Is windswept tree growth negative thigmotropism?

The scientific investigation of the influence of wind on tree growth has been conducted for over 200 years. One influence of wind on trees is the formation of an asymmetric crown, usually characterized as being windswept under moderate windy conditions. As wind exposure increases, the terms applied to this growth form include flag-tree, banner-tree, and krummholz. The modification in crown morphology has been widely recognized and studied, especial in the area of wind prospecting or as a bioindicator of wind speed in environments lacking monitoring stations. However, the causes and physiology underlying this response is little understood. The windswept morphology is consistent with the morphologies associated with other tropisms (i.e. phototropism and gravitropism). Tropisms are defined as a growth response towards (positive) or away (negative) from an environmental stimulus. The asymmetric growth form of windswept trees appears to be a negative thigmotropic growth response to wind. In this review, evidence will be presented to support or reject two hypotheses; H₁ the windswept growth form is the result of a negative thigmotropic growth response or H₂ the windswept growth form is determined by the biophysical properties of wood. It is argued that the windswept growth form is more likely the product of biomechanical properties (accept H₂) than of a physiological thigmotropic growth response (reject H₁). However, proper testing of both hypotheses is still required before a final confirmation can be established.