Monday 29 August 2022

Tree Family Tree

Ash die-back Hymenoscyphus fraxineus is a terrible thing. Leafless branches against the sky-line, especially decorated with corvids, have the look of Mordor or King Lear's blasted heath. At the end of July we escaped from lockdown and went to visit our oldest pals - (both senses of old: being longest in our orbit and eligible for a bus pass). We hadn't had face-time for 3 years! Things have changed, the wind-break line of trees which I helped plant years ago are beginning to look like, well, trees. Like us, like everybody, they have the ash die-back on their acre and were really concerned about a small stand of mountain ash which framed their view across the bay. 

I re-assured them that mountain ash = rowan = caorthann = Sorbus aucuparia was completely unrelated to regular ash = fuinseog = Fraxinus excelsior, soon to be of blessed memory like elms. It's easy, at a glance, to be confused: both trees share the feature of pinnate leaves [rowan shown R with defining saw-tooth edges to each leaflet]. When Linnaeus was putting naming-of-parts order on the natural world he used differences and similarities in the sexual apparatus to decide who was more closely related to whom. That method seemed to have utility because Linnaean classification is largely consonant with the DNA evidence on which all life's wonderful diversity ultimately depends. Similarities in leaves are, literally, superficial.

I was annoyed at myself that I couldn't, off the top of my head, tell our mates just how far distant the two tree species were. But, trained researcher me can defo find out. The definitive way would be to think of gene(s) which were present in all the relevant species, pull the protein sequences out of UniProt, align them with Clustalω, and tally up the differences, drawing a phylogenetic tree of trees. But the good enough way is to trawl through Wikipedia and clip out the taxonomic hierarchy for each species:

Common Genus Family Order Clade
Holly Ilex Aquifoliaceae Aquifoliales Asterids
Elder Sambucus Adoxaceae Dipsacales Asterids
Ash Fraxinus Oleaceae Lamiales Asterids
Olive Olea Oleaceae Lamiales Asterids
Alder Alnus Betulaceae Fagales Rosids
Birch Betula Betulaceae Fagales Rosids
Hornbeam Carpinus Betulaceae Fagales Rosids
Hazel Corylus Betulaceae Fagales Rosids
Chestnut Castanea Fagaceae Fagales Rosids
Beech Fagus Fagaceae Fagales Rosids
Oak Quercus Fagaceae Fagales Rosids
Hickory Carya Juglandaceae Fagales Rosids
Pecan Carya Juglandaceae Fagales Rosids
Walnut Juglans Juglandaceae Fagales Rosids
Aspen Populus Salicaceae Malpighiales Rosids
Willow Salix Salicaceae Malpighiales Rosids
Hawthorn Crataegus Rosaceae Rosales Rosids
Apple Malus Rosaceae Rosales Rosids
Rowan Sorbus Rosaceae Rosales Rosids
Apricot Prunus Rosaceae Rosales Rosids
Blackthorn Prunus Rosaceae Rosales Rosids
Cherry Prunus Rosaceae Rosales Rosids
Damson Prunus Rosaceae Rosales Rosids
Plum Prunus Rosaceae Rosales Rosids
Pear Pyrus Rosaceae Rosales Rosids
Elm Ulmus Ulmaceae Rosales Rosids
Horse Chestnut Aesculus Sapindaceae Sapindales Rosids
Sycamore Acer Aceraceae Sapindales Rosids


  • Most of the trees we know and love, including rowan are roses! But ash, olive, holly and elder are daisies. That's as different as you can be and still be dicots.
  • Rowan is [and a close look at the fruit will make obvious] closely related to apples and pears.
    • ditto hawthorn Crataegus
  • All the stone fruit cherries to plums are in the same genus Prunus
  • Horse chesnut Aesculus and sweet chestnut Castanea are not closely related.
    • Acer - maples and sycamore - is in the same Order as Aesculus.
  • Hickory, walnut and pecan are nutty cousins.
  • Aspen and poplar = Populus are straight and tall willows.
  • I've always believed that holly and oak were closely related. But I was misinformed. Quercus ilex - aka the holm, holly or evergreen oak - has a slightly pointy leaf margin [as R] like holly = Ilex but that is really the only point of similarity.
Homework / quiz. I've not mentioned Citrus because they don't grow hereabouts; but where do they fit in the scheme of things? Answer.

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