Modelling the windthrow risk for simulated forest stands of Maritime pine (Pinus pinaster Ait.)

Since the December 1999 storms, which caused extensive damage in Western Europe, the need for better forest management tools to calculate the risk of wind damage has been highlighted. Therefore, we constructed a computer model which simulates the mechanical stability of Maritime pine (Pinus pinaster Ait.) stands growing under the silvicultural conditions relevant to the Landes de Gascogne region, SW France. This study integrated (i) a tree growth model, PP3, as implemented in the software CAPSIS able to predict the development of Maritime pine stands over time under different silvicultural scenarios and (ii) a wind risk assessment model, geographical analysis of the losses and effects of storms (GALES), as implemented in the decision support system, ForestGALES. The GALES model predicts the critical mean wind speed required to break or overturn the mean tree in a stand. To adjust the species-dependent relationships for Maritime pine in the GALES model, we used data from winching tests. The effect of different spacings, thinning intensity, site fertility index and the effect of soil types on the critical wind speeds (CWS) were analysed for different silvicultural scenarios. The overturning CWS was usually lower than that for stem breakage, therefore overturning should occur more frequently than breakage in the Landes de Gascogne region. Simulations showed an increase in sensitivity to wind with height (age), whatever the silvicultural scenario. Two silvicultural scenarios with the same site index were simulated by contrasting the effect of initial density and intensity of thinning. Trees in the higher density stand showed a higher slenderness ratio and this stand was more resistant to wind up to an age of 45 years. Using the low-density silvicultural scenario, the effect of two different fertility indices was also tested. The highest site index resulted in a greater sensitivity to wind as tree top height was increased. To test different soil conditions, the empirical parameter determining anchorage resistance in GALES was changed and large differences in CWS were found. More studies are needed to improve the calibration of the aerodynamics model in GALES, taking into account the wide tree spacings and specific management practices used in the Landes de Gascogne region. (c) 2005 Elsevier B.V. All rights reserved.