Report – BZ1030 – Zonation Rowes Bay

BZ1030- 991025

Study of Zonation And Successional Processes of

Dune Vegetation At Rowes Bay

By: Stefan Martensson

1. Abstract.

Zonation patterns were studied within a plant community growing in a stressful environment at Rowes Bay, Townsville north Queensland. Observations were made of major environmental factors determining the zonation patterns and the relationship between zonation and succession within the community. Adaptations of plants and evidence of human impacts were also recorded The main zones encountered were either (dead) grasses, herbs and vines, trees, or a mixture of these.

The factors affecting the zones were found to be wind, salinity, pH, and moisture.

The succession pattern that was identified, indicated a predicted pattern with four zones.

2. Introduction

Gradients represent continuous, usually gradual changes in environmental variability that occurs with geographic distance. From the coastline inland there tends to be more or less gradual changes in the environment. All changes are gradual, even if they may look abrupt; it is only a matter of how closely they are observed (Knox et al. 1999).

Zonation – The distribution of the different species of a community into separate zones, which are created by variation in the environment. Competition between species may also contribute to zonation (Oxford 1996).

The wave action and onshore wind build up coastal dunes behind the beach that might or might not be stabilised by the vegetation (Bird, 1997). Humans often affect the stabilised vegetation due to building of roads through or behind the dunes and this decreases the vegetation and leads to erosion, disrupts the natural vegetation and sometimes lead to secondary succession.

Succession – The sequence of communities that develops in an area from initial stages of colonisation until a stable climax community is achieved (Oxford 1996).

The aims of the study are:

1.Describe the main vegetation zonation patterns of the Rowes Bay coastal dune system in tropical northern Queensland.

2.Relate these zonation patterns to various environmental factors, e.g. climate conditions, gradients in soil chemistry/structure, biotic factors (e.g. competition), dune topography.

3.Describe adaptations of plants growing in this area in relation to these various environmental factors.

4.Discuss the zonation patterns in relation to successional processes.

5.Discuss the impacts (negative or positive) of human activity on the dune system – from both an ecological and aesthetic perspective.

3. Methods

Site description

Rowes Bay is a shallow gradient beach, located about 3 km outside the city of Townsville surrounded by a park area, with a fore and hind-dune system extending back over several kilometres into the (Town Common) environmental reserve. See site map in Appendix A

The city of Townsville is located in the dry tropics. I.e. relatively dry winters, with monsoonal rain during the summer months.

Data collection

A transect was established perpendicular to the dune system and extended inland for approximately 160m. This included the fore-dune and at least two hind-dunes up to the fence line making the boundary to the Townsville Golf Course.

A 1 m2 quadrat was used every 1 m interval along the transect to record the dominant species present and the percentage bare ground, and percentage total live and dead cover within each quadrat.

Within the live vegetation category, percentage ground covers each of the following five life forms were recorded: grasses, herbs, vines, shrubs, and trees. The percentage cover of any dominant species within these five categories was also recorded. The percentage cover for the larger shrubs and trees, which were not within the quadrat, but overgrew it, was also estimated

The percentage of vegetation cover could maybe exceed 100% in some areas, due to overhanging of the taller plants. Adaptations of the plant species were also recorded.

Topography of the dunes along the transects was defined by use of an Abney level inclinometer at 1m intervals. Soil texture and structure was measured, and pH was measured at the surface and a depth of 10cm at 3 m intervals along the transects.

Disturbance was kept to a minimum, as the area was a Townsville City Council regeneration site.

Results

The results are presented in Figures 1-4, which show the distribution of different types of vegetation along the transect.

Figure 1 (on separate sheet) shows the distribution in percent of living and patches with dead vegetation across the whole transect. It is very clear that there is an average, relatively low percentage cover of live vegetation in the first 20-22 meters from the beginning of the transect nor between 80 and 130-140 meters. The road is to be found between 87 and 100m occur the footpath can be distinguished in the gap to the right of the road.

Figure 2

The pH decreased the further from the fore-dune the sample was taken. The pH did differ significantly both between the surface sample and the sample taken 10 cm down and along the transect. At the end indicates that the pH is about the same at the surface and at 10 cm. At the start the pH at 10 cm and the surface was about the same, but when the pH dropped at about 20 m the pH at 10 cm’s depth went up to a level of 8.5 which was the highest recorded value. The surface pH stayed the same until a big drop of both values at about 35 meters where the surface value recorded a low value of 6, which was the lowest value on the transect. After the road and the power-line the pH made a large increase with 1.5 units, from pH 6 to a pH of 7.5, and just after about 10 meters the pH decreased with the same ratio. At the end of the transect the pH at 10 cm levelled out at the same as on the surface

Figure 3 (on separate paper) show the major zones of vegetation at Rowes Bay. It shows a clear pattern of four zones and that the zonation pattern start of with a Spinifex zone and zone of mixed grasses vines/herbs and further up along the transect it had a majority of bare ground and disturbed area, e.g. the road and the power-line. The third zone consisted of mixed Acacia crassiocarpa and the last zone a kilometre further up consisted of Eucalypts.

Figure 4 (on separate paper) indicates the variation in height over the start point of the transect,

Table 1.

Species Adaptations
Canavalia Taproot, to get deeper down in the soil to reach the water table.
Spinifex sericeus Low growth – keep out of wind
Ipomea pes-caprae Low growth, reduced and upright leaves to avoid sunlight and evaporation, tolerates high pH and salinity
Allocasuarina Low growth to avoid wind, round vertical leaves to avoid too much sunlight, long taproots in soil for grip and water, tolerates high pH and salinity.
Acacia crassicarpa Round, needle-shaped leaves to avoid too much sunlight and evaporation. Symbiotic N2-fixing bacteria in roots to help uptake of nutrients and water. Nitrogen fixing ability,

Reefer to: Maze et al. (1992), R. Condon, Dept. Bot., JCUNQ, pers. comm.

