Changes in habitat structure, benthic invertebrate diversity, trout populations and ecosystem processes in restored forest streams: a boreal perspective

Changes in habitat structure, benthic invertebratediversity, trout populations and ecosystem processes inrestored forest streams: a boreal perspective

TIMO MUOTKA* , † AND JUKKA SYRJANEN‡

*Department of Biology, University of Oulu, Oulu, Finland†Research Department, Finnish Environment Institute, Helsinki, Finland‡Department of Biological and Environmental Science, University of Jyvaskyla, Jyvaskyla, Finland

SUMMARY

1. Most Finnish streams were channelised during the 19th and 20th century to facilitate

timber floating. By the late 1970s, extensive programmes were initiated to restore these

degraded streams. The responses of fish populations to restoration have been little studied,

however, and monitoring of other stream biota has been negligible. In this paper, we

review results from a set of studies on the effects of stream restoration on habitat structure,

brown trout populations, benthic macroinvertebrates and leaf retention.

2. In general, restoration greatly increased stream bed heterogeneity. The cover of mosses

in channelised streams was close to that of unmodified reference sites, but after restoration

moss cover declined to one-tenth of the pre-restoration value.

3. In one stream, densities of age-0 trout were slightly lower after restoration, but the

difference to an unmodified reference stream was non-significant, indicating no effect of

restoration. In another stream, trout density increased after restoration, indicating a

weakly positive response. The overall weak response of trout to habitat manipulations

probably relates to the fact that restoration did not increase the amount of pools, a key

winter habitat for salmonids.

4. Benthic invertebrate community composition was more variable in streams restored

4–6 years before sampling than in unmodified reference streams or streams restored

8 years before sampling. Channelised streams supported a distinctive set of indicator

species, most of which were filter-feeders or scrapers, while most of the indicators in

streams restored 8 years before sampling were shredders.

5. Leaf retentiveness in reference streams was high, with 60–70% of experimentally

released leaves being retained within 50 m. Channelised streams were poorly retentive

(c. 10% of leaves retained), and the increase in retention following restoration was modest

(+14% on average). Aquatic mosses were a key retentive feature in both channelised and

natural streams, but their cover was drastically reduced through restoration.

6. Mitigation of the detrimental impacts of forestry (e.g. removal of mature riparian

forests) is a major challenge to the management of boreal streams. This goal cannot be

achieved by focusing efforts only on restoration of physical structures in stream channels,

but also requires conservation and ecologically sound management of riparian forests.

Keywords: benthic macroinvertebrates, boreal streams, juvenile trout, restoration assessment, streamrestoration

Correspondence: T. Muotka, University of Oulu, Department of Biology, PO Box 3000, 90014 University of Oulu, Finland.

E-mail: [email protected]

Freshwater Biology (2007) 52, 724–737 doi:10.1111/j.1365-2427.2007.01727.x

724 � 2007 The Authors, Journal compilation � 2007 Blackwell Publishing Ltd

Introduction

During the first half of the 20th century forest industry

grew strongly in Finland and other countries in the

boreal zone. One prominent feature of this develop-

ment was increased exploitation of forest resources in

remote areas. Therefore, the majority of running

waters was dredged to facilitate water transport of

timber, especially in the northern and eastern parts

of the country. In the 1950s and 1960s, this network of

floatways was further expanded, and almost all

streams wide enough for log floating (often no more

than 4–5 m) were dredged, mainly using excavators

(Jutila, 1992; Yrjana, 1998). At its maximum, the total

length of dredged channels in Finland amounted to

approximately 40 000 km, of which 13 000 km were

in use by the 1950s (Lammassaari, 1990). In the 1970s,

water transport of timber was eventually replaced by

road transportation. This marked a turning point in

stream management, with a strong and continuously

growing interest in the restoration of dredged stream

channels. A similar sequence of phases from intense

dredging to restoration can be identified in northern

Sweden, north-western Russia and forested parts of

the northern U.S.A. and Canada (Sedell, Leone &

Duval, 1991; Tornlund & Ostlund, 2002).

Because of the lack of historical data, little is known

about the ecological effects of channelisation in boreal

streams. Nevertheless, channelisation is generally one

of the major causes of habitat degradation in running

waters, and its consequences on stream habitats and

ecosystems are often severe (Allan & Flecker, 1993),

resulting in the loss of structural complexity, simpli-

fied flow patterns and poorly retentive stream

channels. Biologically, the most detrimental effect of

channelisation is the weakening of riparian–aquatic

linkages and reduced retentiveness of allochthonous

organic matter (Petersen et al., 1987). In addition,

channelisation reduces the availability of microhabi-

tats for fishes and other stream organisms (Naslund,

1989; Jutila, 1992).

