Copyright © 2010 by the author(s). Published here under license by the Resilience Alliance.
Acacio, V., M. Holmgren, F. Moreira, and G. M. J. Mohren. 2010. Oak persistence in Mediterranean
landscapes: the combined role of management, topography, and wildfires. Ecology and Society 15(4): 40.
[online] URL: http://www.ecologyandsociety.org/vol15/iss4/art40/
Research
Oak Persistence in Mediterranean Landscapes: The Combined Role of
Management, Topography, and Wildfires
Vanda Acácio 1,2, Milena Holmgren 3, Francisco Moreira 2, and Godefridus M.J. Mohren 1
ABSTRACT. Mediterranean ecosystems have been shaped by a history of human and ecological
disturbances. Understanding the dynamics of these social-ecological systems requires an understanding of
how human and ecological factors interact. In this study, we assess the combined role of management
practices and biophysical variables, i.e., wildfire and topography, to explain patterns of tree persistence in
a cork oak (Quercus suber L.) landscape of southern Portugal. We used face-to-face interviews with
landowners to identify the management practices and the incentives that motivated them. We used aerial
photographs and a Geographic Information System (GIS) to classify vegetation patch-type transitions over
a period of 45 years (1958-2002) and logistic regression to explain such changes based on management
and biophysical factors. The best model explaining vegetation transitions leading to cork oak persistence
in the landscape included both biophysical and management variables. Tree persistence was more likely
to occur on steeper slopes, in the absence of wildfires, and in the absence of understory management. We
identified ecological, ideological, and economical barriers that preclude oak persistence and that are
important to consider in implementing efficient environmental policies for adequate conservation and
reforestation programs of Mediterranean cork oak landscapes.
Key Words: agroforestry system; alternative ecosystem state; Cistus ladanifer; land degradation;
Mediterranean; Portugal; shrub encroachment; succession; Quercus suber; vegetation transition
INTRODUCTION
Mediterranean landscapes have long been shaped
by a history of human management and natural
disturbances (Aschmann 1973, Trabaud 1981,
Conacher and Sala 1998). Understanding the
dynamics of these ecosystems requires an
understanding of how social and ecological factors
interact, and how these interactions change over
time (Lambin 2005, Perz 2007, Plieninger and
Schaar 2008).
Since the 1960s, major socioeconomic changes in
Mediterranean Europe have led to a gradual
abandonment of traditional agriculture and
stimulated rural exodus (Joffre et al. 1991, PintoCorreia 2000). Abandoned farmland was colonized
by shrub and tree species, and a natural succession
process from fallow to forest was observed in many
locations (Mazzoleni et al. 2004). In the drier
southernmost regions, however, succession appears
1
arrested in early pioneer stages (Acácio et al. 2007,
2009, Costa et al. 2009).
The southwestern Iberian Peninsula has been
shaped by a traditional agroforestry land use system
where cork oak (Quercus suber L.), Holm oak
(Quercus ilex L.), or a mixture of both, coexist with
pastures and crops. The rural exodus of 50 years ago
eliminated land ploughing and reduced herbivory
in the understory of the oak savannas and forests
(Pinto-Correia and Mascarenhas 1999, Alves et al.
2003, Pinto-Correia and Vos 2004). In the absence
of human management, the usual pathway of natural
succession in these agroforestry oak systems
involves the gradual colonization of the understory
by shrub species, i.e., pioneer Cistus shrubs,
followed by other shrub communities like Arbutus
and Erica, mixed with oak natural regeneration,
which eventually can lead to forest recovery
(Natividade 1950, Gonçalves 1991). However,
several studies have reported decreasing oak density
Forest Ecology and Forest Management Group, Wageningen University, 2Centro de Ecologia Aplicada “Prof. Baeta Neves”, Instituto Superior de
Agronomia, Universidade Técnica de Lisboa, 3Resource Ecology Group, Wageningen University
Ecology and Society 15(4): 40
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(Ferreira 2000, Vicente and Alés 2006, Vallejo et
al. 2009), high oak seedling mortality (Direcção
Geral das Florestas 1990, Montero et al. 1994), and
lack of tree regeneration both in oak-dominated
patches (Campos et al. 1998, Montero et al. 2000,
Pulido and Díaz 2005), as well as in shrubland
patches (Pons and Pausas 2006, Pausas et al. 2006).
Previous work in southern Portugal found that
shrublands have expanded during the last 45 years
at the expense of other vegetation patch-types, such
as oak savannas and forests, particularly on southern
slopes (Acácio et al. 2009). Although oak
recruitment was observed in the years following the
abandonment of agriculture and pastures, shrub
encroachment progressed fast. In fact, since 1985
cork oak savannas and forests have been turning
into shrublands at an increasing rate (Acácio et al.
2009). Initial cork oak recruitment in old fields after
abandonment, followed by shrub encroachment, has
also been observed in eastern Spain (Pons and
Pausas 2006). Shrub encroachment can limit tree
recruitment through multiple ecological mechanisms
affecting tree seed dispersal, germination, and
seedling survival (Acácio et al. 2007), and it is
particularly severe under drier conditions or with
frequent fires (Acácio et al. 2009).
