The potential of reactivating blowouts for soil quality in the Grey dunes of Eldorado, Terschelling: A comparative analysis between a reactivated blowout and a non-reactivated blowout

2/7/2016

Dana van Bentum

Student number: 10571949

Supervisors:

Annemieke Kooijman

John van Boxel

blowouts for soil quality in the Grey

dunes of Eldorado, Terschelling

A comparative analysis between a reactivated blowout and a non-reactivated blowout

Front page image

From own archive on 30/4/2016: Midsland aan Zee, Terschelling.

BSc Applicant

Dana van Bentum Future Planet Studies Major Earth Science University of Amsterdam Student number: 10571949

Supervisor and First Assessor

Annemieke Kooijman

Institute for Biodiversity and Ecosystem Dynamics University of Amsterdam

Science Park 904, Amsterdam

Second Assessor

John van Boxel

Institute for Biodiversity and Ecosystem Dynamics University of Amsterdam

ABSTRACT

The coastal dune area system of the Netherlands provides important ecosystem services, which have been threatened by high deposition of atmospheric nitrogen, leading to acidification and eutrophication processes within the soil. Blowouts play an important role in counteracting this process, since they distribute fresh sand in their surrounding area which can function as a buffer. The aim of this research was to determine the small-scale potential of the reactivation of blowouts in the Grey dunes of Eldorado, Terschelling on soil development after 25 years. This was determined by comparing soil formation and soil chemical properties of an in 1991 reactivated blowout with one that had stabilized already. Samples obtained in the dunes on Eldorado were analysed for soil characteristics such as pH, electrical conductivity, nutrient content, topsoil characteristics, soil organic matter and vegetation cover, which were analysed in the lab and arcGIS. This resulted in multiple interpolation maps displaying the value distribution of soil properties within the grid area, showing the most significant differences in the reactivated area. Comparing these results with another study, with naturally active instead of reactivated blowouts, led to the insight that acidification and eutrophication of the soil are both slowed down or even counteracted by blowout reactivation. Aeolian activity leads to new succession of soil and vegetation and is responsible for creating a landscape mosaic and a suitable habitat for multiple vegetation types. Grazing in the area, which started approximately five years ago, seemed to have a positive influence on the reduction of grass-encroachment by marram grass and increase of open sand by trampling, which also contributed to higher biodiversity. In naturally active blowouts, eutrophication processes, i.e. accumulation of C and N in the soil, were also counteracted. However, acidification was not affected. In general, reactivation as a management measure in the Wadden district has a long term positive effect on soil chemical properties, leading to less acidification, less eutrophication and a higher species diversity.

DUTCH SUMMARY

Het langetermijneffect van re-activatie van stuifkuilen op de bodem in het Nederlandse duingebied

De grijze duinengebieden van Nederland worden al enkele decennia bedreigd door een teveel aan stikstof in de atmosfeer, veroorzaakt door industrie en landbouw. Deze stikstof zorgt voor vergrassing en verzuring in de duinen, waarbij veel biodiversiteit verloren gaat. Het reactiveren van stuifkuilen in de duinen zorgt voor verstuiving van vers zand, dat als een buffer functioneert. Het doel van dit onderzoek was om de potentie van het reactiveren van stuifkuilen in kaart te brengen door te kijken naar chemische eigenschappen van de bodem in het gebied na het verstuiven van kaal zand, welke in kaart werden gebracht met behulp van ArcGIS na een lab-analyse van grondmonsters. Een vergelijkend

onderzoek tussen een gereactiveerde kuil en een niet-gereactiveerde kuil concludeert dat deze maatregel de verzuring en eutrofiëring van de grond vermindert of zelfs kan tegengaan. De maatregel zorgde voor het verhogen van de pH en het verlagen organische stof in de bodem, zoals koolstof en stikstof, met significante verschillen tussen de stuifkuilen. Dit leidde tot het opnieuw koloniseren van pioniervegetatie in het gebied. Deze nieuwe vegetatie successie zorgt voor een divers landschap met kolonisatie van verschillende vegetatietypen. In het algemeen kan worden geconcludeerd dat re-activatie maatregelen een positief langetermijneffect hebben op de bodemeigenschappen en biodiversiteit in Eldorado, Terschelling.

Table of Contents

ABSTRACT...3

DUTCH SUMMARY...3

TABLE OF FIGURES/GRAPHS/TABLES...5

1. INTRODUCTION...6

2. THEORETICAL FRAMEWORK...7

2.2 Aeolian dynamics of blowouts... 7

2.3 Nitrogen deposition – eutrophication and acidification...8

2.4 Stabilization and development of blowouts... 8

3. FIELDWORK AREA...9

4. METHODS...10

5. RESULTS...14

5.3 Soil chemical processes... 16

6.1 The influence of bare sand... 19

6.2 The relationship between soil and vegetation...20

6.3 comparison with an active blowout... 21

6.4 THE INFLUENCE OF GRAZERS... 22

7. CONCLUSION...23

REFERENCES...24

TABLE OF FIGURES/GRAPHS/TABL

Figure 2 Formation and migration of the depositional lobe (Jungerius, 2008)...7

Figure 3A. Open community with Corynephorus canescens 3B. Heathland community with Empetrum nigrum 3C. Grass community with Ammophila Arenia... 8

Figure 4 The location of Terschelling in the Netherlands. Source: Google Earth....9

Figure 5 Fielwork area overview, including the transects and the outer edges of the grid... 9

Figure 6 Methodological workflow...10

Figure 7 Fieldwork area overview. Blowout R (reactivated) in the east, blowout N (non-reactiaved) in the west, surrounded by a polygon representing the border of the grid...10

Figure 8 Grid points... 11

Figure 9 Vegetation cover in percentages, including the deflation zones of both blowouts... 14

Figure 10 Soil profile with and without OM layer...15

Figure 11 Distribution of the thickness of the OM layer [cm]...15

Figure 12 Distribution of the thickness of the Ah layer [cm]...15

Figure 13 Double soil profile [120 cm]...16

Figure 14 Accumulation layer [cm] of new parent material after reactivation...16

Figure 15 Interpolation pH map showing the deflation zones of both blowouts...17

Figure 16 Distribution of N content in the soil, gr/m2 _{till a depth of 5 cm...17}

