Acer macrophyllum Pursh

As both its vernacular name and specific epithet suggest, Acer macrophyllum has the largest leaves of any maple, typically up to 30 cm long and wide but they can be more than double this on vigorous or basal shoots. The tree is also one of the largest of all maples; where afforded the required space a crown may spread almost as wide as the tree is tall, and in the view of Bean (1976) A. macrophyllum is ‘in many respects … one of the noblest of maples’. A bold statement, but the sight of a full-crowned, mature specimen can leave a lasting impression. Most striking is to see the tree in the rich rainforests of the Pacific Northwest, where the branches of old trees are often laden with epiphytic ferns, mosses and lichens. The species carries more epiphytes than any other tree species in the Pacific Northwest (Pojar & Mackinnon 2014) and an individual in California’s Humboldt Redwoods State Park, measured at 48 m tall in 2012 and christened ‘Humboldt Honey’, is speculated to be the tallest (living) maple in the world (Vaden 2012). A hollow-trunked tree with a girth of more than 11 m grows, appropriately, in Mapleton, Oregon (monumentaltrees.com 2018).

Utilised for its timber, Acer macrophyllum is used for veneer production though more notably for musical instruments. Wood exhibiting quilted patterning on cut slices, caused by a distorted grain, is particularly valued for guitars (Ondich 2021). The species has also been used for maple syrup, though to a far lesser degree than A. saccharum and its relatives. However, with a flavour distinct from Sugar Maple syrup, interest in Bigleaf Maple syrup is increasing, and it is now offered by some commercial producers (West Multnomah Soil & Water Conservation District 2022).

In cultivation, the species unsurprisingly prefers a maritime climate, though it requires adequate space. Fast growing in youth, in optimal conditions it can attain near maximum height in little more than a few decades. However, such quick growth produces trees with soft, brittle wood that is prone to breakage. Trees sometimes develop a poor branching structure that compounds these issues. Somewhat tolerant of both flooding and drought, but thriving in neither (Niemiec et al. 1995), warmer summers may favour the species in parts of Europe that do not become too dry as a result of climate change. Historically at least, trees in parts of Europe have lacked the necessary heat required to ripen their wood (van Gelderen et al. 1994).

According to several authors (e.g.Bean 1976; van Gelderen et al. 1994; Harris 2000), the species was introduced to Britain by David Douglas in 1826, and like Pseudotsuga menziesii was observed more than 30 years prior by fellow Scot Archibald Menzies (Bean 1976; Mitchell 1996). However, material collected on the Lewis and Clark Expedition of 1804 was grown in North American east coast nurseries, and Mitchell (1996) speculates that alternative introduction dates of 1812 and 1824 may relate to material acquired here by John Fraser (1812) on returning from his last trip to the Carolinas, and Douglas (1824) from his first collecting trip to New England in 1823.

In the often wet and humid conditions of Britain and Ireland Acer macrophyllum has attained a similar stature to that of wild trees, but is unlikely to be as long-lived. In the wild, trees may reach 300 years of age, though the former UK and Irish Champion at Westonbirt, which was thought to represent an early introduction, was felled for safety reasons in 2017. The current UK and Irish Champion grows at Kilcroney House, Wicklow, Ireland with a girth of 5.27 m in 2016 (The Tree Register 2023). At Arboretum Tervuren, Belgium, multiple specimens have exceeded 30 m, the tallest measured at 38.4 m in 2021 (Monumental Trees 2024). A specimen at Bochum-Mitte Stadtpark, Germany was measured at 20 m tall in 2018 (Monumental Trees 2024).

Acer macrophyllum has occasionally attracted the interest of commercial foresters in Britain, with experimental plots established in several locations, though apparently never scaled up to commercial levels (T. Christian pers. comm.). Recent introductions to UK collections include that made by Ben Jones from Redwood National Park, California in 2011 (BMJ 002), examples of which at Westonbirt have reached 7 m tall by 2024 (pers. obs). Collections involving personnel from the Royal Botanic Garden Edinburgh and Howick have sampled A. macrophyllum from British Columbia to California since the mid-1990s, with large numbers of plants originating from different accessions planted at Benmore Botanic Garden, Argyll, where it will be interesting to follow the progress of various provenances (Royal Botanic Garden Edinburgh 2024).

Distinct enough in leaf, Acer macrophyllum is rarely confused with other maples, though it shares the character of lactiferous sap with members of Section Platanoidea, and some species in Section Lithocarpa, within which it has previously been treated. From Section Platanoidea, it is most easily separated by its pendulous inflorescences and nutlets with stinging hairs. From members of Section Lithocarpa it can be distinguished by its lobulate to dentate margins (vs. dentate to serrulate) and buds with 5–8, rather than 8–10, scales. Comparable in leaf size to A. velutinum var. vanvolxemii, A. macrophyllum is distinguished from that species by its lactiferous sap, while the leaves of A. velutinum var. vanvolxemii have coarsely serrate margins.