Primary succession

Sunday 14 August 2011

Change of communities

This past week I took some time away from the computer, the phone, the TV and the news of the falling stock market and went to the Hohe Tauern mountains of Austria.   The target area in this vast national park was the Krimml waterfall which is the highest waterfall in Europe, falling over 380m in three massive cascades.  But above and beyond the waterfall lies the glacial valley, the Krimmler Achental. 

While I was hiking up the valley, I noticed evidence of primary succession on the boulders and hillsides and thought of the topic in Option G - Ecosystems & biomes.  First, on approaching the end of the valley, I could see the glacier perched high up on the slope with the bare rock below.  Much further below, vegetation had become established where the glacier once had been. 

How much time was needed for the plants to take root here? 

Photos from 1920 on the walls in the Warnsdorfer Hutte at  an altitude of 2338m show the glacier at the bottom of the slope almost 400m lower than in this picture!  So this indicates the speed at which the glacier has retreated up the slopes as well as how quickly plants invade the new territory.

What is primary succession?

Primary succession is the change of the communities living in an area over time as each community is succeeded by another.  If one were to stand for last 500 years at the side of these mountains and watch the glacier retreat and the vegetation advance, one would notice the series of plant populations living and being replaced by other communities as time past.

What are the stages of succession?

Ecologists call the various stages of succession "seres" and there is a relatively fixed pattern of seres for a specific type of succession.  Succession from open water or bare rock will have different plant communities but each type of succession proceeds pretty much the same way.

Which plants can live on bare rock without any soil? 

In this example of primary succession, the parent rock is granite with quartz intrusions.  The physical action of the glacial ice produces the various sizes of rocks, stones, pebbles, sand and silt.  The finer particles get washed away with snow melt and rain, flowing down the rivers.  The large rocks and boulders are exposed to spores of lichens which are a combination of fungus and algae living together in a symbiotic relationship.  These organisms can attach to surfaces and grow with light, CO2 from the air and water. Whatever minerals they need, they obtain from dust and the surface on which they grow.  Lichens are tenacious and are the first "plants" to colonize new bare rock surfaces.  I say "plants" because they really aren't but they form the first sere of primary succession of plant communities.  Life on the bare rock is very difficult as water quickly evaporates or runs off the surface.  Erosion is quite extreme at this point as there is nothing to hold the particles.  But as lichens grow, their bodies trap dust and the remains of their own dead tissues.  This material accumulates under the lichen body and as time progresses holds a little bit of water for longer periods of time.  This is the beginning of the changing conditions characteristic of primary succession.  Each community changes the abiotic conditions which then enables other communities to take hold.   With the build up of primitive soil- a mixture of mineral and organic matter from the life and death of the lichens, spores of other plants which can survive in these conditions can be trapped. Mosses have no true roots, leaves or stems as they have no vascular tissue.  These plants must live close to the surfaces with water and minerals as they depend on diffusion of these materials.  On this boulder, one can see the patches of black and grey lichens with the green moss.

What is next?

As mosses live and die, they contribute to the accumulation of organic matter and the formation of soil.  Mosses hold water within and between the plant  bodies so creating a moister environment.  Mosses grow over and on top of the lichens, shading the lichens from the sunlight needed to live.  Consequently, lichens will die out as mosses replace them. This is the succession of communities.  As more and more oragnic matter builds up with the death and decomposition of the mosses, wind dispersed seeds of small plants can germinate.  Grasses are typically the next sere.  With the taller grasses, small animals can find shelter and food.  Some of these animals will bring in more seeds stuck to their bodies.   Seeds of shrubs and bushes come next.  More and more organic matter is added to the soil.  Shade from the taller plants moderates the temperature on the surface so less extreme conditions exist.  Roots from plants creep into cracks where water is.  As more and more plant species get established, the mosses are out competed and replaced.  Grasses get covered by shrubs and trees grow over the shrubs.  Little by little, the communities of plants have changed from lichens to mosses to grasses to shrubs to trees.

Does succession ever end?

Succession is continuous but many factors contribute to more and more stable conditions - soil formation and less erosion.

With the soil formation as organic matter accumulates, greater plant species diversity is possible.  The soil holds more water so plants with vascular tissue in roots and stems can survive.  As plant roots hold the soil, less erosion will occur.  More plant growth enables more species of  animals to survive too.  These organisms also contribute to the changes in the community.

Climatic conditions establish the final sere or climax community in the local area.  The high altitude of the Krimml Achental will exert an effect on the plant community so that only those plants that can tolerate long, cold, snowy winters and short summers will survive.  So the spruce and pine trees are the last sere in this succession.  But as I descended from the high valley down to the 1000m level below the waterfall, birches, aspen, populars, maples and many other trees appeared.  In the lower altitudes, with somewhat less extreme conditions, greater plant diversity exists.  The lower altitudes were covered by the same glacier but more than 10000 years ago, so the seral progression has advanced much more than at the top of the valley.


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