The cork oak in the Mountains of Palermo (Italy): ecological insights from the south-eastern edge of its distribution range - Sisef
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iForest Research Article
doi: 10.3832/ifor3360-013
vol. 13, pp. 336-344
Biogeosciences and Forestry
The cork oak in the Mountains of Palermo (Italy): ecological insights
from the south-eastern edge of its distribution range
Emilio Badalamenti (1), The uneven presence of the cork oak (Quercus suber L.) within its distribution
range is not only determined by its climatic requirements but also by specific
Riccardo Scalenghe (1), edaphic needs. Although most of the natural populations thrive in acidic soils
Tommaso La Mantia (1), deriving from metamorphic or volcanic rock outcrops, some cork oak popula-
Rafael da Silveira Bueno (1), tions are found growing in soils deriving from calcareous bedrock, which are
Giovanna Sala (1), considered less suitable. We carried out a multidisciplinary investigation at the
south eastern edge of the Q. suber distribution range (Mountains of Palermo,
Giuseppe Maria Pizzurro (1), NW Sicily), including soil, floristic, and vegetation surveys, aimed at: (i) assess-
Andrea Giaimo (1), ing the native or introduced origin of some peculiar cork oak populations; (ii)
Salvatore Pasta (2) describing the associated plant communities and soils; (iii) identifying the eco-
logical factors which could explain the local adaptation to soils deriving from
calcareous bedrock; (iv) discussing the ecological role played by this species in
the study area and within its distribution range. The collected information sug-
gest that the detected cork oak stands are native to these areas. We hypothe-
sized that the co-occurrence of high fire frequency, high soil calcium content
and erosion, which caused the intermixing of different parent materials, might
favor its competitive interactions with other Mediterranean tree species, thus
accounting for the local presence of Q. suber. The study of cork oak popula-
tions thriving in peculiar substrates at the driest end of the range could be of
great importance for the future conservation of this species, which is ex-
pected to face growing threats in the coming decades.
Keywords: Mediterranean Evergreen Forest, Soil Chemistry, Vegetation Sci-
ence, Wildfire, Landscape, Tree Species, Quercus suber
Introduction The cork oak shows a discontinuous geo- Soil physical-chemical characteristics are
The cork oak (Quercus suber L.) is one of graphical distribution, much more restrict- regarded as being among the ecological
the most important tree species in the ed and fragmented than might be ex- factors most correlated to the occurrence
Mediterranean Basin, where its presence pected based on its climatic requirements. of the cork oak in its natural range (Pet-
is of high ecological, landscape and socio- However, compared to other evergreen roselli et al. 2013). Q. suber generally pre-
economic importance (Pollastrini et al. oaks, Q. suber is characterized by a more fers sandy soils derived from siliceous
2018, EUFORGEN 2019). Cork oak forests restricted ecological niche, as particularly rocks with acid reaction, while usually
cover altogether more than 2 million ha demanding in terms of soil humidity (Pet- avoids soils from base-rich, calcareous bed-
(Matías et al. 2019), almost 1.5 million of roselli et al. 2013). The species grows in rocks such as dolomitic limestones, calcare-
which in Europe and 700,000 ha in North warm-humid Mediterranean areas with ous sandstones, and marls (Serrasolses et
Africa (Houston Durrant et al. 2016). Its dis- oceanic features, receiving at least 450 mm al. 2009). Indeed, the importance of vari-
tribution area encompasses the western of mean annual rainfall, from sea level up ous substrate-related variables, particularly
portion of the Mediterranean Basin and to 2000 m a.s.l., although most cork oak lithology and soil type (Costa et al. 2008),
the southern portion of the European At- stands are found below 800 m a.s.l. (Hous- for the distribution of cork oak has been re-
lantic coasts (Houston Durrant et al. 2016). ton Durrant et al. 2016). peatedly acknowledged. The preference
for well drained soils with coarse textures
is known (Laakili et al. 2016). Conversely,
(1) Department of Agricultural, Food and Forest Sciences, University of Palermo. v.le delle reduced organic matter availability or soil
Scienze Ed. 4, I-90128 Palermo (Italy); (2) Institute of BioSciences and BioResources (IBBR), acidification have negative effects on cork
Italian National Council of Research (CNR), Unit of Palermo, c.so Calatafimi 434, I-90129, Pa- oak regeneration in the longer term (Dias
lermo (Italy) et al. 2016).
There is increasing evidence of a declining
@ Tommaso La Mantia (tommaso.lamantia@unipa.it) trend in cork oak populations, especially at
the edge of the species’ natural distribu-
Received: Jan 27, 2020 - Accepted: Jun 06, 2020 tion range (Vessella et al. 2017, Matías et al.
