Growth prestresses and regulation of movements in living trees

The mechanisms that lead to prestresses (usually called "growth stresses") in wood of growing trees are described and modelled. The functional advantages of these prestresses are described. A special emphasis is put on the asymmetry (i.e. the circonferential heterogeneity) of wood differentiation, that is commonly observed and allows bending movements of the stem. then, a biornechanical model is proposed for the regulation of the longitudinal form of the tree stem (considered as a bent growing beam) form. (By "longitudinal form", we mean leans and curvatures). The biological mechanisms involved in the regulation and their analogy with engineering devices are discussed. The biornechanical model is applied to the righting movements of a leaning trunk. A previous experiment has shown a two step - righting process: first, the whole stem bent up and curved-, then, the lower part went on bending up while the upper part straightened itself by a bending down movement. Such a two-step process have been observed also in the apical zone of trees or in herbaceous plants during primary growth. This leads to sinuous but quite vertical stems. It is important to notice that the upper part of the plant anticipates the curvature of the basis and begins to bent down when the whole stem is still severely inclined. Therefore, the righting process shows complex regulations that involves both proportional and differential perception. Finally, this model is discussed in relation to other biomechanical works dealing with the regulation of growth and wood formation in tree stems. (Résumé d'auteur