ecosystem recovery in restored headwater streams: the role of enhanced leaf retention

Journal of Applied Ecology

2002

39

, 145–156

© 2002 British Ecological Society

Blackwell Science Ltd

Ecosystem recovery in restored headwater streams: the role of enhanced leaf retention

TIMO MUOTKA and PEKKA LAASONEN

Department of Biological and Environmental Science, University of Jyväskylä, PO Box 35, 40351 Jyväskylä, Finland; and Oulanka Biological Station, 93999 Kuusamo, Finland

Summary

1.

There is controversy over how the success of ecological restoration should be meas-ured. Traditionally, emphasis has been placed on species diversity and other communityattributes, whereas the restoration of ecosystem processes has received less attention.Here, we combine replicated field experiments and a field survey to provide an ecosystem-level measure of stream restoration success.

2.

Numerous headwater streams in Finland, and in many other parts of the world, havebeen channelized for timber transport, resulting in channels with simplified structureand flow. Recently, programmes have been launched to restore these streams to theirpre-channelization condition. While the efficacy of restoration in improving fish habitathas been tested, little is known about effects on other stream biota or on the retentionof leaf litter, despite its importance in trophic dynamics of forested headwater streams.Using a before-after-control-intervention (BACI) designed experiment with multiplereference and experimental streams, we examined restoration-induced changes inretention efficiency by conducting leaf-release experiments before (1993) and after(1996) restoration.

3.

Substrate heterogeneity increased, but moss cover decreased dramatically followingrestoration. Retention efficiency in restored streams was higher than in channelized, butlower than in natural, streams. Algae-feeding scrapers were the only macroinvertebrategroup whose density increased significantly after restoration.

4.

Aquatic mosses were a key retentive feature in both channelized and natural streams,but their importance to retention was strikingly reduced by restoration. Duringrestoration work, mosses are detached from large areas of the stream bed, exposingbare stone surfaces for colonization by periphytic algae.

5.

A more effective restoration technique would involve the use of moss transplants, orthe addition of large woody debris, to increase retentiveness and thus enhance theavailability of organic material to benthic consumers. This case study on rivers illustrateshow restoration projects benefit from an ecosystem perspective and from measures ofecosystem processes in assessing restoration success.

Key-words

:

aquatic mosses, benthic invertebrates, ecosystem-level responses, foreststreams, leaf litter, stream restoration, substrate heterogeneity


(2002)

39

, 145–156

Introduction

During the last few decades, ecosystem restoration hasattained a central role in natural resource management,and this trend has been paralleled by a dramaticexpansion of restoration ecology as a scientific dis-cipline (Young 2000). Although many practitioners still

view restoration as a ‘poor second to the preservationof original habitats’ (Young 2000), its potential as amanagement tool, or even as a means of preventing thecontinual loss of biodiversity (Wilson 1992), has beenwidely acknowledged. Unfortunately, restoration deci-sions are often based on intuition rather than rigorousscience. For example, although it is generally recognizedthat each restoration project should have a clearlydefined goal, there is still considerable controversy overhow to assess restoration success. Should it be evalu-ated through the measurement of species richness,

Correspondence: Timo Muotka, Department of Biologicaland Environmental Science, University of Jyväskylä, PO Box35, 40351 Jyväskylä, Finland (e-mail [email protected]).

JPE_698.fm 45 Thursday, January 17, 2002 2:57 PM

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T. Muotka & P. Laasonen

© 2002 British Ecological Society,


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diversity or other community attributes (= structuralrestoration endpoints), or should we aim to restoreecosystem processes (= functional endpoints) (Palmer,Ambrose & Poff 1997)? Assessing restoration success isnot easily amenable to hypothesis testing using con-trolled experiments. At the scales involved, there areproblems with balanced experimental design, lack ofcontrols, and the time–course of experimentation(Michener 1997). Nevertheless, restoration providesan opportunity to monitor ecosystem-level, human-controlled changes at relatively large spatial or temporalscales, and therefore its potential contribution to test-ing general ecological theory is obvious (Bell, Fonseca& Motten 1997; Palmer, Ambrose & Poff 1997).

Factors threatening the biodiversity of headwaterstreams are numerous, but habitat loss and degrada-tion probably rank highest (Allan & Flecker 1993).Channelization is one of the major causes of habitatdegradation in headwater streams. Streams have beenchannelized for diverse purposes, but consequences forstream habitat structure and ecosystem functioning arethe same: channelized streams have lost much of theirnatural heterogeneity, being characterized by simplifiedflow patterns, poorly retentive channels and weakeningof the aquatic–terrestrial linkage (Petersen

et al

. 1987).The biotic communities of headwater streams in

temperate forested areas are heavily dependent onallochthonous detritus, which enters the stream mainlyin the form of autumn-shed leaves. Once retained ontothe stream bottom, leaves enter a processing sequencewhereby they are integrated into detrital food webs.Many detritivorous invertebrates have their majorgrowth period in winter, coinciding with the peak avail-ability of well-conditioned leaf detritus (the ‘shredderresponse model’ of Cummins