2019). Many factors have been invoked to
Citation: Badalamenti E, Scalenghe R, La Mantia T, Bueno RS, Sala G, Pizzurro GM, Giaimo A, explain the ongoing process, which is
Pasta S (2020). The cork oak in the Mountains of Palermo (Italy): ecological insights from the bound to increase in the forthcoming de-
south-eastern edge of its distribution range. iForest 13: 336-344. – doi: 10.3832/ifor3360-013 cades based on current climate change sce-
[online 2020-08-07] narios (Matías et al. 2019), and due to
other growing threats, such as overgraz-
Communicated by: Francisco Lloret Maya ing, soil pathogens and land-use changes
(Dettori et al. 2018). In the Mediterranean
© SISEF https://iforest.sisef.org/ 336 iForest 13: 336-344
Badalamenti E et al. - iForest 13: 336-344
region, climatic forecasting seem to indi- cies’ range, they are particularly prone to Materials and methods
iForest – Biogeosciences and Forestry
cate a rise in temperatures and a reduction the effects of climate change, and could
in precipitation, which will probably de- suffer from a marked reduction and local Identification and characterization of
crease the potentially suitable areas for extinction in the next future (Vessella et al. the study areas
cork oak, especially in the southern edge 2017). For this reason, they are of particular In order to detect any cork oak popula-
and at lower altitudes; accordingly, north- interest for the future dynamics of cork tion, either as forest stand, small nucleus
ward and upward shifts could occur (Ves- oak in the whole Mediterranean Basin. In or isolated individual, the main botanical lit-
sella et al. 2017). Indeed, the expected re- specific reference to the cork oak, accord- erature concerning the Mountains of Pa-
duction in soil water availability could favor ing to Aronson et al. (2009): “Knowledge lermo (the mounts surrounding Palermo
other co-occurring and more drought-toler- of species requirements and constraints city) was consulted (Marcenò & Trapani
ant woody species, such as holm oak can be used to promote conservation and 1978). The historical information gathered
(Quercus ilex L. – Petroselli et al. 2013). restoration efforts or to expand its current was used to identify potential areas cov-
Aridity has been found to play a key role in distribution limits in view of the current cli- ered by Q. suber. After being validated
Q. suber regeneration ability, markedly hin- mate and land use changes”. through dedicated field surveys, the de-
dering seedling recruitment chance and In this study, we aimed to gain ecological tected areas were characterized by adopt-
originating over time an aged demographic insights from the cork oak stands occurring ing the hierarchical classification devel-
structure (Matías et al. 2019). in NW Sicily in soils deriving from calcare- oped by Blasi et al. (2000); this method en-
In Italy, the most extensive cork oak ous bedrock, which are generally consid- tailed overlapping the pedological map
forests occur in Sardinia (82.7% of total na- ered to be less suitable for the species. (scale 1:250.000 – Fierotti 1988) with the
tional area), followed by Sicily, Tuscany and In particular, we characterized soil and geological map (scale 1:250.000 – Lentini &
Calabria, while more discontinuous and plant communities associated to cork oak Carbone 2014), the bioclimatic map (scale
smaller nuclei are found in Campania, Lazio stands, occurring at the southeastern edge 1:250.000 – Bazan et al. 2015), and the map
and Apulia (INFC 2007). In Sicily, Quercus of its natural distribution range. The main of Sicilian potential vegetation (scale
suber is a native species which covers ap- objectives of the study were to: (i) identify 1:250.000 – Anonymous 1996; see Tab. S1 in
prox. 19,000 ha, from sea level up to 1000 and quantify the area covered by cork oak Supplementary material). Three macro-ar-
m a.s.l. It accounts for approx. 7% of total populations in the Mountains of Palermo eas, geographically distinct from one other
regional forest cover (Camerano et al. (NW Sicily); (ii) investigate the effective ori- and fairly homogeneous in terms of sub-
2011), in some cases characterizing old gin of these populations (whether natural strate (geology and soil types), were identi-
growth stands (Badalamenti et al. 2018). or artificial) based on historical and floristic fied (Tab. 1): (i) macro-area A: “Pianetto”
Only 41.3% of the Sicilian cork oak stands information; (iii) identify the ecological fac- (municipalities of Altofonte and Santa Cris-
are currently managed for production and tors responsible for the presence of the tina Gela); (ii) macro-area B: “Cozzo Secco”
the progressive abandonment of cork culti- species in soils that developed on calcare- (municipality of Borgetto); (iii) macro-area
vation is considered to be a threat to the ous substrates; (iv) propose a synthetic in- C: “Colline della media valle del fiume Ore-
conservation of these ecosystems on a re- terpretation of the ecological role played to e del Monrealese” (municipalities of Al-
gional scale. Furthermore, since these pop- by the cork oak within local potential vege- tofonte and Monreale).
ulations grow at the driest end of the spe- tation. The occurrence of the cork oak had al-
Tab. 1 - Main characteristics of the study areas. Sources: (1): Fierotti (1988); (2): Lentini & Carbone (2014); (3): Bazan et al. (2015).
