Effects of undergrowth clearing on the bird communities of the
Northwestern Mediterranean Coppice Holm oak forests
Jordi Camprodon a,b,*, Lluıs Brotons a,b
a Centre Tecnologic Forestal de Catalunya, Pujada del Seminari s/n 25280 Solsona, Catalonia, Spainb Centre d’Ecologie Fonctionelle et Evolutive-CNRS, 1919 Route de Mende, 34293 Montpellier Cedex, France
Received 23 November 2004; received in revised form 6 September 2005; accepted 3 October 2005
Abstract
Undergrowth clearing is a widespread forest management technique used in manyMediterranean regions to reduce dense vegetation in order to
prevent fire or to facilitate other forest exploitation activities. Here, we analyze the effects of undergrowth clearing on biodiversity by focusing on
the variations in bird diversity in Holm oak forests in Catalonia (north-east Iberian Peninsula) under different forest management regimes: (1)
coppice Holm oak forests where the undergrowth layer has been completely cleared, (2) partially cleared forests and (3) cleared and tree thinned
forests and finally, (4) undisturbed forests. The synchronic comparison approach was used in conjunction with a before–after control impact
(BACI) experiment in which the effects of undergrowth clearing were explicitly measured. Complete undergrowth clearing resulted in the almost
complete disappearance of threeWarbler species (SubalpineWarbler, SardinianWarbler and GardenWarbler). Partial clearing also led to a marked
reduction in the numbers of these three species, but the presence of Sardinian and GardenWarblers was maintained in the treated forests. Complete
undergrowth clearing accompanied by tree thinning also led to a decrease in amount of undergrowth species and involved additional negative
effects for species such as Wren, Robin, Blackbird and Blackcap. Only one species, the Nightjar, appeared to benefit from undergrowth clearing,
while many others increased their numbers only when clearing was applied together with tree thinning: Turtle dove, Mistle thrush and Cirl bunting.
Undergrowth clearing brought about a significant simplification in the vertical structure of the forest, which probably reduced foraging
opportunities and breeding resources for most undergrowth species. These effects became more pronounced when tree thinning was applied
together with undergrowth clearing. To reconcile forest management and bird diversity, undergrowth clearing should be applied selectively to
ensure that a certain number of undergrowth patches, preferably corresponding to different shrub species, are not affected by management
treatments.
# 2005 Elsevier B.V. All rights reserved.
Keywords: Clearing; Undergrowth vegetation; Quercus ilex; Birds; Thinning; Forestry
www.elsevier.com/locate/foreco
Forest Ecology and Management 221 (2006) 72–82
1. Introduction
Mediterranean forests have traditionally been subject to
strong human pressure through activities such as forestry, coal
production or vegetation clearing (Blondel and Aronson, 1999).
However, recent socioeconomic changes in some Mediterra-
nean regions have induced a reduction of most of these
activities typically leading to an accumulation of vegetation,
especially in the shrub and lianas layers of the undergrowth. In
this context, undergrowth clearings are currently the main
management activity effecting forests in many regions
* Corresponding author. Tel.: +34 973 481752; fax: +34 973 481392.
E-mail addresses: [email protected] (J. Camprodon),
[email protected] (L. Brotons).
0378-1127/$ – see front matter # 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.foreco.2005.10.044
particularly in the north-westernMediterranean basin. Mechan-
ical undergrowth clearing aims to reduce the densely developed
shrub and liana vegetation layers and are mainly used to
eliminate the combustible load but also to control tree
competition and favour sapling regeneration or access to the
forest (Garolera, 1991; CPF, 1992; Meson and Montoya, 1993).
The increase in the number and impact of forest fires during
recent decades (Terradas, 1996) has boosted the use of
undergrowth clearing, although, the effectiveness of this
forestry technique to fight the spread of fire has been
questioned because of its low cost effectiveness (Camprodon,
2001).
Although the influence of forestry, fire or grass management
on biological diversity in the Mediterranean scrubs, garrigues
and maquis has been studied (Blondel, 1981; Prodon, 1988;
Pons, 1998), the effects of undergrowth clearing have been
J. Camprodon, L. Brotons / Forest Ecology and Management 221 (2006) 72–82 73
largely neglected and are not currently used in the criterion
which guides forest management in the region. Clearing is
likely to have a strong impact on biodiversity, since it entails a
significant elimination of forest biomass. In Mediterranean
forests, more than 40% of the total floristic richness, as
quantified by forest plants, is located in the shrub layer.
Furthermore, most of the arthropod biomass inhabiting Holm
oak forest, essential for the undergrowth feeding of diverse
faunal groups, also occurs in the shrub undergrowth (pers. obs.).
Vegetable biomass is often a good indicator of the density and
specific bird composition of forest habitats (MacArthur, 1964;
Wilson, 1974; Levin, 1976; Prodon and Lebreton, 1981; James
and Wamer, 1982). In particular, a recurrent positive relation
between bird richness, density and extension of the under-
growth layer has been found (for example, Prodon and
Lebreton, 1981; Canterbury and Blockstein, 1997; Donald
et al., 1997; Dıaz et al., 1998). Crawford et al. (1981) concluded
that the crown cover and the height of the undergrowth layer
were the most important factors in explaining the distribution of
forest passerines. These studies linking undergrowth vegetation
and bird communities suggest that an intense modification of
the shrub layer either by mechanical means, grass or prescribed
burning, may have a significant impact on bird fauna. In fact,
the situation after a total clearing of the undergrowth can be
compared to one after a forest fire in which trophic availability
and nest site availability are limited, forcing territorial birds to
breed in unusual locations, consequently increasing their
exposure to predation or reducing their food supply (Renwald,
1977; Wright, 1981; Winter and Best, 1985; Petersen and Best,
1987). In spite of these predicted effects, coppice regeneration
may favour rapid nest establishment even before the develop-
ment of a more dense vegetation layer by growth of new seeds
(Brooker and Rowley, 1995; Pons, 1996). However, unlike
burning, clearing of the undergrowth does not affect tree
canopies and therefore, undergrowth recovery tends to be
slower unless clearing is accompanied by tree thinning.
