Springer 2005
Hydrobiologia (2005) 533: 45–59
Benthic invertebrate community structure is influenced by forest succession after clearcut logging in southeastern Alaska O. Hernandez1,*, R.W. Merritt1 & M.S. Wipfli2 1
Department of Entomology, Michigan State University, East Lansing, MI 48824-1115, USA. Alaska Cooperative Fish and Wildlife Research Unit – USGS, Institute of Arctic Biology, University of Fairbanks, Fairbanks, AK 99775–7020, USA (*Author for correspondence: Tel.: +1-517-355-6514, Fax: +1-517-353-4354, E-mail:
[email protected]) 2
Received 17 July 2003; in revised form 11 May 2004; accepted 14 June 2004
Key words: red alder, forest succession, benthic macroinvertebrate, functional feeding group, headwater stream, timber harvest
Abstract To assess the effects of timber harvesting on headwater streams in upland forests, benthic community structure was contrasted among four dominant forest management types (old growth, red alder-dominated young growth, conifer-dominated young growth, clearcut) and instream habitats (woody debris, cobble, gravel) in southeastern Alaska. Benthos in streams of previously harvested areas resulted in increased richness, densities and biomass relative to old growth types, particularly in young growth stands with a red alder-dominated riparian canopy. Woody debris and gravel habitats supported a combination of higher densities and biomass of invertebrates than cobble habitats. In addition, woody debris also supported a richer and more diverse invertebrate fauna than either cobble or gravel substrates. Maintaining both a woody debris source and a red alder component in regenerating riparian forests following timber harvesting should support greater invertebrate densities and diversity following clearcutting.
Introduction Clearcut logging is a disturbance that affects both physical and biological characteristics of adjacent streams and rivers. One of the major changes occurring after clearcutting is a decrease in the allochthonous energy base on which headwater streams and benthic macroinvertebrates depend on (Vannote et al., 1980; Duncan & Brusven, 1985a). The effects of clearcutting on benthic macroinvertebrates have been investigated before and after harvest. Garman & Moring (1993), found fewer Ephemeroptera, Plecoptera and Odonata, in addition to a threefold increase in Chironomidae after a harvest. Hartman et al. (1996) described the negative impacts of harvests on macroinvertebrate densities. The effects of harvests also have been investigated in comparative studies with a reference
stream. Hawkins et al. (1982), showed that canopy type had a greater influence on invertebrate communities than instream substrates, with greater abundances in streams with clearcut canopies. Similarly, Newbold et al. (1980), found greater total densities and lower invertebrate diversity. Another approach used the comparison of macroinvertebrate community structure in streams with riparian vegetation in different successional stages (Haefner & Wallace, 1981; Murphy et al., 1981). Anderson (1992) compared adult invertebrate emergence from old growth, second growth and clearcut streams and found old growth areas to have the greatest richness and evenness among Ephemeroptera, Plecoptera and Trichoptera. He also found a greater percentage of grazers in
46 clearcut areas, and the greatest percentage of detritivores in second growth areas. Stone & Wallace (1998) found the percentage of scrapers initially present increased after logging, following an increase in algal production, and then declined with the decline in primary production. In addition, they showed an initial decline in the percentage of shredders present and a successive increase following the return of allochthonous inputs. Commercial harvesting began on Prince of Wales Island in the 1950s (Swanston, 1967) and has given rise to regenerating pure and mixed red alder-conifer forests in different stages of succession (Deal & Orlikowska, 2002). The influence of harvesting and subsequent forest succession on associated headwater stream communities is not well understood (Wipfli et al., 2002). Most studies on the effects of clearcutting have focused on spawning and rearing habitats of commercial sport fish species such as coho (Onocorhynchus kisutch Walbaum), pink (O. gorbuscha Walbaum), chum (O. keta Walbaum) and sockeye (O. nerka Walbaum) salmon (Duncan & Brusven, 1985b; Murphy & Milner, 1997). However, due to the mountainous topography of southeastern Alaska, there are many high-gradient, fishless headwater streams in upland forests that drain into downstream salmonid habitats and subsequently affect communities in higher order streams lower in the drainage (Gomi et al., 2002). Piccolo & Wipfli (2002) demonstrated that upland forest management in southeastern Alaska alters this subsidy with an increase in the amount of food transported to foodwebs lower in the drainage in areas where regenerating riparian forests contain red alder (Alnus rubra Bong.). In addition, Wipfli & Musselwhite (in review) demonstrated that streams with a greater density of riparian red alder exported significantly greater numbers and biomass of invertebrates than sites with little red alder. The objective of this study was to contrast benthic invertebrate community structure among: (1) headwater streams with four different forest management types: (a) recent clearcut (CC), (b) red alder-dominated young growth (YA), (c) conifer-dominated young growth (YC), (d) old growth (OG),
(2) instream habitats: (a) woody debris, (b) cobble, (c) gravel, (3) interaction between instream habitat and forest management type.
