1
The long term effect of blowout reactivation
on the vegetation biodiversity in the grey
dunes of Eldorado, Terschelling
Bachelor Thesis
Mara van den Berg (10386939)
Supervisors Annemieke Kooijman John van Boxel
2
Table of content
1. Abstract
2. Summary for the general public
3. Introduction 4. Theoretical framework 4.1 Aeolian activity 4.2 Grey Dunes 4.3 Nitrogen deposition 5. Relevance
6. Main question and sub questions 6.1 Main question 6.2 Sub question 7. Hypothesis 8. Methods 8.1 Fieldwork 8.1a Transects 8.1b Grid mapping 8.2 Lab work
8.2a Soil samples
8.2b Vegetation samples 8.3 Data analysis
8.3a Statistical analysis 8.3b GIS analysis 9. Results 9.1 Vegetation cover 9.2 C and N content 9.3 Vegetation types 10. Discussion 11. Conclusion 12. References
APENDIX A: Field data and Lab data APENDIX B: Matlab script
3
1. Abstract
Due to increased nitrogen deposition since the second half of the 20th century, the soils in the Netherlands acidified. This also happened in the dunes along the Dutch coast. Following this acidification of the soil the dunes became grass-encroached. This suppressed pioneer vegetation. Pioneer vegetation is rare and has to be saved from extinction. The government has taken measures to prevent this. One of these measures was the reactivation of blowouts in the Dutch coastal dune areas. On Terschelling some blowouts in the Eldorado area were reactivated in 1991. Research has been conducted to investigate which effect the reactivation after 25 years still has on the vegetation biodiversity and in relation to that on the soil in the Eldorado area. A reactivated blowout, a blowout that was still active 25 years ago and two stable, non-reactivated blowouts have been studied. The reactivated blowout houses 4 different types of vegetation, while the non-reactivated blowout houses only 2. The reactivated blowout has a significantly higher herb rich grassland cover than the non-reactivated blowout of almost 40 percent against 15 percent. Pioneer vegetation is mainly found in the
reactivated blowout. The pH in the reactivated blowout is 5 on average and 5.3 on locations with pioneer vegetation, while the pH in the non-reactivated blowout is 4.5 on average. The range in C and N content in the vegetation is higher, while the C and N content in the soil and vegetation are on average lower in the reactivated blowout. The grass encroachment has been reduced due to grazers introduced in the area.
Reactivation in combination with grazers in the area has led to stable, herb rich , dune grasslands. When this is desirable, small scale reactivation and grazers are a sufficient measure.
2. Summary for the general public: Het effect van stuivend zand op de biodiversiteit in de duinen
De hoeveelheid stikstof die wordt opgenomen in de bodem is sinds de tweede helft van de vorige eeuw gestegen. Hierdoor verzuurden de bodems in Nederland, waaronder die in de duinen. Dit zorgde ervoor dat snelgroeiende grassen de oorspronkelijke vegetatie verdrongen. De oorspronkelijke duinvegetatie is echter zeldzaam en moet daarom worden beschermd tegen uitsterven. Om het verdwijnen van de oorspronkelijke
duinvegetatie tegen te gaan heeft de Nederlandse overheid 25 jaar geleden maatregelen getroffen. Dichtgegroeide stuifkuilen langs de kust, waaronder een aantal op
Terschelling, werden afgegraven zodat het zand opnieuw kon stuiven. Het stuiven van zand heeft een positieve invloed op de biodiversiteit. Om er achter te komen of de reactivatie van de stuifkuilen na 25 jaar nog invloed heeft op de bodem en vegetatie is een onderzoek uitgevoerd. De resultaten wijzen uit dat reactivatie heeft geleidt tot een minder ontwikkelde bodem, waarin minder stikstof aanwezig is. Ook zijn er meer verschillende soorten vegetatie in de gereactiveerde kuil, waaronder ook meer pionier vegetatie. Het landschap verstuift echter niet meer actief.