The objective of this study is to determine the association
between forest birds and the structure of the shrub layer in
Mediterranean forests and test the hypothesis that undergrowth
clearing has a strong impact on the occurrence of forest birds,
especially those known to rely on low vegetation layers.
Furthermore, we explored the combined impact of undergrowth
clearing with other forestry practices (i.e. tree thinning) on the
bird community. We conclude by suggesting some recom-
mended guidelines aimed at making forest management
compatible with the conservation of species associated with
Mediterranean shrubby vegetation.
2. Methods
2.1. Study area
The study area, of about 2000 km2, is located in the Catalan
Pre-littoral System (north-east Iberian Peninsula). It has a
relatively rainy coastal climate with a short cool winter (de
Bolos, 2001). Coppice Holm oak forests (Viburno-Quercetum
ilicis) are dominant in most mountain areas up to 1400–1500 m.
The arboreal layer of coppice Holm oak forests is rather short
and the trees never reach a great size: 8.5 m average maximum
height, 12.4 cm average diameter at breast height (dbh), with
few trees reaching 35 cm dbh. Due to their limited size, trees
encounter strong competition for light and soil resources from
the well developed ligneous undergrowth which is dominated
by shrubs such as the Strawberry tree (Arbutus unedo),
Laurustinus (Viburnum tinus), Heather (Erica multiflora),
Lentiscus (Pistacea lentiscus) and Rockrose (Cistus salviifo-
lius) and lianas such as Bindweed (Smilax aspera), Ivy (Hedera
helix) and Bramble (Rubus ulmifolius). Sprouts of Holm oak
may also appear in the coppice forest undergrowth. The
arboreal layer, with close crown contact, is dominated by the
Holm Oak (Quercus ilex), accompanied in some areas by
Evergreen oak (Quercus humilis), Cork oak (Quercus suber) or
more occasionally Aleppo pine (Pinus halepensis).
The main forest management technique applied to Holm oak
forests in the area is sprout selection. This technique reduces
the number of sprouts per tree leaving from between two to four
stumps. These are eventually harvested for firewood once they
reach between 18 and 25 cm dbh (Garolera, 1991; CPF, 1992).
Cut rotation time ranges from 10 to 15 years. Usually, before a
sprout selection, the undergrowth layer is cleared with a chain
saw or a bush breaker. This undergrowth clearing is the main
disturbance affecting the shrub layers in these Holm oak
forests.
A particular case of forest management in areas used for cow
pasture and ewe grass consists of undergrowth clearing applied
in conjunction with the thinning of the Holm oak forest by
means of strong selective cutting. These forests have a low
density, open forest structure, since thinning has eliminated
most stems and only the best stumps have been preserved.
Overall, when taking into account the intensity and time
elapsed since undergrowth management, Holm oak forests vary
in the degree of cover and height of the shrub and liana layers.
When considering undergrowth structure, coppice Holm oak
forests where sprout selection is the main management
technique can be broadly classified a priori into four different
groups (Fig. 1): (1) Coppice Holm oak forests where the
undergrowth layer has been completely cleared. The entire
shrub and lianas layers having been eliminated by mechanical
practices. (2) Partially cleared coppice Holm oak forests which
maintain approximately one-third of the shrubs and lianas. This
clearing is not uniform and often clumps of shrubs and lianas
may have been left behind after undergrowth clearing. (3)
Cleared and thinned Coppice Holm oak forests in which all of
the undergrowth is eliminated and most of the arboreal layer is
felled by selective thinning. (4) Dense coppice Holm oak forest
where neither clearing nor thinning have taken place. These
have a high shrub and lianas cover (70–90%) and dense young
growth stand.
2.2. Location of census plots
Using data from the national forest inventory, forest maps
and administrative data on forest management histories in the
study area (Camprodon, 2001), we selected a number of forest
J. Camprodon, L. Brotons / Forest Ecology and Management 221 (2006) 72–8274
Fig. 1. Example of study habitats in the Finestres Mountain within or adjacent to the natural park of Zona Volcanica de la Garrotxa (Catalan Pre-littoral System, NE
Iberian Peninsula). Upper left, a dense Holm oak forest (DH); upper right, a partially cleared Holm oak forest (PCH); bottom left, a completely cleared Holm oak
forest (CCH); and bottom right, a cleared and thinned Holm oak forest strongly felled by selective thinning (CTH).
tracts with a history of forest management and homogeneous
vegetation structure. We classified such forest tracts into one of
the four forest types described above and made sure they were
uniformly distributed within the study area in order to avoid
spatial biases. Within the central section of each selected forest
tract, we located census plots in which the vegetation structure
and the bird community was sampled. Selected forest tracts
were located in forests larger than 200 ha in size, contained at
least 10 ha of homogenous vegetation of the corresponding
forest type and were located at least 1 km from the nearest
forest tract of any other type containing a census plot. Due to
the difficulty in locating adequate, large cleared, thinned
forests, we located, for this forest type, up to three census plots
per forest tract. In this case, census plots were located a
minimum of 400 m from the nearest plot to avoid pseudo-
replication (Hurlbert, 1984). All census plots were located at
least 500 m from the nearest forest edge to avoid interference
from surrounding non-forest habitats.
2.3. Bird census
We used point counts to estimate bird abundance at each
census plot (Tellerıa, 1986; Bibby et al., 2000). Point counts
lasted for 20 min during which all audible and visual contact
was registered in three radiuses of 100 m around the central
point of the census plot. All point counts were conducted within
3 h from sunrise (between 6:30 and 9:30 a.m.). Censuses were
conducted between the third week of April and the last week of
June during the years 1999–2002. We conducted one point
count per season at each census plot. We estimated that given
the long duration of the sampling (20 min), one point count per
season was enough to detect most species present and therefore
accurately sample different forest types. Point counts were
conducted in a random order across different forest types and
years of study reducing any possible temporal bias in data
collection. Larger species of raptors which were occasionally
observed in or around the census plots (Circaetus gallicus,
Buteo buteo, Accipiter nisus, Strix aluco and Asio otus) were
not included in the analyses.
