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Bachelor thesis
The long term impact of active blowouts on vegetation characteristics in the
Grey dunes of Eldorado, Terschelling
Figure 1) dunes on Terschelling (Gemeente Terschelling, n.d.)
Name: Marleen Boersma Student number: 10557407
Course: Bachelor Project Aardwetenschappen FPS Date: 4th of July 2016
Supervisor: Dr. Annemieke Kooijman Co-assessor: Dr. John van Boxel
2 Index (page) Index 2 Nederlandse samenvatting 3 Abstract (English) 3 Introduction 3 Methods 5 Results 9 Discussion 16 Conclusion 19 References 20 Appendices 21
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Abstract
The Grey dunes are an important biodiverse ecosystem that has been negatively affected by nitrogen deposition, which increased extremely around the 1970s. Over the last decades the deposition did not increase anymore, but remained very high. This led to increased acidification of the soil and grass-encroachment. To see if the habitat conditions in the grey dunes improve when blowouts are active, the difference in vegetation characteristics between a stabilized and a more active, only recently stabilized blowout have been researched. Soil and vegetation characteristics have been determined with fieldwork, laboratory analyses and a grid analysis. Several vegetation cover maps have been made, to visualize the difference between the two blowouts. There were also many significant differences between the two blowouts. Vegetation cover was lower and bare sand cover higher in the recently stabilized blowout. Biodiversity was also higher in the recently stabilized blowout, probably due to occurrence of pioneer vegetation. The nitrogen concentration in the vegetation was lower, especially when looking at C/N ratios. The results of this study show that active blowouts, in the presence of cattle grazing, can be used to counteract acidification and grass-encroachment in Grey dunes.
Nederlandse samenvatting
Dit onderzoek gaat over de verschillen tussen actieve en stabiele stuifkuilen en hun invloed op de vegetatie kenmerken in de grijze kalkarme duinen van Terschelling. Dit is relevant aangezien de biodiversiteit in de Nederlandse duinen afneemt door de hoge stikstofdepositie e actieve stuifkuilen kunnen een oplossing zijn voor de toename van vergrassing en de verzuring van de bodem. Met behulp van bodem en vegetatiemonsters zijn variabelen zoals de koolstof- en stikstofconcentraties bepaald. Er zijn kaarten gemaakt voor verschillende vegetatiebedekkingen, zoals gras mossen en struiken. Er kan geconcludeerd worden dat er veel significante verschillen tussen de stuifkuilen bestaan als het gaat om de vegetatie. De hoeveelheid vegetatiebedekking is lager in langer actief gebleven stuifkuilen en er is een hogere mate van kaal zand aanwezig. De biodiversiteit is hoger in de recent gestabiliseerde stuifkuil en de stikstofconcentratie in de vegetatie is lager. Hieruit volgt de conclusie dat een combinatie van grazers en reactivatie van stuifkuilen kan zorgen voor
verbeteringen in de habitat condities in de grijze duinen.
Introduction
The Dutch dunes are an important ecological system. In the coastal dunes, 70% of the national flora is housed, of which approximately 15% is exclusive (Veer & Kooijman, 1997). Open dune grasslands are important habitats in the coastal dune landscape. These open dune landscape consists of a variation of vegetation, such as low vegetation and open sandy spots, which are rich in plant and animal species. Open dune grasslands are also known as Grey dunes or as habitat type H2130, and they are a priority type in the Habitat Directive of the European Union (European Commission Environment, 1992).
In the Netherlands, Grey dunes are highly threatened by high atmospheric deposition of nitrogen and acidifying agents (Van der Meulen et al. 1996, Kooijman et al. 2005). “The Netherlands is among the countries with the highest nitrogen deposition levels worldwide.”(Jones et al., 2004). The main sources of this nitrogen are the industry and the agricultural sector, and the deposition of nitrogen has great impact on the Grey dunes (Jones et al, 2014). Natura2000 came up with the maximum critical load for nitrogen which was estimated to be about 10-15 kilogram nitrogen per hectare per
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year. When exceeding this critical load there could be unchangeable damage to different habitat types. However, in 1993 the highest nitrogen deposition were measured and since the 1990s nitrogen deposition in the Netherlands has an average of 27-30 kg nitrogen per hectare per year (Meijer et al., 2013).
