Abstract
The genus Cistus comprises a group of about 20 shrub species found in wide areas throughout the whole
Mediterranean region to the Caucasus. Being one of the main constituents of the Mediterranean-type maquis, this
plant genus is peculiar in that it has developed a range of specific adaptations to resist summer drought and frequent
disturbance events, such as fire and grazing. In addition, it can form both ectomycorrhizas and arbuscular mycorrhizas. In this paper, we review the information available on the ectomycorrhizal fungi of Cistus across its entire
geographic range, as gathered and critically sifted from both published literature sources and personal observations. Although the resulting data matrix was based primarily on accounts of sporocarp inventories in the field, existing knowledge of the features of Cistus natural and synthesized ectomycorrhizas was also included and discussed. In total, more than 200 fungal species belonging to 40 genera have been reported so far to be associated with
Cistus. An analysis of the pattern of ectomycorrhizal diversity and host specificity revealed that members of the
Cortinariaceae and Russulaceae make the most of both Cistus-aspecific and Cistus-specific mycobionts. Further
studies are needed to expand our preliminary knowledge of the mycorrhizal ecology and biology of Cistus and its
fungal associates, focusing on topics such as mycobiont diversity, host specificity, fungal succession, mycorrhizal
influence on stress tolerance, and impact of disturbances, while comparing the findings with those from other
ecosystems.
Introduction
Although Europe and adjacent areas are relatively wellknown from the mycological point of view, some ecosystems have received comparatively little attention,especially concerning the role played by mycorrhizal symbiosis. In particular, given the ecological importance of host specificity for plant ectotrophic communities and the
associated mycota, studies describing the specificity patterns occurring in selected ecosystems are of premium
significance, as they can contribute to a better definition of the environmental biotic and abiotic factors that affect
specificity phenomena, and how the specialization of ectomycorrhizal fungi and plant hosts originated and
evolved (Molina et al. 1992; Erland and Taylor 2002; Van der Heijden and Sanders 2002). A significant example
of these “neglected mycorrhizal niches” is the Cistusdominated maquis. Cistus harbors a group of about 20
woody, evergreen or semi-deciduous shrub species found in wide semi-arid areas from Madeira and the Canary
Islands throughout the whole Mediterranean region to the Caucasus (Arrington and Kubitzki 2003). Some species,
such as C. carthaginensis in Spain (Boscaiu and Guemes 2001), have a very limited or even punctiform range and
are severely threatened of extinction.
Cistus species have evolved specific adaptations to resist severe summer
drought stress and to regenerate rapidly after fire, a key ecological factor influencing the evolution and dynamics of
the Mediterranean vegetation. Thanks to their ability as early colonizers after disturbance, Cistus species often
form pure stands in vast areas heavily subjected to fire and/or grazing (for an extensive bibliography on various
aspects of Cistus ecology and biology, see (http://www.cistuspage.org.uk).Cistus may form both ectomycorrhizas and vesicular arbuscular mycorrhizas, the other widespread type of mycorrhizal association (Smith and Read 1997). Although most plant species form only one type of association, the dual mycorrhizal status is not unique of Cistus, but is also present in Populus, Salix, Alnus, and Eucalyptus (Smith and Read 1997), and also in some tropical tree genera such as the myrtaceous Ixora, Syzygium, and Tristania (Reddell et al. 1996; Moyerson et al. 2001). In these hosts,
ectomycorrhizas and arbuscular mycorrhizas can either co-occur at a comparable level in the root system or one
type can be predominant, but it is not yet clear which factors favour dominance by each functional association.
In the case of Cistus and Eucalyptus, both genera inhabiting fire-susceptible ecosystems, it has been suggested that mycorrhizal plasticity may represent an important adaptive trait to the cyclical pattern of accumulation and loss of organic resources due to fire (Smith and Read 1997). At variance with the cistaceous host genus Helianthemum (Yu et al. 2001), no reports of ectendomycorrhizas formation by Cistus species are known to date.To expand current knowledge of mycorrhizal biology of Cistus, a new research program at our institution is focusing on the isolation and full characterization of the ectomycorrhizas formed by the fungal symbionts exclusively or prevalently associated with Cistus spp. (Nuytinck et al. 2004; Rinaldi and Comandini, unpublished observations). While screening published data on Cistus mycorrhizas to select suitable candidates for ectomycorrhizal characterization, we were struck by the fact that information on Cistus-associated mycota is generally widely dispersed in the mycological literature, and no recent
comprehensive accounts exist on the topic. In the present review paper, we attempt to fill this gap by providing an
updated checklist of fungal species reported to establish ectomycorrhizas on Cistus spp. on the basis of field
observations. The cases where the association has been confirmed experimentally, either through direct observation of naturally occurring or synthesized mycorrhizas, and by molecular approaches, have been highlighted. We also
attempt to discuss the ecological significance of Cistus mycorrhizas in relation to what is known on the role played
by ectomycorrhizal diversity in other better-studied ecosystems. Finally, we indicate those we believe are the
main research needs to fully disclose Cistus mycorrhizal ecology.
