Abstract
One of the famous Aesop’s fables nicely illustrates two different strategies the plant may use to withstand the wind forces comparing the oak, which trusts in its strength (stress tolerant strategy) and the reed that bends with the wind to survive (stress avoidant strategy). However each oak was once a small sapling which had to bend as well as the reed before its stem was resistant enough to withstand the wind force in erected state. The stem resistance is primarily determined by the stem diameter and is therefore controlled through the radial growth while the flexibility of a stem of a given diameter depends on the wood mechanical properties, namely the limit of elastic deformation. Further, the drag force exerted by the wind depends on the tree height and crown or sail area. To change the strategy from the stress avoidant to the stress tolerant one, the growing tree needs to adjust its growth together with the wood mechanical properties. Radial changes in wood properties were intensively studied by wood technologists however many different patterns were observed depending on the species, sylvicultural treatment or growth condition without further explanation about the reason for its occurrence. Only recently, the possible adaptive value of the radial variation of wood material properties was hypothesized. In the present work, we investigate the ontogenetic trajectory of a risk of a tree break due to wind on a sample of 18 Douglas-fir trees aiming to capture the effect of social status and also of the thinning treatment. Stem analysis is used to retrospectively obtain the height and diameter growth together with the crown height. Crown area is then predicted from the stem diameter and crown height. On the material level, radial variation of density and microfibril angle is measured to predict the modulus of rupture and the limit of elastic deformation of wood. Simple mechanical model of beam submitted to bending is used to estimate the maximal wind speed the tree is able to withstand without damage. The objective of the work is to investigate if radial variation of wood mechanical properties in particular in young stages may significantly help the growing tree to deal with the wind forces.