The Grey dunes on Terschelling are very sensitive to nitrogen deposition (Meijer et al., 2013). The Grey dunes have suffered from grass-encroachment and soil acidification, which means the pH of the soil decreased. Grass- encroachment means that there is a dominance of a few tall grass species, which overgrows the smaller and more characteristic plants (Veer & Kooijman, 1997). Due to this acidification, eutrophication and grass-encroachment the biodiversity in the Grey dunes on Terschelling decreased. The acidification and grass-encroachment in the Grey dunes is not only caused due to nitrogen deposition but it was also reinforced by large-scale stabilization measures that were applied until the 1980s (Kooijman et al., 2005). This stabilisation of coastal dune blowouts was done to reduce aeolian activity in blowouts and prevent wind erosion, but it might also occur spontaneously (Kooijman et al., 2005).
Since the 1990s, management measures have been applied to counteract acidification and grass-encroachment (Van der Meulen et al. 1996, Kooijman et al. 2005). One of these management measures was stimulation of aeolian activity by reactivation of former blowouts (Van Boxel et al. 1997, Van der Meulen et al. 1996). The main function of the reactivation of former blowouts is the exposure and distribution of fresh sand on the soil surface for a new start of soil and vegetation succession (Witz, 2015).
On Terschelling in the area called Eldorado eleven blowouts were reactivated in 1991, but they stabilized with time. In 2000, the blowout area consisted of 15 hectare, of which 10 hectare had a very low activity, and 5 hectare with almost no activity. In 2014, the blowout area had decreased to 11 ha, and only showed very low activity. At present the aeolian activity is very low in all blowouts, and no difference is noticeable in aeolian activity between the blowouts that were reactivated 25 years ago and the non-activated blowouts. Nevertheless, even when stabilized, blowouts still contribute to improvement of habitat conditions. During the time that they were active, fresh sand was deposited in the surrounding areas at least for some time (Van der Meulen et al. 1996, Kooijman et al. 2005). Active blowouts cause an increase in fresh sand on the soil surface, which means that the soil will be rejuvenated (Jungerius & Van der Meulen 1997). Therefore, active blowouts contribute to higher pH, lower amounts of soil organic matter, and suitable habitat conditions for characteristic plant species as well as higher variability and an increase in animal diversity (Isermann, 2005). The soils and the sand on Terschelling are low in lime content, which means that the soils have a low buffer capacity against acidification. Fresh sand depositions from active blowouts might increase the buffer capacity (Meijer et al., 2013). It is however not clear how large the deposition zones have been for the former blowouts, and how long the rejuvenation effects would last.
Research on the topic of active blowouts and their impact on habitat conditions is very relevant. This is not only because of the ecosystem services of the coastal dunes, but also because of the impact of nitrogen deposition on the biodiversity of the flora and fauna on Terschelling. Grass-encroachment limits aeolian activity of blowouts but also the biodiversity. It is therefore important to assess the longer term effects of the reactivation of former blowouts on the habitat conditions. New insight on this topic might also contribute with new information to the PAS (Programmatische aanpak stikstof) policies that were initiated by Natura2000 (Meijer et al., 2013). This research contributes to a research on a larger scale. Therefore, it is important to focus not only on reactivation of former active blowouts, but look into the effects of natural active blowouts as well. The aim of this research is to discover the effects of natural active blowouts on the vegetation in the blowout and the surrounding areas.
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Research
It is expected that the habitat conditions in the Grey dunes of Eldorado on Terschelling have improved after reactivation. Active blowouts might be a good counteraction strategy against grass-encroachment and acidification of the soil. In more active blowouts or recently stabilized blowouts the pH of the soil in accumulation zones surrounding the blowout is expected to be high compared to areas without fresh sand, which means less acidification of the soil. Also pioneer vegetation species are expected to be present in the areas surrounding the recently stabilized blowouts, since the soil is less acid due to the fresh sand. Grass encroachment is expected to be less in recently stabilized blowouts, since there is more bare sand cover and less vegetation cover present.
The question is how long do these effects last and is the stabilization of reactivated blowouts similar to stabilization of natural active blowouts. This bachelor thesis compares the impact of active blowouts on vegetation with stabilized blowouts, and uses recently stabilized blowouts as a natural reference in order to advise about the use of reactivation. The main research question for this research is: What is the impact of recently stabilized blowouts on the habitat conditions when looking at vegetation characteristics, in comparison with stabilized blowouts in the Grey dunes on Terschelling?
