American oaks stop one degree short of equator

Flora de la Real Expedición Botánica del Nuevo Reino de Granada (1783–1816), vol. XIV, p. 26

Panama is just about where the southward journey of the oak through the Americas stops. While the closing of the isthmus between North and South America between 3.5 and 5 million years ago allowed oaks into a small area of north-western South America, only one species extends into Colombia. 

Which means oaks barely have a toehold in South America, and never cross the equator. They get close, to about one degree of latitude north. After splintering into some 240 species, the oaks of the Americas end their conquest and evolution in the Americas with Andean oak, a species named by French botanist Aimé Bonpland to honour his travelling companion and German – let’s say – natural philosopher extraordinaire, Alexander von Humboldt. Hence Quercus humboldtii.

Monsieur Bonpland and Herr Humboldt travelled through these parts between 1799 and 1804, with Bonpland describing twenty new species of Quercus in 1809 about half of which are still accepted today. Thankfully, the oak named after Humboldt remains current. 

Andean oak grows a little in the south of Panama but mostly in the mountains of Colombia at 1,500 to 3,400 m, with rainfall up to three metres each year. 

Herbarium specimen of Quercus humboldtii collected on the Mutis expeditions (Herbarium of Real Jardín Botánico de Madrid)

It's a Red Oak (classified in the subgenus Quercus section Lobatae), although White Oaks (subgenus Quercus section Quercus) do travel almost as far south, just not into Colombia. In high altitude forests, Andean oak is an evergreen or semievergreen tree to 40 m in height. The edges of those leaves are crinkly but not lobed, and despite being a Red Oak, there are no bristly extensions. 

Andean oak as a species is not at risk, despite declining numbers in some areas due to land clearing. Although there is new threat emerging - a phytoplasma (a bacterial like organism). This is a cause of some concern for the nearly 30,000 planted specimens in Bogotá.

Close up of Quercus humboldtii in the herbarium

Which may be a good time to mention the Spanish botanist, priest, physician, mathematician etc., José Celestino Mutis, born in Cádiz, Spain in 1732. He was a ‘disciple’ of Swedish botanist and father of botanical nomenclature, Carl Linnaeus, sent out into the world to collect and classify plants. He ended up establishing a botanic garden - now called Jardín Botánico de Bogotá José Celestino Mutis - and a rich botanical library in Mariquita, 150 km north-west of Bogotá.

Mutis also accumulated a herbarium collection of more than 24,000 plants, mostly from his leadership of the Real Expedición Botánica del Nuevo Reino de Granada from 1783 to his death in 1808.  These he used to write his massive and, in his lifetime, unpublished 'Flora de Bogotá o de Nueva Granada'.

When I was visiting Real Jardín Botánico de Madrid in 2022, the then director and friend, Esteban Manrique, arranged for a tour of their herbarium and library. I was well into my obsession with oaks by then so sought out any references to Quercus I could find.

Happily, for this post, I was shown Mutis's herbarium specimens and original prints of Andean oak, as well as a recently published copy of volume XIV of Flora de la Real Expedición Botánica del Nuevo Reino de Granada (1783–1816). This Mutis's original work, brought up to date by a collective of Spanish botanists. On page 26 of this volume, under the headings of Fagaceae, then Quercus, is an illustration by Francisco Villarroal, a botanical artist who joined Mutis on at least some of his expeditions. 

The original illustration was labelled by Villarroal as Quercus granatensis, which I don't think is a published species of oak, but it has since been confirmed as Quercus humboldtii - Andean oak - by the lead editor of this new publication, Daniel José Duarte Rojas. A pleasing happenstance given Humboldt stayed with Mutis for two months while in Bogota, admiring his extensive collection of preserved plants.

Given I have no photographs of living plants of the Andean oak, I've used pictures taken on that tour supplemented with a couple sent to me subsequently, to illustrate my post. 

Volume XIV of Flora de la Real Expedición Botánica del Nuevo Reino de Granada (1783 – 1816) 

Original illustration by Francisco Villarroel; Quercus humboldtii, labelled as Quercus granatensis

As to why the oaks don’t cross the equator, there is no answer other than that’s the way it is. You may as well ask why there are none in Australia. The answer there is to do with where the oaks started and how they managed to extend their range, with a barrier between them and Australia – a large ocean.

Here there is no obvious physical barrier so perhaps it’s better to think of species filling ever available niche they can, until they can’t. South of Colombia, at the time the oaks got there, perhaps there just weren’t opportunities for them to spread into suitable habitat.

What I can tell you, is that oaks did cross the equator without our help, but it wasn’t in the Americas. That's for another day. 

Note: A big thank you to Esteban Manrique, former director of the botanic garden in Madrid, and his archivist, Esther García. They helped me access the collections on my visit and were generous in providing more information and links to the work online as I prepared this post. I should also note that the specimens have been scanned since my visit, with the official images now looking like this:

Scan of Quercus humboldtii herbarium specimen (accessed 5 April 2024)