landscape effects in bird assemblages differ between plantations and broadleaved forests in a rural...

ORIGINAL ARTICLE

Landscape effects in bird assemblages differ between plantationsand broadleaved forests in a rural landscape in central Japan

Yuichi Yamaura Æ Hitoshi Tojo ÆYasumasa Hirata Æ Kenichi Ozaki

Received: 15 February 2007 / Accepted: 9 April 2007 / Published online: 13 June 2007

� The Japanese Forest Society and Springer 2007

Abstract It is important to conserve forest-dependent

organisms not only in broadleaved forests but also in plan-

tation forests. We examined how surrounding forest areas

affect forest bird assemblages inhabiting conifer plantations

and broadleaved forests in a rural landscape in central Japan.

Surrounding forest areas were measured separately as plan-

tation area and broadleaved forest area within 200 m of each

sampling site. We used hierarchical partitioning to analyze

the effects of surrounding forest areas and stand structures

(stand height and understory coverage) on the occurrence of

four species groups. We especially focused on mature forest

users that are most sensitive to loss of broadleaved forests.

Occurrence of mature forest users inhabiting plantation sites

was positively affected both by plantation area and broad-

leaved forest area whereas the occurrence of mature forest

users inhabiting broadleaved forests was affected by stand

height only. These results suggest that surrounding forest

areas were more important to mature forest users in planta-

tions than in broadleaved forests. To conserve mature forest

users, increasing surrounding forest areas are important in

plantations whereas increasing stand heights would be

effective in broadleaved forests.

Keywords Bird assemblages � Broadleaved forests �Conifer plantations � Heterogeneous landscape �Stand height

Introduction

Loss of broadleaved forests negatively affects many forest-

dependent organisms as a result of edge effects (Batary and

Baldi 2004; Ries and Sisk 2004; Parker et al. 2005),

reduction of patch area (Bender et al. 1998; Connor et al.

2000), and habitat fragmentation (Opdam 1991; Andren

1994). These negative effects have been reported when

broadleaved forests are replaced both by open areas

(Freemark and Merriam 1986; Blake and Karr 1987) and

by plantation forests (hereafter plantations) (Enoksson

et al. 1995; Jansson and Angelstam 1999). Compared with

open areas, however, plantations are not totally unsuitable

for many forest-dependent organisms (Hartley 2002). It is,

therefore, important to conserve forest-dependent organ-

isms in plantations, especially where most of the broad-

leaved forests have disappeared (Lopez and Moro 1997;

Dıaz et al. 1998; Brotons and Herrando 2001).

Increasing patch size is an important management

option for conserving forest birds in plantations (Lopez

and Moro 1997; Dıaz et al. 1998; Brotons and Herrando

2001). Planted conifer trees have few herbivorous insects

(Yui 1988), which are important food resources for bird

species preferring mature forests (hereafter mature forest

users: Holmes et al. 1986; Holmes and Schultz 1988).

This suggests the availability of resources for mature

forest users are lower in plantations than in broadleaved

forests. Species depending on sparse resources require

large patch area (Telleria and Santos 1995; Newton

1998). Area requirements of mature forest users would

therefore be larger in plantations than in broadleaved

forests (Telleria and Santos 1995; Newton 1998). This

predicts that surrounding forest areas affect mature forest

users more in plantations than in broadleaved forests. No

studies have compared the effects of surrounding forest

Y. Yamaura (&) � H. Tojo � K. Ozaki

Forestry and Forest Products Research Institute,

Matsunosato 1, Tsukuba 305-8687, Japan

e-mail: [email protected]

Y. Hirata

Shikoku Research Center,

Forestry and Forest Products Research Institute,

Kochi, Japan

123

J For Res (2007) 12:298–305

DOI 10.1007/s10310-007-0020-1

areas on forest birds in plantations and broadleaved

forests, however.

In Japanese rural landscapes original broadleaved for-

ests have largely been replaced not only by open areas but

also by conifer plantations. These landscape modifications

create small fragments of plantations and broadleaved

forests, both of which are surrounded by open areas

(Yamaura et al. 2005). To conserve bird diversity in such

landscapes it is important to determine whether the effects

of surrounding forest areas differ between plantations and

broadleaved forests. In this work we examined the effects

of surrounding forest areas on forest bird assemblages

inhabiting plantations and broadleaved forests in a Japa-

nese rural landscape. We focused especially on mature

forest users that are most sensitive to the loss of broad-

leaved forests (Yamaura et al. 2005).