Figure 17 Cross section of the long and short transects and their according pH values and bare sand coverage [%]...19

Figure 18 Vegetation succession in inland dunes derived from Hasse (2005) by Sparrius (2012)... 20

Figure 19 Vegetation category distribution along transects in reactivated blowout (right) and non-reactivated blowout (left)...20

Y Graph 1 Soil Horizon thickness [cm]...15

Graph 2 Correlation between pH and bare sand...16

Graph 3 Carbon and nitrogen content in the soil in gr/m2...17

Graph 4 Average PH values of all four blowouts...21

Graph 5 Mean values of the carbon an nitrogen content of all four blowouts in gr/m2_{... 22}

Table 1 Mean values of all blowout variables, plus t-test results...14

Table 2 Mean values of the C/n ratio...18

Table 3 Mean values of the deflation and accumulation zones...19

Table 4 Mean C/N ratio’s of the different blowouts...22

1. INTRODUCTION

The Netherlands is one of the smaller European countries with a relatively large coastal dune area. The dunes are of great value for nature conservation and biodiversity in the Netherlands. They are home to 70% of the national flora of which 15% is practically exclusive (De Molenaar, 1986). Until 1990, a continuous increase in atmospheric deposition of nitrogen has occurred (De Haan et al., 2010). This led to acidification and eutrophication of the dune soils, caused by the exhaustion of the buffer capacity of the Dutch coastal Grey dunes. Blowouts, bowl-shaped or elongated erosional depressions largely free of vegetation, can play an important role in counteracting this process. They are seen as the most conspicuous erosional impact of wind on the Dutch coastal dunes.

After a period of management measures focussed on stabilization of blowouts, Veer & Kooijman (1996) concluded that encroachment of tall grass species in open dune vegetation, which stabilizes the dunes, is considered unfavourable for nature conservation.

Since the 1990’s, management measures have been applied to counteract acidification and grass-encroachment (Van der Meulen et al. 1996). One of these measures was stimulation of aeolian activity by reactivation of former blowouts, since they became rare as a result of centuries of management practices directed at stabilization (Van Boxel et al. 1997). In 1991, Staatsbosbeheer reactivated 11 former blowouts in Eldorado, Terschelling. Blowouts expose and distribute fresh sand on the surface. This leads to changes in pH, electrical conductivity, nutrient content, soil organic matter and vegetation cover within these areas. This exposure of sand with low nutrient content allows for new start of both soil and vegetation succession (Witz, 2015). Since blowouts tend to not spread from their original location (Kooijman et al., 2005), it is possible to draw conclusions about the long term effect of reactivation of former blowouts, which is unique.

The outcomes of this research can contribute to the project executed by the PAS (Program Approach Nitrogen) of the Netherlands. This collaboration program has been active since 2015 to work on the balance between a vibrant economy and

healthy nature in the Netherlands. One of their guidelines to reduce the effects of nitrogen deposition is the reactivation of blowouts in the period of 2015-2021 along the Dutch coastline.

The aim of this research is determining the small-scale potential of the reactivation of blowouts on the soil in the Grey dunes of Eldorado, Terschelling. The main research question is as follows: What are the long-term effects of

reactivation of former blowouts on soil processes in the Grey dunes of Eldorado, Terschelling? The research will be divided into different sub-questions:

1. What is the long -term effect of reactivation of former blowouts on the soil development within the Grey dunes?

2. What is the long-term effect of reactivation of former blowouts on acidification in the Grey dunes?

3. What is the long-term effect of reactivation of former blowouts on the eutrophication in the Grey dunes?

These questions will be answered by conducting a comparative analysis between a reactivated and non-reactivated blowout. Soil chemical properties will be analysed from samples obtained in the dunes on Eldorado after analyzation in the lab and GIS. Moreover, these results will be compared to the results form a research comparing a naturally active and a stable blowout, which will lead to an insight in the long-term effect of the reactivation of blowouts along the Dutch coastline.

2. THEORETICAL FRAMEWORK

2.1 GREY DUNES

Grey dune habitats are a priority type in the Habitat directive of the European union. Some species are limited to these specific ecosystems and therefore need protection. The Grey dunes are also known as “fixed coastal dunes with herbaceous vegetation” (Provoost et al., 2004). The growth of vegetation leads to the stabilization of these dunes. Soil formation processes start with the accumulation of soil organic matter, which eventually leads to the formation of a distinct A-Horizon (Provoost et al., 2004). Mobilization and leaching processes create complex nutrient dynamics within these landscapes. Grey dunes can be found land inwards from the first dunes, which are the non-stabilized white dunes. Grey dunes are characterized by a micro-relief leading to high variability in environmental patterns on a relatively fine scale. This increases the biodiversity in these areas.

2.2 AEOLIAN DYNAMICS OF BLOWOUTS

Blowouts are formed by wind erosion, the most important transport again in coastal dunes, in a pre-existing sand deposit. Wind can transport sand in two ways: by suspension of particles smaller than 0.05 mm and by saltation of the larger particles (Jungerius, 2008). For the uptake of sand grain, a minimum wind speed of approximately 6

FIGURE 1 SCHEMATIC DIAGRAMS OF A SAUCER AND A THROUGH BLOWOUT WITH TYPICAL WIND FLOW PATTERNS INDICATED (HESP, 2002).

m/s is required (Van Boxel et al, 1997). Blowouts are the most common landscape feature in coastal dune areas. Wave erosion along the seaward face of the dune, topographic acceleration of airflow over the dune crest, climate change, vegetation variation in space or change through time, water erosion, high velocity wind erosion and human activities can all initiate the formation of a blowout (Hesp, 2002), since these activities remove the topsoil layer and expose the bare sand underneath. Their presence is most common in the fore dunes where aeolian processes are the most dominant. Blowouts also occur in Grey dunes in combination with water erosion. After water eroded the upper part of the slope, wind can subsequently erode the exposed sand beneath (Jungerius, 2008). Blowouts can form within a few days and can grow several meters in depth (Witz, 2015). Most blowouts can be divided into two types, a saucer- or through-shaped depression (Hesp, 2002), as can be seen in figure 1.

