1,354,137 research outputs found
Influence of gap size on wind damage variables in a forest
No full textWindstorms are the major disturbance factor in tropical and European forest ecosystems. An airflow model can provide the basis to interpret spatial patterns of wind damage on trees, and guide strategies with respect to that concern. In contrast to recent advances on modelling of perturbed canopy flows, few studies have considered the effects of canopy inhomogeneity on the pattern of flow statistics, which in its turn describes the relative risk of wind damage on trees. An atmospheric boundary-layer two-equation closure model SCADIS based on transport equations for turbulent kinetic energy (E) and specific dissipation (omega = epsilon/E, where E is the dissipation of E) (E-omega model), which accounts for the flow dynamics within a plant canopy [Sogachev, A., Panferov, O., 2006. Modification of two-equation models to account for plant drag. Bound. Lay. Meteorol. 121, 229-266] was used to carry out a series of numerical experiments with gap sizes from 3 to 75 tree heights, h, in a modelled forest. Spatial variations of integral wind loading presented as a sum of static and dynamic (gust) components on trees around the gaps were estimated from modelled data. To quantify the changes of wind load characteristics due to gap growth relatively to the undisturbed forest they were normalized by the correspondent values for that forest. The results show that for round gaps the maximal static wind loading on trees surrounding the gap as large as 75h increases up to 14 times of that for undisturbed forest. The maximal static load is located on the exposed (or downwind) gap edge independently of gap size. The maximal value of the gust component increases with the gap diameter up to the gap size of 20h only, where it is 2.6-3.0 times higher than for undisturbed forest, and remains constant for larger gaps. With the growth of gap size the area of maximal values of E shifts from downwind gap edge to the lateral borders of the gap increasing the contribution of gust loading there. Thus, the integral wind loading increases nonlinearly with gap size and for the gap size of 75h it can be up to seven times higher than that for undisturbed forest. The spatial distribution of maximal values of integral loading is similar to that of static loading up to gap size of 20h. For larger gaps the location of integral loading maximum shifts gradually towards lateral borders with increasing of gap diameter. (c) 2008 Elsevier B.V. All rights reserved
Changes of forest stands vulnerability to future wind damage resulting from different management methods
No full textThe structure of forests stands changes continuously as a result of forest growth and both natural and anthropogenic disturbances like windthrow or management activities – planting/cutting of trees. These structure changes can stabilize or destabilize forest stands in terms of their resistance to wind damage. The driving force behind the damage is the climate, but the magnitude and sign of resulting effect depend on tree species, management method and soil conditions. The projected increasing frequency of weather extremes in the whole and severe storms in particular might produce wide area damage in European forest ecosystems during the 21st century. To assess the possible wind damage and stabilization/destabilization effects of forest management a number of numeric experiments are carried out for the region of Solling, Germany. The coupled small-scale process-based model combining Brook90 [1] and SCAlar DIStribuiton turbulence model [2-4] is implemented. The SRES climate scenarios A1B and B1 dynamically downscaled by Climate Local Model CLM [5] are used to project the future climate conditions in the area. The experiments are performed for two tree species (spruce and beech) and a mixed stand and for two target diameter harvesting scenarios. The results show considerable increment of wind damage risks towards 2100 compared to “present climate conditions”, caused by the combination of weak increase of wind speed and precipitation and strong increase of air and soil temperature. The effect is stronger for coniferous species than for deciduous ones. It is shown that management activities have a strong destabilizing effect on forests due to joint influence of climatic factors and decrease of stand density
Application of a three-dimensional model for assessing effects of small clear-cuttings on radiation and soil temperature
No full textA three-dimensional model Mixfor-3D of soil–vegetation–atmosphere transfer (SVAT) was developed and applied to estimate possible effects of tree clear-cutting on radiation and soil temperature regimes of a forest ecosystem. The Mixfor-3D model consists of several closely coupled 3D sub-models describing: forest stand structure; radiative transfer in a forest canopy; turbulent transfer of sensible heat, H2O and CO2 between ground surface and the atmospheric surface layer; evapotranspiration of ground surface vegetation and soil; heat and moisture transfer in soil. The model operates with the horizontal grid resolution, 2 m × 2 m; vertical resolution, 1 m and primary time step, 1 h. The model was tested against meteorological data obtained at a small clear-cutting area in Otterbach in central Germany during summer 2005. The meteorological data including air temperature and humidity, precipitation, solar radiation, wind speed and direction, soil temperatures at 10 and 20 cm depth were measured by five automatic stations within the clear-cut area. One reference station was placed about 100 m from the clear-cut inside the forest stand. Comparisons of modelled and measured solar radiation fluxes and soil temperature profiles showed that the model adequately describes the spatial heterogeneity and dynamics of these variables under different weather conditions. The model can be used to explore solar radiation and soil temperature patterns within heterogeneous forest plots, with applications to various silvicultural tasks
Feedbacks of windthrow for Norway spruce and Scots pine stands under changing climate