et al

. 1989). However, theamount of benthic organic matter is not regulatedsolely by litter input, but also, and perhaps even moreimportantly, by the capacity of a stream to retain ter-restrial inputs (Cummins

et al

. 1989). It has frequentlybeen shown that debris dams dramatically alter theretention characteristics of a stream, thereby regulat-ing the abundances of benthic organisms, especiallydetritivorous invertebrates (Bilby & Likens 1980;Angermeier & Karr 1984; Smock, Metzler & Gladden1989; Trotter 1990; Ehrman & Lamberti 1992; Wallace,Webster & Meyer 1995a; Wallace

et al

. 1999). Whilemost studies addressing the role of bed retentivity tostream ecosystem dynamics have been based on addi-tion or removal of large woody debris or macrophytes(Koetsier & McArthur 2000), Dobson & Hildrew(1992) observed enhanced detritus availability onstream beds following the addition of small leaf traps.Dobson

et al

. (1995) suggested that such retentiondevices could be used as a management tool to increaseinvertebrate production of headwater streams. Al-though such small, non-woody, structures clearlyenhance bed retentiveness, their potential use in therestoration of channelized headwater streams hasremained largely unexplored.

In Finland, as also in vast areas of north-westernRussia (Jutila 1992) and forested parts of northernUSA and Canada (Sedell, Leone & Duval 1991),numerous streams have been dredged to facilitatetimber floating. Water transport of timber ceased inFinland in the late 1970s, and extensive restorationprogrammes were launched thereafter to rehabilitatethese streams to their original pre-channelizationcondition. Most of these streams are unregulated and,as most boreal rivers have restricted floodplains (Petersen,Gislason & Vought 1995), the return of a natural flowregime (Poff

et al

. 1997; Robertson, Bacon & Heagney2001) is not an issue in their restoration. Instead,restoration mainly aims at providing better habitat forimportant game fish such as brown trout

Salmo trutta

L. It is usually conducted using a bulldozer, and themost commonly used restoration measures includeinstallation of boulders, flow deflectors, cobble ridgesand other enhancement structures. Furthermore,cobble-to-pebble sized stones are used to create nurseryhabitats for juvenile trout (Yrjänä 1998). While theefficacy of these measures for fish habitat improvementhas been tested using hydraulic modelling (Huusko &Yrjänä 1997), very little is known about their long-termeffects on other stream biota and ecosystem processes.Obviously, increased habitat heterogeneity shouldenhance the retentive capacity of a stream, which in turnshould lead to increased densities of benthic organisms,especially those directly dependent on benthic detritus.

Our aim in this study was to examine restoration-induced changes in the retentive characteristics ofboreal headwater streams. Leaf litter and its retentionare critical for the energy flow and trophic dynamics offorest streams, providing an ecosystem-level perspectivefor our study. Specifically, we addressed the followingquestions. (i) Is the retentive efficiency of a streamenhanced by installing restoration structures? (ii) Arethere any adverse effects of restoration related tochanges in stream habitat structure? (iii) Is the effect ofvarying discharge on retention efficiency similar inchannelized, restored and natural streams? (iv) Dobenthic invertebrates respond to habitat restoration aspredicted based on changes in retention efficiency?We believe that our results are not specific to borealstreams channelized for timber floating but, due to thepotentially far-reaching effects of enhanced leaf reten-tion on stream ecosystem functioning, they should beapplicable to a much wider range of channelized streamsor, more generally, streams with reduced substrateheterogeneity.

Methods

The study was conducted in eight headwater streams innorth-eastern Finland (65

°

35

′

to 67

°

31

′

N; 27

°

58

′

to29

°

31

′

E). All drain forested lowland areas, and their

JPE_698.fm Page 146 Thursday, January 17, 2002 2:57 PM

147

Recovery of restored streams



,

39

,145–156

riparian zones are dominated by deciduous trees, espe-cially birch

Betula

pubescens

Ehrh., alder

Populus

trem-ula

L., European aspen

Alnus

incana

L. and willows

Salix

spp. They are second- to third-order streams withcircumneutral, oligotrophic and often slightly humicwater. Four of the streams (Kutinjoki, Kosterjoki,Loukusajoki and Poika-Loukusa) were heavily dred-ged in the 1950s to facilitate log transport. Thesestreams were reconstructed by the OstrabothnianEnvironment Centre in 1993 to restore their originalheterogeneous bed structure. We selected these streamsrandomly from a large number of channelized head-water streams in northern Finland for which a restora-tion scheme was available by 1992. As references, weused four approximately similar-sized natural streams(Aventojoki, Kalliojoki, Merenoja and Putaanoja)from the same area, also selected randomly from alarge pool of unmodified streams. The main differencebetween the two sets of streams was that the naturalstreams had undisturbed, highly heterogeneous,stream beds. A feature common to nearly all headwaterstreams in our study area was that their riparian zoneshad been under heavy forestry practices (e.g. timberharvesting and forest drainage), especially during the1950s–1970s. The riparian zones of our study streamshave now remained intact for about 30 years, which,however, is far too short an interval for streamside for-ests to mature. Thus, input of large woody debris hasbeen minimal and the streams, including the ones withunaltered channels, contained very few debris dams.The term ‘natural’ is used here to refer to the streamchannel, not the surrounding terrestrial landscape.Obviously, in a truly pristine condition, boreal foreststreams should contain abundant debris dams. This isindicated by the fact that streams draining across theFinnish–Russian border typically harbour large woodydebris (LWD) densities almost 20 times higher on theRussian side where the impact of forestry on riparianzones has been negligible (P. Liljaniemi, personal com-munication; Vuori, Luotonen & Liljaniemi 1999).