Common features of a blowout are the depositional lobe (1), the deflation basin (2) and the erosional wall (3). The depositional lobe is the area where the sand is deposited. It moves in upwind direction, as can be seen in figure 2. The deflation basin is the deepest point of the blowout, where the wind picks up the sand and transports it to the depositional lobe. The erosional walls are the side borders of the blowout, parallel to the wind direction.

2.3 NITROGEN DEPOSITION – EUTROPHICATION AND ACIDIFICATION

The Netherlands is among the countries with the highest nitrogen deposition levels worldwide (Steffen et al, 2015). The two main causes are the greenhouse gases emitted by the industrial sector

and the intensive manure production in the agricultural sector (Van Breemen & Van Dijk, 1988). In 1994, the deposition of nitrogen in the coastal dunes of the Netherlands was 27-30 kg N/hectare/year, while the critical load for nitrogen deposition in the dunes is 10 kg N/hectare/year (Kooijman et al., 2005). A high deposition of nitrogen does not necessarily lead to a decrease in pH. There are two buffer mechanisms in the Grey dunes that can stabilize the pH: The calcium carbonate buffer and the cation exchange buffer (Kooijman et al, 2005). Dunes contain calcium carbonate, which function as a buffer for acidic compounds. The maximum buffer capacity is determined by the amount of calcium carbonate in the soil but also the stage of decalcification of the soil. The calcium carbonate buffer only works at a pH of 8.3-6.2. At a pH lower than 6.2 all of the calcium carbonate will be dissolved and the cation exchange buffer starts to work, until the pH drops lower than 4.2. The Grey dunes in the Wadden district are lime poor, which means that the maximum calcium carbonate buffer is reached in an early stage. Due to the low calcium carbonate buffer capacity, the cation exchange buffer is the most important buffer in the Wadden district. However, the buffer capacity of cation exchange buffer is lower than the buffer capacity of the calcium carbonate buffer. Exceeding the critical load of nitrogen deposition in coastal dunes means exceeding the threshold values of the system, which leads to eutrophication and acidification of the soil. This takes the system to a new equilibrium, with a lot of grass-encroachment and acidified soils.

2.4 STABILIZATION AND DEVELOPMENT OF BLOWOUTS

FIGURE 2 FORMATION AND MIGRATION OF THE

DEPOSITIONAL LOBE (JUNGERIUS, 2008)

The specific behaviour of a blowout depends on the location in the landscape and local climate. They tend not to move from their original location (Kooijman et al., 2005). Due to the exposure of fresh sand they create the opportunity for both new succession in soil development and vegetation growth. This process increases the biodiversity and landscape diversity of the Grey dunes. In general, blowouts start to stabilize after one year of formation or reactivation, with only a few reaching to full maturity (Van Boxel et al., 1997). During the stabilizing process, a distinct A-Horizon will be formed due to the accumulation of soil organic matter in the soil. During the beginning stage of development, the pH will be between 7 and 8, which is most suitable for different plant species. Therefore, this stage is characterised by a high plant diversity. Marram grass (Ammophila arenia) is one of the most common pioneer species. As soon as the soil starts to become more acid over time by decalcification and acidification, the presence of mosses starts to decrease (Olsson et al., 2009) and the presence of grasses will increase. There are three sorts of communities in the Dutch Grey dunes on Eldorado, which are characterized by a specific vegetation (Van der Meulen et al., 1996). The grass communities and open communities are typical for the beginning stage of stabilization, with mosses, lichen and grass species in

one area. The

distribution of fresh sand is important for these communities. The heathland community with crowberry are typical for the last stage of stabilization, where the pH is lower and there is less fresh sand available.

3. FIELDWORK AREA

The fieldwork area is located on Terschelling, one of the Wadden Islands of the Netherlands located in the northwest of the country. These dunes on this island belong to the coastal Grey Dune area of the Netherlands. Eldorado, the region on the island where this research was performed, is about 80 hectares and is located at 500 metres inland from the first dunes. Between 1945 and 1980, blowouts started to stabilize due to strong stabilization policies executed by

FIGURE 3A. OPEN COMMUNITY WITH

CORYNEPHORUS CANESCENS

3B. HEATHLAND COMMUNITY

WITH EMPETRUM NIGRUM 3C. GRASS COMMUNITY WITH

AMMOPHILA ARENIA

FIGURE 4 THE LOCATION OF TERSCHELLING IN THE NETHERLANDS. SOURCE: GOOGLE EARTH

Staatsbosbeheer. Due to this process, about 20% of the area is covered with fossilised blowouts (Van der Meulen et al., 1996).

This coastal dune area is a lime poor area. 11 former blowouts were reactivated in this area in 1991 (Van der Meulen et al., 1996). The blowouts in Eldorado are approximately 500 m2 _{in size.}

Now, 25 years later, this research will focus on the differences in soil and vegetation characteristics of a reactivated and non-reactivated blowout, which are located next to each other in the north-western part of the region. These locations had to be approved by Staatsbosbeheer, because this area houses protected bird and animal species. Galloway cows and Norwegian fjord horses graze in the area to decrease grass encroachment in the dunes which maintains an open dune vegetation.

The general orientation of the blowouts is in north-western direction, which is related to the dominating north-western wind. A south-western wind is also present in the area, which is a less powerful wind than the north-western wind, since it arrives over land instead of directly from over sea.

These two winds together have formed the landscape of Eldorado. This can be seen in figure 5, where the location and direction of the two blowouts are visualized. The eastern reactivated blowout has a southwest orientation and the western non-reactivated blowout has a northwest orientation.

The Netherlands has a temperate maritime climate (Cfb, according to Köppen). The climate on Terschelling is mainly influenced by the North Sea, with cool summers and moderate winters. In West-Terschelling, the average annual temperature is 8.8°C. There is a significant amount of rainfall during the year of about 800 mm annually.