No full textWind damage is one of the major natural disturbances that can occur worldwide in most types of forests. Enhanced management using adequate decision support systems (DSS) can considerably reduce the risk of windthrow. The decision support system 'Forest and Climate Change' (DSS-WuK) which is currently being developed at Gottingen University aims at providing a tool for the quantitative assessment of biotic and abiotic risks for forest ecosystems under the conditions of changing climate. In order to assess the future risks of wind damage the system employs a coupled modelling approach combining the turbulence model SCAlar DIStribution (SCADIS) with the soil-vegetation-atmosphere-transfer (SVAT) model BROOK 90. The present study investigates projections of wind damage in Solling, Germany under climate scenarios A1B and B1, taking into account the windthrow feedbacks-changes of microclimate as a result of tree fall and consequent stabilization or destabilization of a forest stand. The results of the study indicate that in Solling the risk of windthrow for spruce and pine forest stands is likely to increase considerably during the 21st century. The general tendencies indicate that under A1B the probability of damage would be higher than under B1 and that under the same climate and soil conditions the risk for spruce stands would be higher than for pine stands of equal age. The degree of damage and feedback contribution as well as a sign of feedback in each particular case will strongly depend on the particular local or regional combination of climatic and soil factors with tree species, age and structure. For Solling the positive feedback to local climatic forcing is found. The feedback contributes considerably (up to 6% under given conditions) to the projected forest damage and cannot be neglected. Therefore, the adequate projection of future damage probabilities can be performed only with a process-based coupled soil-atmosphere model with corresponding high spatial and temporal resolution
Forests as protection against airborne immissions
No full textThe effect of a spruce forest in the Solling-hills (Germany) on the concentration of airborne trace compounds in the atmospheric boundary layer is discussed. The discussion is based on field measurements of vertical concentration profiles and vertical fluxes of reactive trace gases and particles in and above a spruce forest and on numerical modelling. Measured SO2-, O-3- and PAN-concentrations are 10 % to 20 % lower near the forest floor than just above the canopy. NO2 is emitted from the forest into the atmospheric boundary-layer and NO is transported both from the atmosphere and from the forest floor into the spruce canopy air space. The net NOx-flux between the atmosphere and this spruce forest can be neglected compared quantitatively to other N-fluxes. Numerical experiments using two models show that a 1000 m long spruce forest reduces the near surface concentration of an airborne trace substance with a deposition velocity of about 0.7 cm/s by up to 6 % as compared to the upwind SO2-concentration. If a forest is replaced by a meadow the SO2-concentration at the former downwind side of a forest increases by about 10 %
Modification of two-equation models to account for plant drag
No full textA modification of the most popular two-equation (E-phi) models, taking into account the plant drag, is proposed. Here E is the turbulent kinetic energy (TKE) and phi is any of the following variables: El (product of E and the mixing length l), epsilon (dissipation rate of TKE), and omega (specific dissipation of TKE, omega = epsilon/E). The proposed modification is due to the fact that the model constants estimated experimentally for 'free-air' flow do not allow for adequate reconstruction of the ratio between the production and dissipation rates of TKE in the vegetation canopy and have to be adjusted. The modification is universal, i.e. of the same type for all E-phi models considered. The numerical experiments carried out for both homogeneous and heterogeneous plant canopies with E-phi models (and with the E-l model taken as a kind of reference) show that the modification performs well. They also suggest that E-epsilon and E-omega schemes are more promising than the E-El scheme for canopy flow simulation since they are not limited by the need to use a wall function. In addition, a new parameterization for enhanced dissipation within the plant canopy is derived. It minimizes the model sensitivity to C mu, the key parameter for two-equation schemes, and whose estimates unfortunately vary considerably from experiment to experiment. The comparison of results of new modified E-epsilon and E-omega models with observations from both field and wind-tunnel experiments shows that the proposed parameterization is quite robust. However, because of uncertainties with the turbulence Prandtl and Schmidt numbers for the E-epsilon model within the canopy, the E-omega model is recommended for future implementation, with the suggested modifications
Numerical analysis of flux footprints for different landscapes
No full textA model for the canopy - planetary boundary layer flow and scalar transport based on E-is an element of closure was applied to estimate footprint for CO2 fluxes over different inhomogeneous landscapes. Hypothetical heterogeneous vegetation patterns - forest with clear-cuts as well as hypothetical heterogeneous relief - a bell-shaped valley and a ridge covered by forest were considered. The distortions of airflow caused by these heterogeneities are shown - the upwind deceleration of the flow at the ridge foot and above valley, acceleration at the crest and the flow separation with the reversed flow pattern at lee slopes of ridge and valley. The disturbances induce changes in scalar flux fields within the atmospheric surface layer comparing to fluxes for homogeneous conditions: at a fixed height the fluxes vary as a function of distance to disturbance. Correspondingly, the flux footprint estimated from model data depends oil the location of the point of interest (flux measurement point) and may significantly deviate from that for a flat terrain. It is shown that proposed method could be used for the choice of optimal sensor position for flux measurements over complex terrain as well as for the interpretation of data for existing measurement sites. To illustrate the latter the method was applied for experimental site in Selling, Germany, taking into account the complex topography and vegetation heterogeneities. Results show that in certain situations (summer, neutral stratification, south or north wind) and for a certain sensor location the assumptions of idealized air flow structure could be used for measurement interpretation at this site, though in general, extreme caution should be applied when analytical footprint models are used in the interpretation of flux measurements over complex sites