Methods

Study area and sampling sites

The study area was located in Ishioka-city and Tuchiura-

city (36�11¢N, 140�10¢E), Ibaraki prefecture, in central

Japan. The elevation ranges from 30 to 360 m. Average

monthly temperatures in this area are between 3.7 and

26.6�C with an annual precipitation of 1,138 mm (Tsu-

chiura meteorological station in 2005). The landscape

consisted of open areas, conifer plantations, and broad-

leaved forests. Broadleaved forests had been intensively

managed as coppices until the 1980s (Yamamoto 2001).

The main canopy species in broadleaved forests were

Japanese chestnut (Castanea crenata), sawtooth oak

(Quercus actissima), and konara oak (Quercus serrata).

Most broadleaved forests in the study area had developed

shrubs and understory (Table 1). Shrub species in broad-

leaved forests included shirakashi oak (Quercus myrsi-

naefolia), arakashi oak (Quercus glauca), eurya (Eurya

japonica), and yamatsutsuzi (Rhododendoron obtusm).

Forest understory was covered by dwarf bamboo (Pleiob-

lastus chino). Japanese cedar (Cryptomeria japonica) and

Hinoki cypress (Chamaecyparis obtusa) were the major

conifer plantation species. Main shrub species in planta-

tions were Q. myrsinaefolia, Q. glauca, E. japonica, and

Japanese aucuba (Aucuba japonica). Open areas were

mainly groves, paddy fields, arable fields, grasslands, and

residential areas.

We selected sampling sites in 18 broadleaved forests

and 30 plantations, ranging from large to small isolated

fragments, to cover the whole range of forest area (Fig. 1).

Sampling sites were at the centers of stands. Each sampling

site was positioned with a GPS receiver.

Table 1 Ranges of four

explanatory variables and

occurrences of birds within each

group in broadleaved forests

and plantations

a Values in the table indicate

minimum values, 25, 50, 75%

percentiles, and maximum

values from left to rightb Total occurrence and species

richness were recorded for all

species except open-area users,

raptors, and non-breeding

species

Forest types Mina 25% 50% 75% Max

Environmental variables

Stand height (m) Plantation 11 15 16 18 25

Broadleaved 9 11 13 13 17

Understory coverage (%) Plantation 0 1 10 20 40


Broadleaved forest area (ha) Plantation 0.0 1.0 2.0 3.2 11.3

Broadleaved 0.6 1.3 2.5 3.3 10.0

Plantation area (ha) Plantation 0.6 1.3 2.3 4.5 10.1

Broadleaved 0.0 1.3 2.4 8.6 10.2

Bird occurrences

Total occurrencesb Plantation 0 1 4 6 12


Species richnessb Plantation 0 1 2 4 6


Mature forest users Plantation 0 1 2 3 8


Shrub users Plantation 0 0 0 2 4


Grass users Plantation 0 0 0 1 4


Ground users Plantation 0 0 0 0 1


J For Res (2007) 12:298–305 299

123

Bird and stand structure surveys

Four trained persons conducted bird surveys during the

breeding season between 16 May and 7 June 2006 (sunrise

to 10:00 a.m.). Birds were surveyed using a fixed-radius

point count method (Hutto et al. 1986). In accordance with

Drapeau et al. (1999) we visited each sampling site twice

and spent a total of 60 min at each sampling site. The first

visit was conducted between 16 and 19 May and the second

visit was conducted between 5 and 7 June. If one visit was

conducted between 9:00 and 10:00 a.m., the other visit was

conducted before 7:30 a.m. Surveys were not conducted

when it was raining. During each visit the same observer

conducted two 15-min counts sequentially rather than one

30 min count, so four 15-min counts were conducted at

each site. In this way we increased statistical power by

increasing number of bird counts (Verner 1984). The

presence/absence of each species was recorded in each

count to avoid double counting the same individuals in

each count. The radius of a point count was set to 20 m to

avoid counting birds outside small stands. The distances to

birds were estimated with the aid of a range finder. Birds

that flew over sampling sites were not recorded.

Stand structure within 20 m of sampling sites was sur-

veyed when the sampling sites were selected (18 April to

13 May 2006). Stand height (average height of canopy

trees) was measured using a range finder and a clinometer.

Coverage of shrubs and dwarf bamboos was estimated

visually to nearest 5% at 1-m intervals from ground to 5-m

height. By summing the coverage of shrubs and dwarf

bamboos across five intervals, understory coverage was

obtained. The survey of stand structure was conducted by a

single person (Yuichi Yamaura) to reduce surveyor bias.

