A Wind Erosion Case Study in an Alpine Environment (Davos, Switzerland) Compared to Wind Tunnel Experiments with Live Plants

GRAF, Franka

a WSL Institute for Snow and Avalanche Research SLF, CH-7260 Davos Dorf, Switzerland. graf@slf.ch

Abstract — It is generally accepted that the (re-)establishment of a protective vegetation cover is the most promising and efficient measure in restoring degraded land in the long term. Sustainable protection against wind erosion requires adequate information about suitable plant species regarding ecological aspects as well as with respect to their proper contribution to wind erosion control. The latter, however, is widely lacking. The goals of the presented field study are to record reliable data on windblown erosion rates under natural alpine conditions and to cross-link the findings with the results of wind tunnel experiments. A wind erosion test field was established at 2409 m a.s.l. in an alpine meadow including two test tracks. One track is left as is, representing the naturally alpine vegetated soil (10-20% plant cover). The other track is equipped with a plastic covering sheet, mimicking desertified soil (0% plant cover). Blue and red quartz sand was spread on the vegetated and sheet-covered track, respectively, to visualise and measure the effect of vegetation on wind erosion control. Compared to the bare soil it was found that only small amounts of sand from the vegetated plot were transported, even during heavy wind events. Related to the seasonal course, the overall ratio varied from 1:19 to 1:717. Qualitatively similar findings, however quantitatively less pronounced, resulted from the wind tunnel experiments (ratio = 1:15). Under consideration of all available information, the comparison with data from the field experiment considering only the configuration that best coincide with the wind tunnel set-up yields at least a 70-fold higher impact of plants on wind erosion control under natural conditions. The difference implies that the sheltering effect of vegetation in nature is much higher than found for wind tunnel runs, even when using live plants.

Keywords — vegetation, wind erosion control.

1  Introduction

Worldwide and, in particular, in the Himalayas and on the Tibetan Plateau, snow and glaciers are important both on a regional scale, providing water supply, and on a global scale, indicating climatic changes. Observations in the field and experiments have consistently been confirming that thin debris layers accelerate the ablation rate of underlying snow or ice (Han et al. 2006). The rapid increase in aeolian sediment transport due to large-scale devastation of protecting vegetation has been verified (Wang & Shen 2009). Wind erosive processes considerably intensify desertification and contribute to glacier decline and speed-up of snow melting both inducing climate change, erosion and sliding (Fujita et al. 2007). The increase in desertification, snow and ice melting as well as soil instabilities are cross-linked phenomena. Thus, successfully inhibiting the main trigger contributes to the amelioration at large.

Re-vegetation is accepted as the most efficient strategy to combat wind erosion and desertification in the long term. Intact vegetation considerably contributes to the stability of the near-surface soil structure (Burri et al. 2011b, Sutter-Burri et al. 2013) with important consequences regarding the soil-atmosphere interface including water and radiation balance and topography. Moreover, vegetation shelters soil from the impact of wind by reducing its erosive force, trapping windborne particles, and providing loci for deposition (Okin et al. 2009). With a certain lag, vegetation recovery and the reduction of aeolian sediment transport should have a decelerating effect on the "sediment-driven" climate, with a positive feedback mechanism on plant development and on the regain of ar-able land (Okin et al. 2009). Related to the presented field study, it was recently demonstrated in the wind tunnel using for the first time live plants that the total sediment mass flux decreases exponentially with increasing canopy density (Burri et al. 2011a, b). A naturally grown canopy was analysed, mimicking the behaviour of natural vegetation canopies much more accurately than previous studies. However, this set-up, too, was considerably simplified compared to real nature but allowed for concentrating on specific parameters and, in particular, guaranteed the repeatability.