FIGURE 5 FIELWORK AREA OVERVIEW, INCLUDING THE TRANSECTS AND THE OUTER EDGES OF THE GRID

4. METHODS

The methods for this research are related to four different stages during the research period. The research started with two weeks of fieldwork, followed by an analysis of the obtained samples in the lab. These result were visualized in ArcMap 10.1 during the GIS-analysis and analysed in Matlab 2015a during the

last period of statistical analysis and writing. FIGURE 6 METHODOLOGICAL WORKFLOW

4.1 FIELDWORK

The fieldwork was performed in the Grey Dunes of Eldorado, Terschelling. The research site was selected by Annemieke Kooijman, based on information from the earlier reactivation project in 1991. Figure 7 shows the two blowouts chosen for this research.

Two perpendicular transects were drawn in complementary direction through the blowout and their surrounding area, which can also be seen in figure 7. The long transect RL and the short transect RS, 110 and 60 meters long respectively, belong to the reactivated blowout (R). The long transect NL and the short

transect NS, 64 and 32 meters respectively, belong to the non-reactivated blowout (N).

Sampling locations with an equal distance along these transects were determined with a 30-meter tape measure. The long transect RL contained 13 sampling points, NL contained 12 sampling points and the short transects both contained 7 sampling points. The location of these sampling points was established by the use of a GPS.

At every sampling location along the transects, different types of information and samples were collected. At first, the vegetation cover in a plot of 1x1 meter was estimated in percentages of bare sand, mosses, herbs, shrubs, helm grass and buckthorn. Subsequently, vegetation samples were collected on these locations along the two transects to gain more insight in the relationship between soil characteristics and vegetation cover. These vegetation samples were obtained by measuring a quadrant of 50x50 cm when there was a vegetation cover of less than 20% and a 25x25 cm quadrant when more than 20% of the surface was covered with vegetation. Within these quadrants, all of the above ground vegetation was removed with the use of scissors. Leftover soil was removed from the vegetation samples before they were put into plastic bags.

The locations where the vegetation cover was removed were used to obtain soil samples from the first 5 centimetres of the soil with the use of bulk density rings. This was done twice in the same location, which resulted in a soil sample of 200 cm2 _{per location. These samples were put into a plastic bag and stored at room}

temperature.

Close to the location where the vegetation sample was obtained, a soil profile was recorded on a field form after coring to a depth of 120 cm with the use of an auger. The most important data of these soil profiles was the thickness of the OM layer and the Ah layer.

In addition to the transact analysis, a grid point analysis was conducted. The outer edges of the grid were established at a distance of approximately 20 meters from the outer points of the transects to incorporate both blowouts in the grid, as can be seen in figure 8. At every grid point, the vegetation cover percentages and the thickness of the OM and Ah layers were collected. The outer edges of the grid points were located with a GPS. The grid points in

between were at a distance of 10 meters and were visualized with Google Earth, which resulted in 210 grid points.

FIGURE 7 FIELDWORK AREA OVERVIEW. BLOWOUT R

(REACTIVATED) IN THE EAST, BLOWOUT N (NON-REACTIAVED) IN THE WEST, SURROUNDED BY A POLYGON REPRESENTING THE BORDER OF THE GRID

4.2 LABWORK

This section describes the different types of analysis that were performed in the lab on the soil samples and the vegetation samples.

4.2.1 Soil samples

Before any of the experiments were conducted, all of the soil samples were weighed on a two decimal scale. After extracting a subsample for the calculation of the bulk density, all the leftover soil samples were put into the oven in their plastic bag for 48 hours on 70 ̊c.

- BULK DENSITY

After weighing the soil samples in their plastic bag, a subsample of 8-20 grams was taken from the sample and put into a crucible, which was also weighed. The samples were put into the oven for 24 hours on 105 ̊c. These samples were weighed again. With these results the bulk density was calculated, according to the following formula.

Bulk density=

Total wet sample x Dry subsample

Wet subsample x Volume

For the calculation of pH and EC, the dried material for the bulk density could be reused. All of the soil samples were first sieved with a 2 mm sieve to separate the roots and other large particles from the small particles. From this sieved material, approximately 10 grams of soil sample was mixed with 25 ml demineralized water. This ratio 1:2,5 is ideal to measure both indicators. This mixture was put into the shaker for two hours and had a resting period overnight. In the morning, the shaker was set for another 20 minutes. The measurements were conducted afterwards with an EC electrode and a pH electrode.

- C AND N CONTENT

The dried and sieved material could be used again to measure the C and N content of the soil. A teaspoon of soil sample was ground at rotational speed of 400 rpm for 5 minutes. These subsamples were put into the oven again on 70 ̊c for 24 hours. Subsequently, a fraction of 40-60 mg of the subsamples was put into tin foil, in duplicate, to prepare for the CHNS analyser, type Elementar Vario EL. This machines measures the C and N percentages within the soil sample. These percentages were multiplied with the bulk density to calculate the C and N content of the soil in grams per m2 _{to a depth of 5 cm.}

4.2.2 Vegetation samples - DRY WEIGHT

To determine the dry weight of the vegetation samples they were put into the oven on 70 ̊c for more than 72 hours. The vegetation samples were weighed and these numbers were extrapolated to calculate standing aboveground biomass per m2.

- C AND N CONTENT

For this experiment the dried vegetation was first ground at a rotational speed of 8.000 rpm. The soil samples were put into the oven again on 70 ̊c for 24 hours. After that, 10-15 mg of vegetation sample was prepared in duplicate in tin foil, to be put into the CHNS analyser, type Elementar Vario EL. This machines measures the C and N percentages within the soil sample. These percentages were multiplied with the dry weight to calculate the C and N content of the vegetation in grams per m2_.

