small streams of the Pacific

Northwest, clearcutting can result in increased solar radiation (Bisson & Bilby 1998),

which also increases primary production and the abundance of scrapers (Murphy et

al. 1981, Wallace et al. 1988, Hernandez et al. 2005). The lower percentage of

scrapers found in headwaters on Prince of Wales Island was comparable to results

found in other headwater streams in regenerating forests of the Pacific Northwest,

that is most likely due to canopy closure (Cole et al. 2003, Haggerty et al. 2004).

Although it was difficult to determine the specific role of alder on macroinvertebrate

functional group relationships from this study, changes in feeding group

compositions are typically the result of changes in food resource type and its

availability (Vannote et al. 1980, Merritt & Cummins 1996b). It does appear that

increased organic material from riparian alder vegetation provides more food

resources for some macroinvertebrate taxa in headwater streams on Prince of Wales

Island.


CONCLUSION

In conclusion, managing upland forest riparian areas to include a red alder

component may provide more suitable conditions for stream macroinvertebrate

communities inhabiting wood and therefore, improve the potential of nutrients toreach downstream ecosystems. Historically, forest managers regarded red alder as an

undesirable species and attempted to remove it from riparian and upland forest

stands; however, its ability to supply the soil with fixed nitrogen (Trappe et al. 1968,

Briggs et al. 1978, Gordon et al. 1979) encouraged its use as a management tool to

enhance productivity of young-growth conifer forests in the Pacific Northwest

(DeBell et al. 1978, Binkley 1981). This tree species also can benefit riparian

ecosystems by providing greater structural diversity than homogeneous conifer

stands (Deal 1997). In southeast Alaska, where fresh water ecosystems are generallynutrient limited (Ashley & Slaney 1997, Wipfli 1997), the presence of red alder in

riparian habitats could protect or even improve the productivity of aquatic

organisms—including downstream salmonids.

ACKNOWLEDGMENTS

We would like to thank Ken Cummins, Eric Benbow, John Wallace, Osvaldo

Hernandez, Christian LeSage, Mollie McIntosh, Frank Rinkevich, Todd White, and

Lauren Wright for their assistance, and the USFS, USDA, and the PacificNorthwest Research Station for financial and technical support.

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Received 25 Feb 2008; Accepted 30 July 2008. Publication date 13 Feb 2009.


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