Methods Study sites The study was conducted in 12 streams in the Maybeso Experimental Forest and adjacent Harris River watershed in the Tongass National Forest, Prince of Wales Island, southeastern Alaska (Fig. 1). Vegetation of the island is classified as a temperate rainforest with annual precipitation ranging between 150 and 500 cm, and air temperatures range from )20 to 36 C (Harris et al., 1974; Duncan & Brusven, 1985c). All stream sites were first order, high-gradient, headwater streams of the Harris and Maybeso catchments, and were sampled once between 7–15 July 1998 and 11–14 June 1999. Riparian vegetation differed across forest management types. Vegetation in the clearcuts was in its fifth year of regeneration with salmonberry (Rubus spectabilis Pursh) and Alaskan blueberry (Vaccinium spp.) as the dominant riparian species. The alder-dominated young growth, between 35 and 45 years old, consisted of a dense red alder canopy along the riparian margin with a mixture of conifers and an understory of ferns and mosses. The conifer-dominated young growth, also between 35 and 45 years old, was predominantly Sitka spruce (Picea sitchensis Carriere) and western hemlock (Tsuga heterophylla Sargent) with some red alder and a fern understory. The old growth, that had never been cut, had similar vegetation to the conifer-dominated young growth, but the trees were more mature and less dense, with a more extensive understory comprised of devils club (Oploplanax horridus Miq.), skunk cabbage (Lysichition americanum Hulten and St. John) and ferns. Physical measurements Physical measurements were taken in streams across the four management types in 1999. Water
47
Alaska
Prince of Wales Island
N 1392 m
Maybeso River
1015 m Maybeso drainage 46 km 2
1187 m Salt water
Harris River Harris drainage
0
5km
108 km 2
Figure 1. Maybeso experimental forest, Prince of Wales Island, southeastern Alaska.
temperatures were taken simultaneously with Onset1 Optic StowAway Temperature Loggers for a 3 week period. Discharge was measured using a Marsh-McBirney flow meter by the velocity-area method described in Gore (1996). The percentage of woody debris was visually estimated within 25 m in each of the 12 streams. Algal ash-free dry mass (AFDM) on clay tiles was determined at two streams of each forest management type using the methods in Steinman & Lamberti (1996). Sampling design and analysis The experimental design was a split-plot; management type was the whole-plot factor, habitat was the 1 Use of trade or firm names in this publication is for reader information only and does not imply endorsement by the U.S. Department of Agriculture of any product or service.
