Primary succession

JANUARY 2008 560

Online

Geo file

Alison Rae

Primary succession – theory and case studies Definition A primary succession is one which ‘takes place on a surface where no soil or vegetation has formerly existed’ (Skinner, Redfern and Farmer (2003) Complete A–Z Geography Handbook, 3rd edn, p 228). An area of bare land is an opportunity for the development of a whole new ecosystem. Speed of development can be extraordinarily fast, as case studies in this Geofile will show.

Primary succession (or priseres) can be divided into xeroseres, those in dry environments, and hydroseres, those in wet areas (Figure 1). This Geofile deals with the two types of xerosere, lithoseres and psammoseres.

Succession – the context Succession in an ecosystem is the series of changes which take place in the community over time. A sere is a particular type of plant succession.

Succession can be subdivided into primary (or prisere) and secondary (or subsere), according to where and when it occurs. Primary succession happens first because it takes place on a surface where no soil or vegetation has previously existed. Sand dunes, tidal marshes, outwash plains, deltas, landslips and areas which have experienced a volcanic eruption or lava flow or which have recently been revealed by glacial melting all come into this category. Prisere development can happen after a major physical disaster.

Some human environments also class as priseres – abandoned quarries, spoil heaps from mines and some types of cleared urban land. (Subseres occur on land which has been previously vegetated; soil already exists, so the process is usually quicker.)

The pioneer community is the first group of plants to colonize a newly exposed land area. Typically these are simple, hardy plants, often with particular adaptations to their challenging environment. They alter the environment slightly, adding nutrients when they die and perhaps some shelter to allow less resistant species to cope.

Throughout the succession process the characteristics and species of plants will change and develop until a balance is reached with the environmental conditions. This climax community will not change unless the environmental conditions do so.

Geofile Online © Nelson Thornes 2008

Figure 1: Types of primary succession Prisere

Xerosere

Hydrosere

develops in dry conditions

develops in wet conditions

Lithosere

Psammosere

Fresh hydrosere

Halosere

on bare rock

on sand dunes

in fresh water

in salt water

Figure 2: Succession on land exposed by retreating glacier – Iceland

GeoFile Series 26 Issue 2 Fig 560_01 Mac/eps/illustrator 11 s/s NELSON THORNES PUBLISHING Artist: David Russell Illustration

Lithosere succession on bare rock Mosses and lichens are usually the first species to colonise a bare rock surface, wherever its location. These cling to surfaces even in the most challenging of climatic circumstances. In the very cool wet coastal plain of southern Iceland this pioneer community covers the most recent lava flows, including some from the Laki fissure eruption of 1783. Thick cushions of mosses lie like an even bright green blanket over the hummocky lava flow, creating an eerie and inhospitable scene.

Lichens take their nutrients directly from the rock on which they grow. In so doing they cause biological chemical weathering because the chemical reactions that take place break up the rock structure. This is therefore the very beginning of soil formation and the development of a climax community.

Mosses live primarily on water – even

rainwater contains some useful nutrients. Because they act as a sponge, holding the water in, they keep the rock surface wet, encouraging solution of minerals and hydration processes, both forms of weathering.

Bedding planes, joints, faults and small cracks are all lines of weakness and as such are vulnerable to weathering and erosion processes which enlarge them.

Small pockets of soil are likely to develop within the crack. A degree of shelter from wind and salt is provided, especially in coastal areas. Larger plants are then able to take root in this slightly more favourable environment, continuing the succession and making it more complex.

Some lithoseres are particularly specialised. The surface of a limestone pavement, such as that above Malham Cove in the Yorkshire Dales, is a high, exposed, inhospitable place. The surface of the clints (blocks of limestone) is bare

January 2008 no.560 Primary succession – theory and case studies of soil or vegetation, but down in the grykes (weathered joints) a unique small-scale ecosystem flourishes. The highly localized microclimate – sheltered, humid, alkaline – harbours plants not found anywhere else.

Figure 3: Cliff top succession (photo)

Case studies 1. Land revealed by a melting glacier Iceland Iceland’s glaciers have shown significant melting over the 20th century and since.

