Forest Dynamics – On Wildlife and Ecological Succession
Landscapes are ever-changing, constantly evolving under the influence of both natural and unnatural factors. Landscapes can change suddenly (as in the case of fires or flooding), or very slowly over long periods of time. This slow change to the landscape is facilitated by members of the ecological communities within it – including wildlife, plants, and microorganisms.
The plant community and the soil structure and composition its built upon play a large part in shaping the types of wildlife that exist within a landscape. Paradoxically, the wildlife on the landscape play a role in the makeup of plant communities. Wildlife is often a vehicle for the spread, pollination, and even suppression of plant life.
The diversity of plant types and the stages of their maturity vary from landscape to landscape. When speaking of the stages of plant maturity in a specific area, they are often referred to as successional stages, or different levels within the concept of ecological succession, championed by Frederic Edward Clements in “Plant Succession: An Analysis of the Development of Vegetation”.
According to Encyclopedia Britannica, ecological succession is the process by which the structure of a biological community evolves over time. Often, this is referenced in relation to forest succession, particularly in the types and stages of trees and other plants that appear at different stages of a forest’s development.
Succession Stages
Generally, succession occurs in three steps – early succession, mid succession, and late succession. But more specifically, within those three broader categories, the stages of succession are (i) stand initiation, (ii) stem exclusion, (iii) understory reinitiation, and (iv) steady state (Cornell, 2016).
There are exceptions to this step-by step process of succession, as not all landscapes and biotic communities are made alike. Two such exceptions are “secondary” succession (as opposed to primary succession), in which the first stage of succession still contains some remnant members of the community previous to disturbance, and various “disclimaxes,” where a landscape is held back in its successional process by repetitive disturbance (McKelvey, 2015). We’ll touch more on these in Exceptions to the Rule later in the post.
As noted by Duke Forest, the general timeline of forest succession in the North Carolina piedmont region occurs as follows:
- 1st year: Horseweed dominant; crabgrass, pigweed
- 2nd year: Asters dominant; crabgrass
- 3rd to 18th year: Grass scrub community; broomsedge grass, pines coming in during this stage
- 19th to 30th year: Young pine forest
- 30th to 70th year: Mature pine forest; understory of young hardwoods 70th to 100th year: Pine to hardwood transition
- 100th year plus: Climax oak-hickory forest
Though this progression follows the succession stages within the piedmont, it looks similar throughout much of the state, with some regional fluctuations and species differentiation.
As illustrated in the above timeline, these transitions from stage to stage do not occur instantaneously and one after the other, but take place over many years and through many stages of microsuccession.
Let’s take a more extensive look at each stage of succession, how forests make the decades-long transition from stand initiation to steady state, and what each stage means for wildlife.
Early Succession – Stand Initiation
Photo Credit: Sujata Roy, NCWF Photo Contest Submission
Succession stages start following the disturbance of a specific landscape. Such disturbances can include land clearing, development, and natural events like flooding and fires. These disturbances typically remove much of the existing plant cover, exposing the soil to sunlight. This creates an opportunity for remaining plants, dormant seeds in the soil, and newly introduced seeds to establish themselves. This initial phase in succession is known as stand initiation.
During stand initiation, pioneer plants start to appear on the disturbed landscape. These are plants that generally thrive with abundant sunlight and can spread quite rapidly, but provide essential nutrients to the soil to further the successional process. There are many types of pioneer plants that take root at this time, including various grasses (like Johnson grass and sedges), wildflowers (like black-eyed Susan and goldenrod), shrubs (like sumac and blackberry), vines (like poison ivy and Virginia creeper), and certain tree species (like eastern red cedar, sweet gum, and some pines).
This stand initiation phase can last from 15-25 years, if left to continue in its developments without further large-scale disturbance. During this phase in particular, the status of the succession stage can be heavily altered by the presence of herbivores on the landscape (McKelvey, 2015), including white-tailed deer, which thrive in these early and mid-succession forest stages, where browse is prevalent, easily accessible, and holds increased nutritional content (Hobbs and Spowart, 1984).
In addition to deer, this succession stage provides critical habitat for many wildlife species, including small mammals (which use brush for shelter, cover, and forage), pollinators (who thrive on the prevalent blooming wildflowers and grasses in these areas), songbirds (many of which nest primarily or exclusively in brushy landscapes), and more.
Mid-Succession – Stem Exclusion
Photo Credit: George Bissinger, NCWF Photo Contest Submission
As certain species, especially woody trees that have the potential to outgrow smaller plants, continue to mature, they start competing for essential resources such as light, water, nutrients, and—perhaps most critically at this stage—space.
The crowns of these trees begin to encroach upon one another, shading the understory and reducing the light available to the lower plant life, which leads to a decline in understory vegetation and the lower branches of the trees themselves.
During this period, many pioneer species in the understory are displaced, surviving only along the edges of the treeline or in small pockets where light still penetrates.
This phase generally lasts between 20 and 30 years. Given that much of North Carolina has been developed for urban, agricultural, and residential purposes over the past two centuries, with many areas undergoing multiple cycles of use, the current forests largely represent late mid-succession to late succession stages. Few areas remain as old-growth forests.