After the end of log floating, intensive restoration

programmes have been initiated in all parts of the

country to rehabilitate degraded streams close to their

prechannelisation state. Due to the lack of historical

information on the physical appearance of forest

streams before channelisation, the ‘guiding image’

(sensu Jungwirth, Muhar & Schmutz, 2002; Palmer

et al., 2005) for restoration has to be based on

contemporary reference streams or expert knowledge

of pristine stream structure and function. In essence,

the restoration process is the reverse of channelisa-

tion: stones and other obstructions that had been

removed from the stream are replaced, using excava-

tors to construct enhancement structures such as

deflectors, boulder dams, cobble ridges, etc. Further-

more, the course of stream channels is changed to

create meanders, side channels are opened, and gravel

beds are created to enhance spawning grounds for

salmonid fishes (Yrjana, 1998). However, installation

of large woody debris (LWD) is rarely used as a

restoration measure in Finland, although it is a

common practice in many other parts of the world

(e.g. Hunter, 1991; Lisle, 2002). Due to a history of

intensive forestry, headwater streams in northern

Scandinavia do not usually have mature riparian

forests (e.g. Lazdinis & Angelstam, 2005) and are

almost devoid of LWD.

Until recently, stream restoration in Finland has

been mainly single-goal restorations, motivated by the

enhancement of sport fisheries through the provision

of better living conditions for game fish, especially

brown trout (Salmo trutta L.). Responses of fish

populations to restoration have been little studied,

however, and monitoring of non-target biota is almost

non-existent. Some authors have suggested that res-

toration has positive effects on fish stocks (e.g. Jutila,

1992), usually based on a space-for-time substitution

approach where restored sites are compared with

dredged sites in the same stream or similar streams in

the same region. Habitat hydraulic modelling has also

been used to examine the effects of habitat enhance-

ment on the availability of stream habitat suitable for

juvenile trout. Using this approach, Huusko & Yrjana

(1997) showed that habitat area for trout larger than

10 cm increased after the installation of enhancement

structures in a northern Finnish stream. By contrast,

habitat area suitable for age-0 trout decreased in the

same stream at all simulated discharges during both

summer and winter.

If restoration increases the availability of microhab-

itats and retentive efficiency, it could be beneficial not

only for fish, but also for other stream organisms.

Food webs in boreal streams are largely fuelled by the

autumnal input of leaves from the riparian zone

(Malmqvist & Oberle, 1995; Haapala & Muotka, 1998),

and the retention efficiency of a stream is dependent

on the presence and abundance of retention structures

Recovery of restored boreal streams 725

� 2007 The Authors, Journal compilation � 2007 Blackwell Publishing Ltd, Freshwater Biology, 52, 724–737

(e.g. Dobson & Hildrew, 1992; Webster et al., 1994).

Therefore, a restored stream with higher substratum

heterogeneity should retain leaf litter more effectively

than the same stream before restoration. Because

detritus manipulations have bottom-up effects in

stream food webs (Wallace et al., 1997), restoration-

induced increases in retention efficiency may have

far-reaching effects on stream food webs, including

top predators such as trout.

In the mid-1990s, we initiated a research project on

the ecological impacts of stream restoration in Fin-

land, with the aim of combining comprehensive

monitoring data based on Before-After-Control-Im-

pact (BACI) designs with large-scale field surveys and

field experiments in channelised, restored and natural

streams. We have monitored the effects of restoration

on the primary target organisms of restoration, i.e.

fish, especially brown trout, and on other stream

biota, especially benthic macroinvertebrates. Here, we

review our main findings, based partly on published

results (e.g. Muotka et al., 2002; Muotka & Laasonen,

2002), partly on new material. We will focus on four

categories of response variables: (i) in-stream habitat

structures; (ii) population densities of juvenile brown

trout; (iii) species richness and community composi-

tion of macroinvertebrates; and (iv) leaf retention as

an important ecosystem process in forest streams.

Effects of restoration on stream habitat structure

at multiple spatial scales

Patch scale

To test the hypothesis that restoration of physical

structures in streams enhances stream habitat com-

plexity, we carried out a series of BACI experiments in

Finnish headwater streams, examining the effects of

restoration on stream habitat structures at scales

ranging from a few centimetres (within patches) to

tens of kilometres (among streams). First, we used a

bed profiler to quantify streambed roughness in three

parallel reaches of stream Myllykoski, Central Finland

(61�44¢N, 26�9¢E), before its restoration (October 2001)

and again after restoration (October 2002), at the same

water level (±1 cm). At the same time, we made

similar measurements in three nearby streams with

unmodified bed structure: Kohnionpuro (62�15¢N,

25�39¢E), Kiertojoki (61�59¢N, 26�3¢E) and Sallaanpuro

62�11¢N, 25�34¢E). These are all low-order (2nd to 3rd

order) forest streams with relatively unmodified

riparian zones. Our bed profiler was 40 cm long,

consisting of a continuous row of measuring rods

(diameter 7 mm). Measurements at each site were

made across 1.2-m longitudinal transects, each con-

sisting of three successive 40-cm sections. Three

transects parallel to each other and to stream flow

were surveyed and the mean values of bed roughness

for these profiles were used as replicates in statistical

analysis. Permanent iron stakes on stream banks were

used to ensure that measurements were made at the

same locations each time.

