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Emory oak near Young, Arizona © Nanebah Lyndon
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Plant Focus

Quercus texana New Madrid acorn
Disentangling the cultivar published as Quercus texana ‘New Madrid’.

Quercus rotundifolia var. macrocarpa

A few weeks ago a friend offered to send me acorns of a holm oak he had come across "laden with acorns" in the village of Monsaraz in the Alentejo, Portugal (south-east of Lisbon, near the Spanish border). I of course accepted the offer, though Quercus ilex is by no means a rare oak for us in Argentina and Uruguay. However, when the package arrived I was surprised by the large size of the acorns and cups, and the small size and shape of the leaves, which were rounder than what I expected for holm oak.

I looked up Quercus ilex for possible varieties with descriptions that might match these characteristics, but found none. So I wrote to IOS member Francisco Vázquez, who I knew to be an expert on Iberian oaks, and he

Quercus rotundifolia var. macrocarpa in the Estelagem de Monsaraz, Alentejo, Portugal. (Photo: Paul Adams)

immediately replied that what I had been sent was Q. rotundifolia, a close relation of Q. ilex, and specifically, Q. rotundifolia var. macrocarpa (Cout.) F.M. Vázquez, S. Ramos & S. García—and yes, the F.M. Vázquez in that author name was my correspondent, so I had clearly contacted the right person. A distinctive feature of this variety is the large acorns that can weigh over 35 g and are often polyembrionic, i.e. they come with 2-3 embryos and so can sometimes produce 2 or 3 shoots. Another characteristic of these acorns is that they are low in tannins and their taste is neutral or even slightly sweet. (Q. rotundifolia is the species of oak that is most used to fatten pigs for ham in Spain and Portugal.)

I looked up this variety on oaknames.org but found it was not listed. Further research lead to Kew's World Checklist of Selected Plant Families, where several forms and varieties of Q. rotundifolia are listed as synonyms and not accepted by the checklist compiler, Rafaël Govaerts. I noted that the varieties had been

Large acorns can weigh over 35 g Small, rounded leaves

published in the Journal of the IOS Society, and thus came back full circle to Francisco’s presentation at the Winchester IOS Conference: “Diversity of Iberian Oaks”, where several varieties of Q. rotundifolia are mentioned. I asked Francisco why his varieties had not been accepted by Govaerts and he replied that following an exchange of correspondence between them, Govaerts had indeed acknowledged his varieties, and even

The tree at Estelagem de Monsaraz (Photo: Paul Adams)

received acorns of some of them to grow at Kew, but that in his checklist he did not accept any rank below that of subspecies. (I still don’t fully understand how that works, because the checklist accepts Q. aliena var. acuteserrata, which is clearly below subsp. rank.) And so what constitutes a subspecies and what a variety? The large acorns of Q. rotundifolia var. macrocarpa seemed a pretty clear difference to justify discrimination. According to Francisco, it all depends on the amount of genetic differences, whereby forms are distinguished by variation in only one DNA locus, and varieties have larger differences (which means that acorns of varietal taxa will often transmit the parental characteristics) and subspecies have an even greater number of genetic differences. At which stage it got a little too involved for a mere amateur, but the upshot was that I asked Francisco if he would like to write our Species Spotlight for the next issue of Oak News & Notes on Q. rotundifolia and its

Quercus rotundifolia var. macrocarpa laden with acorns 
(Photo: Paul Adams)

varieties. He agreed, so you can look forward to further information in February.

In the meantime, my acorns have sprouted (only single plumules produced so far), and I have to confess that I sacrificed one of the acorns in order to verify that they indeed tasted sweet. I was able to confirm that while they may be edible, and even palatable in comparison to more tannic acorns, raw acorns are definitely an acquired taste!