When more than one host genus was mentioned in the original reference, for example “under Quercus and
Cistus”, or “in Quercus stands with Cistus understory”, the relevant fungal species was generally not considered as
a Cistus symbiont, unless this particular association was confirmed by other sources. When more than one Cistus
species was present, the relevant mycobiont(s) was assigned to all potential hosts. Evidence from studies on
the morpho-anatomical and molecular characterization of ectomycorrhizas formed by taxonomically diverse fungal
species on Cistus spp. were also inserted in the data set, as they support hypotheses from field observations. In
addition to studies concerning naturally-occurring, fieldcollected mycorrhizas, data coming from synthesized
mycorrhizas were also considered.
Despite all efforts to cover as large a number of bibliographic sources as possible, our literature survey
was clearly partial and incomplete, and a number of valuable records may have been missed. However, we are
confident that the assembled data matrix includes the majority of the ecologically relevant information available
on Cistus-associated fungal species, especially when Cistus-specific mycobionts are concerned. As the vast
majority of the data set consists of field observations of sporocarps rather than associations confirmed by direct
inspection of ectomycorrhizas, the limitations inherent to data sources must be emphasized. Indeed, the reports of
putative mycorrhizal relationships based solely on sporocarp associations are obviously subject to an unquantifiable degree of error. Trappe (1962) has explained the criteria that should lead to the exclusion or inclusion of
literature reports when compiling lists of mycorrhizal associations based on sporocarp observations. In compiling
the data set for this study, we strictly observed these guidelines and, also on the basis of our personal experience, discarded or labeled all spurious and/or dubious records. As regards synthesized ectomycorrhizas, it should
be stressed that associations induced in laboratory experiments may not occur under field conditions (Harley and
Smith 1983; Molina et al. 1992). Finally, the identification of some fungi in the references we have considered may
not be correct.
Collecting the data
Data on the association between Cistus spp. and ectomycorrhiza-forming fungi presented in this paper are overwhelmingly based on reports of field observations of sporocarp associations with potential hosts. The data set
contains both information collated from a variety of published sources, including taxonomic monographs of
specific groups of ectomycorrhizal fungal genera and the few previous surveys of fungi associated with Cistus, and
personal collections and observations. Fungal taxa belonging to genera for which the mycorrhizal status is currently
uncertain were not listed. Only records clearly mentioning (potential) Cistus hosts were included in the data matrix
crispus, incanus, ladanifer (often mentioned under the synonym ladaniferus), laurifolius, monspeliensis, populifolius, salvifolius] are explicitly reported in the literature to form specific associations. Most fungal records are referred to common host species occurring over large geographic areas (C. incanus, C. monspeliensis, C.salvifolius), and few records are available for rare or uncommon species such as C. clusii, C. crispus, and C. populifolius.
In most cases, the potential host species was not indicated in the original reference, probably due to the
rather homogeneous morphology of the species within the genus Cistus, with subsequent identification problems for
non-specialists, and to the common occurrence of several Cistus species growing tightly together in a stand, whicmade it difficult, if not impossible, to specify the plant partner in a number of instances. Geographically, the
largest number of records were originally collected in Spain, followed by peninsular and insular Italy and
southern France, and a few in Morocco. Virtually no information is available for other extended regions in North
Africa (e.g., Tunisia and Algeria) and for the eastern side of the Cistus range, i.e., the Balkans, the Aegean Archipelago,
the coasts of Anatolia, the Caucasus, Syria, Lebanon. In Israel, studies on the mycorrhizal status of naturally
occurring C. incanus and C. villosus have been conducted(Litav 1965; Berliner et al. 1986), although the involved
fungal taxa were not identified.