Restoration measures were similar in all fourstreams, consisting of placement of boulder dams andflow deflectors, and digging excavations. Boulder damswere constructed by setting boulders side by side acrossthe river, and the inside of the structure was partly filledwith cobbles and pebbles. Deflectors of approximatelyhalf the width of the channel were created to increasediversity in water velocities and to deepen the channel.Deflectors are known to be effective in guiding the cur-rent in places where water velocity exceeds 60 cm s

–1

(Brookes 1988). The spawning habitats for salmonidfish were improved by using sorted gravel to createspawning grounds in suitable stream areas with relat-ively swift currents. Addition of woody debris is rarelyused as a habitat improvement technique in Finland(Yrjänä 1998).

Because the construction of restoration structureswas restricted by site-specific variation in the availabil-ity of suitable material along or close to stream edges,

the resulting bed structure was not exactly the sameamong the restored streams. Therefore, to assess thewithin-group similarity of treatment and referencestreams, and to determine whether they differed fromstreams in other groups, we measured several habitatcharacteristics within 50-m sections at each stream. Wepositioned transects perpendicular to the flow at 3-mintervals, and for each transect we recorded depth,water velocity, moss cover, substrate size and presence/absence of wood in 0·5-m intervals. To assess whetherrestoration enhanced bed heterogeneity, we quantifiedbed roughness (

k

) at each site using a contour-plottingdevice modified from Young (1993). The device was50 cm long, consisting of a continuous row of measur-ing rods (diameter 0·8 cm). Measurements were madein 1·5-m longitudinal transects, each transect consist-ing of three successive 50-cm sections. To obtain ameasurement, the device was pressed firmly against thebottom and distance from a horizontal support wasmeasured for each rod. The standard deviation for thelength of the rod below the support was calculated foreach transect (Statzner, Gore & Resh 1988). Eight 1·5-m transects, distributed evenly across the study section,were established at each site. Distances from perman-ent reference marks on stream banks were used toascertain that measurements before and after restora-tion were made at exactly the same positions. Relativebed roughness,

k

/

D

(roughness/depth), was used asan indicator of substrate heterogeneity (Gordon,McMahon & Finlayson 1992). Finally, stream flow wasmonitored in each stream through permanent gaugingstations located at the same or a nearby stream.

-

To examine any changes in the retention capacity of astream after restoration, we performed a set of artificialleaf releases. Experiments before and after restorationwere conducted 3 years apart, 1–2 weeks before and 3years after restoration (in September to early October1993 and 1996, respectively). Leaf retention is knownto be controlled by stream discharge, and even a slightvariation in discharge may cause drastic changes inretention (Snaddon, Stewart & Davies 1992). Usingstream flow data, we therefore conducted the releasesat closely corresponding discharges in all restoredstreams on both years (Table 1). By contrast, dischargein two of the natural streams (Aventojoki andPutaanoja) was highly variable in September–October1996, and therefore the experiment could be repeatedin only two of the natural streams (Merenoja andKalliojoki) in 1996 at discharges approximating thoseduring releases in 1993. Due to inadequate replication,only data for the two streams where the experiment wasrepeated on both occasions were included in statisticalanalysis, but data for the other two streams are alsoreported for comparative purposes.

A 50-m long study section was established at a rel-atively uniform riffle section of each stream. We first


148




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surveyed the number of suitable riffles in each stream(three to five per stream) and then randomly selectedthe site to be used for the experiments. We used red andyellow plastic strips (8

×

4 cm) as artificial leaves in ourexperiments. In preliminary trials, we used spray-painted leaves of birch and alder, but were unable torelocate most of the leaves retained by the substratum.We thus decided to use artificial leaves, because they areeasier to find, have been used successfully in previousexperiments, and are known to behave much likefreshly fallen natural leaves entering a stream (Speaker

et al

. 1988). In each experiment, 2000 plastic stripswere scattered across the width of the channel during a5-min period at the upstream end of the study section.The downstream end of the section was blocked witha wire screen (mesh size 7 mm). Three hours after therelease, we counted the number of leaves that hadtravelled through the study section and collected on thescreen, and searched the entire reach for leaves thathad been retained within the section. Speaker, Moore& Gregory (1984) and Petersen & Petersen (1991)have reported that the number of leaves in transportstabilizes within 2–3 h of release.