Persistence of early successional stages may be
intrinsically related to management decisions
affecting the ecological processes involved in
succession. Clearing of large areas for agriculture
and pastures may eliminate nurse shrubs that
facilitate tree seedling recruitment; it may also
eliminate suitable seed dispersal sites and affect the
habitats of the animal species responsible for seed
dispersal. Also, livestock grazing, the second most
common land use, contributes directly to seed and
seedling predation (Herrera 1995, Pulido and Díaz
2002, Gómez et al. 2003, Leiva and Fernández-Alés
2003), preventing forest recovery. Management
actions may interact with ecological processes and
amplify limitations for seedling establishment,
slowing down or preventing the natural process of
succession even after management actions have
been reversed. Although there is some
understanding of the ecological mechanisms
responsible for patch persistence in Mediterranean
ecosystems (Fuentes et al. 1984, Gonçalves 1991,
Montero et al. 1994, Puigdefábregas and
Mendizabal 1998, Segura et al. 1998, Holmgren et
al. 2000, Holmgren 2002, Holmgren and Scheffer
2001, Acácio et al. 2007), and the social factors
involved (Pinto-Correia and Jorge 1996, Mazzoleni
et al. 2004), the ways in which ecological and social
mechanisms interact remains poorly studied and
practically unknown (Rindfuss et al. 2004, Lambin
2005, Perz 2007). We hypothesize that management
variables can play an important role in explaining
persistence of cork oak trees and that they interact
with topography and wildfire occurrence to explain
the condition of arrested succession documented in
cork oak landscapes of the Iberian Peninsula.
In this study, we assess the combined role of
management practices and biophysical variables in
explaining cork oak persistence in a 45-year period
(1958-2002) and relate management decisions with
landowner’s perceptions. This long time frame
allows evaluating tree persistence in a patchy
landscape of forests, savannas, shrublands, and
grasslands, where cork oak is the dominant tree
species. We used a combination of aerial image
analysis, databases with biophysical variables, and
interviews with landowners to evaluate the role of
ecological and management factors and the
motivations behind local land management
decisions.
METHODS
Study area
The study area covers 11,000 ha of the Serra do
Caldeirão (37° 14´ N, 7° 56´ W), a mountain ridge
in the northeastern part of the Algarve province in
southern Portugal. The climate is Mediterranean,
with average annual temperature of 16.6°C (São
Brás de Alportel meteorological station, INMG for
the period 1958-1976, and SNIRHa for the period
1980-2002), and average annual precipitation of
approximately 900 mm (Barranco do Velho
udometric station, INMG for the period 1958-1988,
and SNIRHb for the period 1989-2002). Altitudes
range from 146 to 588 meters above sea level.
Predominant soil type is schist lithosols (SROA
1970). Soils are shallow, with a topsoil of
approximately 15 cm, low fertility, and prone to
erosion. Cork oak is the dominant tree species. This
study area is one of the most continuous remnants
of the southern Mediterranean cork oak landscapes
of the Iberian Peninsula.
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Historical background
Before the 15th century, southern Portugal was
covered with cork oak forests, which were mainly
used by kings and nobility to hunt large game, for
firewood, and for extensive grazing. Between the
15th and the 17th centuries, the cork oak area in
Portugal diminished substantially because of an
increasing demand for wood for shipbuilding,
agricultural land, and pastures. Cork began to be
used for bottling wine after the 17th century,
acquiring an increasing commercial value. From the
18th century onward, cork oak forests were
gradually transformed into an agroforestry system
exploited for cork, crops, and pastures (Mendes
2007). It is likely that the management of such
agroforestry systems throughout the centuries also
included protection of oak natural regeneration and
artificial seeding or planting, to maintain cork
production (Capelo and Catry 2007).
In the first half of the 20th century, traditional
management of cork oak systems in the Iberian
Peninsula included cork extraction, extensive
grazing by pigs and sheep, and crop production.
Shrub clearing and other land operations were done
by manual labor and trees were exploited in unevenaged stands for natural regeneration (Soares et al.
1998). From 1929 until the beginning of the 1960s,
most of the land in southern Portugal was cultivated
for wheat, following a government policy known as
The Wheat Campaign that strongly subsidized
wheat production (Baptista 1993). During the
1950s, wheat cultivation expanded to less suitable
lands with poor and shallow soils, such as oak forest
lands, which tend to be more vulnerable to the
detrimental effects of intensive cereal cultivation
(Baptista 1993). At that time, most of the cork oak
area in southern Portugal was exploited as an
agroforestry system for cork, grazing, and cropping.
The age-structure of stands included older trees
from the 19th century and younger trees that had
been planted at the onset of the 20th century, when
large reforestation projects with cork oak took place
in the south (Costa and Pereira 2007). Decades of
The Wheat Campaign led to soil exhaustion (PintoCorreia and Vos 2004), elimination of cork oak
natural regeneration, and the felling of many adult
trees for cereal cultivation (Mendes 2007). Only
forest areas located on steep slopes were not utilized
as an agroforestry system because steepness made
understory management for crop cultivation
difficult or impossible.
Between the 1960s and 1970s, the country’s
agricultural working population decreased by about
one third (Baptista 1993) because of important
national and international socioeconomic changes,
i.e., industrialization, immigration to cities, and
emigration. After 1975, understory use had already
been abandoned in the least productive lands and
smaller properties in southern Portugal, including
our study area, and cork was maintained as the single
production component (Pinto-Correia and Mascarenhas
1999). At present, agriculture in our study area is
only practiced for local family subsistence around
the few scattered settlements and livestock has been
reduced to only a few flocks of goats and sheep,
approximately 500 animals, feeding mainly on
fallow lands and shrublands (Pires 2000). Land
holdings within the area are very fragmented and
all under private tenure. Cork extraction is currently
the main economic activity of the region, because
it is the major economic asset of cork oak lands
(Pereira 2007).