We used FST-hemispheres (Statzner & Muller, 1989)

to quantify changes in shear stress (lN cm)2) in the

stream Myllykoski, following Statzner & Muller (1989).

We selected a representative riffle section and identi-

fied a sampling site of 10 · 9 m. We then made

measurements in 100 regularly spaced spots (10 tran-

sects perpendicular to the flow, with 10 measurement

spots in each transect), noting the heaviest (densest)

hemisphere moved by the current. The procedure was

repeated both before (October 2001) and after (October

2002) the restoration. Again, permanent references on

stream banks ensured that the measurements were

taken at the same spots on both visits.

Bed profile in the restored stream changed substan-

tially between the sampling occasions (Fig. 1a–c), while

no corresponding change was observed at the reference

sites (Fig. 1d–f) (two-way repeated measures ANOVAANOVA,

Time · Site: F1,4 ¼ 28.6, P ¼ 0.006). Thus, restoration

clearly increased bed heterogeneity. FST measure-

ments showed that the proportion of microhabitats

with low shear stress increased after restoration,

especially in the lateral parts of the channel (Fig. 2).

This stream is regularly used for canoeing, and to allow

for a continued use of the stream for this purpose, mid-

channel areas were left unmodified during restoration.

Therefore, these results are conservative, suggesting

that in most other restored streams bed complexity is

likely to have increased more radically.

Our measurements of shear stress do not provide

conclusive evidence for the enhancement of habitat

heterogeneity following restoration, because our

approach resulted in unreplicated data and lack of

proper controls. Even so, combined with the meas-

urements of bed profiles, the data imply that small-

scale habitat complexity increases after restoration, as

does the availability of microhabitats characterised by

low shear stress.

726 T. Muotka and J. Syrjanen


Reach scale

Next, we assessed the effects of restoration on habitat

structure at the reach scale (i.e. including many riffle-

pool sequences) by estimating the proportion of

different channel units (riffles, runs, pools) in three

forest streams in Central Finland: Rutajoki (61�60¢N,

25�59¢E), Konkkojoki (62�14¢N, 25�16¢E) and Myllyko-

ski. All the streams were restored using similar

methods (see Yrjana, 1998), although the exact amount

of material added to each stream may have varied.

For each stream, the same reach (300–1000 m) was

0.77 0.95 1.41 2.18 3.93 6.82 10.90.0

0.1

0.2

0.3

0.4

0.5

Shear stress (10 µN cm–2)

Freq

uenc

y

0.77 0.95 1.41 2.18 3.93 6.82 10.9Stone

Width (m)

Leng

th (

m)

Before(a)9

87

6

5

4

3

2

1

00 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9

109876543210

After(b)

(c) (d)

Fig. 2 Kriging maps of shear stress

(10 lN cm)2) in a reach of the stream

Myllykoski before (a) and after (b) restor-

ation. Darker shading indicates higher

shear stress. Zero indicates dry land and

white areas indicate still water (no hemi-

sphere moved). Frequency distributions of

shear stress before (c) and after (d) res-

toration are also shown. ‘Stone’ refers to

boulders reaching above water level at

base flow.

Length (cm)

40

20

00

(a) (b) (c)

(d) (e) (f)

20 40 60 80 100 120 0 20 40 60 80 100 120 0 20 40 60 80 100 120

Dep

th (

cm)

Before

After

20

0

-20

Fig. 1 Bed profiles in three restored reaches of the stream Myllykoski before and after restoration (a–c) and in three unmodified

reference streams (d–f).



surveyed both before and 2 years after restoration at

the same discharge by the same person, using the

protocol outlined by Bisson & Montgomery (1996).

The results indicate that the proportion of runs

decreased and that of riffles increased following

restoration, while little change was observed in the

proportion of pool habitats (Fig. 3). Thus, it appears

that the addition of boulders and cobbles to low-

turbulence run habitats changes these runs to highly

turbulent, shallow riffles.

Broad-scale comparison of habitat structure

We conducted detailed habitat surveys in six stream

groups in two adjacent drainage systems in northern

Finland (n ¼ 4–6 streams in each group): channelised

and near-natural reference streams, and streams

restored 1 month, or 4, 6, or 8 years before sampling

(Muotka et al., 2002). A discriminant function analysis

of in-stream and riparian habitat variables showed

distinct recovery trajectories among the streams. The

first axis represented a gradient in moss cover, with

high-cover natural streams (mean cover of 71%) and

recently restored streams with very little mosses

(mean cover: 6%) being the endpoints of the gradient

(Fig. 4). Moss cover in channelised streams was very

close to the reference streams (65%). The second axis

was mainly related to streambed complexity; chann-

elised streams with low bed roughness differed

sharply from all other sites. Importantly, moss cover

increased steadily with recovery time from restor-

ation, and streams restored 8 years before our survey

supported dense moss cover. This suggests that

leaving areas of stream bed intact during restoration

works strongly accelerates re-colonisation by mosses

of nearly denuded stream beds, because mosses

possess effective means of spreading within and

among riffles through vegetative growth and frag-

ment dispersal (Stream Bryophyte Group, 1999).