4.3 GIS

The information found during transect and grid analysis in the field and in the lab could be used to create interpolation maps in ArcGIS. Most of the maps are based on the grid analysis, since this analysis contained more points than the transect analysis and can create accurate interpolation maps. The pH data from the transact analysis was correlated to the bare sand cover along the transects and the N content of the soil to the OM layer. This gave a high correlation, which made it possible to extrapolate the transect data to the grid point data and create and interpolation map. The following steps were taken to create the maps: 1. Found ground layer from Actueel Hoogtebestand Nederland

2. Processed transect and grid point data in Google Earth with coordinates from the field

3. Converted Google Earth KML files to layer files in ArcGIS

4. Used the attribute table in ArcGIS to join the grid and transect field data to the point data features.

5. Made interpolation maps of the following variables:

Vegetation

- Vegetation cover IDW 4

Soil

- OM layer IDW 4

- Ah layer IDW 4

- Ah layer 2 IDW 4

- pH map IDW 4

- N content map IDW 4

- Accumulation thickness IDW 12

The inverse distance weighting interpolation (IDW) method was the best interpolation method to use on this dataset, with number of points = 4 (except for the accumulation map, number of point = 12).

5. Clipped all of the maps to a polygon connecting the edges of the grid, created in Google Earth

4.4 STATISTICS

The results from the lab work analysis were statistically analysed using Matlab 2015a. Explorative analysis methods were used, such as boxplots, minimum and maximum values and the mean, to see how the data of the different variables was distributed. A two sampled t-test was used to see if the two means are significantly different from each other, according to a p-value of 0,05. This is important to draw a conclusion on the relationship between the different variables. Next to the explorative analysis, a correlation between different variables was calculated in order to extrapolate transect data to grid data. This extrapolation made it possible to make interpolation maps with transect data. Since the blowouts do not have the same amount of sampling points, point RL1 in the deflation zone was left out for every statistical test. In this way each blowout had a similar amount of data points in the deflation and accumulation zone, to obtain a clear overview of the differences in soil and vegetation characteristics.

5. RESULTS

Reactivated

Non-reactivated

T-test H =

P-value

OM [cm] 1,34 2,58 1 0,0919 Ah [cm] 2,45 3,00 0 0,2163 Bulk Density [g/cm3_{] 1,09 0,89 1 0,0055} EC [mS/cm] 80,87 105,87 0 0,1715 pH 5,04 4,51 1 0,0035 C % 1,27 2,65 1 0,0046 N % 0,08 0,14 1 0,0043 C/N ratio 15,35 17,92 1 0,0500 C content soil [gr/m2_{] 619,60 1.027,80 1 0,0062} N content soil [gr/m2_{] 38,73 55,74 1 0,0138}

This section contains all of the combined results from the fieldwork, lab work, GIS-analysis and the statistical analysis. The results will be displayed in three sections: vegetation cover, soil horizons and soil chemical processes. The following table 1 gives an overview of the mean values of all the soil

characteristics that were investigated during this research, with a distinction between the reactivated and the non-reactivated blowout. Their according two

sample T-test values are in the last two columns. These values will be used to explain the following maps and processes found in the field.

TABLE 1 MEAN VALUES OF ALL BLOWOUT VARIABLES, PLUS T-TEST RESULTS

5.1 VEGETATION COVER

In 1991, all of the vegetation within the reactivated blowouts was removed from the deflation area. The results of these changes are still present the fieldwork area, which led to some specific findings in the following map. Figure 9 shows the vegetation cover in percentages in the grid area, including the two blowouts in the middle. The polygons refer to the deflation basin of the blowouts. Most of the area is covered with at least 80% of vegetation, which is common for a grey dune landscape (Houston, 2008). A significant difference can be found within the area of the reactivated blowout, which generally shows a vegetation cover of less than 50%. The areas that show the lowest vegetation cover, and thus the highest cover with bare sand, refer to the erosional walls of the reactivated blowout that

were found in the field. This figure also shows that one of the erosional walls of the non-reactivated blowout is still active, due to the presence of rabbits.

5.2 SOIL HORIZONS

Due to the human intervention and disturbing activities, this research expected to find differences between the soil formation in a reactivated blowout and a non-reactivated blowout after 25 years. The following maps will give detailed descriptions about the two common soil horizons in the grey dunes: the OM layer and the Ah layer.

The OM layer is a surficial organic deposit that contains the organic matter, which is the party decomposed litter layer of the plant residues. The Ah layer refers to the layer underneath the OM layer, which is a mineral layer consisting of accumulated soil organic matter derived from the OM layer, including dead plant roots. Graph 1 shows the FIGURE 9 VEGETATION COVER IN

PERCENTAGES, INCLUDING THE DEFLATION ZONES OF BOTH BLOWOUTS

GRAPH 1 SOIL HORIZON THICKNESS [CM] OM [cm] Ah [cm] 0.00 1.00 2.00 3.00 4.00 1.34 2.45 2.58 3.00 Reactivated Non-reactivated

thickness [cm] of both soil horizons and the difference in thickness of the layers between the two blowouts. Noticeable is that both layers are thicker in the non-reactivated blowout, due to a more stable environment.

Sand in the Ah layer is heavier than the organic material in the OM layer. Therefore, as table 1 shows, the bulk density [g/cm3] is higher in the reactivated blowout where there is more bare sand present than in the non-reactivated blowout.

 5.2.1 Distribution of OM and Ah Layers

The distribution of the thickness of the soil horizons can give indications on the stability of the blowouts and the soil formation processes (figures 11 & 12, with transparent polygons referring to the blowouts). Thicker soils often refer to more stable environments where soil formation can follow a natural course. A thicker OM layer can therefore be found in and around the non-reactivated blowout (figure 11), with a significantly thicker layer of > 7 cm in the area behind the erosional wall.

The Ah layer on figure 12 shows the same pattern, with a thicker Ah layer in and around the non-reactivated blowout. There is a smaller difference between the mean thickness of the Ah layers of both blowouts than between the OM layer (graph 1), which is due to the fact that in the reactivated blowout the OM layers are often absent where there is no vegetation. The soil consists of only an Ah layer in these locations, as can be seen in figure 10. This is also the explanation for the t-test results, which concluded that the data from the two blowouts is not significantly different. The soil with an Ah layer of less than 2 cm thick can only

be found in the reactivated area, where soil development could have only occurred in the past 25 years.