For each leaf found, we recorded the distance trav-elled (nearest 1·0 m). We also recorded the retainingobject (boulder, cobble, gravel, coarse woody debris,moss, other aquatic vegetation, backwater, streamedge) for each leaf. The streams were nearly devoidof bole wood, and most of the woody material con-tributing to leaf retention was small twigs < 2 cm indiameter.

The effect of discharge on leaf retention was meas-ured in one randomly selected stream for each streamgroup. Stream Kalliojoki represented a natural stream,Kosterjoki a channelized, and Kutinjoki a restoredstream. In each of these streams, we conducted a leaf-release experiment with 2000 artificial leaves at fouror five discharges, using the methods described above.The discharges for conducting the experiments wereselected to represent the range of flows for each stream.In this experiment, only the number of leaves thatcollected on the downstream screen after 3 h wasrecorded.

Macroinvertebrate samples were collected by kick-sampling (net frame 25

×

25 cm, mesh size 0·25 mm),and four timed samples were taken from each site. Toenable comparisons of benthic densities among thestream types, we standardized our sampling effort asmuch as possible. The distance kicked along thestream was exactly 1·0 m, and the person taking thesample kicked the substratum vigorously for 60 s. Allbenthic collections were made by the same person(PL), and a field assistant ascertained that the 1-m linewas not crossed. Samples were taken 1–2 weeks before,and again 3 years after, restoration, in both treatmentand reference streams. The post-restoration sampleswere collected at the same time of the year as the pre-samples 3 years earlier (September–early October).Samples were preserved in 70% alcohol, and inverte-brates were later sorted in the laboratory. Animalswere identified to species or genus, and they wereassigned to functional feeding groups accordingto Malmqvist & Brönmark (1985) and Merritt &Cummins (1996).

Our study design was a partial before-after-control-intervention (BACI)-type design (Underwood 1994):we had replicates for ‘control’ sites (here, the naturalreference streams) and ‘impacted’ sites (the restoredstreams); however, our ‘before’ and ‘after’ sampleswere unreplicated through time. Therefore, becausewe lacked a true temporal trajectory of changes inbenthic communities before and after restoration, itmust be remembered that any differences betweenthe before (1993) and after (1996) samples in thenatural streams could potentially be due to otherchanges that happened to be coincident with the res-toration works.

Leaf transport distances were fitted to the negativeexponential model of

L

d

= L

0

e

–kd

, where

L

0

is the numberof leaves released into the reach and

L

d

is the number ofleaves in transport at distance

d

from the release point.

Table 1. Habitat characteristics (mean and range; n = 4 for each stream type) of the study streams. Relative bed roughness iscalculated as the ratio of bed roughness/water depth (k/D; Gordon, McMahon & Finlayson 1992). Stone size was measured asthe largest stone diameter

Stream type

Channelized Restored Natural

Discharge (m3 s–1) 0·93 (0·64–1·11) 0·90 (0·55–1·11) 0·63 (0·35–1·10)Stream width (m) 6·0 (5·5–7·0) 7·0 (6·0–8·0) 5·5 (5·0–6·5)Depth (cm) 30 (26–32) 33 (28–36) 24 (20–30)Current velocity (cm s–1) 33 (25–37) 31 (27–36) 27 (22–31)Stream gradient (%) 0·64 (0·56–0·71) 0·64 (0·56–0·71) 0·74 (0·63–0·95)Stone size (cm) 16·7 (11·5–22·3) 18·5 (12·3–31·8) 29·8 (20·9–38·8)Bed roughness (cm) 5·8 (5·5–7·0) 8·0 (6·7–9·5) 9·1 (8·4–11·4)Relative roughness 0·20 (0·18–0·22) 0·26 (0·23–0·28) 0·38 (0·30–0·45)Moss cover (%) 35 (16–44) 9 (5–19) 47 (31–58)


149




,

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,145–156

The slope,

–k

, is the instantaneous leaf-retention rateand 1/

k

is the average distance travelled by a leaf in thestream before its retention (Speaker, Moore & Gregory1984).

The aim of restoration is usually either to restorea system to its near-natural pre-disturbance state, orsimply to improve its current condition. Thus, thekey questions when testing for restoration-inducedchanges are: (i) was a response variable positivelyaffected by restoration, and (ii) was the status of anundisturbed near-pristine system achieved? Therefore,we were mainly interested in two comparisons: differ-ences between (i) channelized and restored streams;and between (ii) restored and natural streams, whereascomparison between channelized and natural streamswas deemed less interesting. Our basic approach was toperform planned, a priori, comparisons using

t

-tests(Winer 1971; Day & Quinn 1989) for each responsevariable to test for the predetermined patterns of dif-ferences among our ‘treatment’ (stream) groups. Thesecomparisons are non-orthogonal (restored streams areincluded in both comparisons) but, as argued by Winer(1971), ‘the meaningfulness of a comparison is moreimportant than its orthogonality’. The response vari-ables examined were: key habitat variables (substrateheterogeneity, moss cover), retention efficiency, reten-tion coefficient and densities (log-transformed) offunctional feeding groups of benthic macroinverte-brates. All statistical tests were performed using SPSS7.5 for Windows (SPSS Inc. 1997).