Macro-area
Characteristics
A B C
Locality Pianetto (Cozzo Giammeri and Cozzo Secco Colline della media Valle del fiume
Costa Zolia) Oreto e del Monrealese
Stand description Small, isolated and discontinuous, Scattered and discontinuous Isolated plants and small nuclei,
grazed, pure stand or mixed with stands, grazed, mixed with discontinuous, pure stands or
holm and downy oak evergreen thermophilous mixed with evergreen
sclerophyllous species or with thermophilous species
downy oak
Municipality Altofonte, Santa Cristina Gela Borgetto Monreale, Altofonte, Marineo,
Montelepre, Giardinello
Regional Technical Map 595130 and 608010 594140 594120 and 594160
1:10.000 (Section no.)
Maximum range of E 2373518-2374629 E 2356966-2357797 E 2370475-2371368
UTM Coordinates N 4206720-4208280 N 4209665-4210804 N 4213457-4217808
Area (ha) 251 50 240
Mean altitude (m a.s.l.) 720 560 480
Altitudinal range (m a.s.l.) 597-835 432-659 240-680
Slope (%) 20 20 30
Main aspect NO-SE SW None
Soil type1 Leptosols, Regosols, Phaeozem Leptosols (Association 6) Leptosols, Regosols, Phaeozem,
(Association 7) Cambisols (Associations 21 and 29)
Lithotype2 Calcilutites and marls, Silicoclastic deposits, yellowish Radiolarites and Spongolites with
calcarenites grading up to graded quartzarenites and brown redeposited calcarenites and
doloarenites and dolorudites clays (Geology 8 and 30) calcirudites, and red marls
(Geology 42) (Geology 41)
Bioclimate3 thermo-Mediterranean upper sub- thermo-Mediterranean lower sub- thermo-Mediterranean upper sub-
humid humid humid
337 iForest 13: 336-344
Ecological insights from the south-eastern edge of Quercus suber range
ready been reported in macro-area A (at means of a Trimble® GPS (Trimble Inc., Sun- Floristic and vegetation surveys
iForest – Biogeosciences and Forestry
Dingoli site) by Marcenò & Trapani (1978), nyvale, CA, USA). Data referring to the Floristic and phytosociological surveys
while cork oak forests have been present UTM plane coordinate system, timed to were carried out during spring 2004. Analy-
in macro-area B at least since XIII century, share information, latitude and longitude, sis of cover-value ranges available from
when the territory was owned by the were exported as shapefiles to be treated vegetation surveys on evergreen forests in
Abbey of “Santa Maria di Altofonte” (Bresc with QGIS® ver. 3.10 (QGIS Development Sicily (see Brullo et al. 2009 and literature
1983). Conversely, no previous historical Team) and were overlapped with sections therein) suggested the use of 20% as a valu-
data was available on the occurrence of Q. no. 594140, 595130, 594120, 594160 and able threshold for the cover of Q. suber.
suber in macro-area C. 608010 of the Regional Technical Map Full vegetation surveys, within 100 m 2-wide
Subsequently, we searched for local data (scale 1:10.000). The shape relative to punc- plots, were carried out only within the sub-
on wildfires, as they are an important fac- tual distribution files of cork oak was then populations where cork oak cover exceed-
tor explaining the natural distribution of overlapped with the soil, geological and ed 20%. Four plots were established in
the cork oak in the Mediterranean, as well bioclimatic maps. macro-area A, and seven plots were estab-
as in our study areas. Information collected lished in each of the macro-areas B and C,
by the Special Office of the Firefighting Soil surveys which included the most interesting cork
Forest Fire Service (USSAB 2005), for the The choice of the most suitable areas for oak stands. For each plant species, the cov-
period preceding our field surveys, and by the pedological surveys was based on two er/dominance scale proposed by Braun-
the Command of the Forest Service of steps: after superimposing the cork oak Blanquet (1932) was considered, with mod-
Sicily for more recent data (2007-2019 – distribution map with the geological map, ifications subsequently proposed by Pig-
https://sifweb.regione.sicilia.it/arcgis/rest/s direct field observations of the soil fea- natti & Mengarda (1962). The Shannon di-
ervices/Censimento_Incendi/MapServer), tures and plant species assemblages (aci- versity index (H’ – Shannon 1948) and
was considered. The oldest data are sorted dophilous vs. basiphilous) were carried out. Evenness index (Pielou J) were calculated
by municipality and they provide informa- The benchmark pedons (=soil profiles) rep- after converting Braun-Blanquet values ac-
tion on the occurrence of wildfires in Sicily resentative of the main soil types (Tab. 1) cording to Tüxen & Ellenberg (1937). Classi-
in the years 1986-2002 (USSAB 2005). In de- were described in the field after direct soil fication of the vascular plants followed Pig-
tail, in the framework of the Regional plan excavation and according to the genetic natti et al. (2017) and hierarchical classifica-
for the protection of vegetation from fires, horizons defined by Schoeneberger et al. tion of the plant communities followed Gu-