We used species richness and abundance (number of
individual contacts) per point count as variables in further
analyses. We also classified bird species into different
categories (Table 3) according to the patterns of micro-habitat
choice as described in the literature (Muntaner et al., 1983;
Snow and Perrins, 1998; Shirihai et al., 2001): trunk-foraging
species (Woodpeckers, Nuthatch and Short-Toed Treecreeper),
other cavity nesters (tits, genus Parus), birds using tree
canopies (Wood Pigeon, Golden Oriole, Jay, Phylloscopus
Warblers, Firecrest, Chaffinch and some thrushes), species of
the shrub layer of the undergrowth (Wren, Robin, Blackbird
and most Sylvia Warblers) and birds associated with the agro-
forest open habitats (Nightjar, Turtle Dove, Serin, Greenfinch,
Goldfinch, Cirl Bunting). Due to their strong dependence on the
undergrowth layer, we also used another sub-guild within the
shrub layer category composed only of the species of the genus
Sylvia (Table 3).
2.4. Habitat inventory
We characterized vegetation structure for each forest type by
a set of measurements conducted at each census plot. We
described structural and floristic variables within a radius of
50 m around the central point of the corresponding census plot.
The cover of different vegetation layers was visually measured
at different interval heights (CV25: 0–0.25 m, CV50: 0.25–
0.5 m, CV1: 0.5–1 m, CV2: 1–2 m, CV4: 2–4 m, CV8: 4–8 m,
J. Camprodon, L. Brotons / Forest Ecology and Management 221 (2006) 72–82 75
CV16: 8–16 m), a model proposed by Prodon and Lebreton
(1981). The precision of this visual method is acceptable, with
the coefficient of variation estimated to be around 7% (Prodon,
1988). The cover of cleared, cut or pruned dry branches on the
ground (Bran) and rock cover (Rock) were also recorded using
the same method. The diameter of the trunk at breast height
(dbh) was also measured for trees using diametrical classes (D)
at 5 cm intervals. We grouped dbh classes into four categories:
lesser trees (<D5, dbh < 2.5 cm), small trees (D5–15, dbh
between 2.5 and 17.5 cm), medium sized trees (D20–30, dbh
between 17.5 and 32.5 cm) and big trees (D > 30, dbh from
32.5 cm). Final variables used an expressed number of trees/ha
for each of the corresponding tree size categories.
2.5. Experimental removal of the understory
To further explore the effect of undergrowth clearing on the
bird community, we employed a before–after control impact
approach on a section of Holm oak forest before and after
undergrowth clearing management (BACI, Stewart-Oaten
et al., 1986). In December–February of 1999 a complete
mechanical clearing of all shrub and lianas layers in 12 ha of
Holm oak forest with dense and homogenous undergrowth was
carried out. This forest was located north of the study area, in
the Finestres Mountain at a height of 600 m with an east-
southeast aspect.
We established two controlled forest sections, one adjacent
to the experimental one, and the second, of similar size,
altitude, orientation and vegetable structure, located 2 km from
the cleared section. Bird occurrence during the breeding season
within the sections was monitored for a period of 5 years: 2
years prior to the undergrowth clearing (1997 and 1998) and 3
years following the intervention (1999–2001). The census was
conducted by walking inside the forest section and locating all
bird contacts on a map. We conducted 10 visits to each forest
section and territories were identified on the basis of the spatial
clustering of simultaneous song and call contacts (i.e. mapping
method, IBCC, 1969; Tellerıa, 1986; Bibby et al., 2000). When
territories were not entirely within the studied forest section, we
only considered the area which lay within the boundaries of the
studied section.
In each forest section, we also monitored changes in
vegetation during the 5 years by means of 25 permanent plots
located 50 m from each other. Within each of these plots
vegetation variables were measured and described using the
same methodology employed at bird point counts. Climatic
conditions within forest sections were similar for the 5 years of
the project, with warm temperatures (20 � 1.2 8C as the
average for the month of July) and moderate rainfall
(790 � 207.5 mm annual).
2.6. Data analysis
In order to assess overall differences in species richness and
abundance between the four forest types with regard to the
vegetation structure and management regimes, we used
ANOVA and tested differences between the two groups by
means of the Tukey honest significant difference (HSD) (Sokal
and Rohlf, 1995). In cases in which assumptions of normality
were not met, we used generalized linear modelling (McCul-
lagh and Nelder, 1989) with a Poisson or binomial (for scarcer
species) error distribution.
Principal components analysis (PCA) with a varimax
normalized rotation, which maximizes the correspondence
between the factors and the original variables, was used to
simplify vegetation structure variables. The variables of tree
density were excluded from the factorial analysis because they
were not correlated with each other or with the cover variables.
Presence and abundance of bird species in each plot were
correlated to the habitat structural variables by means of the
canonical correspondence analysis (CCA). This is a statistical
technique derived from the correspondence analysis (CA),
designed to analyze tables of double entrance that contain some
measurement of correspondence between rows (sampling
points) and columns (species) (Greenacre, 1984). Like the
CA, CCA generates gradients in the original data by means of a
maximization of the correlation between the values corre-
sponding to the species and the values corresponding to the
used sampling points. But in the case of the CCA, the values
corresponding to the sampling points are constrained to be
linear combinations of a series of environmental variables of
interest (Prodon, 1992). CCA is especially sensitive to the
possible effects of the colinearity between variables. With the
object of reducing the number of variables in the final analysis,
it proceeds with a forward selection process in which only
significant variables in the identification of environmental
gradients are evaluated. All the census plots (n = 145), 24 bird
species (excluding those with less than a total of three contacts),
and 13 environmental variables were analyzed in the CCA
(Table 1).
To further investigate habitat selection patterns of different
guilds, we used the generalized linear model. This technique
considers the dependency of discrete variables (census of birds)
on a group of independent variables or indicators (i.e. habitat
structure: in this case, the factors obtained by PCA). We used
presence/absence of individual bird species in each census plot
as a dependent variable and used logistic regressions (binomial
error distribution) to evaluate species habitat relationships. In
addition, we used the number of species per plot for each
different guild and used Poisson error distribution to establish
guild–habitat relationships. A forward step-wise procedure was
applied to select significant variables only ( p-level threshold
p = 0.05).
