Long-distance edge effects in a pine forest with a deep and sparse trunk space: In situ and numerical experiments

Abstract

Abstract As forest edges are a major source of heterogeneity in fragmented landscapes, the atmospheric flow over forested areas is often under their influence. Understanding how far the upstream edge has an impact on the turbulent wind flow in a forest canopy is important, in particular for scalar flux measurement. In this study, edge and stand flows over a maritime pine forest characterized by a dense crown layer located above a deep and sparse trunk space are analysed in detail from in situ measurements and large-eddy simulations (LES). The LES model used here appears to simulate remarkably well most characteristics of the turbulent wind flow for this particular canopy structure. It is shown that the main characteristics of the edge flow in this case differ from those usually observed in forests with a more uniform vertical foliage distribution. The main differences are (i) the development of turbulence above the canopy occurring closer to the edge, (ii) the absence of a well-defined enhanced gust zone around the top of the canopy, (iii) the presence of a large secondary wind maximum within the trunk space, and (iv) the development of a positive momentum flux layer below the crown layer. Most of these differences are related to the presence of a substantial sub-canopy wind jet induced by the wind flow through the trunk space at the edge. The secondary velocity maximum induced by this wind jet differs from that observed in homogeneous stand conditions, where it seems to be related to the mesoscale pressure gradient. The wind jet appears to decrease very slowly with distance from the edge, so that edge effects are still significant at 9 h from the edge (where h is the mean canopy height). The length of the adjustment region is shown to be greater than 10–15 h, and to depend on the depth of the trunk space. In very fragmented forested areas with deep and sparse trunk space, within-canopy flow may always be under the influence of edges.