Field studies offer the possibility to address the full complexity of a particular wind erosion situation, however, at the expense of unambiguous assignment of response and explanatory variables as well as comparability. As a consequence, the results of such investigations are strongly coupled with their specific site characteristics, and the general conditions of the individual experiments. Thus, it is rather difficult to find a sound correlation between data of wind tunnel and field investigations. Therefore, the extrapolation from wind tunnel based models to natural scales is still not straightforward. Here we combine the natural conditions of an alpine test field with parts of the set-up applied in the wind tunnel experiments, namely using the same sandy soil and comparing bare and vegetated soil of the same plant cover (15-20%). The objective of the field study was to reliably measure windblown erosion rates under natural alpine conditions in order to validate the findings of the associated wind tunnel experiments. The synthesis of the two studies aims to assess the extent and intensity of desertification and to evaluate the effectiveness of counter-measures, i.e. to quantify the impact of natural plant cover on wind erosion control (Zobeck et al. 2003).

2  Material and methods

A wind erosion test field with two tracks was established at 2409m a.s.l. in an alpine meadow dominated by Phleum alpinum L. and Poa alpina L. (Fig. 1). One track was left as is, representing the naturally alpine vegetated soil (15-20% plant cover). The other track was equipped with a plastic covering sheet (10x2 m2), mimicking desertified soil (0% plant cover). A direct link to the wind tunnel experiments (Burri et al. 2011a, b) was provided as experiments with bare soil and comparable vegetation cover were performed. Blue and red quartz sand (♯: 0.2-0.6 mm) was spread on both tracks (2x2 m2) to visualise and measure the effect of vegetation on wind erosion control. Leeward of the two test tracks, 4 sets of panels and ground-plates were installed in 3 lanes each, equipped with sticky foils to trap and quantify particle transport (Fig. 1, 2). The left and right lanes diverge with an angle of 15° related to the centre lane. Each lane consists of four vertical panels (10x50 cm2) mounted at a height of 40 cm with the corresponding ground-plate (30x3 cm2) installed at their foot (Fig. 2).

Figure 1: Test field with the 2 tracks with coloured sand, the meteorological stations and sand trapping devices.
Figure 2: The 3 lanes of sand trapping devices; note the red sand in the first row of the centre and left lane.

The distances between the panel-plate sets in the three lanes were 3, 5, 10, and 15 m measured from the end of the test tracks and 7, 9, 14, and 19 m measured from the front of the two sand sources. In addition, wind direction, wind speed at 50, 100, and 200 cm height, air temperature, humidity, incoming and reflected short- and long wave radiation, as well as precipitation were recorded (Fig. 1). Periods of sediment transport were supposed to meet the subsequent requirements: wind speed (h=50 cm) >6 m s-1, wind direction 230-290°, no precipitation, humidity >6 %, and temperature >0 °C.

Table 1: Time periods of potential wind erosion risk with corresponding mean and maximum wind velocities during the three experiments.
experimentperiod12345678910 
ex. 15: 21-25/06 duration [min] 1030 30 703010 1010∑= 200
 v mean [m s−1] 7.0 7.2 7.2 8.1 6.6 6.8 6.2 6.8 weight. = 7.3
 v max [m s−1] 7.0 7.5 7.8 9.8 7.2 6.8 6.2 6.8 max = 9.8
ex. 21: 27/07-04/08 duration [min] 10 1030 30 200 10 10 30 20∑350
 v mean [m s−1] 6.2 6.8 6.3 6.8 7.6 6.2 6.2 6.8 6.8weight. = 7.1
 v max [m s−1] 6.26.8 6.5 6.8 10.0 6.2 6.2 7.2 7.2max = 10.0
ex. 33: 09-14/09 duration [min]100 101020105010202010∑260
 v mean [m s−1] 7.7 6.2 7.0 7.6 6.5 7.6 6.5 7.0 6.8 6.5 weight. = 7.3
 v max [m s−1] 9.0 6.2 7.0 7.8 6.5 8.2 6.5 7.2 7.2 6.5 max = 9.0
Table 2: Number and ratio of blue and red sand particles trapped on the plates in the different rows (1-4) and lanes (left, centre, right) during the three experiments (ex. 15, ex. 21, ex. 33). Framed: ct1 (first trap of centre lane) that best corresponds to the wind tunnel set-up of Burri et al. (2011b).
ex. 15 row number (dist. from sand source [m]) 
lane colour 1 (7) 2 (9) 3 (14) 4 (19) sum row ratio ∑ blue ∑ red
leftblue 3 1 0 0 4 1:2’240  
 red 5’943 2’751 248 19 8’961
centre blue 8 5 1 4 18 1:862   
 red 10’809 4’205 399 108 15’521   
right blue 22 13 20 28 83 1:45  
 red 2’355 1’071 230 42 3’698   
  lane ratio 1:579 1:422 1:42 1:5   1:268 10528’180
ex. 21 row number (dist. from sand source [m]) 
lane colour 1 (7) 2 (9) 3 (14) 4 (19) sum row ratio ∑ blue ∑ red
leftblue 4 4 0 0 6 1:6’897  
 red 34’480 6’903 0 0 41’383
centre blue 2 13 0 2 17 1:4’327   
 red 20’970 34’557 12’000 6’030 73’557   