Finally, changes in relative abundance of individual species
after clearing treatments were evaluated using the BACI
approach by applying the method of Stewart-Oaten et al.
(1986). Abundances were log-transformed (ln(x + 1)), to render
the residuals homoscedastic and normally distributed. For each
species, the difference in (log) abundance between each
treatment and the mean value of the two controls for each
sampling period was calculated, and these differences were
then subjected to analysis of variance of repeated measures
(ANOVA). Effects were fitted for treatment (cleared-control),
before/after, species and the interaction between these. Single
J. Camprodon, L. Brotons / Forest Ecology and Management 221 (2006) 72–8276
Table 1
Average values and standard deviations of the more representative environmental variables and results of ANOVA test between different structures from Holm oak
forest
Variable DH PCH CCH CTH F p
CV0.25 (%) 50.6 � 19.62 a 42.7 � 14.91 b 27.2 � 14.16 c 41.9 � 9.22 b 15.23 <0.001
CV0.5 (%) 40.5 � 16.00 a 30.9 � 9.17 b 19.7 � 8.45 c 33.9 � 5.61 b 23.83 <0.001
CV1 (%) 56.5 � 15.94 a 29.5 � 8.09 b 10.1 � 7.02 c 27.1 � 11.07 b 112.09 <0.001
CV2 (%) 61.7 � 14.83 a 26.8 � 9.42 b 11.0 � 5.32 c 13.1 � 7.46 c 101.32 <0.001
CV4 (%) 53.2 � 13.85 a 33.1 � 14.58 b 28.9 � 12.60 b 12.3 � 3.84 c 65.04 <0.001
CV8 (%) 44.6 � 14.34 a 48.8 � 12.85 a 47.4 � 12.68 a 27.9 � 4.58 b 20.46 <0.001
CV16 (%) 4.3 � 9.35 9.7 � 11.12 7.4 � 9.60 4.2 � 5.49 2.97 n.s.
Bran (%) 4.2 � 5.25 b 16.3 � 16.46 a 18.5 � 15.20 a 0.0 � 0.18 b 20.36 <0.001
Rock (%) 5.2 � 5.73 b 3.2 � 6.78 b 5.4 � 7.13 b 15.3 � 7.94 a 20.17 <0.001
D < 5 trees/ha 511.7 � 932.14 a 214.7 � 420.69 ab 191.9 � 438.53 ab 0.0 � 0.00 b 4.75 0.004
D5–15 trees/ha 2312.5 � 1103.68 a 1518.1 � 805.05 b 1706.1 � 8880.97 b 139.0 � 103.22 c 39.53 <0.001
D20–30 trees/ha 67.6 � 85.04 b 151’8 � 106.44 a 131.4 � 104.43 a 162.3 � 69.61 a 7.72 <0.001
D > 30 trees/ha 3.2 � 12.06 7.3 � 13.59 7.1 � 22.97 10.6 � 29.30 1.09 n.s.
TD (trees/ha) 2383.3 � 1064.05 a 1749.9 � 760.60 b 1878.9 � 802.04 ab 311.9 � 112.28 c 40.91 <0.001
n 40 39 36 30
Different letters identify forest types groups according to differences in mean values of a given variable as tested by Tukey HSD. CV0.25–CV16: vegetation cover
classes, Bran: dead branch cover, Rock: rock cover, D < 5: density of smaller trees, D5–15: density of small trees, D20–30: density of medium trees, D > 30: density
of large trees. TD: total density. DH: dense Holm oak forests, PCH partially cleared Holm oak forests, CCH: completely cleared Holm oak forests, CTH: cleared and
thinned Holm oak forests.
degree-of-freedom contrasts were then calculated for each
species, to assess whether there was a significant change in the
difference between the band and control forest sections from
before-to-after the application of the clearing treatment.
3. Results
3.1. Structural differences between forest types
The cover of the understory was significantly related to
forest type. In dense, reference forests, shrub cover reached,
below 2 m in height, values above 50%. This cover decreased to
26–42% in partially cleared forests and to 10–27% in
completely cleared forests (Table 1). Total tree density showed
similar figures between different cleared forests, with
abundance of sprouts, ranging between 1.750 and
2.400 trees/ha, largely within the 5–15 cm class and rarely
Table 2
Principal components analysis performed on variables describing vertical cover of th
(Rock)
F1 (high shrub) F2 (low tree) F3 (high t
CV25 0.156 0.133 0.118
CV50 0.469 �0.289 0.003
CV1 0.795* �0.175 0.000
CV2 0.908* �0.002 0.010
CV4 0.863* 0.371 �0.077
CV8 0.098 0.931* 0.149
CV16 �0.042 0.133 0.982*
Bran �0.085 0.017 �0.046
Rock �0.090 �0.161 �0.058
Expl. var. 2.472 1.180 1.013
% from total 27.5 13.1 11.3
Factor loadings for each individual variable are shown for the six factors obtained u
93.1%.* p < 0.05.
surpassing 30 cm. On the other hand, cleared and thinned Holm
oak forests had, on average, as little as 300 trees/ha, mainly
within the classes 15–30 cm (Table 1). They presented an
overall aspect of grazed forest, with trunks not in direct contact
with the shrub and liana layers.
PCA allowed us to distinguish six independent structural
factors associated with: (1) a decrease in shrub cover (up to
0.5 m high), (2) high shrub cover (between 0.5 and 4 m), (3)
low arboreal layer (between 4 and 8 m) and (4) high arboreal
cover (more than 8 m). Factor (5) was associated with a
gradient in rock cover while factor (6) identified a gradient in
dry branch abundance (Table 2).