A more detailed comparison between a third-order

(width: 4–6 m) forest stream (Kosterjoki) in north-

eastern Finland (67�31¢N, 29�31¢E), before and after

its restoration, and a close-to-pristine reference

stream in an adjacent catchment (Fig. 5) provides

additional insight into the effects of restoration on

stream habitat structure. Changes in habitat struc-

ture were mainly positive: a uniform depth

distribution of the channelised stream shifted to a

much more complex one, resembling that of the

reference stream. Similarly, velocity distribution after

restoration matched that in the reference stream,

with a strong skew towards slow-velocity habitats

(Fig. 5).

A dramatic, undesirable change occurred in the

cover of aquatic mosses in response to restoration

works. The frequency distribution of moss cover in

the reference stream was bimodal, indicating that

mosses were overall abundant but patchily distri-

buted. Log floating in the restored stream Kosterjoki

ended about 20 years prior to our sampling, so

mosses had had ample time to recover. Accordingly,

both the total area covered by mosses and the spatial

variability of moss cover in stream Kosterjoki before

restoration was quite close to that of a regional

reference site. Excavator that works along the stream

bed during restoration had a strong impact on mosses:

moss cover declined to one-tenth of the pre-restor-

ation value, and the stream was nearly devoid of

mosses the first year after restoration (Fig. 5). This

phenomenon has been repeatedly observed in Finnish

streams following restoration (Laasonen, Muotka &

Kivijarvi, 1998; Muotka et al., 2002; Korsu, 2004). We

therefore strongly recommend that heavy machinery

0

20

40

60

0

20

40

60

0

20

40

60

Rutajoki

Freq

uenc

y of

cha

nnel

uni

t (%

)

Könkköjoki

Before

After

Run Riffle Pool

Myllykoski

Fig. 3 Frequency distributions of different channel unit types in

three streams in Central Finland before and after restoration.



and procedures that destroy mosses should not be

used for stream restoration in the future.

Effects on juvenile brown trout

In the mid-1990s, we started to monitor trout popu-

lations, with an emphasis on age-0 trout, in two

streams in Central Finland (Rutajoki, Konkkojoki)

2–3 years before their restoration, and in three refer-

ence streams (Muuratjoki (62�08¢N, 25�40¢E), Saajoki

(61�58¢N, 25�23¢E) and Kohnionpuro). All these

streams are second- to third-order tributaries of Lake

Paijanne. They drain forested catchments that have

been modified to some degree by human activities

(forestry, agriculture, urbanisation), but they have

almost intact riparian zones. The streams are mesohu-

mic but their water quality is generally good, with low

levels of nutrient concentrations and circumneutral

pH. All the streams supported relatively abundant,

naturally reproducing, mainly resident, populations

of brown trout, even before restoration (see Fig. 6).

Monitoring started at slightly different times in

different streams, and continued for 6–10 years. Two

to five reaches, amounting to 400–900 m2, were

sampled with backpack electrofishing gear in each

stream. Sampling was always conducted at the same

time of the year (September–October). We used the

three-pass removal method to collect fish, and fish

densities were estimated using the Junge and Libos-

varsky equation (Bohlin et al., 1989). To obtain more

pre-restoration data points, we back-calculated fish

0 20 40 60 80 100

Channelized Restored Natural

Depth (cm)0 20 40 60 80 100

0

10

20

30

40

0 20 40 60 80 100

0 20 40 60 80 100

Current (cm s–1)

0 20 40 60 80 1000

10

20

30

Fre

quen

cy (

%)

0 20 40 60 80 100

0 20 40 60 80 1000

20

40

60

80

0 20 40 60 80 100Moss Cover (%)

0 20 40 60 80 100

x = 50CV = 82%

_

x = 29CV = 105%

_

x = 36CV = 54%

_x = 42CV = 69%

_x = 31CV =33%

_

x = 26CV = 83%

_x = 42CV = 61%

_

x = 5CV = 265%

_x = 42CV = 60%

_

Fig. 5 Frequency distributions of depth, current velocity and moss cover before (channelised) and after (restored) restoration of stream

Kosterjoki, and in an unmodified reference stream (Merenoja), north-eastern Finland.