Management and social variables
Management and social data were derived from
face-to-face interviews carried out by a single
interviewer with local landowners. Landowners
were defined as proprietors of the land although they
may not necessarily live in the study area. Because
there is no available landowner data for this region
of Portugal, we selected landowners to be
interviewed from the registration system of the
Association of Forest Producers of Serra do
Caldeirão (AFPSC). We were able to contact 20
landowners owning 31 parcels. This landowner
sample is representative for the study region,
because it includes about 20% of all members of
AFPSC, who own about 10% of the land throughout
the whole study area, according to data from
AFPSC. The interviews included a combination of
open-ended, qualitative questions along with
closed, structured questions. Interviews were
grouped in three main categories, each with
questions covering several variables:
1. Land management between 1958 and 2002,
using the following variables: 1) understory
management, i.e., crops cultivation and/or
livestock; 2) total livestock units; 3) livestock
composition; 4) cutting adult cork oak trees;
5) shrub clearing; and 6) investments in land,
including cork oak planting.
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2. Landowner profile, using the following
variables: 1) age; 2) level of education; 3) type
of management decisions, i.e., decisions
made by owner or others; and 4) profession.
3. Individual perceptions: 1) factors related with
forest cover change, using closed questions
on cork oak natural regeneration and shrub
encroachment, and open-ended questions on
erosion, cork oak mortality, and wildfires; 2)
open-ended questions on the main problems
faced in local forest management; and 3)
open-ended questions on what is missing for
local forest management.
It is well known that when questions refer to an event
that occurred long in the past, interviewees may not
accurately recall their motivations and decisions and
may even be confused as to which management
strategy was implemented in each parcel (Rindfuss
et al. 2004). To account for this, national historical
events, such as elections of certain governments,
etc., were used as dates to help the landowner
remember the past management practices and
circumstances around it. Therefore, the years 1975
and 1985 were used as key years to detect
management changes because 1975 was the first
year following the revolution that ended the
dictatorship period in the country and also marked
the end of the land abandonment period. Also, 1985
was an important political year followed by the
adhesion of Portugal to the European Economic
Community, today’s European Union (EU), in
1986, with important land management changes.
Management variables were derived from
subquestions relating to the respective time periods
and answers showed that management changes
coincided within these specific periods for most of
the farmers.
From the initial six land management variables
(category 1), we derived a set of three variables
summarizing the more relevant information for
assessing vegetation patch-type transitions:
1) understory management before 1975; 2) tree
cutting after 1975; 3) tree planting after 1985.
Understory management existed only for the period
before 1975. For all parcels except one, owners
stopped understory use for livestock or crops after
1975. Understory management before 1975 was
positively associated with the presence of livestock
(χ² = 10.2, P = 0.0014), and this latter variable
resulted from a combination of the variables total
livestock units and livestock composition, which
were therefore excluded from further analysis.
Understory management before 1975 was also
positively associated with the variable shrub
clearing (χ² = 21.6, P < 0.001), so the latter variable
was excluded from further analysis.
For all types of investments in land, such as
acquiring machinery and equipment, pruning trees,
improving forest road networks, and building water
points, tree planting and cutting were the ones that
had a higher visible impact in forest cover changes.
Tree planting only took place after 1985 following
EU subsidies. Adult cork oak trees were only cut
after 1975 because of mortality.
Collection of data on vegetation patch-type
transitions
Vegetation patch-type transitions were characterized
using aerial photographs from 1958 and
orthophotos, i.e., a rectified copy of an original
aerial photograph, from 2002, thus covering a 45year period. Aerial photographs were previously
scanned and then orthorectified and georeferenced
with ENVI 3.4 (Research Systems 2001) to produce
geometrically correct images and project them onto
the same coordinate references as the digital
orthophotos (Transverse Mercator projection,
datum WGS84). Digital elevation models with a
spatial resolution of eight meters were used for
altitude coordinates (orthorectification process),
and between 10 and 15 ground control points per
photo were taken from the 2002-orthophoto for map
coordinates (georeferentiation process). We
obtained a RMS Error of about 10 meters. We
overlaid a regular 0.5 x 0.5 km grid of points over
the entire study area, and selected all the points that
were contained inside the parcels of the landowners
who were interviewed. We selected 35 points from
the 31 parcels (larger properties had more than one
point) and photo interpreted on screen a 50 m-radius
circle (sampling unit) around each photographic
point. Photointerpretation was performed on a
photo-by-photo basis. The grid was created with
ArcView GIS 3.2 (Environmental Systems
Research Institute 1999) and laid over the aerial
photographs and orthophotos (imported into the GIS
as scanned images). For 1958 and 2002, each of the
35 sampling units was classified into one of four
vegetation patch-types (Table 1).
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When more than one vegetation patch-type occurred
in a sampling unit, it was classified based on the
predominant type. Sampling units with only one
kind of patch-type were predominant (>50% of the
total units) and when not, we could usually clearly
assign one dominant class to each unit. We used tree
cover percentage rather than tree density because in
many cases it was not possible to count the number
of trees on the photographs. Boundaries for forest
cover (10% and 30% of ground cover) follow the
National Forest Inventory norms (Direcção Geral
das Florestas 2001).
data for every year between 1984 and 2002
(provided by the Tropical Research Institute for
1984-1989 unpublished data, and by the General
Directorate of Forests for 1990-2002, http://www.a
fn.min-agricultura.pt/portal/dudf/cartografia/cartografareas-ardidas-1990-2009). The limits of the burned
area per year were stored as an ArcView GIS layer
and overlaid with the sampling units to identify the
sampling units that were affected by fire between
1984 and 2002. The maximum number of times a
unit was affected by fire in this period was two.