3.2. Bird–habitat relationships and forest management
We recorded a total of 30 bird species in the Holm oak
forests we analyzed. In dense forests, we detected 20 species,
e vegetation (CV25–CV16), the cover of dry branches (Bran) and the rock cover
ree) F4 (rock) F5 (dead branches) F6 (low shrub)
�0.002 �0.100 0.918*
�0.038 �0.216 0.683*
0.005 �0.167 0.495
�0.101 �0.077 0.252
�0.048 0.049 �0.009
�0.169 0.014 0.001
�0.057 �0.043 0.094
�0.122 0.969* �0.176
0.972* �0.119 �0.016
1.006 1.048 1.659
11.2 11.6 18.4
sing a varimax normalized rotation. The percentage of accumulated variation is
J. Camprodon, L. Brotons / Forest Ecology and Management 221 (2006) 72–82 77
whereas in totally and partially cleared Holm oak forests 18–19
species were sighted and 26 in cleared and thinned Holm oak
forests. Species associated with the shrub layer were abundant
in dense Holm oak forests and tended to become scarcer in
forests in which the shrub layer had been reduced as a result of
the undergrowth clearing management (Table 3). However,
these trends varied according to the species in question.
Whereas species such as the Wren and the turdids (Robin and
Blackbird) appeared in cleared zones in which the arboreal
layer had been unaffected by the management, Sylvia Warblers
were very scarce in any type of cleared forests. For instance, the
Subalpine Warbler was completely absent from totally and
Table 3
Species abundance, total richness and abundance and richness of each guild by po
Scientific name Common name Guild
Species richness
Abundance
Undergrowth species Und
Sylvia Warblers Syl
Trunk-foraging species Tru
Tits Par
Tree-canopy species Cro
Open habitats species Ope
Columba palumbus (Copa) Wood Pigeon Cro
Streptopelia turtur (Sttu) Turtle Dove Ope
Caprimulgus europaeus (Caeu) Nightjar Ope
Upupa epops (Upep) Hoopoe Ope
Picus viridis (Pivi) Green Woodpecker Tru
Dendrocopos major (Dema) Great Spotted Woodpecker Tru
Troglodytes troglodytes (Trtr) Wren Und
Erithacus rubecula (Erru) Robin Und
Turdus merula (Tume) Blackbird Und
Turdus philomelos (Tuph) Song Thrush Cro
Turdus viscivorus (Tuvi) Mistle Thrush Cro
Sylvia cantillans (Syca) Subalpine Warbler Und Sy
Sylvia melanocephala (Syme) Sardinian Warbler Und Sy
Sylvia borin (Sybo) Garden Warbler Und Sy
Sylvia atricapilla (Syat) Blackcap Und Sy
Phylloscopus bonelli (Phbo) Bonelli’s Warbler Cro
Phylloscopus collybita (Phco) Chiffchaff Cro
Regulus ignicapillus (Reig) Firecrest Cro
Muscicapa striata (Must) Spotted Flycatcher Cro
Aegithalos caudatus (Aeca) Long-tailed Tit Cro
Parus cristatus (Pacr) Crested Tit Par
Parus ater (Paat) Coal Tit Par
Parus caeruleus (Paca) Blue Tit Par
Parus major (Pama) Great Tit Par
Certhia brachydactyla (Cebr) Short-toed Treecreeper Tru
Oriolus oriolus (Oror) Golden Oriole Cro
Garrulus glandarius (Gagl) Jay Cro
Fringilla coelebs (Frco) Chaffinch Cro
Serinus serinus (Sese) Serin Ope
Carduelis chloris (Cach) Greenfinch Ope
Carduelis carduelis (Caca) Goldfinch Ope
Emberiza cirlus (Emcr) Cirl Bunting Ope
n
Different letters identify forest types groups according to differences in mean values
dense Holm oak forests, PCH: partially cleared Holm oak forests, CCH: completely
significant.* p < 0.05.** p < 0.01.*** p < 0.001.
partially cleared forests (Table 3). The Blackcap occupied
totally cleared forests, but it was significantly less abundant in
dense forests (Table 3). When the clearing was accompanied by
selective thinning, the Wren, the Robin and the Blackcap were
less frequent and the rest of the Sylvia Warblers disappeared.
Only Blackbird abundance seemed to remain relatively
unchanged despite the undergrowth vegetation structure
(Table 3).
Common ground foragers in the dense forests like TheWood
Pigeon, the Jay and the Chaffinch, did not increase their use of
forest habitats with reduced shrub cover (Table 3). Only the
Chaffinch increased in abundance when the arboreal canopy
int count, according to the degree of undergrowth clearing or arboreal cutting
DH PCH CCH CTH p
8.30 a 6.36 b 5.44 b 8.60 a ***
10.30 a 7.95 ab 6.86 b 9.97 a ***
5.05 a 2.56 b 1.78 c 1.17 c ***
2.80 a 0.82 b 0.22 c 0.03 c ***
0.13 b 0.35 b 0.26 b 1.50 a ***
0.68 b 1.03 b 0.94 b 1.60 a ***
2.28 2.15 2.14 2.07 n.s.
0.17 a 0.21 a 0.28 a 2.07 b ***
0.03 0.21 0.11 0.13 n.s.
0.20 *
0.14 0.10 *
0.10 ***
0.05 b 0.05 b 0.06 b 0.40 a ***
0.03 b 0.37 a ***
0.63 a 0.41 a 0.39 a 0.03 b ***
1.58 a 1.51 a 1.39 a 0.37 b ***
0.80 0.79 0.69 0.93 n.s.
0.05 0.07 n.s.
0.05 b 0.30 a ***
l 1.13 ***
l 0.98 a 0.28 b ***
l 0.65 a 0.13 b ***
l 0.78 a 0.56 ab 0.31 b 0.03 c ***
0.05 0.13 0.17 0.07 n.s.
0.50 a 0.31 a 0.36 a ***
1.05 a 0.79 a 0.81 a 0.17 b ***
0.20 ***
0.18 0.21 0.19 0.17 n.s.
0.08 b 0.23 b 0.19 b 0.43 a **
0.05 0.06 n.s.
0.28 b 0.36 b 0.28 b 0.43 a ***
0.38 0.41 0.44 0.43 n.s.
0.13 b 0.33 a 0.22 b 0.57 a ***
0.05 b 0.06 b 0.20 a **
0.48 0.44 0.42 0.33 n.s.