Landscape metrics and explanatory variables

A landscape map of the study area was generated with

object-oriented segmentations and classification techniques

using eCognition (Definiens Imaging 2001). Two kinds of

satellite data were used: 2.8-m resolution QuickBird data

acquired on February 2003 and 2.5-m resolution Baseim-

age (NTT data Geo-contents, Japan). Images were seg-

mented on the basis of results from field observations,

forest-planning maps produced by the Forest Agency, and

interpretation of satellite images. Land cover was classified

into three types: conifer plantations, broadleaved forests,

and open areas. Because the landscape map had unclassi-

fied and misclassified forests, landscape classification

within 200-m radius circular buffers (12.6 ha) generated

from each sampling site were manually modified on the

basis of ground truth and 1:5,000 forest distribution maps.

The forest distribution maps were managed by the Forestry

Division in Southern District Head Branch, Ibaraki Pre-

fecture. Because of a difference between the resolution of

the object-oriented classification technique and the manual

modification, it was difficult to analyze edge length.

We measured the areas of plantations (plantation area)

and broadleaved forests (broadleaved forest area) within

each 200-m radius circular buffer using the landscape map

and ArcMap 9.0 (ESRI 2004). We treated plantation area

and broadleaved forest area as surrounding forest areas.

The radius of the circular buffers was set to 200 m because

this area (12.6 ha) is larger than the home ranges of most of

the species recorded. For example, the size of the home

range of the long-tailed tit (Aegithalos caudatus) is 12 ha

(Nakamura 1972), and that of the great tit (Parus major) is

1.6 ha (Saitou 1979). Another reason for this radius is that

broadleaved forest areas and plantation areas within 200-m

Fig. 1 Location of sampling sites and landscape map of the study

area in Ishioka-city and Tsuchiura-city, Ibaraki prefecture, central

Japan. Dark gray, conifer plantations; light gray, broadleaved forests;

black, unclassified forests. White areas indicate open areas. Two-

hundred-meter radius circular buffers generated from each sampling

site represent the area in which landscape metrics were measured. It is

noted there are misclassifications outside 200-m radius circular

buffers

300 J For Res (2007) 12:298–305

123

circular buffers were highly correlated with those areas

within 100-m radius circular buffers (broadleaved forest

area: r = 0.82; plantation area: r = 0.87). In total, four

explanatory (environmental) variables were used (Table 1).

The first two variables were stand height and understory

coverage, which relate to stand structure. The other two

variables were broadleaved forest area and plantation area.

Understory coverage was arcsine-transformed and broad-

leaved forest area and plantation area were log-transformed

to prevent high leverage problems (Wintle et al. 2005).

Because broadleaved forest area and plantation area have

upper limits, arcsine transformation may be more appro-

priate than log-transformation (Legendre and Legendre

1998). Log-transformation reduced outliers of broadleaved

forest area better than arcsine transformation, however.

Stand height was not transformed. None of the pairs of

explanatory variables were highly correlated within the

same forest type (r < 0.42).

Group categorization and response variables

Open area users which, according to Higuchi et al. (1997),

forage and/or nest in non-forests (e.g. rice and arable

fields, and urban areas) were excluded from subsequent

analysis (Table 2). Raptors and non-breeders were also

excluded. Although the oriental turtle dove (Streptopelia

orientalis) and the brown-eared bulbul (Hypsipetes

amaurotis) appear in or around woodlots or along tree-

lined roads, these species were categorized as open-area

users because oriental turtle doves forage in arable fields

and brown-eared bulbuls nest in awnings (Higuchi et al.

1997). The other species were categorized into four species

groups, in accordance with Higuchi et al. (1997), on the

basis of their foraging resource types—mature forest users

(consisting of canopy users, stem probers, and flycatchers),

shrub users, grass users, and ground users (Table 2). Al-

though Japanese white-eye (Zosterops japonicus), Japa-

nese pygmy woodpecker (Dendrocopos kizuki), and great

tit are observed in and around urban woodlots, these spe-

cies were categorized as mature forest users because they

forage and nest mainly in forests (Higuchi et al. 1997). To

increase the power of statistical analysis, the presence (1)

or absence (0) of each species in each count was summed

across the four 15-min counts within each species group.

We treated the resulting totals as occurrences of each

species group for each site.