4
3. Introduction
Conservation of the dune ecosystem in the Netherlands is of high importance since the Dutch dunes house 70 percent of the national flora. The dunes are a species rich
ecosystem and 15 percent of all vegetation in the dunes does only exist there (Veer and Kooijman, 1997). Originally, the dune landscape consisted of variable patches ranging from highly vegetated to non vegetated. Over the last decades the distribution of typical dune vegetation (pioneer vegetation) decreased, while the distribution of marram grasses increased. Previous research has been done on these changes in vegetation in the Dutch dunes. The changes in vegetation may have been caused by increasing
nitrogen deposition. The nitrogen deposition peaked in the 90s of the previous century (De Haan et al., 2008). This led to eutrophication and acidification which caused grass-encroachment on vegetation and soil in the Dutch dry dune grasslands (Veer and Kooijman, 1997).
Non vegetated areas as blowouts already started to stabilise in the 40s. Blowouts are bowl-shaped or elongated erosional depressions in the landscape. The first stabilisation of bare sand was caused by algae. Algae stick sand particles together and make the surface less prone to wind erosion (Van den Ancker et al., 1985). Due to the stabilisation of the blowouts and the increasing nitrogen deposition the landscape became
overgrown by grasses. At the outset it was thought that this was a good, stabilising, development of the dune landscape, which would make the Dutch coast better resistant to the sea. Later it was understood that this led to a decrease of the biodiversity because the process of repeating succession was stopped. 25 years ago management measures were taken on Terschelling to increase the biodiversity again. This was done by the stimulation of aeolian activity by reactivation of former blowouts. The surface in the blowouts was excavated which led to the exposure of the surrounding surface to fresh sand, distributed by the wind. This meant that soil and vegetation succession could start over new, which gave pioneer vegetation a new chance. Moreover, grazers were
introduced in the area as a management measure (Van der Meulen et al., 1996). This research has been conducted to investigate what the effect of the reactivation of blowouts in the Eldorado area on Terschelling is on the soil and vegetation 25 years after reactivation. In May and June 2016 research has been done on one reactivated blowout, one blowout that was still active 25 years ago, and two blowouts that were stable and have not been reactivated. On the picture below the research area is indicated.
5
Figure 1. Overview map of Terschelling with fieldwork area (Topografische dienst/kadaster, 2015).
The reactivated blowout that has been studied is blowout 3 (see figure 2 and 3). This blowout was stripped of vegetation in the south-east and in the north (Van Bolhuis, 1995). Blowout 12 has been studied as a natural reference, since it was still active 25 years ago. Blowout 13 and a small blowout north of Blowout 3 were already stable 25 years ago and have not been reactivated (Van der Meulen et al., 1996). Comparing these blowouts with the reactivated blowout can give insight in the effect of reactivation.
Figure 2. Map of Eldorado area with reactivated blowouts within the circles (Van der Meulen et al. 1996).
6
In this research the results of the comparing study on vegetation between blowout 3, the reactivated blowout, and the stable, non-reactivated blowout, north of blowout 3 will be presented. In the discussion the results of this study will be related to the results of the comparing studies of the other blowouts.
Figure 3. Reactivated blowout 3 within red contour, non-reactivated blowout within blue contour.
The aim of this research is to get deeper insight in the effect that reactivation can have on soil properties and the distribution of pioneer vegetation in the dunes. From this insight an advice has been formulated on reactivation measures. This advice is intended for bodies engaged in decision-making and operational activities related to the Dutch dunes.
4. Theoretical framework 4.1 Aeolian activity
Due to aeolian activity the geomorphology of the dunes changes fast. Water and gravitational erosion have an effect on the change of the geomorphology as well. However, these factors are better predictable and have less influence on the coastal dunes. The wind is the great shaper of coastal dunes. Since the wind changes direction often the landscape becomes very dynamic (Jungerius and Van der Meulen 1997). One of the landforms created by the wind is a blowout. This is a depression in the landscape, where the soil has eroded away. It occurs mostly close to the coast but can also occur further inland where the soil has stabilised already. This can be caused by erosion due to water and gravity. If this erosion leads to exposure of bare sand the wind can start to erode, for which a minimum wind speed of 6 m/s is needed (Van Boxel et al. 1997). Erosion will go faster and faster if more bare sand gets exposed to the surface. A
7
blowout can be formed in a couple of days (Jungerius 2008) and indicates that the landscape is dynamical (Provoost et al. 2004).
