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Excerpts from Jim Conrad's
Naturalist Newsletter
Entry dated April 19, 2024, from notes taken about 1.5km northeast of Puerto de los Velazquez, Municipality of Pinal de Amoles; N21.138°, W99.665°, elevation ~2780 meters (~9120 feet); oak/pine forest on limestone bedrock; in the Eastern Sierra Madre Mountains of east-central Querétaro state, MÉXICO
ANDEAN ALDER
Along a ridgetop, leaning over the face of an old roadcut, the above small tree's blackish branches gracefully arched over the seldom-used road and my backpack.
The tree's leaves, shown at the right, had secondary veins which went straight to the blades' finely toothed margins without curving or branching into reticulating tertiary veins. The combination of a tree leaf with such veins and finely toothed blade margins brings to mind the Birch Family, the Betulaceae, of which three genera occur in this part of upland central Mexico. These three genera comprise various species of birches, alders and hop-hornbeams, which are kinds of trees mostly occurring in the Temperate Zone, but high elevations in the tropics serving for some.
The tree bore numerous small, blackish cone-like structures, as seen above. There, the brown, wormlike items on the right are immature catkins of male flowers. Catkins of female flowers occur on the same tree in much smaller catkins. On the picture's left, somewhat woody, cone-like structures are what remains of female catkins having enlarged, and the catkins' scales having spread apart, shedding their seeds.
Below the tree, the road was strewn with the female flowers' cone-like structures, looking like little pine cones. In the picture, the grainy things in two clusters are seeds. Among the three genera of the Birch Family found in this part of Mexico, the only one producing such rounded, woody, cone-like structures is the genus Alnus, the alders. The Birch family goes back to the time of dinosaurs, the late Cretaceous, and this fact relates to its flowers being so simple without corollas, and fruits looking like gymnosperm cones, which have an even older history.
The split trunk, heavily covered with mosses and lichens because this ridgetop zone was borderline cloud forest, otherwise was fairly smooth, except for narrow, horizontal, wrinkles in the bark.
In our upland central Mexican region known as the Bajío, two alder species are documented, Alnus acuminata and Alnus jorullensis. Various subspecies, varieties and forms have been described for each species. The species and subtaxa are based on leaf and cone shape, the conspicuousness of glands atop the leaves, how pointed the leaves are, number of leaf veins, and such. I don't see clear distinctions between the two species, much less the subtaxa, and our tree's features are fairly intermediate. Of the two species, mostly by comparing our pictures with those on the Internet, and taking special notice of how close our tree's leaf veins are, the number of veins, and leaf texture, our tree tilts toward being ALNUS ACUMINATA, often called the Andean Alder.
Both of highland central Mexico's species were discovered by Humboldt and Bonpland, so apparently they thought there were two different species. I find no mention of hybridization between species. However, the 2004 study by Zhiduan Chen and Jianhua Li entitled "Phylogenetics and Biogeography of Alnus (Betulaceae) Inferred from Sequences of Nuclear Ribosomal DNA ITS Region," support the idea that Alnus species arose in Asia and migrated to the Americas across the Bering Land Bridge into western North America, and then southward. This idea is supported by fossil evidence.
Mexico's two species, which don't occur north of the Mexican border, are very closely related -- sister species on the phylogenetic Tree of Life, thus arising from a common ancestor. Alnus acuminata extends into South America's Andes, while Alnus jorullensis only reaches as far south as Honduras. With this mind, surely there was a time when the two species's ancestor lived in Mexico, with one arm of that ancestral population expanding into South America, maybe changing along the way, while the home population evolved in various ways. I see no reason why intermediate forms shouldn't persist here in Mexico.
Still, our tree does seem more Alnus acuminata than the other, and finding plants on the Internet requires a name, so I'm filling this page under that one.
Alder trees have a long history of various traditional medicinal uses in many cultures. In Mexico, the online Flora Medicinal Indígena de México reports that the Otomí people around Texcatepec, Veracruz, to treat fever, boil the fruits, leaves and bark and with the water bathe the lower half of the body.
An online web page sponsored by the Mexican Government lists many uses of the tree's wood. Also, it mentions that the bark is rich in tannins, thus providing treatments for skin and venereal diseases. The leaves are used as poultices for skin wounds, and fruit extracts for throat inflammation.
Roots of all alder species form nodules in which actinomycete bacteria of the genus Frankia live symbiotically. This bacterium changes, or "fixes," nitrogen gas, which forms 78% of the atmosphere but is unusable to plants, into ammonia, a nitrogen-bearing molecule in which nitrogen becomes easily available not only to the alder, but also other organisms in the ecosystem. Because of this, in Mexico the species is regarded as an important tree to plant for land reclamation.
Good citizens, these alder trees, no matter what taxonomists call them.