Location
The location of the research and the fieldwork is the dune area Eldorado on the north-western part of Terschelling. The blowouts are located approximately 500 meter from the sea.
Figure 2) The red circle indicates he study area Eldorado on Terschelling.
In the figures below the study area Eldorado with blowouts of which some where reactivated in 1990s are shown. In this area, part of the blowouts have been reactivated in 1991, and a few have not been reactivated (Van der Meulen et al. 1996).
Figure 3) Study area Eldorado, NW-Terschelling. a) Map of blowout locations on Eldorado (Van der Meulen et al. (1996)), b) Topographic map of blowouts on Eldorado (stabilized blowout: yellow, recently active blowou: red).
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Methods
Fieldwork
During a fieldwork trip to the grey dune area Eldorado on Terschelling, soil and vegetation samples have been collected. These samples have been taken in two blowouts, one stabilized blowout and one recently stabilized blowout. In each blowout two transects were made, as shown in figure 4 below.
Figure 4) two transects per blowout, Elodorado dunes Terschelling (Google Earth)
Letter A represents the sample points in the stabilized blowout and B represents sample points in the more recently stabilized blowout, letter L stands for the long transect and K represents the short transect perpendicular to the long transect.
In each of the two blowouts, 2 transects were established, these transects go through the blowout, the deposition zones and unaffected dunes further away. However, the accumulation zone was affected and partly removed, due to the man-made lake constructed recently. In each blowout there were 19 sample points, which means a total of 38 sample points. At each sampling point we made a detailed soil profile description, as well as a description of the surface characteristics such as the cover of bare sand, the cover of the moss layer, the cover of the herb layer and the cover of the shrub layer. The cover of bare sand and the moss and grass species was recorded in percentages. At each sampling point, a soil sample was collected with metal pF-rings for further analysis of soil characteristics in the lab, such as the bulk density, pH and electrical conductivity (EC), and carbon and nitrogen content. Two samples per location were taken with the bulk density rings, which means the volume of the soil sample was 200 cm3.
Also, above ground vegetation at each sampling point on the transect was sampled. Mostly 25x25 cm plots were sampled for analysis of dry weight per m2 and carbon and nitrogen content. When there
was only a little bit of vegetation present, 50x50 cm plots were used. Only aboveground vegetation was cut and the moss layer has not been sampled. The moss layer at the soil surface contained too much soil and sand, which will be hard to remove and would have influenced the results of the lab analysis.
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In addition, detailed measurements of surface, soil and vegetation characteristics have been conducted in a grid. This grid contained the two blowouts as well as the surrounding area and consisted of approximately 200 grid point measurements in total.
The location of each grid point has been recorded with a GPS, and characteristic surface and topsoil features were described, such as the cover of bare sand, the cover of the moss layer, the cover of the herb layer and the cover of the shrub layer.
Figure 5) These Google Earth images show the locations of the grid points, in the second image the manmade lake is also visible (Google Earth, 2005 and 2010).
Lab work
All samples taken along the transect points have been analysed in the lab in order to estimate different soil and vegetation characteristics. This detailed lab work description is based on Witz (2015).
Soil samples
- Bulk density
Each soil sample has been placed in the oven to dry at 70˚C for 48 hours, and then be weighted. A sub sample was weighted before and after it was placed in an oven at 105 ˚C for 24 hours. The bulk density has been calculated using the following formula:
Total wet weight x Dry subsample weight
Bulk density = --- Wet subsample weight x Volume
Whereas the volume depends on the number of pF-rings that were taken per sampling point, in this case it is 200 cm3.
- pH and Electrical conductivity of the soil and vegetation
The dried material has been sieved with a 2 mm sieve and a subsample was used for estimating both pH and electrical conductivity of the soil. Approximately 10 grams of each sample material was mixed with 25 ml of demineralized water. This was done to obtain a mixing ratio of 1 : 2.5, which is an ideal ratio to measure these indicators. The mix was shaken for about 2 hours followed by a resting period overnight and another shaking of 20 more minutes before the measuring, to completely dissolve all soluble particles in the soil. Values were measured by using a pH electrode and an EC electrode.
8 - Carbon and Nitrogen
A fraction of the soil sample with less than 2 mm in diameter has been ground at a rotational speed of 400 rotations per minute for approximately 5 minutes. Afterwards, the subsamples dried for another 24 hours at 70 ˚C, and then about 50 mg of each sample has been prepared in duplicate for analysis. Carbon and nitrogen values were both measured with a CHNS analyser of the type
Elementar Vario EL. The results can be used to calculate the carbon and nitrogen values in the top 5 cm layer of the soil per square meter.