a regional map of fire risk was developed, (2002). Overall, 5 pedons, sampled in bulk arino et al. (2017). Each plant species sur-
not only considering the factors which en- and undisturbed soil horizons, were de- veyed was included in one of the following
hance the likelihood of wildfire (e.g., vege- scribed: four pedons belonging to macro- phytosociological alliances: (i) Erico-Quer-
tation types or climate traits), but also tak- area A, and one pedon belonging to macro- cion ilicis, including pure evergreen or mix-
ing into account the frequency of wildfires area B. No pedon was considered in macro- ed ever- and summer-green oak forest spe-
occurring in a given area, as well as the av- area C because it was not possible to ac- cies linked to neutrocline-acid soils (EQi);
erage affected area and the average af- cess the privately owned areas. Pedon 1 (ii) Quercion ilicis, including pure evergreen
fected wooded area, thus including a quan- (Code Ag1) and pedon 2 (Code Ag2) were or mixed ever- and summer-green oak for-
titative assessment. In particular, this map located at Cozzo Giammeri (38° 00′ 23.80″ est species (Qi); Oleo-Ceratonion siliquae,
was elaborated through the overlapping of N, 13° 20′ 34.41″ E), pedon 3 (Code Az3) and including maquis species (OCs); (iii) Pruno-
different base maps, including a statistical pedon 4 (Code Az4) were located at Costa Rubion ulmifolii, including mantle species
risk map, a vegetation risk map, a climate Zolia (37° 59′ 50.75″ N, 13° 20′ 49.64″ E), (PRu); (iv) Cisto-Ericion multiflorae, includ-
risk map, and a morphological risk map. and pedon 5 (Code B) was located at Cozzo ing garrigue species (CEm); (v) Hyparrhe-
For a climatic and bioclimatic characteri- Secco (38° 01′ 31.15″ N, 13° 09′ 07.21″ E). For nion hirtae, including xerophilous grassland
zation of the study areas, the thermoplu- each pedon, three soil samples were col- species (Hh); (vi) Other species belonging
viometric stations of Partinico (189 m a.s.l.) lected at several soil depths. Bulk soil sam- to other syntaxa.
and Monreale (310 m a.s.l. – Duro et al. ples were air dried, crushed and 2-mm Where only a few scattered individuals of
1993, SIAS, http://www.sias.regione.sicilia. sieved. Particle size distribution was deter- cork oak occurred in open and discontinu-
it/) were considered the most suitable mined by sieving and sedimentation (ISO ous plant communities (cork oak cover
(Tab. 2). Taking into account their location, 11277:2009): total carbonates volumetrical-Badalamenti E et al. - iForest 13: 336-344
of species characteristic of Erico-Quercion of the model (Levene’s test: p = 0.69, 0.50 more or less dense cork oak forests, cover
iForest – Biogeosciences and Forestry
ilicis could support the past occurrence of and 0.12, respectively). All statistical analy- approximately 540 ha. Concerning fire oc-
cork oak forests. The authors of all scien- sis was performed in R v. 3.6.1 (R Core currence, all investigated areas fall within
tific epithets concerning plants mentioned Team 2019) and spatial autocorrelation was areas classified as high or very high fire
in the text are provided in Tab. S2 (Supple- assessed with GEODA 1.8.16 (Anselin et al. risk, according to the regional classification
mentary material). 2006). scheme (USSAB 2005). This means that
these areas not only have a high likelihood
Statistical analysis Results of suffering from wildfire but they have
To test the effects of the environmental been effectively burned with high fre-
variables, namely altitude, main aspect Distribution and characterization of the quency at least at the end of the last cen-
(north or south) and lithology (calcareous study areas tury (USSAB 2005). Such a trend has been
or flysch) on richness, diversity (Shannon The distribution map of the areas where confirmed by recent data (2007-2019),
H’) and evenness (Pielou J), we used a gen- cork oak occurs in the Mountains of Pa- showing a high mean annual frequency of
eral linear model (GLM). Before running lermo is shown in Fig. 1. Macroarea A is wildfire passage, ranging from one wildfire
the model, we first checked that the three mainly characterized by calcilutites, marls, every two years elsewhere in macro-area B
dependent variables followed a normal dis- and calcarenites grading up to doloarenites to one wildfire per year elsewhere in mac-
tribution (Shapiro-Wilk test: p = 0.55, 0.23 and dolorudites, and Leptosols, Regosols, ro-area C. Hence, although the total area is
and 0.85, respectively). Spatial autocorrela- and Phaeozem (Fierotti 1988, Lentini & Car- considerably large, the intense human
tion across the sampling plots using Mo- bone 2014). Macroarea B is mainly charac- pressure may represent a serious threat for
ran’s I index, calculated from three differ- terized by Silicoclastic deposits, Numidian the conservation of these forest ecosys-
ent weight matrices (i.e., distance to the Flysch, and mainly Leptosols (Fierotti 1988, tems.