Evapotranspiration and heat fluxes over a patchy forest - studied using modelling and measurements
No full textMost forests in Europe are too small to fulfill strict fetch requirements associated with idealized flux observations. As a consequence of limited fetch, the flux measured above the canopy will often deviate from the source strength underlying the measurements, i.e. observations of sensible and latent heat flux above forest downwind of a forest edge show these fluxes to be larger than the available energy over the forest (Klaassen et al. 2002, Theor. Appl. Climatol. 72, 231-243). Because such flux measurements are very often used for calibration of forest parameters or model constants, further using these parameters without a proper interpretation in mesoscale or global circulation models can results in serious bias of estimates of modelled evapotranspiration or heat fluxes from given area. Since representative measurements focused on heterogeneous effects are scarce numerical modelling can be used to interpret the measurements. Recently, the atmospheric boundary layer (ABL) model SCADIS (Sogachev et al., 2002, Tellus 54B, 784-819) has been successfully applied to analyze the mechanisms of CO2 flux formation near a forest edge for neutrally stratified conditions (Sogachev et al., 2008, Ecological. Appl. 18, 1454-1459). In the present work, we apply the SCADIS with enhanced turbulence closure including buoyancy for investigation of the spatial distribution of latent and sensible heat vertical fluxes over patchy forested terrain in Denmark during selected days in the summer period. A closer look at the result shows that though the meteorological mast is located in the middle of a forest patch by size about 1x2 km2, it is not free from uncertainties regarding energy balance closure. Comparing observed and SCADIS (1D and 3D) simulated data for the mast confirms that caution is needed when interpreting measured flux data. The approach used in this work can be utilized in interpretation of already existed experimental data and in the planning of future experiments.Most forests in Europe are too small to fulfill strict fetch requirements associated with idealized flux observations. As a consequence of limited fetch, the flux measured above the canopy will often deviate from the source strength underlying the measurements, i.e. observations of sensible and latent heat flux above forest downwind of a forest edge show these fluxes to be larger than the available energy over the forest (Klaassen et al. 2002, Theor. Appl. Climatol. 72, 231-243). Because such flux measurements are very often used for calibration of forest parameters or model constants, further using these parameters without a proper interpretation in mesoscale or global circulation models can results in serious bias of estimates of modelled evapotranspiration or heat fluxes from given area. Since representative measurements focused on heterogeneous effects are scarce numerical modelling can be used to interpret the measurements. Recently, the atmospheric boundary layer (ABL) model SCADIS (Sogachev et al., 2002, Tellus 54B, 784-819) has been successfully applied to analyze the mechanisms of CO2 flux formation near a forest edge for neutrally stratified conditions (Sogachev et al., 2008, Ecological. Appl. 18, 1454-1459). In the present work, we apply the SCADIS with enhanced turbulence closure including buoyancy for investigation of the spatial distribution of latent and sensible heat vertical fluxes over patchy forested terrain in Denmark during selected days in the summer period. A closer look at the result shows that though the meteorological mast is located in the middle of a forest patch by size about 1x2 km2, it is not free from uncertainties regarding energy balance closure. Comparing observed and SCADIS (1D and 3D) simulated data for the mast confirms that caution is needed when interpreting measured flux data. The approach used in this work can be utilized in interpretation of already existed experimental data and in the planning of future experiments
Numerical assessment of the effect of forest structure changes on CO2 flux footprints for the flux tower in Solling, Germany
No full textThere are many natural and anthropogenic reasons why a gap can occur inside the forest. When a gap appears within a studied stand (e.g. near a flux tower which operated for some time, providing information about the ecosystem-atmosphere exchange), an assessment of new measurement conditions should be carried out. Using a three-dimensional approach for footprint estimation based on numerical solution of Reynolds-Averaged Navier-Stokes (RANS) equations, we investigated possible changes in air flow and CO2 flux footprints resulting from two suggested forest management activities - clear-cut and stripe-cut - around the flux tower located in 130-year-old spruce forest in the Soiling highland, Germany. The model results show that degree of changes in flux footprints depends on the chosen management strategy. The clear-cut strategy produces the largest changes and the stripe-cut leads to weaker changes of investigated characteristics. The role of remote canopy sources increases, while the contribution of remote soil sources decreases with increased share of removed trees. In general, the investigated characteristics change differently for summer and winter due to the combined effects of phenology and upwind topography. (C) 2010 Elsevier B.V. All rights reserved
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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