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P. Grossoni, P. Bruschi, F. Bussotti, M. Pollastrini & F. Selvi
The taxonomic interpretation of Mediterranean oaks of Quercus sect.
Quercus (Fagaceae): uncertainties and diverging concepts
Abstract
Grossoni, P., Bruschi, P., Bussotti, F., Pollastrini, M. & Selvi, F.: The taxonomic interpretation
of Mediterranean oaks of Quercus sect. Quercus (Fagaceae): uncertainties and diverging con-
cepts. — Fl. Medit. 31 (Special Issue): 271-278. 2021. — ISSN: 1120-4052 printed, 2240-4538
online.
The high degree of polymorphism found in the genus Quercus is certainly the main cause of the
divergent taxonomic treatments that have often generated more uncertainties, if not confusion,
than clarity. However, in recent years, also thanks to the use of molecular investigation tech-
niques, several doubts have now been overcome. In this paper we summarized the main litera-
ture on the topic and provide a synthetic evaluation of the systematic position of the Italian taxa
belonging to the so-called “Quercus pubescens group” (“downy oaks”).
Key words: Quercus pubescens complex, downy oaks, Mediterranean Basin.
Introduction
Oaks (Quercus L., Fagaceae Dumort.) are widely distributed in the Northern
hemisphere playing important roles in providing forest products (timber, fuel wood,
cork, mushrooms, extracts and derivatives, etc.), in conservation of biodiversity, pro-
tection of landscape and all related ecosystem services. In Italy, oak forests occupy
about 2,873,000 ha, corresponding to 29.4% of the entire national forest area (Corona
& al. 2004). The genus Quercus includes a large number of species. Camus (1936-39)
quoted about 800 taxa, while Krüssmann (1986) reported about 450 species. The most
recent taxonomic revisions tend to reduce the number of species by putting more
emphasis on similar characters and adopting a wider species concept, so that current-
ly between 300 and 350 species are recognized. Denk & al. (2017) provide a list of
858 names among species, synonyms, subspecies, hybrids, and dubious species.
The classification of oak species is under debate since centuries. Recently Denk &
Grimm (2010) and Denk & al. (2017) revised the past classifications (Ørsted 1871;
Schwarz 1936-39; Camus 1936-39; Nixon 1993) and proposed a new one based on a
phylogenetic and molecular approach. According to this recent classification,
Quercus is subdivided into two subgenera, Q. subg. Quercus and Q. subg. Cerris,
Fl. Medit. 31 (Special Issue): 271-278
https://doi.org/10.7320/FlMedit31SI.271
Version of Record published online on 19 September 2021
Article
both including several sections. Oak species have very different general habit (trees
to shrubs), phenology (evergreen, deciduous, semi-deciduous), and ecological
requirements (from tropical to boreal and Mediterranean). Several species display
strong polymorphism and a remarkable ability to form hybrids, making their taxo-
nomic limits, and the species concept itself, quite elusive.
As early as in 1911, Antonino Borzì had deemed that the genus Quercus did pro-
vide “[...] the perfect negation of the concept of species [...]”, followed by the state-
ment “[...] it is an immense chaos [...]”. Still in the last quarter of the last century,
Burger (1975) stressed the difficulty of applying the biological species concept in
Quercus, due to relatively frequent gene exchanges between different species. A few
years later, referring to the “ability” of Q. alba L. and Q. stellata Wangenh. to form
hybrids with other 11 species of the eastern United States, Whittemore & Schaal
(1991) confirmed that oak species are easily interfertile due to weak and incomplete
interspecific reproductive barriers. On the other hand, the lack of sharp morphologi-
cal delimitations, together with low interspecific genetic differentiation, have often
been interpreted as the consequence of natural hybridisation in Quercus (Bruschi &
al. 2000; Salvini & al. 2009). Incorporation of heterospecific alleles, through
hybridization and regression, would yield reduced interspecific distances compared
to values expected for taxa that had been fully isolated genetically subsequent to spe-
ciation. Similar considerations can be found in Le Hardÿ de Beaulieu & Lamant
(2010), who published an illustrated world monograph of Quercus, that can be con-
sidered evolved from the work of Aimée Camus (1936-1939). Based on data collected
across the genus in different continents, these two authors also stressed the difficulty
of applying the biological species concept to oaks. In fact, several complexes of sym-
patric “species” exist in which taxa are capable of frequent gene exchange, thus
reducing their morphological divergence. According to Le Hardÿ de Beaulieu &
Lamant (2010) it would be more correct to use the term “multispecies” when refer-
ring to these complexes.
Gene flow and hybridization have certainly played an important role also in the
evolutionary history of oaks of the Mediterranean Basin. This region is considered a
hotspot of plant biodiversity (Médail & Quézel 1999), resulting from its biogeograph-
ic history and natural heterogeneity of habitats and environmental conditions. Events
like migration, micro-evolution, isolation in glacial refugia, range disjunctions and
others have favoured the formation of many endemic species but also local phenotyp-
ic variation in populations sometimes classified as distinct entities or taxa. Moreover,
the strong and long impact of human activities on habitats, landscape and vegetation,
especially forest fragmentation, has contributed to promote hybridization and intro-
gression between taxa in many plant groups (Médail & al. 2019).
