Scholarly article on topic 'Factors affecting the distribution of Scirpus nodosus plants in a dune slake community'

Factors affecting the distribution of Scirpus nodosus plants in a dune slake community Academic research paper on "Biological sciences"

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South African Journal of Botany
OECD Field of science
{"Dune slake" / "salt spray" / " Scirpus " / "soil factors" / "wind effect"}

Abstract of research paper on Biological sciences, author of scientific article — R.A. Lubke, A.M. Avis

A survey was undertaken to show the distribution of Scirpus nodosus plants in a dune slack community on the eastern Cape coast. Plants were recognized as belonging to three separate height classes and the position of the individuals was related to various environmental factors. Although soil pH, conductivity and percentage soil moisture are strongly interrelated it is suggested that soil moisture is the major controlling factor in the establishment of these plants. The effect of wind-borne salt spray may also limit the establishment of dune slack plants on the foredunes as field observations showed that although large amounts of salt spray reach the dunes, the amount deposited in the dune slacks is negligible.

Academic research paper on topic "Factors affecting the distribution of Scirpus nodosus plants in a dune slake community"

Factors affecting the distribution of plants in a dune slack community



R.A. Lubke and A.M. Avis

Department of Plant Sciences, Rhodes University, Grahamstown

A survey was undertaken to show the distribution of Scirpus nodosus plants in a dune slack community on the eastern Cape coast. Plants were recognized as belonging to three separate height classes and the position of the individuals was related to various environmental factors. Although soil pH, conductivity and percentage soil moisture are strongly Interrelated it is suggested that soil moisture is the major controlling factor in the establishment of these plants. The effect of wind-borne salt spray may also limit the establishment of dune slack plants on the foredunes as field observations showed that although large amounts of salt spray reach the dunes, the amount deposited in the dune slacks is negligible. S. Afr. J. Bot. 1982, 1: 97-103

'n Opname is gedoen om the verspreiding van Scirpus nodosus-plante in 'n tussenduinholte-gemeenskap op die Oos-Kaapse kus aan te dui. Daar is gevind dat plante aan drie versklllende hoogteklasse behoort en dat die posisie van individué deur verskeie omgewingsfaktore bepaal word. Alhoewel grond-pH, geleidlngsvermoe en die persentasie grondvog sterk onderllng verbonde is, wil dit voorkom dat grondvog die belangrikste beherende faktor by die vestiging van hierdie plante is. Die uitwerking van windgedraagde soutsproei mag ook die vestiging van tussenduinholte-plante beperk aangesien veldwaarneming aantoon dat, alhoewel groot hoeveelhede soutsproei die duine bereik, die hoeveelheid wat in die tussenduinholtes neergeslaan word, onbenullig is.

S.-Afr. Tydskr. Piankt. 1982, 1: 97-103

Keywords: Dune slack, salt spray, Scirpus, soil factors, wind effect.


The coastal strip between the Kleinemonde and Riet Rivers, an area of about 350 ha and 4 km long, consists of shifting sand dunes with little vegetation cover (Lubke 1982). This section of the coast is one of the most windy areas in South Africa, predominant winds being easterlies and north-easterlies in summer, with more intense winds from the west or south-west in winter. It appears that wind and sand movement, among other environmental factors, play an important role in limiting vegetation on the foredunes (Lubke 1982). However, more species occur in the dune slacks and wind and sand movement is possibly not as important in this environment. Other factors, such as availability of water, salinity and pH of the soil and intensity of sea spray may play a role in controlling the distribution of dune slack plants. The aim of this paper is to show how environmental factors may affect the distribution of Scirpus nodosus Rottb. individuals within the dune slack.

Study area

The section of the coast under study is about 12 km east of Port Alfred and was subjected to an intensive vegetation survey by Lubke (1982). The dune slack community studied (Figure 1) was immediately east of the line transect through the foredunes and dune slack (Figure 2, Lubke 1982).

The dune slacks form within parabolic dunes which are concave to the direction of the prevailing wind (Chapman 1976). In the area of study, the west side was stabilized by the rush, Juncus kraussii (Figure 1). S. nodosus plants occurred in a hollow, protected from the prevailing winds by fairly large dunes on either side, and a large dune on the seaward side. This site was chosen because the plants had only recently become established, with S. nodosus as the dominant species of the community.