Before photo classification (2003) we visited the
study area several times to familiarize ourselves
with the different vegetation types and compare
them with the patterns on the aerial photos of 2002.
We estimated transitions between vegetation patchtypes by counting the number of sampling units of
any vegetation patch-type that changed into any
other between 1958 and 2002. Vegetation changes
where there was maintenance or increase in tree
cover were registered as presence of cork oak
persistence; whereas patch-type transitions toward
absence or decrease in tree cover were registered as
absence of cork oak persistence (Table 2).
Statistical analyses
Biophysical variables
Three environmental variables were registered:
slope, exposure, and wildfire occurrence. Slope
determines soil depth and erosion risk, which in turn
affect tree growth and survival. The exposure of a
slope determines the amount of incident irradiance
and therefore is strongly correlated with plant water
stress, e.g., southern exposures are drier, and often
plant survival. Wildfire directly affects tree and
seedling mortality and indirectly influences several
ecological processes involved in tree seedling
recruitment and vegetation succession.
The topographical variables, slope and exposure,
were derived from digital elevation models
(Portuguese Military Geographic Institute, IGeoE)
and stored as two ArcView GIS layers. We assigned
one class of slope and exposure to each sampling
unit based on the most common type. The slope
categories were: low to medium (0-20%); steep
(20-30%); very steep (>30 %). The exposure
categories were: north facing slopes (including NW,
N, and NE), south facing slopes (including SW, S,
and SE) and others (E and W). Wildfire occurrence
was available as spatially referenced and digitized
Logistic regression (Hosmer and Lemeshow 2000)
was used to model the influence of biophysical and
management variables on oak tree persistence
(coded as 1=presence/0=absence) during the period
1958-2002. Because of the small sample size in the
lower slope class, only 2 slope categories were
considered, ≤30% and >30%. Exposure and wildfire
were also considered as categorical variables.
To assess the relative importance of biophysical and
management variables on the probability of cork
oak persistence, we used an Information Theoretical
Approach (ITA; Burnham and Anderson 2002). The
ITA looks for simplicity and parsimony of several
working hypotheses and is based on finding the
strength of evidence of each model, for a set of
candidate predictive models. The AIC adjusted for
small samples (AICc) was used as a measure of
information loss of each candidate model, with the
best fitting model having the lowest AICc and the
highest Akaike weight (wi). The latter measures the
relative likelihood that a given model is the best one,
given the data and the set of competing candidate
models. AICc differences (∆i) between each model
and the model with the smaller AIC (AICmin) can be
used to assess the relative support for the different
alternative models (Burnham and Anderson 2002,
Rushton et al. 2004).
To avoid overfitting and finding spurious effects
when working with small sample sizes, the number
of candidate models should be small (Anderson
2008). Thus, we used stepwise regression as a way
of keeping the number of candidate models low,
relative to our sample size, prior to the ITA
evaluation. In this sense, we did not use null
hypothesis testing to compare models, but we used
it to restrict the number of candidate models in our
set to be evaluated by the IT approach. Overall, we
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Table. 1 Classification of vegetation patch-types.
Vegetation patch-type
Description
Cork oak forests
High cork oak (Quercus suber L.) density (more than 100-150 trees/ha) and a diverse
shrubby layer, e.g., Arbutus unedo, Viburnum tinus, Erica arborea, Pistacia lentiscus,
Rhamnus alaternus, Phillyrea latifolia; this class corresponds to a tree cover higher than
30%
Cork oak savannas
Low cork oak density (less than 100 trees/ha) with sparse shrubs in the understory; this
class corresponds to a tree cover between 10% and 30%
Shrublands
Dominated by Cistus ladanifer shrubs, pure or mixed with Ulex argenteus, Genista hirsuta,
and Lavandula stoechas shrubs; this class corresponds to a tree cover lower than 10%
Grasslands
Dominated by cultivated croplands, semi-natural pastures, or fallow land; this class
corresponds to a tree cover lower than 10%
built 12 models: a) separate models for each of the
predictor variables (6 models); b) models for each
set, i.e., biophysical and management, of variables
(2 models); c) stepwise models, forward and
backward stepwise selection yielded similar
models, for each set (2 models); and d) stepwise
models, forward and backward selection, pooling
all 6 variables from both sets (2 models). The
relative importance of each variable was estimated
by summing the Akaike weights across all models
that contained that variable (Burnham and Anderson
2002). Model fit and predictive performance of each
model were evaluated through the likelihood ratio
statistic (full model χ²) and by calculating the area
under the receiver operating characteristics (AUC)
curve (Pearce and Ferrier 2000).