In the figure below the process of the migration of blowouts is made visible. The wind comes from the left and erodes sand particles away at the trailing edge. These sand particles are deposited on the leading edge. Due to this both the trailing edge and the leading edge migrate in the direction the wind comes from (Jungerius, 2008).
Figure 4. Upwind migration of a blowout (Jungerius 2008).
The process of erosion and formation of the blowout strengthens itself when the blowout becomes larger and the surface becomes more even. Factors that withhold the formation of blowouts are SOM, roots, soil moisture, vegetation and algae surface crusts (kooijman et al. 2005). Active blowout processes can alter the developing processes in the dunes in such a way that the biodiversity could increase again and that pioneer vegetation could get a chance.
4.2 Grey dunes
The dunes behind the white, sandy, coastal dunes are called the grey dunes. These dunes are more land inwards and have developed some soil. At least there is a Ah layer
present. In these areas soil development activities such as leaching of calcium carbonate and nutrient enrichment with nitrogen and phosphorus take place. Atmospheric
nitrogen deposition has even increased nitrogen enrichment of the soil. This is causing acidification of the soil. The pH gets lower and nutrients in the soil increase. Due to low decomposition rates plant material (humus) accumulates. Nowadays a lot of the grey dunes are encroached with fast growing grasses that flourish on the nutrient enriched and acidified soil. They suppress the original vegetation; typically there were herbs, mosses and bare spaces present as well. This allowed a rich biodiversity to establish in the grey dunes in the past (Provoost et al. 2004).
In the grey dunes on Terschelling the vegetation succession is disturbed by the process described above. This has caused the disappearance of pioneer vegetation. The
vegetation succession in the grey dunes is comparable with the vegetation succession in figure 5.
8
Figure 5. Succession of ‘stuifzanden’ (Sparrius et al., 2012).
4.3 Nitrogen deposition
Together, nitrogen and phosphorus are limiting factors for plant growth in the grey dunes. however, nitrogen deposition has increased since the second half of the 20th century and has exceeded the critical amounts (Jones et al., 2014). Since the 1990s the atmospheric nitrogen is decreasing but still remaining too high. The effects of it, like grass-encroachment, go on even after the reduction of nitrogen in the atmosphere (De Haan et al., 2008). The three main impacts of atmospheric nitrogen on the soil are eutrophication, acidification and toxification (Jones et al. 2014).
The acidification and eutrophication of the soil cause the lime content to decrease. Lime, normally high due to shells and lime rich sediments, is dissolved and leaches out of the system. However, on Terschelling the lime content in the soil is naturally low (Kooijman et al., 1998).
5. Relevance
This research contributes to a better understanding of the long term effect on
biodiversity of the reactivation of blowouts. Grass-encroachment has limited the aeolian activity in the dunes and due to this the biodiversity has decreased. Retaining the
biodiversity is highly important in the dune areas because coastal dune areas are rare and have specific ecosystem services (Provoost et al. 2004). Since the increased nitrogen deposition is the underlying cause of the decreasing biodiversity, the Dutch government has established a program, ‘Programmatische Aanpak Stikstof’ (PAS), to stop nitrogen enrichment in the dunes. This program was established in 2015. This research adds to this program and the earlier research conducted by Kooijman et al. (1998),
commissioned by the ministry of agriculture, nature and fishery.
6. Main question and sub questions 6.1 Main question
What is the long-term effect of reactivation of blowouts, with respect to vegetation and soil properties, on the vegetation biodiversity in the grey dunes of Eldorado, disturbed by increasing nitrogen deposition since the second half of the 20th century?
9
6.2 Sub questions
What is the difference after 25 years between a reactivated and a non-reactivated blowout with respect to the distribution of different types of vegetation?
What is the difference after 25 years between a reactivated and a non-reactivated blowout with respect to the distribution of pioneer vegetation?