Most of the newly exposed rock is from previous lava flows, so it weathers to form high quality soil. It is an uneven surface where sheltered hollows exist, allowing plants an extra advantage.

Figure 2 shows a new ecosystem after 60 years of development. There are many species, including some shrubs reaching over a metre high.

2. Coastal lithoseres Cliff coastlines provide new bare rock surfaces for vegetation to try to take hold. Iceland probably has more bare rock surface per unit area than any other

European country, due to its location along the Mid-Atlantic Ridge and the consequent lava flows. Its coastline has many cliff areas where lithosere succession is in progress.

The southern Icelandic coast has high vertical cliffs, home to many sea birds; it is a bleak and desolate place, except perhaps on an unusually sunny day as in Figure 3). On the cliff tops the hard basaltic lava flows are gradually weathering and resistant plants can use small patches of regolith starting to become soil. A few plants take root in cracks on the cliff face, giving the birds some limited shelter. In turn, the birds defecate on and around the cliffs, providing nutrients for the developing ecosystem.

3. Volcanic eruptions a) Krakatoa (Indonesia – 1883) Krakatoa, a well-known volcanic island in Indonesia, is a caldera so its eruptions are particularly violent. Its last huge eruption in 1883 was so explosive that it:

Figure 4: Primary succession, Krakatoa, 1983 Krakatoa 800

fe rn s, mosses , Cyr tandra shrubs and orchids

Height (m )

600 Neonauclea trees 400

200

Casuarina with dense grass on steepest slopes

Neonauclea trees with fig and macarandra trees Terminalia trees , beach plants, macarandra and Barringtonia, fig Terminalia, Casuarina

beach plants, Barringtonia, coconut

rainforest

cliffs

0 Climate Temperatures are high and constant.

Most months a verage 2 8°C, giving a ver y low annual range . Rain is heavy, falling in convec tional storms most afternoons throughout the year .

800

800 Cyrt andr a shrubs , fe rn s, mosses an d orchids , mosses , fe rn s, fe rn s, Cyr tand ra orchid s, Cyr tand ra small trees shrubs , mosses and shrubs, woodland in orchid s ravines increasing number 600 of Neonauclea t rees mi xe d woodland savanna grassland, coarse grassland

400 fe rn s, shrubs, dense grass, some fe rns growing an d macarandra and blue-green bacteri a fig s

200

beach plants, Barringtonia

0

Number of plan t species

1883 0

1886 26

grass 3 m high 400 rain fo rest climax: increasing number Neonauclea trees Neonauclea with fig, macarandra and of macarandra and tak ing ov er from Neonauclea trees , macarandra and figs Terminalia 200 fig s

beach plants, coconuts beach plants , coastal woodland Barringtonia, Barringtonia ,tussock climax (types as 1918) tussock grass grass , coconut

1908 115

1918 132

1933 271

Barringtonia, beach plants , Casuarina

Height (m )

Height (m )

600

Year

Note : The rainforest climax vegetation here does not contain as man y species as the rainf orests on surrounding islands .

0

1983

Year

?

Number of plan t species

Source: D. Waugh (2000) Geography: An Integrated Approach, Nelson Thornes, p.289 Fig 11.8 Geofile Online © Nelson Thornes 2008

Fig 11.7

January 2008 no.560 Primary succession – theory and case studies • was heard right across the Indian Ocean • reduced the island to one third of its previous size • covered the area in 50m of ash, destroying all previous vegetation and wildlife • killed all life in the surrounding sea • set up a tsunami which reached all shores of the Indian Ocean, killing 30,000 people and destroying innumerable other ecosystems.

In the hundred years following, the return of soil and vegetation was amazingly quick. Obviously the equatorial climate with its typical high temperatures and rainfall helped the processes involved. Figure 4 shows that after 50 years vegetation had developed to such a degree that true rainforest could start to develop. By 1983, 100 years after the catastrophic eruption, rainforest had developed up to 400 m above sea level around the whole mountain, and as high as 500 m in parts.