R.A. Lubke* and A.M. Avis

Department of Plant Sciences, Rhodes University, Grahamstown 6140, Republic of South Africa

*To whom correspondence should be addressed Accepted 8 July 1982


Vegetation sampling

The study area was triangular in shape and three points, A, B and C (Figures 1 & 2) were located at the extremes. To aid mapping of the vegetation the area of 220,5 m2 was divided into quadrats. A scale map of the study area was produced on which the vegetation in each quadrat was plotted (Figure 2). In order to calculate density and percentage frequency

Figure 1 The study area in the dune slacks near Kleinemonde. Salinity traps in the centre of the study area are 99 m from mean sea level.

• • • • • •


Scirpus nodosus plants: Less than 5 cm 5 to 30 cm Greater than 30 cm

Other Species:

Helichrysum praecinctum

Helichrysum venereum Senecio elegans Hebenstreitia cordata Scaevola thunbergii Arctotheca populifolia Heteroptilis suffruticosa Myrica cordifolia Sporobolus virginicus Chironia decumbens

Salinity traps



Figure 2 Map of the vegetation of the study area.

Salt spray

The 'salt trap' (consisting of a 20 x 20-cm square of cheesecloth with a square decimeter marked in the centre) was used for measuring the amount of wind-borne salt (Oosting & Billings 1942). The cloth was stretched onto light wooden frames by means of thumb tacks (Figures 1 & 3). Three of these traps were set up facing the prevailing wind at sites 1-4 (Figure 4a). After 24 h the cheesecloth was removed and the square decimeter cut out and stored in a glass vial. Wind speed was measured at each site when the traps were set up, and when removed. The cheesecloth pieces were soaked in 200 ml distilled water for 10 min and the amount of chloride present was measured using an E1L Chloride electrode and T & C pH meter. A standard curve for chloride was produced to convert the readings from millivolts to CI" concentration in mg dm"3.


Distribution of plants in the dune slack The vegetation occurring in the dune slack was plotted in detail on scale map (Figure 2). S. nodosus plants <30 cm mainly occur in the centre of the site showing that pioneer plants first became established in the centre, probably by wind-borne seed. Plants of height class 5-30 cm and <5 cm are also found in the centre. These were younger seedlings which had germinated later. S. nodosus of height class 5 - 30 cm is found mainly on the two edges furthest from the sea, and no plants > 30 cm are found in this area. These plants became established in this slightly drier area by seed, showing that S. nodosus is spreading and becoming more abundant throughout the study area. Seedlings are widespread except at the extreme points.

Of the eleven species found in the study area some are common dune species whereas others favour the moist dune slacks. In the former category Scaevola thunbergii, Myrica cordifolia, Hebenstreitia cordata, Senecio elegans and Heteroptilis suffruticosa occur in the drier margins of the study area. The two Helichrysum spp. are common in dune slacks, H. vellereum occurring in drier areas and H. praecinctum in the moist areas, along with Chironia decumbens and Scirpus nodosus. Sporobolus virginicus is the second most abundant species, and is fairly evenly spread over most of the moister areas in the study site. Sporobolus virginicus and Arctothecapopulifolia are abundant pioneer species of dune slacks along the eastern Cape coast. The latter is not very common in the study site as it favours drier habitats. These two species are found both in dune slacks and on the foredunes (Lubke 1982).

of the dune slack species, 20 quadrats of 1 m2 each were taken from the bottom left of each large quadrat marked 1-20 on Figure 2. The number of individuals of each species was recorded for each quadrat. Because the S. nodosus population consisted of numerous plants of different ages it was divided into three height classes, namely, plants <5 cm, plants 5-30 cm and mature plants >30 cm (Figure 3).

Figure 3 The three height classes of Scirpus nodosus in the study area. A majority of smaller height class (< 5 cm) in foreground and taller plants (5 - 30 cm) in middle region and tallest plants (> 30 cm) behind the salinity traps.

Soil factors

Soil samples were taken from quadrats 1 -20 (Figure 2). Quadrats 11,12 and 20 were located in the centre of the sample site, which was very moist. Water samples from these points were taken and pH and conductivity of the water was measured.

Soil pH, conductivity and moisture content was measured for each sample following the methods of Lubke (1982). Using a soil auger a hole was made at each quadrat and the depth from the soil surface to the water table was measured.