+0 +4

+6+8

NA

CH

Moss cover (0.63)

Rel

ativ

e ro

ughn

ess

(0.7

6)

Fig. 4 Discriminant function analysis (DF 1 vs. DF 2) of

in-stream and riparian habitat variables in six stream types (n ¼4–6 streams per stream type). Numbers refer to years elapsed

since restoration; +0 streams were sampled 1 month after res-

toration. CH ¼ channelised streams, NA ¼ near-natural refer-

ence streams. Modified from Muotka et al. (2002).



densities for up to 2-year classes, based on the

densities of 1- to 2-year-old fish in the first sampling

year. For this purpose, we used the average year-to-

year survival estimates for each site and age-group

across the whole sampling period.

For statistical analyses, we split the data in two

separate time series with equal monitoring periods in

each series. We thus compared trout densities in

Rutajoki (restored in 1997) with densities in one

reference stream, Kohnionpuro. This analysis was

carried out as a BACI paired-samples model (Stewart-

Oaten, Murdoch & Parker, 1986) by calculating the

difference (i.e. delta-value) in mean densities between

the impacted site (Rutajoki) and the reference site

(Kohnionpuro) separately for each year. The pre-

restoration delta-values were then related to post-

restoration values with independent samples t-test.

Correspondingly, trout densities in Konkkojoki

(restored in 1999) were tested against two control

streams, Muuratjoki and Saajoki, with a beyond-BACI

model (see Underwood, 1994). The analysis was run

as an asymmetrical two-way ANOVAANOVA, with interaction

between time (B; before vs. after) and location (I;

restored vs. unmodified) revealing a possible impact

of the treatment (Underwood, 1994).

In the stream Rutajoki, the average densities of age-

0 trout were slightly lower after restoration, whereas

densities in the reference stream during the same

period increased slightly (Fig. 6a). The difference in

delta-values among the streams was non-significant

(t6 ¼ 0.52, P ¼ 0.62), indicating no impact of restor-

ation on trout densities. In Konkkojoki, trout density

increased after restoration, compared with the refer-

ence streams (Fig. 6b), and the time · location inter-

action (B · I) approached significance (F1,18 ¼ 4.06,

P ¼ 0.06), indicating a weakly positive response of

trout populations to restoration. It should be noted,

however, that there was a more than twofold increase

in post-restoration trout densities in stream Konkko-

joki (Fig. 6b). Densities of age-groups 1 and 2 showed

no change in response to restoration in either of the

two restored streams.

Other fish species in the study streams were either

very rare (and therefore not included in the analysis)

(pike Esox lucius L., burbot Lota lota L., minnow

Phoxinus phoxinus L., stone loach Noemacheilus barbat-

ulus L., and grayling Thymallus thymallus L.) or did not

show any responses to channel alteration (perch Perca

fluviatilis L., roach Rutilus rutilus L., and common

bullhead Cottus gobio L.).

We suspect that the rather weak response, or even

lack of response, of trout to stream restoration

measures is due to modest changes in some aspects

of stream habitat structure important for trout, espe-

cially the fact that the amount of pool habitats did not

increase. Pools with abundant woody debris are key

features of salmonid habitats in summer (e.g. Bridcut

& Giller, 1993; Urabe & Nakano, 1998; Rosenfeld,

Porter & Parkinson, 2000) and even more so in winter

(Cederholm et al., 1997; Jakober et al., 1998; Harvey,

Nakamoto & White, 1999). If, as suggested by many

authors (Cunjak, 1996; Quinn & Peterson, 1996; Maki-

Petays, Muotka & Huusko, 1999; Solazzi et al., 2000),

winter represents a bottleneck for the survival of

juvenile salmonids in boreal streams, restoration

schemes aiming at enhancing trout survival and

production should ensure provision of suitable over-

wintering habitat and unrestricted movement bet-

ween summer rearing habitats and overwintering

areas (Cunjak, 1996). Although some studies have

shown a positive effect of stony enhancement struc-

tures on trout populations (Naslund, 1989; Linlokken,

1997), adding wood to streams devoid of LWD is

probably a more effective short-term management

option, providing trout with suitable overwintering

habitats with deep, slowly flowing pools, and thereby

increasing their overwintering success (Sundbaum &

1992 1994 1996 1998 2000 2002 20040

5

10

15

20

(b)

(a)

Rutajoki Köhniönpuro (ref.)

1994 1996 1998 2000 2002 20040

5

10

15

20

Könkköjoki Muuratjoki (ref.) Saajoki (ref.)

Abu

ndan

ce (

indi

vidu

als

per

100

m2 )

Fig. 6 Densities of age-0 brown trout (annual averages) in

restored and reference streams in Central Finland. Arrows

indicate the time of restoration in each stream. (a) Before-

After-Control-Impact (BACI) paired-samples design, with one

treatment and one reference stream; (b) beyond BACI design,

with one treatment and two reference streams.



Naslund, 1998; Roni & Quinn, 2001; Lehane et al.,

2002).