Table 2 List of species

recorded in the bird survey and

assignment of the species into

seven groups

MFU, mature forest user; SU,

shrub user; GU, grass user;

OAU, open area user; GDU,

ground user; NB, non-breedera Number of sampling sites in

which each species was

recordedb Exotic species

Family Species Scientific name Group # Sitesa

Accipitridae Goshawk Accipiter gentilis Raptor 1

Columbidae Oriental turtle dove Streptopelia orientalis OAU 4

Picidae Japanese pygmy woodpecker Dendrocopos kizuki MFU 16

Motacillidae White wagtail Motacilla alba OAU 1

Campephagidae Ashy minivet Pericrocotus divaricatus MFU 1

Pycnonotidae Brown-eared bulbul Hypsipetes amaurotis OAU 44

Troglodytidae Wren Troglodytes troglodytes NB 1

Turdidae Brown thrush Turdus chrysolaus NB 1

Sylviidae Short-tailed bush warbler Urosphena squameiceps SU 3

Bush warbler Cettia diphone SU 26

Muscicapidae Narcissus flycatcher Ficedula narcissina MFU 1

Black paradise flycatcher Terpsiphone atrocaudata MFU 2

Aegithalidae Long-tailed tit Aegithalos caudatus MFU 6

Paridae Varied tit Parus varius MFU 8

Great tit Parus major MFU 14

Zosteropidae Japanese white-eye Zosterops japonicus MFU 34

Emberizidae Siberian meadow bunting Emberiza cioides GU 17

Black-faced bunting Emberiza spodocephala NB 4

Japanese gray bunting Emberiza variabilis NB 1

Fringillidae Oriental greenfinch Carduelis sinica OAU 18

Ploceidae Tree sparrow Passer montanus OAU 27

Sturnidae Gray starling Sturnus cineraceus OAU 6

Corvidae Carrion crow Corvus corone OAU 1

Jungle crow Corvus macrorhynchos OAU 14

Phasianidae Chinese bamboo partridgeb Bambusicola thoracica GDU 6

Timaliidae Red-billed leiothrixb Leiothrix lutea NB 2

J For Res (2007) 12:298–305 301

123

Five response variables were used. The first three vari-

ables were the occurrences of mature forest users, shrub

users, and grass users. Ground users were not analyzed as a

separate group because of the small number of occurrences

(Table 1). The fourth and fifth response variables were the

total occurrences and species richness of all bird species

through the four 15-min counts.

Statistical analyses

Differences between total occurrences, species richness,

and occurrences of the three species groups in broadleaved

forests and plantations were tested using generalized linear

models (GLMs) with Poisson distribution. Forest-type ef-

fects were tested by using a dummy explanatory variable

(i.e. broadleaved forest, 1, or plantation, 0) in the models.

The relative importance of each environmental variable

on each response variable was examined by use of hier-

archical partitioning (MacNally 1996, 2000), using the

‘‘hier.part’’ R package (Walsh and MacNally 2005).

Hierarchical partitioning calculates incremental improve-

ments of the goodness of fit (here R2) by incorporating

each explanatory variable in models. The incremental

improvements for each explanatory variable are averaged

over all combinations of other explanatory variables. The

resulting average values are measures of the independent

effects (i.e. not confounded by other explanatory variables)

for each explanatory variable (MacNally 1996, 2000). Here

independent effects represent the explained percentage

(independent R2) of total variation of response variables.

Such independent effects are tested by using randomization

tests (MacNally 2002). The relative importance of each

explanatory variable is therefore indicated by independent

R2 and by their significances with 1,000 permutations. The

direction of the effects of each explanatory variable was

also obtained, from models with all four explanatory

variables. These analyses were conducted for broadleaved

forest sites and plantation sites separately. Occurrences of

shrub users and grass users in plantation sites were quite

small (the average values of occurrences were <0.5) with

many zero values. Values >1 were therefore truncated to 1

and truncated values were analyzed using GLMs with

binomial distribution. Other response variables were ana-

lyzed by using GLMs with Poisson distribution. Because

five response variables were analyzed for two forest types

separately, ten response variables were analyzed using

hierarchical partitioning.

Because the sampling sites were spatially aggregated

and some circular buffers overlapped, there might be a

spatial autocorrelation problem (e.g. Legendre 1993). Be-

cause it is difficult to determine, a priori, which analysis

suffers from a spatial autocorrelation (Wiens 1989; Koenig

1999), however, before conducting statistical analysis we

first constructed a model with all explanatory variables for

each response variable. If the residual deviation was larger

than the residual degree of freedom, we considered that

such response variables had a spatial autocorrelation prob-

lem. We analyzed these response variables with GLMs with

a quasi-Poisson or quasi-binomial distribution (Burnham

and Anderson 2002; Crawley 2005). All statistical analyses

were tested at 5% level and conducted using R Ver. 2.2.1

(R Development Core Team 2005).