Density and frequency of plants

The density and percentage frequency of all species is indicated in Table 1. With respect to S. nodosus plants, those < 5 cm have the highest density and percentage frequency because of the large number of scattered seedlings. Those plants of height class 5 - 30 cm have a high density and percentage frequency. Density is high because there are many plants evenly distributed. There are fewer of the larger plants (> 30 cm), concentrated mainly in the centre of the area, and their density is therefore low, although their frequency is high because of their large size.

Salt Trap 1

Salt Trap 2

Salt Trap 3

Salt Trap 4



70 80 90


Figure 4a Location of salt traps along the transect from the sea. Salt traps 1 and 2 were on the foredunes and 3 and 4 in the dune slacks. Salt trap 3 is situated within the study area.


Figure 4b Change in wind velocity at different distances from the sea. (- A - measured at 12h00 on 12.9.1981 - O - measured at 12h00 on 13.9.1981)

Table 1 Mean density and percentage frequency for all species in the dune slack quadrats


Density (plants m 2) (mean ± S.E.)

Percentage frequency

Scirpus nodosus <5 cm 104,1 ± 37,1 56,3

Scirpus nodosus 5-30 cm 9,6 ± 6,9 37,5

Scirpus nodosus > 30 cm 2,1 + 1<3 31,3

Scaevola Ihunbergii 0,4 * 6,3

Sporobolus virginicus 4,9 ± 1,8 43,8

Myrica cordifolia 0,4 * 6,3

Helichrysum vetlereum 0,8 0,4 25

Hetichrysum praecinctum 1,6 ± 1,1 25

Hebenstreitia cordata 0,06 * 6,3

Arctotheca populifolia 0,06 * 6,3

Chironia decumbens 3,06 + 2,5 12,5

"Occurred in only one quadrat.

Of the other species, Sporobolus virginicus has the highest density and frequency, spreading over the area by means of stolons and rhizomes. Chironia decumbens has an anomalous high density because of the large number of plants concentrated in only two quadrats. The density of the other species is not really significant in this study, but does show that the number of species in this relatively young community is higher than most other slacks in the area which have only 3 or 4 species present (Lubke 1982).

Soil factors

The density of Scirpus nodosus and the related soil factors in the 20 sampled quadrats is shown in Table 2. To show the relationship between the soil factors and Scirpus nodosus, isolines of pH, percentage moisture and conductivity were plotted on a map (Figure 5). Data were available at grid points and along the margins of the study area and isolines were extrapolated from these points.

The pH is lower in the centre of the study area where the plants have been established for a longer time (Figure 5A). An increase in the amount of vegetation results in more litter and a higher organic content, which is known to decrease the pH. Salisbury (1925) states that older dunes become more acidic as leaching proceeds and hydrogen ions replace the bases. The increased moisture content and

higher conductivity in the centre of the dune slack has a significant bearing on the relatively low pH values recorded in this region. The significant correlation coefficients between the three soil factors are shown in Table 3.

Soil water content is probably one of the most important environmental factors affecting the growth and development of S. nodosus, which is one of the primary pioneer species of dune slacks. Most of these dune_slack species must become adapted to either an excess or deficiency of water because the amount of water present in a dune slack fluctuates greatly depending on the rainfall. In this area the rainfall is very erratic (Dyer 1937), and the alternating level of the water table could be a limiting factor in the germination, growth and establishment of plants on dune slacks.

The percentage moisture isolines are shown in Figure 5B. These values may be slightly higher than the norm, because samples were taken shortly after a rainy spell.

In the centre, where the S. nodosus first became established, there is a relatively high percentage moisture. The water table was close to the surface in this area, being 11 cm below soil surface at quadrat 20 (Table 2). This 20% moisture isoline encloses almost the entire central section of the study area. The 10% and 5% moisture isolines are in the extremes of the sample site. Only a few of the

Figure 5 Map of the study area showing the distribution of Scirpus nodosus plants and the isolines of: (A) pH, (B) percentage moisture, and (C) conductivity (micromhos cm"2).

S. nodosus seedlings are found at the extremes. We observed a positive relationship between the density of S. nodosus plants and percentage soil moisture but the correlation coefficient (/-=0,486) was not significant because of the small sample size.