What is the difference after 25 years between a reactivated and a non-reactivated blowout with respect to vegetation dry weight and C and N content and C/N ratio in the vegetation?
How does the difference in vegetation between a reactivated blowout and a non-reactivated blowout relate to the soil?
7. Hypothesis
For the first three sub questions It is hypothesized that there is less vegetation cover in a reactivated blowouts than in a non-reactivated blowout after 25 years. This means that there is less vegetation per m2, measured in dry weight, in the reactivated areas. Thus there will be, per m2, less carbon and nitrogen content in the vegetation. It is also expected that there will be a larger variety of vegetation in the reactivated blowout, and more pioneer vegetation.
For the last sub question it is hypothesized that a reactivated blowout will show less soil development after 25 years. It is thus expected that soil pH is higher in a reactivated blowout and that there is less organic matter. Carbon and nitrogen in the soil increase with soil development and will thus be lower in the reactivated and less stabilized blowout.
This means that for the main question it is hypothesized that reactivation has a positive, increasing, effect on the vegetation biodiversity.
8. Methods 8.1 Fieldwork
To find answers to the main and sub questions a fieldwork of 2 weeks has been carried out on Terschelling in the Eldorado area. This study focusses on the fieldwork done in a reactivated blowout, blowout 3, and a stable, non-reactivated blowout north of blowout 3.
8.1a Transects
In each blowout that has been studied sampling points have been set on two crossed transects (see figure 6), according to previous research done by Witz (2015). In the
10
reactivated blowout 20 and in the non-reactivated blowout 19 sampling points have been examined.
On the sampling points soil samples have been collected with a pF ring (or bulk density ring). The content of 2 rings was enough soil to carry out all the research that has been done in the lab. This means that on each sampling point 200 cm3 soil was collected in plastic bags that are strong enough to go in the oven. Due to this less soil was lost and results are more accurate.
On each sampling point a soil profile description was made. Also the surface cover, bare sand cover, moss cover, herb cover and shrub cover, marram grass cover and buckthorn cover have been recorded on each sampling point.
At last the vegetation has been sampled on each sampling point. Where vegetation is more developed, a vegetation sample was taken on a plot of 25 x 25 cm2. When the vegetation was more open it was be sampled on a plot of 50 x 50 cm2. This could be translated to the vegetation that is present on a m2. Vegetation has been cut and carried in plastic bags to do further research in the lab. Vegetation like mosses have not been sampled because it has too much sand in it, which would bias the results (Witz, 2015). Moreover, on each sampling point a list of all the occurring vegetation was made.
Figure 6.Transects in fieldwork area.
8.1b Grid mapping
In and around the blowouts 210 grid points have been set (see figure 7), according to previous research done by Witz (2015).
On each grid point the thickness of the Ah layer has been measured and the percentage of vegetation and bare sand has been estimated.
Exact locations of the grid points have been recorded with a GeoXM GPS to be able to interpolate the data and make maps out of it.
11
Figure 7.Grid in fieldwork area.
8.2 Lab work
8.2a Soil samples
To calculate the bulk density (weight/volume) each sample has been dried for 24 hours at 40°C. Then each sample was weighted to determine the bulk density.
Each soil sample was sieved in a 2 mm sieve. A part of the smaller section was ground for 5 min. with a rotational speed of 400 rpm. This material has been dried again at 70°C for 24 hours. About 40 - 60 mg of each sample was prepared in duplicate for analysis with a CHNS analyser. This machine analyses the carbon and nitrogen content.
Then 10 grams of the sieved soil sample were used to determine the pH and EC. The soil was mixed with 25 ml demi water and shaken for two hours. Then the samples had to rest a day and be mixed again for 20 minutes. After this all soluble particles are dissolved. pH and EC values have been measured with electrodes.
8.2b Vegetation samples
All vegetation samples have been dried for 24 hours on 40°C and weighted to determine the dry weight of biomass per m2. A representative part of the plant material has been ground at 8000 rpm. This material has been dried again at 70°C for 24 hours. 10 - 15 mg of each sample has been prepared for analysis with the CHNS analyser to determine the carbon and nitrogen content.
