This is a large plant, around 6' tall and fairly wide. Note the large black-charred burl at the plant base, which survived a fire and resprouted into the now robust plant seen here. I wonder how old the present above-ground incarnation is? And also how old the continuously-living clonal entity is...starting from its initial germination from a seed (i.e. from prior to the fires it's survived)?

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Historically, this was the fifth species of Arctostaphylos to be recorded from San Bruno Mountain, and its occurrences there are few. It did not appear in the original 1968 edition of ''A Flora of the San Bruno Mountains'', since it wasn't found and vouchered on the mountain until 1972 (see the two earliest vouchers here).

Manzanitas can be notoriously difficult to place to species, as variation and overlap of character-states can sometimes make key couplet choices too ambiguous to clearly interpret & decide. Such character-state variation & overlap also makes it very difficult to construct a consistent and workable key! And as problems with keys (and characterizations) are recognized, revised treatments appear which can impact circumscription of species, and thus specimen ID's. All these factors come into play here.

Note that vouchers from this population were initially determined as A. crustacea in 1972, and then later annotated as A. glandulosa in 1979 by the well-respected Arctostaphylos devotee Walter Knight...which is how the plants were listed in the 1990 CNPS paperback edition of 'A Flora of the San Bruno Mountains'. More recent ID efforts by various workers, using updated treatments, have placed the plants back in A. crustacea again.

The critical key characters separating A. glandulosa and A. crustacea occur at couplet 2 in the Jepson eFlora 'Key to Arctostaphylos':

2. Leaves with stomata generally only abaxially, surfaces generally differing in color and/or hairiness

vs.

2' Leaves with stomata on both surfaces, occasionally fewer adaxially, surfaces generally the same in color and/or hairiness (except Arctostaphylos pacifica)

...where, for emphasis, I've displayed the ''wiggle-room'' qualifiers 'generally' and 'occasionally' in red.

Note that those key characters are often difficult to both ascertain and interpret. The stomata of Arctostaphylos leaves typically appear as exceedingly tiny pin-pricks on the leaf surfaces and are treacherously easy to overlook...especially when they're obscured by the presence of hairs. And the gradations of hairiness in a nominally 'hairy leaf' can very from obvious to virtually nil, especially if you have a species with 'glabrate leaves' (meaning the hairs abrade or fall off over time, a fairly common occurrence). These factors, together with the presence of the 'wiggle-room' qualifiers, can sometimes make an objective decision at a key couplet impossible. In such cases, the best approach is often to simply 'go both ways' and see which alternative yields the result that best reconciles with the plant material at hand. In the successful scenario, following one alternative will eventually lead to one or more inconsistencies; while the other will go through...and you can then check your specimen against the candidate species description to verify the determination. That's how I arrived at A. crustacea ssp. crustacea here.

Interestingly, this is the only manzanita species on the mountain which is commonly found at many other stations in the region surrounding the San Francisco Bay. Of the other four Arctostaphylos species on San Bruno Mountain, two are entirely endemic to the mountain (A. imbricata and A. pacifica), another (A. montaraensis) has a few plants on the mountain and is otherwise only known from its main stations on Montara Mountain about 9 air miles to the southwest. The last species, A. uva-ursi, is circumpolar and the most widely distributed species in the genus...but the San Bruno Mountain population is fairly disjunct from other populations (the nearest to the south being along the Big Sur coast of Monterey Co., and to the north at Pt. Reyes, Marin Co.).