right

blue 0 0 101 42 1431:28  
 red 3’036 544 351 138 4069   
  lane ratio 1: 9’748 1: 2’800 1:122 1:140 1:717 166 119’009
ex. 33 row number (dist. from sand source [m]) 
lane colour 1 (7) 2 (9) 3 (14) 4 (19) sum row ratio ∑ blue ∑ red
leftblue20 15 0 0 35 1:367  
 red 11’099 1’751 8 0 12’858
centre blue 182 88 7 0 277 1:66   
 red 12’804 5’090 356 0 18’250   

right

blue 832 493 75 0 1’4001:12  
 red 1’383 311 38 0 1’732  
  lane ratio1: 24 1: 12 1:5 NA   1:19 1’712 32’840

For the present article only the foils of the ground-plates of three selected experiments (ex. 15, ex. 21, ex. 33) were considered. A step by step practice was applied to the jpg-files resulting from the scanning procedure of the sticky foils (Xerox WorkCentre 7345: 300 dpi). The software ImageJ (ver. 1.46 m; http://imagej.nih.gov/ij/) was used and the different steps were: 1) edge cropping (100 pixels on each side), 2) subtraction of particles > 0.6 mm, 3) colourthresholding (red and blue particles) based on the HSB colour space limiting Hue, Saturation, Brightness, and 4) filtering particles between 0.2 and 0.6 mm. The field experiments presented were performed in 2011 from 21-25 June (ex. 15), 21 July – 4 August (ex. 21), and from 9-14 September (ex. 33). The wind tunnel experiments with bare sand and sand planted with live Perennial Ryegrass ( Lolium perenne L. ) were performed at the WSL Institute for Snow and Avalanche Research SLF. For providing a vegetated surface, eight 1 x 1 m2 trays were planted with L. perenne and aligned in the 8 m test section of the wind tunnel. Sediment sampling was conducted with a modified WITSEG sampler (Dong et al. 2004) positioned 7 m down-wind of the test section. In this article the results of the runs with bare sand and the configuration with a canopy density of  25 tussocks per square meter yielding a plant cover of 15-20% are considered. The mean free stream velocity measured at h=60 cm was  15 s−1 (Burri et al. 2011b).

Figure 3: Distribution of windblown sand of ex. 15 (left: vegetated soil, right: bare soil). Rhombuses indicate the test tracks and filled symbols the correspondingly coloured sand. The blue and red dotted lines mark the first row of sediment traps at a distance 7 m off the front of the sand sources. Width: distance between sediment traps (rows); length: distance off the front of the sand sources.

