The effects of dune dynamics on the
establishment of A. arenaria
Thomas Meerwijk
A thesis for the degree Bachelor of Science at the University of Amsterdam, UvA
Administered by the Netherlands department for sea research (NIOZ) department of coastal systems
Supervised by Carlijn Lammers
Abstract
Dune systems are coastal systems which are largely structured by dune building plants, with many abiotic stressors like nutrient deficiency, drought and sand dynamics limiting the establishment of coastal vegetation. Dune systems are highly dynamic with changing stressors over a dune gradient, changing the way vegetation establishes. This study highlights how the establishment success of Ammophila arenaria changes over a dune gradient and how this is affected by germination source, either seeds or rhizomes. We conducted a field experiment where we divided the dune gradient into three zones, the foredune zone, embryonic dune zone and the zone were A. arenaria grows closest to the shore and compared the effect of nutrient deficiency and germination source on establishment success in each of these zones. We also conducted two surveys one in which we measured the natural occurrence of seeds and rhizomes to correct for their natural occurrence in the field experiment. Secondly we looked at the natural establishment from either seeds or rhizomes and compared this with the results from the first survey. In our field experiment we had shoot emergence in a minimal number of plots, with only up to three shoots per plot, and no shoot emergence in the embryonic dune zone. This was likely due to very low amounts of precipitation during the experiment. The surveys resulted in seeds and rhizomes concentrating in marine dispersed heaps of rhizomes. Newly established plants also concentrated in these heaps of rhizomes with clonal expansion increasing in dominance further from the shore. These rhizome heaps are important for facilitating the establishment of new plants and maintaining genetic diversity.
Introduction
Coastal ecosystems play an important role in protecting coastal communities worldwide by capturing sediment and reducing erosion (Spalding et al., 2014). The role of these ecosystems is getting more important as coastal regions are under increased risk of rising sea water levels and increased wave activity due to climate change (Masselink et al., 2016). However, human interference has caused coastal ecosystems to decline worldwide (Silliman et al., 2015). The restoration of these ecosystem for coastal protection has been getting increasing attention (Narayan et al., 2016). It is therefore important to get a better understanding in how these ecosystem function.
One of these coastal ecosystems which can play a key role in coastal protection are dune systems (Reijers et al., 2019). Dune systems are biogeomorphic landscapes which are a result of an interaction between physical and biological processes i.e. accumulation of wind-blown sand onto clumps of vegetation (Durán & Moore, 2013). The formation of a dune system begins with halophytic pioneer-species establishing themselves in non-vegetated areas. When vegetation settles, sand can accumulate around the vegetation and form incipient foredunes (Maun, 2009). Once these incipient foredunes are formed, the sand accumulation results in a positive selection mechanism for burial-tolerant vegetation (Durán & Moore, 2013). After these burial-tolerant plants have settled the dune stabilizes and grows in height and expands horizontally to form an established foredune ridge which creates the first form of coastal protection (Maun, 2009).
Before these feedbacks loops are formed, the early pioneer species are not protected from physical stressors and are therefore under a lot of stress from environmental conditions. Maun (1994) identified that nutrient deficiency, lack of moisture, soil salinity, sand burial and sand erosion are important stressors affecting vegetation in biogeomorphic landscapes like dune systems. These stressors can affect plant growth and therefore dune formation.
The vegetation is especially more susceptible to these physical stressors during the seed germination and seedling growth stages (Fay & Schultz, 2009). But if these stressors are below a certain threshold for long enough the seeds will be able to germinate and the plant can establish itself in the environment, subsequently feedback loops can be formed, and vegetation growth can stabilize. The time period in which these stressors are below the germination threshold is called the plants’ “Window of Opportunity” (WoO)(Balke, Herman, & Bouma, 2014). The dune area is highly dynamic with changing stressors over a small distance, changing the frequencies WoO (Gormally & Donovan, 2010). The general trend in coastal dune areas is that nutrient availability, soil salinity, sand transport and sand deposition decrease with distance from the shoreline (Gormally & Donovan, 2010) (Reijers et al., 2019). This difference in environmental factors can produce a change in germination success of dune vegetation locally. Nonetheless research on plant dynamics over a gradient usually focusses on grown plants and the effect on early establishment has not been fully understood.
