CONCLUSIONES, RECOMENDACIONES Y ANEXOS

Marinka van Puijenbroek obtained her PhD at the Plant Ecology and Nature Conservation group at Wageningen University in 2017. She currently works as a researcher on dunes and saltmarsh vegetation at Wageningen Marine Research.

Dissertation title:

"Dunes, above and beyond: The interaction between ecological and geomorphological processes during early dune development." PhD supervisors:

Frank Berendse (Wageningen University) Juul Limpens (Wageningen University)

The development of new dunes begins with the establishment of vegetation on the beach. The vegetation traps the sand, preventing it from blowing away, which results in a small embryo dune. Plant species that build dunes are specialized in catching the wind-blown sand and can grow with the trapped sand. In the Netherlands, there are two main dune-building grass species,

marram grass (Ammophila arenaria) and

sand couch grass (Elytrigia juncea) (Figure 1).

Over time, when the vegetation grows and catches more sand, the embryo dune will transform into an established foredune. The process of a bare beach developing into a foredune depends on the establishment of plants, the growth of the dune, and the dunes surviving storms.

The establishment of plant species on the beach depends on various environmental factors. The beach can be a harsh

environment with high soil salinity, low soil moisture, salt spray, sand burial and erosion. Soil salinity and salt spray are higher closer to the sea, whereas sand burial is higher close to the dunes. These abiotic factors do, however, depend on the beach morphology: on higher and steeper beaches, the soil salinity, salt spray, and soil moisture will be lower than on lower and gradual beaches. The Sand Motor has a distinctive morphology, since the beach is very wide (800 m) and several meters above sea level (4–6 m above mean sea level).

From studies of natural beaches, we know that more embryo dunes can develop on wider beaches. Large embryo dune complexes can develop on beaches wider than 300 m; this indicates the high potential for embryo dune development on the Sand Motor. The high elevation of the Sand Motor could, however, affect abiotic conditions, Figure 1.

The two main dune- building grasses in the Netherlands.

Figure 1a. Marram grass, Ammophila arenaria.

Figure 1b. Sand coach, Elytrigia juncea.

Figure 2a. Overview of the Sand Motor. The presence of dune- building species on the Sand Motor and the location of the five transect in which we monitored the vegetation more extensively.

Figure 2b.

Aerial photograph of the Sand Motor, the points indicate the plots in our field experiment, the number indicate the locations number. 0 0,25 0.5 1 Kilometers E le v a ti o n ( m N A P ) 15 8 -1 2015 2016 a b

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locations from sea to dune. The locations were selected to represent the entire Sand Motor (Figure 2b). We established four plots of 50 x 50 cm at each of these locations, and planted 20 plants of either marram grass or sand couch grass in each. Over time, we measured the number of plants at each plot. The same experiment was conducted at a natural coast at the Hors, Texel to compare plant growth.

In the first summer, the growth of planted dune-building grasses was initially lower on the Sand Motor than on the natural coast of Texel (Figure 4), probably due to the lower soil moisture on the beach. This would make the dune-building grasses more dependent on the precipitation here than on a natural coast. In winter, average plant mortality was lower on the Sand Motor than on the natural coast, eventually resulting in higher vegetation growth on the Sand Motor. This clearly indicates the positive effect of the high elevation, where there is no storm erosion, the most limiting factor for dune development. The only field experiment locations which showed substantial plant mortality were those closest to the sea . Here, vegetation disappeared due to the structural erosion of the Sand Motor by the sea. Due to the rapid growth of the dune- building species in our field experiment, the planted dune-building species created quite impressive dunes at the Sand Motor within three and a half years (Figure 5).

Overall, the Sand Motor has a huge potential for dune development. The wide beach of the Sand Motor gives a lot of space for embryo dune development, and its high elevation protect dune-building species against storms. This potential is already visible at the Sand Motor, where new embryo dunes develop and become bigger every year.

The distribution of the dune building grasses does not seem affected by the elevation; the absence of vegetation on some parts of the Sand Motor could be because the dune-building grasses cannot reach these areas. The dune-building species on the Sand Motor mainly occurred close to the foredune, where these species were already present. In fact, there is hardly any vegetation on the seaward ridge of the Sand Motor, which is much further away from any vegetation. Dune building species mainly disperse via rhizomes (root parts) and seeds. Rhizomes are mainly dispersed after a large storm, when the rhizome is broken off from the maternal plant. The rhizome can then establish on the location where it washes up, thus initiating the development of a new dune. Storms can therefore have a positive effect on the spread of dune- building species.

High waters as a result of storms do not reach most parts of the Sand Motor, which means that plant material of dune-building species cannot be deposited there. This could explain the absence of vegetation on higher elevations like the most seaward ridge. The seeds of marram grass and sand couch grass are relatively small, and it is unlikely that they could reach the whole of the Sand Motor by wind dispersal. In addition, the seeds might be trapped by the lake in the middle of the Sand Motor, which might reduce the availability of seeds. However, these seeds could drift to the edge of the lake and find suitable conditions due to the higher soil moisture there. This could explain the abundance of dunes in the area around the lake. The seaward ridge has been eroding since 2014, which could have a negative effect on the germination of seeds; seeds might be blown away from the seaward ridge, or seedlings might be unable to survive due to sand erosion.

Since the distribution of the dune-building grasses could be due to dispersal, we conducted a field transplant experiment, thus excluding the effect of dispersal and allowing us to compare the Sand Motor with a natural coast. For the field transplant experiment, we planted marram grass and sand couch grass in five transects at four Figure 3. (top left)

Frequency distribution of the elevation (m above sea level) for both the whole of the monitored transects and for points were natural vegetation actually established on the Sand Motor. Vegetation mainly occurred at higher elevations at the Sand Motor.

Figure 5. (bottom)

Images of vegetation growth over 2.5 years. A photograph of one of our field experiment locations in 2014 four months after planting, and a photograph 2.5 years later. This shows how fast embryo dunes can develop on the Sand Motor.

Figure 4. (top right)

Growth expressed as the number of plants per plot of dune building grasses Marram grass (left) and Sand couch grass (right) planted at different distances (locations) from the sea on a natural coast (Texel) and on the engineered coast at the Sand Motor. Location I is closest to the sea and location IV furthest from the sea. Although plant growth was initially lower on the Sand Motor, the high survival of vegetation during winter by the absence of storms resulted in an overall higher plant growth.

Elevation (m NAP) F re q u e n c y d is tr ib u ti o n Area

Sand Motor transects Vegetation points N u m b e r o f P la n ts

Marram grass Sand couch grass

Te x e l S a n d M o to r 2014 2017

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Small developing dune covered with beach grass

Trapping of wind-blown sand by beach grass

Denser and taller beach grass traps more sand

More vigorous growth of beach grass due to sand trapping

(burial)

Dune building