3  Results

During the season the vegetation cover of the planted plot changed from  15% (ex. 15) to  20% (ex. 21) to  10% (ex. 33). Concomitant, mechanical properties of the plants (flexibility/stiffness) have been changing, altering their behaviour towards wind and sediment trapping. The flexibility of the shoots increased slightly from ex. 15 to ex. 21 and was strongly decreased in ex. 33. The proposed requirements for potential wind erosion risk and, thus, sediment transport were met during eight (ex. 15), nine (ex. 21) and ten (ex. 33) periods lasting from 10 to 200 minutes. The corresponding total time of potential sediment transport summed up to 200, 350, and 260 minutes. The maximum wind velocities varied between 6.2 and 10 m s−1 (Tab. 1).

The respective overall number of trapped grains for the three experiments (ex. 15, ex. 21, ex. 33) was 105, 166, and 1’712, for blue and 28’180, 119’009, and 32’840 for red sand. The corresponding ratio between blue and red sand – i.e. the reduction of sediment transport due to vegetation – was 1:268, 1:717, and 1:19 (Tab. 2). If rows and lanes are considered individually, the factor for the reduction in sediment transport by plants ranged from 5 to 9’748 for the rows and from 12 to 6’897 for the lanes. In all three experiments significantly less sand was transported from the vegetated compared to the bare soil (p-value < 0.001).

Referred to the wind tunnel set-up with sediment sampling at 7 m downwind in the middle of the test section (Burri et al. 2011b) the first row and the centre lane of the field configuration and, in particular, the centre trap of the first row (ct1) conform most closely. The corresponding grain numbers of ct1 for the experiments 15, 21, and 33 are 8, 2, and 182, for blue and 10’809, 20’970, and 12’804 for red sand. Likewise the ratios yield 1:1’351, 1:10’485, and 1:70 (Tab. 2).

In Figure 3 the detailed sand grain distribution from bare and vegetated soil is shown for ex. 15. The total sediment flux [kg m-2 s-1] in the wind tunnel experiments was 0.261 for bare and 0.017 for vegetated soil, yielding a ratio of 1:15, i.e. a 15-fold reduction in sediment transport due to vegetation. Related to the overall trapped sand grains of each of the three field experiments, it may be concluded that under natural conditions this positive effect of plants on wind erosion control is 1.3 to 48 times higher than found in the wind tunnel. Restricted to corresponding ct1 (first sediment trap of centre lane) the factors range from 4.7 to 700. Taking further into account the seasonal changes of plant cover and plant mechanical properties in the field, the wind tunnel set-up and field configuration most soundly coincide with ex. 15 (Fig. 2). Consequently, the positive effect of vegetation on wind erosion control is roughly 90 times higher in nature (ct1 of ex. 15: 1:1’350) compared to the findings in the wind tunnel (1:15).

4  Added value to integrative risk management

It has impressively shown that plants are indispensable in stabilisation and restoration concepts for regions prone to wind erosion. Additionally, seasonal variations as well as the important interactions between plants and their living environment both below and above ground and referring to succession processes need to be integrated in the planning from the start. If, and only if these key factors are set, wind erosion control can be tackled reliably with respect to long-term success.

5  Discussion and conclusion

Wind tunnel and field experiments point into the same direction, emphasising the importance of plants in wind erosion control. However, the positive effect of appropriately applied vegetation is still deeply underestimated. Discrepancies may have several sources. Apart from variations in flow characteristics (vertical profile, turbulence structure, eddies, …), differences in soil structure are most decisive. Although live plants were used in the wind tunnel, the short growing phase (4-5 months) and restriction to one species did not allow for a functional soil ecosystem. As against nature, interactions between plants and micro-organisms as well as hydrological processes (hydraulic lift, evapo-transpiration, …) were drastically reduced or lacking. Compared to the field investigations that account primarily for an alpine environment having undergone natural pedogenesis and maintaining functional interactions, the wind tunnel experiments rather represent a first development stage in a re-colonisation process of a completely unstructured and abiotic substrate, e.g. the abiotic part of the spoil of road construction. In contrast, under natural conditions at least parts of the soil structure, organisms (living individuals, propagules), and organic matter, particularly in deeper layers, remain even after heavy erosive processes.

