Habituation to wind as a prime driver of secondary growth in forest trees: How trees filter chronic wind-signals to acclimate strongly to high winds
Résumé
Controlled experiments have shown that secondary growth is extremely sensitive to static bending strains. However, the strain regime in nature is more complex, due to a full spectrum of winds. We hypothesized that ii) the sensing of wind-induced strains is a major driver of wood production in forest conditions and ii) trees avoid overreacting by responding only to winds which bring information on exposure to high wind.To assess hypothesis 1, the strain regimes of beech trees in an un-thinned acclimated stand was manipulated by comparing trees free to sway with guyed trees (as a control treatment for the effects of all the factors but wind-induced trunk bending). Radial growth was then monitored during 3 years using dendrometers. Depending on the wind regime of each year, wind-induced strains increased radial growth by +37% à +95 % by. Wind is thus a major driver of wood growth even in conditions of competition for the access to light.To investigate possible wind habituation, we submitted the unguyed trees to artificial bending with strain magnitudes mimicking the magnitude of wind with various mean return time during the leaf-on period, ranging from the mean daily (thermal) winds to the strong winds that occur once to twice a year [2]. It was found that trees only responded to the unusual wind events, but responded massively to it ( + 76 to + 156 % depending on the social status). Tree acclimation to wind is thus forced through a strain filter enhancing the detection of unusual wind events. It is argued that such habituation requires a memory, and allows trees to get a more relevant information for the estimation of their exposure to wind hazards.