Gymnocarpium Ferns of PNW
Gymnocarpium disjunctum and G. dryopteris are very hard to distinguish reliably from photos. The only perfectly reliable guide is based on genotype (G. dryopteris is an allotetraploid with one of the parents being diploid G. disjunctum).
G. dryopteris leaves are generally smaller, but hard to distinguish from young G. disjunctum.
The pinnules (leaflets) of the second pinna are usually highly asymmetric in size in G. disjunctum but similar in size or only mildly asymmetric in G. dryopteris. See https://www.inaturalist.org/observations/176866915. According to Hitchcock and Cronquist, >1.5x length difference is G. disjunctum and less is G. dryopteris. I would say that if they are nearly equal in length call it G. dryopteris and if they are >2x in length call it G. disjunctum, otherwise leave ID at genus level.
The basal pinnae of mature G. disjunctum are often 3-pinnate or 2-pinnate with moderately to deeply lobed terminal leaflets (3-pinnatifid).
Both ferns usually present in the field as somewhat dispersed groups of single fronds, each arising from a long wandering rhizome. If you can observe the second pinna pair in multiple such fronds the distinction should be clearer but it is annoying to photograph lots of them.
There are other small differences listed in Hitchcock and Cronquist but they are even subtler or microscopic (spore size).
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When I'm photographing Gymnocarpium I try to look over the plants in a grouping and select ones that are fully mature, with sori. Photographing immature leaves often leads to ambiguous observations that can only be identified to genus.
In addition to the asymmetry of the basal basioscopic and acroscopic pinnules of the 2nd pinnae, mature leaves of G. disjunctum are more divided, 3-pinnate-pinnatifid. Immature leaves, and the leaves of G. dryopteris are 2-pinnate-pinnatifid.
The size of the leaves can be helpful, although there is overlap. G. dryopteris leaf blades are 3-14 cm, with 9-28 cm petioles. G. disjunctum leaf blades are 8-24 cm, with 12-44 cm petioles.
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