0.45 b 0.38 b 0.44 b 1.33 a ***
0.10 b 0.10 b 0.06 b 1.07 a ***
0.05 0.05 0.06 0.20 n.s.
0.05 0.08 0.06 n.s.
0.50 ***
40 39 36 30
of a given variable as tested by Tukey HSD or generalized linear modelling. DH:
cleared Holm oak forests, CTH: cleared and thinned Holm oak forests. n.s.: not
J. Camprodon, L. Brotons / Forest Ecology and Management 221 (2006) 72–8278
Fig. 2. The arrangement of samples in the two first axes of the canonical
correspondence analysis. Dense Holm oak forests are represented by filled
squares; partially cleared Holm oak forests by open circles, completely cleared
Holm oak forests by triangles and cleared and thinned Holm oak forests by
crosses.
was also thinned, together with open habitat ground foragers
which are absent or very rare in dense forests, such as the Turtle
Dove, the Hoopoe, the GreenWoodpecker, theMistle Thrush or
the Cirl Bunting. Only the Nightjar appeared in completely
cleared forests, being entirely absent from dense forests.
Finally, trunk-foraging species and birds from agro-forest open
habitats increased in abundance in cleared and thinned Holm
oak forests remaining less frequent in denser forests (Table 3).
Bird community structure in different types of forests was
significantly captured by the environmental variables used. The
first CCA axis, accounting for 11.2% of total variation in bird
community, represented a gradient based on structural variables
related to cover of shrubs and small trees (i.e. from dense to
cleared forests), whereas the second gradient was positively
related to tree density and negatively to rock cover (Figs. 2 and
3). The two first axes accounted for 14.9% of total variation.
The presence of three shrub layer bird species, the Subalpine
Warbler, the Sardinian Warbler and the Garden Warbler, which
were then more abundant in dense Holm oak forests, was
Fig. 3. The relationship between the bird species and the environmental
variables selected by the canonical correspondence analysis. The variables
have been selected from a progressive evaluation of the meaning of the different
variables, by means of the Monte Carlo test. Only the variables with a p-level of
at least 0.05 are included. In order to interpret the environmental variable and
the species acronym’s, see the text and Table 3, respectively.
strongly associated to the first gradient. The rest of the shrub
layer species, such as the Wren, the Robin or the Blackcap, and
tree-canopy birds were associated with totally or partially
cleared forests. On the other hand, birds from agro-forest open
habitats, such as the Turtle Dove, the Hoopoe or the Cirl
Bunting (Fig. 3), were mainly associated with cleared and
thinned Holm oak forests.
Habitat selection models for undergrowth species showed a
strong association of this group with PCA factors describing the
low and high shrub layer and the low arboreal layer (Tables 2
and 4). Within this group of birds, the Wren and the Blackbird
also selected the accumulation of dead branches on the ground
(Table 4). Furthermore, we detected a negative association of
the occurrence of species such as the Wren, the Blackbird and
the Sardinian Warbler with the highest tree cover and also for
the Wren, the Blackbird, the Blackcap and the Garden Warbler
with rock cover. Trunk foraging species and tits were negatively
associated with the density of small trees. Tits were the only
group which had a positive association with rock cover. We did
not find any significant model for tree-canopy species. Finally,
birds of agro-forest open habitats tended to negatively select
plots with high tree densities and dense sub-arboreal layers
(Table 4).
3.3. Effects of experimental clearing on the bird
community
The average cover of shrubs and lianas before the
undergrowth clearing in the experimental forest section
(12 ha in size) was 57% for the vegetation layers between
0.25 and 4 m. These layers were dominated by Heather,
Strawberry tree, Binweed, Lentiscus, Bramble, Liguster and
Old Man’s Beard. After the clearing, the average cover of these
layers decreased to about 6%, before increasing progressively
again to about 16% in 2002 four years after the treatment.
Before the clearing the experimental forest section hosted
between 2 and 7 territories of different undergrowth bird
species (Fig. 4). We found a highly significant interaction
between the control-impact and the before–after factors
(F1,21 = 224.25, p < 0.001) indicating that while within the
control forest sections the number of occupied territories
remained constant, the number of bird territories within the
treatment section decreased sharply overall after the clearing of
the understory (Fig. 4). We also found that the effect of the
clearing was indeed species specific (F6,21 = 34.18, p < 0.001)
and therefore not all species were eliminated from the plots
after the undergrowth clearing (Fig. 4). During the breeding
season following winter clearing, the Subalpine Warbler, the
Sardinian Warbler and the Garden Warbler apparently
abandoned their territories and remained absent during the
years following the disturbance (1999–2001, Fig. 4). From
circumstantial detection of singing males, territories of Sylvia
Warblers seemed to be maintained in areas surrounding the
experimental forest section and the control sections throughout
these years. In contrast to other Sylvia Warblers, the Blackcap
continued to use to some degree the cleared zone, but this
species modified microhabitat use within the territories,
J. Camprodon, L. Brotons / Forest Ecology and Management 221 (2006) 72–82 79
Fig. 4. Representation of temporal changes in mean number of territories within the experimental (black circles) and control forest sections (white squares) for each
species commonly using the forest undergrowth: species specific contrast tests showed that the interaction between control-impact and before–after factors are
significant for all species except Sylvia atricapilla, Troglodytes troglodytes and Erithacus rubecula. The arrowmarks the point in timewhen the undergrowth clearing
treatment was applied to the experimental forest section during winter 1998–1999.