RESULTS
Land management and landowner’s profile
Before 1975, most landowners used their lands as
an agroforestry system for cork, pastures, and crops
(70% of respondents at 26 sampling units). Herd
sizes of mostly sheep and pigs varied between 50
and 100 animals per herd, which grazed on each
landowner parcel, with an average parcel of 10 ha,
but also throughout the study area. After 1975, adult
dead trees were cut at 29 sampling units (about 80%
of total sampling units) and after 1985, cork oak tree
planting was carried out at 14 sampling units (40%
of all sampling units). All interviewees are
responsible for the decisions and implementation of
their own land management practices, however,
only 10% work directly on the land, 50% are retired,
and 40% have a different profession than land
management. Most landowners were older than 60
years (60% of interviewees), and only one (5% of
interviewees) was younger than 45 years old. Half
of the interviewees had only a primary school
education and 20% were college-educated.
Landowner’ perceptions
Table 3 shows the frequency, i.e., number and
percentage, of landowner responses regarding the
main causes of soil erosion, tree mortality, and fire,
changes in natural regeneration, and changes in
shrubland cover since 1960. Table 4 shows direct
quotations from landowners regarding difficulties
and needs on local forest management.
For landowners, soil erosion was mostly caused by
the use of disk harrowing, rippers, and heavy
machinery (21.2%), followed by heavy rains.
However, opposite reasons such as cereal
cultivation in the past and lack of cereal cultivation
were also pointed out as responsible for soil erosion
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Table 2. Types of vegetation transitions (1958-2002) grouped into presence of cork oak (Quercus suber
L.) persistence or absence of cork oak persistence (Fo: forest; Sa: savanna; Sr: shrubland; Gr: grassland)
Presence of cork oak persistence
(maintenance or increase in tree cover)
Absence of cork oak persistence
(absence or decrease in tree cover)
Type of
transition
% from the total sampling units classified as
presence of oak persistence
Type of
transition
% from the total sampling units classified as
absence of oak persistence
Fo→Fo
48
Fo→Sr
33
Sa→Sa
13
Sa→Sr
17
Sa→Fo
13
Sa→Gr
8
Sr→Fo
9
Sr→Sr
17
Sr→Sa
4
Gr→Sr
17
Gr→Sa
13
Gr→Gr
8
(15.2% and 9.1% of total causes, respectively). We
also found that 35% of interviewees perceived that
soil erosion was not a problem at the study site. On
the other hand, 7.7% of farmers perceived soil
erosion as a cause of oak mortality (Table 3).
The landowners identified a long list of main causes
to explain cork oak mortality but by far the most
frequent one was disease (24.4% of all responses
corresponding to 95% of all interviewees). The
disease to which landowners are referring is the one
caused by the fungi Phytophthora cinnamomi,
which has been correlated with cork oak decline in
several recent studies (Moreira and Martins 2005).
Perhaps surprisingly, lack of traditional land use that
combines wheat cultivation and livestock, was
mentioned as the second most frequent cause of cork
oak mortality (15.4% of all responses corresponding
to 60% of interviewees). Landowners further
explained that wheat cultivation and livestock
provided fertilizers and manure to the soil, and the
lack of such management is responsible for cork oak
mortality today. Drought and management changes
from traditional manual labor to the use of disk
harrowing and heavy machinery are the two next
most frequent responses for cork oak mortality
(12.8% and 9%, respectively; Table 3).
Most interviewees (80%) considered that oak
natural regeneration increased since 1960 after
understory cultivation ceased, whereas 20% of the
interviewees stated that natural regeneration has
decreased since that date (Table 3). Most
landowners stated that they do not protect natural
regeneration during silvicultural interventions such
as shrub clearing, which can lead to the destruction
of seedlings. Landowners also claimed that the use
of inadequate techniques, such as bad terrain
preparation and insufficient supervision during the
first years, in planting were responsible for low
seedling survival rates. Furthermore, during the
course of the interviews, several landowners stated
that direct oak seeding should be used instead of
planting because it would allow higher seedling
survival, but EU programs have only been
subsidizing planting operations. Two of those
landowners included seeding with selected acorns
and subsidies for seeding when asked about “what
is missing for local forest management” (see Table
4). Interestingly, most interviewees (80%) reported
that Cistus shrublands have increased since 1960
and only one affirmed that shrubland area has
decreased because of subsidized shrub clearing in
the last decades. Three respondents stated that there
were no changes in shrubland area (Table 3).
Cork oak mortality, including disease, and wildfires
were pointed out as the two main difficulties for
local forest management by 35% and 30% of all
respondents, respectively (Table 4). Setting fire
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Table 3. Landowner’s individual perceptions (frequency of responses). Total number of responses per
question (in Q1, Q2, and Q3) is higher than number of interviewees (20) because each interviewee usually
provided more than one response to each question.
Question asked
Response
Frequency of
response
Number
%
Q1. What are the main
causes of soil erosion
at the study area?
(open-ended)
Lack of fertilizer and lack of cereal cultivation
Drought
Cereal cultivation in the past
Disk harrowing, use of rippers and heavy machinery
Heavy rains
Complete shrub clearing in steep terrains
Decreasing tree density
Wildfires
Total
3
5
5
7
6
2
1
4
33
9.1
15.2
15.2
21.2
18.2
6.1
3.0
12.1
100
Q2. What are the main
causes of cork oak
mortality at the study
area?(open-ended)
Disease
Drought
Wildfires
Complete shrub clearing
Lack of traditional land use with wheat cultivation and livestock (lack of
fertilizer and manure)
Pesticides
Disk harrowing and ploughing, shrub clearing with heavy machinery
Less soil water
Old age of cork oak trees
Soil erosion
Pollution
Clayey soils
Inadequate human management
Total
19
10
5
3
12
24.4
12.8
6.4
3.8
15.4
1
7
1
1
6
5
2
6
78
1.3
9.0
1.3
1.3
7.7
6.4
2.6
7.7
100
Arson
Lack of shrub clearing and fuel management
Drought
Lack of firemen capacity: bad coordination among firemen during combat;
firemen lack forest fire fighting techniques and have bad knowledge of the
terrain
Negligence
Accidents
Total
19
5
3
5
52.8
13.9
8.3
13.9
2
2
36
5.6
5.6
100
Q4. How did cork oak
natural regeneration
change since 1960?
