Paleo-autochtonous species (3): Gleditsia

When autumn arrives in the Parque del Retiro in Madrid and the deciduous trees begin to lose their leaves, stands out among them a tree of very different size from its neighbors and whose branches armed with dissuasive thorns hang enormous dark legumes that soon end up covering the soil at the foot of the tree. The nature of the fruit suggests that it is a legume, but this tree belongs, however, to an essentially tropical subfamily, of which there are very few representatives in temperate cold areas. The honey locust (Gleditsia triacanthos) and the Kentucky coffeetree (Gymnocladus dioicus) are, in fact, the only species of the subfamily of the Cesalpinioideae grown in our country that truly support the cold. Both species share some characteristics that are characteristic of this subfamily, such as bipinnate leaves and the presence of glands (extrafloral nectaries) on the rachis of the leaves and leaflets, the actinomorphic flowers, etc. Unlike most species of this subfamily, however, both species have unisexual flowers. The acacias also belong to this subfamily, but are not as resistant to the cold as these two species.




An interesting feature of the Geditsias is that the same individual may have pinnate and bipinnate leaves or, as can be seen in this photograph, exceptionnally have partially pinnate and bipinnate leves. This leaf belongs to Gleditsia sinensis (Royal Botanic Gardens of Madrid).

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The genus Gleditsia, composed of about 14 species, is another clear example of taxon with disjunct distribution area, with 3 species present in the Americas and all the others in Asia. One of these species is present in the southern Caspian Sea and this allows us to intuit that this genus was probably present throughout the Northern Hemisphere before the glaciations. As shown in the map below, there is little fossil evidence of this kind in the Pliocene or in the European Pleistocene, and these are not sufficient to know its spatial and temporal distribution. But it confirms, however, that the genus had a holarchic distribution before the ice ages. The return of this genus to the European continent, curiously, was not made from its stronghold in the basin of the Caspian Sea, but of the American continent. The honey locust (Gleditsia triacanthos), originally from a large area of North America, it is now naturalized in much of the European continent. It is even even cultivated in the area where the Gleditsia caspica originates and it is verified that both species hybridize very easily. A recent study (1) conducted in Azerbaijan has shown that even in nature reserves many of the observed individuals are already first-generation hybrids. This example shows that despite the long time that both species have separated, the speciation process has not yet succeeded in preventing or hindering the crossing of the different strains of this genus.




The honey locust is an interesting case of an orphan species. Provided with dissuasive spines on a large part of its trunk and its branches, it produces fruits with a sugar pulp highly appreciated by herbivores, it is very interesting to note that there is currently no herbivore species that preferentially feeds on its fruits or leaves and which actually co-evolved with this species. In North America, it is suspected that they are probably elements of the recently disappeared megafauna, such as giant sloths, which ate their fruits and contributed to spread the seeds (2). In Europe, it is not so clear what kind of herbivore it would interact with, although observations in Argentina suggest that other large herbivores could also have fed on their vegetables. Livestock in this region are the main contributors to the spread of this species.




Male flower of Gleditsia sinensis (Royal Botanic Garden of Madrid). The flowers are actinomorphic. Petals and sepals are very similar to each other.

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	Gleditsia	Familia: Fabaceae	Orden: Fabales
	Trees or shrubs, deciduous. Trunk and branches usually with stout, simple or branched spines. Leaves alternate, often clustered, simply paripinnate and bipinnate often on same plant; stipules caducous, small; rachis of leaves and pinnae sulcate; leaflets numer­ous, subopposite or alternate, base oblique or subsymmetrical, margin serrulate or crenate, rarely entire. Inflorescences axillary, rarely terminal, spikes or racemes, rarely panicles. Flowers polygamous or plants dioecious, light green or greenish white. Receptacle campanulate, outside pubescent. Calyx 3-5-lobed; lobes subequal. Petals 3-5, slightly unequal, ca. as long as or slightly longer than calyx lobes. Stamens 6-10, exserted, slightly flat, broad, with crisped hairs from middle downward; anthers dorsifixed. Ovary sessile or shortly stalked; ovules 1 to numerous; style short; stigma terminal. Legume ovoid or elliptic, flat or subterete. Description: Flora of China

Legumes of Gleditsia triacanthos in the Park of Retiro (Madrid).