12
8.3 Data Analysis
8.3a Statistical analysis
After the lab work datasets on the different soil and vegetation properties were generated. To compare the different properties of the soil and vegetation between the reactivated blowout and the non-reactivated blowout a t-test has been used.
A clustering program, TWINSPAN, has been used to group all the occurring vegetation into vegetation types. To test if these vegetation types had significantly different soil and vegetation properties a one-way-ANOVA and a post hoc test were used.
8.3b GIS analysis
ArcGIS has been used to create surface maps of the spatial distribution of all the different vegetation types and some soil properties. The data from the grid points has been interpolated with IDW to create such maps.
Moreover a pH map could be generated because there was a correlation between pH and bare sand cover on the transact sampling points.
9. Results
9.1 Vegetation cover
Maps on the vegetation cover and on different types of vegetation are created from the grid analysis. In each map the deflation and accumulation zone of the non-reactivated blowout are indicated on the left and the deflation and accumulation zone of the reactivated blowout are indicated on the right.
The map on vegetation cover shows a clear difference between the two blowouts. There is only a little spot with less than 100 percent vegetation cover in the non-reactivated blowout, while there are large areas with low vegetation covers, until 30 percent, in the reactivated blowout. These low vegetation cover areas are mainly located on the slopes inside the blowout, where erosion takes place easier than on flat locations.
However, there is no significant difference (p-value is 0.0543) between the two blowouts according to the comparison made from the transect data.
13
Figure 8. Vegetation cover
Another way to estimate the differences in vegetation between the blowouts is to compare the amount of vegetation present in each blowout in g/m2. The average dry weight of vegetation is 383 g/m2 in the non-reactivated blowout. In the reactivated blowout the average vegetation dry weight is 225 g/m2. This clear difference is tested to be significant with a p-value of 2.3332e-04.
The cover of marram grass does not significantly differ between the two blowouts. Looking at the map there might me a bit more marram grass in the reactivated blowout, but the transect data does not show a clear difference since the p-value is 0.9506. Spots with a higher marram grass cover correspond with areas with less vegetation cover.
Figure 9. Marram grass cover
The moss cover does also not differ between the blowouts. The comparison of the transect data results in a p-value of 0.8480. Although, on the map a difference is visible. It can be clearly seen that the moss cover is higher on places with a high vegetation cover, if the maps are compared.
14
Figure 10. Moss cover
The herb cover turns out to be significantly higher in the reactivated blowout with a p-value of 0.0034. The map also shows darker colours (more herbs) in the area of the reactivated blowout. Overall, the percentage of herbs seems to be higher on flat areas. Since herbs root less deep than marram grass or shrubs this is plausible.
Figure 11. Herb cover
Buckthorn is only found in and around the reactivated blowout. It appears mainly on places with a relatively high herb cover or places with a relatively high bare sand cover. The largest plants and most concentrated buckthorn cover were found in the
accumulation zone of the reactivated blowout. On the other spot, the buckthorn plants were relatively small.
15
Figure 12. Buckthorn cover
In the reactivated blowout are significantly more shrubs. Almost all of the non-reactivated blowout is covered with high amounts of shrubs, while the non-reactivated blowout shows large areas with a very low shrub cover, especially on the slopes. The p-value from the comparison of the transect data is 0.0174
Figure 13. Shrub cover
The diagram in figure 14 shows a quick overview of the average cover of each vegetation type per blowout.
16
Figure 14. Diagram showing the average cover per vegetation type.
9.2 C and N content
In figure 15 and 16 the results of the C and N analysis are presented. The C and N content in the vegetation in g/m2 are both higher in the non-reactivated blowout, as expected. The percentage of C in the vegetation lies around 50%. The percentage of N in the vegetation ranges from 0.9 % to 2.8 %.
On the other hand, the range in C and N content in the vegetation is higher in the reactivated blowout.
The comparison of a recently stabilized blowout with a blowout that is already stable for a long time shows opposite results. The C and N content in the vegetation in g/m2 are higher in the recently stabilized blowout (Boersma, 2016).
Figure 15. Boxplots showing the C and N content in the vegetation in g/m2 in the reactivated blowout (1) and in the non-reactivated blowout (2).
