Chemistry is very variable but the presence of norstictic acid can be used as a diagnostic character in many cases. Treatment of chemotypes also varies by different authors (Purvis et al. 1992; Halonen et al. 1999; Fos & Clerc 2000). The following chemotypes are admitted here: (1) chemotype with norstictic acid (K+ red, Pd+ orange) as main substance and different accessory substances such as other compounds of the stictic acid complex, protocetraric and diffractaic acids is the most common in Europe (Purvis et al. 1992; Halonen et al. 1999); (2) chemotype with salazinic and norstictic acids (K+ red, Pd+ yellow to orange) plus accessory substances has been reported from Fennoscandia (Halonen et al. 1999); (3) chemotype with norstictic and squamatic acids as main compounds was notified from Spain (Fos & Clerc 2000). Two further chemotypes are reported by Tõrra & Randlane (2007): (4) with an unknown substance (Rf class 2 in A, dark fingerprint-like spot in UV 254, blue in UV 366, dark yellow after treatment with H₂SO₄, dark blue in UV 366 after acid and heating, not belonging to the stictic acid complex as checked using two-dimensional TLC compared to Parmotrema crinita) as a main substance (K–, Pd–); (5) without medullary substances (K–, Pd–).
In Estonia chemotype (4) is the most usual (n=12) and chemotypes (1) and (2) are not rare (both n=5) while chemotype (5) has been recorded once (Tõrra & Randlane 2007).

Close to U. glabrescens and U. wasmuthii: they all have shrubby thalli, isotomic-dichotomous branching, and branches without foveolae or depressions. U. fulvoreagens always lacks isidiomorphs and is often characterized by bracelet-like soralia which may totally surround terminal branches, becoming excavate when mature. Both U. glabrescens and U. wasmuthii may have isidiomorphs on young soralia while their soralia are somewhat different in the shape: U. glabrescens has young punctiform soralia, becoming later regularly rounded but still staying discrete; U. wasmuthii has typically oblong-cylindrical soralia. Identification of medullary compounds usually helps to distinguish U. wasmuthii which is represented in Estonia by two chemotypes (with barbatic acid as a main substance, and barbatic and salazinic acids as two main substances). U. fulvoreagens usually contains either norstictic acid or an unknown substance as a main compound, and U. glabrescens contains either norstictic only or both norstictic and salazinic acids as main compounds.

Europe (map legend)
Corticolous. Recorded in Europe: Austria, Belgium, Bulgaria, Czech Republic, Estonia, Finland, Germany, Great Britain, Hungary, Ireland, Italy, Latvia, Lithuania, Luxembourg, Netherlands, Norway, Poland, Portugal, Romania, Russia, Slovakia (probably extinct), Slovenia, Spain, Sweden, Switzerland, Ukraine.

Estonia
Majority of examined specimens grows on conifers (on Pinus 55%, on Picea – 33%), rarely recorded on Betula (8%) and other deciduous trees (4%). This is different from the main phorophites recorded for U. fulvoreagens in East Fennoscandia where 35% of specimens grow on Alnus, 18% on Betula and 13% on Picea (Halonen et al. 1999). Frequent in Estonia (30 specimens), known mainly from the northern coast and south-eastern part of the mainland (Tõrra & Randlane 2007).