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Species cultivated in our parks for a long time and from which also developed a variety without thorns. It seems, however, that its classification as an invasive species condemns for now this species to stop being used as an ornamental tree. Its wide naturalization, however, guarantees for the moment its survival in our flora.

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		(1) Schnabel1 A. & Krutovskii K. (2004) / Conservation genetics and evolutionary history of Gleditsia caspica: Inferences from allozyme diversity in populations from Azerbaijan / Conservation Genetics, Vol.5, pp. 195–204, 2004
		(2) Whit Bronaugh (2010) / The Trees That Miss The Mammoths / American Forests (Winter 2010 issue)

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Author: Adrián Rodríguez
Translation: João Ferro

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Paleo-autochtonous species (2): Nyssa

Leaves of Nyssa sylvatica in autumn.

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Little known and still little used in our country (Spain), the tupelos (Nyssa), are deciduous trees that are highly valued in other countries for their magnificent autumnal colors. There are currently 8 species recognized in this genus, all with fairly high water requirements. The most commonly cultivated and with the largest distribution area (N. sylvatica and N. sinensis) are typical of wet and riverine forests. Others, such as N. aquatica, N. biflora or N. ogeche (all north american), are adapted to live in marshy areas and have a broad base very typical of the trees that grow in this type of environment, such as the bald cypress (Taxodium distichum) where they live together in many places. In our country (Spain) it is still rare to see them, although it is already possible to obtain N. sylvatica in some nurseries.



The tupelos flowers are not very flashy. These belong to N. sylvatica. / Picture: Using georgia native Plants

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The genus Nyssa is another taxon with a disjoint distribution area at the continental level, with 5 species in eastern North America and 3 in East Asia. The genus, as might be expected, was also present on the European continent before the ice age, where different fossil species more or less related to the present species were described. As can be seen on the map, the most recent citations come from the Mediterranean basin (Middle Pleistocene) and the Caspian Sea region, where they seem to have survived until the last interglacial period (Eemian) in the river Emba delta. It really lacked them very little to survive on our continent.

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Together with other small genera (Camptotheca, Davidia, Diplopanax, Mastixia), this genus constitutes the small family of Nyssaceae, related to the family of Cornaceae, in which they were still included in the APG III classification. Nisaceae, like most Cornales, still have primitive features such as free petals and arboreal size, which can also be observed in the neighboring order of the Ericales, for example.

	Nyssa	Family: Nyssaceae	Order: Cornales
	Trees dioecious or functionally dioecious (with anthers of bisexual flowers that do not dehisce); bark gray brown, rough, ridged; twigs with transverse diaphragms; winter buds scaly. Leaves alternate, simple, often crowded near ends of branches, estipulate; petiole terete or winged; blade usually elliptic to oblanceolate or obovate, rarely ovate, base cuneate to rounded. Flowers unisexual, usually in heads or short racemes, in axil of a bract with 2 bracteoles; sepals forming a low rim; petals greenish to greenish white. Male flowers 5-merous, pedicelate or not. Stamens 10, arranged in two alternate whorls; filaments linear; anthers 2-celled, dorsifixed, with lateral lengthwise slits; disk pulvinate. Female flowers (4 or)5-merous, not pedicelate; staminodes usually present. Ovary inferior, 1(or 2)-loculed, 1-ovuled; style subulate or conic, bifid, with stigmatic tissue at inside of stylar arms. Fruit drupaceous, ± laterally flattened, with persistent calyx and disk, usually blue-black (sometimes reddish purple in N. aquatica; yellow, orange, or red in N. ogeche); mesocarp juicy, acidic. n = 22. Description: eFloras

The only species that is occasionally cultivated in the Iberian Peninsula is Nyssa sylvatica which, however, is sensitive to frost when young and difficult to transplant. This makes it relatively rare and one has to go to arboretums or botanical gardens to observe it.



Nyssa sylvatica fruits / Picture: Great Plains Nursery

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Author: Adrián Rodríguez
Translation: João Ferro

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