In western Europe the two primary dune building species are Elytrigia juncea and Ammophila arenaria. E. juncea is the early, halophytic pioneer species and A. arenaria is the secondary burial tolerant species. A. arenaria is widely used in coastal protection programs in western Europe because of its great dune building capabilities (Puijenbroek et al., 2017). In this study we will focus on how A. arenaria can establish itself over a dune gradient. A. arenaria has three different ways in which it can be established in a dune area. It can disperse locally by clonally expanding itself with the use of rhizomes or it can reproduce sexually and disperse over a larger distance through the use of wind dispersed seeds. The third way of dispersal is when a heavy storm reaches the dune system, breaking the rhizome structures and washing them away. This can be either loose pieces of rhizome or entire heaps of entangled rhizomes. The rhizomes can then be transported by fluvial and coastal processes and wash ashore in a new area where the plant can establish itself (Hilton & Konlechner, 2011). The consensus is that the clonal reproduction
through rhizomes is the primary way of expanding and that the dispersal through seeds is rare (Mclachlan, 2014) (Huiskes, 1977) (Pope, 2006). But the effect of marine dispersed rhizomes has not yet been studied thoroughly.
There are some mechanistic explanations as of why the growth from rhizomes is so dominant over the germination from seeds. Van Der Putten (1990) found that plants which grew from loose pieces of rhizome had a better recovery of nutrients, this manifested itself in an overall higher aboveground biomass production in plants which grew from rhizomes than seedlings. Another explanation could be that rhizomes are less affected by burial, when seeds are buried more than 1cm their germination success reduces with 50 percent (Huiskes, 1977) whereas shoots still emerged from rhizomes when they were buried up to 40 cm (Konlechner, Hilton, & Orlovich, 2013). Erosion can cause exposure of the root system and total dislodgement of the seedling or rhizome, this is detrimental for the plants establishment (Balke et al., 2014) (Maun, 1994). However, rhizome structures or heaps of rhizomes can stabilize the sand which reduces the effects of erosion and enhance conditions for shoot development increasing the establishment success (Hilton & Konlechner, 2011).
This study consists of a field study which investigated the change in establishment success of A. arenaria over a dune gradient and if the different modes of establishment changed the establishment success. Because the germination source has an effect on the efficiency of nutrient recovery, treatments with and without fertilizer were compared. The field experiment was then compared with the natural establishment of A. arenaria from either seeds or rhizomes in different levels over the dune gradient. However, before the experimental establishment and the natural establishment can be compared, the natural occurrence of seeds and rhizomes in the area have to be determined. Therefore, a preliminary survey was performed filtering the sand and estimating the amount of seeds and rhizomes. When the amount of seeds and rhizomes were established, the
Methods
Study site
The study was performed in the foredune area on the north-western part of the island of Texel (53.16°N, 4.82°E). The experiment was performed a few hundred metres from the entrance to the beach to reduce
anthropogenic interference. The area had a clear distinction between the foredune area and embryonic dune area. The foredune area mainly consisted of big patches of A. arenaria with non-vegetated areas in between. In the embryonic dune area there grew a mix of less dense patches of A. arenaria and E. juncea. Away from the embryonic dune area and closer to the shore there were freestanding shoots of mainly E. juncea and some A. arenaria.
Field experiment
To determine the establishment success of A. arenaria over a dune gradient, we divided the site into three different zones which represent three vastly different areas where A. arenaria can establish, with different stressors affecting the plant (Fig 1). The first zone was on the border where A. arenaria could grow closest to the sea (FA, First Ammophila). The second zone was in the embryonic dune area (ED), as previously described. The last zone was in the foredune area (FD). In each of these zones we tested if germination source and nutrient deficiency had an impact on establishment success. We did this by either burying 600 grams of rhizomes or sowing 100 seeds in plots (1x1m) and crossing this with a fertilizer treatment. The rhizomes were extracted from the dunes on site at the end of April. The rhizomes were kept in plastic bags and planted the day after. The seeds were obtained fromJelitto Perennial seeds on 06-04-2020. Each plot was treated with either
no fertilizer or24 grams of conventional fertilizer (Pokon bio gazonmest). Before the treatments were applied, the top layer of the plots was raked in order to filter out any residual rhizomes. Excess plant growth 20 cm from the plot was removed and underground rhizomes were severed. In total there were 4 combination of treatments, seeds – fertilizer, rhizomes – fertilizer, seeds - no fertilizer and rhizomes - no fertilizer. Each of these
combinations were replicated 8 times per zone. The plots were divided using a random block design and placed in a single row per zone perpendicular to the shore. The coordinates and elevation of the plots were taken using an
RTK-GPS tracker. The plots were monitored every 1 – 2 weeks, where the number of shoots per plot were counted and the shoot length measured. At the end of the experiment we determined if the shoots were growing into the
Figure 1: Schematic overview of the experimental setup (adapted from
plots from outside plants or if they came from the planted material. The experiment ran from the first of May until the June 22nd.