Results

Stream discharge during the leaf-release experimentswas closely similar in streams before and after restora-tion. The four natural streams, however, had slightlylower discharges (Table 1). This was unavoidablebecause practically all larger streams in our study areahave been channelized for log floating, and only thesmallest streams (generally less than 5 m wide) havebeen left undisturbed. Nevertheless, size differencesamong our streams were not too pronounced: the aver-age discharge before and after restoration was 0·93(1993) and 0·90 m

3

s

–1

(1996), respectively, while it was0·63 m

3

s

–1

(1993) in the natural streams.The most distinct differences in habitat structure

between the stream types were related to substrateheterogeneity and moss cover (Table 1). Restoredstreams had significantly higher substrate complexitythan channelized streams (planned comparison test,

t

= 2·46,

P

= 0·036, d.f. = 7), and did not differ fromnatural streams (

t

= 1·33,

P

= 0·217, d.f. = 7). Mosscover in restored streams was significantly lower thanin both channelized (

t

= 3·67,

P

= 0·005, d.f. = 7) andnatural streams (

t

= 3·92,

P

< 0·001, d.f. = 7). Overall,natural streams were characterized by relatively largeheterogeneous substrates with extensive moss cover,

whereas the channelized streams had fairly homogene-ous substrates composed mainly of cobbles and smallboulders. Restoration clearly enhanced substrate het-erogeneity, approaching the level of natural streams.Moss cover in the channelized streams was, however,drastically reduced following restoration (Table 1).The two variables most relevant to leaf retention (mosscover, bed roughness) showed no temporal trendsbetween the sampling occasions (

P

> 0·20 for bothvariables).

-

Restoration enhanced the retentive capacity of astream to leaf litter inputs, although the pre- vs. post-restoration difference in retention efficiency was notquite significant (planned comparison test

t

= 2·06,

P

= 0·069, d.f. = 7). Natural streams retained leavesmore efficiently than the restored streams (

t

= 3·87,

P

= 0·040, d.f. = 7) (Fig. 1a), but they showed con-siderable variation in their retention efficiency (Fig. 1b).Calculated across all sites, number of leaves retainedwas not significantly correlated with stream discharge(

r

s

= 0·32,

P

= 0·310,

n

= 12) but was significantlycorrelated with substratum heterogeneity (

r

s

= 0·98,

P

< 0·001,

n

= 12).Leaf-retention curves conformed generally well to a

negative exponential model, all coefficients of determ-ination (

r

2

) being > 0·90 (Fig. 2). Nevertheless, differ-ences in retention patterns among the stream typeswere distinct. Retention rates were significantly higherin the natural (mean

k

±

1 SE 0·0203

±

0·0061,

n

= 2)than in the restored streams (0·0075

±

0·0025,

n

= 4;planned comparison test

t

= 2·63,

P

= 0·027, d.f. = 7),and also in streams after restoration compared withbefore (0·0020

±

0·00031,

n

= 4) restoration (

t

= 3·09,

P

= 0·013, d.f. = 7). In the channelized streams, leaveswere retained at relatively uniform rates throughoutthe study reach, whereas in the natural streams and insome, but not all, restored streams, retentive structureswere patchily distributed (Fig. 2). The average distancetravelled by a leaf was almost an order of a magnitudehigher in the channelized than in the natural streams(mean distance of 528·5 vs. 65·2 m, respectively), butintermediate (mean: 179·0 m) in the restored streams.

Cobbles and mosses were the most retentive struc-tures in the channelized streams, whereas the role ofmosses was negligible in the restored streams where themajority of leaves was retained by cobbles (Fig. 3). Inthe natural streams, mosses were the most importantretentive feature, followed by boulders. The only signi-ficant difference between the stream types was foundfor mosses, which were less important in the restoredthan in the channelized (planned comparison test onarcsin-transformed data

t

= 2·74,

P

= 0·023, d.f. = 7)or natural streams (

t

= 2·32,

P

= 0·045, d.f. = 7).Woody debris, stream margins and aquatic vegetation(other than mosses) retained 15–20% of the leaves, withnegligible differences between the stream types.


150




,

39

,145–156

Linear regression models provided good fit to data onleaf-retention efficiencies at various discharges in allthree streams studied (Fig. 4). Slopes of regression linesvaried significantly among the streams (

F

2,8

= 8·35,

P

=0·011). Retention efficiency decreased with increasingdischarge more dramatically in stream Kalliojoki (anatural stream) than in the other two streams (Tukey’stest for multiple comparisons

P

< 0·05), whereasslope for the restored stream (Kutinjoki) did not differsignificantly from that of the channelized stream(Kosterjoki; Tukey’s test

P

> 0·05).

