During a pre-congress excursion (23 July 2019) to the Sintra mountain range, led by the organizers of the XXII Symposium of Cryptogamic Botany, we found foliicolous (epiphyllous) lichens growing abundantly in the ‘enchanted forest’ of Peninha (38°46′9.06″N, 9°27′32.27″W), c. 40 km north-west of Lisbon. Foliicolous lichen communities are principally tropical but do occur to a limited extent in subtropical and oceanic-temperate regions where high humidity, relatively mild conditions, and adequate non-deciduous leaf substrata are found (Santesson Reference Santesson1952; Lücking Reference Lücking2008). In Europe, foliicolous lichen communities are mainly restricted to highly localized microsites, particularly protected gorges and ravines. Such localities have been reported within, and north and south of, the Pyrenees (Vězda & Vivant Reference Vězda and Vivant1972; Sérusiaux Reference Sérusiaux1993; van den Boom & Sérusiaux Reference van den Boom and Sérusiaux1996; Llop & Gómez-Bolea Reference Llop and Gómez-Bolea2006, Reference Llop and Gómez-Bolea2009), in southern Italy (Puntillo et al. Reference Puntillo, Bricaud and Sérusiaux2000) and in the Black Forest (Lücking et al. Reference Lücking, Wirth and Ahrens2009). While foliicolous lichen communities are known from the laurisilva forests of Madeira and the Azores, previous reports from continental Portugal appear limited to a collection of Byssoloma subdiscordans (Nyl.) P. James on cycad leaves in Monserrate Park, Sintra (as B. rotuliforme; Santesson Reference Santesson1952). Other foliicolous species from continental Portugal have been cited mainly on bark substrata (Santesson Reference Santesson1952; Sérusiaux Reference Sérusiaux1996, Reference Sérusiaux1998; Breuss Reference Breuss2016).

The presence of a foliicolous lichen community at the Peninha site is hardly surprising. The north-eastern-facing forested slope is bathed in mist-laden winds from the Atlantic, strongly reminiscent of the wet Macaronesian laurisilva forests. Amidst a profuse cover of Hedera helix ssp. canariensis (Willd.) Cout., the predominant tree supporting the foliicolous lichens is also strikingly similar in appearance to those of the laurisilva forest. However, while diverse indigenous tree species comprise laurisilva, the ‘enchanted forest’ of Peninha is principally just Pittosporum undulatum Vent., an Australian tree introduced to the area in the 19th century. The Serra de Sintra itself has a unique microclimate and a considerable degree of autochthonous biological diversity and endemism, owing to its substantial elevation (to 528 m) and exposure to year-round Atlantic fog that effectively doubles the moisture supplied by precipitation (Pinto da Silva et al. Reference Pinto da Silva, de Bacelar, Catarino, Correia, Escudeiro, Serra and Rodrigues1991). The indigenous vegetation includes Cupressus lusitanica Mill., as well as Quercus pyrenaica Willd. and the more Mediterranean Q. faginea Lam. and Q. suber L. Bryophytes are notably diverse (Cacciatore et al. Reference Cacciatori, Garcia and Sérgio2015). However, centuries of human exploitation, followed by the introduction of many exotic species into the vast gardens of local aristocrats, has taken a very heavy toll on the original plant communities (Pinto da Silva et al. Reference Pinto da Silva, de Bacelar, Catarino, Correia, Escudeiro, Serra and Rodrigues1991).

Elsewhere in the Iberian Peninsula, where foliicolous lichens occur on native vegetation, the main substratum is usually boxwood leaves (Buxus sempervirens L.), but that shrub/tree was not seen at the Peninha site. An interesting question is what, if any, native substrata might allow these lichens to persist in the area in the absence of the introduced Pittosporum upon which they are currently thriving. A number of authors have considered the role of non-native tree plantations in potentially reducing epiphytic lichen diversity (Quine & Humphrey Reference Quine and Humphrey2010; Nascimbene et al. Reference Nascimbene, Nimis and Benesperi2012, Reference Nascimbene, Lazzaro and Benesperi2015; González-Montelongo & Pérez-Vargas Reference González-Montelongo and Pérez-Vargas2019). At the Peninha site, by contrast, an introduced exotic is providing the substratum for a lichen community not reported previously for the region. It is interesting to compare this situation with that of other reported foliicolous communities in Iberia and Western Europe, which frequently occur in gorges or narrow ravines with hyperhumid conditions and mild temperatures (Puntillo et al. Reference Puntillo, Bricaud and Sérusiaux2000; Llop & Gómez-Bolea Reference Llop and Gómez-Bolea2006). These relatively inaccessible habitats typically preserve native vegetation that would be vulnerable to any alterations in the microclimate. Thus, while foliicolous lichen communities may serve as important indicators of unique microclimatic conditions that are significant for biodiversity, their presence does not necessarily correlate with the conservation of native plant community structure.