nearest neighbor) was also checked (Dor- Lentini & Carbone 2014). Macroarea C is
mann et al. 2007). We set the distance mainly characterized by Radiolarites and Soil surveys
range at 2300 m, which was the minimum Spongolites with redeposited calcarenites Most of the investigated soil profiles
distance to allow all plots to have at least and calcirudites, red marls, and Leptosols, showed marked limitations in root growth,
one neighbor. We then set the range at Regosols, Phaeozem, and Cambisols (Fie- mostly belonging to Leptosols. The five pe-
4300 m to connect all plots inside the same rotti 1988, Lentini & Carbone 2014). dons showed a taproot plus shallow hori-
macro-area, and finally we used a “queen” Macro-area A has a surface of about 251 zontal roots. The main portion of root
matrix, searching for the first closest ha and is localized in the south-east sector biomass is located in the root crown just
neighbor without a maximum distance of the map. Macro-area B is around 50 ha below the trunk base at a soil depth < 0.3
threshold, thus connecting at least one wide and is localized in the south-west sec- m. As we sampled by horizontal profiles,
neighbor from all macro areas. To test for tor of the map. Macro-area C is approx. 240 the horizontal extension of the whole root
differences in richness, diversity, and even- ha wide, encompassing much more frag- system, including roots thinner than 1 cm,
ness between the three macro-areas, we mented and isolated stands, localized in is unknown. In macro-area A, pedon 1 and
used one-way ANOVA, after verifying the the northern sectors of the map. Overall, pedon 2 (Ag1 and Ag2) belonged to Lep-
homogeneity of variances in the residuals the areas under investigation, covered by tosols / Regosols (sporadically Phaeozems)
soil associations, while pedon 3 and pedon
4 (Az3 and Az4) belonged to Leptosols.
These soils evolved from lithological sub-
strates made of siliceous shales, radiolar-
ites and marls, with interbedded lenticular
resedimented breccias. Graded and lami-
nated calcareous sandstone with elements
from Middle-Upper Triassic Cretaceous
platforms occurred in pedon 3 and pedon
4. Cozzo Giammeri profiles (Ag1 and Ag2)
evolved on slopes with a gradient of 15 and
25%, respectively, while Costa Zolia profiles
(Az3 and Az4) evolved on slopes with a
gradient of 10%. The pedon differed in
terms of stoniness and rockiness. The main
land use was overgrown pastureland. In
macro-area B, pedon 5 belonged to Lep-
tosols, which evolved on lithological sub-
strates made of graded doloarenites and
dolorudites, and re-sedimented dolomite
breccias with megabreccias from Triassic
carbonate platforms. This profile was open
on one side in a slope with a gradient of
35%, stoniness of 25%, rockiness of 20% and
moderately rapid drainage. A morphologi-
cal description of all pedons is given in Tab.
S3 (Supplementary material). Average soil
pH ranged between 6 and 7 in all study
sites. Variability of soil pH increased almost
linearly with a soil depth of up to 80 cm
(ranging from 5.0 to 7.8); conversely, it de-
Fig. 1 - The cork oak distribution map in the Mountains of Palermo. The different let- creased progressively at higher soil depths
ters indicate the three macro-areas (A, B and C). Up on the left, the natural distribu - (Fig. 2). Overall, the large variability in soil
tion range of Quercus suber is reported (EUFORGEN 2019). pH (from 4.6 to 7.8) showed good plastic-
ity of cork oak to this edaphic factor. Na/Ca
339 iForest 13: 336-344Ecological insights from the south-eastern edge of Quercus suber range
molar ratios were constantly higher than 1,
iForest – Biogeosciences and Forestry
Fig. 2 - Soil pH vari-
Mg/Ca ratio exceeded 0.6, while Na/K ratio
ation with soil
exceeded 20 (Fig. 3).
depth (average of
pedons 1-5). The
Floristic and vegetation surveys
dashed line repre-
Despite evenness being autocorrelated in
sents the median
the two close range matrices, source vari-
of pH values while
able richness as well as diversity were not
whiskers indicate
autocorrelated in all matrices (Moran’s I p
the range. The
> 0.05 in all cases – Tab. S4), suggesting
shaded area repre-
high local scale variability. GLM indicated
sents the range of
that richness, diversity and evenness are
soil pH in Quercus
not predicted by altitude, aspect and lithol-
suber stands found
ogy (p > 0.05 in all cases – Tab. 3), and they
in literature.
did not differ between the macro areas
(richness: F = 0.24, p = 0.78; diversity: F =
0.74, p = 0.49; evenness: F = 1.01, p = 0.38).
However, significant differences (F = 7.92,
p < 0.01) were found in the richness of Fig. 3 - Ionic
species belonging to the alliance domi- species within the
nated by Q. suber (EQi), which was signifi- saturated extracts
cantly higher in macro-area B than in C, al- from soils. Aver-
though not differing from A (after Tukey age concentra-
HSD test). High degradation and fragmen- tions in nM (x-axis)
tation of forest habitats in the surveyed ar- and depth (y-axis)
eas can be inferred from the number of in cm.
plant taxa not framed within any alliance
(see “Other Species” in Fig. 4 and Tab. S2
in Supplementary material), as well as from
those belonging to the alliance Hh. In all
macro-areas, plant taxa belonging to
“other species” represented the dominant
component of plant communities, ranging
from 38.6% in macro-area B, 39.8% in C and
up to 42.5% in A (Fig. 4).