sub-plot factor. Three streams were sampled in each of the four management types. Within each stream, three random macroinvertebrate samples were collected in 1998 while two were collected in 1999, from each of the following habitats: (1) woody debris, characterized by being of cedar origin, >10 cm diameter and conditioned (in water long enough to be suitable for invertebrate colonization); (2) completely submersed riffle cobble (64–256 mm diameter); and (3) gravel between 2 and 16 mm diameter. A total of 108 samples were collected from 12 headwater streams in 1998, and 72 samples were collected from 12 headwater streams in 1999. The experimental design may be interpreted differently than a split-plot because forest management types were not randomly allocated. However, an analysis of the main effects using alternative designs would not lead to very different results. Residuals were tested for normality, and results were log transformed or square root transformed
48 where necessary. Multiple ANOVAs were generated contrasting richness, diversity, density and biomass versus forest management type (OG, YA, YC, CC) and habitat (wood, cobble, gravel). ANOVAs were generated for the analysis of discharge and algal AFDM versus forest management type (SAS Institute, 1996). Means were separated using Fishers LSD at p ¼ 0.05. Although results were transformed, they will be presented in untransformed fashion in graphs and tables. Macroinvertebrate sampling The woody debris samples were randomly selected from the first six suitable pieces encountered upstream of logging roads. Length and diameter measurements were taken to estimate surface area. Each piece of woody debris was hand collected from the stream and placed in a 19 l bucket and pressure sprayed with water from a hand-pumped lawn sprayer to remove invertebrates. The macroinvertebrate sample was then rinsed through a 250lm sieve and transferred into Whirl-Paks, preserved in 80% ethanol, and returned to the lab for sorting under a dissecting scope. All invertebrates were picked from each sample, counted and identified mostly to generic level for Insecta (except Chironomidae) using Merritt & Cummins (1996a). Chironomidae were subsampled and identified to subfamily. Non-insect invertebrates were not identified beyond order level. Cobble samples were removed from submersed cobble of riffle areas. Each cobble habitat was selected from the center of the first three riffle areas encountered upstream of logging roads. Sample collection and processing were similar to that of woody debris. The cobble were labeled and returned to the lab to estimate surface area. Surface area was determined by wrapping the cobble in foil paper, removing the foil, and tracing it onto paper. Surface area was calculated from the paper using a Li-Cor portable leaf-area meter ModelLi-300. Gravel core samples were collected from the center of each of the first three gravel areas encountered upstream of logging roads. Samples were collected with a core sampler (6 cm · 6 cm · 6 cm). The core sampler was inserted into the gravel area to a depth of 6 cm and the contents scooped out with a 250-lm mesh net. The entire
sample was transferred to a Whirl-Pack and returned to the lab for processing as above. Invertebrate density was estimated from abundance and surface area calculations for the woody debris and cobble samples and was converted to number per 1 m)2. Density was calculated from abundance and sample volume for the gravel core samples and was converted to number per 1 m)3. Dry mass of invertebrates was estimated according to Benke et al. (1999). Richness was measured as mean number of taxa present and diversity was measured using the Shannon–Weiner diversity index (Hauer & Resh, 1996). Macroinvertebrates were designated a functional feeding group status (shredders, filtering-collectors, gathering-collectors, scrapers and predators) according to Merritt & Cummins (1996b). Dry mass of Oligochaeta were not determined and therefore omitted from biomass and functional group analysis. Results Physical measurements Daily stream temperature maximums were greatest in the OG forest management type and lowest in the YC. Daily temperature minimums were lowest in the YC management type and highest in the OG. The greatest differences in maximum and minimum daily temperatures were found in the CC and smallest differences were found in both YC and OG forest management types (Table 1). Discharge ranged from 0.004 to 0.01 m3 s)1 across all streams and was similar among forest management types. Cobble and gravel habitats comprised the greatest proportion of available instream habitat for invertebrate colonization. The YC management type had the largest proportion of wood pieces, followed by OG, YA and CC forest management types (Table 2). Algal AFDM was greatest in the CC forest management type. It was significantly greater than in the OG, YA, and YC forest management types ( p < 0.05) (Fig. 2a). Interaction of habitat and forest management type There were no significant interaction effects on benthic invertebrate community structure at an a level of 0.1.
49 Table 1. Average daily maximum, minimum and differences in streamwater temperature (C) 1999 OG
CC
YA
YC
Ave daily max
10.44
9.96
9.13
7.71
Ave daily min Ave max minus min
10.03 0.41
9.03 0.93
8.38 0.75
7.35 0.36
OG – old growth, CC – clearcut, YA – alder-dominated young growth, YC – conifer-dominated young growth.
Table 2. Percentage of instream woody debris, cobble and gravel habitats within forest management types Forest management type Woody debris
Cobble Gravel
OG
12.1
46.3
40
CC
3.3
53.8
32.1
YA YC
5.4 20.9
22.1 24.1
67.5 34.2
Totals do not equal 100% in cases where fine woody debris and bedrock are present. OG – old growth, CC – clearcut, YA – alder-dominated young growth, YC – conifer-dominated young growth.
