Old Field Succession

            The form of ecological succession which occurs on abandoned farmland is referred to as old field succession.  When a plowed field is abandoned, it represents a new habitat for plant and animal species to colonize, but because it is basically bare soil, it is a stressful habitat for many plants.  There are no trees to provide shade or to serve as windbreaks.  Continuous exposure to sun and wind may cause high rates of evaporation and water stress for plant species.  On the other hand, there are plenty of light and mineral nutrients available in these sites.  This environment is first colonized by a group of species called pioneer species.  Pioneer species are usually characterized by having long-lived seeds capable of remaining dormant in the soil for many years, long-range dispersal ability, and the ability to utilize resources rapidly, allowing them to grow and reproduce quickly.  Many of them are included in that group of species commonly called weeds.  Among the earliest species to arrive are ragweed, crabgrass and foxtail.  After a year or two, these are joined by various species of asters.  These pioneer species change the environment; as they die, dead plant material (plant litter) accumulates on the soil, and this helps to hold water in the soil.
            When an old field is three to ten years old, perennial grasses and herbs gradually replace the annual and biennial pioneer species.  A typical dominant perennial herb in old fields of eastern North America is goldenrod.  Many of the grasses are bunch grasses, which grow in clumps; little bluestem (Andropogon) is common in New Jersey fields.  The perennial herbs outcompete the pioneer species, which are unable to germinate and grow under the dense perennial cover.  After about ten years shrubs begin to reach noticeable size.  Early in this stage the field will be a mixture of perennial grasses and small shrubs.  Common old fields shrubs include sumacs, multiflora rose, cherries, bayberry and spicebush.  As the shrubs grow they shade the ground and many of the shade-intolerant perennial grasses and herbs become less numerous.  Eventually the filed will be nearly uninterrupted growth of tall shrubs.  bines such as poison ivy, Virginia creeper and Japanese honey suckle may also grow among the woody species.
Eventually tree seedlings begin to sprout among the shrubs.  The seedlings of most tree species of the mature community are shade-tolerant, so that they can begin to grow under the cover of shrubs.  As the seedlings grow, the field will become a mixture of shrubs and young trees, and it will begin to resemble a woodland.  As the tree species that dominate the mature community begin to reach their maximum size, they shade out the shrubs beneath.  In a mature forest, other shrubs which are shade-tolerant will grow beneath the canopy trees; maple leaf viburnum and flowering dogwood are the most common ones in this area.
As succession proceeds the communities differ in a number of ways.  First stage successional communities consist of a relatively few species of herbs, most of which are annuals.
These plants are quick-growing, and are most successful in full sunlight.  Species diversity is relatively low, and most of the plants have similar growth habit, that is they are herbaceous.  This community lacks vertical structural complexity and provides only a few types of habitats for animal species. During the next stage of succession, perennial grasses, perennial grasses and herbs replace the annuals and species diversity increases, but most of the plants are still herbaceous and there is not much structural diversity.  Still later in succession, shrubs begin to grow to noticeable size and both vertical structural complexity and species diversity increase.  The mixture of shrubs and herbaceous forms presents a greater diversity of food sources and habitats for animal species.
In the final stage of succession, the mature forest, species diversity and vertical structural complexity reach their peak.  Forest structure is distinctly layered.  The tallest trees form the uppermost canopy layer.  Beneath the canopy there may be one or more subcanopy layers, usually comprised of young canopy trees.  Below the subcanopy is a shrub layer, and beneath the shrubs the plants on the forest floor make up the herbaceous layer.  The plant species that make up the layers beneath the canopy are shade-tolerant species, since much of the sunlight is blocked by the canopy species.  Dead and decaying material on the forest floor is called the litter layer.  The diversity of species and plant growth forms present in the mature forest in turn provide habitats for a greater number of animal species.

Succession is a continuous process and does not always proceed in neat states.  You may find annuals like ragweed and perennial grasses growing near each other; the transition from annuals to perennials is a gradual and patchy one.  You may notice that different parts of the same old field appear to be different stages of succession, or that some parts of older fields seem to be at an earlier stage of succession than parts of younger fields.  One of the most common reasons for this is secondary disturbance, especially fire.  Fire may not destroy enough of the vegetation to return a field to the initial "bare ground" condition, but it may allow pioneer species to successfully reproduce, maintaining the site at an earlier stage of succession.  You may also find plants in the mature forest that are typical of earlier successional stages.  These often colonize gaps which are created when mature trees die or are blown down during storms.  Succession will occur in these gaps on a small scale.  A mature forest actually contains a patchwork of gaps in different stages of recovery.