(closed)
Increased
Decreased
Total
16
4
20
80.0
20
100
Q5. How did
shrubland area change
since 1960?
(closed)
Increased
Decreased
No changes
Total
16
1
3
20
80.0
5.0
15.0
100
Q3. What are the main
causes of wildfires at
the study area?
(open-ended)
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Table 4. Management difficulties and needs identified by landowners. Table shows direct quotations from
landowners.
Interview
Number
Main difficulties for local forest management
What is missing?
1
Properties are too small to invest
Government subsidies
2
Topography of the landscape
More shrub clearing to prevent wildfires
3
Wildfires, cork oak disease
(no answer)
4
Wildfires, lack of machines, high costs of shrub
clearing
Roads, machines, seeding with selected acorns, prescribed burning in
the winter
5
Wildfires and disease
Manpower
6
Cork oak mortality, lack of cadastre, small
property
Landowner cooperatives and concerted management, scientific
research; environmental instruments are too rigid concerning fuel
management
7
Cork oak mortality, long time for subsidies
payment; acorn production has been decreasing
More communication between landowners and researchers, livestock
to feed on the grass and shrubs (fuel management)
8
Wildfires
(no answer)
9
(no answer)
(no answer)
10
Lack of land profitability, land abandonment
(no answer)
11
Lack of land profitability, decreasing cork
production because of disease
Compensations for restrictions from environmental instruments such
as Natura 2000, cadastre
12
Bureaucracy (e.g., long time to get an
authorization to cut a dead cork oak)
There are no subsidies for seeding, (which is more successful than
plantations), landowners cooperatives, more decision making power
for Associations of Forest Producers
13
Cork oak mortality (still without a cure for the
disease), market competition with plastic stoppers
(no answer)
14
Lack of forest fire fighting techniques; too much
bureaucracy for investments with subsidies and
long time waiting for money
Subsidies for livestock (which is good for fuel management and
fertilizing the soil); improvement of inspection
15
Cork oak mortality, decreasing cork production,
money has to be invested before receiving
subsidies
Find a cure for the cork oak disease
16
Cork oak mortality
Manpower, people working the land and living there
17
Land abandonment, lack of profitability of small
landowners
Forest management, faster payment of subsidies and less bureaucracy
18
Small property and land fragmentation
Livestock for fuel management, landowners cooperatives, multiple-use
forest management
19
Wildfires
Less taxes for managers and businesses; incite production
20
Wildfires
Landowner cooperation
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intentionally was pointed out as the main reason for
wildfires (52.8% of total responses), followed by
lack of firemen capacity to fight forest fires and lack
of fuel management (both with 13.9% of total
responses; see Table 3). Bureaucracy, i.e., the long
wait for subsidies payment and for authorization to
cut a dead cork oak tree (20%), small property size,
and lack of land profitability (both with 15%) were
the next most frequent answers. Many other issues
were quoted as problems for local forest
management, including decreasing acorn and cork
production, land abandonment, lack of cadastre, and
market competition with plastic stoppers (Table 4).
Landowner cooperatives, subsidies for seeding,
livestock, and compensation for restrictions from
environmental programs, livestock for fuel
management, manpower, and communication
between researchers and landowners were quoted
as the most important needs for local forest
management (Table 4). Five landowners did not
respond when asked about what is missing.
Determinants of tree persistence: management
or biophysical variables?
Vegetation patch-type transitions indicating
presence of tree persistence were observed at 23
sampling units (65.7% of all units) mostly because
of persistence of forest and savannas (48% and 13%
from the total sampling units registered as presence
of oak persistence, respectively) and transitions
from savannas to forests (13% from the total
sampling units registered as presence of oak
persistence). Vegetation patch-type transitions
indicating absence of tree persistence were observed
at 12 sampling units (34.3% of all units) mostly
because of changes from forests, savannas, and
grasslands to shrublands (33%, 17%, and 17 % from
the total sampling units registered as absence of oak
persistence, respectively), and shrublands persistence
(17% from the total sampling units registered as
absence of oak persistence; see Table 2).
The best model for tree persistence, i.e., persistence
of forest and savannas, transitions from any patchtype to forests, and transitions from shrublands and
grasslands to savannas, during the period
1958-2002, included both biophysical, i.e., slope
and wildfire, and management variables, i.e.,
understory management and tree planting, and had
an Akaike weight of 0.76 (model 1 in Table 5).
According to Anderson (2008), models within ∆i up
to four have substantial support, thus the second
model (∆i = 2.304) is also considered an
alternatively good fit to the data. This second model
(Akaike weight of 0.24) is similar to the first one
but with the exclusion of slope. All remaining
models have much higher ∆i and residual Akaike
weights. The evidence for the first two models is
expressed by summing their Akaike weights,
showing that they represent an approximate 99%
confidence set, i.e., we have 99% confidence that
these two statistical models represent the best fit
among the set of models compared. In these two
models, wildfire occurrence, understory management
before 1975, and cork oak planting were the most
important variables. These three variables were
equally important, each with an Akaike weight of
0.99. Slope followed in importance (Akaike weight
= 0.76) whereas the remaining variables were not
relevant for the models (Akaike weights <0.01). The
two best models showed that cork oak persistence
was more likely in areas where understory
management, using livestock or cultivation, and
cork oak plantings were never carried out, in steeper
slopes and in the absence of wildfires (Table 5).
