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Disturbance and Succession

Succession is the term that scientists use for the vegetation changes in plant communities over time. Succession works at many scales, from the thousands of acres burned in Yellowstone National Park several years ago to the comparatively tiny gap in the canopy left when a tree routinely topples in a forest.

Early ecologists believed that there was a fixed and predictable end point to succession, which they termed the climax stage, and that this climax stage was stable and self-perpetuating for long periods of time. They believed that vegetation changes in a plant community are inherent in the community itself, and outside factors such as storms or fire play little or no role in the process. Today, ecologists realize that no self-perpetuating end point is ever really reached, and that periodic disturbance - fire, flooding, damage by insects and diseases, and windstorms, to name a few - plays a critical role in maintaining the diversity of species and habits in a region. Rather than an anomaly that occasionally disrupts climax communities, disturbance is now viewed as the key recurring factor that keeps a mosaic of habitats in different stages of vegetation development in fairly close proximity to one another. This, in turn, assures the presence of a diverse mixture of plants and animals that characterize each phase of the change from, say, bare land to mature forest.

Although how vegetation succession or change occurs is more complicated than previously thought, what will ultimately happen in most places is still generally predictable. It certainly is true that specific types of vegetation will eventually predominate on most sites in particular floristic provinces.

In the Eastern Deciduous Forest province, vegetation change on abandoned farmland left undisturbed for many years will work something like this, with regional variations: Millions of seeds that lay dormant in the exposed soil germinate, causing an explosion of physiologically tough, aggressive annuals like horseweed and common ragweed. These plants, called pioneer species, dominate the first season. In a few years, biennials (today, many of them non-natives such as common mullein and Queen Anne's lace) become common, along with a few perennial wildflowers like asters and goldenrods. After five years or so, grasses and wildflowers turn the area into a meadow. Within a few years young maples, ashes, dogwoods, cherries, pines, and cedars, many present as seedlings in the earliest stages, rapidly transform the meadow into "old field," an extremely rich, floriferous blend of pioneer trees, shrubs, and herbaceous species particularly favored by wildlife. Given enough time without major disturbance, perhaps several centuries, a mature or old-growth forest will once again be found on the site.

Succession and Disturbance in the Garden

The twin forces of succession and disturbance are constantly at work in the garden, just as they are in the native landscape. When you plant on bare earth you are creating a plant community. This community is a human invention, but it will change with time. You not only set in motion the forces of succession but ultimately become the agent of disturbance as well.

In traditional ornamental gardening, the hand of the gardener must always be at work to quell the influence of the dominant native plant community. Consider what happens when you weed your flower border. When you weed, you are thwarting succession. Annual weed seeds exposed by tilling and planting germinate and grow to cover the bare soil. Wildflowers such as goldenrods may blow in as seeds on the wind and find an empty spot in the garden. In forested regions, tree seedlings germinate and start the long process of reforestation. Without constant tending, weeds, unplanted wildflowers, and tree saplings would quickly overtake the garden, turning it into a thicket.

Even in the natural habitat garden, it is often beneficial for the gardener to guide change by thwarting succession, in this case by substituting for natural agents of disturbance such as fire. Meadows remain open grasslands because annual mowing keeps trees out. A meadow would otherwise soon start to become a forest. Prairies are renewed by wildfires that burn off the thatch, clearing the soil and releasing nutrients. Thankfully, most of us do not have wildfires in our gardens. To keep a prairie healthy, however, we must either mow it, or do a careful prescribed burn. We must lend mother nature a helping hand. The California chaparral is still another fire-dependent community. One of the reasons wildfires are so devastating in southern California is that regular fires are suppressed. Without regular burning, the large amounts of flammable litter accumulate. When a fire finally does come, it has so much fuel that it spreads out of control, consuming houses as well as native vegetation.

When we build a garden in a woodland, we are acting as agents of disturbance, too. If we thin the canopy or remove a tree, we are initiating what ecologists call gap phase succession. We let in more light and alter the dynamics of that piece of forest. When we plant, we disturb the soil, allowing new seeds to germinate. All our actions as gardeners have consequences.

In the interests of both low maintenance and environmental harmony in the garden, it is important to understand and work with natural forces like succession.