 5.2.2 Accumulation map

After the reactivation of the blowouts in 1991, the aeolian activity within the blowouts FIGURE 10 SOIL PROFILE WITH AND WITHOUT OM LAYER

FIGURE 11 DISTRIBUTION OF THE THICKNESS OF THE OM FIGURE 12 DISTRIBUTION OF THE THICKNESS OF THE AH

FIGURE 13 DOUBLE SOIL PROFILE [120 CM]

came into force again. Sand was transported by the northwestern and southwestern winds to the accumulation zone of the blowout, where it created a double soil profile (figure 13). Underneath the OM and Ah layer, the C layer with weathered parent rock (sand) can be found. In the accumulation zone another OM and/or Ah layer were found underneath this C layer. The following map shows the thickness [cm] of the new soil profile that was created on top of the old soil profile, starting with the redistribution of sand due to aeolian activity. After stabilization, a new Ah and OM layer could be formed on top of the weathered parent material. The average thickness of the sand accumulation is 4,15 cm. The polygon represents the accumulation zone, based on google earth imagery and field observations. Most of the double soils are present in the accumulation zone of the blowout, which is logical due to the wind direction in the area. The thickest accumulation layer (> 7 cm) was found in northwestern direction, in the center of the accumulation zone (figure 14). Other double soils were found in the southern and eastern part of the grid, where other reactivated blowouts were present.

FIGURE 14 ACCUMULATION LAYER [CM] OF NEW PARENT MATERIAL AFTER REACTIVATION

5.3

SOIL CHEMICAL PROCESSES

 5.3.1 Acidification - pH

The pH was measured according to the soil samples obtained along the transects. However, to create an interpolation map for the pH, a high correlation must be found between transect information and grid information. A high correlation was found between the pH and the percentage of bare sand, with an R value of 0,7006. The pH for the grid analysis

could be calculated with the use of the

formula in graph 2. After interpolating these pH values, figure 15 could be made, which shows the distribution of the values of the pH in the grid area. Although this map looks a lot like the vegetation cover map, important conclusions can be drawn. At first, there is a significant difference in mean values between the blowouts, as can be seen in table 1. From the map it is clear that the highest pH’s in the field (> 5,8 pH) are present in the deflation basin of the reactivated blowout. Secondly, there are two centres where the pH is highest, which refer to the deflation lobes within the reactivated blowout. These locations contain a 100% coverage of bare sand and aeolian activity is still present. The erosional wall in the non-reactivated blowout also shows a different pH where bare sand was present in the field, due to the presence of rabbits.

GRAPH 2 CORRELATION BETWEEN PH AND BARE SAND

0 10 20 30 40 50 60 70 80 3 3.5 4 4.5 5 5.5 6 6.5 f(x) = 0.03 x + 4.63 R² = 0.49

pH

 5.3.2 Eutrophication – Carbon and Nitrogen content

The carbon and nitrogen content of the soil are important variables to indicate the eutrophication in the dunes. The input of carbon and nitrogen in the soil strongly depends on the vegetation in the area. The decay process starts with the input of plant litter and leads to the formation of soil organic matter in the OM layer. In general, organic material derived from plants contains 50% of carbon and 1% of nitrogen (Overstreet & DeJong-Huges, 2009). As seen before, the vegetation cover in the reactivated blowout is lower than in the non-reactivated blowout. This led to the following results in graph 3, which shows that both the C and N content

are significantly lower in the

reactivated blowout, due to a lower input of organic material via plant litter. Therefore, a high correlation of 0,56 was found between the OM layer and the N content in the soil, and the following interpolation map could be produced. Figure 16 shows the distribution of the presence of N content in the soil. The locations with less nitrogen in the soil correspond to the locations where bare sand was found in the field, which follows the theory of the formation of soil organic matter with FIGURE 15 INTERPOLATION PH MAP SHOWING THE DEFLATION

ZONES OF BOTH BLOWOUTS

GRAPH 3 CARBON AND NITROGEN CONTENT IN THE SOIL

FIGURE 16 DISTRIBUTION OF N CONTENT IN THE SOIL, GR/M2 _{TILL A DEPTH OF 5 CM}

C content soil [gr/m2] N content soil [gr/m2] 0.00 200.00 400.00 600.00 800.00 1,000.00 1,200.00 619.60 38.73 1027.80 55.74 Reactivated Non-reactivated

the input of plant litter. The soil in the reactivated blowout generally contains half the amount (30-40 gr/m2_{) of N content in the soil as the non-reactivated blowout}

(60-70 gr/m2_).

 5.3.3 C/N ratio

The C/N ratio refers to the amount of carbon relative to the amount of nitrogen present in the soil. There is always more carbon present in the soil than there is nitrogen, as the previous section confirmed. However, the C/N ratio is not only influenced by these minerals, but also by micro-organisms such as algae and bacteria in the soil. These contain a much lower C/N ratio, of about 10:1, while plant litter mostly contains a C/N ratio of 25:1. Younger soils, such as in the reactivated blowout, receive less carbon and nitrogen input from vegetation, since there is less vegetation present in these areas (figure 9). In this case the presence of micro-organisms is more

relevant for the C/N ratio within the soil. This is also what the C/N ratio shows in table 2. The ratio is lower in the reactivated blowout, due to the lower amount of C content in the soil.

TABLE 2 MEAN VALUES OF THE C/N RATIO

Reactivat

ed

Non-

Reactivated

6. DISCUSSION

6.1 THE INFLUENCE OF BARE SAND

The reactivation of the blowouts was performed to increase the presence of bare sand in the area. It is lime-rich and nutrient-poor sand from deeper layers. Van der Meulen et al. (1996) stated that the most important characteristics of bare sand biotopes are the access to the soil, the quick heating of the soil by solar radiation and the minimum physical resistance of the environment while propelling. This creates a suitable habitat for lots of fauna and flora. Specific soil characteristics are also important for these habitats, such as the pH and EC. Decomposition and mineralization rates are slowed down in acid soils (pH < 4.5 to 5), which results in an increasing humus content (Aggenback & Jalink, 1999). A higher pH in the reactivated blowout means less acidification of the soil, which is beneficial for the living conditions of certain plant species (Kooijman et al., 2005) as will become clear in section 6.2. Not only the difference in acidity between the two blowouts was apparent, but also the difference between the acidity in the deflation and the accumulation zones. The following graphs in figure 17 show a cross section of all the transects points containing the pH values and bare sand percentages on these locations. A significant difference was found between the deflation area (middle area) and the accumulation zone (table 3), which is due to a higher lime content of the soil by the presence of sand in the deflation area (Kooijman et al., 2005). After 25 years, the pH in the deflation zone is still higher than the pH in the accumulation zone of the non-reactivated blowout, which points out that the reactivation still has consequences on soil properties in the area. Figure 17 shows that the highest pH values were found at the locations with the highest bare sand percentages.

