Phegopteris excelsior and P. connectilis are very similar and there is overlap in the distinguishing morphological characters, so we placed particular emphasis on spore size for species determination (see Patel et al. 2019a for measurements). Phegopteris excelsior tends to have spaces between all of its pinnae (Fig. 1), whereas in P. connectilis they are closer together, with their pinnae filling the frond's plane (Gilman 2020). We found this feature particularly helpful in the field and herbarium as a rapid character to select candidate P. excelsior, which we then verified by taking spore measurements using scanning electron microscopy. We aimed to image and measure at least 10 spores from each specimen. The images for Chapman-Lam 5753 did not show enough spores, but it is one of several P. excelsior collections made in Bilberry Creek, making the species otherwise well documented from the site. Overall, average spore length in our P. excelsior collections ranged between 62.54 – 69.95 μm, with extreme minimum and maximum lengths of 54 and 80 μm, respectively (Table 1). Patel et al. (2019b) summarised spore ornamentation in the Thelypteridaceae, which can sometimes help distinguish taxa. No distinctions in perine morphology between P. connectilis and P. excelsior were readily apparent to us (Fig. 2), although this was not a focus of our study.

The ecological conditions at these newly reported P. excelsior sites are consistent with those previously reported (Patel et al. 2019; Gilman pers. comm. 2023). Generally, the new records are from deeply shaded toe slopes, on banks just above streams or lakes. The Algonquin Park specimen differs in being collected from igneous rock along a road cut. If north-facing, it could possibly provide similar cool conditions.

More work is required to determine the distribution, abundance, and conservation status of P. excelsior. NatureServe (2024) has calculated some provincial status ranks in Canada (see Master et al. 2012 for rank definitions): P. excelsior is considered of conservation concern in Quebec, where its provincial status rank is S3? (Vulnerable), and in Prince Edward Island it has now been ranked S1S3 (Critically Imperilled to Vulnerable). However, NatureServe (2024) has not yet provided an overall global status rank, and its rank is either SU or SNR (Unrankable or Not Ranked) in nearly all provinces and states where it is known. This indicates a high degree of uncertainty around its conservation status both in those jurisdictions and globally. The discovery of three widespread eastern Ontario occurrences demonstrates we have much to learn in Ontario as well. Its habitat is much more widespread than the presently documented distribution of P. excelsior, suggesting there could be additional occurrences elsewhere in central and northern Ontario.