The only feeding group whose densities increasedsignificantly after restoration was algae-scrapinginvertebrates, with densities almost three times higherin the restored than in the channelized streams(Fig. 5; planned comparison test

t

= 2·40,

P

= 0·048,d.f. = 7). Densities of detritivores (shredders, filterersand collector-gatherers) changed little during the 3-year period between the pre- and post-restoration sam-ples. Densities in natural streams did not differ betweenthe years (paired sample

t

-tests on log-transformeddata,

P

> 0·30 for all groups). All feeding groups had a

tendency towards maximum density in the naturalstreams, but due to large among-stream variation dif-ferences between the restored and the natural streamswere significant only for shredders (

t

= 2·89,

P

= 0·037,d.f. = 7) and scrapers (

t

= 3·41,

P

= 0·019, d.f. = 7) andnearly so for predators (

t

= 2·23,

P

= 0·061, d.f. = 7).

Discussion

The in-stream enhancement structures used for habitatrestoration in our study streams caused a generalincrease in the capacity to retain leaf litter. Neverthe-less, after 3 years, restored streams did not approachthe retentive efficiency of natural streams. Comparedto most previous studies, our channelized streamsshowed extremely poor retentivity, which was clearlyrelated to their highly simplified bed structure. Our fig-ure of 8% of leaves retained was much lower than the40% reported by Petersen & Petersen (1991) for chan-nelized agricultural streams in southern Sweden. How-ever, our natural streams were also less retentive thantheirs, and the difference between these two studies ismainly explained by the fact that our streams werelarger, with higher discharges, than the headwaterstreams studied by Petersen & Petersen (1991).

Leaf transport curves conformed well to the negativeexponential loss model, although a linear model pro-duced nearly as good a fit for all channelized and most

Ret

entio

n ef

ficie

ncy

( %

)

0

20

40

60

80

0

20

40

60

80

(a)

(b)

KU LO PL KO KA ME PU AV

Before restorationAfter restorationNatural streams 1993Natural streams 1996

Before restorationAfter restorationNatural streams 1993Natural streams 1996

Fig. 1. Retention efficiency (percentage of leaves retained out of 2000; mean ±1 SE) of the three stream types (a), and of each studystream separately (b). Streams before and after restoration: Kutinjoki (KU), Loukusajoki (LO), Poika-Loukusa (PO), Kosterjoki(KO); natural streams: Kalliojoki (KA), Merenoja (ME), Putaanoja (PU), Aventojoki (AV). Sample size is four for each streamtype, except for natural streams in 1996 where n = 2.


151




,

39

,145–156

restored streams. This means that retentive structureswere much less patchily distributed in the channelizedand the restored streams than in streams with unmodi-fied bed structure. A highly patchy distribution ofretention sites is typical of streams with debris dams,which contribute markedly to the distribution oforganic matter, resulting in extreme patchiness of com-munity and ecosystem functions in these streams(Smock, Metzler & Gladden 1989). Although debrisdams occurred rarely in our study sites, our naturalstreams were characterized by the presence of a fewexceptionally retentive patches (Fig. 2). This suggests

that, in the absence of large woody debris, other bedstructures assume a key role in the retention process ofunmodified headwater streams. The only striking dif-ference in the relative importance of retention struc-tures among our stream types was the reducedimportance of mosses in recently restored streams,compared with channelized and natural streams. Itmay indeed be that mosses are the key retentive featurein many headwater streams lacking woody debris,although differently sized stones are also important.Overall, our results suggest that structures used forstream enhancement in Finland do not effectively

(a) Kutinjoki

0

20

40

60

80

100

(b) Loukusajoki

(c) Poikaloukusa

% le

aves

in tr

ansp

ort

0

20

40

60

80

100

(d) Kosterjoki

(e) Kalliojoki

0

20

40

60

80

100

(f) Merenoja

(g) Putaanoja

0 10 20 30 40 500

20

40

60

80

100

(h) Aventojoki

0 10 20 30 40 50

BE: y = 99·9 e(–0·00273x), r 2 = 0·98AF: y = 97·6 e(–0·00715x), r 2 = 0·93

BE: y = 99·0 e(–0·00141x), r 2 = 0·92AF: y = 97·0 e(–0·00492x), r 2 = 0·99

BE: y = 100·5 e(–0·00224x), r 2 = 0·94AF: y = 102·9 e(–0·01147x), r 2 = 0·90

BE: y = 100·1 e(–0·00167x), r 2 = 0·99AF: y = 98·4 e(–0·00329x), r 2 = 0·92

y = 89·3 e(–0·02448x), r 2 = 0·92 y = 99·4 e(–0·03580x), r 2 = 0·91

y = 94·0 e(–0·00926x), r 2 = 0·96 y = 102·0 e(–0·01143x), r 2 = 0·96

Distance from release point (m)

Fig. 2. Relationship between leaf retention (percentage of leaves in transport) and distance travelled from the release point in eachstudy stream. The regression line represents fit to the negative exponential model. (a–d) streams before (BE, black dots) and after(AF, open dots) restoration; (e–h) natural streams. Due to extensive overlap, only data for 1993 have been presented for the twonatural streams (e, f ) where the release experiment was repeated in both years.


152




,

39

,145–156

mimic the physical complexity of naturally retentivestream channels.