Macro-area A is characterised by a com-
plex and heterogeneous landscape, due to
extreme variability in local topography (as-
pect and slope), continuous shifts from
carbonate (compact calcarenites and calci-
lutites) to acid (quartz sandstones) out-
cropping rocks, as well as to major distur- tied to open patches of perennial grassland cion ilicis, due to the high number of spe-
bance factors (mostly cattle grazing and dominated by Ampelodesmos mauritanicus cies referred to this alliance, in addition to
fires). Here, cork oak stands, pure or mixed or of acidophilous garrigue dominated by acidophilous species such as Lavandula
with a number of evergreen or deciduous Cistus spp. and Lavandula stoechas, while stoechas, Limodorum abortivum, Pteridium
trees, are intermingled with more open the current intensity of cattle grazing is un- aquilinum, and Tuberaria guttata. We may
thermophilous maquis on S-facing ridges derlined by high numbers of mantle and hypothesize that local forest communities
and summits. Quercus suber does not seem grassland species. Only near rocky out- belonged in the past to the Erico arboreae-
to play a dominant role in the absence of crops, especially along ridges and on hill- Quercetum virgilianae, rather widespread in
human disturbance. These forest commu- tops, a higher frequency of Mediterranean the Tyrrhenian sector of Sicily (Brullo et al.
nities show excellent potential for recovery maquis species was recorded. In small val- 2009).
and future development, although the pos- leys, where soils are deeper and the effect Macro-area C is subdivided into four sub-
sibility of establishing the potential vegeta- of wind is slightly mitigated, some scat- areas geographically disjoint from each
tion (Quercetalia pubescenti-petraeae) with tered individuals of downy oak (Quercus other (Fig. 1). The most interesting cork
downy oak as the dominant tree species pubescens Willd. s.l.) co-occur with cork oak stands occur in the southeast sub-area,
(Anonymous 1996) is currently low. In fact, oak, while the shrubby understory is domi- where the semi-natural landscape appears
the current floristic composition of local nated by Cytisus villosus. This plant commu- rather uniform: a series of ridges whose S-
communities is rich in plants linked to nity may be framed within the Erico-Quer- facing slopes usually host thermophilous
rocky habitats and overgrazed pasture-
lands, showing clear signs of soil degrada-
tion and thinning processes. Some cooler Tab. 3 - Summary of the results of the GLM testing the effects of the three environ -
and shady microhabitats are suitable for mental variables on richness and diversity.
Cephalanthera longifolia, Quercus congesta
and Iris foetidissima, which are rare species Richness (R² = 0.22) Diversity (R² = 0.13) Evenness (R² = 0.05)
Variable
on the Mountains of Palermo (Pasta & Estimate t p Estimate t p Estimate t p
Troìa 1994).
In macro-area B, the cork oak stand is un- Intercept 35.506 3.1Badalamenti E et al. - iForest 13: 336-344
downy oak (Blasi et al. 1997). In our study
iForest – Biogeosciences and Forestry
areas, three main factors, probably acting
in synergy, could account for the scattered
but persistent occurrence of Q. suber: (i) lo-
cal fire disturbance; (ii) the peculiar charac-
teristics of local soils; and (iii) the effect of
erosion processes on the local geological
framework.
High fire frequency, as observed in all
macro-areas, could have hindered evolu-
tion towards more mature forest ecosys-
tems, favouring the more fire-resistant
cork oak, once a minor component of pris-
tine mixed oak forests (Barberis et al.
2003). Indeed, Q. suber is a fire-resistant
tree species with a thick bark acting as an
insulating material and protecting the inner
woody structures. After fire passage, the
cork oak is also able to resprout both from
Fig. 4 - The relative contribution (in %) of each surveyed phytosociological alliance stem and basal buds (Pausas 1997), show-
within each macro-area (A, B and C). (EQi): Erico-Quercion ilicis; (Qi): Quercion ilicis; ing a notably high survival rate (Sirca et al.
(OCs): Oleo-Ceratonion siliquae; (PRu): Pruno-Rubion ulmifolii; (CEm): Cisto-Ericion mul- 2015). The marked effect of human distur-
tiflorae; (Hh): Hyparrhenion hirtae; (Other): species belonging to other syntaxa. bance on local plant communities can also
be derived by the fact that none of the abi-
taxa and assemblages, while N-facing same is true for our study sites, as the cal- otic factors assessed (altitude, aspect and
slopes host some remnant nuclei of Medi- careous nature of the substrates is differ- lithology) proved to be a predictor of the
terranean oak forests. It is therefore quite ent from the typical siliceous rocks with observed richness, diversity and evenness
difficult to assign local plant assemblages acid reaction, where most of the cork oak of the cork oak stands.