DISCUSSION
Cork oak persistence best explained by both
management and biophysical variables
The analysis of alternative statistical models shows
that cork oak persistence is better explained by
models including both biophysical and management
variables than by those based only on one set of
variables alone. Previous work at the study area had
identified two ecological variables, wildfire and
exposure, as the two most important biophysical
variables explaining patch-type transitions (Acácio
et al. 2009). Our results now clearly demonstrate
that vegetation changes are better explained by also
taking management variables into account. Cork
oaks are most likely to persist in areas where
understory management, using livestock or
cultivation, and cork oak plantings were never
carried out, especially on steeper slopes and in the
absence of wildfires. This indicates that
management is a significant driver of vegetation
change and a significant variable to explain patchtype transitions and tree persistence patterns in this
Mediterranean landscape.
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Table 5. Statistical models for tree persistence. For each model, the table shows the predictor variables
entering the model (remaining variables did not enter), respective direction of association (positive + or
negative -) with the response variable (presence of cork oak persistence), the AICc value, AICc differences
(∆i) and Akaike weights (wi), and model fit (full model χ²) and area under curve (AUC±s.e.). Models are
ordered by increasing ∆i, with the best model having ∆i=0 (model 1) and the worst model having ∆i=29.0
(model 12). The type of model is also coded according to the method used: (a) separate models for each
variable, (b) models for each set of biophysical and management variables, (c) stepwise models for each
set of biophysical and management variables, (d) stepwise models with the whole set of variables. Codes
for management variables: underst= understory management pre 1975; cuttree= tree cutting post 1975;
afforest= tree planting post 1985.
Variables
afforest
AICc
∆i
wi
χ²
AUC
-
-
22.618
0.000
0.759
34.45
0.98±0.019
-
-
24.922
2.304
0.240
29.41
0.95±0.035
3c
-
-
37.062
14.444
0.001
14.71
0.83±0.069
4b
-
-
38.583
15.965
0.000
15.75
0.85±0.066
39.933
17.315
0.000
11.84
0.80±0.084
40.265
17.647
0.000
9.11
0.69±0.087
41.612
18.994
0.000
7.76
0.73±0.097
43.947
21.329
0.000
5.43
0.70±0.096
44.481
21.863
0.000
12.59
0.81±0.080
45.902
23.284
0.000
3.47
0.66±0.100
48.278
25.660
0.000
1.01
0.56±0.100
51.661
29.043
0.000
0.12
0.52±0.103
Model
aspect
1d
slope
fire
underst
+
-
2d
5c
+
cuttree
-
-
6a
-
7a
-
8a
9b
- (S)†
10 a
+
+
11 a
12 a
†
-
- (S)†
less probability of tree persistence in southern exposures
The role of management
The fact that tree persistence was found to be more
likely in areas where understory management had
not occurred in the past can be explained in several
ways. In southern Portugal, decades of intensive
wheat cultivation and grazing pressure under oak
savannas and forests led to severe soil fertility loss,
still evident today (Natividade 1950, Oliveira 1955,
Gonçalves 1991, Pinto-Correia and Vos 2004).
Agroforesty practices in southern Spain had similar
consequences (Montero et al. 1994). Intensive
understory cultivation and grazing reduce perennial
vegetation cover, increasing water and soil nutrients
losses by water run-off and soil erosion that lead to
soil degradation (Lima et al. 2000). Additionally,
trampling by grazing herds causes soil compaction,
which reduces water infiltration and further
contributes to soil degradation (Pulido and Díaz
2002). Poor, degraded soils and higher insolation in
nonvegetated sites make conditions for seedling
establishment very difficult and thus halt tree
Ecology and Society 15(4): 40
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persistence and recruitment. These positive
feedbacks can maintain semiarid ecosystems in
persistent states of low biomass (Westoby et al.
1989, Rietkerk and van de Koppel 1997, Holmgren
and Scheffer 2001).
We also found that absence of tree persistence and
transitions toward shrublands were more likely
where cork oak planting had taken place, which
seems a contradictory result at a first glance.
However, cork oak plantations in the Algarve
province have registered very low survival rates,
especially on drier southern exposures (Lima et al.
2000). Inadequate planting techniques have been
pointed out as a major reason for poor seedling and
sapling recruitment, such as inadequate terrain
preparation and shrub management and poor
monitoring during the first years (Lima et al. 2000).
Also, the low genetic quality of the reproductive
material used in many cork oak reforestation
projects in Portugal has been identified as a
contributing cause of low reforestation success
(Almeida et al. 2006). In Spain, oak plantings also
suffer from high mortality rates because of summer
drought (Alloza and Vallejo 1999, Castro et al.
2002).
Furthermore, initial vegetation clearings for
reforestation projects not only destroy existing
seedlings but eliminate nurse plants that can
facilitate seedling recruitment (Castro et al. 2002).
As a result, unvegetated sites are quickly invaded
by Cistus shrubs that compete with the remaining
seedlings for soil water and nutrients, increasing oak
mortality and subsequently decreasing tree
persistence.