The depth of the water table also follows a similar trend to that of percentage moisture. Most of the water is from rainwater seepage from neighbouring dunes. Large plants in the dune slacks decrease evaporation and the increased moisture content of the soil results in lower soil temperatures. This is an important factor because in this area of the coastline the summer soil temperatures may exceed 40 °C on the surface. The temperature fluctuation in the dune slacks is generally less than that experienced on the foredunes (Lubke 1982), and is thus one of the contributing factors to a greater divesity of species in the dune slacks.

To give an indication of salt accumulation in the soil, conductivity isolines were plotted on the map (Figure 5C). These isolines show the same trends as those of moisture. The highest conductivity of 365 micromhos cm"2 corresponds to a high percentage moisture of 19,20% (quadrat 7, Table 2). The 250 micromhos cm"2 isoline encloses the entire central area and correlates well with the 20% moisture isoline. As with the lowest moisture isolines (5 or 10%) the lowest conductivity isolines (200 and 150 micromhos cm"2) are also at the edges of the study area.

There is a positive correlation (Table 3) between percentage moisture and conductivity of the soil. A high water content indicates that these areas in the dune slack are lower than the surrounding sample sites. Water and salts leached from the sands will accumulate in these areas resulting in high conductivity and soil moisture readings (Stone, pers. comm.). High conductivity readings of the water samples (quadrats 11,12 and 20, Table 2) indicate that the water has a relatively high concentration of inorganic salts. It is unlikely that this concentration of salts has any effect on the germination aftd establishment of S. nodosus seedlings as there was found to be no correlation between conductivity and density of all plants (r = -0,089). The advantage of having more water present in the centre of the area is probably greater than the disadvantage of a higher salinity.

Salt spray

The observation of a visible spray of salt water from the sea during high onshore winds suggested that wind-deposited salt on the sand surface might accumulate in the soil, thus increasing its osmotic concentration, and also that salt might be deposited on the leaves of plants, thus resulting in a reduction of growth. This would serve as a selective factor in determining the distribution of dune slack plants.

The chloride concentration in the salt traps and the wind velocity measured at the four stations are recorded in Table 4. Decrease in wind velocity with respect to distance from the sea is shown in Figure 4b. Chloride concentration decreases with distance from the sea. Thirty-three metres from the sea 71,6% of the total CI" measured was intercepted and only 0,17% at 129 m from the sea. The decrease is related to a decrease in the wind velocity (Figure 4b) in the dune slack region (r = 0,92; prob <0,05).

Table 2 Density of S. nodosus and related soil factors in the twenty sampled quadrats

Density of 5. nodosus

(plants m"2) Soil factors

Position in study area Quadrat No. < 5 cm 5-30 cm > 30 cm pH conductivity (micromhos cm"2) % moisture depth of water table (cm)

1 0 0 0 8,98 128 1,53 *

2 0 0 0 8,77 177 1,44 *

3 245 0 0 8,93 205 1,18 *

4 63 0 0 8,92 205 2,43 *

5 0 0 0 8,93 164 1,83 *

6 36 9 3 8,77 184 13,23 *

7 9 0 6 8,22 365 19,20 25

8 0 0 0 8,37 258 19,53 20

9" - - - 8,96 135 12,84 45

10 82 2 0 8,75 220 5,68 47,5

11 371 4 20 8,19 260 21,92 16,5

12 299 23 1 8,48 242 19,31 19

13a - - - 8,40 169 16,14 25

14 0 0 0 8,92 111 1,02 71

15 435 6 0 8,24 - 20,94 16

16a - - - 8,73 248 13,88 39

17 0 0 0 8,97 140 2,24 60

18a - - - 8,82 200 7,10 50

19" - - - 8,89 222 0,64 70

20 127 110 3 11

Water factors

11 7,62 695 - 16,5

12 7,96 740 - 19

20 7,53 960 - 11

* Not measured because of the rocks in the area

"Density of plants not recorded as quadrat lay on the edge of the study area

Table 3 The relationships between the soil factors

Correlation Regression

Soil factors Mean ± S.E. coefficients (r) equation

pH 8,72 ± 0,06 - 0,739 ,y= 1674,6-168,8* Conductivity (micromhos cm"2) 201,8 ± 14,4 (Prob < 0,001)

Moisture (%) 8,96 ± 1,86 - 0,865 >> = 8,99-0,029* pH 8,72 ± 0,06 (Prob < 0,001)

Moisture (%) 8,96 ± 1,86 0,607 y= 159,8 + 4,69* Conductivity (micromhos cm"2) 201,8 ± 14,4 (Prob < 0,01)

Oosting and Billings (1942) found a similar relationship in North Carolina. They also reported an increase in chloride concentration on dunes behind the dune slacks. In our study, salt spray was not measured on the dunes above the dune slack.

