Forest edges and the soil-vegetation- atmosphere interaction at the landscape scale: the state of affairs

Abstract

Although the soil-vegetation-atmosphere exchange of momentum and heat is fairly well understood for many types of homogeneous surfaces, the disturbances created by transitions of one surface type to another remain to be analysed more fully. This is especially true for the impact which a large transition such as the forest edge has on the average fluxes in a small-scale heterogeneous landscape with forest. Recently acquired experimental evidence appears to some extent contradictory and at variance with conventional concepts.Key words: forest edges, momentum fluxes, energy balance, land surface heterogeneity, advection, area averaging of fluxes. I IntroductionObservations on the water balance of forests have shown that the generally high water use of forests -as compared to, for example, low agricultural crops -is mainly caused by high evaporative losses of intercepted rainfall. One-dimensional modelling, reviewed by Veen and Dolman (1989), suggests that a high evaporation rate of intercepted water is explained to a large extent by the high aerodynamic roughness of forests. This can be easily understood. Strong absorption of the kinetic energy of the air flowing over the rough canopy creates strong turbulence. The upward motions in the eddies very quickly transport water vapour away from the wet, evaporating surface. When the leaves are dry, turbulence is no less intensive. But in a dry canopy the stomata at University of Groningen on October 12, 2011 ppg.sagepub.com Downloaded from 293 regulate the transpiration rate. The surface resistance of forests is, therefore, much larger than the aerodynamic resistance. As in low vegetation it increases with decreasing availability of -among others -light and soil moisture. Water losses through transpiration in forests have turned out to be similar to those of other vegetation types.The notion that forests present a very rough surface to the flow of air is important not only for the fluxes of water vapour but also for the other quantities that are transported by turbulent diffusion. The downward motions in eddies of the strongly turbulent wind field over forests transport gases and aerosols towards the land surface more efficiently than in the much quieter flow patterns across smooth, i.e., low vegetation. The flux of acidifying air pollutants from the atmosphere towards the forest soil is, therefore, significantly larger than the flux to arable soils. Measurements and calculations, for example, in the Dutch ACIFORN project (Van Aalst and Erisman, 1991) have demonstrated this.Aerodynamic roughness also plays a role in the influence of forest on climate. The direct effect of forests on the atmosphere stems from high roughness in combination with low albedo. Low albedo causes much radiative energy to be available for conversion into sensible and latent heat. High roughness makes for low resistance to heat fluxes from the forest to the atmosphere. Extensive regions covered by forest significantly affect the properties of air masses overhead. At the continental scale...

The Spatial Variability of Turbulence above a Forest

“…This complicates the transition process (Gardiner et al, 1995;Irvine et al, 1997). Forest edges cause strong local perturbation of the¯ow (Veen et al, 1996). Decreases of diffusivity down to 60% can occur after a grass-forest transition, which slowly recover to normal values at more than ten times the canopy height (Kruijt, 1994).…”

The impact of broadleaved woodland on water resources in lowland UK: III. The results from Black Wood and Bridgets Farm compared with those from other woodland and grassland sites

“…Gash, 1986;Gardiner et al, 1994), a good basis does not yet exist on which to predict the exact extent of the edge of individual forests and woodlands in terms of turbulent exchange (Veen et al, 1996) While studies in native broadleaved woodlands in New Zealand by Young and Mitchell (1994) and Davies-Colley et al (2000) showed that microclimatic influences into woodland from surrounding vegetation can penetrate only about 50 m, Chen et al (1995) reported that edge effects on microclimate may extend as far as 240 m into a Douglas fir forest. So, although it could be postulated that this penetration of edge microclimate might influence evaporative conditions for any understorey and the lower parts of tree canopies, no measurements are known to support such suggestions.…”

Micro- and mesoclimatology

“…Some other studies of small-scale landscape complexity concern the environment of forest edges (Liu et al, 1996;Veen, 1996), the energy budgets of complex plant canopies (McInnes et al, 1996;Tournebize et al, 1996), the spatial variability of soil properties (Kim and Stricker, 1996;Lewan and Jansson, 1996;Mahmood, 1996) and evapotranspiration and energy budgets for a multispecies plant canopy in a semi-arid watershed (Flerchinger et al, 1996).…”