Trees

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The following was taken from an ecology listserv:

Dear ECOLOGGERS:

Apologies for maybe waxing a bit too poetic here but I used to use cellular responses as a way to detect and measure stressors. Looking out my window, I now see trees with that same world view and wonder “How are trees (or any organism’s) forms informing us?” What I see  (and it doesn’t really matter where or what they are) are odd forkings, broken spars, and twisted limbs. To what degree can the shapes of living things tell us about their life and ecosystem?

Just wondering what literature there might be or what your thoughts are if you’ve pondered such things yourself.

Cordially yours,
GW Patton
Silver Spring, MD

Hi, GW Patton,

Trees are excellent organisms to consider with regard to your question.  For one thing, a great part of the young tree is contained within the old tree, buried under layers of newer wood.  Even a tree whose bole is clear because it has shed its earlier branches still contains within it the traces of branches (i.e., knots) that tell when and where they were initiated and lost.

Even without cutting into the wood, one can learn from trees much of the history of their environment.  For instance, a tree growing in the open will typically have a broad, spreading crown (like a tree left in a meadow to provide shade for cows).  If the meadow is abandoned for grazing, the progeny of that tree (and others) will grow into a dense second-growth forest.  Now the spreading-crowned meadow tree finds itself surrounded by and soon out-topped by its tall, slender progeny.  The response, in trees with sufficient plasticity (which is most), is to put up tall, vertical stems off of the largely horizontal, spreading branches, resulting in a candelabra -like form that will be recognizable for decades in the forest.

Tree form also reveals the logging history of a forest stand.  Many years after a forest is logged species such as oaks, that produce stump sprouts, will present not as scattered individuals, but rather as groups of two, three, or four closely spaced trunks, which are the surviving sprouts of the  parent stump, and are of course, in genetic terms, still the original tree.  These clustered trunks will have few and small branches directed into the center of the group and will have most of their branches pointing outward, together forming a complete crown.

Trees show land-use changes in many ways.  When a road cut is put through a forest, species that don’t re-sprout well, such as pines, will show a long, clear boles, very different from the form a pine would achieve if it grew from seed at the edge of a clearing.  Trees that DO sprout well will put new branches into the new clearing, but it will be obvious that these are secondary branches because they will be initially small compared to the mature branches higher up in the crown.  Cutting into the tree will show that the knots of these branches do not extend all the way into the center of the tree, but rather originate in the growth ring of the year of the clearing event, or a little later.

Trees growing on wind-swept ridges show a flag-form, with most of the branches pointing down-wind, and trees in salt-spray zones are “pruned” to air-foil shapes reflecting the prevailing winds..

Examining individual twigs tells much about the recent history of a tree’s environment.  By measuring the distance between bud-scale scares one can learn how much the shoot has grown over many years.  This record is reliable until the bud-scale scars become unrecognisable due to radial expansion of the shoot.  For an axis that is producing short extensions and little diameter growth, the scars may be countable for up to thirty or more years.  One will sometimes see a switch to producing long extensions, probably reflecting some change in the immediate environment, such as the death of a nearby branch, letting in more light that stimulates growth.  Occasionally, such an axis will revert to producing short extensions, probably reflecting the growth response of surrounding branches closing the temporary light gap.

For most common temperate-zone tree species the normal growth habit is to produce rather straight twigs.  When one sees twisted, gnarly branches, typically in mature trees that are no longer growing vigorously, this reflects bud mortality.  Every twist or turn in the branch is caused by the terminal bud having died (from insect damage, fungal attack, freezing, squirrel bites, etc.), de-repressing a near-by (basipetally) axillary bud, which changes the direction of extension until another tip die-back sends another bud off in a new direction.  As the branch thickens the details of these mortality evens are not visible on the surface, but careful dissection with a sharp chisel reveals them buried in the wood.

Hey, I could go on and on, but I hope all this addresses the question you raised.

Martin M. Meiss

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