I suggest A2c+C1
This fungus causes a brown pocket rot in living Calocedrus decurrens (Incense cedar) trees, and only fruits high up on old (> 200 years) trees. The tree host is restricted to the mountains of Oregon and California (USA) and the fungus is widespread through most of its range but is absent from the drier eastern part of the range. Its fruitbodies are rarely observed and collected, as they are short-lived, and grow high up in the tree. Over-harvesting of old trees, and the drought and consequent wildfires severely threaten the occurrence of mature individuals. Assessed as Vulnerable under criterion A2 (based on a severe reduction of the host trees of an age that supports fruitbody formation), and C1 (based on an estimated number of mature individuals, and a projected and continuing decline of the available host tree, with a decline of at least 10% over the coming 50 years (corresponding to 3 generations of Tyromyces amarus).
The correct name for this species is Postia amara; it is also known as Oligoporus amarus.
Endemic to USA. Only occurring in the Sierra Nevada Mountains and Cascade Range in California, and Oregon, within the range of the host tree Calocedrus decurrens (Gilbertson & Ryvarden 1986), but lacking in the eastern and drier parts of its host tree´s distribution (Wagener & Bega 1958; Wood 2003).
Tyromyces amarus is only found in the range of incense ceder (Calocedrus decurrens) and only fruits in mature over 200 year old trees. Old-growth forests with incense ceder have become very rare. Mycoportal only lists 75 voucher specimens from around 30 collecting events (mycoportal.org, Feb 2018), this is extremely low for an easy to recognize polypore species. There are only 5 observations in the last 10 years posted on Mushroom Observer (mushroomobserver.org, Feb 2018). Tyromyces amarus is estimated to be present on less than 5000 trees, corrsponding to less than 10 000 mature individuals (cf. Dahlberg & Mueller, 2011). The total population is estimated to be less than Mycelial occurrence is often observed as pocket rots in felled trees, but mature, reproducing specimens are very rare. It is apparently a rare fungus which have significantly been affected about declining number of old insense cedars, a decline that is ongoing. Three generations of T. amarus is considered to correspond to 50 years (cf. Dahlberg & Mueller, 2011). The past decline of mature old incense ceder is estimated to exceed 30%. The future decline is projected to exceed 10% in the coming 50 years.
Tyromyces amarus cause pocket brown rot in living Calocedrus decurrens (incense cedar) trees of more than 100 years old (Wood et al. 2003). It occur in the mountains of Oregon and California, on Calocedrus decurrens, though the tree species is more widely distributed (see http://www.fs.fed.us/database/feis/plants/tree/caldec/all.html#DISTRIBUTION AND OCCURRENCE). Fruiting high up in trees that are over 200 years old, and trees that are less than 150 years old are in general free of the fungus (Wood et al. 2003). The fruitbodies are annual conks, and they decay during the winter.
The species is widespread and is commonly seen as a pocket brown rot in felled tree, but it fruits rarely, as it needs old trees (> 200 years old). Logging of incense cedar (high value timber resource), and habitat loss due to climate change, drought, combined with insect infestations, and forest fires are the main threats. Incense cedars that are infected by the species loose their commercial value (Boyce 1920), and logging of trees less than 100 years old is practice, which means that the number of old trees that can harbour the mature specimens of the fungus are rare and have become rare because of the logging of old-growth forests. Forest fires have increased in intensity and scope in the period after 2000 in comparison with the years before that (see e.g. https://en.wikipedia.org/wiki/List_of_California_wildfires), after years of fire suppression (North 2012). Older trees of incense cedar are increasingly absent, so the occurrence of mature individuals will continue to decrease significantly.
Restriction of logging of old-growth forests. Forest fire mitigation measures to prevent massive raging wild fires. Trees of less than 200 years old should not be logged when the fungus is present in them, but left to age so that the fungus can fruit.
Boyce, J.S., 1920. The dry-rot of incense cedar. USDA Bulletin 871 (http://www.fs.fed.us/psw/publications/documents/usda_series/usda_bull871.pdf).
Gilbertson R.L. & L. Ryvarden, 1986. North American Polypores II. Fungiflora, Oslo, Norway
North, M., 2012. Managing Sierra Nevada Forests. PSW-GTR 237.
Wagener, W.W. & R.V. Bega, 1958. Heart-rots of Incense Cedar. Forest Pest Leaflet 30.
Wood D.L., T.L. Koerber & R.F. Scharp, 2003. Pests of the native California conifers. University of California Press.