CA, Southern California, 20070620
Madison Heights, lecanora. 

Finally studying this damned lichen properly, using key in Nash, proper razor for sectioning, proper cover slip so I can use 1000x objective, and Lugol's to stain ascus tip.

Lecanora hagenii [] [voucher] 

Fillmore, between Euclid and Alley, N side, stump of camphor that was just cut down.  Up until recently, was shaded, and at the edge of the daily sprinklers' spray zone, on bark.  Pretty nice growth of it, actually, although still nothing like the population in front of the Huntington Hospital.

THAL

either endophloeic or highly degraded, lt grey around apo rims, K-, C-, P-

MED

white

ALG

green

APO

lecanorine, rims eroding from rough to jagged to almost non-existent, very crowded, heavily white pruinose over sl reddish brown, epihym brown, hymen clear

ASC

thick unfeatured uniformly stained tip

SPORE

8 per ascus, clear, lots of bubbles, 1 cell, ellip, 9-10x5-6um

Still don't have any polarizing filters, so can't observe crystals.  Keys to either L. hagenii or L. hybocarpa in Nash.  Former seems more likely, since the latter is supposed to have a visible thallus and K+ yell (according to Brodo). Lastly, Nash shows record on L. hagenii in LA.

Poking about on the web turns up a report about lichens in Zmir, Turkey, in which they show that L. hagenii is "extremely toxitolerant", indeed it "seems to be the most toxitolerant species in the study area". [1] And I found a few additional mentions of it being singled out as an example of a pollution-tolerant species.  Apparently it is thought to be nitrophytic, i.e. require or at least tolerate high levels of nutrients (including nitrogen).

Other tolerant species include L. dispersa, Physcia adscendens, and Xanthoria parietina.  Pollutants include SO2, NOx, O3, particularly SO2. Also Amandinea punctata, Lecidella elaeochroma, Phaeophyscia nigricans, Physconia grisea, Punctelia ulophylla, Xanthoria polycarpa.  I'll bet the black dot one (on Magnolia just N of Alpine on W side, and on Hillcrest and Encino at the base of that clump of palms) is Amandinea.  But none of these looks like the gold dust  Chrysothrix candelaris, presumably.  (Nash doesn't have any record of Candelariella efflorescens in LA.) Wait, Psilolechia lucida is another leprose gold lichen, which is apparently "relatively insensitive to... pollution". Chrysothrix candelaris is "normal" to "moderate" on the Hawksworth/Rose scale.  Hawksworth/Rose 1970 did study on recolonization of lichen flora of London after air quality has improved. Acidification, eutrophication.  Releve'.  SO2 no longer sole factor.

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