Vegetation samples
The vegetation samples taken from the fieldwork area have also been analysed in the lab. - dry weight
The vegetation samples have been dried in an oven at 70˚C for at least 24 hours and then the dry weight of vegetation per square meter was calculated. All left over soil particles as well as woody plant parts will be removed, with a sieve. Afterwards, the material was ground at a rotational speed of 8.000 rotations per minute.
- The carbon and nitrogen values
The carbon and nitrogen concentrations and ratio have been estimated after the grounded vegetation material has been in the oven at 70˚C for more than 24 hours. Then about 10-15 mg of each sample has been prepared in duplicate for analysis. The nitrogen content and carbon content in percentages and the C/N ratio were measured with the CHNS analyser of the type Elementar Vario EL. The dry weight of vegetation per square meter was calculated and afterwards used to estimate the content of nitrogen and carbon in grams per square meter.
GIS analysis
The data collected during the lab work as well as the grid point data set collected in the field have been used to create surface maps for the field areas, showing different surface features. These maps were produced in ArcGIS 10.2.
Different interpolation functions have been used depending on the spatial distribution and the value span of the data. The Inverse Distance Weighted interpolation technique has been used to construct most of the vegetation cover maps. Inverse Distance Weighted (IDW) is a method of interpolation that estimates cell values by averaging the values of sample data points in the neighborhood of each processing cell. The closer a point is to the center of the cell being estimated, the more influence, or weight, it has in the averaging process (ArcGIS, HelpDesktop, 2016). Another interpolation function, called Spline, was used for the more pointy data for some of the features. It is useful to smooth out the area and it might give an indication of aggregated values avoiding sharp transitions between points (Burt, Barber & Rigby,2009).
A significant correlation between bare sand cover and the measured pH of the soil has not been found, therefore we were unable to produce a surface map for the pH of the soil in the blowouts in this area.
Statistical analysis
In order to check if the differences found between the two blowouts are significant, a t-test has been conducted using MATLAB. The data estimated during the lab work has been used to calculate the significance of the soil and vegetation characteristics. To check for a significant correlation between pH and bare sand cover we used MATLAB functions as well. An vegetation description and
categorising along the transects point has been conducted by Annemiek Kooijman. With the use of a TWINSPAN technique four vegetation types have been determined. ANOVA functions have also been used to calculate mean values of soil and vegetation characteristics per vegetation type.
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Results
The results are divided into three parts, the first part focusses on the maps that are made from the grid data and differences between the two blowouts. Secondly the results based on the data
obtained in the lab are presented. The last part focusses on the different vegetation types that were estimated and the soil and vegetation characteristics for each of these vegetation types.
Vegetation cover maps
This table shows minimum, maximum and mean values for the different vegetation covers in percentages. A represents the stabilized blowout and B represents the recently stabilized blowout. The existence of a significant difference between the two blowouts is tested with a t-test and the corresponding p-value is provided to show how strong the differences are. These values are all based on all grid data, that were used to make surface maps.
cover type in percentages minimum value in A maximum value in A minimum value in B maximum
value in B Mean A Mean B
Significant
difference P value
Bare sand
cover 0 100 0 100 4,62 19,10 yes 2,93E-06
Vegetation cover 20 100 0 100 95,38 80,91 yes 0,005 Moss 0 70 0 70 29,86 24,86 yes 0,047 Herbs 0 30 0 40 16,43 13,57 yes 0,002 Shrubs 0 75 0 50 12,03 7,29 yes 0,018 Grass 0 70 0 60 22,58 17,76 no 0,190 Marram grass 0 45 0 70 11,98 14,86 no 0,056 Buckthorn 0 30 0 30 2,78 2,00 no 0,338
Table 1) Grid data of vegetation, mean, minimum, maximum and significant differences with p-value. This first map shows the vegetation cover in percentages, the mean value of the stabilized blowout on the right side of the map is 95,4% and for the recently stabilized blowout it is 80,9%, The difference between the blowouts is tested to be significant. The highest vegetation cover values were found in the most northern part of the area.
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Map 1) Vegetation cover map in percentages of stabilized and recently stabilized blowout and surrounding area.