Natural occurrence of seeds and rhizome fragments
To correct for the naturally occurring seeds and rhizomes and determining their spatial distribution in the system we performed a field survey in which we filtered the top layer of sand and determined the amount of seeds and rhizomes which were present. We did this by randomly placing 6 plots of 0.5 by 0.5 metre in each of the zones, differentiating between plots with heaps of rhizomes and plots without heaps of rhizomes. In each plot we filtered the top 20cm of sand. This was done using a shovel and a filter with a grid size of 1mm. The sand was filtered with seawater and the residues were dried. After drying the residues the length of the rhizomes was measured and the nodes and the seeds were counted. In the ED and FA zones heaps of marine dispersed rhizomes were present, but not in the FD zone. 2 – 3 plots with heaps of rhizomes were sampled in the ED and FA zone respectively as the rhizome heaps decreased with distance from the shore.
Natural establishment from seeds or rhizomes
To investigate how A. arenaria establishes itself over a gradient we sampled freestanding young plants over an area of 90 metres per zone perpendicular to the sea. Each plant was excavated and the germination source, either rhizomes or seeds, was determined.
Statistical analysis
All of the statistical analyses were done using the programming software RStudio (version 3.6.1). To test the difference in establishment success the number of emerged shoots per zone in the field experiment were compared using a Kruskal-Wallis test of significance as the data was not normally distributed. The normality of the data was tested using a Shapiro test of normality, all the data henceforth was checked for normality with this test. A pairwise Wilcox test was performed to differentiate between the zones. The difference in seeds and rhizomes found between the zones in the filtration survey was also analysed using a Kruskal-Wallis test. To differentiate between the zones a pairwise Wilcox test was performed. To investigate if there were more seeds or rhizomes present in the rhizome heaps a Kruskal-Wallis test was performed. Lastly to test how the the total proportion of seeds and rhizomes found was compares to the proportion of seedlings or rhizome grown plants a proportion z-test was used.
Results
Field experiment
In the field experiment a very low germination rate was measured, with shoots emerging in less than 10% of the plots. There was only shoot emergence in plots in the FD and FA zone (2 and 6 out of 32 plots respectively) and no shoot emergence in the ED zone (Table 1). In the plots where plants germinated, only between 1 and 3 shoots emerged per plot, with an average length of 9.17±6.46 cm. At first shoots only emerged from rhizomes, however in the last measurement shoots germinated from seeds in 3 plots. At the end of the experiment no significant difference was found in average number of shoots per plot (P = 0.1667). Also no significant difference in number of shoots between plots with fertilizer and plots without fertilizer (P = 0.4491) these are therefore not highlighted in table 1. Natural occurrence of seeds in the field was minimal in comparison to the sown seeds, 100 seeds were sown and an average of 5.67±1.62 seeds per 1x1 m were found, so correction for the natural occurrence of seeds led to no changes in the data as numbers were already limited. This correction only included plots without rhizome heaps as we selectively placed the plots outside of the rhizome heaps. There was also no significant difference in number of shoots between plots with seeds and plots with rhizomes (P = 0.1484). After the plots were made, a dry period of 35 days without precipitation followed. On June 5th and June 6th there was a precipitation event with 197mm and 147mm of rain respectively (Data recovered from https://www.knmi.nl/nederland-nu/klimatologie/daggegevens on 25-06-2020).
Table 1: Plots with shoot emergence in the different zones from either seeds (S) or rhizomes (R).