Many wildlife species thrive in this successional stage, as it offers a balance between early and late successional conditions, supporting species that depend on a mix of plant types from each stage.
This phase provides ample nesting opportunities for songbirds, birds of prey, and squirrels, and also supports species like ruffed grouse and black bears in mid-succession temperate forests.
Late Succession – Understory Reinitiation and Steady State
Photo Credit: Dave Kiefer, NCWF Photo Contest Submission
As species progress into mid and late succession stages, they enter the understory reinitiation phase. During this period, trees reach a level of maturity and intensify their competition for resources. Their crowns, growing closer together, are buffeted by the wind, causing branches to break and smaller, less competitive trees to be displaced.
As these branches and trees are removed from the overstory, light is able to penetrate through the newly created gaps and reach the forest floor, allowing dormant seeds and understory plants to re-emerge.
At this stage, certain tree species may become dominant, depending on their lifespan and competitive abilities. Over the decades, as the forest continues to mature, dominant species may shift. The duration of the understory reinitiation phase and the transition from mid to late succession can vary widely, ranging from 60 to over 100 years before the forest enters the steady state.
The steady state is characterized by a mature forest with large, fully-grown trees, leading to the decline of early succession plants and sporadic presence of understory vegetation in the small openings where light reaches the forest floor. During this phase, the forest landscape remains relatively stable, with dominant species reproducing and excluding many other plant types.
This steady state persists until the forest is disturbed by human activities or natural events.
Several wildlife species depend on the mid to late succession stage of the forest, such as the red-cockaded woodpecker, which relies on mature longleaf pine trees (60+ years old) for nesting cavities. The loss of intact old-growth stands is a significant threat to this keystone species.
Other species that depend on mature forests include flying squirrels and various cavity-nesting birds.
Exceptions to the Rule of Succession
Though the general course of succession is the most common mode of forest development, there are certain factors that may contribute to alterations in the successional process, or the hindrance of it.
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Secondary Succession:
As previously noted, the successional process begins on a disturbed landscape. In the case of primary succession, this generally starts on virtually “lifeless” soil, such as soil cleared for construction purposes. However, some landscapes start at a different level of disturbance, where part or most of the plant diversity on the landscape still remains after the disturbance – or at the very least, the integrity and nutrition of the soil is still present in the landscape. Because of this “head start”, secondary succession often occurs faster than primary succession, though it may appear differently at various stages due to its fractured successional status.
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Disclimax:
There are various types of disclimax. But generally, disclimax refers to the continuous disturbance of a forest type during the process of succession, whether by repeated clearing of varying degrees, animal grazing, or other prolonged or repeated disturbance types.
Threats to the Process of Succession
Ecological succession is a natural process by which forests have developed over millennia. The slow progression of species and landscapes is essential for the right functioning of forest ecosystems, and cannot be rushed to reach a state of maturity.
But there are ways in which the process of succession can be interrupted, weakened, or otherwise altered that can cause problems for ecosystems and the species within them.
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Invasive Threats to Forest Communities
Many invasive plant species are known to take root in disturbed landscapes, finding a niche and then expanding even into more mature forest types. Certain invasive pest species thrive on trees of all stages, with high die-off rates in native tree populations.
When these invasive species gain a foothold in a forest ecosystem, the die off of trees promotes the unnatural intrusion of species from other succession types, staggering the succession stages of a single forest region.
As more invasive and exotic species are introduced to these environments, it is hard to predict how a certain forest type will respond to the alteration (McKelvey, 2015).
Though there are many invasive species that threaten these communities, here is a list of just a few:
- Hemlock Wooly Adelgid (Adelges tsugae)
- Emerald Ash Borer (Agrilus planipennis)
- Kudzu (Pueraria)
- Japanese Honeysuckle (Lonicera japonica)
- Laurel Wilt Disease
To see a full breakdown of threats facing native trees and forest communities of North Carolina, check out our blog post: Treetop Wildlife and Forest Habitats Face Threats from Invasive Species.
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Development
Development is an aggressive disruptor of the succession process, either destroying a forest at a certain level in its maturation, or stalling it for a long period of time – an extreme form of disclimax. This development takes many forms, including harvesting lumber, agricultural expansion, and urban sprawl.
In order for a forest to move through the process of succession, it must be able to move along without aggressive disturbance. However, there are some small-scale disturbances in the form of land and habitat improvement strategies like thinning and prescribed fire that may manipulate successions stages. These practices can actually improve the health of forests at various stages of succession, and provide habitat for the wildlife species that call them home.
Sources:
“Wildlife and Forest Stewardship” by North Carolina State University, 2019
“Forest Succession and Management” by Peter Smallidge, Cornell Small Farms Program, 2016
“Forest Succession” by Duke Forest, Duke University, 2024
Written by:
– Bates Whitaker, NCWF Communications & Marketing Manager
– Dr. Liz Rutledge, NCWF VP of Wildlife Resources