Impact of stand density on tree morphology and growth stresses in young beech (Fagus sylvatica L.) stands

Key message According to biomechanical processes, tree morphology (trunk inclination, height-to-diameter ratio and crown area) explains statistically silvicultural effects on growth stress variation. Abstract Growth stresses constitute the main mechanism allowing the tree to control its posture against its mechanical environment, but are also among the most important factors contributing to the depreciation of timber value. This study aimed at assessing the link between stand planting density and growth stress level in European beech (Fagus sylvatica) stands. Beech seedlings were planted in four plots corresponding to four planting densities: 2,500, 5,000, 10,000 and 40,000 stems/ha. They were left to grow for 26 years without any intervention, resulting in trees with highly different morphologies but of the same age and provenance. After 26 years of growth, both the tree morphology and growth stress indicators were measured on the standing trees in each plot and an attempt was made to establish a link between them. Our results showed that initial stand density influences growth stresses of the first order as a result of its impact on tree morphology. The best predictors of high growth stresses were high trunk inclination, high height-to-diameter ratio (slenderness factor) and low crown area. According to mechanosensing theories, these morphological criteria emphasised that growth stresses are due to a global mechanical stimulation rather than to local stem inclination alone. Research now has to be undertaken on new methods that combine the integrative assessment of tree morphology as well as its monitoring over time.