Zone
Source
19-mei
8-jun
17-jun
22-jun
FA
R
0
1
2
2
FA
S
0
0
0
0
ED
R
0
0
0
0
ED
S
0
1
0
0
FD
R
1
2
3
3
FD
S
0
0
0
3
Natural occurrence of seeds and rhizome fragments
No significant difference between the zones in the amount of seeds that were present were found (P > 0.08) (Fig 2a). Nodes were used as a way of normalizing the amount of rhizomes as the resources to measure dry weight were not at our disposal. Rhizome length or amount of rhizomes were not used as they would break during transfer and data would be altered. A significant difference was found in the amount of rhizome nodes in each zone, where the amount of rhizomes nodes significantly decreases with distance to the shore
(
FA-ED (P < 0.03), FA-FD (P < 0.02) and FD-ED (P < 0.02)) (Fig 2b). This correlation can be a result of rhizome heaps decreasing with distance from the shore. This is further supported by the fact that the plots with rhizome heaps did have significantly more rhizome nodes than the plots without rhizome heaps (P < 0.0003) (Fig 3a). The plots with rhizomes heaps also accumulated significantly more seeds than plots without rhizome heaps (P < 0.006) (Fig 3b). But this effect was still too little to cause a difference in amount of seeds between the zones.Figure 2a: Mean number of seeds found in the Embryonic dune (ED), First Ammophila (FA) and Foredune (FD) zone during the filtration survey, error bars depict the standard errors per zone.
Figure 2b: Mean number of nodes found in the Embryonic dune (ED), First Ammophila (FA) and Foredune (FD) zone during the filtration survey, error bars depict the standard error per zone.
Figure 3a: Mean amount of nodes per plot, found either in rhizome heaps (Y) or outside of rhizome heaps (N), in the filtration survey. Error bars depict the standard error.
Figure 3b: Seeds found, either in rhizome heaps (Y) or outside of rhizome heaps (N), in the filtration survey. Error bars depict the standard error.
Natural establishment from seeds or rhizome fragments
There were almost no freestanding newly established plants in the ED and FD zone. Instead they were concentrated in heaps of rhizomes in the ED zone and in the FD zone dispersal was almost primarily clonal. One heap of rhizomes with newly established plants was present in the FD zone. Thus, we almost exclusively sampled plants which grew from the heaps of rhizomes in these zones. The plants in the FA zone were freestanding young plants. A significant difference between the plant growth from rhizomes and from seeds was found. Plant growth from rhizomes made up between 94 and 100 percent of the sampled plants over the whole gradient. The ratios of rhizome grown plants or seedlings stayed the same over the entire gradient with no significant difference in rhizome grown plants (P = 0.3679) or in seedlings (P=0.3679) between the zones (Table 2). The proportions of seedlings and seeds were compared and did not significantly differ from each other (P = 0.7796). This means that each node on the rhizome has the same germination rate as each individual seed found.
Table 2: Amount of rhizome grown plants, seedlings, rhizomes and seeds found per zone in the field surveys with their total proportions.
Zone
Rhizome grown
Seedling
Rhizomes nodes
Seeds
FA
15
1
1344
34
ED
85
3
135
25
FD
18
0
9
7
Total
118
4
1488
66
Total%
96.72%
3.28%
95.75%
4.25%
Discussion
This study investigated how local changes in the environment in a dune area affected the establishment of A. arenaria. This was done by means of a field experiment which was then compared to the natural establishment of A. arenaria. The hypothesis was that rhizomes had lower requirements for their WoO in order to germinate and that this would result in a higher ratio of rhizome grown plants than seedlings, in the field experiment as well as in their natural occurrence. Low soil nutrient levels, soil salinity, erosion and burial were thought to be the main factors affecting the WoO for A. arenaria.
The results from the field experiment were very little, only a few plants were able to grow and there was no significant difference found between the zones. However the lack of difference between the zones is probably due to
the low germination rate, as previous research found that changes in environmental factors affect plant germination and the different zones have different stressors which could affect the establishment of A. arenaria (Gormally & Donovan, 2010) (Reijers et al., 2019) (M. A. Maun, 1994). The low germination rate indicates that the requirements for the WoO of A. arenaria were not met. One factor limiting the germination rate could be a high soil salinity level. However, soil salinity in dune areas decreases with height as flooding is less often (Puijenbroek et al., 2017). Puijenbroek et al. (2017) found that in higher regions, 2.06 – 3.17 metres above NAP, soil salinity had a low probability to affect spatial distribution of A. arenaria. It is therefore not likely that the low germination rate in our experiment is due to a high soil salinity, as our plots were at a height of between 2.41 and 3.00 metres above NAP.
Nutrient deficiency is also not likely the cause of the low germination rate, as no significant difference was found between the plots with fertilizer and the plots without fertilizer. However soil analyses have not yet been performed in our study area, thus, soil salinity or nutrient deficiency cannot be definitively excluded as reasons for the low germination rate. Further examination of the soil must be performed in order to ascertain that the soil salinity is indeed low and that the fertilizer did increase soil nutrient levels.
