"Iris Cohen" wrote
The two endings have entirely different uses. -ense means from. Planta
chinense or sinense means plant from China. -ana or -anum or -anus means in
honor of. It can be added to the name of a country or a person. There are
many different plants named rothschildiana, honoring the Rothschilds, who
financed numerous botanical expeditions.

Martin Rand schreef
There do seem to be quite a few exceptions to this. And the Latin root of
'-anus' only implies 'of' or 'pertaining to', not particularly distinct from
'originating from' (cf. 'montanus', 'montana').

E.g I'm sure Aira caryophyllea ssp armoricana is named because it's found
in Brittany, not particularly to honour the place.
Argemone mexicana?
Genista monspessulana / Acer monspessulanum?
Limonium transwallianum?

Anyway, St. Louis code:
"60D.1. An epithet derived from a geographical name is preferably an
adjective and usually takes the termination -ensis, -(a)nus, -inus,
or -icus."

(Which fails to mention '-acus' BTW.)

It doesn't seem to make any usage distinctions between these.

+ + +
Indeed there is no distinction in usage, otherwise there would be two or
more forms of epithets ("sinense" and 'sinanum'?!?) for popular geographical
names. As a rule there is one epithet for one geographical name.

There does appear to be a pattern as to what kind of geographical names have
what kind of epithet. I don't believe there is a place in Italy that takes a
"-ensis" ending. The further away from Italy a place is the more likely it
appears to get an epithet ending on "-ensis".
PvR