The species is closely related to Arthonia ilicina Taylor and Arthonia ilicinella Nyl., but distinguished by thicker apotecia and smaller generally 4-septate ascospores becoming brown and warty when old. It is considered the termophilic vicariant of A. ilicina mainly in the hilly and submountain belts where it grows mostly on Fraxinus ornus L.
This species occurs only locally in humid riparian forests of Central-Southern Italy. It is threatened by unintentional effects from wood harvesting and by fire escaped from agricultural lands.
Criterion A: There are no reports about the trends of population size over long time periods, so it is not possible to apply this criterion. However a past and ceased reduction (57% from 1931 to now) of the habitat is known for one of the locations.
Criterion B: B2 could be applied as the the area of occupancy (AOO) is 36 km2. However, the species does not fit the condition of also meeting two subcriteria. It only fits the subcriterion B1a (severe fragmentation and/or number of location ≤10). However no information is available regarding continuing declines and extreme fluctuations.
Criterion C: No information is available about the total number of individuals of this species. Thus, subcriteria C1 and C2 are not applicable.
Criterion D: The total number of individuals is unknown, the area of occupancy (AOO) is bigger than 20 km2 and the number of locations is 9 (>5). Therefore, the species does not fit this criterion.
Criterion E: No quantitative analyses have been carried out.
The species is assessed as NT until more research on the species, particularly the trends of habitat quality over long time periods, is carried out. It is possible because the taxon meets the area requirements under criterion B for threatened (EOO <20,000 km2 and/or AOO <2,000 km2), it is severely fragmented and occurs at 9 locations but the population is not declining and there are no extreme fluctuations.
This species occurs in three widely disjunct regions of Central-Southern Italy: 1) Latium Coast with a single subpopuation on the Tolfa mountains; Adriatic-Ionian Coas where many subpopulations are severely fragmented and dispersal; 3) Calabria with 3 subpopulations, scattered across a single ridge system (Aspromonte). The total area of occupancy in this ca. 90,000 km².
This species is considered only locally abundant but no detailed assessments of population size and possible decline exist.
Population Trend: Uncertain
Arthonia calabrella is an epiphytic species growing mostly on smooth, base-rich bark of broadleved deciduous trees (e.g. Fraxinus) in humid riparian forests.
Threatened by unintentional effects from logging and wood harvesting (5.3.3. according IUCN –CMP United Classification of Direct Threats) and by fire escaped from established agricultural lands (7.1.1. according IUCN –CMP United Classification of Direct Threats).
The location on the Ionian coast is within the borders of protected areas and it is known a strong reduction of the habitat in previous decades. It is likely that intensive forestry has led to a population decline.
Most (55%) of the locations are within the borders of protected areas.
Additional research is requiring on population size, distribution and trend
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