The next map shows the bare sand cover in percentages, this map is almost the exact opposite of the vegetation cover map. This is mainly due to the fact that we determined that there was bare sand cover if there was no vegetation cover. The locations with high values for bare sand cover are at the same locations as areas with low vegetation cover values. The differences between the blowouts are tested to be significant, with higher values for the recently stabilized blowout.
The high values of bare sand and the low vegetation cover values in the most southern part of the grid can be explained by the manmade lake that was constructed recently. This caused an increase in bare sand cover.
Map 2) Bare sand cover map in percentages of stabilized and recently stabilized blowout and surrounding area.
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Map 3) Shrubs cover map in percentages and locations of willow trees in stabilized and recently stabilized blowout and surrounding area.
Map 3 visualises the shrubs cover in the blowouts as well as the locations of willow trees. We tested the shrub cover to be significant different between the two blowouts with higher values for the stabilized blowout on the right side of the area. We labelled the willow trees as a different category, since it only occurred on two specific locations in both blowouts. That is also the reason we did not calculate the significant difference between the willow cover.
The map below shows the marram grass cover, there was no significant difference found between the blowouts.
Map 4) Marram grass cover map in percentages of stabilized and recently stabilized blowout and surrounding area.
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Map 5 shows the buckthorn cover in both blowouts, there was no significant difference found. However, most buckthorn was found in the accumulation zone and in the valley of the stabilized blowout.
Map 5) Buckthorn cover map in percentages of stabilized and recently stabilized blowout and surrounding area.
This map shows the grass cover for our grid area, and there was no significant difference found between the two blowouts, although there is a large area in the recently stabilized blowout without any grass cover, which is at the location that has almost 100% bare sand cover.
Map 6) Grass cover map in percentages of stabilized and recently stabilized blowout and surrounding area.
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The next map is the maps that demonstrate the herbs cover, and we tested the difference to be significant again. Which means that the cover of herbs was higher in the stabilized blowout.
Map 7) Herbs cover map in percentages of stabilized and recently stabilized blowout and surrounding area.
The last map below demonstrate the moss cover in the area. The difference between the stabilized and recently stabilized blowout is significant, the area around the stabilized blowout and the blowout self have a higher moss cover. Moss seems to be present throughout the area, except for the areas with high bare sand cover and the areas were willow trees grow.
Map 8) Moss cover map in percentages of stabilized and recently stabilized blowout and surrounding area.
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Vegetation characteristics
This part of the results contains information about the transects points, most of this data is obtained after laboratory work.
Based on the samples we took along the transect points, several vegetation characteristics were estimated. In the table below the nitrogen and carbon content are given for each blowout, with minimum, maximum, and mean values, the p value and whether or not there is a significant difference between the two blowouts. A represents the stabilized blowout and B represents the recently stabilized blowout.
vegetation characteristics minimum value in A maximum value in A minimum value in B maximum
value in B Mean A Mean B
Significant difference P value N content (%) 1,06 4,00 0,00 3,54 2,14 2,96 yes 0,005 C content (%) 35,80 49,48 43,42 54,05 45,21 47,80 no 0,328 C/N ratio 10,64 45,15 12,37 44,24 24,52 29,87 yes 0,037
dry weight (gram/m2) 5,64 201,92 22,80 216,80 79,05 92,36 no 0,444
C content(gram/m2) 2,34 96,49 10,84 104,04 36,23 44,54 no 0,328
N content(gram/m2) 0,15 2,85 0,65 7,41 1,48 2,71 yes 0,005
Table 2) Vegetation characteristics based on lab work: mean, minimum, maximum and significant differences with p-value.
The nitrogen content in percentages and the nitrogen content in grams per square meter were significant different between the two blowouts, with higher values in the stabilized blowout. However, the mean for the C/N ratio in the recently stabilized blowout was significantly higher. A higher ratio means a relatively lower nitrogen concentration which means that the nitrogen content in the vegetation must be relatively lower in this blowout.
Vegetation types
An vegetation description and categorising along the transects point has been conducted by Annemiek Kooijman. With the use of a TWINSPAN technique four vegetation types have been determined.
1) Vegetation that occurs in recently stabilized areas (pioneer vegetation) 2) Vegetation that occurs in stabilized areas
3) High moisture vegetation 4) Wet valley vegetation
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In these cross images it is visualized which vegetation type occurred at which point along the transects.