Table 3: Ratio of different data sets of the mass sediment flux between bare (*: mean value of three runs) and vegetated soil (medium-density configuration: 16% plant cover) from wind tunnel experiments (Burri et al. 2011b) and from the centre lane (clt) including all 4 traps and the first trap of the centre lane (ct1) of the experiments 15, 21, and 33 of the field investigation.
Study typedata setvegetated soilbare soil ratio
wind tunneltotal sediment flux [kg m−2 s−1]0.0170.261*1:15
field investigationex. 15: clt [number of grains]1815’5211:862
 ex. 15: ct1 [number of grains] 8 10’8091:1’351
 ex. 21: clt [number of grains]273’5571 : 4’327
 ex. 21: ct1 [number of grains]1720’9701:10’485
 ex. 33: clt [number of grains]27718’2501:66
 ex. 33: ct1 [number of grains]182 12’804 1:70

The extreme difference in sediment transport between the blue coloured sand of the vegetated and the red one of the bare soil is reflected by an overall ratio of 1:268 (ex. 15), 1:717 (ex. 21), and 1:19 (ex. 33), however, with a huge variation within the different experiments depending on the location of the traps and among the three experiments indicating a seasonal trend which is related to a change in vegetation cover as well as to plant mechanical properties, particularly the elasticity of the above ground biomass. The vast variation within the short horizontal distance of 17 m and the small covering area of  275 m2 reflects to a certain extent the complexity of wind erosion field experiments.

The considerable difference in sediment transport between the blue sand of the vegetated and the red one of the bare soil fits well with the total mass flux results of the wind tunnel experiment. In turn, the exponential decrease in total sediment mass flux with increasing canopy density found in the wind tunnel study with the grass Lolium perenne is in accordance with field observations by Allgaier (2008), Hesse and Simpson (2006), Lancaster and Baas (1998), and Li et al. (2007). The relationship between plant cover and erosion reduction is similar to the one found for Salt Grass (Distichlis spicata ) by Lancaster and Baas (1998) and does not significantly differ from their proposed model.

Though, the data of the present field investigation gives evidence to suggest that the sheltering effect of vegetation is even more pronounced than worked out by the wind tunnel experiment. Based on the overall ratio between bare and vegetated soil of the field investigation, the findings of the wind tunnel experiment with the comparable medium-density configuration (ratio = 1:15) are surpassed by a factor ranging from 1.3 to 48.

In the wind tunnel experiment, sediment sampling was conducted with a WITSEG sampler according to Dong et al. (2004) positioned near the end of the test section, at approximately 7 m down-wind (Burri et al. 2011b). From this perspective and to better sustain comparison with the data of the field investigation, only the sticky foils of the centre lane (clt), starting in a distance of 7 m off the front of the two sand sources (blue and red sand) and, in particular the first trap of the centre lane (ct1) should be taken into consideration. To take these circumstances as a basis, the difference in sheltering effect of the vegetation between wind tunnel and field experiments diverges even more. The mean ratio of bare to vegetated soil of clt and cl1 of the three experiments ranges from 1:66 to 1:4’327 and from 1:70 to 1:10’485, respectively compared to 1:15 in the wind tunnel experiment and, therefore, amounts to an increase by up to three orders of magnitude (Tab. 3).

However, the wind tunnel experiments were conducted with reference wind speeds of 15.3 m s-1 during 2 minutes for the bare soil and of 15.7 m s-1 during 10 minutes for the vegetated soil (medium-density configuration). In the field study, only one 10 minutes measurement interval reached a maximum wind speed of 10.0 m s-1 with an average wind speed of 7.1 m s-1. The corresponding total wind erosion period was 350 min. Due to the lower maximum wind speed compared to the wind tunnel experiments, it can be speculated that the protection function is overestimated in the field. On the other hand, during the field experiments the wind exposed period with potential wind erosion risk was many times longer with 3 h 20 min. for ex. 15, 4 h 20 min. for ex. 21, and 5 h 50 min. or ex. 33 (Tab. 1).