to a plant association. On more acidic sub- stands occur. Regarding pedological traits, we found
strates, like quartz sandstones and argil- Our study sites fall within areas consid- large plasticity in Q. suber to substrate con-
lites, local forest communities probably be- ered to be of particular interest for the fu- ditions, including the pH of the rhizosphere
longed to the Erico-Quercion ilicis alliance, ture dynamics of cork oak, as they could and the geological substrate. Average soil
or alternatively to the Erico arboreae-Quer- face growing risk of local extinction, or a pH ranged between 6 and 7 in all study
cetum virgilianae plant association. Else- significant reduction in the population size sites, which is consistent with values re-
where, the high frequency of woody spe- (Vessella et al. 2017) due to many interact- ported in the literature (Cucchi 1963). Our
cies typical of Oleo-Ceratonion, which rep- ing detrimental factors (climate change, results also match the preference of Q.
resents the potential vegetation of this fire frequency, etc.) driven by man (Matías suber for soils with acid or sub-acid reac-
area (Anonymous 1996), supports the hy- et al. 2019). Our field observations and tion, together with a tendency to avoid
pothesis that cork oak locally belonged to wildfire data clearly proved that the sur- limestone soils (Duque-Lazo et al. 2018).
the Oleo sylvestris-Quercetum virgilianae. veyed cork oak stands are highly exposed However, the total calcium content in soils
Most of the cork oak stands in this macro- to these risks. and its active portion should be distin-
area are very scattered and open, forming The occurrence of species typical of the guished. The cork oak, like the sweet chest-
a patchwork with xeric perennial grass- Erico-Quercion ilicis (EQi) may represent an nut (Castanea sativa Mill.) and the common
lands dominated by Ampelodesmos mauri- effective proxy for assessing the native sta- bracken (Pteridium aquilinum (L.) Kuhn),
tanicus or with mantle communities phys- tus of local Q. suber populations. A high may tolerate a rather high content of solu-
iognomized by Crataegus monogyna, Cyti- frequency of base-avoiding plant species ble limestone (Ozenda 1982). Interestingly,
sus infestus, Osyris alba, Daphne gnidium, typical of the EQi was recorded in the un- recent research has found natural cork oak
Spartium junceum, Pistacia terebinthus and derstory of the surveyed cork oak stands. populations growing in soils with an active
P. lentiscus. Intense and widespread soil Since many of these species show high fi- lime content below 26% and soil pH close to
erosion processes, and frequent wildfires delity to cork oak stands throughout the 8 (Duque-Lazo et al. 2018). Q. suber rarely
caused extreme soil and habitat degrada- whole island (Brullo et al. 2009), all the develops in podzols and regosols (Simões
tion. The high degree of soil thinning may macro-areas have very good potentiality et al. 2016), whereas more frequently is
also be inferred by the high frequency of for Q. suber. As historical data (Bresc 1983, found in cambisols (Aponte et al. 2013)
species adapted to cliffs, ledges and Gussone 1842, Lojacono-Pojero 1888, Parla- with a moderately thick humic horizon and
screes, as observed in the small stand of tore 1848) testify the local occurrence of subacidic soil pH. In the pedon profiles, the
“Contrada Loghi Vecchi” (Monte Caputo – Q. suber at least since the Middle Ages, and presence of nitrates proves that rhizo-
see Tab. S5 in Supplementary material). considering that the first afforestations in sphere activity is mostly concentrated in
Sicily started around 1920s, cork oaks the first 30 centimeters of soil depth. Sub-
Discussion should be considered locally native in these sequently, the scarcity of phosphorus, the
Our study provides ecological insights areas and the hypothesis of a past intro- excess of calcium and sodium, counterbal-
from Quercus suber stands occurring on duction in pre-existing woodlands should anced by similar concentrations of chlo-
the Mountains of Palermo, at the south- be discarded. rides and sulphates, characterize all the pe-
east edge of its distribution range. The Cork oak stands growing on alkaline or don profiles. The average depth of the de-
pedological characteristics obtained by di- neutral-alkaline soils have been already ob- scribed pedons falls within the range re-
rect soil excavation and pedon observation served in western and southeastern Sicily ported in the literature (Dettori et al. 2001).
(Corona et al. 2005) allowed to infer some (Minissale et al. 2006), as well as in south- Soluble ions, particularly sodium, do not
interesting indications on the ecological ern Latium (central Italy), where they have disturb the cork oak, though excess cal-
range of Q. suber in terms of soil variables. been interpreted as steady “disclimax” cium most likely does (Maghnia et al. 2017).