The predominant foliicolous lichen of the Peninha forest is distinct from those seen in other Iberian or northern European localities. It consists of tiny, raised areolae thoroughly encrusted with white crystalline material. Apothecia are abundant and may have a volcano-like appearance where the surrounding areola is almost bullate-swollen and the apothecium appears as a sunken crater (Fig. 1A & B). The excipulum often has a distinctive orangey-brown pigment encircling the disc, which is frequently bright green with numerous photobiont cells embedded in the epithecium and hypothecium (Fig. 1C). Asci contain a single, large, hyaline, muriform ascospore measuring (22–)22.1–32.2–42.3(–45) × 5.6–11.9–18.2(–19) μm (Fig. 1D). Pycnidia are scarce, located at the thin marginal areas of the thallus and easily mistaken for young apothecia. Pycnoconidia are bacilliform, measuring 2–3 × 0.8(–1) μm. Asexual diahyphal propagules are characteristic of the genus Gyalectidium, each a roughly isodiametric bundle of sausage-like macroconidial chains with photobiont cells intermixed (Fig. 1E). They form at the base of one (or occasionally several) small, membranous hyphophore scales narrowing to a point at the tip; they are located at or near the periphery of thallus areolae (Fig. 1F–H). The hyphophores are infrequently observed, however, and their scales are inconspicuous without the use of higher magnification with a dissecting microscope. The material appears to represent Gyalectidium minus Sérus., an obligately foliicolous taxon described from the Canary Islands (Ferraro et al. Reference Ferraro, Lücking and Sérusiaux2001) and also known from a single site in southern Italy. The Italian material was previously recorded as G. caucasicum (Elenkin & Woron.) Vězda (Puntillo et al. Reference Puntillo, Bricaud and Sérusiaux2000), a taxon once thought to occur rarely in France and Spain (Sérusiaux Reference Sérusiaux1996), but excluded from Europe in a more recent treatment (Ferraro et al. Reference Ferraro, Lücking and Sérusiaux2001). Since G. minus has not been recorded for Madeira (Sérusiaux Reference Sérusiaux1996) or the Azores, the Peninha site would also represent its first report in Portuguese territory. It appears to show a typical tethyan distribution (Ferraro et al. Reference Ferraro, Lücking and Sérusiaux2001; Lücking Reference Lücking2003), with a presence in Macaronesia (Canary Islands) and the western Mediterranean region (Iberian and Italian peninsulas). In the single known locality in Italy, G. minus is considered a threatened species (Ravera et al. Reference Ravera, Nimis, Brunialti, Frati, Isocrono, Martellos, Munzi, Nascimbene, Potenza and Tretiach2011).

Fig. 1. Gyalectidium minus Sérus. from Peninha (Sintra), Portugal. A & B, thallus areolae with apothecia; C, hand-cut section through apothecium, showing orange pigment in excipulum (e) and photobiont layers (p) above and below immature hymenium; D, ascospore; E, diahyphal propagule with photobiont cells mixed within dense bundle of conidial chains; F, hyphophore with subtending scale (s) and several diahyphal propagules (d); G & H, thallus areolae with hyphophores subtended by pointed scales (arrows). Scales: A, B, G & H = 100 μm; C–F = 10 μm.

Gyalectidium minus is distinct from the two other species of the genus known to occur in foliicolous localities of northern Spain, southern France and the Black Forest (Germany). Gyalectidium setiferum Vězda & Sérus. has very long, slender, seta-like hyphophore scales that are abundant and prominent; apothecia are unknown in this taxon. Gyalectidium puntilloi Sérus. has separate, white, polygonal encrustations of crystals surrounded by a thin green thallus; apothecia are ‘very rare’ (Ferraro et al. Reference Ferraro, Lücking and Sérusiaux2001). It should be noted that these are the current phenotype-based species concepts, which, as in other genera of foliicolous lichen fungi, await evaluation by molecular sequence comparisons.

Gyalectidium minus grows alone on many of the leaves examined. However, a number of leaves are also colonized by additional foliicolous species. The most common companions of G. minus in the Peninha ‘enchanted forest’ are Bacidina apiahica (Müll. Arg.) Vězda, Fellhanera seroexspectata Sérus. and Fellhaneropsis myrtillicola (Erichsen) Sérus. & Coppins. Less frequent associates include Byssoloma croceum Sérus. & Puntillo, Fellhanera bouteillei (Desm.) Vězda, Phylloblastia fortuita Llop & Gómez-Bolea and Porina hoehneliana (Jaap) R. Sant. All the aforementioned species are new reports for continental Portugal, while B. croceum is a new report for the Iberian Peninsula (Breuss Reference Breuss2016). The distribution pattern of the latter species appears, at present, similar to that of G. minus.

Considering the markedly oceanic character of the western Iberian Peninsula, it seems plausible that other overlooked foliicolous communities could occur in Portugal and north-western Spain, and these might well include unexpected taxa as reported here.