Effect of management type A total of 37 insect genera comprising four orders (Ephemeroptera, Plecoptera, Trichoptera, Diptera) were collected from headwater streams of the Maybeso and Harris River watersheds (Table 3). In addition, three subfamilies of Chironomidae were identified from 1999 subsamples. Orthocladinae comprised the largest percentage across the four forest management types, followed by Tanypodinae and Chironominae (Table 4). Mean richness was greatest in the YA forest management type (Fig. 2b). Clearcut and YC were intermediate in their mean taxa richness, while OG had the lowest mean number of taxa. Mean diversity was greatest in both YA and YC forest management types and significantly less in CC and OG (Fig. 2b). Mean density of macroinvertebrates collected in samples from wood and cobble habitats were significantly greater in the CC and YA management types than in the OG and YC. In addition, biomass in the YA was significantly greater ( p ¼ 0.03) than in the OG management type (Fig. 3a). Both mean density and mean biomass of macroinvertebrates collected from gravel core
samples were greatest in CC management types and lowest in OG (Fig. 3b). Previously harvested sites differed in macroinvertebrate biomass, both functionally and taxonomically, relative to the OG. The OG forest management type was characterized by the dominance of the predatory stonefly Sweltsa (Chloroperlidae) and the gathering-collector mayfly Baetis (Baetidae). This was followed by the scraper mayfly Cinygmula (Heptageniidae) and the shredder Tipulidae (Fig. 4). The CC forest management type was similar to the OG with the dominance of predators Sweltsa and the gathering-collector Baetis. However, the dominant scrapers were the mayfly Drunella (Ephemerellidae) and the shredder stonefly Despaxia (Leuctridae). Filtering-collectors were represented by the caddisfly Dolophiloides (Philopotamidae). Similar to the clearcut, the YA forest management type had a large proportion of the same predators, but differed from the OG in that the scraper mayfly Cinygmula was equally represented. The gathering-collector mayfly Paraleptophlebia (Leptophlebiidae) comprised 11% of the biomass, followed by the shredder Despaxia and the filteringcollector Dolophiloides. Lastly, YC forest management type also had predators as the dominant functional group, followed by the scraper Cinygmula and Chironomidae gathering-collectors. Similarly, shredders were represented by Despaxia and filtering-collectors were primarily black fly larvae (Simuliidae). Effect of instream habitat Mean richness was greatest on wood and significantly greater than on gravel ( p < 0.0001) and cobble ( p < 0.0001) habitats. Taxa richness also was significantly greater on gravel than on cobble habitats ( p ¼ 0.0005) (Fig. 5a). Mean diversity was lowest on the cobble habitat, and significantly lower than on wood ( p < 0.0001) and gravel ( p < 0.0001) habitats (Fig. 5a). Mean invertebrate density on wood was significantly greater than on cobble habitats ( p < 0.0001). In addition, mean biomass was also significantly greater on woody debris than on cobble habitats ( p ¼ 0.0003) (Fig. 5b).
50 0.08
algal afdm (mg per m2)
0.06
a
0.04
0.02
b
b
(a)
b
0.00 14
3.0 richness diversity
a
12
2.5
richness (mean number of taxa)
10
b ab
2.0
b ab
8 1.5
b 6
1.0 4
0.5
2
0
(b)
mean shannon-weiner diversity
a
a
0.0 OG
CC
YA
YC
forest management type
Figure 2. (a) Mean algal ash free dry mass (±se) across four management conditions. (b) Macroinvertebrate mean richness (±se) and mean Shannon–Weiner diversity (±se) across four management conditions. OG – old growth, CC – clearcut, YA – alder-dominated young growth, YC – conifer-dominated young growth. Bars with same letters are not significantly different from each other, Fishers LSD ( p ¼ 0.05).
Scrapers, followed by gathering-collectors and shredders, represented the most abundant functional feeding groups on wood, based on biomass (Fig. 6). Predators and filtering-collector functional groups comprised the remaining biomass. Similarly, scraper and gathering-collector functional groups dominated the cobble habitat. However, filtering-collectors were the next most abundant group, followed by predators and shredders. In contrast, predator functional group dominated the gravel habitat, followed by scrap-
ers, gathering-collectors, shredders and filteringcollectors.
