Observations of turbulence downwind of a forest-heath interface

Gash, J. H. C.

doi:10.1007/bf00118661

Cited by 91 publications

(72 citation statements)

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“…The heavy line outlines forest. Note the plume-like structure immediately downwind of the edge (after Kruijt, 1994) Gash (1986a), and is consistent with the results published by Gardiner et al (1994). If the momentum flux measurements at canopy level by Gardiner et al are Figure 7 Schematic representation of hypothetical plume of maximum momentum flux downwind of a forest edge (after Kruijt, 1994) at University of Groningen on October 12, 2011 ppg.sagepub.com Downloaded from 306 situation is the reason why the observation by Kawatani and Sadeh (1971) has to be considered support for, rather than proof of the occurrence of a momentum flux plume occurring in nature.…”

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confidence: 90%

“…For a 'dry' to 'wet' change in surface conditions, it is usually assumed -on the strength of the measurements by Rider et al (1963) Gash (1986a) was mentioned in the previous section. His measurements relate to the flow across an interface between heather (predominantly Calluna) and a 9-11 m high forest with pines (P. sylvestris and P. nigra) and larch (Larix decidua).…”

Section: Introductionmentioning

confidence: 99%

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

Veen

Klaassen

Kruijt

et al. 1996

Progress in Physical Geography: Earth and Environment

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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...

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confidence: 90%

Section: Introductionmentioning

confidence: 99%

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

Veen

Klaassen

Kruijt

et al. 1996

Progress in Physical Geography: Earth and Environment

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“…VCT also allowed us to consider two unique landscape-scale predictors of wind damage associated with proximity to, and direction from, forestnonforest edge. The negligible importance of direction of wind exposure and dominant importance of proximity to forest-nonforest edge is likely a consequence of increased turbulence as wind enters (Irvine et al 1997) and exits (Gash 1986) forested areas in addition to asymmetrical tree growth near forest edges (Brisson 2001). Locally, windward forest-nonforest edges may be more susceptible, but overall, simple forest-nonforest edge was the most significant.…”

Section: Discussionmentioning

confidence: 99%

Ecological importance of intermediate windstorms rivals large, infrequent disturbances in the northern Great Lakes

et al. 2011

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Abstract. Exogenous disturbances are critical agents of change in temperate forests capable of damaging trees and influencing forest structure, composition, demography, and ecosystem processes. Forest disturbances of intermediate magnitude and intensity receive relatively sparse attention, particularly at landscape scales, despite influencing most forests at least once per generation. Contextualizing the spatial extent and heterogeneity of such damage is of paramount importance to increasing our understanding of forested ecosystems. We investigated patterns of intermediate wind disturbance across a forested landscape in the northern Great Lakes, USA. A vegetation change tracker (VCT) algorithm was utilized for processing near-biennial Landsat data stacks ) spanning forests sustaining damage from four recent windstorms. VCT predominantly maps stand-clearing disturbance and regrowth patterns, which were used to identify forest boundaries, young stands, and disturbance patterns across space and time. To map wind damage severity, we compared satellite-derived normalized difference vegetation index (NDVI) values calculated from pre-and post-storm Landsat imagery. A geographic information system (GIS) was used to derive wind damage predictor variables from VCT, digital terrain, soils/landform, land cover, and storm tracking data. Hierarchical and random forests regressions were applied to rank the relative importance of predictor variables in influencing wind damage.A conservative estimate of aggregate damage from the intermediate windstorms (extrapolated to ;150,000 ha, ;25,500 severe) rivaled individual large, infrequent disturbances in the region. Damage patterns were relatively congruent among storms and became more spatially heterogeneous with increasing disturbance intensity. Proximity to forest-nonforest edge, stand age, and soils/landform were consistently important damage predictors. The spatial extent and distribution of the first two damage predictors are extremely sensitive to anthropogenic modifications of forested landscapes, the most important disturbance agent in the northern Great Lakes. This provides circumstantial evidence suggesting anthropogenic activities are augmenting and/or diminishing the ecological effects of the natural wind disturbance regime. Natural disturbances of intermediate size and intensity are significant agents of change in this region, and likely in other regions, deserving more attention from ecologists and biogeographers.

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“…In fact, it is possible that the catches of the ground level gauges, 30m from the upwind forest edge, were affected by the change from forest to moorland (Gash, 1986) causing windflow to drop over the ground level gauges, enhancing their catch and exaggerating the apparent difference with the canopy gauges. There is a long-term ground level gauge (D1W) about 350 m from the forest edge at a similar altitude to the two ground level gauges used, here, and its catch over the period is within 2% of the mean of the four canopy gauges.…”

Section: (C) Comparison Of Canopy and Ground Level Gaugesmentioning

confidence: 99%

Measuring rainfall to a forest canopy: an assessment of the performance of canopy level raingauges

Robinson

Grant

Hudson

2004

Hydrol. Earth Syst. Sci.

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Accurate rainfall measurements are crucial for water resource and environmental assessments but can be difficult to achieve in extensive areas of forest. This paper reviews the different techniques for measuring rainfall to a forest area and presents the results of a 5-year experiment comparing the catches of gauges installed above a forest canopy with those of ground level gauges outside the forest. It examines the consistency of the catches between the canopy gauges, the sensitivity of the catch to the height of the gauge rim above the forest canopy, and compares the canopy gauge catches to the true rainfall in ground level gauges just outside the forest. The study shows that suitably designed and maintained canopy gauges can provide consistent measures of rainfall to a forest surface that are sufficiently accurate for most purposes and offer a preferable alternative to measurements in small clearings.

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Observations of turbulence downwind of a forest-heath interface

Abstract: The growth of the equilibrium layer downwind of a forest -heath interface has been observed using eddy correlation measurements made in real time. The atmosphere adjusts more quickly to the transition from heath to forest than to the transition from forest to heath.

Cited by 91 publications

References 12 publications

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

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

Ecological importance of intermediate windstorms rivals large, infrequent disturbances in the northern Great Lakes

Measuring rainfall to a forest canopy: an assessment of the performance of canopy level raingauges

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