Table 4
Selection of environmental variables on the undergrowth species and the different guilds, from generalized linear models, by means of the procedure of forward
stepwise (whole p to enter: 0.05; whole p to remove: 0.05)
Species Model x2 d.f. % p
Undergrowth species 1.44 � 0.002D20–30 + 0.33 � high shrub + 0.17
� low tree + 0.20 � high tree �0.15 � rock
92.51 5 47.72 ***
Sylvia Warblers �0.16 + 0.79 � high shrub + 0.28 � low tree
� 0.30 � rock + 0.18 � low shrub
126.76 4 50.57 ***
Trunk-foraging species 0.38 � 0.001D5–15 54.04 1 36.06 ***
Tits 0.49 � 0.001D5–15 + 0.21 � rock 35.19 2 19.04 ***
Open habitats species 0.51 � 0.01D5–15 � 0.58 � low tree
� 0.33 � low shrub
156.86 1 63.08 ***
Wren �0.87 + 0.72 � high shrub + 0.49 � low tree
� 0.67 � high tree � 0.59 � rock + 0.63 � branc
42.05 5 22.67 ***
Robin 0.72 + CD10–15 + 1.78 � high shrub + 0.91 �high tree + 0.72 � low shrub
62.28 4 39.43 ***
Blackbird �1.60 + 0.001D < 5 + 0.01D20–30 � 0.47 � rock
+ 0.39 � branc � 0.48 � high tree
26.54 5 13.66 **
Sardinian Warbler 2.53 + 3.06 � high shrub � 1.20 � high tree 97.54 2 62.71 ***
Subalpine Warbler �1.28 + 2.45 � high shrub + 0.87 � low shrub 86.61 2 48.66 ***
Garden Warbler �1.98 + 0.98 � high shrub � 0.74 � rock 23.67 2 17.75 ***
Blackcap �0.92 + 1.04 � high shrub + 0.70 � low tree + 0.65
� high tree � 0.83 � rock + 0.60 � low shrub
55.54 5 29.53 ***
All the species follow a binominal distribution, while the guilds follow a Poisson distribution.** p < 0.01.*** p < 0.001.
J. Camprodon, L. Brotons / Forest Ecology and Management 221 (2006) 72–8280
showing a tendency to concentrate its activity along the areas
neighbouring non-cleared zones. The Blackcap used the
cleared zones basically for feeding and, secondarily, as singing
sites, but located the nest in the non-cleared zones.
However, the distribution of Wren contacts remained
basically identical before and after the clearing and occupied
vegetation along small dry streams. This species took
advantage of the accumulation of dead branches resulting
from forest clearing and used this substrate for feeding and
singing whereas it bred on the ground or rock margins. The
Robin also maintained a constant use of the forest after
clearing. However, more than half of the audible contacts for
this species along the border of the experimental forest section
corresponded to the non-cleared vegetation. Finally, the
Blackbird used the cleared zones as feeding substrates by
foraging on the ground. Most of the species activity also
concentrated on the non-cleared parts. Nearly half of the
Blackbird contacts in the cleared area corresponded to young
birds. The tree-dwelling species showed no change in numbers
after disturbance compared to the control.
4. Discussion
Change in perceived habitat quality is the key factor
determining birds’ response to disturbances (Sousa, 1984;
Wiens, 1989; Bazzaz, 1991). Prodon and Lebreton (1981)
showed that as small changes in vegetal cover occur, significant
impacts on the bird community may be detected when overall
vegetal cover in the undergrowth is low. However, if ligneous
vegetation is dense enough, clearing of the vegetation may have
negligible effects on birds. In line with these predictions, the
abundance of Sylvia Warblers, birds morphologically adapted
to life within dense shrubs, diminished in cleared forests, but
especially so when clearing of the undergrowth vegetation was
almost complete. The results obtained after the experimental
undergrowth clearing of a forest reinforced the trends observed
in the synchronous study based on point counts regarding
changes in the abundance of Sylvia Warblers following the
clearing disturbance. Among this group of birds, the Subalpine
Warbler appeared to be the most demanding in terms of
undergrowth vegetation structure, since it prefers higher and
denser shrubs and is the less arboreal of the Sylvia Warblers in
the Holm oak forests, whereas the Blackcap was the most
flexible species, probably due to its preferential use of tree
canopies compared to other Sylvia Warblers (Fuller, 1995;
Snow and Perrins, 1998; Shirihai et al., 2001). If the
undergrowth clearing is accompanied by selective thinning,
the combined effects of these two factors on the bird
community appear more intense. This management practice
diminishes the abundance of other undergrowth bird species in
addition to the Sylvia Warblers, but also the thinning induces
changes in the composition of the tree-canopy birds and
benefits other species associated with agro-forest open habitats
(Fig. 3).
The disappearance of the undergrowth vegetation induced
by the clearing results in important limitations for birds. First,
species will encounter a decrease in the availability of trophic
resources within the undergrowth vegetation (i.e. arthropods
and fruits), a basic substrate used for feeding (Snow and
Perrins, 1998). According to our own unpublished data,
arthropod biomass contained in undergrowth ligneous vegeta-
tion can be very high, even higher than that recorded in the
canopy layer of a mono-specific arboreal layer such as that
dominating in Holm oak forests. Even the piles of accumulated
fine dead branches on the ground after the clearing are a
substrate apt for feeding. Second, a reduction in vegetation load
may induce the loss of singing and breeding sites or other
locations that may provide refuge from predators. Sites within
the arboreal layers can act as an alternative for flexible species
but not for selective species which focus on the use of low
shrubs. Finally, changes in vegetation structure lead to changes
in the microclimate of otherwise humid, dense Holm oak
forests. These effects may be important for hygrophylic species
especially when clearing is accompanied by tree thinning
(Wren, Robin and Blackcap). In fact, microclimate has often
been considered as an influential abiotic factor on bird
distribution (Janzen, 1986; Lovejoy et al., 1986; Wiens,
1989), either through changes in the interactions with other
species or by placing restrictions on the species physiological
needs. When thinning is combined with undergrowth clearing,
the loss of trophic resources and refuge substrates for more
flexible undergrowth birds is enhanced and accompanied by a
complete loss of forest interior microclimate. On the other
hand, open conditions lead to the appearance of agro-forest
habitat species, which is favoured by the fact that open
meadows are often found relatively near (500–1000 m) thinned
and cleared forests. This is to facilitate the use by cows of
alternative pasture habitats (pers. obs.).
The structure of the remaining vegetation after clearing is
another basic factor determining post-disturbance habitat use
by birds. The height of the shrub layer between 1 and 2 m
appears to have a critical role for undergrowth species such as
the Sylvia Warblers (see also Blondel, 1981 for ‘‘garrigue’’ bird
assemblages). The low height of the arboreal layer prevailing in
Holm oak forests may help to mitigate the negative effects of
sparse undergrowth for flexible species (i.e. species capable of
using tree canopies to some extent) such as the Robin, the
Blackbird and the Blackcap.