Discussion

The aggregation of plants generally, form three zones of vegetative cover in coastal dune systems –

1. A pioneer type composed mostly of herb like plants & grasses.

2. A heath/shrub type of woody shrubs, vines and small trees with a few associated herbs.

3. A woodland or forest type dominated by trees.

Nearly all the plants in the first two zones of vegetation are tolerant of salt spray, strong on-shore winds and various degrees of soil salinity. (Beach Protection Authority 1984)

At Rowes Bay five different zones were encountered, the reason why Beach Protection Authority just mentioned three zones was that during this study subdivisions of the two first zones were done. The transect started with a zone of the pioneer grass species Spinifex.

The mixed second zone, containing bare ground, Chloris gayana grass, other species of herbs, veins and later Allocasuarina trees, replaced the Spinifex zone abruptly and with a distinct line in the vegetation. The third zone was made up of a mixed Allocasuarina trees, with some patches of herbs/veins and grasses. The Acacia crassicarpa zone had also patches of grass, further up along the transect the grass coverage increased to full coverage until the fence line interrupted both of the species and they disappeared. The fifth and last zone was the Eucalypts forest it was a couple of kilometres inland and was encountered during this study. (A. Webb pers. comm.).

This sequence reflects long term succession and is related to advancing dune systems following in the Bay over the past 5-6 thousand years. This type of succession is common in the Australian tropics.

The Spinifex zone is found as a pioneer species because it has a natural resistance for salt spray, which the Spinifex is exposed to during most of the year. (Maze et al. 1992) It is also believed that salt sprays contain all the nutrients necessary for plant growth except nitrogen and phosphorus (Rozema et al. 1985). Grasses are especially good at binding sand on beaches and the binding prevents erosion and sand/soil movement (Beard, 1997, and Eisma, 1997). Where the second zone with Chloris gayana, herb and vein started the Spinifex grass abruptly disappeared, the mixed zone gradually continued into the third. Which had Allocasuarina trees that are adapted to dry conditions (Table 1) with long roots and needle-shaped leaves, the zone had an abrupt end at the fence line at 80 meters.

In the fourth zone secondary succession was evident, that is where an area has been disturbed and is being re-colonized by pioneer species, were encountered due to the human interference by the footpath, road and the power line. When building the power line devastation of the vegetation was done and re-generation is evident. Chloris gayana grass was encountered and little further up (30m) Acacia crassicarpa trees started to re-colonise after the interference. The human impacts are obvious in this zone with both the footpath power line and the road. After the road there was a drastic elevation in pH with 1.5 units if that was due to the road runoff or something else could not be determined.

The Chloris gayana grass was found abundant around the edges of the footpath and the road, however the area was in general bare and dead grass but some patches of live grass were found. The pH at 10 cm depth decreased to the same value as the pH on the surface; this was thought to be due to increased and deeper organic material and because it is deeper, the soil gets more homogenised and therefor the change to an equal pH.

The variation in pH can be explained by the difference in depth of organic matter, e.g. in the beginning of the transect there was no or a very thin layer of organic matter which made the pH high due to the salt spray from the ocean. This sandy soil was alkaline at 10cm depth throughout the transect until the end of the transect (160m) were the organic matter became thicker and due to that the pH dropped to an acid level (pH 6) same as on the surface. The pH on the surface was in general 1-1.5pH units lower throughout the transect at 10cm depth.

The whole transect was sandy and even 1 km further up into the Eucalypts forest the soil was sandy but now more like sandy loam, the pH at this location was about 6 e.g. the same as at the end of the transect. This indicates that the soil stabilized around 160m.

Conclusion

The human impacts, e.g. footpath, road and the power line, was evident. The environment could cope with the power line though secondary succession was encountered, at the footpath and the road the environment have not coped with the major change.

The succession encountered in the dune system that was related to advancing dune systems following in the Bay over the past 5-6 thousand years is likely to continue with out major disturbance from the road and the other human impacts. Non-human impacts that are likely to happen is storms, cyclones, but the environment will re-generate from that.

References

  • Beach Protection Authority (1984) Mulgrave Shire Northen Beaches: A detalied study of coastline behaviour in north Queensland, Australia. Beach Protection Authority, Qld.
  • Bird E.C.F. (1997) General features of the coastline, in Dry Coastal Ecosystems, Vd. 2C (ed. van der Maarel, E.) Elsevier, Amsterdam. Pp 1-10.
  • Daintith J., Isaacs A. and Martin E. (1996) Concise Science Dictionary, 3rd Ed. Oxford University Press, Oxford.
  • Eisma D. (1997) Physiography of beaches and coastal dunes, in Dry Coastal Ecosystems, Vd. 2C (ed. van der Maarel, E.) Elsevier, Amsterdam. Pp 11-28.
  • Knox B., Ladiges P. and Evans B. (1999) Biology. McGraw-Hill Book Company Australia Pty Limited, Roseville.
  • Maze K.M. & Whalley R.D.B (1992) Effects of salt spray and sand burial on Spinifex sericeus R. Br. Australian Journal Ecology 17: 9-19.
  • Rozema J., Bijwaard P., Prast G. and Broekmann R. (1985) Eco-physiological adaptations of coastal halophytes from fore-dunes and salt-marshes. Vegetatio 62, 499-521.

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