17
On average, the C/N ratio in the vegetation is lower in the reactivated blowout than in the non-reactivated blowout. This means that on average there is, relative to C, more N available in the reactivated blowout. However, the range in C/N ratio is again higher in the reactivated blowout, reaching C/N ratios higher and lower than in the
non-reactivated blowout.
The comparison of the two other blowouts gave the opposite result. The recently stabilized blowout has a higher C/N ratio (Boersma, 2016).
Figure 16. Boxplot showing the C/N ratio in the vegetation in the reactivated blowout (1) and in the non-reactivated blowout (2).
9.3 Vegetation types
With TWINSPAN (Two-Way-INdicator-SPecies-ANalysis), a program that clusters different vegetation species that occur together, 4 different vegetation types that are characteristic for this area are determined.
1. Pioneer vegetation
Characteristic species: Cerastium semidecandrum (zandhoornbloem) 2. Stabelized grassland rich in herbs
Characteristic species: Luzula campestris (gewone veldbies), Hypochaeris (biggenkruid) and Holcus lanatus (gestreepte witbol)
3. Stabelized shrubs
Characteristic species: Empetrum nigrum (kraaiheide) 4. Wet valley vegetation
18
The diagram in figure 17 shows the presence of each vegetation type per blowout along the transect. Underneath the diagram a picture of each of the different vegetation types is present to give an indication of what they look like.
Figure 17. Diagram showing the occurrence of each vegetation type per blowout and underneath it a picture of each type.
The 4 vegetation types differ from each other in terms of pH, bulk density of the soil, C content in the soil and N content in the soil. In the resulting ANOVA tables below it becomes clear that the pioneer vegetation and valley vegetation type both have a significantly higher pH and bulk density, and that the C and N content in the soil are lower on locations with these vegetation types.
0 2 4 6 8 10 12 14 16 18
Pioneer Vegetation Stabelized, Grasses and herbs
Stabelized, Shrubs Wet Valley Vegetation Reactivated Blowout Non-Reactivated Blowout
19
Figure 18. Statistical results on the analysis of the differences between the four vegetation types. PH distribution on the left and bulk density distribution on the right and the
corresponding ANOVA tables above them.
Figure 19. Statistical results on the analysis of the differences between the four vegetation types. C content on the left and N content on the right and the corresponding ANOVA tables above them.
20
In the schematic overviews of the two blowouts below it is clearly visible that the reactivated blowout has a higher biodiversity than the non-reactivated blowout. The reactivated blowout has more pioneer vegetation and 4 different vegetation types. The non-reactivated blowout has only two vegetation types and very little pioneer
vegetation.
In the long and recently stabilized blowout are also 4 vegetation types determined. However, these vegetation types are not the same as those in the reactivated and non-reactivated blowout. There is no real pioneer vegetation type present in the long and recently stabilized blowout. However, vegetation type 1 houses some pioneer
vegetation. Although the recently stabilized blowout has the highest biodiversity of the two, the long and recently stabilized blowout both have a higher biodiversity than the non-reactivated blowout. Moreover there is more lichen present in the recently
stabilized blowout (Boersma, 2016).
Reactivated Non-reactivated Long stabelized Recently stabelized Figure 20. Scematic overview of the different blowouts and the distribution of vegetation types.
10. Discussion
The results from this research are broadly in line with what was expected and hypothesized.
The resulting maps on vegetation cover are quite accurate and show clear differences between the blowouts. In the cases of vegetation cover, marram grass cover and moss cover there were no significant differences. The location and number of the sampling points, might cause the insignificant statistical results not to be representative and therefore the results to be inaccurate.
The grid data were not used to compare the blowouts since it was not indicated which grid point belonged to which blowout and because more grid points were located in and around the reactivated blowout than in and around the non-reactivated blowout. If a statistical test would have been used to compare the blowouts with the grid data there would probably have been a significant difference in vegetation, moss and marram grass
21
cover since there are more grid points than transect points and because the grid points are evenly distributed over the blowout.