Recent research has emphasized the need communities favoured by century-long fire In fact, at each depth, molar ratios Na/Ca
for very detailed field data when studying disturbance to the detriment of local “cli- were always higher than 1, while Mg/Ca ra-
cork oak stands (Vessella et al. 2017). The max” forests dominated by holm and/or tio was at maximum 0.6, and Na/K ratio
341 iForest 13: 336-344Ecological insights from the south-eastern edge of Quercus suber range
was permanently higher than 20. It is plau- straints. Mixed oak woods must have been pecially at the edge of its natural range,
iForest – Biogeosciences and Forestry
sible that the high Ca content, deriving the rule before man favoured holm oak, due to overgrazing, high fire frequency,
from parent material, may represent a ma- cork oak and manna-ashes (Fraxinus ornus) soil pathogens and land-use changes. We
jor obstacle for the colonization of forest to the detriment of deciduous oaks (e.g., studied several cork oak populations on
species typical of more mature stages, thus Quercus pubescens and Quercus cerris) by soils deriving from calcareous bedrock,
favouring the cork oak. Furthermore, cork coppicing native woodlands. Conversely, it which are generally less suited to the spe-
oak may produce changes in soil nutrient is well known that regressive processes af- cies. Based on our results, we hypothe-
content, particularly N and Ca, affecting fect these ecosystems when the distur- sized that the local presence of Q. suber
the composition of soil microbial commu- bance is more intense and/or more fre- could depend on high fire frequency, the
nity (Aponte et al. 2013). This would ex- quent than the cork oak’s regeneration peculiar biogeochemistry of local soils
plain the good establishment and natural and recovery ability. Due to the combined (e.g., high Ca content), and erosion, which
regeneration even on dolomitic limestones effects of overgrazing and wildfires, cork caused the re-emergence of calcareous
found on Mt. Gibilmesi, Mt. Petroso and oak stands are initially replaced by species- substrates. The evolution of these stands is
Costa Sant’Anna near Mt. Cuccio, where Q. poor and structurally simplified mantle and hardly predictable as strictly linked to dis-
suber was planted during the second half shrubland communities, dominated by fast turbance frequency and intensity (mostly
of the twentieth century (Cutino et al. resprouters such as Cytisus infestus, Spar- wildfires). Monitoring activities and protec-
2018). In agreement with its general eco- tium junceum and Euphorbia characias, but tion measures are highly necessary to con-
logical behaviour, on the Mountains of Pa- also by species that are largely dispersed trast the likely future shrinkage of these
lermo Q. suber seems to play a dominant by wild animals, such as Pistacia lentiscus, populations, which are threatened by ex-
role only in macro-area B, characterized by Pistacia terebinthus, Osyris alba, Prunus tremely high fire frequency and overgraz-
acidic soils and rocky outcrops. This finding spinosa, Pyrus spinosa, Rubus ulmifolius and ing, as well as the likely negative effects of
suggests some peculiar adaptation to local Crataegus monogyna (La Mantia et al. climate change on regeneration ability and
conditions in the other macro-areas; for in- 2019). If the disturbance regime is further survival rate.
stance, the ability of cork oak seedlings to intensified, perennial grasslands domi-
withstand iron deficiency has been proved nated by Ampelodesmos mauritanicus are Acknowledgements
(Gogorcena et al. 2001). the most likely fate of Q. suber stands, as The individual contributions of authors
As far as soil erosion is concerned, the has largely occurred in other areas of the are the following: RS, TLM and SP con-
calcareous substrates colonized by Q. su- western Mediterranean basin (Vilà et al. ceived the study and established method-
ber are in tight contact with (and once 2001). Contrarily, open and scattered cork ology; EB, RS, TLM, RSB, GS, GMP, AG and
were underneath) the flysch outcrops, oak stands could evolve towards more SP collected and analyzed data; GMP, AG
mainly quartz sandstones and argillites (Ca- complex woodlands if grazing pressure and SP carried out field surveys; RSB per-
talano et al. 1978). Due to the heavy impact and fire frequency decrease (Bueno et al. formed the statistical analysis; EB, RS, TLM,
of human activities since prehistoric and 2020). However, a similar scenario, consis- RSB, GMP and SP wrote the manuscript.
protohistoric times, the upper lithological tent with the hypothesis of many authors This work was financially supported by
layers (flysch) may have been eroded, re- who attribute the meaning of steady discli- the PRIN2010-11 “CARBOTREES” (Climate
maining obliterated or reduced to small max to cork oak stands, maintained by change mitigation strategies in tree crops
pockets in cracks of the underlying carbon- moderate disturbance regimes, especially and forestry in Italy – national coordinator:
ate layers. In any case, if the general condi- wildfires (Barberis & Mariotti 1979), should R. Valentini), funded by the Italian Ministry
tions for establishment of the species ex- be supported by further qualitative and of Education, University and Research, and
ist, the plasticity of the cork oak (Maghnia quantitative investigations. by Tommaso La Mantia’s funding Pj autf
et al. 2017) may induce deep transforma- As far as our study sites are concerned, 008567.
tions on soil characteristics over time. they have been particularly prone to in- We thank the reviewers for significantly
The issue of how to interpret the dynamic tense and frequent human pressure (most- improving the quality of the manuscript
role played by cork oak in the vegetation of ly wildfires and overgrazing) for millennia. and for providing us with useful sugges-
the thermo- and meso-Mediterranean is Natural woodlands, including cork oak tions. Thanks are due to Broni Hornsby for
still under debate (Brullo et al. 2009). In stands, have been exploited or cleared the revision of the English.
the progressive succession of the Mediter- since proto-historical times. The land
ranean vegetation, many authors (Di Bene- patches between these forests have been References
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