It is possible, however, that the apparently weak

responses of trout to restoration are because of our

still rather short time series and lack of statistical

power, as the number of replicate streams was low

and interannual variation in densities remarkably

high. In line with this idea it has been suggested that,

unless the population change is very large, more than

10 years of post-treatment monitoring may be needed

to detect a population response by salmonids to

restoration (Korman & Higgins, 1997; Roni et al.,

2002). In any kind of environmental monitoring,

however, strict reliance on hypothesis testing may

be misleading, and more emphasis should be placed

on the size and direction of the biological effects

(Stewart-Oaten, 1996). Consequently, in streams

where the primary motivation for restoration is the

enhancement of salmonid fisheries, long-term monit-

oring is indispensable, because a positive response by

fish populations is a key measure of restoration

success in these streams.

Effects on benthic biodiversity

In a study examining the effects of stream restoration

on benthic invertebrate diversity, we sampled nine

second- to third-order streams in the Iijoki drainage

basin, northern Finland, 4, 6 or 8 years after their

restoration, three streams in each group. In addition,

we had two types of reference streams: channelised

and natural streams (n ¼ 3 for each) in the same or

adjacent river system. We used kick-sampling to

collect the samples in October 1997, four 1-min

samples being taken at each site. Animals were

identified to the lowest possible taxonomic level,

which usually was species. The sampling protocol and

sampling sites are described in detail in Laasonen

et al. (1998) and Muotka et al. (2002).

We observed detectable responses of invertebrate

communities to restoration. Community composition

was more variable in streams restored 4–6 years

before sampling than in unmodified reference streams

or streams restored 8 years before. Interestingly,

different stream groups had different indicator taxa

(indicator value analysis; Dufrene & Legendre, 1997):

in channelised streams, most indicator species were

filter-feeders or scrapers, while in restored streams

most of the indicator taxa were shredders. The

reference streams were characterised by a mix of

functional groups and rather weak indicators, includ-

ing filter-feeders, collector-gatherers and predators.

Because all the study streams were located within a

spatially restricted area (two adjacent catchments), the

regional species pool was the same for all the streams

studied. This suggests the presence of an ‘anthropo-

genic environmental filter’, selecting for species with

suitable traits to persist in conditions typical of

channelised streams (simplified substratum structure,

homogeneous flow patterns and low retentive poten-

tial; Muotka et al., 2002).

The few studies conducted thus far on the

responses of macroinvertebrates and other benthic

organisms suggest that restoration causes an initial

drop of abundance and diversity, followed by a

relatively rapid recovery (i.e. within a few years) to

pre-restoration levels (Tikkanen et al., 1994; Biggs

et al., 1998; Friberg et al., 1998; Muotka et al., 2002).

Studying the effects of restoration of headwater

streams in northern Sweden, Lepori, Palm & Mal-

mqvist (2005a) did not detect any differences in the

abundance or species richness of shredding inverte-

brates in leaf bags placed in restored, channelised and

unimpacted streams. Similarly, Negishi & Richardson

(2003) observed no significant impact of boulder

placements on taxonomic richness of benthic inverte-

brates in a second-order forest stream in British

Columbia, Canada. Furthermore, Lepori et al. (2005b)

suggested that the lack of response of fish and

invertebrate communities that they observed in

response to increased structural heterogeneity in

streams was because restoration did not create new

habitats at scales relevant to these organisms. They

also suggested that factors effective at regional and

catchment scales may overwhelm any effect that

restoration of stream channels might have on stream

communities.

Some authors have suggested that, owing to the

inherently high variability among sites, community

parameters may be ineffective at detecting gradual

changes in community composition between reference

and impacted sites, and should therefore not be used

as indicators of recovery in monitoring programmes

(Brooks et al., 2002, Negishi & Richardson, 2003). The

structure of macroinvertebrate communities may

indeed vary remarkably at fairly small spatial scales,

e.g. among riffles within a stream (Brooks et al., 2002;

Heino, Louhi & Muotka, 2004), thus posing problems



for their use in assessing recovery from restoration.

Nevertheless, enhancing benthic biodiversity is often

one of the key goals of stream restoration, and

therefore changes in the diversity and community

composition of benthic organisms must be included,

by definition, in assessment programmes (Nilsson

et al., 2005). To this end, our results, which show a

remarkable long-term recovery potential of macro-

invertebrate communities from restoration (Muotka

et al., 2002), are encouraging. They also show, how-

ever, that monitoring programmes need to be relat-

ively long to detect changes in community structure,

some of which may be quite subtle.

