You've probably noticed that if your neighbor stops mowing their lawn regularly, it doesn't take long for the weeds to spring up. And, if you've driven by an abandoned farm house, you may have seen shrubs growing up in what was once fertile pasture. In most parts of the world, if a plot of land is left alone long enough, it will naturally pass through a series of plant stages and eventually become covered with trees. This somewhat predictable pattern of changing communities of plants becoming more complex over time was formally described by H. C. Cowles as the process of succession.
In developing his theory, Cowles noted that, at each stage of succession, one can expect certain characteristic groups of plants to occur together. He called these stages seres. Anyone who has done any gardening at all can describe what happens to a plot of bare ground if it is left alone for a while. With rare exceptions, Nature never leaves the ground uncovered.
In a typical successional scenario, the first plants to spring up are the pioneers - plants that are specially adapted to grow quickly on bare soil. We used to call them weeds, until we understood that they have an important role in the ecosystem. Pioneering plants generally produce lots of small seeds that are easily dispursed far and wide by the wind and by animals. The seeds also have the ability to lie dormant for a long time - sometimes years - so that they can wait for a time when conditions are right. When the tiny seeds eventually feel the Sun and the rain, they can sense that an opportunity has arrived, perhaps due to a space opening up in the vegetative cover above them. They germinate and grow quickly, covering the ground, which has the effect of helping to conserve moisture and protect the soil from erosion by falling rain and wind.
A bit slower to germinate and to grow are the grasses. They fill in between the pioneers and spread by underground rhizomes, developing a dense network of strong roots that help to hold the soil together and also add to its moisture holding capacity. The pioneer plants add to the fertility of the soil as well, some by pulling nitrogen out of the air and fixing it in the soil, others by growing deep roots into the subsoil and bringing nutrients up to the surface. All of these plants together build thriving soil communities; their networks of roots loosen the soil and provide pathways for the many microscopic and larger organisms that begin to cycle nutrients back and forth.
This improved soil with sheltering tall grasses and beneficial pioneer plants becomes the ideal place for shrubs to sprout. Known as perennial plants because they continue to grow year after year, the shrubs can grow much larger than the plants that came before them - assuming that nobody comes along with a mower. Their spreading branches provide a more permanent shelter for the soil, further improving moisture retention. Many of the shrubs are also good at gathering nitrogen or sub-soil elements. Their nutrient-rich leaves eventually drop onto the soil below, beginning to build a layer of mulch on the ground, which is composted by soil organisms, further feeding the web of life below ground and above, and building the humus-rich soil which is a characteristic of the forest.
In this rich, moist soil with deep shelter, tender young trees are able to germinate and grow. The faster growing softwood trees are the first to emerge above the shrub layer. They may take 5 to 10 years to reach maturity but, for a while, these trees will dominate the developing forest in a sere that is often called a woodland forest. Most of our favorite fruit trees are softwoods, and many important trees in this group also specialize in fixing nitrogen or accumulating nutrients from deep and far, continuing to build healthy forest soils.
In a story that is much like the Tortoise and the Hare, hardwood trees are much slower to grow - often taking more than 20 years to reach maturity - but they typically grow much larger than the softwoods. Towering above the smaller softwood tree layer, hardwoods slowly form the canopy layer in the maturing forest. Because of their slower growth habits, the wood of hardwood trees is typically stronger and often highly valued as construction material. Many hardwoods produce nuts that can be an important staple crop, and many are also nitrogen-fixers. The huge amounts of nutritious leaf drop from the big trees continues to drive nutrient cycling in the soil, further building healthy forest soils.
In time, the canopy of hardwood trees fills in high above and eventually shades out most of the other species of plants that have helped to build a mature forest. The theory of succession suggests that this sere of mature hardwood trees could continue in this state indefinately. Cowles called this state a climax forest.
An important element of the theory of succession is the concept of ecological disturbance. From time to time, limited parts of a forest will experience an event that causes the plants living there to be reset to an earlier stage of succession: an old tree falls and leaves a gap in the canopy above, or a severe storm knocks down a group of tall trees on a ridge-top, a large forest fire can kill many trees in an area, leaving little but charred ground. In each case, the disturbance is an opportunity for plants that thrive in an earlier sere. Seeds of pioneering plants, blown in on the wind, soon take root amid the ashes and begin the process of building a forest again.
A disturbance need not be one that takes an entire ecosystem back to bare ground, but could be as small as a browsing deer taking the budding tip off of a young branch. In each case, Nature begins again from where it finds itself, and the patterns of succession continue. As a result, a large forest will likely never reach a uniform stable state with all of its trees belonging to the most mature sere, the climax forest. In the real world, the land will be covered in a mosaic of plant communities in various stages of the process.
Succession was sometimes taught as a linear process, with nature following a pre-determined path from beginning to end, but it seems that Cowles understood it as more of a fluid process, somewhat like the eddies of a stream. Although time moves along like the stream moves downhill, many places swirl from time to time and move in an opposite direction to the expected flow. We are learning that this constant mixing contributes to the synergies that occur in functional plant communities and that disturbance is an important feature of a healthy natural ecosystem.