Wind effects on trees

Abstract

The interactions between airflow and trees and forest stands are diverse. They include the reduction in nearsurface wind speed and production of turbulence by trees. Near-surface wind conditions affect physiological processes in trees, tree growth, and survival (Ennos 1997; Eugster 2008). Turbulent components of the flow field dominate the tree response behaviour (Mayer 1987; Gardiner 1995; Schindler et al. 2010) and drive the scalar exchange at the forest–atmosphere interface (Finnigan 2000). Wind–tree interactions take place at a wide range of temporal and spatial scales (de Langre 2008). Aerodynamic drag at all surfaces of the aerial parts of trees—from individual leaves (Vogel 1989) to whole tree crowns (Kane et al. 2008)—perturbs the airflow inside forest canopies (Shaw et al. 1974; Baldocchi and Meyers 1988; Turnipseed et al. 2003). Therefore, detailed information on forest structure is an essential precondition for understanding wind–tree interactions and the successful application of flow models to tall canopies. In flow models, forest structure is often represented by the mean vertical profile of the plant area density. Queck et al. (2011, this issue) present a method that can be used to record detailed 3D stand structure from terrestrial laser scanning. They investigated the relationship between wind speed, aerodynamic drag, and plant area density and show how 3D laser scanner data can be used to derive turbulence parameters for flow models. The structure of windward forest edges (Mitscherlich 1973; Dupont and Brunet 2008a, b) as well as the stand structure (Gardiner et al. 1997; Marcolla et al. 2003; Dupont and Brunet 2008b, c; Queck and Bernhofer 2010) affects the flow field within and above forests. In the nearedge region, pronounced gradients of flow quantities provoke high wind load on trees (Stacey et al. 1994; Peltola 1996; Gardiner et al. 1997), which may trigger damage in strong wind conditions. Although Gardiner and Stacey (1996) as well as Dupont and Brunet (2008a) report that tapered forest edges reduce wind loading and related bending moments of trees near the edge, it is still not completely clear, how changes in edge and stand structure affect the flow field at the canopy near forest edges, and whether these changes can contribute to the mitigation of damage in high wind conditions. In a detailed wind tunnel study, Ruck et al. (2011, this issue) investigated the effect of changes in taper angle of windward forest edges on the flow field for different stand densities. Their results demonstrate the impact of edge shape and stand density on quantities of the flow field near the canopy top, the region most relevant for tree failure in strong wind conditions. The aerial parts of trees start to vibrate in response to wind excitation (Sellier and Fourcaud 2005; Rodriguez et al. 2008). In forests, dynamic tree responses are not only observed at the single tree level (Mayer 1987; Gardiner 1995; Peltola 1996; Flesch and Wilson 1999; Rudnicki et al. 2008; Schindler et al. 2010) but also at the tree group level (Rudnicki et al. 2001, 2003; Schindler et al. 2011, this issue). At both levels, the resulting tree response patterns are complex and complicated. So far, even in horizontally homogeneous terrain, not all physical and biological This article belongs to the special issue ‘Wind Effects on Trees’.