In 1997 some of us (Bussotti & Grossoni 1997) published a comparative analysis
of the differences between several classifications of taxa of Quercus described from
the Mediterranean region (Camus 1936-1939; Greuter & al. 1986; Krüssmann 1986;
Nixon 1993; Pignatti 1982; Schwarz 1964, 1993). They found that these classifica-
tions had several nomenclatural and taxonomic discrepancies, the use of names and
synonyms being inconsistent and based on contrasting species concepts. This resulted
in doubts, uncertainties and contradictory conclusions. That analysis was subsequent-
272 Grossoni & al.: The taxonomic interpretation of Mediterranean oaks of Quercus ...
ly repulished in the journal Forêt Méditerranéenne (Bussotti & Grossoni 1998). It
pointed out that the greatest perplexities and uncertainties concerned the groups “Q.
pubescens - Q. petraea” and “Q. faginea - Q. lusitanica”, in which a high genetic
diversity is associated with the tendency to differentiate numerous local phenotypes
along almost continuous morphological gradients, without any clear correlation with
ecological or geographical factors, thus making it very difficult to separate well-
defined taxa.
The present contribution aims at revisiting the past and current classifications of
the species belonging to Q. subg. Quercus in Italy, with special reference to the group
Q. pubescens – Q. petraea group and related entities of the so-called “downy oaks”.
Taxa of Quercus sect. Quercus in Italy
Among the species currently placed in Quercus sect. Quercus, Q. robur was the
only one described by Linnaeus (1753). Fiori (1923-1925) still used this name for the
whole complex, albeit dividing it into numerous infraspecific taxa, many of which are
currently obsolete. Subsequently, Di Tella (1930) and Merendi (1930) distinguished
the English oak (“Q. pedunculata Ehrh.”) from the sessile oak, in which they includ-
ed both “Q. sessilis Ehrh.” and “Q. lanuginosa Lam.”. All these entities were placed
by Fiori (1930) in Q. robur and were generically called referred to as “querce roveri”.
More recent classifications are summarized in Table 1, which shows a persistent
variability in the number of species, as well as the presence of numerous doubtful
species. The unequivocally recognized species are Q. robur L., Q. petraea (Matt.)
Liebl., Q. pubescens Willd. and Q. frainetto Ten., while Q. pyrenaica Willd., already
considered as rare and present only in the Val di Susa (Piemonte) in the first edition
of Flora d’Italia (Pignatti 1982), was excluded from the native Italian flora in the
second edition (Brullo 2017) and by Brullo & al. (1999) and Brullo (2017), whereas
Bartolucci & al. (2018) still include Q. pyrenaica in their list of the Italian native
flora. These authors subdivided each Q. petraea and Q. robur into two subspecies,
while Brullo (2017) considered Q. brutia Ten., omitted from Table 1, as a “phantom
species”.
Recently, Bussotti (2020) summarised a synoptic table with the main morphologi-
cal characters of the three principal species of the «robur-petraea-pubescens» com-
plex; among them, Q. robur presents the lowest identification difficulty, due to the
uniqueness and stability of the characters that consistently separate it from all other
species. On the other hand, the distinction between Q. pubescens and Q. petraea is
often uncertain due to the lack of stable and reliable morphological characters con-
cerning twigs (shape, size, presence/absence of pubescence), leaves (size, shape,
pubescence) and fruits (cupule of the acorns). Indeed, most of these characters show
a continuous series of intermediate states connecting one species to the other and are
often widely variable even within the same population or geographic area.
Flora Mediterranea 31 (Special Issue) — 2021 273
The downy oaks
Quercus pubescens is a very polymorphic species, and its circumscription includes a
swarm of closely related entities, that are currently still considered as independent taxa in
some recent accounts of the Italian flora, as reported in Table 1.
Considering the species within Q. pubescens “lato sensu”, compared to those recog-
nized by Brullo (2017), the list by Bartolucci & al. (2018) does not include Q. virgiliana
(Ten.) Ten., nor Q. amplifolia Guss., while Q. ichnusae Mossa & al. and Q. leptobalana
Guss. are included. Schwarz (1993) also reported Q. sicula Borzì ex Lojac., although, at
the time, it had already been qualified as a “mistake” by Antonino Borzì (Brullo & al.
1999). There is also an inconsistency in the spelling of one epithet: “leptobalana” in
Bartolucci & al. (2018) and “leptobalanos” in Brullo & al. (1999) and Brullo (2017), a dis-
crepancy which has its roots in the 19th century and, evidently, persists to date (both names
274 Grossoni & al.: The taxonomic interpretation of Mediterranean oaks of Quercus ...
Table 1. List of native taxa in Quercus subgen. Quercus accepted and quoted for Italy in relevant
recent literature and web sources.
are attributed to Guss. 1844)., whereas Q. pubescens has been definitively accepted instead
Q. humilis Mill. (Q. humilis is now accepted as synonymous only for Q. pubescens subsp.
pubescens).
