Acer rufinerve (grey snake-bark maple)

Acer rufinerve, commonly known in English as grey snake-bark maple or redvein maple or as urihadakaede (melon-skin maple) in its native Japan, is a medium-sized deciduous tree reaching a height of 15-20 m. This early successional species thrives in forest gaps and edges and in the understorey of open woodlands. It is planted in numerous arboreta and green areas in temperate climatic conditions. It is a very adaptable tree species that may regenerate vegetatively or through the numerous seeds it produces. Although this species is traditionally not considered to be invasive in neither its native nor introduced ranges, recent field observations in three Belgian acidic forests revealed that it may be an aggressive colonizer, strongly regenerating and producing dense thickets up to distances exceeding 50 m from seed trees. Species richness of the herbaceous layer and regeneration of light demanding tree species are strongly reduced in these areas. Similar invasion events are likely to occur in other countries sharing similar eco-climatic conditions to those found in Belgium.

Acer is one of the largest tree genera in the broadleaved deciduous forests of the northern hemisphere, with about 200 species. Species of Acer have been subdivided into various sections. Acer rufinerve, first described and published by Philipp Franz von Siebold and Joseph Gerhard Zuccarini in 1845, belongs to section Macrantha, which also includes other weedy species like A. capillipes, A. davidii, A. grosseri and A. pensylvanicum. This monophyletic group of 18-22 species can be distinguished from those of other sections by striped bark, stalked winter buds, two pairs of bud scales with the outer one covering the inner completely, and racemose inflorescences with two subtending leaves. Section Macrantha is mainly distributed in eastern Asia, with a single species, A. pensylvanicum, in eastern North America (Zhang et al., 2010).

A. rufinerve is a medium-sized deciduous tree growing to 10-15(-20) m. The mature bark is grey-green with narrow grey-white longitudinal stripes. On older specimens the stripes are intersected by diamond-shaped beige patches. Branches are green and new shoots are glaucous, with a bluish-grey bloom. Leaves are opposite, 8–15 cm long and wide, with either 3 or 5 shallow lobes. Where there are 5 lobes the lower ones are small and indistinct, whereas the main lateral lobes are large, forward pointing and acuminate. The terminal lobe is slightly larger than the main lateral lobes and also acuminate. Leaf bases are weakly rounded to shallowly cordate, the leaf margin is irregularly double-serrate and petioles are (2-)5(-6) cm long. The underside of the leaf is paler than the dark green upper side. Leaf veins are impressed on the upper side, and on the lower side covered with brown to rust-coloured hairs, particularly in the axils of the veins, though these disappear with age. Flowering occurs in April and May either at the same time or after leaf emergence. Flowers are in terminal, 5-10 cm racemes with 10-20 flowers. Sexual expression is complex, with some trees changing sex with age, so that in a given population dioecious and monoecious trees can be found. The 5 petals and 5 sepals are pale lime green, held on a short 1-5 mm pedicel. Male flowers have 8 stamens, shorter than the petals and sepals. Samaras are 2-3 cm long with wings spreading at angles of 70-120° (Yamazaki et al., 1993; van Gelderen et al., 1994; Ushimaru and Matsui, 2001; Nanami et al., 2004).

A. rufinerve is native to Japan where it is widely distributed in the temperate zone; it is found on the islands of Honshu, Shikoku, Kyushu and Yakushima in mountain forests up to 2500 m elevation (Ogata, 1999; van Gelderen et al., 2004).

This species, probably the most common snake-bark maple in cultivation worldwide (van Gelderen et al., 1994), has been introduced into North America, Europe and Australia and is cultivated for ornamental purposes in public and private, as well as botanical gardens (BGCI, 2014; University of Copenhagen, 2015) in climatic zones ranging from dry to temperate and cold. However, it is described as invasive in Belgium (Rafalowicz et al., 2009; Branquart et al., 2011) and potentially so in Denmark (Nielsen and Leverenz, 2002). 

A. rufinerve has been introduced from Asia as an ornamental plant species to many botanical gardens around the world. It is available as an ornamental from the catalogues and websites of commercial nurseries and botanical gardens (Halford et al., 2011; BGCI, 2014). It was introduced to nurseries in Denmark in 1880 (University of Copenhagen, 2015) and Belgium in 1881 (Halford et al., 2010). The first reported observation in the wild outside its native range dates back to 1990 in Belgium (Branquart et al., 2011) where, in Hainaut province in the west of the country, a few A. rufinerve trees had been planted by foresters in the Bon-Secours forest near Mons in the period 1950-70. By the first decade of the 21st century they had colonized over 60 ha (Rafalowicz et al., 2009). Possible invasive populations have also been observed in forests in Soignes and Halle (Halford et al., 2010).

A. rufinerve is a common ornamental tree in Europe, North America and Australia. It is cultivated in areas with dry, temperate and cold climatic conditions and has shown itself able to naturalize in temperate regions (e.g. Belgium). Considering the species’ availability from commercial nursery catalogues and websites (Halford et al., 2011), the risk of secondary introduction into the wild is potentially important. In Denmark, the species was mentioned in 2002 as noteworthy for its locally abundant natural regeneration close to source trees in Hørsholm Arboretum, indicating its potential ability to spread in that country (Nielsen and Leverenz, 2002).

