44: Succession- from rock to forest

Roses are red, violets are blue, but how do they end up growing in that old quarry down the road which used to be covered in rocks? Complex plant communities are able to appear in places that were previously barren due to the biological process of succession, without which earth would be a pretty barren rocky place. This week we will be looking at succession and how bare rock can be transformed over time into a lush forest. Curious? Read on!

Dwarf fireweed and moss, pioneer species starting to grow on bare rock in Kenai Fjord National Park (Picture: USGS/ Bruce Molna)

What is succession?

The process of a biological community changing over time is known as succession. This begins with a barren lifeless area such as desert or bare rock. This could perhaps be after a volcano has erupted, spewing lava over a landscape which hardens over time, or when a glacier has retreated leaving only rocks behind. These areas have no soil or plants, yet life always finds a way, and slowly the very hardiest of plants called pioneer species start to move in. This includes things like lichens which are able to grow without soil instead inhabiting the bare rocks, and lyme grass which can grow in sand.

Instead of getting it’s nutrients from the soil, the lichen extracts nutrients out of rain droplets running off the land, and out of the rock on which it sits. In this process the lichen will start to break down the rock, and some parts of the lichen may die and start to decompose. Slowly, very slowly, this process of pioneer species growing, spreading and dying will occur, often over the space of hundreds of years. Throughout this the bare rock is reduced to many smaller rocks with very old decomposed plant litter scattered about, i.e. a very basic soil.

With this thin soil layer new plant species are now able to move in, including things like hardy grasses, and the types of plants you get on road verges. All these plants are pretty small, over time growing and dying too, creating a thicker soil layer. This in turn allows bigger plants to move in who need this deeper soil to survive. These bigger plants often create so much shade for the little earlier plants that they cannot survive, and so the plant community changes from tiny annuals to more established large grasses and perennials that come back each year.

And so the process continues, the bigger plants grow and die, alter the soil conditions making it deeper, richer and less stony. Shrubs start to be able to survive as do shade intolerant trees such as pines, which can survive as they are the tallest plants around. Not for long though! The shrubs and pine trees have created a new taller community, and the changes they make to the soil might have changed what can grow there, once again shifting out some of the older hardy but not-very-competitive species. However, these big plants are about to be foisted out themselves. This is by the final stage in succession, where large mature forests made up of shade-tolerant trees rule. This includes trees like oak, maple and hickory, which block out the light from the shade intolerant plants making it hard for them to survive.

This final, tall stable community is known as a climax community and is the end point of succession. As a quick aside- forests are usually the climax community in more humid parts of the planet, however other communities like grasslands can be the climax community in less humid environments.

Why everything isn’t forests?

If nature is constantly going along this process from bare rock to mature forests, why isn’t the whole planet covered in these forests? That’s because the natural world is not stable. Whilst over time plant communities dutifully march along the process of succession, every now and then they get knocked back. Perhaps it is dramatic- a volcano erupts completely covering a plant community in molten lava, killing it and replacing it with bare rock again once the lava cools and starting succession from scratch. Maybe it’s not so dramatic- a forest fire burns all the trees meaning the mature forest disappears and the shade intolerant plants earlier in the stage of succession are no longer overshadowed. The littler plants can survive and grow again, until slowly over time succession changes the community back into a mature forests. These events cause succession to reset to an earlier point where it must start progressing from again.

Maybe shrubs and tree saplings are eaten away by grazing animals like sheep or kudu each year. This means the plant community is constantly being reset to an earlier stage and so doesn’t have time for succession to occur before it is reset again. The Scottish Highlands are a good example of this resetting- it is a region that was formerly largely forest covered, yet human felling of trees followed by widespread introduction of sheep mean that it has been shifted from a stable climax community to what is nowadays a stable intermediate community or largely grassy moors much earlier on in the process of succession.

The succession which occurs from barren land such as rock and sand is known as primary succession, whilst succession that happens in an area that has received smaller scale disturbance like grazing or even a forest fire but hasn’t had the entire soil community wiped out is known as secondary succession.

Heavy grazing by sheep in the Scottish Highlands has halted succession, which is one of the reasons why much of the area is no longer covered in forests

How do we study it?

All this happens over variable timescales, but it can be really really long. Especially the earlier stages of succession can take hundreds of years. Not only do pioneer species need to arrive, often doing this by wind or being somehow dropped, moved or pooped out by an animal, they also then need to grow, die, decompose, have offspring that grow, die and decompose and so on. The later successional stages can take shorter amounts of time once soil is more established and more animals are attracted to the area carrying and dropping more seeds in the process. But so if this all happens over quite a long period of time, how do we study it?

One lucky occurrence really helped scientists understand this process. This was a volcanic eruption that happened in 1963. This eruption happened on the sea floor off the southern coast of Iceland, and lasted for four years spewing out lava which formed a volcanic island. This island, named Surtsey, became a really useful model for studying succession. As a remote, uninhabited island it was not disturbed by humans and was protected by being designated a World Heritage site.

Being a recent formed island scientists could track exactly when and how the plant community changed. With periodic trips to observe the island, the scientists tracked which were the first plants to arrive (mosses then lichens) and how they arrived (carried by birds who fished in the seas surrounding the island), then how long they took to grow, when and by what they were replaced and so on. By 2004 there were 60 species of vascular plants (e.g. grasses), 75 bryophytes (moss like), 71 lichens, 24 fungi and 89 different species of birds. Whilst shrubs are now present on the island it is still marching along in the process of succession and being studied and scrutinised along the way.

The volcanic island of Surtsey off the south coast of Iceland (Picture: http://www.arctic-images.com)

The natural world is constantly changing in one way or another, whether it is the cycling of the seasons, the cycle of life in birth, death and decomposition leading to new life, or through the shifts experienced by entire plant communities in succession. As for climax communities, one could argue that there is no real 100% stable climax community as even old oak forests are disturbed by weather and animals and humans. Also assuming that because succession moves in the direction of climax communities these are the ideal plant communities isn’t really true either. Sure, they move in that direction, but unless you are asking a very specific question such as which plant community is better for me to grown my petunias in, one stage of succession e.g. a mature forest isn’t necessarily inherently better or more ideal as an ecosystem than a disturbed grassy area filled with little plants. They’re just different.

For more info:

A virtual time lapse of an island experiencing succession:

Music: kongano.com and MrSethSongs

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