Succession to a climax ecology - annuals to trees and soil formation
Bare soil is rare in nature. Seed blown in from annual wildflowers - or dormant in the soil - germinates and begins the battle to survive by competing for light, nutrient, and moisture. Thus begins the succession from annual through perennial, scrub and briar, and shrubby woodland to tall canopy trees. Annual plants are always the pioneers on bare soil. They do not waste too much time in competing with each other, but produce as much seed as they can. Thus they are OPPORTUNISTS. The soil is improved by the annuals covering and protecting the surface, loosening the soil and adding to its fertility when they compost-in as they die off.
Perennial wildflowers appear after annuals and they gradually squeeze out the annuals. They do this because they can spread so persistently by expanding the size of the clump, sending up new shoots each year and building a large, permanent, root mass. This large root mass acts as a storage organ allowing them to last through the winter and immediately start regrowing in the spring. Perennials want to hold on to their gains and live for a number of years - they are called EQUILIBRIUM species.
The permanency of perennials and their root depth allows soil production to take place, and also probably accumulation of nitrogen from fixation and then mineralisation. This creates conditions in the soil that begins to favour the more woody species such as briars, gorse and other thorny shrubs. Woody species ripen to produce bark and hard wood each year and this takes a lot of energy. However, it has advantages in that it creates more robust plants that can clamber and reach up. Thus they can climb over the perennials, blocking their light until finally they in turn are also overshadowed as the canopies of trees begin to dominate. Silver birch is often seen as the first tree to appear because of the massive quantity of seed released each year and its quick growth. Ash and hawthorn are also fast growing trees, but the success or dominance of any one species depends on the soil conditions and the nature of its competitors. It is probable that the succeeding plant communities of annuals, perennials and shrubs are necessary to condition the soil and therefore make it ready for trees – the CLIMAX SPECIES – to thrive.
Soil was formed from weathering of bedrock over many thousands of years to produce smaller particles and free minerals. Soil is still being made today by trees in the cyclical process of drawing up minerals from the topsoil and subsoil, and returning them to the surface of the soil through the fall of leaves, fruit and other detritus onto the ground. The bacteria, fungi and small creatures then incorporate these leaves and other litter into the soil.
Mark Fisher - Permaculture Design course handout notes