Despite these inconveniences, it seems most likely that in nature the sheltering effect against wind erosion processes is more pronounced than eventuated by the wind tunnel experiments, notwithstanding live plants were applied. The uniform planting pattern and the homogenous wind flow of the wind tunnel set-up differ considerably from natural conditions and are comprehensible reasons behind the differences between wind tunnel and field experiment data. Further explanation is found in the field of the mutually dependent hydrologic balance and soil structure, which are additional key aspects in wind erosion control by plants. In the wind tunnel investigations the 1 cm thick quartz sand layer was on top of 6 cm thick 3:1 mixture of crushed sand and earth (Burri et al. 2011b) filled in wooden trays with drainage holes to prevent waterlogging. The plants have been grown in there for about 4-5 months. Although the substrate was well rooted, the stage of development and stability of the soil matrix and pore structure were far behind and the water cycle not as properly functioning compared to the soil of the field investigation. There, the 5 cm thick blue quartz sand layer of the vegetated plot was on top of an alpine meadow whose soil has been developed for decades. Correspondingly, it complied much better with the requirements of the subsequently discussed processes in view of stability and resistance against wind erosion.

In dry sand, the angle of repose and, thus, the stability in general and of the surface grains in particular is determined by their shape as well as by friction forces. An increase in stability through wetting processes is primarily due to adhesive binding (apparent cohesion) associated with interstitial liquid bridges between grains and, therefore, directly related to the architecture of the matrix and pore structure of the sandy substrate (Barbasi et al. 1999, Scheel et al. 2008). The more stable the substrate, the slower the desiccation process and, therefore, the longer lasting the additional stability effect of the interstitial liquid bridges which is all the more decisive at the wind-exposed surface.

The mechanisms underlying these processes and stabilising effects were not addressed in the present studies. However, based on previous research with different substrates, it is likely that mycorrhizal hyphae, together with the associated plant roots play a considerable part in contributing; enmeshing sand grains and organic particles by acting as "sticky string bags" (e.g. Degens et al. 1996; Rillig and Mummey 2006, Graf and Frei 2013). In the course of the wind tunnel study of Burri et al. (2012 a, b) the first experimental demonstration was provided that mycorrhizal fungi are able to increase soil resistance to wind erosion. Although total root length of 2 month-old mycorrhized plants (Anthyllis vulneraria) was significantly smaller than of the non-mycorrhized ones, the wind-induced soil loss of the mycorrhized root balls was significantly reduced compared to the non-mycorrhized control samples (Burri et al. 2011a).

In the vegetated plot of Latschüelfurgga the blue sand was distributed above a naturally developed rhizosphere with site-adapted plant species. Different to the red sand on the plastic covering sheet – mimicking bare soil – and to the set-up of the wind tunnel, it was in direct contact with the functional hydrologic cycle of the subjacent soil’s matrix and pore structure which are more stable and provide better retention capacity than the sand itself. Amongst other things, this superior stability is due to the interactions of plant roots with soil organisms, in particular with their associated mycorrhizal fungi, which were lacking not only in the control plot at Latschüelfurgga but also in the wind tunnel experiment.

Related to the stability of the pore structure, the interactions between roots and soil micro-organisms, evaporation, transpiration as well as hydraulic lift are certainly key functions with respect to water flux and the distribution of moisture along soil depth.

Soil evaporation is a significant loss or depletion from the water balance and maximized if there is a shallow groundwater table, a hot and dry climate, a uniform fine-grained soil, and bare surface exposed to sunlight and wind. The shallower the water table is, the more continually the water will be supplied upward to the soil surface. In this case soil evaporation is controlled largely by climatic conditions at the soil surface. This scenario was met quite well on the vegetated plot in the alpine environment at Latschüelfurgga. Hence, a more or less continuous water flux during the potential wind erosion risk periods may have been taken place. Consequently, the near surface zone of the blue sand in the field investigation was probably rarely completely desiccated and the additional stability through interstitial liquid bridges effective almost always, at least up to a certain degree. In contrast, the sand surface of the planted (16% cover) configuration in the wind tunnel experiment was much drier due to a lacking water table and missing of substantial radiation; the consequence being that the apparent cohesion collapsed earlier, at a faster pace, and on a larger scale.

