Ecosystems proceed through a series of stages (called "seres") as they age. In general, they progress in this order: Disturbance, grasses, shrubs, deciduous trees, and lastly evergreen trees. As with most things in nature though, the local variation is endless (this will be discussed at each sere, with the Pacific Northwest at the end. We're a little weird). The main driving forces in succession is access to sunlight, water, and nutrients. The availability of each of these changes as the ecosystem passes through each sere.
Seres of Succession
Disturbances can be anything that sets a portion of the ecosystem to a blank slate. This can be as simple and local as a large tree falling, or as widespread as fires. If the disturbance is a natural cause, such as fire, insect infestation, landslides, windstorms, or earthquakes, the following succession is called, "Primary Succession." If the disturbance is human-caused, such as logging or clear-cutting for space, the following process is called, "Secondary Succession." What happens after the disturbance is essentially the same whether it is primary or secondary succession. It is more of a distinction between human- or nature-caused successions.
After a complete disturbance, all organic matter can be removed (such as in severe fires or massive landslides). In this case, there is little nutrients to support most complex plant species. Grasses can grow in more nutrient-poor soils than many plant species, and provide the services of holding the soil together with their roots, and adding organic matter to the soil as they die. In this way, larger and more nutrient-demanding plants can grow in their place. Many dry ecosystems only get this far in succession, such as grasslands, steppes, and deserts.
As the grass' roots hold the soil together, water from rainfall is trapped in the soil more easily. With more water and organic matter available, larger and more complex plants start coming in. Shrubs are generally described as multi-branched plants ranging up to 15'-20'. Basically, they're taller than grass, but shorter than trees. They vastly add organic matter to the soil as they die, as well as bring in a lot more biodiversity to an ecosystem. Many shrubs produce berries, seeds, and leaves to attract primary consumers (birds, deers, mice, etc). As they grow taller and denser, they shade out the floor of the ecosystem, which often promotes plant growth in areas with too much sunlight. In some ecosystems, the sun causes the rainwater in the soil to evaporate before the plants have a choice to absorb it. Shading the ground allows the water to stay around for longer periods, allowing the plants to grow taller and more complex.
With plenty of water and organic matter in the soil, trees are able to grow. Fast growing and photophilic (sun-loving) deciduous trees quickly establish themselves. Their falling leaves add even more nutrients and organic material to the forest floor. Their closed canopy and frequently densely packed trunks help to moderate the conditions inside the forest, reducing the drastic changes in temperature caused by winds and sunlight. Shading out the floor and providing so much nutrients paves the way for the large and very nutrient-demanding evergreen trees. On the East side of the country, it generally rains while the deciduous trees retain their leaves, so they are able to stay in dominance over the evergreen trees (see the section on evergreen trees).
Many evergreen trees do not do well with too much sunlight, and are generally shade-philic. They grow up under the closed canopy of the deciduous trees, fed by the nutrients in the soil left by previous plants that had died off. Some are more tolerant of sun and nutrient poor soil than others, so there is frequently a predictable order of species that appear as the conditions change within the ecosystem. In some ecosystems, evergreen species can outcompete deciduous trees, shading them out entirely. In those ecosystems, the rain that sustains the trees' growth comes in the fall and winter seasons, when the deciduous trees have shed their leaves. This means that the evergreen trees are growing year-round, whereas the deciduous trees are only growing in the drier spring and summer months.
Pacific Northwest Forest Succession
As mentioned, our forests behave a little oddly. We have no native grasses, so skip that sere entirely. Most of our shrubs are shade-philic, so they even wait until the deciduous trees appear. Our main pioneer species is the Red Alder. When you find a stand of Red Alder trees, it is very likely that the forest had been disturbed recently. Alders have the added benefit of a bacteria on their roots to fix nitrogen from the air and add it to the soil, making it available for the trees' roots. Big Leaf Maple* begins appearing once enough Nitrogen has been added back into the soil. This step is accelerated if the disturbance did not remove the organic matter from the soil., so the Nitrogen left over from the previous plants is still available.
Once the alders have started shading the forest floor, shrubs such as Salal, Oregon Grape, Sword Fern Salmonberry, and Vine Maple start appearing. Within the shrub species, there is a range of sun and shade tolerance, with the evergreen shrubs tending toward the shade, and the deciduous plants growing more in the sunnier areas. As mentioned in the section above, these plants add a great diversity to the forest, both in terms of the variety of shrub species, but also in the number of consumers they draw into the forest.
Once the canopy has been closed, the evergreen trees start appearing. The fast growing Douglas Fir and water loving Western Red Cedar* show up, followed by the slow and steady Western Hemlock. In our particular ecosystem, a large majority of our rain comes in the fall and spring, when the deciduous trees have shed their leaves. Because of this, our evergreen trees get the leg up on the Alder and Maple trees, and in many ecosystems can shade them out completely, often leaving stands of purely evergreen trees. The shade-tolerant and -philic shrubs often stick around, causing the dense, dark, and green forests that we are well known for.
* There is some debate among the IslandWood staff as to the exact order of trees that appear in the Pacific Northwest. Most can agree though that Red Alder appears first, and Western Hemlock last. Much of the confusion lies where Douglas Fir and Western Red Cedar fall. A lot of the disagreement can be explained by local variation, since the ecosystem's topography, water availability, soil and substrate, slope, aspect, and the disturbance all play a factor is what species appear when. So many of the answers discussed by staff could be correct, depending on the particular ecosystem.