Tree in Seattle Japanese garden

The kingdom Plantae and where trees fit in

Life on earth is classified into different categories and the process of doing this is the science of taxonomy. There is a lot of interesting history associated with biological taxonomy but my goal with this post is to show where trees fit into the taxonomy of life. Plant taxonomy is governed by the International Code of Nomenclature for algae, fungi and plants. They define the ranking of taxonomy as kingdom, division or phylum, class, order, family, genus, and species.

As of 2015 there are seven kingdoms – Animalia, Plantae, Fungi, Bacteria, Chromista, Protozoa and Archaearef – viruses aren’t considered living so they have their own separate grouping.

Trees fall into the kingdom Plantae, which comprises “all organisms possessing plastids with double envelopes that are free in the cytoplasm”.ref Which must be one of the most brain-exploding definitions of plants you will ever read! What it means is that members of the kingdom Plantae are dependent on chloroplasts, organelles with the green pigment chlorophyll, which can photosynthesise, AND these chloroplasts have specific morphology and location. The reason there is this qualifier is because the kingdom Chlorista, which includes some algae, also has chloroplasts, although they are different to those in Plantae.

Anyway even though ‘tree is only a way of being a plant‘, it’s not as far as we know a way of being a chlorista, so we will focus on Plantae for this post.

(Bear with me on this next bit – to drill down yourself you can refer to this site). Plantae is broken into 8 phylla, one of which is Tracheophytaref (plants which have a vascular system); within Tracheophyta are 8 classes, including:

  1. Cycadopsida includes cycads & palms (not so relevant for bonsai)
  2. Ginkgoopsida includes only one member – the ginkgo
  3. Gnetopsida includes nothing we care about from a bonsai point of view (Gnetum, Welwitschia, and Ephedra)
  4. Liliopsida – also known as the monocots, includes lillies & bamboo (not bonsai relevant unless you’re into bamboo)
  5. Lycopodiopsida – club mosses (not for bonsai)
  6. Magnoliopsida – also called the dicots or angiosperms – includes all the flowering plants, including many trees we use for bonsai
  7. Pinopsida – also known as conifers, again, a favourite of bonsai enthusiasts.
  8. Polypodyopsida – leptosporangiate ferns (maybe an accent plant, but not a bonsai)

Angiosperms evolved a lot later than gymnosperms – their last living ancestor was no earlier than 140–250 million years ago, whereas their joint ancestor with gymnosperms was 310–350 million years ago.ref They evolved to produce flowers which could be pollinated by insectsref, and to have lower reproductive costs overallref, making angiosperms the most successful plant family today. There are 444k angiosperm species entries in the Global Biodiversity Information Facility databaseref, which constitutes 74% of the total species in the database. You can see them all here.

Pinophytes are a bit easier to get your head around as the only surviving order within it is Pinales. Pinales contains all the conifers in six families (along with some extinct families and species which I haven’t listed):

  1. Araucariaceae (including monkey puzzles and the Wollemi pine)
  2. Cupressaceae (including cypress, juniper, redwood)
  3. Pinaceae (including pines, cedar, spruce, hemlock & fir)
  4. Podocarpaceae (mainly southern hemisphere evergreens including Buddhist Pine) and Phyllocladaceae (celery pines)
  5. Sciadopityaceae (Japanese umbrella pine is the only member in this family)
  6. Taxaceae (yews) including Cephalotaxaceae (plum yew)

Here’s a useful chart which was created on genetic analysis of 64 protein-coding genes to determine the position of dawn redwood relative to other conifers (you can see it there in the Cupressaceae family) :

Quite often trees are moved from one classification to another, specially once their DNA has been sequenced and this has been very much the case in angiosperms – an updated classification is created by the Angiosperm Phylogeny Group every few years. Here is a summary of the most recent changes made in 2016.