The role of biophysical variables
Cork oak persistence was most likely to occur in the
absence of wildfires. Frequent fires or intense ones
can kill adult cork oak trees especially when burning
occurs immediately after cork extraction (Cabezudo
et al. 1995, Pausas 1997, Moreira et al. 2007), which
takes place in the summer months, coincident with
the start of the wildfire season in Mediterranean
Europe. Because the abandonment of traditional
agroforestry led to shrub colonization and the
accumulation of dead wood, former cork oak
systems experienced an increase in wildfire hazard
(Pausas and Vallejo 1999, Piussi and Farrell 2000,
Pardini et al. 2004). Higher fire frequency increases
the loss of soil fertility by accelerating erosion
(Pardini et al. 2004). Moreover, because colonizing
Cistus shrubs are well adapted to very poor soils
(Gonçalves 1991) and rely on fire to spread their
seeds, this triggers the conditions for a positive
feedback where cork oak recruitment is difficult and
the system remains in an arrested early successional
stage dominated by pyrophytic Cistus shrubs.
The best statistical model also shows that cork oak
persistence was more likely to occur on steeper
slopes, although slope presents a lower Akaike
weight than understory management and wildfire.
This is probably because on these slopes, forests
were less vulnerable to human interventions,
especially ploughing, which is practically
impossible on very steep slopes.
Landowner’s perceptions behind management
decisions
From an ecological perspective, the landowners’
perceptions on the ecological conditions of these
systems and the mechanisms that explain them
appear perhaps surprising: 15% of the landowners
believe that the absence of traditional land use
combining wheat cultivation and livestock is one of
the main causes of oak mortality; 9% believe that
the lack of wheat cultivation is in fact responsible
for soil erosion; and 35% of them believe that soil
erosion is not a problem.
Landowners are likely to have a landscape
preference for agroforestry systems because this is
the type of traditional landscape they perceive as
appropriate and they may find it difficult to accept
its transformation (Gómez-Limón and de Lucío
Fernández 1999). Therefore, they deny the
existence of a problem, such as soil erosion. This
ideological barrier (Scheffer et al. 2000) can prevent
landowners from choosing adequate management
practices, such as adequate techniques for shrub
clearing or protection of natural regeneration, which
could certainly contribute to increase cork oak
persistence in the study area.
Landowner behavior within the region is also
conditioned by the perception that their holding is
too small and that further improvement through
investment would not pay dividends. On the other
hand, some types of land operations are subsidized
by the EU, which facilitates management actions
that may not be appropriate. For instance, although
landowners believe that seeding is the best method
Ecology and Society 15(4): 40
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for cork oak artificial reforestation, they chose to
use planting operations instead because these are
economically subsidized. In fact, subsidies are
included on the list of needs for local forest
management. Subsidies play an important role in
management decisions and can contribute to further
degradation of tree cover in some cases (PintoCorreia and Vos 2004). In our statistical analysis,
planting was a significant variable contributing to
absence of tree persistence and was also identified
by many landowners as a problem.
CONCLUSION
Our results show that the dynamics of
Mediterranean landscapes are better understood as
coupled social-ecological systems requiring an
integrated approach that combines interdisciplinary
tools from ecological and social sciences. We found
that land management, i.e., intensive understory use
and inadequate planting operations, in combination
with biophysical factors, i.e., wildfire and slope,
facilitated shrub encroachment and prevented tree
persistence or recovery between 1958 and 2002. It
is likely that oak savannas and forests lost resilience,
as a result of past land uses damaging soil and
vegetation, and are currently additionally
threatened by more frequent wildfires. As a result,
natural succession appears halted into a pioneer
state dominated by Cistus shrublands where cork
oak forest recovery can become very difficult even
through assisted reforestation. Our study highlights
a series of ecological, ideological, and economical
barriers that contribute to this process. If the cork
oak systems are to be maintained, forest
regeneration should be stimulated by addressing
those barriers in a concerted way.
Responses to this article can be read online at:
http://www.ecologyandsociety.org/vol15/iss4/art40/
responses/
Acknowledgments:
This research was funded by the Portuguese
Foundation for Science and Technology (Fundação
para a Ciência e a Tecnologia), fellowship number
SFRH/BD/5008/2001 to V. Acácio. M. Holmgren
thanks the Dutch NWO Meervoud Programme
(836.05.021). We thank the Associação dos
Produtores Florestais da Serra do Caldeirão
(APFSC) for kindly providing the information on
land property. We also thank: the Portuguese
Military Geographic Institute, the Portuguese
Geographic Institute, and the National Pulp
Industry Association for kindly providing the
photographic material; Rute Palmeiro, Susana
Pereira, Tiago Dias, and Miguel Porto who helped
with photo interpretation work; João Carreiras and
Filipa Marques who assisted with ENVI and
ArcView software. Information on burned areas
between 1984 and 1989 was kindly provided by
Maria José Vasconcelos (project "Spatio-temporal
fire patterns in Portugal 1984-2003. Impacts, risk
assessment, and fuel dynamics", POCTI/
AGG/44942/2002, Tropical Research Institute,
IICT, Lisbon, Portugal). Information on burned
areas between 1990 and 2002 was kindly provided
by the General Directorate of Forests (DGRF Direcção Geral das Florestas). Special thanks to
Frances Westley for her suggestions on an earlier
version of this manuscript.
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