TABLE 3 MEAN VALUES OF THE DEFLATION AND ACCUMULATION ZONES

This higher lime content due to the presence of bare sand accounts as a buffer for minerals such as

nitrogen and carbon that enter the soil, which also reduces the eutrophication in the dunes. Not only has bare sand an effect on eutrophication, but also on the electrical conductivity of the soil. The locations where bare sand was

FIGURE 17 CROSS SECTION OF THE LONG AND SHORT TRANSECTS AND THEIR ACCORDING PH VALUES AND BARE SAND COVERAGE

Reactivat

ed Non-Reactivated

Deflation 5,44 4,54

Accumula

found showed a significantly lower EC, due to the absence of an organic matter layer. This is also confirmed by the high correlation between the OM layer and EC, with a value of 0.58 (p < 0.05).

6.2 THE RELATIONSHIP BETWEEN SOIL AND VEGETATION

The previous paragraph showed the influence of bare sand on different soil chemical properties and processes, such as the reduction of acidification and eutrophication of the soil. The importance of these different soil chemical properties and processes becomes more clear while looking at their influence on the vegetation in the area. Figure 18 shows that vegetation succession starts with the growth of grasses in areas with bare sand. With an increasing input of soil organic matter (in relation with a decreasing pH and increasing N content), vegetation such as mosses and lichens starts to propel in the area, with as a last stage of succession the dwarf shrubs vegetation. This process of vegetation succession can also be detected in the reactivated blowout on Eldorado, in

relation to the pH changes and C and N content of the soil. This research was conducted by van den Berg (2016) during the same period. The general relation between the soil and vegetation in the research area will be discussed.

Four different vegetation categories were identified in the field along the transects: 1. pioneer vegetation (Cerastium semidecandrum), 2. grasses and herbs (Luzula campestris, Hypochaeris radicata, Holcus lanatus), 3. stabilized

shrubs (Empetrum nigrum) and 4. moss vegetation (Brachythecium rutabulum),

in order of succession stage. The most common vegetation category, found at each transect location, can be seen in figure 19. Bare sand has a low pH which creates suitable habitat conditions for pioneer vegetation (Kooijman et al., 2005). Therefore, a process of rejuvenation of the vegetation starts in the reactivated blowout. In the accumulation zones of the blowout, pioneer vegetation established. At locations with a relatively low input of sand, little pioneer species will occur, such as the little mouse-ear. At locations with a relatively high input of sand, helm grass will grow (Kooijman et al., 2005). These pioneer species are the starting point of a new vegetation succession. The transect location characterized with vegetation type 1 and 2 in figure 19 correspond to the locations where bare sand was found (see vegetation map figure 9). Pioneer vegetation was even found on the erosional wall of the non-reactivated blowout, due to the presence of rabbits. FIGURE 18 VEGETATION SUCCESSION IN INLAND DUNES DERIVED FROM HASSE (2005) BY SPARRIUS (2012)

FIGURE 19 VEGETATION CATEGORY DISTRIBUTION ALONG TRANSECTS IN REACTIVATED BLOWOUT (RIGHT) AND NON-REACTIVATED BLOWOUT (LEFT)

Vegetation type 3 was most common in the non-reactivated blowout, which was covered in crowberry. The key element for soil development is the production of biomass, which can already happen in early moss dune stages (Provoost et al., 2004). Therefore, the most developed soils were found on locations where vegetation type 3 and 4 were present. A high correlation (0,45, p-value < 0,05) between the OM layer and the vegetation cover confirms this process. This strong correlation between biomass and soil minerals also means that grass encroachment can strongly react to mitigation polities (Kooijman et al., 2005). Vegetation type 4 was only found in the valley within the reactivated blowout. Due to less disturbance by aeolian activity, a higher water content and a more stable soil formation, a later category of vegetation succession was found in these locations. In general, reactivation of the blowout created a mosaic of different ecosystems in the area with different pH’s and soil mineral contents, which gives a suitable habitat for all different kinds of flora in the dunes.

6.3 COMPARISON WITH AN ACTIVE BLOWOUT

All of the previous sections discussed the differences between a reactivated blowout and a non-reactivated blowout. However, looking at the differences between a reactivated blowout and a naturally active blowout can tell us even more about the effectiveness of the reactivation measures taken in 1991 (Van der Meulen et al., 1996), in terms of counteracting acidification and eutrophication. Martens (2016) and Boersma (2016) conducted the research of the comparisons between a naturally active blowout and a stabilized blowout in the same area.

6.3.1 Acidification

Acidification can be reduced by the presence of lime in the soil, which increases with the distribution of bare sand. The presence of bare sand in relation to the pH has already been discussed in section 6.1. The dunes in the Wadden district have a primary lime content of 0.5-2% (Kooijman et al., 2005), which is lower than other dune districts in the Netherlands. Stabilized dune soils decalcify due to continuous chalk (CaCO3) leaching, starting from the upper soil layers (Provoost et al., 2004).

The lime content in the soil is too low to maintain lime rich soils for a longer period (Kooijman et al., 2005). After CaCO3 disappears from the soil, the pH

shows an abrupt and steep decline which is not in line with the gradual removal of lime content from the soil (Provoost et al., 2004). Before comparing the results from an active blowout and reactivated blowout, it was therefore expected to find lower pH values in the active blowout, since leaching processes have had more time to remove the chalk from the soil. Graph 4 shows the differences in average values of pH of all four investigated blowouts. This shows that the reactivation, even after 25 years, is still visible when looking at the soil characteristics. 5.04 4.51 5.29 5.12

pH

The mean values of the naturally active and stabilized blowout were not significantly different, in contrast to the differences between the blowouts investigated in this research. From this part, the conclusion can be drawn that reactivation has a significant positive long term effect on the increase in pH values. On the contrary, there is no clear evidence that a naturally active blowout seems to counteract the acidification process.