Discussion Forest management type Results of this study showed that taxonomic and functional differences in macroinvertebrate composition occurred between harvested sites and old
51 Table 3. Checklist of insect taxa collected from upland forest headwater streams of the Maybeso Experimental Forest and adjacent Harris River watershed, Prince of Wales Island, southeastern Alaska Order
Family
Genus
Presence
Ephemeroptera
Baetidae
Baetis
abcd
Heptageniidae
Cinygma Cinygmula
abcd abcd
Epeorus
abcd
Ironodes
abcd
Rithrogena
c
Ephemerellidae
Drunella
abc
Leptophlebiidae
Paraleptophlebia
abc
Nemouridae
Zapada
abcd
Visoka
d
Leuctridae
Despaxia
abcd
Chloroperlidae
Sweltsa
abcd
Philopotamidae
Dolophiloides
abcd
Hydropsychidae
Arctopsyche
acd
Rhyacophilidae
Rhyacophila
abcd
Brachycentridae
Micrasema
bcd
Limnephilidae
Cryptochia Chiranda
abcd c
Moselyana
d
Pseudostenophylax
a
Psychoglypha
ad
Goeridae
Goeracea
abcd
Uenoidae
Neophylax
cd
Glossossomatidae
Anagapetus
d
Plecoptera
Trichoptera
Coleoptera
Ptilodactylidae
Diptera
Thaumaleidae Ceratopogonidae
c b Atrichopogon Probezzia
Chironomidae
abc abcd abcd
Dixidae
Dixa
abcd
Psychodidae
Pericoma
d
Simuliidae Tipulidae
Prosimulium Dicranota
abcd abcd
Limonia
a
Hexatoma
bc
Pedicia
a
Tipula
abcd
Chelifera
abcd
Clinocera
abcd
Oreogeton
abc
Empididae
(a – old growth, b – clearcut, c – alder-dominated young growth, d – conifer-dominated young growth)
growth stands. The changes in the availability of food resources (e.g. algae, labile allochthonous in-
puts and fine particulates) in the harvested conditions likely caused changes in macroinvertebrate
52 Table 4. Percent distribution of Chironomidae subfamilies collected from woody debris, cobble and gravel habitats in forest management types Habitat
Woody debris
Cobble
Gravel
Taxa
Forest management type OG
CC
YA
YC
Orthocladinae
59.3
91
85.6
83.4
Tanypodinae
33.3
6.1
13.1
10.9
Chironominae
7.5
2.9
1.3
5.7
Orthocladinae
100
100
100
94
Tanypodinae
0
0
0
2.7
Chironominae
0
0
0
3.4
Orthocladinae
100
86.2
80.2
95.9
Tanypodinae
0
2.8
4
0
Chironominae
0
11.1
15.8
4.1
OG – old growth, CC – clearcut, YA – alder-dominated young growth, YC – conifer-dominated young growth.
composition. Anderson (1992) found that based on the emergence of aquatic insects, streams in OG forest management types had greater evenness and number of taxa than from recent clearcut and young growth deciduous riparian forests. In contrast, richness of macroinvertebrates in our OG management type was lower than in the YA forest type. Old growth sites were lacking three taxa (Hexatoma spp., Micrasema spp., Neophylax spp.) that were present at the other sites. In addition, two dipterans (Thaumaleidae and Limonia spp.) were present only in the CC sites, and three other taxa were only present in the YA forest management type. Vegetation in the OG consisted primarily of more refractory coniferous allochthonous material, while all other management types had sources of more labile deciduous allochthonous inputs (i.e. summer red alder leaf litter and catkins, salmonberry, etc.) that may be more readily available for aquatic hyphomycete colonization and use by the caddisfly shredders Micrasema and Chyranda and by the crane fly shredder, Limonia (Peterson & Cummins, 1974). However, our results also may have differed from those of Anderson (1992) because of greater taxonomic resolution in his study. Mean algal AFDM was greatest in the CC types, which was similar to the findings by Murphy et al. (1986) who reported their clearcut streams averaged 130% greater periphyton AFDM than in buffered and old growth streams, which they attributed to an increase in amount of light reaching the stream. Increased algal AFDM in our