Figure 6) The occurrence of vegetation types at transect points in the stabilized blowout (left) and recently stabilized blowout (right).
Vegetation type 1, also known as the pioneer vegetation occurred more often in the recently stabilized blowout, and is mainly located on slopes. Vegetation type 4 , which consist of wet valley vegetation occurred at the same locations as the willow trees. Vegetation type 2 consists of
vegetation that prefers stabilized areas and occurred in accumulation zones of both blowouts, which is roughly transect point 10 till 12.
The number of occurrences of each vegetation type per blowout is also demonstrated in this graph.
Figure 7) A comparison between the stabilized and recently stabilized in occurrence of vegetation types.
An ANOVA test between the soil and vegetation characteristics and the vegetation types was conducted. This provided mean values per vegetation type and information about the significant difference between vegetation types. In table 3 mean values per soil or vegetation characteristic are given. It also answers if the mean of the vegetation types significantly differ from each other.
0 1 2 3 4 5 6 7 8 9 10
Recently stabilized Stabilized High moisture vegetation
Wet valley vegetation
occurance of vegetation type
16 Mean VegType 1 Mean VegType 2 Mean VegType 3 Mean VegType 4 p-value significant different O layer 0,46 0,63 1,07 2,33 0,05 1 and 4 Ah layer 7,54 9,77 12,14 12,67 0,46 no Vegetation cover 93,85 87,00 96,43 100,00 0,35 no
Bare sand cover 6,15 13,00 3,57 0,00 0,35 no
pH 5,39 5,03 5,23 5,30 0,02 1 and 4 C content soil(gram/m2) 565,69 896,47 1261,20 1896,70 0,00 all C content vegetation(gram/m2) 56,21 40,40 20,96 17,03 0,01 3 and 4 from 1 N content soil(gram/m2) 51,85 71,61 93,93 164,10 0,00 all N content vegetation(gram/m2) 3,09 1,70 1,39 1,44 0,01 2 and 3 from 1 C/N ratio vegetation 32,80 29,35 18,29 12,90 0,00 3 and 4 from 1 C/N ratio in soil 10,46 11,63 13,36 14,80 0,03 1 and 4 Table 3) Mean values per vegetation type of different soil and vegetation characteristics and significant differences with p-value.
Pioneer vegetation and vegetation that occurs in wet valleys differed significantly for most
vegetation characteristics, which can be seen in table above. The carbon and nitrogen content in the soil in grams per square meter differed between all four vegetation types. The vegetation or bare sand cover as well as the Ah layer does not seem to affect the different vegetation types, since there is no significant difference found between the means of the vegetation types.
According to table 3, pioneer vegetation occurs on less developed soils with a thin O and Ah layer. pioneer vegetation prefers soils with a high pH but low carbon and nitrogen content in grams per square meter. However, the carbon and nitrogen content of the pioneer vegetation in grams per square meter is high in comparison with other vegetation types. Vegetation that occurred in stabilized areas also occurred in the area with the lowest vegetation cover and highest bare sand cover. This vegetation type also occurred on soils with low pH. The high moisture vegetation type has the lowest carbon and nitrogen content in grams per square meter. The wet valley vegetation
occurred on more developed soils with a thick O and Ah layer and a low pH value. The preferred area for wet valley vegetation has no bare sand cover and contains high values of carbon and nitrogen in grams per square meter in the soil.
An example of the difference between the four vegetation types is visualized in the graph below. This graph displays the mean pH value per vegetation type and the corresponding standard
deviation. The pH per vegetation type was significant different, with the highest value for vegetation type one(pioneer vegetation).
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Figure 8) Mean pH value and standard deviation per vegetation type.
According to the vegetation description, the recently stabilized blowout contained more lichen species. Lichen species that were found in the studie area are: Cladonia ciliate, Cetratraria aculeate, Cladonia fimbriatum, Cladona foliacea, Cladonia furcate and Cladonia portentosa.
Cetracul Cladcili Cladfimb Cladfoli Cladfurc Cladport
number of
occurrence cover (%)
stabilized blowout 0,00 1,26 0,16 0,32 0,11 0,11 3,58 0,19
recently stabilized
blowout 0,05 3,32 0,32 1,47 0,11 0,05 3,89 0,21
Table 4) mean values of times lichen species occurred per blowout.
The difference in lichen cover between the blowouts was tested to be not significant, but the mean value of the number of times lichen species occurred per blowout is significant.
