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Wind Action on Trees
For years we have been researching different ideas to enable us to produce very hardy trees. We recognize that efforts must be made to nurture a more adapted tree in response to our rapid climactic changes. Trees and shrubs will have to withstand more severe wind events, more droughts, and more extreme variability of weather.
So how do we approach this? The first step was to research two different ideas: 1) factors affecting wood structure above the ground and 2) factors affecting root architecture and stability below the ground.
Looking at how trees respond to wind action is not a new idea – just one neglected by the nursery industry. The Greek culture was pondering wind stimuli as far back as 300 BC and coined it , ‘thigmomorphogenesis’. This means , ‘the response of plant growth and development to mechanical stimulation.’ Cities around the world are starting to take wind loading and tree failures and windthrows seriously. As urban centers grow, land scarcity is becoming an issue. Shortages of land is forcing cities to have trees closer and closer to urban structures. The risk to personal safety and building damage are increasing.
Trees respond to wind stimuli at multiple levels. It is truly a remarkable adaptation. These adaptations only occur when trees experience wind loading forces. Also, the duration of these wind loading forces is important where a certain minimum amount of time, ‘ presentation time ‘ , is required where the trees is bent and then allowed to return to vertical posture. Trees must have wind swaying. Developmental changes induced by wind swaying are more overall compact tree form, increase in stem taper, shorter branches and smaller leaves.
Even the type and amount of wood the tree produces is in response to wind load. I always thought, wood was wood. So wrong! The first response is the formation of ‘flexure’ wood on the lee side of the tree. This flexure wood is laid down along the trunk, branch bases and branch nodes. In areas of high wind, trees will initiate the formation of ‘ reaction ‘ wood (the strongest type) in order to decrease stress to the tree. Even the rate of formation of these woods is determined on the wind stimuli. The more stimuli, or stress, the tree senses the faster these specialty woods are laid down.
Trees create props, called buttress roots, which act as a mechanical lever to help hold up a tree. These buttress roots are found around the base of the tree and are a woody growth on the upward side of a lateral root. These props transmit tension and wind load and compression forces down to the roots. Even the shape and structure of roots changes in response to wind stimuli. Tree roots are known to move in the soil as trees sway in the wind. The shape of the roots becomes T shaped and, also, thicker . This T formation occurs on the lee side and trees allocate more root mass on the lee side. The T shape acts like an anchor and helps absorb the compressive forces transmitted to the soil. The windward side of the tree develops I shaped roots. These roots are better designed to flex in response to winds.
Lastly, foliage is affected by wind. Leaves are swept away from the direction of the wind causing the leaves to cluster on the lee side. The clustering of leaves results in a decrease of photosynthesis. Leaves rub together causing distortion or tearing, leading to water loss and stress for the tree. Therefore, the tree responds by producing smaller leaves and changing leaf patterns.
It would seem that we should be producing trees exposed to wind so that natural adaptive processes will occur. This is turn will create strong trees more capable of enduring extreme weather events. Unfortunately, the public demands absolutely perfect trees, especially in trunk straightness. Industry creates these perfect trees by staking.. Unfortunately, staked trees often have thinner roots and once older guyed trees have their support removed, stems break or the tree topples over with high winds.
Here, at Bee Sweet Nature Co, we made the decision to never stake trees and let wind loads shape the tree and initiate strength hardiness. Hopefully, one day, the public will realize that perfect in shape is not always the best tree. As the poet Douglas Malloch wrote,
The tree that never had to fightDouglas Malloch
for sun and sky and air and light
But stood out in the open plain
and always got its share of rain
Never became a forest king
but lived and died a scrubby king.
Good timber does not grow with ease
the strong winds, the stronger the tree
The further sky, the greater length
the more the storm, the more the strength
By sun and cold, by rain and snow
in trees and men good timber grow.