Zelkova Spach

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Credits

Owen Johnson (2023)

Recommended citation
Johnson, O. (2023), 'Zelkova' from the website Trees and Shrubs Online (treesandshrubsonline.org/articles/zelkova/). Accessed 2024-06-18.

Family

  • Ulmaceae

Common Names

  • Zelkovas

Glossary

Critically Endangered
IUCN Red List conservation category: ‘facing an extremely high risk of extinction in the wild’.
IUCN
World Conservation Union (formerly the International Union for the Conservation of Nature).
Near Threatened
IUCN Red List conservation category: ‘does not qualify for Critically Endangered Endangered or Vulnerable now but is close to qualifying or is likely to qualify for a threatened category in the near future’.
abscission
Shedding of a part of a plant (e.g. leaf flower) due to old age or stress.
berry
Fleshy indehiscent fruit with seed(s) immersed in pulp.
clone
Organism arising via vegetative or asexual reproduction.
drupe
A fleshy dehiscent or indehiscent fruit with one to several seeds each enclosed in a hard endocarp (the stone).
endemic
(of a plant or an animal) Found in a native state only within a defined region or country.
family
A group of genera more closely related to each other than to genera in other families. Names of families are identified by the suffix ‘-aceae’ (e.g. Myrtaceae) with a few traditional exceptions (e.g. Leguminosae).
fastigiate
(of a tree or shrub) Narrow in form with ascending branches held more or less parallel to the trunk.
hybrid
Plant originating from the cross-fertilisation of genetically distinct individuals (e.g. two species or two subspecies).
key
(of fruit) Vernacular English term for winged samaras (as in e.g. Acer Fraxinus Ulmus)
pollen
Small grains that contain the male reproductive cells. Produced in the anther.
pollination
Act of placing pollen on the stigma. Various agents may initiate pollination including animals and the wind.
samara
Dry indehiscent winged fruit usually with a single seed (as in e.g. Acer Fraxinus Ulmus. Also called a ‘key fruit’.
variety
(var.) Taxonomic rank (varietas) grouping variants of a species with relatively minor differentiation in a few characters but occurring as recognisable populations. Often loosely used for rare minor variants more usefully ranked as forms.

Credits

Owen Johnson (2023)

Recommended citation
Johnson, O. (2023), 'Zelkova' from the website Trees and Shrubs Online (treesandshrubsonline.org/articles/zelkova/). Accessed 2024-06-18.

Six species of deciduous tree (one of ancient wild hybrid origin), often suckering. Buds alternate, small, globular to conic; twigs slender, often zig-zagging and never with corky wings or spines. Bark smooth and grey or sandy at first, then often flaking in small shallow scales to show a yellow to reddish underbark, or scaling more coarsely and fissured in age. Leaves often in two flattened rows, ovate to lanceolate, often rough to the touch above and sometimes slightly oblique at the base, the major side veins nearly always terminating in one large tooth; petiole short. Flowers in spring, inconspicuous, yellow-green, scented; male flowers clustered in the lower leaf axils on younger shoots, female and bisexual flowers 1(–2–4) in the upper leaf axils. Fruit a small drupe with some hard dry flesh, ripening in autumn and falling sometimes with c. 4 leaves attached, which act as wings. (Bean 1981; Ainsworth 1989; Andrews 1994; Fu, Xin & Whittemore 2003; Certini et al. 2020).

The genus Zelkova must rank among the most ‘collectable’ of plant groups. Its representatives tend to be particularly graceful trees, with the slender twigs that are characteristic of the elm family (Ulmaceae), and with attractive leaves that are striking because unlike elms they have just one, big marginal tooth for each major side vein, and with a bark that is usually though not always colourful, its sandy-grey surface flaking in fine scales to show bright amber to cinnamon tints, like evening sunlight dappling a forest floor. Zelkovas never flower or fruit showily, but even this can be considered an advantage whenever there is a pavement or patio that might need to be swept; however, the flowers of Z. abelicea (Lam.) Boiss. at least are scented (Edwards & Marshall 2019). The six species form a select group all of whose taxa are in cultivation in the west; none is particularly abundant as a wild tree, and the IUCN’s latest assessments range from Near Threatened (Z. serrata (Thunb.) Makino) through to Critically Endangered (Z. sicula Di Pasq., Garfi & Quézel: a species discovered in eastern Sicily as recently as 1991). Anybody growing a zelkova will therefore be contributing in some degree to ex situ conservation efforts; the international Project Zelkova was set up in 2010 to promote and co-ordinate study and conservation of this genus (Kozlowski et al. 2018). Among their few cultivars, zelkovas can also boast what is perhaps the largest and most magnificent of all fastigiate trees – a clone of Z. carpinifolia (Pall.) K.Koch which through a strange history of misunderstanding and oversight has never been named and is described below for the first time, as ‘James Gordon’.

The word Zelkova was derived by François André Michaux in 1831 (Michaux 1831) from the Georgian name for Z. carpinifolia, which translates roughly as ‘column of stone’ (dzeli and kva) and commemorates this species’ strong, hard timber and often straight trunk (Kozlowski & Gratzfeld 2013); it was published for the genus as a whole by the French botanist Édouard Spach ten years later. Zelkova is in fact an example of the ‘conservation’ of a familiar botanical name, since Abelicea had been published as early as 1808 by James Smith, in a description that remained little known (Royal Botanic Gardens Kew 2023). Abelicea derives from the Greek name for Z. abelicea, αμπελιτσιά, which had been adopted in pre-Linnean nomenclature as early as 1594 by Onorio Belli, writing to Charles de l’Écluse (Bean 1981). Until 1841, most botanists had placed the two known members of Zelkova within Ulmus or Planera, or even in the unrelated genera Quercus or Rhamnus. In English the vernacular name ‘false elm’ is occasionally recommended, but seems at best superfluous.