A more likely explanation is that drought limited the germination of A. arenaria because there were only two significant precipitation events for the entire duration of the experiment. After the precipitation we see a slight increase in shoot emergence, however this was only from rhizomes. Only at the end of our experiment did shoots emerge from seeds and this was without a preceding precipitation event. This could mean that the WoO is different for rhizomes than for seeds, but this should be examined in more detail in future research. Desiccation could play an increasingly bigger role in limiting the establishment of A. arenaria in the future as climate change causes dry periods like this to increase and precipitation events to be more extreme. This decreases the frequency of opportunity for A. arenaria to establish itself.
Burial and erosion could also affect plant establishment, seed germination more than growth from rhizomes (Konlechner et al., 2013) (Huiskes, 1977). While you have more sand transport and deposition closer to the sea, the amount of burial and erosion was observed to be more extreme further from the sea, due to vegetation either capturing sand, or creating a sand blowouts. These were observations made relatively to the poles which were used to mark the plots ( ±20cm aboveground). In the FD zone some poles were almost completely buried and others were more dug out, whereas in the FA zone no changes in the aboveground length of the poles was observed. When desiccation is not limiting all growth, burial and erosion could lead to lesser germination rates further from the sea and cause rhizome grown plants to be even more dominant in this area.
Two surveys were performed to investigate the natural occurrence of seeds and rhizomes in the area and how this correlates to the natural establishment of A. arenaria in nature. This was done by means of a filtr The number of seeds found was very low in comparison to the amount of rhizome nodes. When the proportion of seeds was compared to the proportion of seedlings found, there was no significant difference found, meaning that the seeds have the same viability as the rhizome nodes. In the filtration survey no significant difference in seeds were found between the zones, but the amount of rhizome nodes did differ in the different zones. The amount of rhizome nodes decreased with distance from the shore. This is due to the fact that the amount and size of rhizome heaps decrease with distance from the shore, which is a result of the rhizome heaps being deposited by marine processes. The rhizomes can be transported over long distances, as bud viability still remains high even after long term submergence in salt water (Aptekar & Rejmánek, 2000). Subsequently, these rhizome heaps could be an important input of genetic variability into other populations as sexual reproduction through seeds was seldom found. Furthermore, the structure these rhizome heaps form can trap sand and create positive feedback loops for A. arenaria mimicking the facilitative effect of E. juncea. The rhizome heaps contained significantly more seeds than the surrounding area, but seedlings were still rare. Though this is a consequence of the very limited amount of seeds compared to the amount of rhizomes as the proportion of the found seeds did not differ from the proportion of seedlings. The amount of seeds may increase later in the year, as only as late as July was A. arenaria observed producing seeds.
The sand trapping and seed capturing ability of the rhizome heaps led to a concentrated amount of newly established plants in the rhizome heaps. The establishment of A. arenaria can result in the formation of incipient foredunes. These results about the effect of marine dispersed rhizomes on dune building is in line with the study of Hilton & Konlechner (2011) but studies are scarce and should be examined in more detail.
There was a clear difference between the shoot emergence in the field experiment and the shoot emergence in the natural setting found in the survey. This is probably due to the changes in the microclimate of the rhizome heaps. The newly established plants concentrated in these rhizome heaps meaning that the environment inside the rhizome heaps is more favourable for germination than outside of these rhizome heaps. These changes in microclimate should be examined in future research to get a better understanding of what limits the germination of A. arenaria.
This experiment was performed on only one location and in a relatively short time period, but dune systems are highly variable in their plant ratios and geomorphology. To better understand the effect of local environmental changes on the establishment of A. arenaria, research has to be done in a variety of places and over a longer time scale.
Data accessibility
Data used in the report is made available and can be accessed at: https://doi.org/10.5281/zenodo.3959096
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Supplementary figures and tables
Table 3: Number of shoots per zone from either seeds (S) or rhizomes (R).
Zone
Source
19-mei
8-jun
17-jun
22-jun
FA
R
0
1
2
4
FA
S
0
0
0
0
ED
R
0
0
0
0
ED
S
0
1
0
0
FD
R
1
3
5
4
FD
S
0
0
0
6
Figure 4: Locations of the field experiment and the surveys at paal 28 Texel. The white points are the locations of the plots from the field experiment, four points were taken per plot in each of the corners. The red points are the locations of the filtration survey. The yellow points represent the locations of the newly established plants from the second survey.