Effects on ecosystem processes

Because of the often variable nature of community

parameters, indicators directly linked to ecosystem

processes have been suggested in the assessment of

restoration success (e.g. Brooks et al., 2002). In forest

streams strongly dependent on the autumnal leaf

input, leaf retention might serve as such an indicator

ecosystem process. To examine changes to the retent-

ion capacity of a stream after restoration, we used a

BACI design with four reference and four experimen-

tal streams to conduct a release experiment with

artificial leaves (plastic strips) just before and 3 years

after restoration (Muotka & Laasonen, 2002). Care was

taken to conduct the releases at closely corresponding

discharges in all streams on both occasions. We

established a 50-m long study section in each stream,

and the downstream end of the section was blocked

with a wire screen. In each experiment, 2000 plastic

strips were scattered across the width of the channel

at the upstream release point. Three hours after the

release, we counted the number of leaves that had

travelled through the study section and collected on

the screen, and searched the entire reach for leaves

that had been retained within the section. For each

leaf found, we recorded the distance travelled and the

retaining object.

Retention efficiency in the reference streams was

high, with 60–70% of the 2000 leaves released being

retained within a 50-m study section. By contrast,

channelised streams were poorly retentive (c. 10% of

leaves retained), and the increase in retention fol-

lowing restoration was modest (14%; Fig. 7a). Aqua-

tic mosses were the key retentive feature in both

channelised and natural streams. However, as dis-

cussed above, the importance of mosses was drastic-

ally reduced immediately after restoration, explaining

the modest increase in retentiveness in the restored

streams. Woody debris (mostly fine debris with a

diameter <5 cm) did not contribute importantly to

retention in any of the streams studied (Fig. 7b).

Enhanced short-term retentive capacity correlated

with higher organic matter storage, as shown by the

greater standing crop of benthic organic matter in the

restored compared with the channelised streams

(Laasonen et al., 1998; Haapala, Muotka & Laasonen,

2003; see also Negishi & Richardson, 2003).

Leaf breakdown is another obvious candidate for an

indicator process in stream bioassessment (Gessner &

Chauvet, 2002). Other measures of ecosystem func-

tioning such as stream metabolism and nutrient

retention (Young & Huryn, 1999; Sweeney et al.,

2004) can also be used for assessing restoration

success in streams but, to our knowledge, they have

not been used for this purpose. Lepori et al. (2005a)

used a litter-bag experiment to compare leaf break-

down rates in channelised, restored and reference

sections of headwater streams in northern Sweden.

They were unable to detect differences between

restored and reference sites, whereas breakdown

was slightly faster in channelised sites. However, this

difference was attributed mainly to mechanical frag-

mentation resulting from faster currents in the chann-

elised sites during high-flow events. Leaf breakdown

has been suggested as an integrative approach to

assess the condition of stream ecosystems facing

various anthropogenic stressors (Gessner & Chauvet,

2002) and appears to work well for some types of

stresses, such as stream acidification (Dangles et al.,

2004). However, the complexity of factors, both biotic

and abiotic ones, regulating leaf breakdown in

streams restored through the addition of enhance-

ment structures may make this method less useful for

assessing restoration success in previously channel-

ised boreal streams.

Mosses as key organisms in boreal forest streams

As mosses clearly are a key retentive structure in

boreal forest streams, the loss of mosses during

restoration works may have far-reaching effects on

stream ecosystem dynamics. Few invertebrates are

able to directly consume mosses, but mosses still have

an important role in stream food webs by trapping



organic material, both fine and coarse, thus providing

a rewarding feeding arena for many aquatic insects

(Suren & Winterbourn, 1992; Lee & Hershey, 2000). By

altering near-bed flow regimes (Nikora et al., 1998),

mosses may also provide benthic animals with

hydraulic refugia during high-flow events. Further-

more, mosses are used extensively by juvenile brown

trout to provide underwater cover (Maki-Petays et al.,

1997; Heggenes & Saltveit, 2002).

Loss of mosses is the probable reason for the rather

weak positive response of detritivorous invertebrates

to restoration in our study streams; in fact, scrapers

were the only functional feeding group of macro-

invertebrates whose densities increased significantly

during the 3-year recovery period following restor-

ation (Muotka & Laasonen, 2002). Apparently, the

removal of mosses from large areas of the stream bed

indirectly favours the colonisation by periphytic

algae, the main food resource of scraping inverte-

brates. With a longer recovery period, mosses even-

tually recover (see above), and this change in habitat

structure will likely be reflected in a shift towards a

detritivore-dominated macroinvertebrate community.

However, the time scale for this community-level

change is currently unknown.

Mosses mainly rely on vegetative reproduction, and

recolonisation after removal by disturbance is mainly

through fragments drifting from upstream areas. This

process can be relatively fast, provided that colonisa-

tion sources are available upstream (Stream Bryo-

phyte Group, 1999). Most restoration projects in

Finland, however, are carried out on entire streams,

leaving the whole stream nearly devoid of mosses,

and almost all recolonisation must take place from

other streams in the region. Little is known about the

dispersal of mosses among streams (Stream Bryo-

phyte Group, 1999), but it is likely to be low. Thus, an

evident management implication is to cause as little

damage to mosses during restoration works as poss-

ible. However, where mosses are rare, as is the case

also in some boreal streams, replacement of boulders

may enhance stream retentiveness sufficiently to

allow for the recovery of ecosystem functioning after

restoration (Lepori et al., 2005a).

