Malus trilobata

Taxonomy and evolution

Taxonomic history

The taxonomic history of Malus trilobata is complex, reflecting its unique morphology. The species was originally described by Jean Louis Marie Poiret as a kind of hawthorn, Crataegus trilobata, based on a specimen collected by Jacques Labillardière in Lebanon, in 1810. Since then, the species has also been placed within the genera Pyrus, as Pyrus trilobata (Poir.) DC. Prodr., Sorbus, as Sorbus trilobata (Poir.) Heynhold, Cormus, as Cormus trilobata (Poir.) Decne., and Malus, as well as in Eriolobus (Ser.) Roem., whereby the latter was variably considered to include only M. trilobata or also M. florentina. Many authorities have sought to emphasise the species' distinctiveness by assigning it to unique genera, sections or subsections. Traditionally, many researchers have further classified the species within its own section, Eriolobus, but the validity of these subgroupings has been questioned more recently.

Both positions appear to be supported by research findings. M. trilobata resembles other Malus species in floral morphology, and it is similar to some Malus species in leaf shape. For these reasons, Kazimierz Browicz (1969) considered it to be most closely related to Malus doumeri and, to a lesser extent, to Macromeles tschonoskii (which is also often placed in Malus), both of which are native to East Asia. On the other hand, M. trilobata differs in the grit cells in its fruits, which are more commonly associated with Pyrus, even though they also occur in some other Malus species. Furthermore, phytochemical studies have shown that, uniquely in Malus, trilobatin replaces phloridzin as the main phenolic compound of the leaves in M. trilobata, which may similarly be taken to indicate a need for separation.

Some researchers recognise two varieties: Eriolobus trilobatus var. trilobatus (Labill. ex Poir.) M.Roem., which occurs throughout the species' range, and Eriolobus trilobatus var. sorgerae Browicz, which is only found in Gömbe, Antalya Province, Turkey, and is distinguished by having much smaller leaves than the type variety. Current consensus, however, recognises no sudivisions.

Modern taxonomy and evolution

Phylogenetic studies based on plastid and nuclear DNA indicate that Malus trilobata falls between other species normally classified as Malus, and it appears to be sister to M. florentina, with both species often, but not always, positioned together on a branch that also contains the North American species prairie crabapple (Malus ioensis), southern crabapple (M. angustifolius) and sweet crabapple (M. coronaria). In Liu et al. (2022), this branch, called Clade II, is shown to either be basal to the rest of Malus, in nuclear phylogenies, or to be sister to Pourthiaea and thus closer to other Maloideae such as Sorbus and Aronia than to Malus proper, in plastid phylogenies. This discordance between nuclear and plastid phylogenies, the authors proposed, could be due to incomplete lineage sorting, allopolyploidy, or hybridisation, all of which were important mechanisms underlying the evolution of the Maloideae. The authors proposed hybridisation as the most likely scenario, whereby the ancestor of Clade II hybridised with the ancestor of Pourthiaea, so that all its descendants, including M. trilobata, inherited Pourthiaea's chloroplast DNA through a process known as chloroplast capture.

According to Liu et al. (2022), Malus originated in North America and East Asia, most likely in the middle Eocene, around 43.58 million years ago.

Description

Malus trilobata is a medium-sized deciduous tree. It initially assumes a narrow, upright habit with horizontal or ascending branching, but becomes more rounded with age. The species usually attains a mature size of 6-10 m or up to 18 m height by 7 m width. The leaves are deeply 3-lobed, with each lobe usually again divided into several pointed lobes, so that the leaf may almost appear palmate. In leaf shape, the species resembles certain species of maple (Acer) as well as the chequer tree (Torminalis glaberrima).

The tree blooms relatively late — usually between May and June in the European part of its range — bearing large flowers with white petals and up to 4 cm in diameter, with flowers appearing in loose umbels of up to 10. The apple fruits ripen in fall between October and November. They are roundish and large, being around 2.5 cm in diameter and weighing around 12 g. Yellow in colour, they are edible and reportedly have a good taste. The pulp has grit cells, which is otherwise atypical of apples, but common in pears. The seeds are dark and around 7 cm in diameter. The bark of the tree is dark grey, deeply fissured and scaly with age, and it develops a deep root system.