Zelkovas can be characterised as relict trees. Fossils referable to the genus are found widely through much of the Tertiary period, with the earliest dating from about 55 MYA in what is now western North America. Out of this putative ancestral heartland – where no wild Zelkova survives – the genus seems to have spread through Asia and Europe, before the ranges of the surviving species contracted to a few mountain refugia in the face of Ice Age related climate change (Kozlowski & Gratzfeld 2013). The three east Asian species currently enjoy wide though scattered distributions; the western ones have continued to inch closer to extinction. Zelkova carpinifolia is scattered along the Caucasus mountains and in the mountain forests from eastern Turkey to western Iran, Z. abelicea is endemic to the mountains of Crete, and Z. sicula is confined to two tiny clonal populations in Sicily. These three lineages may have diverged as long as 20 MYA, though intermediate populations existed until much more recently, with one species known only from fossils surviving in the mountains of central Italy until about 30,000 years ago (Kozlowski & Gratzfeld 2013).

Zelkova sicula in particular owes its survival to the genus’ remarkable ability to reproduce clonally from root suckers, as most elms can: it is a triploid of ancient hybrid origin and seems to be sterile (Kozlowski et al. 2018). But one feature of zelkovas as a whole is how little effort they seem to put into sexual reproduction: the flowers, and fruit, are so inconspicuous and sparingly produced that many observers will probably never have noticed them. Given most zelkovas’ threatened status, it is perhaps surprising how little research has yet been conducted into their methods of seed dispersal. The fruit – unlike the winged samara of the elms – is a drupe: berry-like, with a little hard, dry flesh. (This used to be taken to indicate the genus’ evolutionary relationship to Celtis, the hackberries and nettle-trees, but phylogenetic analysis has recently promoted the removal Celtis from Ulmaceae to the family Cannabaceae; the similarity of these two genera’s fruit turns out to be an example of convergent evolution). In Crete, Z. abelicea co-ordinates its fruit production in occasional ‘mast-years’, as many oaks and beeches do, but most of the drupes are infertile (Fazan 2018). The drupe presumably evolved to be eaten by some animal or bird, but in the case at least of Z. abelicea the method of dispersal is remarkable: a specialised abscission zone allows the drupe to drop while still attached to a length of twig with about four dry leaves attached, and these leaves act like helicopter wings to carry the seed away from the parent plant (Certini et al. 2020). Perhaps as a result of this adaptation, the leaves on flowering Zelkova shoots tend to be smaller than on non-flowering ones – a feature which can be particularly remarkable in cultivated plants of Z. serrata (Thunb.) Makino. The drupe is largest (and most edible?) in the Chinese endemic Z. sinica C.K. Schneid.

Like most relic trees, zelkovas no longer have many pests or diseases to trouble them. They usually resist honey fungus (Edwards & Marshall 2019). They are related closely enough to elms for their foliage to be palatable to elm bark beetles (Scolytus spp.), and during severe outbreaks of Elm Disease, when the beetles breed in dying elms and become abundant, zelkovas are liable to get infected by the beetles with the Elm Disease fungus (Ophiostoma spp.) and to die in turn (Clarke 1988). However, the elm bark beetles only seem to feed on zelkovas as a last resort, meaning that when most of the elms in an area have been reduced to coppice growth and the bark beetle population collapses, Elm Disease ceases to be a significant threat to the many zelkovas which survive (Andrews 1994). This is the current situation across most of Europe and North America.

Zelkovas prefer a fairly fertile soil which is not waterlogged; due to the microclimates in which they have recently become marooned, Z. abelicea and Z. sicula are moderately well adapted to severe summer drought. In the wild zelkovas tend to grow over limestone, and they succeed reliably when planted in the most reactively chalky soils. As they are forest trees, they can survive in considerable shade, but their often elegant habits are best expressed in more open situations. All are fairly hardy, though the limits of the southerly species Z. abelicea and Z. sicula have yet to be tested.

Partly because they produce little pollen and few seeds, and partly because the surviving species are of ancient, distinct ancestry, zelkovas have proved refreshingly unwilling to dilute their specific features by interbreeding when grown together. A single clone, the cut-leaved Zelkova ‘Verschaffeltii’, appears to have arisen in the Netherlands as a hybrid of Z. carpinifolia with Z. serrata; however, an experimental attempt by the American botanist Frank Santamour to hybridise Z. carpinifolia and also Z. schneideriana Hand.-Mazz. with Z. serrata using controlled pollination failed to produce any viable seed (Santamour 1983). This said, the genetic separateness of the six species does not necessarily make them visually distinctive, and the three east Asian species in particular can be hard to separate in the field. Zelkovas from the Taroko Gorge in Taiwan were originally described by Bunzō Hayata in 1920 as a new species (Z. formosana or Z. tarokoensis) before becoming conventionally treated as a subpopulation of Z. serrata (Royal Botanic Gardens Kew 2023), but recent genetic analysis has placed them comfortably within Z. schneideriana (Naciri et al. 2019; this Taiwanese population is probably not yet represented in cultivation in the west).

In addition to being the most widespread – or the least rare – zelkova in the wild, Z. serrata is the species best represented in cultivation, particularly in the United States where the tree thrives in most conditions. It is also the only member of its genus, so far, to have produced a variety of interesting yellow-leaved, variegated and dwarf sports. With their rather small foliage, the east Asian Zelkova are favourite subjects for bonsai work.

The key below is partially adapted from one devised by Jan De Langhe which is available (here). It should be noticed that foliage features within wild populations (of five of the six species at least; Z. sicula only survives as two clones) may be more diverse than the range displayed within the more limited, cultivated gene pools of these trees.