The difference between the pH’s in both areas can be explained by the types of vegetation present in the field. The

presence of crowberry in this research area produces large amounts of plant

litter and therefore causes a significant decrease in pH. This type of vegetation is less present in the research area of Boersma (2016) and Martens (2016), which results in a higher pH. The difference between the pH of both active blowouts is not in line with the theory about decalcification (Provoost et al., 2004). This difference could be explained by the disturbance of animals, which will be discussed in section 6.4.

6.3.2 Eutrophication

The process of eutrophication is a results of the amount of C and N content in the soil. As graph 5 shows, the amounts of C content are very much in line with the previously discussed relationship between vegetation succession and soil mineral content. The lowest values are found in the reactivated blowout, followed by the naturally active blowout and the highest values in the stable blowouts. The mineralization rate is a result of the vegetation succession in the area, which increases the availability of nitrogen for plant growth.

On the other hand, the N content of the soil within the blowouts shows a different pattern. The N content of the blowouts found in the area investigated by Marleen and Pauline were much higher. This can be explained by looking at the C/N ratio’s in table 4.

TABLE 4 MEAN C/N RATIO’S OF THE DIFFERENT BLOWOUTS

Reactivated

Non-Reactivated Active Stable

GRAPH 4 AVERAGE PH VALUES OF ALL FOUR BLOWOUTS

GRAPH 5 MEAN VALUES OF THE CARBON AN NITROGEN CONTENT OF ALL FOUR BLOWOUTS IN GR/M2

C content soil [gr/m2] N content soil [gr/m2] 0.00 200.00 400.00 600.00 800.00 1,000.00 1,200.00 619.60 38.73 1027.80 55.74 775.30 62.59 1083.60 89.94

C and N content

C/N ratio 15,35 17,92 12,4 12,04

The presence of crowberry caused a higher C/N ratio in the reactivated blowout and non-reactivated blowout, due to the fact that crowberry plant litter has a low N content and is hard to decompose. The lower C/N ratio in the naturally active and stable blowout are much lower due to the fact that plant litter in these blowout contains a higher N content. In general, the conclusion can still be drawn that more active blowouts, with a higher bare sand percentage, contain lower C and N contents in their soil.

6.4 THE INFLUENCE OF GRAZERS

Helm grass is one of the first vegetation types that reoccurred two years after reactivation, at locations where the helm vegetation used to grow in the accumulation zones (Van der Meulen, 1996). Due to the characteristics of the plant, it has been able to be the main cause of grass-encroachment before and after the reactivation, but mainly due to their strong reduction of the availability of daylight at the ground floor (Kooijman & Van der Meulen, 1996). After 25 years, it seems that there is less grass encroachment in the area, which is related to the presence of horses and cows in the area over the five past years. Nonetheless, grazing can maintain vegetation structure, but it does not stop the succession of vegetation (Provoost et al., 2004). Kooijman & de Haan (1995) showed that the presence of grazers in tall grass communities can result in a larger number of species and a significantly lower height of the vegetation, mainly due to the increased availability of light for other vegetation. This results in the succession of the heathland communities in the area (Kooijman et al., 2005). Other research by Kooijman et al. (1996) concluded that grazing is a reasonably effective measure against grass-encroachment in terms of vegetation structure, whereas in this case it could have led to a decrease in tall grass abundance (Ammophila arenia). As a result of a more open vegetation, bare sand could have been exposed to the surface again. Trampling of these animals also contributes to the increase of open sand. These processed might have caused an increase in pH levels in the research area of Marleen and Pauline, since the grazers were often present in that area. On the contrary, cows were less present in the area of this research since the most abundant vegetation type in the stabilized areas was crowberry, which cows do not like to eat. Further research in the area should be conducted to understand the direct influence of grazers on flora and fauna in Eldorado, Terschelling.

7. CONCLUSION

The two investigated blowouts in Eldorado, Terschelling showed some remarkable differences 25 years after the reactivation process. Bare sand appeared to be one of the cause of the differences between the reactivated and non-reactivated blowout, due to its buffer capacity function. The presence of bare sand led to a higher pH, an important soil chemical property for vegetation succession, which allowed pioneer species to colonize the area. The redistribution of sand also led to a decline of carbon and nitrogen content in the

soil, in combination with the diminished input of organic matter by the removal of vegetation. Organic matter input stimulated soil formations, as was showed with a high correlation between vegetation cover and the thickness of the OM layer. Therefore, OM layers were significantly less present in the reactivated blowout. The deflation area of the blowouts is mostly affected by the reactivation. Stabilization processes have already occurred in the accumulation zones of the blowout, where younger soil profiles were found. After comparing these results to the results from a comparative analysis between a naturally active and stabilized blowout, the following general conclusion can be drawn: Eutrophication and acidification of the soil are both slowed down or even counteracted by reactivation in this area. Naturally active blowouts can also slow down or counteract eutrophication processes, but there is no real evidence which shows a significant influence on acidification processes in these blowouts. Aeolian activity is mainly responsible for creating a landscape mosaic, leading to new vegetation succession and a suitable habitat for multiple vegetation types. Grazing in the area seemed to be having a positive influence on the reduction of grass-encroachment by marram grass, which resulted in a heathland vegetation succession and an increase in biodiversity. In general, reactivation as a management measure in the Wadden district has a long term positive effect on soil chemical properties, leading to less acidification, less eutrophication and a higher species diversity.

ACKNOWLEDGEMENT

A research of this scope could not have been done without my fellow students Mara van den Berg, Pauline Martens and Marleen Boersma. A special thanks goes to the supervisor of this project, Annemieke Kooijman, for helping and supporting us in during the fieldwork and while writing this bachelor thesis. I would also like to thank Chiara Cerli, Jorien Schoorl and John de Vos for the support in the laboratory at the University of Amsterdam. This fieldwork was made possible due to the approval of Staatsbosbeheer on Terschelling. Furthermore, I would like to thank John van Boxel for additional information and feedback as my second supervisor.

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