The C and N content in the soil as well as in the vegetation are on average lower in the reactivated blowout. This indicates that the soil is less developed and that there is more pioneer vegetation, less dependent on N availability (Ketner-Oostra & Sýkora, 2000). Furthermore, the fact that the range in C and N content in the vegetation is higher in the reactivated blowout indicated that there is a greater variability in Vegetation types, which is the case. All 4 vegetation types encountered in the whole study area are
present in the reactivated blowout, while only 2 types are present in the non-reactivated blowout.
It was expected that the comparison of the recently stabilized blowout and the stable blowout would show the same kind of results as the comparison of the reactivated and the non-reactivated blowout. However, this is not the case. In terms of N and C content in the vegetation the results are the opposite. Following, the results from the reactivated blowout could be attributed to the reactivation, as this has caused the vegetation
succession and N fixation to start from the beginning again.
Pioneer vegetation is found on locations with relatively more bare sand. The soil is little developed in these areas and there is no OM layer present. In these areas the pH is also higher than in the rest of the reactivated blowout and higher than in the non-reactivated blowout. On Average the pH is 5.3 on the locations with pioneer vegetation, with values higher than 6.1 on some spots. The pH is 5 in the reactivated blowout on average and 4.5 in the non-reactivated blowout on average (Van Bentum, 2016).
The fact that the pioneer vegetation type is present in the non-reactivated blowout can be attributed to the rabbits, active in the study area. These rabbits make their holes in the walls of the blowouts. This causes a range of sand around each rabbit hole. This gives pioneer vegetation the chance to develop.
If the results of this research are compared with the situation before reactivation it is most striking that the marram grass encroachment is heavily reduced. Where 25 years ago the whole area was overgrown with marram grass there is now only 10 percent marram grass cover on average. This reduction has been caused by the grazers that were also introduced in the Eldorado area as a management measure (Van der Meulen et al., 1996).
From this can be concluded that rabbits and grazers are of great influence on the vegetation distribution in the area. The grazers stopped and declined the grass encroachment. The reactivation of blowouts initiated pioneer vegetation. However, without rabbits and grazers, the reactivated areas would probably be already
encroached or stabilized again since this is already happening but slowed down by the movement of the animals. Animals maintain bare areas open and make bare paths along the walls of the blowouts and through the whole Eldorado area.
After reactivation in 1991, reactivated blowout 3 looked like this in 1995. So the areas stripped of vegetation already began to stabilize. At the moment there are no bare areas like this anymore. There is not a square meter without vegetation. However, there are a
22
lot of animal paths, causing constant erosion and bare sand on all the walls of the reactivated and, but less, non-reactivated blowout. On the picture a large area seems to be densely vegetated with shrubs. If this was the case this has been altered over the years, cause the shrub percentage inside the reactivated blowout is mostly less than 10 percent now.
Figure 21. Reactivated blowout 3 in 1995 (Van Bolhuis, 1995).
Although accumulation thicknesses until 9 cm are found in the accumulation zone the shrubs that were growing there, which was mainly crowberry, is still present.
Crowberry is a very resistant shrub that is not disturbs by burial of 5 till 10 cm (Van Bolhuis, 1995). However, due to the accumulation of bare sand on top of the existing vegetation, other vegetation had a chance to grow. Now there is also a lot of buckthorn, herbs and pioneer vegetation present in the accumulation zone.
11. Conclusion
Reactivation in 1991 of some blowouts led to more pioneer vegetation in the Eldorado area on Terschelling. The reactivated blowouts are quite stable again, however. The decline in marram grass is caused by grazers in the area. The combination of grazers and reactivation led to stable, herb rich, dune grasslands. The soil in reactivated areas has a higher pH and has less C and N content in the soil and vegetation then non-reactivated areas.
If it is desired to achieve stable, herb and pioneer rich, dune grasslands it is advised to use small scale reactivation in combination with grazers. I fit is desired to maintain a dynamic dune system other measures should be taken, since reactivation did not led to dynamic blowouts after 25 years.