CC sites may have resulted in a suitable food source for the caddisfly scraper, Goeracea, and the dipteran scraper, Thaumaleidae. Lastly the presence of filtering-collectors in managements other than the OG forests suggested the presence of higher quality fine particulates derived from alder and salmonberry in young stands and sloughed algal cells in the clearcut stands, than particulates derived from conifer stands. In addition, fine particulates may enter the stream through bank erosion, or lateral inputs from runoff and resuspension as Anderson & Sedell (1979) described. Webster et al. (1990) also showed that disturbed streams exported significantly more particulate organic matter than reference watersheds. Although diversity of macroinvertebrates has been reported to be lower in harvested streams (Newbold et al., 1980), our results showed mean diversity was not lower in CC than in OG streams. In addition to lower macroinvertebrate diversity, streams within harvested areas have generally been found to have greater macroinvertebrate densities throughout the lower United States (Hawkins et al., 1982; Silsbee & Larson, 1983), in the Pacific Northwest (Murphy et al., 1981), and Alaska (Duncan & Brusven, 1985b). However, Hartman et al. (1996) described negative impacts of harvesting on macroinvertebrate densities in Carnation Creek, Vancouver Island. In our study, large numbers of Chironomidae in the OG forest management type and large numbers of midges and Baetis spp. in the CC type, as well as fewer taxa,
53 6000 density biomass
500
a
4000
400
a 3000
300
ab a 2000
200
ab 1000
b
b
biomass (mg per m2)
density (mean number per m2)
5000
600
100
(a)
0
0
700x10
3
600x10 3
600x10
3
500x10 3
3
a 400x10 3
400x10
3
300x10
3
a 300x10 3
ab
200x10 3
200x10 3
ab 100x10
ab ab
3
b
100x10 3
b 0
0 OG
(b)
biomass (mg per m )
500x10
density biomass
3
density (mean number per m3)
b
CC
YA
YC
forest management type
Figure 3. (a) Macroinvertebrate mean density (±se) and mean biomass (±se) from large woody debris and cobble habitats across four management conditions. (b) Macroinvertebrate mean density (±se) and mean biomass (±se) from gravel habitats across four management conditions. OG – old growth, CC – clearcut, YA – alder-dominated young growth, YC – conifer-dominated young growth. Bars with same letters are not significantly different from each other, Fishers LSD ( p ¼ 0.05).
were primarily responsible for lower diversity. The increase in densities after a harvest was largely due to large numbers of Baetis spp., Chironomidae and the mayfly Drunella spp. in CC forest management types. Similarly, Wallace & Gurtz (1986) reported increased numbers of Baetis spp. in their harvested streams and attributed it to increases in autochthonous production. The high macroinvertebrate densities in our YA were due to large numbers of Zapada spp. (Nemouridae) and Micrasema spp., both of which are shredders and presumably use allochthonous inputs from the red alder riparian vegetation in the summer. Culp & Davies (1985) also found higher macroinvertebrate
abundances in substrate patches with alder detritus as compared to hemlock detritus. We found that headwater streams of the Maybeso Experimental Forest and adjacent Harris River watershed had greater invertebrate densities following a harvest, with CC sites having the greatest densities. Young growth forest management types had intermediate densities, and OG sites had the lowest invertebrate densities. Mean biomass was also greater in harvested areas, particularly in the YA and recently CC forest management types. This greater invertebrate biomass in harvested areas may be the result of a greater nutrient availability through increased leaf
54 YC
OG
S c ( 19. 7%)
Pr (36. 4%)
Fc ( 1.9%) Gc (14. 7%)
S c ( 24. 3%) Sh (6.8%)
Sh ( 6.4%) Gc ( 32 .5%) Pr (57. 3%)
CC
YA Fc (2.1%) Gc ( 10.8 %)
Pr (51.9 %) S c ( 40. 5%)
Sh ( 5.4%)
S h (3.7%) S c ( 12%)
Pr (41.2%)
Gc (31.5%) Fc ( 0.9%)
Figure 4. Proportion of functional feeding groups, based on biomass, across four management conditions. OG – old growth, CC – clearcut, YA – alder-dominated young growth, YC – conifer-dominated young growth, Shredders (Sh), Gathering-collectors (Gc), Filtering-collectors (Fc), Scrapers (Sc), Predators (Pr).