In general, all the larger and ecologically key ectomycorrhizal fungal genera harbor a significant number of
species that are associated with Cistus. Cortinarius (29 species) and Russula (28) are the best represented in the
list, followed by Inocybe (23), Amanita (22), Hygrophorus(13), Lactarius (12, excluding the probable spurious report
of L. deliciosus, see below), Hebeloma (12), Boletus (10),Tuber (10, excluding T. californicum), Tricholoma (10). At
the family level, Cortinariaceae (for a comment on the concept of families in the Agaricales, see notes in Table 1)
and Russulaceae, clearly form the prevalent groups. The association of Cistus with numerous hypogeous ascomycetes (Table 1) seems to be a common feature of the Cistaceae as a whole, as other genera, such as Helianthemum, also show similar mycorrhizal preferences (e.g., Malloch and Thorn 1985).
“Broad host range” fungi, which can form mycorrhizal with host plants from diverse plant families, orders, and
even classes (Molina et al. 1992), occur frequently in the list, and include Amanita vaginata, Hebeloma crustuliniforme, Paxillus involutus, Telephora terrestris, and Tricholoma sulphureum. Many other listed fungal species
are more frequently associated with Quercus spp., and their association with Cistus, when reliably identified, is
probably sporadic and/or occasional. These include, for example, several of the Lactarius species reported here,
such as L. atlanticus, L. chrysorrheus, L. ilicis, L. mairei,and L. zonarius (Basso 1999). However, these records are
important because they provide valuable information concerning the mycorrhizal ecology of selected fungal
species, revealing their tendency to switch hosts when
Diversity and host specificity of Cistus ectomycorrhizal fungi
Current knowledge about Cistus ectomycorrhizal fungal diversity is based mostly on above-ground observations of
fungal fruitbodies (Table 1). About 230 fungal species belonging to 40 genera are listed, belonging to both
Ascomycota and Basidiomycota, a number of Cistus associates considerably larger than previous accounts
(Malloch and Thorn 1985; Lavorato 1991; Ballero et al. 1992; Vila and Llimona 1999, 2002). Early studies
reviewing ectomycorrhizal fungi and relevant plant hosts overlooked Cistus and its mycoflora (e.g., Trappe 1962),
partly due to a focus on forests rather than shrublands. On the plant side, nine different Cistus species [albidus, clusii,
these share the same environment—a feature that has certainly played a major role in the evolution of host
specificity among ectomycorrhizal fungi—and broaden our perspective of Cistus-compatible fungi.
The analysis of the assemblage of the fungal taxa more closely linked to Cistus also reveals an interesting pattern.
Two different groups of mycorrhizal fungi can be discerned here. The first comprises “narrow host range” fungi, i.e.,
fungal species forming mycorrhizas only in association with a single plant genus (Molina et al. 1992), Cistus in this
case. About 35 Cistus-specific fungi are identifiable to date, all of them being epigeous basidiomycetes. These
include Leccinum corsicum, Amanita cistetorum, Hebeloma album, H. cavipes, H. cistophilum, H. erumpens,
Inocybe cistobulbipes, I. rocabrunae, Hygrophorus pseudodiscoideus var. cistophilus, Lactarius cistophilus, L.
cyanopus, L. tesquorum, Russula cistoadelpha, R. monspeliensis, R. tyrrhenica, and a number of Cortinarius
species. Again, a few genera within the Cortinariaceae and,on a minor scale, Russulaceae, seem to comprise almost all
the fungal species highly specialized to Cistus. A second group has those fungal species which although not
associated exclusively with Cistus, commonly establishfunctional symbiosis with this host, and are often a
component of the macromycetous fungal flora in Cistusdominated plant communities. Tuber melanosporum, Amanita muscaria, Hebeloma hiemale, Laccaria laccata fo.pseudobicolor, and L. proxima belong to this group.
Among the macromycetes putatively linked to Cistus spp., it is apparent that some of the claimed associations are
very unlikely. This is the case, for example, of the related Suillus collinitus and Rhizopogon spp., both genera being
currently accepted as specific to conifers (almost exclusively Pinaceae) (Kretzer et al. 1996; Molina et al. 1999).