23
12. References
Ancker, V., Den, J. A. M., Jungerius, P. D., & Mur, L. R. (1985). The role of algae in the stabilization of coastal dune blowouts. Earth Surface Processes and Landforms, 10(2), 189-192.
Boersma, M. (2016). The Long term impact of active blowouts on vegetation characteristics in the Grey dunes of Eldorado, Terschelling.. Bachelor Thesis. UvA Haan, B. D., Kros, J., Bobbink, R., Jaarsveld, J. V., De Vries, W., & Noordijk, H. (2008). Ammoniak in Nederland. RIVM, Bilthoven.
Jones, L., Provins, A., Holland, M., Mills, G., Hayes, F., Emmett, B., et al. (2014). A review and application of the evidence for nitrogen impacts on ecosystem services. Ecosystem Services, 7, 76-88.
Jungerius, P. D. (2008). Dune development and management, geomorphological and soil processes, responses to sea level rise and climate change. Baltica, 21(1-2), 13-23.
Jungerius, P., & Van der Meulen, F. (1997). Aeolian dynamics in relation to vegetation in a blowout complex in the meijendel dunes, the netherlands. Journal of Coastal
Conservation, 3(1), 63-70.
Ketner-Oostra, R., & Sýkora, K. V. (2000). Vegetation succession and lichen diversity on dry coastal calciumpoor dunes and the impact of management experiments. Journal of Coastal Conservation, 6(2), 191-206.
Kooijman, A., Besse, M., Haak, R., Van Boxtel, J., Esselink, H., ten Haaf, C., et al. (2005). Effectgerichte maatregelen tegen verzuring en eutrofiering in open droge duinen. Eindrapport Fase 2. Directie Kennis, Ministerie van Landbouw, Natuur en
Voedselkwaliteit.
Kooijman, A. M., Dopheide, J. C. R., Sevink, J., Takken, I., & Verstraten, J. M. (1998). Nutrient limitations and their implications on the effects of atmospheric deposition in coastal dunes; lime‐poor and lime‐rich sites in the Netherlands.Journal of Ecology, 86(3), 511-526.
Martens, P. (2016). Assessing the long term influence of active blowout processes on soil quality in the Grey dunes of Eldorado. Bachelor Thesis. UvA.
Provoost, S., Ampe, C., Bonte, D., Cosyns, E., & Hoffmann, M. (2004). Ecology,
management and monitoring of grey dunes in flanders. Journal of Coastal Conservation, 10(1), 33-42.
Sparrius, L. B., Sevink, J., & Kooijman, A. M. (2012). Effects of nitrogen deposition on soil and vegetation in primary succession stages in inland drift sands. Plant and soil, 353(1-2), 261-272.
24
Van Bentum, D. (2016 The potential of reactivating blowouts for soil quality in the coastal Grey dunes of Eldorado, Terschelling. Bachelor Thesis. UvA
Van Bolhuis, A. (1995). Effect van wind op de grootte van accumulatiezones eldorado terschelling. Fysisch Geografisch en Bodemkundig Laboratorium, afdeling
landschapsecologie, Universiteit van Amsterdam,
Van Boxel, J., Jungerius, P., Kieffer, N., & Hampele, N. (1997). Ecological effects of reactivation of artificially stabilized blowouts in coastal dunes. Journal of Coastal Conservation, 3(1), 57-62.
Van der Meulen, F., Kooijman, A.M., Veer. M.A.C. & Van Boxel, J.H., 1996. Effectgerichte Maatregelen tegen Verzuring en Eutrofiering in Open Droge Duinen; Eindrapport Fase 1, 1991-1995. Fysisch Geografisch en Bodemkundig Laboratorium, Universiteit van
Amsterdam, 232 pp.
Van der Meulen, F., & Salman, A. H. P. M. (1996). Management of Mediterranean coastal dunes. Ocean & Coastal Management, 30(2), 177-195.
Veer, M., & Kooijman, A. (1997). Effects of grass-encroachment on vegetation and soil in dutch dry dune grasslands. Plant and Soil, 192(1), 119-128.
Witz, L. (2015). The potential of small-scale blowout activity for landscape diversity in the Dutch grey dunes. Master Thesis. UvA.
