I’ve been trying to research this, but I haven’t found the answer on my own.
Both common and pacific silverweeds come up often in observations along the Oregon coast. How can I tell the difference between the two species? At first I thought one was odd-pinnate (A. anserina) and the other even-pinnate (A. pacifica), but now in researching, it appears that perhaps A. anserina is can be either (true?). This older observation is an example of the problem: https://www.inaturalist.org/observations/830168
This plant comes up often in the area where I help with IDs and it would be great to know if there is a way to differentiate between the two.
My impression is the taxonomy is not fully agreed upon by folks (e.g. Argentina vs Potentilla. The FNA treatment considers Potentilla pacifica as a subspecies of Potentilla anserina, but does offer a key to the subspecies here:
The first couplet separates the two taxa you would expect there (ssp. anserina and ssp pacifica):
Epicalyx bractlets equal to or longer than sepals, often 2-fid or dentate; leaflet surfaces: abaxial with cottony-crisped hairs dense, long hairs common to abundant, on and between veins, adaxial glabrous or sparsely to densely hairy; achenes with dorsal groove; inland and seashore plants → A. anserina
Epicalyx bractlets shorter than sepals, usually entire, rarely 2-fid or dentate; leaflet surfaces: abaxial with cottony-crisped hairs absent or sparse to dense, long hairs absent or sparse, on veins, adaxial usually glabrous, rarely sparsely to densely hairy; achenes without dorsal groove; seashore or near-coastal plants → A. pacifica
Not my region of expertise, but looking at nearby references the Jepson Manual adds a couple more differentiating features
Leaf 3–15 cm, generally densely hairy adaxially; pedicel generally 2–7 cm; 1200–2600 m; […subsp. anserina]
Leaf 3–50(75) cm, ± glabrous adaxially; pedicel generally 5–30 cm; generally < 150 m; [… subsp. pacifica]
I looked at a couple vouchers to convince myself that the costal vs mountain difference does not hold in Oregon, and it seems to be generally accepted that both taxa occur coastally in Oregon. The greener upper sides of leaves may be useful in images (as a metric of how hairy adaxially the leaf is) but there may be a lot of observations where the evidence in the picture is simply not enough to determine species. That is the drawback of not having a physical specimen to reexamine, but we shouldn’t be expecting every observation of a difficult group with disputed taxonomy to be fully identifiable.
It’s entirely possible that the species distinction simply does not hold at the Oregon coast. That what are being named as two species are simply polymorphism in one interbreeding population. That is the position that some good botanists have held over the years.