Quercus acerifolia (E.J.Palmer) Stoynoff & W.J.Hess ex R.J.Jensen

Maple-leaf Oak has become something of a rock star for oak groupies. Found only on a handful of bluffs in Arkansas, it was considered for many years one of the rarest oaks in the United States. Add to that a long-standing controversy regarding its status as a species or variety, and some attractive features (pleasing habit and dimensions, unique leaf proportions, and outstanding autumn colour). It has been better known in the US than in Europe, but that is changing, and concern for its Endangered status has meant it has been well collected and distributed in ex-situ collections in several important arboreta.

E.J. Palmer first discovered this oak in 1923 on Mount Magazine, Logan Co., Arkansas. He described it as a ‘very distinct-looking Oak, in appearance quite unlike any other I know of’ but treated it as a variety of Q. shumardii. Despite characteristics that set it apart from other red oaks (leaves broader than long, small acorns with shallow cups, generally shrubby habit), he hesitated to describe it as a species till it could be studied further (Palmer 1927). Its status has not been free from controversy. Stoynoff and Hess (1990) rushed to elevate it to species status in time to draw attention to what they suggested ‘may well be the most rare species of oak known,’ put further at risk by the creation of a state park on Mount Magazine (while parks are designed to protect natural areas, they also invite human activity that can unintentionally harm fragile species). Johnson (1992, 1994, 1999) was not convinced and reported as Q. shumardii var. acerifolia further findings of the taxon in three other sites in Arkansas. In the same vein, Smith (1994) considered it a ‘phase’ of Q. shumardii. In 1998 Hess and Stoynoff presented a detailed analysis that, in their view, showed the oak to be distinct from its close cousins Q. shumardii, Q. buckleyi and Q. texana and supported its recognition at species level. This did not suffice to persuade Ladd and Thomas (2015), who suggested that it was an upland variant of Q. shumardii, perhaps caused by the environmental conditions of the exposed upland sites. (They further stirred up the hornet’s nest by declaring that the type for var. schneckii, another variety of Q. shumardii, was in fact Q. texana, and that var. acerifolia, as the next available name, should now be applied to everything that had been erroneously known as var. schneckii.) However, most authors now agree that Q. acerfiolia deserves species status, and lately Wu (2022) showed that even ex-situ specimens continued to display the morphological characteristics that distinguish this taxon, indicating that they are mostly genetically conserved, rather than environmentally induced.

For many years Mount Magazine was the only known location for this species. Other sites in Arkansas were found by George P. Johnson in 1991 (Pryor Mountain, Montgomery Co. and Sugarloaf Mountain, Sebastian Co.) and 1993 (Porter Mountain, Polk Co.) (Johnson 1992, 1994). Russell (2020) reported finding specimens in two additional locations on private property in Ouachita National Forest, one of which, near the Arkansas-Oklahoma border, held over 100 specimens; it is possible that more sites will be located in future.

No sooner was this oak discovered than it was in cultivation. Palmer’s type collection, which he collected on a second visit to the Mount Magazine site, the year following his discovery, included seeds that were subsequently germinated at the Arnold Arboretum. A tree from this seed grows there; it has reached a dbh of 49 cm (Arnold Arboretum 2022), but is badly shaded. A seedling propagated from this tree’s acorns was planted in 1950 at the Morton Arboretum and has developed well; it is likely the finest specimen in cultivation, with a nicely rounded crown from heavy basal limbs (Grimshaw & Bayton 2009). Though the species is usually multistemmed, this specimen at the Morton has a single trunk with lateral limbs emerging quite close to the ground. It usually displays attractive autumn colour.

It is grown in several collections in the US, mostly from wild-origin accessions. Specimens from a 2017 wild-collected accession are held in a consortium including the US National Arboretum, the Morton Arboretum, the Dawes Arboretum, and the Arnold Arboretum. At Stephens Lake Park Arboretum in Columbia, Missouri, a conservation grove of Q. acerifolia, consisting of wild-origin seedlings and grafted trees using wild-collected scions – all sourced from the currently known locations of the species – is currently in preparation (Russell 2020, Arnold Arboretum 2022).

Quercus acerifolia is rarer in Europe but can be found in specialist collections including Buckingham Palace (8.2 m × 23 cm dbh in 2017), Sir Harold Hillier Gardens, grown fom Arkansas seed collected in 1989 (7.2 m × 23 cm in 2017, up from 5.1 m × 12 cm in 2006, but it was 6.6 m × 25.4 cm in 2023) and Chevithorne Barton (2 m tall in 2012) in the UK (The Tree Register 2022, Grimshaw & Bayton 2009, The Oaks of Chevithorne Barton 2022). At Arboretum des Pouyouleix, France, there are two trees, one grafted which fruits regularly and was 7.5 m tall and 22 cm in diameter in 2021, and one of wild source collected in 2011 from shrubby plants on the summit of Pryor Mountain in the Ouachita Mountains, Arkansas, that was 6 m × 6 cm in 2021 and fruited for the first time in 2020. It is also at Arboretum du Passadou (France), Iturraran Botanic Garden (Spain), and Trompenburg Arboretum (Netherlands).