Fragmented landscape: Impact on atmospheric flow and tree stability

The wind is responsible of more than half of the damage affecting European forests, e.g., in 2009, Klaus storm directly destroyed 43.1 Mm3 of timber in the Landes Forest in South-west France equivalent to 14% of the standing volume. At present only local stand level factors are taken into account in calculating forest wind risk (Gardiner et al., 2008). However, it is known that the wind is strongly affected by the surfaces over which it has previously flowed. Forest edges in particular play an important role in determining the characteristics of the atmospheric flow by generating turbulence and coherent tree scale structures and represent region of high damage risk (Brunet et al. 2009). In a fragmented landscape, consisting of surfaces of different heights and roughness, the multiplicity of edges may have cumulative effects at the regional scale leading to increased forest damage during storms.

Our main hypothesis is that the level of forest fragmentation affects the characteristics of the atmospheric flow (wind speed, turbulent kinetic energy, etc…), which then have an impact on the level of mechanical stress on trees in the forest. To test this we conducted experiments using a well characterized wind tunnel model with different levels of fragmentation that mimic interfaces from a “short grass” surface to a “tall forest” canopy (Raupach et al., 1986). Measurements were made using 3D Laser Doppler Velocimetry at high spatial resolution. This set of experiments has been used to calibrate a Large-Eddy Simulation (LES) airflow model accounting for the canopy drag (Dupont and Brunet, 2008). The LES is then used to predict the potential impact and risk levels of fragmentation on forest damage in order to try to understand observations of recent storms damage to the forests of South-west France.