Results

Variation in environmental variables

Broadleaved forest area and plantation area varied widely

between sampling sites, and the ranges of the two variables

were similar for broadleaved forest sites and plantation

sites (Table 1). The area within each 200-m radius circular

buffer (12.6 ha) was not occupied by a single forest type

because forests in the study area were highly fragmented.

Stand height was higher in plantation sites than in broad-

leaved forest sites (t = 5.4, P < 0.00001) and understory

coverage was greater in broadleaved forests sites than in

plantation sites (t = –8.3, P < 0.00001).

Responses of birds to forest types

A total of 26 bird species including eight species of mature

forest users were recorded in this study (Table 2). Differ-

ences between bird occurrences and species richness in the

different forest types were tested by using GLMs with a

quasi-Poisson distribution. Total occurrences and species

richness for all bird species (except for open-area users,

raptors, and non-breeders) were significantly greater in

broadleaved forest sites than in plantation sites (the total

occurrences: slope = 0.43, s.e. = 0.19, t = 2.30, P = 0.03;

species richness: slope = 0.42, s.e. = 0.17, t = 2.48,

P = 0.02). Occurrences of mature forest users, shrub users,

and grass users, however, were not significantly different in

the two types of forest (mature forest users: slope = 0.31,

s.e. = 0.25, t = 1.22, P = 0.23; shrub users: slope = 0.61,

s.e. = 0.33, t = 1.85, P = 0.07; grass users: slope = 0.30,

s.e. = 0.50, t = 0.61, P = 0.55).

Responses of birds to four environmental variables

In broadleaved forest sites occurrences of mature forest

users were highly affected by stand height, which inde-

pendently explained 29% of the total variation (Table 3).

The other three variables in total explained only 10% of the

total variation and the independent effect of each variable

was quite small and not significant. In contrast, occurrences

302 J For Res (2007) 12:298–305

123

of mature forest users in plantation sites were highly af-

fected both by broadleaved forest area and plantation area,

each of which explained more than 10% of total variation.

The other two variables explained a total of only 5% of the

total variation. In contrast with mature forest users,

occurrences of shrub users were negatively affected by

plantation area in broadleaved forest sites. Occurrences of

grass users were negatively affected by stand height in

plantation sites, and by broadleaved forest area in broad-

leaved forest sites.

For the results for all bird species, total occurrences

were positively affected by understory coverage in plan-

tation sites and by stand height in broadleaved forest sites.

Species richness was positively affected by broadleaved

forest area in plantation sites and by stand height in plan-

tation sites.

Discussion

Responses of mature forest users

Occurrences of mature forest users were highly affected

both by broadleaved forest area and plantation area in

plantation sites. In contrast, occurrences were not affected

by either of the areas in broadleaved forests sites. These

findings support the prediction that effects of surrounding

forest areas are greater for mature forest users inhabiting

plantations than for those inhabiting broadleaved forests.

Resources needed for this species group include tree cav-

ities for nesting and food in the canopy, both of which are

abundant in broadleaved forests but are only sparsely dis-

tributed in plantations (Yui 1988; Newton 1994). Thus in

plantations some of the mature forest users may not persist

in small isolated fragments because mature forest users

would require the additional resources provided by adja-

cent broadleaved forests or plantations (Dunning et al.

1992).

In broadleaved forest sites occurrences of mature forest

users were not affected by broadleaved forest area. Al-

though this finding supports the prediction that effects of

surrounding forest areas are small for mature forest users

inhabiting broadleaved forests, Lindenmayer et al. (2002)

reported that species richness in broadleaved forests in-

creased with broadleaved forest area. This inconsistency is

probably because broadleaved forests in our study area

were small and fragmented, so that some area-sensitive

species have already been disappeared. For example, the

Japanese green woodpecker (Picus awokera) and the Jap-

anese gray thrush (Turdus cardis), the most area-sensitive,

large bodied species in Japanese rural landscapes (Askins

et al. 2000; Kurosawa and Askins 2003), were not recorded

in this study. These birds are likely to be found in larger

forests. This suggests that if we examine broadleaved for-

ests larger than those in this study these area-sensitive

species would be detected and mature forest users would

be more abundant in larger broadleaved forests. Another

possible reason why the Japanese green woodpecker did

not appear is that broadleaved forests were too young for

this large-cavity nester (Fuller and Henderson 1992;

Newton 1994).

