Influence of gap size on wind damage variables in a forest

Windstorms are the major disturbance factor in tropical and European forest ecosystems. An airflow model can provide the basis to interpret spatial patterns of wind damage on trees, and guide strategies with respect to that concern. In contrast to recent advances on modelling of perturbed canopy flows, few studies have considered the effects of canopy inhomogeneity on the pattern of flow statistics, which in its turn describes the relative risk of wind damage on trees.

An atmospheric boundary-layer two-equation closure model SCADIS based on transport equations for turbulent kinetic energy (E) and specific dissipation (ω=ɛ/E, where ɛ is the dissipation of E) (E–ω model), which accounts for the flow dynamics within a plant canopy [Sogachev, A., Panferov, O., 2006. Modification of two-equation models to account for plant drag.

Bound. Lay. Meteorol. 121, 229–266] was used to carry out a series of numerical experiments with gap sizes from 3 to 75tree heights, h, in a modelled forest. Spatial variations of integral wind loading presented as a sum of static and dynamic (gust) components on trees around the gaps were estimated from modelled data.

To quantify the changes of wind load characteristics due to gap growth relatively to the undisturbed forest they were normalized by the correspondent values for that forest. The results show that for round gaps the maximal static wind loading on trees surrounding the gap as large as 75h increases up to 14 times of that for undisturbed forest.

The maximal static load is located on the exposed (or downwind) gap edge independently of gap size. The maximal value of the gust component increases with the gap diameter up to the gap size of 20h only, where it is 2.6–3.0 times higher than for undisturbed forest, and remains constant for larger gaps.

With the growth of gap size the area of maximal values of E shifts from downwind gap edge to the lateral borders of the gap increasing the contribution of gust loading there. Thus, the integral wind loading increases nonlinearly with gap size and for the gap size of 75h it can be up to seven times higher than that for undisturbed forest.

The spatial distribution of maximal values of integral loading is similar to that of static loading up to gap size of 20h. For larger gaps the location of integral loading maximum shifts gradually towards lateral borders with increasing of gap diameter.