Natural ecosystems are unstable and remain in a constant state of change. Where laypeople see a static forest, plant succession specialists see a single frame from a never-ending film. Succession is the gradual change of one community of living organisms into another. For instance, in just a few years, a pine forest can transform into a fir forest. These changes happen under the influence of various internal and external factors.

Successions can be of two kinds: primary and secondary. The biggest succession event began with the birth of life, when organisms started to interact with each other. Depending on the environment, primary successions can be xerarch or hydrarch in type: xerarch successions occur in dry surroundings, while hydrarch successions occur in areas abundant with water.

Secondary successions are categorized by influencing factors. Autogenic successions happen under the influence of internal forces (a fallen tree is replaced by a new one), while allogenic successions happen due to external factors (such as if the tree is felled by humans).

Xerarch succession

Xanthoria, or the orange wall lichen, is the first sign of a xerarch succession. On their own, fungi and algae cannot survive in dry conditions; but if they combine their forces, as is the case with lichen, they help each other stay alive. By excreting chemicals, the fungus slowly destroys the substrate, producing a micron-thin film of soil, which is just enough for the algae to take hold.

The next stage involves green moss, the kind you’ll find on pretty much any boulder in the Leningrad Oblast. But succession can happen in the most unusual places, too; you’ll sometimes see stone steps and masonry covered with moss in abandoned parks and estates. The destruction of architectural landmarks due to succession is inevitable in our climate. Xanthoria can quite literally consume marble and granite. St. Petersburg, for instance, is home to more than 100 types of marble-eating lichen. Humans have no way of fighting it, either, as even a micron-size piece of lichen will keep growing back. Instead, people have come to develop ways of using the first stage of succession for aesthetical purposes; you’ll often see concrete walls decorated with artworks made of moss.

Marble statues in St. Petersburg's Summer Garden. Credit:
Marble statues in St. Petersburg's Summer Garden. Credit:

Hydrarch succession

One of the examples of hydrarch succession is the Lake Suuri in Karelia. The water level dropped two meters, but the silt level remained the same; as it continued to accumulate, many plants that were previously unable to sprout by the shore began to grow. This process continues even despite high precipitation or rising water levels. In ten years, parts of the lake will become a swamp.

Another example are floating bogs. Those are bits of vegetation on water surface that stick together and, over time, become a single floating “pillow” up to 20 square meters in size. It may take some 50 years for one to form, but it only takes a moment to destroy it by flipping it over.

Floating bogs will often connect with each other into a single system, and trees will even grow on them. If you’re walking through the woods and find yourself in a clearing with squishy, unstable soil under your feet and swaying trees, you’re not actually standing on ground, but rather a bog floating on top of a lake up to ten meters deep. The trees in such places can be felled with a single push.

The Lammin Suo peatland near St. Petersburg. Credit:
The Lammin Suo peatland near St. Petersburg. Credit:

Autogenic succession

Birch trees provide young fir trees with shelter from snow, which is why you’ll often see old, withering birch trees being “hugged” by sturdy young firs. What actually happens is that the latter taps into the former’s root system and saps it of nutrition. After a while, the birch trees in the area die away and are replaced with a thick fir forest.

This succession, in turn, changes the grass level below the trees. First comes the green moss, but once the microclimate becomes drier (as happens every ten years), the green moss is replaced by blueberry bushes since their longer roots allow them to spread quicker than other plants.

Allogenic succession

One of the most noticeable and radical types of succession is allogenic. It happens under the influence of external factors and may sometimes lead to pyrogenic succession (i.e. one that is caused by fires). If a single tree falls, ants and other insects cause it to rot and fall apart, and a new tree eventually takes that spot. Conversely, if a sudden windfall causes many trees to fall, nature is unable to process them on its own. A build-up of dry tree material creates the conditions for a ground fire, which spreads through forests with the speed of a train. Such fires are not put out, but rather localized. Once an area has burned out completely, new plants sprout from the ash: pines, fireweed, raspberry, strawberry and others. Seeds that had once planted themselves in the soil and failed to sprout are reawakened by the warm soil.

For the next fifty years, the area is home to herbs, grasses and sprouting trees. It is diverse at first, but later becomes more homogenous due to allelopathy: an ability of some organisms to excrete chemicals that suppress or dampen the growth of others. In 100 years, a dominating tree species emerges in the area and, finally, 200 years later pyrogenic succession ends with the arrival of firs.

Pastoral succession

Livestock grazing exerts a great deal of stress on the soil. The first stage of pastoral succession is the growth of various thorn plants which aren’t eaten by livestock and are thus more likely to survive. As more of the area becomes covered with thorns, goats are brought to pasture due to their ability to eat them. As these animals tear plants out of the ground, the soil becomes damaged and fails to evaporate water. The resulting “pillow” swells up and slides off during heavy storms.

Olga Vershinina
Olga Vershinina

Effects of succession

The main result of succession, the so-called “climax community”, is a fully-developed, harmonious biological community. Such communities consist of various soil types, a lush undergrowth and a diversity of grass and shrubbery. Climax community is less one thing and more a system of co-existing subclimax communities.

Biologist Olga Vershinina delivered a lecture on the topic as part of the talk show “Break Down by Atoms”, organized by the St. Petersburg Information Center for Nuclear Energy.