Also Amphinema byssoides has been reported as being restricted to coniferous tree species (Erland and Taylor
1999). Another clear example of a most likely spurious report is that of the common Pinus-associated Lactarius
deliciosus from Greece (Zervakis et al. 1998) and Spain (Ortega and Esteve-Raventós 1999). It is, thus, possible
that some fungi in the list are in reality not compatible with Cistus, but rather with other hosts present in the same
community, most frequently Quercus spp. (see above) and Pinus spp. The absence of Cenococcum in Table 1 is also
noticeable, as this symbiont is almost ubiquitous on the roots of most ectomycorrhizal hosts in a range of ecological
situations (LoBuglio 1999). Given the little attention devoted so far to the observation of Cistus ectomycorrhizal
morphotypes (see below), however, it cannot be excluded that Cenococcum forms mycorrhizas with Cistus.
In spite of the fairly large number of ectomycorrhizal macromycetes linked to Cistus spp. in Mediterranean-type
ecosystems, information on the features of relevant mycorrhizas is remarkably limited. To date, only very
few accounts of Cistus ectomycorrhizal types exist in the literature. Most studies focused on the ectomycorrhizas
formed by hypogeous ascomycetes, such as Tuber and Terfezia with Cistus spp., both under natural and cultured
conditions. These investigations resulted in the full
characterization of the ectomycorrhizas formed by Tuber spp. on C. incanus (Fontana and Giovannetti 1979;
Giovannetti and Fontana 1982), and in several other preliminary and/or not exhaustive descriptions of mycorrhizal morpho-anatomical details (Chevalier et al. 1975; Fusconi 1983; Leduc et al. 1986; Wenkart et al. 2001;
Roth-Bejerano et al. 2003). For basidiomycetes, descriptions of the mycorrhizas formed by L. laccata and Boletus
rhodoxanthus on C. ladanifer (Torres et al. 1995; Hahn 2001), by Hebeloma sacchariolens on C. salvifolius
(Rosell 1981) and, more recently, by Lactarius tesquorum on Cistus sp. (Nuytinck et al. 2004), are available.
Common host-dependent features of all the ectomycorrhizal types described so far on Cistus spp. are (for the
specific terminology used to characterize ectomycorrhizae, see Agerer 1986, 1987–1998, 1991): simple or monopodial-pinnate ramification systems; small diameter of ectomycorrhizal tips; thin mantle thickness; cortical cells
generally present in two rows, tangentially rectangular, either radially rectangular orientated or square to radially
rectangular orientated; Hartig net generally uniseriate, surrounding one–two rows of cortical cells and rarely
reaching the endodermis (“cortical Hartig net”, for a description of this peculiar structure, see Smith and Read
1997).
In addition to the ectomycorrhizal types mentioned above, very recent observations of Cistus roots excavated
in Sardinia (Italy) have led to the isolation of ectomycorrhizas of Lactarius cistophilus and of a morphotype
unequivocally formed by a sebacinoid mycobiont (Rinaldi and Comandini, unpublished observations). The latter
finding adds to the increasing evidence that members of the Sebacinaceae, a family assigned to the heterobasidiomycetous order Auriculariales, are common symbionts in various ectomycorrhizal communities. Typical ectomycorrhizas formed by these fungi have been recently detected by both molecular and morphological analyses on several temperate deciduous and coniferous trees, including Carpinus, Corylus, Fagus, Tilia, Picea, and Abies (Selosse
et al. 2002; Urban et al. 2003; Comandini, unpublished observations), and also in Australian Eucalyptus Mediterranean-type forests (Glen et al. 2002).
In recent years, the application of a range of molecular tools has greatly enhanced our knowledge of ectomycorrhizal communities, expanding well beyond classic morphotyping our possibilities to track and identify ectomycorrhizas and to compare the above- and belowground mycorrhizal fungal components of ecosystems (Horton and Bruns 2001). Unfortunately, this “revolution” has only marginally impacted research on Cistus ectomycorrhizas. To date, indeed, only the mycorrhizas of Lactarius tesquorum on Cistus sp. have been fully characterized from both a molecular and morphological point of view (Nuytinck et al. 2004). Clearly, the confirmation of many putative Cistus mycobionts and the unambiguous identification of their mycorrhizas await a more general recourse to molecular methods.
