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What is a tree?

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  • Richard Morris
    Dear all, In the developing permaculture info project where we are trying to create a knowledge base for permaculture information see the mailing list at
    Message 1 of 2 , Sep 8, 2004
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      Dear all,

      In the developing permaculture info project where we are trying to
      create a knowledge base for permaculture information
      see the mailing list at
      http://lists.ibiblio.org/mailman/listinfo/permacultureinfo
      for more info.

      As part of the development of that we have been working on the structure
      of a plant database and how to represent plants and their
      characteristics systematically.

      I've been reading a book on XML topic maps which describes a way of
      constructing diagrams of related concepts (a topic map is very like a
      touchgraph graph). In one of the examples they devise a hierarchy of
      plants where "an oak tree is a type of tree". At first sight this seems
      a fairly uncontroversial statement.

      But think for a moment how this matches with the botanical plant
      hierarchy. Trees are liberally dispersed over a wide range of families
      from Tree Ferns to Conifers and Ginkgos to Monocots (Yucca) and
      liberally dispersed among the dicots. However trees have a patchy
      coverage with some families having a wide range of trees and non-trees.

      The pea family (Leguminosae) is a particularly interesting example which
      has 69 annuals (eg peas), 133 perennials (clover), 96 shrubs (gorse) and
      39 trees (acacia) mentioned in the Plants For A Future database.
      How come a family can have woody plants like gorse and acacia
      and other like peas and clover which are very far from treeness?

      So it seems that treeness does not really tie well with the typical
      plant systematics. Which leeds to a question "what makes a tree a tree?"
      Is there a tree gene? How come some plants switch it on and other plants
      don't?

      Treeness is a very different sort of thing to "has a backbone"
      a feature which is is inherited though a large part of the animal
      kingdom, there was some root animal which had a backbone and all its
      descendants: fishes, amphibians, reptiles, birds, mammals all share this
      property. Flowering plants also fit well with the botanical systematics,
      all flowering plants (grasses, lilies, orchids, brasicas, roses, oaks
      willows, the pea family, ...) belong to the grouping Angiosperm a well
      define section of the plant kingdom.

      For trees there may have been some very ancient plant which was a tree
      (probably a tree fern). But it's descendants are not all trees, some are
      some are not. (Its possible that all its descendants may share some deeper
      characteristic such as a particular cell structure).

      Perhaps treeness is more akin to having an eye. This is a property which
      has developed independently several times through history, vertebra have
      them, insects have them, squid have them, but none of these share a
      common root which had an eye, they all developed independently (I
      believe its developed twelve different times).

      Or is it more like a true habit like swims in water? Fish do, dolphins
      do it, water snakes do it. All inhabit the same ecological niche
      but have arrived at different ways to fill the niche.

      I don't think the independent evolution idea fits well for trees though.
      If you compare the structure of an oak tree
      and a pine tree they are very similar in terms of cell structure.
      There does seem to be a stronger link between the two than just
      the fact that they inhabit a similar ecological niche. Woody growth
      is a strong shared characteristic of all trees.

      So you could take the radical proposition that all plants (from tree
      ferns onwards) are really trees, just that they have not decided to
      become woody and branch at the base. Or more scientifically their might
      be a "tree gene" shared by a good part of the plant kingdom, but certain
      plants decide to switch this gene on and others don't.

      So in conclusion the best answer might be that the concept of a tree is
      a really complicated thing it means
      perennial
      woody
      branches higher up
      inhabits a particular ecological niche (keep leaves high up).
      In other words a tree is not an atomic thing, unlike flowering plant
      And should not form the base of any serious hierarchy of plant. The
      concept of a tree should be treated with caution.

      ttfn

      Rich

      Object oriented programmers note: A tree is a poor candidate for a base
      class for trees. Perhaps better though of as an interface which certain
      plants can implement. Or maybe don't use the concept tree at all, stick
      to the constituent parts, such a woody, perennial, not branching at base
      and fits a particular ecological niche.

      --
      Plants for a Future: 7000 useful plants
      Web: http://www.pfaf.org/ same as http://www.comp.leeds.ac.uk/pfaf/
      Post: 1 Lerryn View, Lerryn, Lostwithiel, Cornwall, PL22 0QJ
      Tel: 01208 872 963 / 0845 458 4719
      Email: webmaster@...
      PFAF electronic mailing list http://groups.yahoo.com/group/pfaf
    • Richard Morris
      ... Hi Sara, Good to hear from you. Yes I quite agree that we should keep tree in the the database, for all the reasons you discussed. The point of the post
      Message 2 of 2 , Sep 13, 2004
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        Sara Watson wrote:
        > Hi, Richard.
        >
        > I'm new to the group and just read your email and agree that perhaps
        > "treeness" is not the best basis for attributes; however, you have a
        > wide array of users, from those who know the correct botanical
        > terminology to some sloppy endusers, like myself, who still rely on
        > some very basic and general characteristics as a means of
        > researching--so I think for me, searching for "tree" in addition to
        > other characteristics like deciduous or environ, is as valuable as say
        > Fabaceae etc. And if the database is to be looked at as a means of
        > educating the public, it seems important to include the lowest common
        > denominator in addition to very specific informatin for the
        > well-informed.
        >
        > Is the database so limited that it wouldn't provide for mutiple
        > attributes? I'll have to check it out.
        >
        > Thanks,
        >
        > Sara W. Burgess
        >

        Hi Sara,
        Good to hear from you. Yes I quite agree that we should
        keep "tree" in the the database, for all the reasons you discussed.

        The point of the post was really a bunch of random thoughs about what
        the concept of tree means and some musings over quite how patchy the
        occurences of trees are in plant family tree, a fact that I find rather
        curious.

        No need to worry!

        Rich
        --
        Plants for a Future: 7000 useful plants
        Web: http://www.pfaf.org/ same as http://www.comp.leeds.ac.uk/pfaf/
        Post: 1 Lerryn View, Lerryn, Lostwithiel, Cornwall, PL22 0QJ
        Tel: 01208 872 963 / 0845 458 4719
        Email: webmaster@...
        PFAF electronic mailing list http://groups.yahoo.com/group/pfaf
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