Discussion
In this part the results of the research will be discussed in different chapters.
Vegetation cover
The recently stabilized blowout has a low vegetation cover, because this blowout was recently active and has still a large area of bare sand on the slopes in the blowout. Since we only made a choice between vegetation or bare sand, the maps are almost the exact opposite of each other, and for both surface covers a significant difference between the blowouts was estimated.
Grass cover as well as Marram grass do not significantly differ between the blowouts. However, it is noticeable that the grass cover is highest in the valley and more stabilized areas of the blowouts. This is caused by the preference of grass for more moistures soils, and according to our observations the soil was wetter in the valley of both blowouts. Therefore, grass was almost entirely absent in the drier area with high bare sand cover. There is not really a specific area noticeable with mainly marram grass. However it was not located in the wet dune valley area with the willow trees, this is because marram grass prefers dry and sandy soils (Provoost et al, 2004).
In the last 25 years the cover of marram grass has decreased enormously, since there has not been any reactivations in these blowouts we assume that this is mainly caused by the grazers. In the area Eldorado, Galloway cattle and Exmoor Ponies were placed by Staatsbosbeheer, with the intension to
5,39 5,03 5,23 5,30 4,60 4,80 5,00 5,20 5,40 5,60 1 2 3 4 pH Vegetation types
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decrease the grass encroachment. It seems to have worked since 4 Galloway cows were removed because there was not enough food left for all cattle (Staatsbosbeheer, 2015). Since, the marram grass roots grow until 50cm depth, they might easily return after grazing (Bekker et al, 1999). Therefore, it is expected that the amount of marram grass cover will increase when these animals leave.
The stabilized blowout has a better developed soil and a thicker Ah layer and O layer, this means that it is better suitable for bigger vegetation such as shrubs. This corresponds with the results, since there were significantly more shrubs in the stabilized blowout than there were in the recently stabilized blowout. The willow trees form a separate group since they only occur at two locations in each blowout. The area were willow trees grow, stabilized most likely before the reactivation started and has not been active for at least 25 years. The Buckthorn cover is described as a separate group, mainly because it occurred so much. However there is no significant difference found between the blowouts. Which is a bit odd since buckthorn is known for being a pioneer specie, therefore we expected to find more buckthorn cover in the recently stabilized blowout. The high cover of buckthorn is mainly located in the most southern part of the grid area, due to the increased bare sand cover. Buckthorn is known to be a specie that grows on sandy soils and might exclude all other vegetation (Bekker et al, 1999). This corresponds with the occurrence of almost only buckthorn on the slopes in the accumulation zones.
The high moss cover in the stabilized blowout, suggest that moss prefers to grow in stabilized areas. Moss species such as Brachythecium rutabulum are labelled as species that prefer moistures soils, therefore it is not present in areas with dry sandy soils and high bare sand cover (Ketner-Oostra, 2004). Which explained why there is more moss found in the stabilized areas.
Vegetation characteristics
The nitrogen content in grams per square meter was significant different between the two blowouts. Also the C/N ratio was significant different, with a higher mean for the C/N ratio in the recently stabilized blowout. Higher values for C/N ratio means that the nitrogen content is relatively lower in comparison with carbon content in the recently stabilized blowout. This confirms the hypothesis that nitrogen deposition in vegetation is lower in more active or recently stabilized areas than in stabilized blowouts (Jones, 2014).
Since there was less vegetation present in the recently stabilized blowout, it is expected that the carbon and nitrogen content per square meter should be lower as well (Carboni et al, 2009).
However, we found higher nitrogen and carbon contents per square meter in the recently stabilized blowouts. This might be due to sampling that we did, we took relatively more vegetation in the recently stabilized blowout. Whereas we were unable to sample everything in the stabilized blowout, since there were a lot of shrubs that we could not cut, and a lot of moss we did not sample since moss contained too much sand.
Vegetation types
As seen in the results pioneer vegetation occurred more often in the recently stabilized blowout. This confirms the hypothesis that active or recently active areas might cause an increase in the
occurrence of pioneer vegetation (Grootjans et al, 2013). However, pioneer vegetation also occurred in the stabilized blowout, but this can be explained. In the stabilized blowout the pioneer vegetation type occurs mainly on the slopes of the blowout, on this slopes there was relatively more bare sand than in the rest of the blowout. This bare sand is on the soil surface due to the paths that are made
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on the slopes, these paths are mainly made by the animals that live in this area, such as the Galloway cattle and Exmoor ponies.