litter processing rates (Stone & Wallace, 1998), particularly red alder litter which is processed quicker than conifer needles and contains higher levels of nitrogen (Sedell et al., 1975). Stone & Wallace (1988) suggested that these processes could lead to increased production of macroinvertebrates in harvested streams. Changes in the functional feeding group composition of streams have been shown to be the result of changes or differences in food availability (Vannote et al., 1980). For example, Grubbs & Cummins (1996) showed that shredder populations are well matched to the pattern of riparian litter inputs. Duncan & Brusven (1985b) found a tendency for increased scrapers from spring to summer in their studies on Prince of Wales Island. In this study, large proportions of predators across management types may have resulted from an under representation in the non-predator fauna due to low sampling efficiency, since all available habitats (fine woody debris, root wads, mosses, bedrock) were not sampled. Another possible reason for the presence of a large proportion of predators is a prey base with a rapid
turnover in generation time. For example, large numbers of Harpactacoida copepods, with presumably short generation times, were collected from all forest management types, primarily from woody debris and gravel habitats where the predator Sweltsa spp. were more commonly found. Streams with alder-dominated riparian vegetation had a functional and taxonomic similarity to OG forest management types. However, YA sites also had an abundant, richer and more diverse fauna. Therefore, streams in YA types were potentially contributing a greater amount and variety of benthic invertebrates to larger fishbearing streams lower in the drainage, as demonstrated in studies by Piccolo & Wipfli (2002) and Wipfli & Musselwhite (in review). Instream habitats In the event of a disturbance such as timber harvest, Gurtz & Wallace (1984) concluded that invertebrates responded by increasing their abundance on physically larger substrates that required more en-
55 16
3.0 richness diversity
12
2.5
a
a
a 2.0
10
b
8
1.5
b
6
c
1.0
4
mean shannon-weiner diversity
richness (mean number of taxa)
14
0.5 2 0
(a)
0.0 wood
cobble
gravel 400
5000
4000 300
a
a 3000
200
b
2000
biomass (mg per m2)
density (mean number per m2)
density biomass
100 1000
b 0
0 wood
(b)
cobble
instream habitat type
Figure 5. (a) Macroinvertebrate mean richness (±se) and diversity (±se) across instream habitats. (b) Macroinvertebrate mean density (±se) and mean biomass (±se) across large woody debris and cobble habitats. Bars with same letters are not significantly different from each other, Fishers LSD ( p ¼ 0.05).
ergy to move. In this study, the physical stability, structural complexity and ability to retain organic matter resources for invertebrate consumption, may all have been important. Invertebrate richness and diversity were greatest on wood followed by gravel and cobble substrates. This could have been due to a combination of physical stability and structural complexity of the substrates. Woody debris is physically larger than either cobble or gravel substrates, and has greater structural complexity than cobble substrates, possibly allowing for a more stable habitat, greater variety of food
resources and refugia for invertebrates. Wallace et al. (1995) showed an increase in coarse and fine particular matter accumulation after log additions to a stream in North Carolina. In addition, large number of taxa have been found in association with wood as shown by Dudley & Anderson (1982) who recorded 56 taxa in the Pacific Northwest closely associated with woody debris and another 129 species as facultatively associated. Although mean invertebrate densities and biomass were not comparable between gravel and the other two habitats because their numbers were
56 wood
grav el
cobble
Fc (1.1%) Fc (5.0%) Sc (26.8%)
Sc (54.4%) Sc (45.6%)
Gc (19.9%)
Gc (21.7%) Sh (4.9%) Pr (11. 6%)
Sh (17.1%)
Gc (20. 7%)
Sh (2.1%)
Fc (18.1%)
Pr (48%)
Pr (3.6%)
Figure 6. Proportion of functional feeding groups, based on biomass, across instream habitats. OG – old growth, CC – clearcut, YA – alder-dominated young growth, YC – conifer-dominated young growth, Shredders (Sh), Gathering-collectors (Gc), Filteringcollectors (Fc), Scrapers (Sc), Predators (Pr).