Discussion and Conclusions

The absence of plants from most areas of the dune systems can be explained by the migratory nature of the sand dunes owing to the persistent winds. Within the dune slacks wind is of less importance in the establishment of vegetation because this environment is more sheltered. The flora of wet slacks consists of many plafits found in salt marshes commonly found outside the dune system (Salisbury 1952).

Few species are confined only to the dune slacks, because the habitat has similar attributes to both sand dunes and salt marshes. Since dune slacks are relatively isolated, pioneer plants such as Scirpus nodosus can only become established by wind-borne seed from neighbouring slacks.

The study area was separated from an adjacent dune slack, which had S. nodosus and Juncus kraussii as dominant species. Seeds from the neighbouring slack could have been carried into this area. The presence of more mature plants in the middle of the slack suggests that S. nodosus plants colonized that area first. Once established, they increased in number by rhizomes and also from new seed once the slack had become relatively stable. This is evident from the presence of plants in the smaller height

Table 4 Wind velocity and chloride concentration measured at four sites on the sand dunes and dune slacks (See Figure 4a)

Wind velocity

Site no. Distance from sea (m) 12.9.81 12h00 13.9.81 12h00 CL conc. (mg dm-3) total CL intercepted at 4 sites

I 33 4,28 2,87 "F = 7,108 7,436 7,647 7,397 71,6%

2 63 3,32 2,23 ~x = 1,970 3,052 2,511 24,3%

3 99 2,21 1,60 ~x = 0,142 0,312 0,755 0,403 3,9%

4 129 1,35 0,92 ~x = 0,021 0,025 0,007 0,018 0,17%

classes. While S. nodosus pioneered the moister areas, other plants such as Arctothecapopulifolia and Sporobolus virginicus became established in the drier areas. This resulted in a successional development of the slack and the invasion of the other species.

There is a pH gradient from 8,0 in the centre to 9,0 on the margins in this particular slack (Figure 5A). Plants affect the pH of the soil by increasing the acidity (Salisbury 1925) and although we found that as the density of S. nodosus increased there was a corresponding decrease in pH, the correlation was not statistically significant (/* = -0,483). Further studies are required to substantiate any relationship between density and pH.

In the study area, pH was found to be negatively correlated with percentage moisture and conductivity (Table 3). However, there is a positive correlation (r = 0,607) between percentage moisture and conductivity. In spite of this interrelationship between these soil factors it appears that percentage moisture is the more important controlling factor in the establishment of vegetation in the dune slack. Rainfall is an important determinant of soil moisture

content, and since the amount and periodicity of rainfall is very erratic in this area of the coast, the water supply to plants of the dune slack will vary considerably. For this reason not many slacks in these vast dune systems have become colonized. The depth of the water table is also related to the percentage moisture in the soil, being closer to the surface in the centre of the study area.

S. nodosus plants are the largest and most dense in areas of high conductivity and high soil moisture. This is the lowest lying part of the study area where moisture accumulates and thus seedlings can become established. Most dune slacks do not experience any flooding by high spring tides and the salts present in the soil are due to rainwater seepage or wind-blown spray. The levels of salt, as indicated by conductivity readings, did not appear to have an adverse effect on seedling establishment. Using salt traps it was found that there was a marked decrease in the amount of wind-borne salt as the distance from the sea increased (Table 4), which is also related to the decrease in wind velocity (Figure 4b). It is therefore possible that dune slack plants are less tolerant of salt spray on their leaves than plants of the foredunes. Further studies were therefore initiated to test the effect of salt spray on the growth of S. nodosus plants (Lubke & Avis 1982).


The assistance in the form of facilities and equipment provided by the Departments of Plant Sciences and Geography, Rhodes University, is gratefully acknowledged. We would also like to thank Mr W.O. West and Mrs D.S. Wiggett for their assistance with the diagrams and Miss H.J. Kew for typing the manuscript.


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