Clevea hyalina Lindb.

Clevea hyalina (Sommerf.) Lindb.

SPECIMEN EXAMINED. — Tunisia. Tunisian Dorsal, Siliana Governorate, Delegation of Siliana South: Jbel Serj, N side in the Park, sterile, 35°56’33.65”N, 09°32’57.28”E, 1018 m a.s.l. (site 2020- 32), 12.III.2020, Ben Osman & Hugonnot (TUN[TUN2020-8]).

Clevea hyalina grows in the Jbel Serj National Park on the north side of a calcareous cliff covered by rupicolous and grazing-resistant plants, on a sub-horizontal soil pocket. This strongly calcicolous species is known to occur in two distinct latitudinal belts, corresponding to two extremely dissimilar habitats. In the Arctic region, it occurs on polygons and solifluction slopes with such cold-adapted species as Arnellia fennica (Gottsche) Lindb. and Sauteria alpina (Nees) Nees (Schuster 1992). In warmer regions, at low elevations, it is usually restricted to steep, shaded, densely-forested slopes and cliff bases (Schuster 1992; Damsholt 2002), which remarkably, is the same ecological setting as the one observed in Tunisia. Associated species include xerothermophytes such as Encalypta vulgaris Hedw. and Targionia hypophylla L.

In this context, there are two major categories of strongly disjointed distributional ranges. Firstly, the so-called ‘arcticalpine taxa’ occur both in boreo-arctic regions and at high elevations on the southernmost mountain ranges (Stevanović et al. 2009). Their disparate ranges are thought to be the product of the warming at the end of the last glaciation, which triggered both northward and upward migrations of the populations occupying previously the Europe’s lowland steppes. Secondly, taxa at different levels of ploidy often present disjointed distributional ranges that could be associated with different ecological requirements. The proportion of polyploids in mosses also increases with latitude in North America as it does in Europe (Kuta & Przywara 1997), and this can be attributed to the heightened ability of polyploids to colonize northern deglaciated areas following the last glaciation (Brochmann et al. 2004). The disjointed distribution of Clevea hyalina may correspond to one (or both) of these patterns, but it is clearly the heritage of its ancient distribution in the Glacial Age.

The species is characterized by its long ventral hyaline scales projecting beyond the margins at the thallus apex, and the blackish ventral scales. The cells surrounding the pores are stellate with thickened radial walls. The Tunisian specimen was sterile.

Jovet-Ast & Bischler (1971) studied intensively the liverwort flora of Tunisia but failed to record Clevea hyalina. One hypothesis is that the species is quite difficult to spot in the field during the dry season (Rubasinghe 2011) or genuinely rare and localized, as indicated by the paucity of North African reports. The species reaches its southern distributional limit in the Maghreb. The species is also present in Algeria and Morocco (Ros et al. 2007).

Clevea hyalina is a circumpolar arctic-montane species occurring in Europe (Hodgetts & Lockhart 2020), North Africa (Bischler 2004; Ros et al. 2007), Central (Rubasinghe 2011) and North Asia (Borovichev & Bakalin 2013), and North America (Schuster 1992).