ABSTRACT
It is proposed that the asymmetrical branches used by many trees, especially trees in the southeastern North American hurricane zone and the glaze ice zone , have evolved in order to protect the tree from the wind damage that breaks the trunk by rhythmical gusts.
INTRODUCTION
Many tree species form branches which are of widely different angles with the trunk, which have different lengths at the same altitude, and which change direction within the limb randomly. This phenomenon is so consistent within a species and so independent of any obvious external forcing agents that it must be genetically controlled. The limb changes direction of growth at random angles in the absence of any obvious external trigger. Indeed, such a growth habit is actually usually counter productive to a plant's struggle for light. Even if there were a trigger, it would still require something genetic to respond to it. I suspect that the evolutionary forces creating this situation are the extreme breakage forces that can be created in a structure when gusts of a high wind are spaced apart in the same frequency as the structure's oscillation frequency.
DISCUSSION
It is known that wind gusts acting in the same frequency as a structure's oscillation frequency can cause severe damage. [Geiger, 1950, p 302 ]. It is such a circumstance which caused the failure of the Tacoma, Washington, bridge. A tree can be especially in danger of breaking off at the trunk or being uprooted if it is weighted down with ice [Bennett, 1959]. These last two are the worst kind of damages a tree can have. A way trees can reduce this risk is by having each limb project out at different angles, lengths, and shapes. Thus each limb has a different oscillation frequency and moves in different directions at the same time. As a result the forces on the trunk and even within the limb cancel each other out. One can easily see this phenomenon in action on a deciduous tree during a high winter wind when the branches move in directions opposite to each other at the same time. The above phenomenon appears to be developed the best in dicotyledon angiosperms. The modern ginko has such a development, also. I do not know about ancient species.
CONCLUSIONS
One would expect that trees using asymmetric growth would be characteristic of areas subject to strong winds. Indeed, such branches are characteristic of the southeast of North American hurricane zone. Even the conifers there utilize such a growth. The pitch pine (Pinus rigida) has fairly well developed asymmetry for instance. It is also characteristic of mid section of that continent where winds can reach speeds greater than 200 kilometers per hour at times. In higher latitudes where light strikes at an angle to the ground, wind is a little lighter, and ground frozen in winter prevents toppling at that time, shedding of snow loads tends to dictate a different kind of growth. Asymmetrical branches are not usually well developed in shrub vegetation there either. Asymmetrical branches are rarely seen on trees that have very flexible trunks. such as eastern red cedar (Juniperous virginiana) or palm trees. It is not usually well developed in climbing vines. One would not expect to see asymmetrical branches in the under story of a dense rain forest where light is the main limitation and plants are shielded from wind or in shrubs which have no trunk. I hope forest ecologists from such areas will observe if this is indeed the case.
REFERENCES are below
Deciduous Forest, a Function of Glaze Ice: Damage from glaze ice creates the temperate deciduous forest.
Origin of the Angiosperms: Angiosperms are proposed as arising on the Ontong -Java Plateau in Permian time.
Did the Wood Roach Cause the Permian-Triassic Coal Hiatus?: Wood roaches may have caused Permian conifer success and reduced mulch, starting in North America.
The Rise of the Termites: Cretaceous termites may have accelerated the spread of angiosperms and conifers and furnished silica for diatoms.
Alkaline Guts of Termites may cause Silica Loss from Soil.: Extreme alkalinity of humus eating termites may be causing silica to leach from tropical soil to produce bauxite and laterite and provide silica for plants.
Cretaceous Soil Phosphorus: Runway building termites may have caused a Cretaceous and Paleocene phosphorus famine, decreased tooth and bone formation because of that phosphorus famine, and also, paradoxically, greater plant diversity.
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REFERENCES
Bennett, I 1959 Glaze, its meteorology and climatology, geographical distribution, and economic effects. Environmental Protection Research Technical Report EP-105 U.S. Quartermaster Research & Engineering Center, Natick, MA, USA.
Geiger, R. 1950 The Climate Near the Ground, Stewart, N.M., translator. Harvard University Press, Cambridge, MA, USA.
This site updated in July 2008