This is how a mature forest is formed

It is well known that living beings change , species evolve over time, responding to a selective pressure applied by the environment that surrounds them and by the interactions between organisms.

And, likewise, ecosystems also change over time . An ecosystem as complex as a forest does not come into existence spontaneously, it is not something created , it is the product of continuous, slow and gradual development, over long periods of time, and through a series of stages, each one which is dominated by a number of organisms.

The process of development of a community over time, and the successive substitutions of the populations of some species for others, is called ecological succession . In general, in terrestrial ecosystems, the different phases of a succession are defined according to the plant communities they present, since being organisms of slow development, relatively high longevity, and generally sessile, they tend to have a more significant presence and defining. However, the entire ecosystem changes in the process, and with it, the associated animals and soil-dwelling microorganisms.

First phase: colonization by pioneer species

When live rock outcrops, when a lava flow from a volcano cools, or when a glacier retreats, exposing a bare valley, there are no plants, animals, or soil in the area. Just rock. A barren, bare rock setting . In the absence of soil, water is not retained; it simply flows to lower elevations, or infiltrates through cracks into the subsoil.

Well, in those environments is where the first colonization occurs . Lichens and mosses , which arrive thanks to the wind that transports their spores or by fragments carried by the water from higher areas, are usually the first forms of life that settle. They cling to the rock, and slowly but gradually, they wear it down. They form small communities, almost insignificant, but as populations develop, die and are replaced by new ones, thin layers of soil are formed. The corpses of these tiny forms of life are deposited in nooks, crannies and holes and filling them with fertile soil, capable of retaining water.

The seeds of the first annual grasses arrive blown by the wind. These plants have small and very shallow roots, so they are able to take root in very small areas of soil. The presence of mosses, which retain moisture, benefits them. They are species that, in a very short time, are capable of creating a large amount of biomass. As they are species that only live during spring and summer, and die after releasing their seeds, their remains when they die form a greater amount of soil than mosses, and in much less time.

Second phase: the intermediate communities

Once the amount of soil is sufficient to support somewhat more complex root systems, the first perennial herbaceous plants colonize the territory that the annuals have previously fertilized. A perennial plant is one that survives from one year to the next, thanks to resistance organs that it maintains under the ground, such as bulbs, rhizomes, thickened roots or tubers, even if it loses its leaves or all its aerial part during the winter.

In fact, although not all perennial plants lose their leaves, those that do are much more common, and this event further helps the process of ecological succession. The leaves that fall in autumn are metabolized by soil microorganisms and generate more soil , more volume of land that can be, in turn, used by larger species.

Gradually, herbaceous populations with more and more complex root systems, or even shrubs, begin to colonize the environment . These plants continue to form more soil, generating increasingly complex interactions between them. Competition appears, and larger animals are installed in the ecosystem.

At a point where soils are already complex and deep enough, the first trees begin to populate the environment. Their strong roots reach deeper than those of herbs and shrubs, to bedrock. They infiltrate between the cracks that the previous communities had opened and break it, allowing the deeper layers of the soil to form, and favoring it to settle.

The forest, the climax community

As conditions improve and all the horizons of a mature soil are established, new tree species replace old ones . Ecological relationships reach maximum complexity, and a balance is established in the ecosystem.

This ecosystem is called forest. —or mature forest, as opposed to the immature forests of intermediate communities—. In the process of ecological succession, it is the last phase and constitutes the most complex ecosystem. the climax community

Of course, this process of ecological succession is different depending on the environment to be colonized. The same thing does not happen in a limestone outcrop in a temperate climate, than in the lava flow of a Canarian volcano , or in the land devastated by the explosion such as that of the island of Krakatoa.

Ecological succession processes also occur in aquatic ecosystems. Depending on the environment, the process will have other species, with other types of communities, but basically it goes through the same phases. The final community of the natural succession of the bottom of the Mediterranean Sea is the Posidonia meadow , and not a forest; but it is still a climax community.

Primary and secondary succession

In ecology two types of succession are distinguished, primary and secondary. The process explained in this article corresponds to primary succession, and defines the change in species composition over time in a habitat that has not previously been populated by any organism. Lava flows or live rock outcrops are key sites where, if an ecological succession process begins, it would be primary. In these places not only is there no previous life, but there is not even a soil.

Secondary succession is one that occurs after a disturbance of a pre-existing ecosystem. It does not matter whether this disturbance is more or less serious, or whether it has a natural or anthropogenic origin. A flood, a forest fire, a massive logging event, the abandonment of a crop field… are events that can devastate an ecosystem, but remains of vegetation, fungi or microorganisms still remain. This type of land still retains some soil, which greatly facilitates the installation of new plant communities, and helps the succession process to be very different.

References:

Chang, C. C. et al. 2019. Ecological succession in a changing world. Journal of Ecology, 107(2), 503-509. DOI: 10.1111/1365-2745.13132

Krivtsov, V. et al. 2000. Indirect regulation rule for consecutive stages of ecological succession. Ecological Modelling, 133(1), 73-81. DOI: 10.1016/S0304-3800(00)00281-7

Pielou, E. C. 1966. Species-diversity and pattern-diversity in the study of ecological succession. Journal of Theoretical Biology, 10(2), 370-383. DOI: 10.1016/0022-5193(66)90133-0