The following was received via email: I have two Acer pectinatum subsp. forrestii, growing here in Oklahoma, but some photos, do not look like my plants, they have very red stems and petioles, but my plants have single lobe leaves. The photo of your plant at UBC shows minor lateral lobes to the leaf. My plants also show the long arching branches that your plant has (best seen in this photo from Davidsonia. One plant was received from Ok. State Un. and was named Acer caudatum, Ung. and the other I purchased from Heronswood. In the book Maples for the Garden, by van Gelderen, A. pectinatum subsp. forrestii looks like your plant, while the more narrow leaf without a lateral lobe looks like A. pectinatum subsp. maximowiczii, but van Gelderen, does not mention red branches, stems, and petiole associated with A. pectinatum subsp. maximowiczii. I have written to Dan (Hinkley at Heronswood) about this but since we don't have leaves on the tree at this time, he has not responded. Thanks for you thoughts and time.
Your plant probably is Acer pectinatum subsp. forrestii. From the image you supplied, it appears to be relatively juvenile, and as such, is extremely difficult to differentiate from other juvenile forms of a number of other related and unrelated maple species. Complicating matters is the variation that is found among plants of this taxon; even when reproductively mature, different plants mave have significantly different degrees of leaf lobe development, cuticular wax deposition, bark striping, petiole and shoot colour, autumn leaf colour, vigour and overall size. The picture from our website accession #35480 is not typical, but the plant may eventually settle down to a more typical "forrestii" look. If you're not convinced of your own plants' identities, at this time of year you can look for "stalked" buds, which are characteristic of all snake barks. At least then you'll know if you're in the ballpark. After three or four years, plants should have a greater proportion of adult leaves, which should make identification easier. In maples, adult leaves are those that are set down in overwintering buds. In general, there are three to five pairs of these preformed leaves. Snakebark seedlings have leaves that are typically unlobed or barely lobed because they did not have the benefit of preformation (and the presumably longer development time responsible for more complicated leaf shapes). These so-called "neoformed" leaves are typical of seedlings and sucker growth, and are also frequently found at the tips of mature branches, especially where there has been significant summer moisture and subsequent summer growth (i.e., no leaf preformation). This is not the full story (of course); from my observations, I suspect there are varying degrees of preformation in these and other maples. In our experience here at UBC, there is considerable confusion regarding the snakebarks. The Acer pectinatum species complex is more confusing and its taxa more ambiguous than many writers will let on, or realize. We have numerous examples of plants that seem to be intermediate between A. pectinatum forrestii and A. p. maximowiczii (and A. p. laxiflorum, A. p. taronense and A. p. pectinatum) for example. These are not garden hybrids, they are from wild collected seed. There is a fair amount of material sold by reputable nurseries that is of garden origin, and hence, probably hybrid, but I know that Heronswood is very careful about this sort of thing. Acer caudatum (not to be confused with A. caudatifolium -- another snakebark species) has yellowish stems (its subsp. ukurunduense has red stems), unstalked buds and its leaves are characteristically hairy underneath (A. p. forrestii is not hairy).