It is well known that stream discharge greatly influ-ences retention efficiency: the higher the discharge,the less retentive a stream is to allochthonous inputs(Speaker

et al

. 1988; Jones & Smock 1991; Snaddon,Stewart & Davies 1992). According to our study,channelized, restored and natural streams differ inhow tightly leaf retention is linked to discharge. Thechannelized stream retained leaves ineffectively atall discharges, whereas the natural stream was highlyretentive at base flows, but much less so at higher dis-charges. A corresponding observation was made byWebster

et al

. (1987) in a set of laboratory trials wheredischarge and substrate were directly manipulated.They found that smooth surfaces with little structuralcomplexity were ineffective in trapping seston on all

discharges, whereas increased discharge greatly reducedretention on more complex (artificial turf, gravel)substrates. It must be noted, however, that the effect ofdischarge on retention is closely linked to water depth,because the probability of a leaf coming into contactwith the substrate decreases with increasing depth(Webster

et al

. 1994). Our natural streams were gener-ally shallower than the other streams, and therefore wecannot exclude water depth as a partial explanation forthe observed differences in leaf-retention rates.

Because most CPOM (Coarse Particulate OrganicMatter) export occurs during major storms (Wallace

et al

. 1995b), the timing and extent of spates may becritical determinants of organic matter storage in streams(Jones & Smock 1991; Maridet

et al

. 1995). The flowregime of boreal streams typically exhibits an annualpeak during spring, with a secondary peak in late

Retention structure

% le

aves

ret

aine

d

0

10

20

30

40

50

Woodydebris

Streammargin

Vegetation Moss Cobble Boulder

Before restorationAfter restorationNatural streams

Fig. 3. The proportion of artificial leaves retained (mean percentage ±1 SE) by various retentive structures in each stream type(n = 4 streams per group).

Discharge (m3 s–1)

0·0 1·0 2·0 3·0

Ret

entio

n ef

ficie

ncy

(%)

0

20

40

60

80

y = 46·5 – 12·9xr 2 = 0·81, F = 12·35, P = 0·039

y = 7·9 – 1·8xr 2 = 0·90, F = 17·7, P = 0·052

y = 82·9 – 28·7xr 2 = 0·93, F = 40·08, P = 0·008

Stream types:ChannelizedRestoredNatural

Fig. 4. Relationship between stream discharge and retention efficiency in a natural (Kalliojoki), channelized (Kosterjoki) andrestored (Kutinjoki) stream.


153Recovery of restored streams

© 2002 British Ecological Society, Journal of Applied Ecology, 39,145–156

autumn, during or immediately after leaf fall (Haapala& Muotka 1998). Thus, the retention efficiency ofchannelized and natural (or restored) streams may dif-fer least during the period of major leaf input, resultingin less distinct differences in resource availability forbenthic consumers than could be expected based ondifferences in retention potential (Laasonen, Muotka& Kivijärvi 1998). The lack of debris dams may furtherreduce differences in organic matter storage amongstreams with differing bed retentivity, because stonesare effective in leaf retention only at low discharges,whereas the role of debris dams increases with risingdischarge (Smock, Metzler & Gladden 1989; Jones &Smock 1991; Raikow, Grubbs & Cummins 1995). Itmust be stressed, however, that this part of our studywas unreplicated, and therefore any generalizationsmust await further experimentation.

The dramatic loss of mosses during restoration worksmay have far-reaching effects on benthic communitiesand ecosystem processes of these headwater streams. Arecurring theme in stream ecology has been a consistentrelationship between retention efficiency and abundanceof detritivorous invertebrates (Rounick & Winterbourn1983; Smock, Metzler & Gladden 1989; Prochazka,Stewart & Davies 1991; Dobson & Hildrew 1992;Wallace, Webster & Meyer 1995a). Therefore, consider-ing the increased bed complexity and a correspondingchange in retention capacity, one would have expectedan increase in the importance of detritivores followingstream habitat restoration. This, however, did nothappen, and the only significant change was an increasein the abundance of algae-scraping invertebrates. Webelieve that this result is connected to the removal ofaquatic mosses during restoration works. Channelized

woodland streams characteristically support abundantmoss growth (Laasonen, Muotka & Kivijärvi 1998),probably because timber floating mainly ceased in the1960s, and mosses have had enough time to recolonizethese streams. Mosses clearly are key retentive featuresin headwater streams devoid (or nearly so) of macro-phytes and debris dams. However, during restorationworks mosses are detached from large areas of thestream bed, and bare stone surfaces are exposed forcolonization by periphytic algae. Thus, the resourcebase for benthic consumers shifts from terrestriallyderived detritus to autochthonous production bybenthic algae, with a concomitant increase in scraperabundance. It is probable that, as mosses recover,retention efficiency of these streams will increase, andbenthic communities will gradually shift to detritivore-dominated assemblages dependent on allochthonousorganic material. The effect of mosses on streamecosystem dynamics goes far beyond their role inparticle retention, however. For example, they affordoptimal ‘nursery’ habitats for the early instars of manybenthic invertebrates (Suren & Winterbourn 1992). Byaltering near-bed flow regimes (Nikora et al. 1998),mosses provide benthic animals with hydraulic refugiawhere environmental conditions remain essentiallyunchanged during high-flow events (Lancaster &Hildrew 1993). Unfortunately, little is known about thecolonization and growth rates of aquatic bryophytes,but it can be safely assumed that the full recovery oflarge canopy-forming species (e.g. Fontinalis spp. andHygrohypnum spp.) most effective in particle retentionwill take many years, if not decades (Muotka et al., inpress). This suggests that if the recovery of benthic com-munities and ecosystem processes to pre-channelization