The entities related to the “downy oak” complex were identified and determined for the
first time in Sicily, Sardinia or southern Calabria; according to Brullo & al. (1999) and
Brullo (2017) the Italian range of Q. pubescens is limited to the peninsular and continental
regions, it being replaced in the large islands and in some areas of the southern peninsula
by other species of the same complex, Quercus virgiliana and Q. dalechampii Ten.; these
have also been reported in various countries of south-eastern Europe and the Balkans.
However, there is much uncertainty as to the correctness of these reports, due to the often
contradictory descriptions of the corresponding entities (Di Pietro & al. 2012) and an over-
ly subjective approach to recognition and classification. Arrigoni (2018), due to the lack of
reproductive isolation, does not recognize Q. virgiliana and considers it as a synonymy of
Q. pubescens, together with all the other putative species of the group (Q. congesta C.
Presl, Q. amplifolia, Q. ichnusae, and Q. leptobalana)
A first morphometric approach to discriminate Q. pubescens from other species of the
same group in Sicily was attempted by Di Noto & al. (1995), without conclusive results.
Based on the study of 20 Q. pubescens populations randomly sampled in central and south-
ern Italy, Bruschi & Grossoni (2004) observed a high morphological and molecular vari-
ability (with populations from Sicily, Sardinia and Calabria showing a higher average
diversity than the others). While morphological characters (those related to the acorn cap)
allowed these authors to identify five different groups, no differences were detected at
molecular level. The same conclusions were drawn by Franjić & al. (2006) in a molecular
study on Q. pubescens populations of Southern Croatia: “This study confirms a high vari-
ability of Q. pubescens populations, but differences were not so big to confirm the opinion
of existence of several species in this area”. Di Pietro & al. (2016) found that all morpho-
logical characters analysed in 24 Apulian populations exhibited continuous variation, so
that none of them could be used as a character to discriminate between populations; they
concluded that it “is unlikely that more than one species belonging to the Quercus pubes-
cens complex occurs in the Apulia region”. Di Pietro & al. (2020a, b, c) expanded this
approach by comparing morphological and genetic aspects of seven taxa of the Q. pubes-
cens group (Quercus pubescens, Q. amplifolia, Q. congesta, Q. dalechampii, Q. ichnusae,
Q. leptobalanos, Q. virgiliana) in southern Italy and the islands (Sicily and Sardinia), con-
cluding that “In light of the results obtained, the taxonomic classification for the pubescent
white oaks currently reported in the major Italian floras and checklists for the study area
was not confirmed by molecular analyses”.
Hybridisation within Quercus sect. Quercus appears to be extensive (Rushton 1993),
and recorded hybrids between Q. petraea and Q. robur (Bacilieri & al. 1995) and between
Q. petraea and Q. pubescens (Salvini & al. 2009) are common and widespread.
The results of a parentage analysis carried out through microsatellite markers on a
mixed Q. petraea – Q. pubescens population (Salvini & al. 2009) showed an asymmet-
rical gene flow with a predominant component in the direction Q. petraea versus Q.
pubescens. These results also showed that intermediate individuals are pollen-receptive
towards both species and their high pollen viability provides potential for fostering high
rates of introgression.
Flora Mediterranea 31 (Special Issue) — 2021 275
Conclusions
The accurate morphological and genetic analyses, carried out by the various research
groups mentioned in this article, allow us to exclude that the multitude of botanical names
associated with the Q. pubescens group is mirroring effective differentiation into well-
defined species, thus supporting the conclusions of Wellstein & Spada (2015): “While some
schools in southern Europe still emphasize the distinctness and the species status of many
taxa described during the earliest botanical surveys, the current trend is toward rejecting
many names and considering them as synonyms”.
Polymorphism is frequent in oak species. Corti (1959), for example, mentioned 175 infra-
specific taxa under Quercus ilex, that is considered a non-controversial species. Q. ilex is
characterised by a set of well-defined and generally accepted characters, whereas the charac-
terization of the “white oaks” is more subtle and their evaluation can be affected by a certain
degree of subjectivity.
In the absence of selective reproductive barriers, hybridization can generate phenotypes
with appreciable morphological and ecological diversity, even with extreme forms that can
sometimes be quite distinctive. However, these phenotypes are usually distributed along con-
tinuous morphological gradients, and have no or little geographic or ecological distinctness,
being often overlapping and mixed in the same locality or forest areas.
However, a taxonomic simplification does not imply a simplification at the genetic and
ecological level. The deciduous oak forests of southern Italy are a large reserve of genetic
variability that is an important asset for the adaptation of European forests to climate change.
Southern provenances of deciduous oaks are good candidates to restore the Central European
forests affected by drought in a context of “assisted migration” (Bussotti & al. 2015).
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Address of the authors:
Paolo Grossoni, Piero Bruschi, Filippo Bussotti*, Martina Pollastrini & Federico Selvi,
Department of Agriculture, Food, Environment and Forestry (DAGRI), University of
Firenze. Piazzale delle Cascine 28, 50144 Firenze, Italy.
* Corresponding author. E-mail: filippo.bussotti@unifi.it
278 Grossoni & al.: The taxonomic interpretation of Mediterranean oaks of Quercus L. ...