Young stems of A. rufinerve form very dense thickets wherein few herbaceous plant species are able to grow. Forest light-demanding herbaceous species like Lonicera periclymenum, Luzula spp. and Pteridium aquilinum are likely to be outcompeted by the invasive tree. However, bramble (Rubus fruticosus) often co-occurs with it, sometimes at high densities. A. rufinerve also significantly decreases natural regeneration of heliophilous tree species like Quercus robur and Fraxinus excelsior (Rafalowicz, 2009; Rafalowicz et al., 2009).

There are several striped-bark Acer species within section Macrantha with which A. rufinerve could be confused, including A. capillipes, A. davidii, A. grosseri and A. pensylvanicum. These species, however, do not display the two following diagnostic criteria of A. rufinerve: rust-coloured hairs on veins on the underside of leaves and the dense coverage of young shoots by a soft bluish-grey bloom with no trace of red. A. capillipes is distinguished from A. rufinerve based on its red petiole, some redness to the bark, less forward-pointing leaf lobes and lack of rust-coloured hairs on veins. A. davidii has entire or only slightly 3-lobed leaves, but can have rufous pubescence on the underside of young leaves. A. pensylvanicum and A. grosseri are similar but lack the red hairs on the underside of leaves and white-blue bloom on young growth (van Gelderen et al., 1994; Vertrees and Gregory, 2010).

Grey snake-bark maple is an early successional and light-demanding species which occupies open habitats, such as forest edges, canopy gaps and also the sub-canopy in secondary forests. Its abundance decreases as succession proceeds (Sakai, 1987; Masaki et al., 1992; Matsui, 1995). It is very shade tolerant at the seedling and sapling stages, but needs the better light conditions found in forest gaps to grow larger (Hiroshi Tanaka, pers. obs.). In its native range, it grows in the middle and upper parts of mountain forest slopes, up to an elevation of 2500 m (van Gelderen et al., 1994; Vertrees and Gregory, 2010).

In Japan, recruitment of A. rufinerve is rather low in spite of strong seed production and is limited by pre-dispersal seed mortality due to predation by larvae of the weevil Bradybatus limbatus (Tanaka, 1995). The new aphid species Trichaitophorus koyaensis (shown by Sugimoto (2013) to be the accepted name for both Periphyllus montanus and Trichaitophorus takahashii) was described from A. rufinerve by Takahashi in 1961, while the root lesion nematode Pratylenchus penetrans was catalogued as a pest of the tree's roots by Goodey et al. in 1965.

Although the extent of economic losses due to the presence of A. rufinerve in managed forests are not known, the costs of management and loss of timber production are not negligible (Halford et al., 2010). Potential impacts due to invasion by the species are poorly understood and difficult to assess because interactions with other tree species need to be studied in the context of long-term plant succession and forest dynamics. As the number of invaded sites is limited and management actions are undertaken to halt invasion, any study of long term vegetation dynamics is difficult. In Belgium impact information may be derived from field observations performed at invaded sites. 

Under acidic growing conditions, A. rufinerve where invasive exhibits prolific regeneration and produces dense thickets up to 50 m from mother trees, usually in the vicinity of forest gaps or under light canopy with intermediate light conditions. As a result, the abundance and species richness of the herbaceous layer is strongly reduced, and regeneration of light demanding tree species is inhibited in areas where these dense thickets form (Rafalowicz, 2009; Verzele, 2010; Branquart et al., 2011).

In its native range, however, dense populations are observed infrequently and A. rufinerve does not form monospecific stands, probably due to control by seed predators and other natural enemies. It is not especially considered as a specific nuisance by foresters although it may exhibit weedy behaviour in young forest plantations (Hiroshi Tanaka, pers. obs.).

Different techniques have been tested in Belgium to control A. rufinerve in invaded forest areas. Due to a shallow rooting system, young stems up to 4-5 cm in diameter can be easily hand pulled (and destroyed). Hand pulling of larger stems is more difficult and could be replaced by cutting combined with chemical treatment of stumps to avoid sprouting. Hand pulling could become very laborious when treating large areas. In such situations, mechanical soil crushing up to a depth of 25 cm may be favoured, but superficial mulching has to be avoided because of the high sprouting capacity of the plant. As soil crushing generates strong soil perturbation, it has to be avoided in protected areas and at sites where this practice may facilitate the development of other invasive plants (Frisson et al., 2010). As with the control of Prunus serotina and other light demanding invasive trees, single stem harvesting has to be favoured over clear-cutting, and shade tolerant trees have to be encouraged in order to reduce light transmission and outcompete invaders (Jacquemart et al., 2010; Invexo, 2012).

Considering the intensive use of A. rufinerve as an ornamental plant in private, public and botanic gardens as well as the high regeneration and dissemination dynamics observed in Belgium, the propagule pressure is expected to be high. The species is certainly not studied enough and its occurrence in managed and natural forests is probably underestimated. In particular, species capacity to escape from cultivated areas should be further investigated, as well as factors potentially limiting its development in the adventive range.