It’s all about the light, or is it?

Plants capture photons of energy from sunlight with some sophisticated photosynthetic chemistry.  But to maximize that energy gain, they have to position their leafy photo-capture surfaces in particular ways.

Rounded dome shape ensures that leaves get maximum exposure to light.

A rounded dome shape of this tree manzanita ensures that leaves get maximum exposure to light at the outer surface of the dome.

I have always been fascinated by the bizarre geometry of oak trees, especially the California Live Oak whose thick horizontal branches seem to defy gravity as they grow outward from the central trunk dozens of feet without any vertical supports.

Live oaks seem to defy any organizing principle for growth as their branches spread horizontally in every direction.

Live oaks seem to defy any organizing principle for growth as their branches spread horizontally in every direction.  

Lateral spread accomplishes at least two important things for oak trees:  it shades out potential competitors, and it avoids the problem of upper leaves shading lower leaves, thus maximizing the photosynthetic surface area.

A grove of live oak present an otherworldly scene.

A grove of live oak presents an otherworldly scene of twisted shapes and dappled light.  Is this an adaptation to this particular environment?

A different strategy is to grow above the canopy of your neighbor to avoid being shaded out — so well illustrated by the giant Redwoods.

From:  http://www.npr.org/blogs/pictureshow/2009/09/redwoods.html

From: http://www.npr.org/blogs/pictureshow/2009/09/redwoods.html , originally published in National Geographic magazine, Oct. 2009.

Normally, branch stem, and root growth are controlled by gravity sensors (statocytes) that stimulate elongation of cells on one side of a stem relative to the other, bending the growth tip. This gravitropic response allows roots to grow downward and shoots to grow upward regardless of their lateral or vertical (e.g., upside down) placement, but it doesn’t explain why oaks preferentially branch out horizontally or maple tree branches grow in a V-shape.

Recently, researchers at Leeds University have identified an “anti-gravitropic” response opposing the gravitropic one on the other side of the stem, such that branches and stems are maintained at a pre-set angle from the main trunk.  The particular angle of growth is adaptive to a set of environmental conditions:  best angle for shedding heavy snow, best angle for resisting strong horizontal winds, etc.

Isn’t nature amazing?

5 thoughts on “It’s all about the light, or is it?

  1. I was up in the high Sierras a couple of weeks ago marveling at the twisted old pines and cedars that survive on the wind-swept granite. Up there the survival strategy seems to be to create a modest, aerodynamic canopy, but otherwise focus on anchoring a sturdy, unbreakable trunk into whatever trace of dirt exists. I wish I could see that root system.

    • Good observation — I have seen those pines, and they are indeed remarkably twisted into spirals. I believe your analysis is exactly right; they survive by avoiding wind damage to limbs and trunk. It looks like the roots are a mirror of the above ground parts, twisted into spirals to resist pulling forces in all directions (check google images of bristlecone pine roots). Thanks for writing.

  2. I’ve probably said this before, but… the awesome power of evolution!

    Another amazing fact about the oak tree, at least the UK ones, is that they are an ecosystem in their own right, and I believe they support up to 1000 other species of microflora and fauna, insects, mammals and birds. And, of course, generations of children climbing them and hanging rope swings from their branches. I love them!

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