In Japanese rural landscapes, fragments of plantations

and broadleaved forests are surrounded by open areas.

This study indicates that creating plantation patches close

to each other and maintaining broadleaved forests in the

neighborhood of plantation patches are important for

conserving mature forest users in plantations. In broad-

leaved forests, increasing stand height by using, for

example, extended rotations would effectively conserve

mature forest users. Tall stands have much foliage (Helle

Table 3 Independent effects R2 (%) and the direction of the effects of each explanatory variable on the occurrences of the birds

Explanatory

variables

Total occurrencesa Species richnessa Mature forest users Shrub users Grass users

Plantation Broadleaved Plantation Broadleaved Plantation Broadleaved Plantation Broadleaved Plantation Broadleaved

Stand height 2 (+) 26 3 (+) 29 1 (+) 29 4 1 (–) 14 2

Understory

coverage

(+) 12 3 (+) 11 3 4 1 4 2 1 0

Plantation

area

1 9 1 2 (+) 12 3 9 (–) 29 1 5

Broadleaved

forest area

(+) 10 0 (+) 12 1 (+) 13 6 3 1 3 (–) 37

Distributionb QP QP QP Poisson QP QP QB QP QB Poisson

The signs in parentheses, which were shown for variables with R2 > 10%, are the directions of the effects of the corresponding variables. Bold

figures indicate that corresponding explanatory variables had significant effects (P < 0.05)a Total occurrence and species richness were obtained for species other than open-area users, raptors, and non-breedersb Distributions used in GLMs: QP, quasi-Poisson; QB, quasi-binomial

J For Res (2007) 12:298–305 303

123

and Monkkonen 1990) and developed stem bark (Holmes

et al. 1979), and many cavities (Newton 1994). All of

these are foraging substrates or nesting resources for

mature forest users. The effects of surrounding forest

areas should not be undervalued, however, because large

broadleaved forests are important habitats for area-sensi-

tive birds.

Similarities of bird communities in plantations and

broadleaved forests and responses of other groups

Although total occurrences and species richness of all bird

species were significantly larger in broadleaved forest

sites than in plantation sites, occurrences of mature forest

users, shrub users, and grass users did not differ signifi-

cantly between sites in the two forest types. This small

difference between broadleaved forests and plantations is

consistent with studies in Chile (Estades and Temple

1999) and New Zealand (Clout and Gaze 1984). These

studies, however, are inconsistent with studies in Japan

(Ohno and Ishida 1997) and Australia (Lindenmayer et al.

2002) in which bird species richness was much smaller in

plantations than in broadleaved forests. One factor pos-

sibly explaining the small difference is the smallness of

patch area relative to the home ranges of birds (Addicott

et al. 1987). Because broadleaved forests surrounded

small plantations, some birds nesting in broadleaved for-

ests would forage in plantations and contribute to the high

bird occurrences in these sites (Curry 1991; Tubelis et al.

2004). These neighborhood effects (sensu Dunning et al.

1992) would assimilate bird communities in adjacent

patches and prevent small patches from generating unique

bird communities corresponding to intrinsic patch quality

(Wiens 1994).

The occurrences of shrub users were negatively affected

by plantation area in broadleaved forest sites. Occurrences

of grass users were negatively affected by stand height in

plantation sites, and by broadleaved forest area in broad-

leaved forest sites. These results suggest that factors

affecting the occurrence of birds differed among species

groups, and that different management methods are needed

for each species group. Although shrub users mainly use

shrubs and understory vegetation, their occurrence was not

affected by understory coverage. Occurrences of shrub

users did not differ between broadleaved forests and

plantations, although understory coverage developed more

in broadleaved forests than in plantations. This was pos-

sibly because shrub users mainly use shrubs that develop in

forest edges but not inside forests (Imbeau et al. 2003; Ries

and Sisk 2004; Ries et al. 2004). Another possibility is that

understory coverage was too low for the shrub users to use

understory in most sites.

Acknowledgments We thank two anonymous reviewers and

T. Amano for comments on the manuscript, and J. Okazaki and

H. Tanaka for useful suggestions during this study. K. Hosoda,

Y. Mitsuda, S. Sugiura, S. Yamada, and staff in the Forestry Division

in Southern District Head Branch, Ibaraki Prefecture, helped us

construct the landscape map. This study was funded by the Agriculture,

Forestry and Fisheries Research Council of Japan.

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