In addition to soil evaporation the transpiration of water within a plant coming from the roots and subsequently lost as vapour through stomata in its leaves contributes to the upward water movement too. Factors that affect the plant driven evapo-transpiration include their growth stage and age, percentage of soil cover, solar radiation, humidity, temperature, and wind. Whereas the above ground growth performance of the plants as well as the percentage of soil cover were comparable in the field and wind tunnel investigations, differences in solar radiation, humidity, temperature, and wind were observed as well as in rooting. It can, therefore, be taken for granted that evapo-transpiration was by far lower in the wind tunnel experiment, particularly due to insufficient light and radiation as well as the short duration of the wind erosive process (10 min.). As a consequence, in the wind tunnel experiment evapo-transpiration of plants was probably not of importance with respect to the surface stability and resistance to shear forces.

Another process contributing to the upward water movement is hydraulic lift, a passive dislocation of water from roots into soil layers with lower water potential, while other parts of the root system in moister soil layers are absorbing water. Usually, considerable amounts of water are transferred from wetter (deeper) layers to the often drier near surface zone of the soil during the night. This partial rehydration of the upper soil layers provides an additional source for transpiration the following day. Lifted water may also contribute to the availability of water soluble nutrients located most plentiful in the upper soil layers and, therefore, indirectly influences survival and growth performance of the plants and associated organisms. Hydraulic lift may prolong or enhance fine-roots activity in the subsurface layers by keeping them hydrated and thus, buffer the rhizosphere organisms from effects of soil drying during persistent periods of lacking precipitation (Bauerle et al. 2008, Querejeta et al. 2007). A considerable positive influence on sand surface stability by hydraulic lift was most likely restricted to the vegetated plot of the present field investigation. During the wind tunnel experiment this process was probably not active.

Conclusively, it can be summarised that the findings of the wind tunnel experiment and the data of the present field investigation point into the same direction, emphasising the importance of plants in wind erosion control. The data from the wind tunnel experiments may rather represent a point of reference for the very first development stage of the re-colonisation process of a completely unstructured and abiotic substrate. Despite the fact, that for the first time an intact vegetation cover of live plants was applied in the wind tunnel, there are still essential deficiencies in order to appropriately simulate natural conditions. Particularly concerned are soil formation related to development stages and succession phases of the vegetation. The preliminary findings from the field investigations account primarily for an alpine environment having undergone natural pedogenesis. Projected to restoration of degraded soil, the findings from the field investigation represent, therefore, a rather late stage of development.

Interactions between soil organisms and plant roots, hydrological processes and the mechanical properties of the above ground plant parts rank among the most decisive mechanisms in view of resistance against wind erosion. In respect of the latter, seasonal changes of plant cover and the plant’s material properties need to be appropriately taken into consideration if field experiments are envisaged or data of corresponding investigations are analysed and compared. Even with respect to wind tunnel experiments with live plants this might be an important issue to think of.

In order to improve our knowledge on the performance of plants and vegetation types in wind erosion control and in view of adjusting conventional models for sediment transport and climate change at different scales, it is necessary to properly include such processes in future investigations, in the field as well as in the wind tunnel. Consequently, it will be necessary to consider not only one individual grass species but representatives of other plant groups and combinations of species, in particular with respect to root morphology.

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Citation

Graf, F. (2014): A wind rosion case study in an alpine environment (Davos, Switzerland) compared to wind tunnel experiments with live plants. In: Planet@Risk, 2(3): 117-124, Davos: Global Risk Forum GRF Davos.