Outlook

Until recently, most stream restoration projects in

Finland have focused on restoration of in-stream

physical structures to improve brown trout fisheries.

From a biodiversity perspective, such an approach is

only acceptable if trout can be considered as an

umbrella species for forest stream conservation. To

our knowledge, this assumption has not been directly

tested, although preliminary analyses from 24 streams

in Central Finland showed that only mayfly (Epheme-

roptera) species richness was positively (but not

significantly) related to trout presence or biomass,

whereas stoneflies (Plecoptera) and caddisflies (Tri-

choptera) showed no relationship with trout presence

(Ruokonen, 2004). Clearly, restoration (and restoration

assessment) needs to be based on a broader consid-

eration of stream biota than just game fish species, if

the general goal is to enhance the overall biodiversity

of stream ecosystems.

Another major challenge to stream managers and

restoration ecologists is that stream restoration is

shifting its emphasis from stream channels to whole

catchments. Although most forest streams in boreal

areas have been dredged for timber floating, these

streams rarely are heavily modified, so that much of

their structural complexity is still present (Nilsson

et al., 2005). However, although new environmental

legislation in Finland has devoted more attention to

Treatment Reference0

20

40

60

80

(b)

(a)

Woody Streamdebris margin

BeforeAfter

)%(

ycneiciffenoitnete

R

Vegetation Moss Cobble Boulder0

10

20

30

40

50BeforeAfterReference

deniatersevaelfo

egatnecreP

Fig. 7 (a) Mean (±1SE) retention efficiency (% artificial leaves

retained out of 2000 released) of treatment (before vs. 3 years

after restoration) and reference streams in north-eastern Finland.

Number of streams is four, except for the reference streams on

the latter occasion when sample size was 2. (b) Mean proportion

(±1SE) of leaves retained by various retentive structures in each

stream type (n ¼ 4 streams per group). Modified from Muotka

& Laasonen (2002).



sustainable management and protection of riparian

forests (e.g. Virkkala & Toivonen, 1999), the focus is

on the terrestrial component of biodiversity (e.g.

riparian forests as corridors for dispersal of terrestrial

organisms and key habitats for forest biodiversity;

Virkkala & Toivonen, 1999). Given the importance of

riparian vegetation for stream structure and function-

ing, a broadened view that includes conservation of

aquatic biodiversity is urgently needed in riparian

management. As forest streams cannot be restored

without ecologically sound management of riparian

forests, we endorse the adoption of a landscape-scale

approach to stream restoration (e.g. Kauffman et al.,

1997; Jungwirth et al., 2002; Roni et al., 2002; Bond &

Lake, 2003; Lake, Bond & Reich, 2006).

While the establishment of intact streamside forests,

including mature forests that provide natural input of

woody debris into stream channels, should be the

ultimate goal of restoration, a first-aid measure to

streams devoid of LWD could be the purposeful

addition of wood. Placement of wood in streams has

positive effects on fishes (e.g. Lehane et al., 2002), but

effects on invertebrates are less well known and more

variable (Wallace, Webster & Meyer, 1995; Hilder-

brand et al., 1997). In a survey of 71 stream reaches in

Michigan and Minnesota the presence of wood

increased macroinvertebrate taxa richness, although

the effect was significant in only one of the study areas

(Johnson, Breneman & Richards 2003). Guidelines for

assessing the amount of wood needed to achieve

predefined restoration goals (e.g. Lisle, 2002) mainly

use near-pristine streams as a reference. For example,

Liljaniemi et al. (2002) showed that Russian boreal

streams draining unmodified catchments supported

10- to 100-fold higher standing stocks of LWD than

adjacent Finnish streams where catchments have had

a long history of commercial use of forests. Mitigating

the detrimental impacts of forestry is a major chal-

lenge to stream management in forested areas, and

this goal cannot be achieved by focusing efforts and

resources merely on the restoration of physical struc-

tures within stream channels but also requires con-

servation and ecologically sound management of

riparian zones.

Acknowledgments

The research reported in this paper could not have

been possible without the help and support from a

number of people, too many to mention here. We owe

our special thanks to Anssi Eloranta, Antti Haapala,

Pekka Laasonen, Veijo Vosjem and Timo Yrjana for

considerable logistic support at various stages of

the work. We also acknowledge the constructive

comments made by two anonymous reviewers and

M. Gessner on earlier versions of the manuscript. Our

research has been supported by grants from the

Academy of Finland (grants no. 35586, 39134, 47968

and 206151 to TM), Maj and Tor Nessling Foundation

(to T.M.), University of Oulu (Thule Institute) and

Kone Foundation (to J.S.).

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Changes in habitat structure, benthic invertebrate diversity, trout populations and ecosystem processes in restored forest streams: a boreal perspective