In thinned Holm oak forests, the reduction in tree density
and associated lianas, together with the preservation of the
more developed tree trunks seemed to promote the presence of
trunk-foraging species. The rock cover was negatively
associated with different undergrowth passerines and positively
with the tits. The effect of rock cover on undergrowth birds can
be interpreted as an indirect indicator of shrub cover deficiency
in rocky areas (i.e. undergrowth clearing and intense thinning
leave the open and the rocky outcrops visible) or as indication
of dry conditions in the area. A more direct explanation may be
found for tits since this group makes direct use of rocky
outcrops as substrate for nest building which may be otherwise
scarce in managed Holm oak forests (pers. obs.).
A site tenacity effect has been described several times in
undergrowth birds (first described in Emlen (1970), and ratified
by Prodon and Lebreton (1981), Prodon et al. (1987), Pons
J. Camprodon, L. Brotons / Forest Ecology and Management 221 (2006) 72–82 81
(1996, 1998)). According to this effect, birds may persist in the
original territories after disturbances such as fires. The
existence of tenacious birds may cushion the effects of the
disturbance, since only one part of the population disappears
either by death or dispersion. However, in the experimental
forest clearing, the three Sylvia Warblers species most affected
by the disturbance did not show tenacious behaviour after
clearing of the undergrowth, probably because they found
suitable habitats nearby.
To summarize the main effects of clearing and thinning of
Holm oak forests on birds, it is possible to establish a gradient
of response strength according to the use by species of
undergrowth vegetation: (a) species that require high shrub
cover and which are already scarce after partial clearing: the
Subalpine Warbler, the Sardinian Warbler and the Garden
Warbler; (b) species that tolerate clearing to some degree, but
use, preferably, nearby non-cleared areas as breeding sites: the
Blackbird and the Blackcap; (c) a group of species that remain
indifferent to changes in the undergrowth, but are sensitive to
decreases in undergrowth cover when applied together with tree
thinning: the Wren, the Robin and the Firecrest; (d) species
which tended to favour the undergrowth clearing: Nightjar; and
finally, (e) tree-canopy dwelling species that enter the cleared
and thinned Holm oak forests: the Turtle Dove, the Mistle
Thrush, the Spotted Flycatcher, the Golden Oriole, the Serin,
the Greenfinch and the Cirl Bunting.
5. Applied conclusions
The reduction of the vegetal biomass of the undergrowth is a
convenient forestry practice in particular forest management
circumstances. However, maintaining large extensions of
forests with a low combustible load is a titanic and
economically unrealizable task with obvious ecological
impacts (Meson and Montoya, 1993). Consequently, to
improve biodiversity conservation it is important to ensure a
certain number of patches of the different shrubs and lianas
species and to apply clearing to patchy surfaces (10 ha
maximum). The selection of shrubs and lianas for clearing
should be applied plant by plant, so that the ligneous
undergrowth is uniformly distributed, or distributed into
clusters. However undisturbed small sectors no longer than
1 ha, should be reserved.
To maintain undergrowth Warbler species after clearing
would require a minimum of 30–40% of shrub cover above one
meter in height (preferably 1.5–2 m). Sylvia Warblers are one of
the characteristic birds of Mediterranean woodlands, and
therefore, they have a substantial value to conservation, in spite
of not being a threatened species (Tucker and Heath, 1994). It is
worth noticing, that in addition to benefiting Warbler species,
preservation of the undergrowth in Holm oak forests is related
to other ecological and forestry advantages such as the
protection of young saplings from herbivores and the
prevention of soil erosion (Meson and Montoya, 1993).
The most intense clearings should be concentrated in the
sectors with greater risk of fire, for example along forest edges
near farmland areas. A complementary measurement may be to
pile up part of the dry branches in small dispersed piles, of 1–
2 m in diameter and at least 1 m high. These piles simulate live
shrubs and when distributed sparsely throughout the clearing
zone do not increase the risk of fire. In cases where there is a
lack of living ligneous undergrowth, these piles may act as a
refuge for arthropods, small mammals and some undergrowth
birds, especially the Wren, the Robin and the Blackbird.
Finally, the management of small plots of grazed open Holm
oak forest with an arboreal layer below the 400 trees/ha, might
be beneficial for the protection of tree-dwelling birds, that avoid
both dense Holm oak forests as well as opened spaces without
trees, as long as these grazed Holm oak forests are located in
convenient places (preferably neighbouring meadows) and are
at least a few hectares in size. In this case, trees must grow at
low density without technical rotation, until they attain large
trunks and big crowns. These forests play an interesting role in
the local conservation of some scarce bird species that only
occur in open forests, such as Mistle Thrush, Spotted
Flycatcher, Turtle Dove or Nightjar. However, these open
and grazed Holm oak forests are very different ecologically to
semi-natural or dense Holm oak forests, so they should not be
thought of as a general management model, but as comple-
menting the traditional forestry for firewood and wood
production.
Acknowledgements
This study has been carried out thanks to the financing of the
Environmental Department of the Generalitat de Catalunya
(Catalan Government) and the Forest Technology Centre of
Catalonia. At the same time, to carry out the project we have
been able to count on the collaboration and the support of the
technical services of the Environmental Department, particu-
larly Joan Montserrat and Emili Bassols technicians of the
Natural Park of the Zona Volcanica de la Garrotxa (Catalonia).
Also, Enric Batllori, Joan Carles Abella and Marc Albert
Fernandez have collaborated in the field work. We are grateful
also to Xavier Ferrer, Santi Manosa, Raimon Marine, Pere Pons
and Roger Prodon for their participation in the sampling design
and the text revision. This work has been conducted under the
GDRE project ‘‘Mediterranean and Mountain systems in a
changing world’’. LB benefited from a Ramon y Cajal contract
from the Spanish government and JC from a travel grant from
the CTP and the Universities and Research Department of the
Catalan Government.
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