Above this altitude other factors are involved; Krakatoa is over 800 m high, even after the 1883 event, so cooler temperatures due to altitude and other climatic factors inevitably limit rainforest succession.

Today, the number of species within the Krakatoa rainforest are too numerous to be counted accurately, though there may not yet be as many species as in the original forest. Moreover, average tree height in the canopy is not as high as it will become in a few more decades. b) Surtsey (Iceland – 1963) The shield volcano, Surtsey, is located 18 km beyond Heimaey on the periphery of the Vestmann Islands off south west Iceland. This extremely active area along the Mid-Atlantic Ridge has some of the newest land and lithoseres in the world. Surtsey was an undersea volcano until the 1963 eruption increased its height sufficiently to break the ocean surface and form the world’s newest island.

One of the most researched places on Earth, only scientists can go there and they must wear specialist protective clothing. The development of this new lithosere is of huge interest to Icelandic and other scientists; any risk of contamination (bringing in any seed or bacteria alien to this ecosystem) would alter the natural succession. One researcher made the mistake of arriving on the island with a tomato sandwich and seeds dropped to the ground as he ate it. Later, tomato plants were found Geofile Online © Nelson Thornes 2008

growing from a crevice in the lava!

They were uprooted and destroyed, as plants alien to that ecosystem. c) Mt St Helens (USA – 1980) Similarly, there are areas around Mt St Helens where the public are not allowed to go; it is maintained as a National Monument. This is a much larger area than either Krakatoa or Surtsey and not an island, so keeping this lithosere ‘unpolluted’ is impossible. Today, 27 years after the famous eruption, its ecosystem development is closely monitored and will continue to be so.

Scientists have developed some interesting theories on how ecosystems respond to large-scale disturbances like this massive eruption. The area around Mt St Helens is a relatively simple system and gives a great opportunity to investigate developing habitat relationships. It seems that animals from the tiniest insect to the largest elk have a crucial influence on the developing vegetation and plants from all major stages of forest development appear to be establishing themselves simultaneously.

Neither of these phenomena had been observed so clearly before. Seeds of all levels of species are coming in from the surrounding area and even some of the larger plants manage to take hold in situations of limited nutrients. This contradicts classic ecological theory stating that species establish themselves in a certain order, that one species gives way to another (i.e. that mosses are followed by grasses, grasses by shrubs and shrubs by trees). This is very different from studies undertaken in glacial terrain (such as in front of the retreating glacier in Iceland, above), where true succession does seem to take place in terms of number and size of species. The key factor here seems to be whether the lithosphere area is surrounded by other, thriving ecosystems.

Psammosere primary succession Sand dunes may seem one of the most alien environments as far as colonizing plants are concerned. Such an environment is:• Arid: any rainfall drains away quickly through the course sand with large pore spaces in between • Salty: most plants cannot cope with a saline environment • Exposed: there is nothing to protect plants from strong winds sweeping in across the sea.

Nevertheless, primary succession does take place and often remarkably quickly. The plants which succeed in such a difficult situation are those adapted to the particular conditions: • To combat aridity plants must be xerophytic (drought resistant) • To combat salinity plants must be halophytic (salt tolerant) • To combat strong winds plants can have a variety of adaptations, which will be described below.

The Studland Bay dune system, eastern Dorset Much of the land at Studland Bay has been built up by longshore drift and accumulation by constructive waves over the last 500 years. There is a wide sandy beach. Winds are generally onshore so sand is blown inland. If it is caught by an obstacle (driftwood, stone, even litter!), it builds into an embryo dune which is so small that a misplaced foot can destroy it. Grasses such as sea lyme and sea couch can take hold.

These are both halophytic and xerophytic and, as they grow, they trap more sand, building the dune until it is so large these pioneer grasses are overcome by the larger, tougher marram grass.

The relationship between sand dunes and marram grass is truly symbiotic, i.e. each relies on the other and contributes to the welfare of the other. The roots of marram grass grow down through the dune and help hold it together, allowing it to grow. In turn, the enlarging dune encourages the growth of the marram.

This grass protects itself from wind and high consequent rates of transpiration by having all its stomata (pores) on the inside of leaves which curl in on themselves. The tough stringiness of this plant resists the wind, even wind carrying sand, which makes its erosive power so much greater.

