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

Forest edges are a major source of heterogeneity in fragmented landscapes. Consequently, the atmospheric flow over forested areas is often under their influence. Understanding how far a forest edge may influence turbulent wind flow within the canopy is important, in particular for scalar flux measurement. In this study, the edge flow over a maritime pine forest characterized by a high and dense crown layer associated with a large and sparse trunk space is analyzed in detail from in situ measurements and large-eddy simulation (LES). The LES model appears to simulate remarkably well most characteristics of the turbulent wind flow in this particular case of canopy foliage distribution. It is shown that the main characteristics of the edge flow differ from those usually observed in forests with a more uniform vertical foliage distribution. The main differences are (i) the presence of a large secondary wind maximum within the trunk space, (ii) the development of the turbulent region above the canopy occurring closest to the edge, (iii) the absence of a well-defined enhanced gust zone 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 strong sub-canopy wind jet induced by the wind flow through the trunk space at the edge. This wind jet decreases 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). This implies that the length of the adjustment region is probably more than 10-15 h, and therefore that in very fragmented forested areas with large and sparse trunk space, within-canopy flow may always be under the influence of edges.