M. trilobata is diploid, with a chromosome number of 2n=34. File:Eriolobus trilobatus 1.jpg|Flowers of Malus trilobata in Mount Meiron Nature Reserve, Israel File:Eirolobus trilobatus fruits.jpg|Fruit of Malus trilobata File:Malus trilobata leaves 01 by Line1.jpg|Leaf of Malus trilobata File:Malus trilobata trunk 01 by Line1.jpg|Bark of Malus trilobata

Distribution and ecology

Malus trilobata is distributed in mountainous terrain in the eastern Mediterranean, however, its range is remarkably disjunct, consisting of several disconnected populations. A general, albeit outdated

In Europe, it is known from Thrace, specifically Evros Prefecture in Greece and the eastern Rhodopes in Bulgaria. Here, it occurs between 166-380 m in xero-thermophilous communities on acidic siliceous soil, together with deciduous oaks (Quercus pubescens, Q. ithaburensis subsp. macrolepis, Q. frainetto) and Turkish pine (Pinus brutia), fellow Rosaceae (Torminalis glaberrima, Cormus domestica, Pyrus spinosa), mock privet (phillyrea latifolia), Greek strawberry tree (Arbutus andrachne), Oriental hornbeam (Carpinus orientalis), flowering ash (Fraxinus ornus), terebinth (Pistacia terebinthus), tanner's sumach (Rhus coriaria), cade juniper (Juniperus oxycedrus), garland thorn (Paliurus spina-christi), bladder-senna (Colutea arborescens), black bryony (Dioscorea communis), pink rock-rose (Cistus creticus), wild asparagus (Asparagus acutifolius) and traveller's joy (Clematis vitalba).

In Anatolia, the species is distributed in the Aegean and Mediterranean regions, that is, the northwest, southwest, south and southeast of the peninsula*.* The largest populations exist in the Taurus Mountains between the Berit Daǧi and Amanus mountains. A sizeable population exists also in Antalya Province where, however, the species is reportedly not common either.[[File:Cedars of God (Lebanon cedar forests), Lebanon.jpg|left|thumb|Malus trilobata occurs at some frequency in the [[Cedrus libani|Lebanon cedar]] forests of Mount Lebanon]]Apparently the largest population persists in the Levant, especially in the Lebanon Mountains of Lebanon, which is also the type locality of the species. For example, a large population occurs in Horsh Ehden nature reserve. Nevertheless, the species is rare also in Lebanon: a comprehensive survey in 2018 identified a total of 55 localities, the lowest count among the 27 tree species surveyed, with the next lowest count being 92 for silver almond (Prunus argentea). In contrast, Aleppo oak (Quercus infectoria) and kermes oak (Q. coccifera), the two most frequently recorded species, where found at 2092 and 1914 locations, respectively.

Throughout its range, M. trilobata is distributed mainly on rocky slopes and neutral and acidic soils. M. trilobata is insect-pollinated, and, like other Malus, is thought to be pollinated primarily by bees. M. trilobata is a host plant for Ropalopus ledereri, a species of longhorn beetle from the eastern Mediterranean that feeds on the thin terminal branches of Malus species, Crataegus species and kermes oak.

Uses and cultivation

Traditionally, the leaves and fruits of Malus trilobata were used for treatment of a number of ailments, including heart diseases, diabetes, asthma and gastritis, shortness of breath, and hypertension. Additionally, microbiota isolated from the leaves, bulk soil, and roots of M. trilobata could be a source of biocontrol agents for Botrytis cinerea and Penicillium expansum, fungi that colonise apples post-harvest. These microbiota, the study suggested, could help alleviate some of the economic damage these fungi cause, and may represent a future alternative to fungicides. One study also suggested that antibacterial compounds for controlling multidrug-resistant strains of bacteria such as Staphylococcus aureus could be yielded from the rhizosphere of M. trilobata. Furthermore, the fruit of the species has culinary potential. In particular, it has high nutritional value, and is rich in malic acid, chlorogenic acid, epicatechin, rutin, and fructose.

In cultivation, M. trilobata is noted for its robustness, including its frost and drought tolerance, and its insensitivity towards infections.

Status and conservation

Malus trilobata is a rare species, occurring as isolated trees or groups of trees virtually in its entire range. Most recently (2017), it was classified as near threatened (NT) on the IUCN Red List, but with an unknown population trend. This assessment was justified on the grounds that the known subpopulations are small, isolated, and fragmented, with an estimated area of occupancy (AOO) of 2550 km2, approaching the threshold for vulnerable (VU). Known threats include land use changes, cutting, overgrazing, fires, urban expansion and agriculture. Additionally, the species appears to suffer from poor recruitment and a limited ability to colonise new habitats. 58% of which were impacted by extreme wildfires in 2023. The genetic structure of the European population in Evros has been scrutinised, revealing relatively low genetic diversity and population fragmentation due to roads, firebreaks and geographical isolation. The species is represented in several ex situ conservation and germplasm collections,

References

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