As these grasses die they rot down into organic material; this is the beginning of humus for the next generation of more complex colonizing plants. The sand turns greyer in colour due to the addition of this material. This is the start of it becoming soil. Other plants can now cope with what has become a slightly less challenging environment.

At Studland Bay sand dunes this group of plants include: • herbs, i.e. soft-stemmed green plants • rosette plants, which grow flat to the ground to protect themselves from

January 2008 no.560 Primary succession – theory and case studies wind, sandblasting and, in areas of human use, from human feet; plantains are an example • tough, woody-stemmed plants like heather, ling and bramble.

These plants thrive on the back of the larger dunes and in the slacks, the dips between lines of growing dunes. Here, the environment is more sheltered, less saline and has humus available from earlier plants, providing nutrients. Also, because slacks are lower lying, plant roots may be able to reach the freshwater table, solving the issue of effective drought.

As more lines of dunes have developed at Studland (Figure 5), the older ones show:

Figure 5: Cross-section through Studland dunes to show the development of the psammosere Development of sand to soil

Key Rosette plants Sea lyme and sea couch Marram grass Herbs

Yellow sand Humus/grey sand Podsol (acid soil)

East

Sea pH

8 Shelly sand

1

8–7

West

2

Wasting dunes (grey dunes) Slack B 1

Slack 0

Embryo dunes

Gorse Brambles Heather and ling Other shrubs, various trees

Yellow dunes and ridges

Foredunes (newly developing ridge 0)

Open beach

G B H S

B

H

7–6

H

Slack 2 G

6

GH

5

G H G

S S

S

4.5 – 4

Figure 6: Studland dunes climax vegetation

GeoFile Series 26 Issue 2 Fig 560_05 Mac/eps/illustrator 11 s/s NELSON THORNES PUBLISHING Artist: David Russell Illustration

• a greyer colour due to increasing humus content • a decreasing pH value, as more humus increases acidity levels • a greater % of surface covered with vegetation • greater average height of vegetation • increase in number of vegetation species (apart from the climax vegetation – see below).

Eventually this primary succession reaches a climax ecosystem with interesting characteristics. Light woodland of alder and willow (waterloving species), silver birch and stunted conifers is found at the back of the oldest, most decayed dunes (Figure 6).

Here, pH is as low as 4, showing incredibly high acidity; this is about as acidic as a growing medium can be and have plants survive in it. Fieldwork over a number of years shows this low reading to be consistent and accurate.

Shrubs, mainly gorse, brambles and large heathers, cover almost every part of the ground. Measurements using quadrats show the percentage of ground covered by vegetation increases steadily inland from the beach along a transect through Studland’s dunes. Here, the larger species dominate and so drive out the smaller plants. As a result, for the first time in this succession process, the number of species in the ecosystem actually reduces.

Conclusion Lithospheres make fascinating studies.

Depending on the environment, development can be rapid in terms of time and space.

Perhaps this Geofile will open up some fieldwork or coursework ideas for you!

Geofile Online © Nelson Thornes 2008

Focus Questions 1. What makes a lithosere succession distinctive?

2. What makes a psammosere succession distinctive?

3. Using the information at the bottom of Figure 4, draw a line graph to show the changes in species number between 1883 and 1933 in the lithosere succession on Krakatoa. What is the best way to describe the shape of this curve? What does this mean in terms of the development of the succession?

4. Comment on the fact that, by 1933, there were already 720 species of insects identified on Krakatoa.

5. Seeds reached the island of Krakatoa to re-colonize it by wind, drifting in the sea or by being carried by birds. What effects might this relatively random situation have had on the succession?

6. (a) Explain what makes the succession observed at Mt St Helens different from what is considered a ‘normal’ succession. (b) The succession on Krakatoa developed as a result of seeds and insects being blown to the island or carried in the sea. Why did it follow a more normal pattern of succession when Mt St Helens did not do so?

7. Write an essay to compare prisere development with subsere development. Refer to examples and case studies you have studied in detail wherever possible.