The species belongs to a genus with two further species, both being parasites of Hypoxylon species.

Chlorostroma vestlandicum was discovered in 2010 and described from western Norway in 2014. After surveying a large number of sites throughout western/southern Norway in the period 2011-2020 it is currently known from 23 sites (at least 1km apart). Recently, it was also discovered from three sites in western (European) Russia (Popov et al. 2021). C. vestlandicum is a fairly conspicuous and characteristic stromatic pyrenomycete, and is probably a rare species with a limited distribution. It is a highly distinctive species due to a strikingly orange entostroma, a specialized association with the wood-decay fungus Hypoxylon vogesiacum, and is only found in connection with this on dead wood of Ulmus glabra (Wych Elm). In Norway, H. vogesiacum and U. glabra are nationally Red Listed (as Near Threatened and Endangered respectively). The substrate and supposed fungal host of C. vestlandicum will in the future be declining in its main distribution area. Dutch Elm Desease (DED) affects so far only the populations in southeast Norway (one locality, Vestfold) and has not spread to western Norway, but this disease is a significant future threat to C. vestlandicum in its entire distribution area.

There is an estimate of 150 localities in Norway. Outside Norway we estimate the species to be present in fewer than 100 localities. Assuming on average 10 mature individuals/site, this gives an estimate of fewer than 2,500 mature individuals globally.

C. vestlandicum is assessed as Endangered in Norway and not evaluated in Russia. The known population is completely restricted to smaller parts of northern Europe. In its distribution range, it is considered to suffer from a decline in host trees and in the associated H. vogesiacum of 15-30% over three generations (30 years) in the present and future caused by fungal pathogens, Red Deer damage to elm trees, and competition to U. glabra from the invasive Acer pseudoplatanus. C. vestlandicum is assessed as Endangered C2a(i) based on an estimated total population not exceeding 2,500 individuals, an ongoing decline in habitat and population and small subpopulations (<250 mature individuals).

Chlorostroma vestlandicum is known from Norway and Russia. In Norway, 22 of 23 sites are lying in western Norway (including mid-Norway). The last one is an isolated site in southeast Norway. In addition three new records were recently found in European Russia not far from St. Petersburg (Popov et al. 2021). Since C. vestlandicum is a prominent species it would have been described earlier if widely distributed in Europe. It most probably has a much narrower distribution than its host, possibly defined by climatic and/or historic factors.

Chlorostroma vestlandicum was first found in western Norway in 2011. The host of C. vestlandicum, the fungus Hypoxylon vogesiacum, is nationally Red Listed in Norway as Near Threatened, while the tree, Ulmus glabra, which provides the dead wood on which parasitized H. vogesiacum grows, is nationally Red Listed in Norway Vulnerable (Artsdatabanken 2021). U. glabra has been estimated as having an ongoing decline of 30-50% (or maybe 50%) over 90 years. H. vogesiacum has its largest European population in western parts of Norway, where old trees of U. glabra not yet exposed to Dutch Elm Disease (DED) are still common. H. vogesiacum is here estimated as having a decline of 15-30% in 30 years. Within Europe, generally, populations of U. glabra are also declining (10-40% in the past century; 30% in the coming century), and the species is assessed as Vulnerable (Rivers 2017), while globally, U. glabra is Data Deficient (Barstow and Rivers 2017). In western Russiam DED is present with serious impact on urban trees of U. glabra, but good data on wild populations of the tree are lacking (Popov et al. 2021). Based on this, C. vestlandicum is suspected to have an ongoing and future decline of 15-30% during three generations (30 years). Its habitat is under serious threats (see threats).

There are an estimated 150 localities of C. vestlandicum in Norway, based on many searches over the last 10 years. Outside Norway we suppose fewer than 100 localities. Applying the recommended template for assessing wood inhabiting fungi (10 individuals/site; Dahlberg and Mueller 2011) this results in an estimate of fewer than 2,500 mature individuals, in small subpopulations of fewer than 250 mature individuals.

Chlorostroma vestlandicum is associated with, and probably parasitic on Hypoxylon vogesiacum. Its distribution and population size are probably partly delimited by the distribution and population size of its fungal host. C. vestlandicum is found on coarse logs of old trees or on fallen branches of Ulmus glabra, or on dead parts of still living trees. Most Norwegian, and at least one of the three Russian collections, were associated with stromata of H. vogesiacum (Popov et al. 2021). According to currently available information, C. vestlandicum has a much narrower distribution than H. vogesiacum. All findings of C. vestlandicum in Norway were made in deciduous forests with U. glabra, up to 320 m above sea level. The species (like its hosts) seems to prefer relatively warm summers. H. vogesiacum was described from France and is furthermore reported from Sweden, Finland, Austria, Switzerland, and Spain (additional records from North America and Asia could be other species), but C. vestlandicum has never been found in these countries. H. vogesiacum has a hemiboreal distribution in Nordic countries (Granmo et al. 1989, Granmo 1999, Nordén et al. 2014). In Norway, C. vestlandicum is distributed in the warmest parts of southern Norway north to Trøndelag, with its largest populations in western Norway, one of the few regions in Europe where Dutch Elm Disease is still absent.

The main threats are from the decline of suitable Ulmus logs due to changes in habitat management, competition from the invasive Acer pseudoplatanus, and an increase in Red Deer (Cervus elaphus) population affecting re-establishments of new deciduous trees. Dutch Elm Disease (DED) is also an obvious threat which will kill all host trees if, as predicted, it reaches the habitats of Chlorostroma vestlandicum in western Norway (hitherto not discovered here). Such threats are intensified by the species' small and fragmented population.

There is no use or trade of this species.

Both Ulmus glabra and Hypoxylon vogesiacum are nationally Red Listed and declining in Norway (as Endangered and Near Threatened respectively; Artsdatabanken 2021), and Chlorostroma vestlandicum (Endangered in Norway) therefore warrants attention in conservation work. All known sites are in need of area-specific protection and management plans. The known localities are very rich in other rare and nationally Red Listed species. Monitoring the spread of Dutch Elm Disease (DED) is important to stop it from approaching the core area of distribution of C. vestlandicum. Control of excessive Red Deer populations and halting spread of invasive Acer pseudoplatanus are also desirable.

More information is needed about the life strategy of C. vestlandicum and its connection to H. vogesiacum. Since C. vestlandicum is a prominent species it would have been described earlier if widely distributed in Europe. It most probably has a much narrower distribution than its host, H. vogesiacum, and U. glabra, the key associated tree, and that narrower distribution is possibly defined by climatic, historical or ecological factors. C. vestlandicum should be searched for in other areas with a similar climate in Europe. H. vogesiacum is also found on other trees, including Fraxinus excelsior, but C. vestlandicum is only encountered on U. glabra. Can it also live on other trees? Knowledge of the population dynamics of U. glabra and H. vogesiacum has many gaps which should be filled.