Num

ber

of in

divi

dual

s m

-2

0

200

400

600

800

1500

2000

2500

3000Before restorationAfter restorationNatural streams 1993Natural streams 1996

Scrapers Gathering-collectors

Filterers Shredders Predators

Functional feeding groups

Fig. 5. Densities of macroinvertebrate feeding groups in the pre-(1993) and post-(1996) restoration samples, and in the naturalstreams in the same years. Values are means (±1 SE) of four streams per stream type.


154T. Muotka & P. Laasonen


state is to be enhanced, current restoration practicesmust be modified. A more successful rehabilitationscheme might be achieved by relatively simple measures,however, for example by using moss transplants or,better still, leaving relatively large areas of the streambed untouched to serve as colonization centres formosses after restoration.

Given the above reasoning, one might ask why, then,are the densities of scrapers even higher in naturalstreams, although these generally support much higherdensities of mosses than restored streams? This isprobably also connected to the prominent role ofmosses in organic matter retention in natural foreststreams. Mosses retain not only leaf litter, but also finesuspended particles, thus providing rewarding feedingarenas for deposit-feeding invertebrates. Many of theinvertebrates categorized as scrapers are secondarilycollector-gatherers (e.g. Baetis and Heptagenia may-flies), feeding on fine organic material deposited on thestream bed (Huhta, Muotka & Tikkanen 1995; Merritt& Cummins 1996). Woodland stream ecosystems arelargely fuelled by inputs of organic matter from theterrestrial environment, and it may well be that theseinvertebrates attain a different functional role (i.e.collectors instead of scrapers) in natural forest streamswith abundant moss cover.

If the desired goal of restoration is to increase systemproductivity by enhancing a stream’s retention capa-city (improvement of the present, degraded status of thesystem), this can be achieved by a slight modificationof current restoration practices. However, if the moredemanding goal of mimicking the pre-disturbance near-natural status is to be achieved, more radical measureswill be needed. It may be that such fundamental changescannot be achieved without the use of large woodydebris to enhance the retention efficiency and habitatdiversity of previously debris-free channels. No othernatural in-stream structure contributes as importantlyto retention efficiency as do debris dams, especially duringhigh discharges. As the effects of detritus manipulationare known to propagate up in lotic food webs (Wallaceet al. 1997), even single-interest restorations aimed atimproved salmonid fisheries may ultimately benefitfrom restoration measures that enhance the availabilityof organic material to benthic consumers.

In conclusion, our study shows that in-stream restora-tion does enhance stream bed complexity, but thiscomes with a cost: the use of heavy machinery duringrestoration works caused a drastic reduction of mossbiomass. Because aquatic mosses are clearly a keyretentive feature in boreal headwater streams, theretentive capacity of a stream following restorationdoes not increase as much as could be expected basedon enhanced substrate heterogeneity. The retentivepotential of a stream will probably improve as mossesrecover, but because mosses are slow-growing plantsthis may take years or even decades. Therefore, from anecosystem perspective, better results might be achievedby using softer technology, for example by adding moss

transplants or large woody debris to stream channels,to enhance the transference of terrestrially derivedorganic matter to detritus-based food webs. Moregenerally, the ecosystem-level perspective adopted inthis study might serve as a basis for an effective assess-ment of stream restoration. The use of replicated fieldexperiments with multiple reference sites could aid inplacing restored sites along a recovery gradient fromchannelized (or otherwise degraded) to near-pristinestreams. In this respect, our method resembles the‘start site–target site’ approach of Mitchell et al. (1999)for assessing restoration of heathland vegetation. Animportant aspect of our approach is that it combineswell-targeted field experiments (leaf releases) with afield survey of stream habitats and macroinvertebratecommunities, to provide a combined measure ofrestoration success. Indeed, as suggested by Manel,Buckton & Ormerod (2000), such a combination oflarge-scale surveys and site-specific process-orientatedstudies may provide the most powerful tool availablefor ecologists to assess large-scale anthropogenicchanges on ecosystems.

Acknowledgements

We thank M. Erkinaro and P. Hilkuri for help in thefield and Oulanka Biological Station for logisticalsupport. We also acknowledge the two anonymousreferees for their constructive comments on an earlierdraft of the manuscript. This study is part of theFinnish Biodiversity Research Program (FIBRE), andit was financed by the Academy of Finland (grantnumbers 35586 and 39134 to T. Muotka) and the OlviFoundation (to P. Laasonen).

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Received 15 March 2001; final copy received 8 November 2001


ecosystem recovery in restored headwater streams: the role of enhanced leaf retention

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