Comparing ectomycorrhizal diversity in different ecosystems
In addition to forming extensive patches of pure shrublands, Cistus is also a significant presence in other
vegetation communities. These communities include the mixed Mediterranean maquis or garrigue, where Cistus
occurs together with other sclerophyllous (not-ectomycorrhizal) scrubs, such as Olea, Phyllirea, Pistacia, Erica,
Arbutus, and some Quercus and Pinus-dominated communities, where Cistus is an element of the undercanopy
vegetation and colonizes clearings and open areas. The data gathered in this study indicate that the number of
ectomycorrhizal fungal species associated with Cistus is significant, and support the importance of this symbiosis in
these Mediterranean ecosystems. On the other hand, when compared to other better-known host plants such as
Pseudotsuga menziesii, which associates with some 2000 fungal species in North America (Trappe 1977), the
ectomycorrhizal diversity of Cistus might look rather poor.However, it should be stressed that comparing lists of
fungal associates drafted by different authors might be biased to some extent by personal choices for inclusion or
exclusion of those fungal genera for which the mycorrhizal status is uncertain or not fully confirmed. Focusing on the
information available for other better-studied Mediterranean-type ecosystems, some 660 fungal associates, many
of which are endemic, have been recorded for Eucalyptus in natural environments of Australia (Castellano and
Bougher 1994; Bougher 1995). Eucalyptus is also believed to have the potential to associate with the richest flora of
host genus-specific ectomycorrhizal fungi in the world (Molina et al. 1992), while diversity in its plantations is
considerably lower in both Australian and exotic areas (Lu et al. 1998a,b; Giachini et al. 2000) (an extensive account
of Eucalyptus ectomycorrhizal fungi can be found at http://www.ffp.csiro.au/research/mycorrhiza/eucfungi.html). Eucalyptus and Cistus share a small contingent of associated fungi, included in the genera Pisolithus, Scleroderma, and Laccaria, with the characteristics of “early stage” species.These mycobionts appear in the early phases of fungal
successions and represent, in some cases, pioneer fungi,being often typical of disturbed habitats with young trees
and shrubs, or may occur even in freshly cleared areas and often associate with a diversity of hosts. Some Hebeloma
listed in Table 1 are also well known early stage fungi.
What needs to be done
Our knowledge of Cistus mycorrhizal ecology is too limited to draw any simple conclusions for this host plant
and its associated fungi as for many key aspects mentioned above. However, information acquired from studies of
other ecosystems may help to highlight priority areas for future research. In particular, the pattern and role of host
specificity, possibly for each fungal species, in Cistus ectomycorrhizal communities should be investigated, as
the importance of host specificity in other ectomycorrhizal communities has been underlined (Bruns et al. 2002).
Examining the plant-fungal associations for Cistus would also serve to elucidate mycobiont–host distribution and
mycobiont–host species relationships (Newton and Haigh 1998), to see if they have a general validity or, and
eventually how, they change in specialized environments.Many of the Cistus-specific symbionts listed here, e.g.,
Cortinarius spp. and Inocybe spp., have been identified only very recently, which highlights the need for in-depth
taxonomical surveys of these and other fungal genera in Cistus maquis over the entire host geographic range.
Belowground ectomycorrhizal diversity in Cistus natural communities should also be explored to see if it reflects the
Cortinariaceae and Russulaceae dominance detected aboveground, and the morphological and molecular
characterization of the prevailing ectomycorrhizal types should be performed.
Work aimed at confirming the identity of putative mycobionts and extending their number would certainly be
easier if carried out in pure Cistus stands, rather than in areas where it grows mixed with other ectomycorrhizal
hosts. However, the exploration of diversity and biology of Cistus mycorrhizas in mixed plant communities could
provide key information on the structure of both the plant and fungal components of these communities in Mediterranean-type ecosystems. Doubtlessly, molecular tools will play a major role in tackling these complex issues. As data on the succession of mycorrhizal fungi in Cistus ecosystems are totally lacking, community level studies should
also be conducted to address this important aspect, as well as the impact of mycorrhizal colonization upon the fitness
of Cistus and its resilience to natural and anthropogenic disturbances, such as strong water limitation, fire, overcutting, and grazing.
Considering the ecological niches occupied by Cistus and the intermediate position of this host genus in the
vegetation series leading to evergreen Quercus or Pinus climax forests on one side, and to impoverished pastures
and/or desertified lands on the other, a deeper knowledge of Cistus ectomycorrhizal fungal communities may well
prove to be of wider significance and to contribute to understand the role and dynamics of mycorrhizas in inherently unstable ecosystems, especially if integrated into broader ecological investigations. These studies may also provide valuable tools to help shape future programs of protection and management of natural resources in vast
areas across the Mediterranean basin. In general, Cistus holds the potential to develop into an alternative model to
assess the role of mycorrhizal symbiosis in ecosystem functioning of Mediterranean-climate shrublands throughout the world. Clearly, we are just at the beginning of this research journey.