In Europe, high species richness is typically related to high pH levels in the soil. Due to the
degradation of sandy grasslands in the Grey dunes, an opportunity opens up for plant species with high colonization and dispersal capacity to displace the native species. Many of the red-listed plant species today are calcicole species that favour dry soils with high pH and low nutrient content (Mårtensson and Olsson 2010). This is why it is expected and also confirmed that there is more pioneer vegetation present in the recently active blowout.
According to our observations in the field the dune pansy was mainly present in areas with high bare sand cover in the recently stabilized blowout. (Lammerts et al, 1999)
Vegetation type one consist of pioneer vegetation, these species are hardy species which are the first to colonize previously disrupted or damaged ecosystems, beginning a chain of ecological succession that ultimately leads to a more biodiverse steady-state ecosystem (Lammerts et al, 1999). There was no significant difference found in pH between the blowouts (Martens, 2016). According to the ANOVA test the pH of the soil did differ between the different vegetation types. Pioneer vegetation occurred at locations with the highest mean value for pH, this corresponds with the definition given by Lammerts et al. in 1999. The dune pansy (Viola curtisii) is an example of pioneer vegetation, it prefers soils with a pH of 5.0 till 5.8. According to our observations dune pansy was mainly present in areas with high bare sand cover in the recently stabilized blowout.
Other significant differences in vegetation types are mainly based on nitrogen content and carbon content in the soil and vegetation and the thickness of the O layer. These soil and vegetation characteristics were also the variables that significantly differed between the two blowouts.
Different lichen species such as the Cladonia ciliate and Cetratraria aculeate occurred most in the more recently stabilized blowout. Although, the estimated percentages of lichen cover was
estimated to be almost the same in both blowouts, the number of times lichen occurred was higher in the recently stabilized blowout. Lichen occurs in early stages of stabilization and disappears later when grass encroachment increases (Ketner-Oostra, 2004). This is the reason lichen was mainly present in the recently stabilized blowout.
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Conclusion
When looking at the vegetation characteristics and the vegetation cover of different species there are many significant differences between a stabilized blowout and a recently stabilized blowout. The vegetation cover is lower and the bare sand cover higher in the recently stabilized blowout. The moss, shrubs and herbs cover is significantly higher in the stabilized blowout. Buckthorn is mainly present in the accumulation zones of the blowouts on the dry and sandy soils. Marram grass is roughly present throughout the entire area except the wet dune valley areas. Nevertheless, the cover of marram grass has decreased enormously, mainly due to the grazing by cattle.
The samples that were analysed in the lab provided information about the carbon and nitrogen content of the vegetation. The nitrogen concentration in the vegetation per square meter is lower when looking at the differences between C/N ratio. Less nitrogen in the vegetation of the recently stabilized blowout confirms the hypothesis that active blowouts can be part of the solution against nitrogen deposition.
The vegetation description and the TWINSPAN technique estimated four main vegetation types, with means that differ significantly when looking at carbon and nitrogen content in the soil and vegetation in grams per square meter. The pioneer vegetation type occurred most often in the recently
stabilized blowout, as expected. In the stabilized blowout the pioneer vegetation was mainly present at the slopes of the blowout. This is caused by relatively high bare sand cover on the slopes, due to the paths that were made by the Galloway cattle and Exmoor ponies. The occurrence of pioneer vegetation has led to an increase of biodiversity in the recently stabilized blowout compared to the stabilized blowout.
Therefore active blowouts in combination with grazing cattle can be used to counteract nitrogen deposition, eutrophication, acidification and grass-encroachment in the Grey dunes along the Dutch coast on Terschelling. Good managing of Grey dunes is important to increase the biodiversity and improve the habitat conditions. This research demonstrates the importance of active blowouts or more recently stabilized blowouts in comparison with stabilized blowouts. The more recently stabilized blowout contains more bare sand, which leads to improved soil and vegetation conditions and an increased biodiversity. That is why it might be useful to continue reactivation of former blowout in the Grey dune areas. The reactivation in combination with cattle presence should be implemented in the managing policies of Staatsbosbeheer for the Grey dunes. It might be advisable to reactivate different blowouts every 25 or 30 years, since the positive effects of active blowouts on soil and vegetation conditions are still noticeable after 25 years.
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