based on a volume rather than a surface area, gravel substrates were areas of high numbers and biomass of invertebrates. Gravel is more structurally complex than cobble. Cobble is generally smooth surfaced while the gravel habitat has interstitial spaces for refuge and detritus accumulation. Culp & Davies (1985) showed the importance of interstitial detritus in gravel substrates in determining the distribution of macroinvertebrates, with greater number of invertebrates associated with either high or low levels of red alder detritus, as opposed to hemlock. All functional groups were represented in each of the three habitats under study. Scrapers made up a large proportion in each of the habitats; however, each habitat had a dominant functional group associated with it. Woody debris had the highest relative proportion of shredders, reflecting its ability to retain coarse particulates (i.e. leaf litter and detritus) (Bilby & Likens, 1980). Cobble substrates had the highest relative proportion of filtering-collectors suggesting their importance to this group even in the presence of stable woody debris substrates. Gravel substrates had the highest relative proportion of predators, likely due to the presence of high abundances of Harpactacoida copepods and Chironomidae as a prey base. Instream habitat quantification revealed that sections of streams examined were comprised primarily of cobble and gravel substrates. The proportion of woody debris present and available for
macroinvertebrate colonization in OG forest management types averaged only 12% of the total habitat available, and the proportion of woody debris in CC types was also small (3%). Very few large pieces of wood remained in these streams after the harvesting event and the majority of wood that was available was small and appeared to be slash from the harvesting event itself. Management areas dominated by red alder had a small proportion of woody debris (5%). Bilby & Ward (1991) compared woody debris inputs to streams from old growth, clearcut and second growth forests and concluded that large woody debris inputs from young growth forests with red alder riparian stands were minimal. Surprisingly, YC sites had the greatest proportion of woody debris (21%); however, most of the woody debris was larger in diameter than that of the current forest, suggesting that the origin of the wood was not from the young growth stand and was in the streams before the harvest or as a direct result of the harvest. In addition to the functional importance of woody debris in: (1) channel and pool formation (Keller & Swanson, 1979); (2) retention of organic matter (Bilby, 1981); and (3) serving as a habitat for invertebrate colonization (Gurtz & Wallace, 1984), it is biologically important as a food resource for some invertebrates (Anderson et al., 1979; Kaufman & King, 1987). Woody debris habitat contributed significantly to the richness, diversity and
57 abundance of macroinvertebrates in these small southeastern Alaskan headwater streams.
Conclusions The results of this study indicated that forest succession after timber harvest affected macroinvertebrate community structure in the upland forests of southeastern Alaska. This appeared to be the result of changes in food availability relative to the OG forest management type. First, canopy removal has led to increases in sunlight penetration to the stream and consequently to higher autochthonous food resources, which results in greater biomass and densities of scraper and gathering-collector invertebrates in CC forest management types. Secondly, in YA sites, provision of more labile allochthonous organic matter (i.e. red alder litter) has increased the abundance and number of different shredder invertebrates. Lastly, timber harvest has led to the presence of more filtering-collector organisms in all harvested conditions. Although clearcut forest management types had the highest densities of invertebrates, alder-dominated young growth types had high densities of invertebrates in addition to a richer and more diverse fauna. Thus, management of upland forests in southeastern Alaska could provision for red alder riparian vegetation to maximize macroinvertebrate diversity and abundance. The evaluation of instream habitats showed that woody debris and gravel substrates contributed to high densities and biomass of invertebrates to upland southeastern Alaskan headwater streams. In addition, woody debris also contributed to high taxa richness and diversity. Maintenance of both gravel and woody debris substrates within these streams is advantageous for large number of benthic invertebrates that could potentially benefit the diet of downstream economically important fish communities (Wipfli & Gregovich, 2002). Results of this study suggest that increased invertebrate abundance and diversity could be achieved by encouraging red alder along the stream margin while maintaining sources of conifer wood to the stream. Selectively cutting a proportion of the riparian vegetation along headwater streams draining young forests could potentially
provide greater sunlight penetration to stimulate autochthonous production and therefore invertebrate production while maintaining conifer wood recruitment to the stream. Further planting of red alder along the stream margin for labile sources of allochthonous organic matter should also help support more productive and diverse aquatic communities.
Acknowledgements We would like to thank D. Busch, T. Gomi, and B. Graham for assistance in the field, and E. McCoy for help in the lab. Thanks to T. Gomi for providing Figure 1. We would also like to acknowledge the following funding sources for this research: Urban Affairs Program, College of Natural Sciences, Department of Entomology, and the Bill and Melinda Gates Foundation. Lastly, I thank the Craig Ranger District of the USDA Forest Service for logistical support, and the Pacific Northwest Research Station, USDA Forest Service, Juneau, AK, for research funding and logistical support.
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