Acer opalus Mill.
A. italum Lauth
Eng.: Opalus maple. Spa.: Acirón, arce, ásar. Fre.: Érable à feuilles d’aubier. Ara.: Keïkab, keïkob, siqmur, asfendan. Tam.: Tfifia, arich.
Small tree, deciduous, polygamous, up to 10-12(15) m in height, of regular shape, usually rounded when growing isolated. Trunk well defined, straight, somewhat tortuous in rupicolous environments, with greyish-brown bark, thick, fissured. Branches greyish or brown-yellowish, with smooth bark, younger branches greenish or reddish-brown, pubescent. Leaves 2.5-18(20) × 2.5-17(19) cm, opposite, palmate, 5-lobed (smaller leaves sometimes trilobed), coriaceous or not, glabrous, shiny, deep green on the upper side, from ± pubescent —at least along main veins— to densely tomentose on the underside. Lobes unequal, irregularly dentate or serrated, the 2 basal lobes smaller, rarely absent, the upper lobes usually with sinuses of variable depth (15-70% of the leaf blade), from parallel to convergent. Petiole well developed, hairy or glabrous. Inflorescence corymbiform, pendulous, with ± pubescent peduncles. Flowers hermaphrodite or unisexual (male), from yellowish-white to greenish-yellow. Calyx with 4-5 free sepals. Corolla with 5 petals arranged alternately with the sepals, borne on a fleshy disk, on which also 8 stamens are borne, very exserted in male flowers. Fruit a paired samara, with a glabrous or hairy seminiferous area and wings that form a generally acute angle, rarely obtuse (30°)40°-100°(120°).
Flowering:
March to June.
Fruiting:
September to November.
Habitat:
Forests on generally limestone soils, more rarely on siliceous substrates, in cool and humid environment. Occasionally as subrupicolous. In subhumid to humid bioclimate, on mesomediterranean and supramediterranean floors.
Distribution:
S of Europe, from the Iberian Peninsula to the Balkans; N of Morocco and Algeria, Crimea, Caucasus, Anatolia, Syria, Lebanon and N of Iran.
Observations:
The taxonomy of the aggregate A. opalus is extremely complex due to the existence of numerous intermediate forms between different subspecies. The current tendency is to consider at least 3 subspecies in the western Mediterranean area, up to Dalmatia and the Balkan Peninsula (Sánchez-Gómez & Güemes, in Flora iberica, 2015).
In North Africa 2 subspecies can be distinguished:
A. opalus subsp. granatense (Boiss.) Font Quer & Rothm. (A. granatense Boiss.; A. opalus subsp. hispanicum (Pourr.) Jahand. & Maire; A. hispanicum Pourr.), endemic to the Betic-Rif area reaching the Iberian-Levant mountains to Montserrat (Barcelona) and the Balearic Islands (Majorca). Further towards the N, in the pre-Pyrenean region, there is introgression with the type subspecies. In North Africa it is locally common, even abundant, in the limestone mountains of the western Rif, forming part of the mixed forest of firs, cedars and oaks (Quercus faginea). It differs from the other subspecies of the western Mediterranean by presenting usually smaller leaves, up to 8(10) cm, upper lobes with ± deep and parallel sinuses, and smaller fruits, with wings forming an acute angle up to 70°.
A. opalus subsp. neapolitanum (Ten.) Sánchez-Gómez & Güemes (A. obtusatum Waldst. & Kit. ex Willd.), present in Algeria, where it forms small populations (eastern Tellian Atlas from the Tazekka Massif to the Edough Massif). This subspecies is thought to have reached Algeria through the Italian Peninsula, where it is most represented, and it is recognised for its very large leaves, up to 18(20) cm, densely tomentose on the underside and large fruits that form a more obtuse angle than the other subspecies.
In Algeria, also, A. opalus s.l. is found in isolated populations (Djurdjura, SW of Algiers, in Zaccar and Schuler Cheliah, in Aures Massif) and, even rarer, A. hyrcanum Fisch. & C.A.Mey. has been found in the last location (Quézel & Santa, 1963). These populations are deviants, unlike subsp. Neapolitanum, and they correspond to forms of somewhat smaller leaves and glabrescent undersides; depending on the authors, they have been interpreted as belonging to the type subspecies (subsp. opalus), or as introgressed forms with A. monspessulanum (Quézel & Santa, op. cit.). As a whole, although they have a converging leaf morphology with the type subspecies, they should be interpreted as isolated populations of ancient origin or, perhaps, populations introgressed with any of the above subspecies for North Africa. In fact, in the Moroccan Rif populations, deviants specimens of the subsp. granatense can also be seen, that could have gone through introgressive processes.
Conservation status:
Common and widespread species. In the IUCN Red List of Threatened Species Acer opalus s.l. is listed as Least Concern (LC) at global level (Crowley et al., 2018). In the Livre Rouge de la flore vasculaire du Maroc (Fennane, 2021) it has been considered as Vulnerable (VU). In Algeria it is included in the List of protected non cultivated flora (Executive Decree 12-03 on 4-Jan-2012).