Ecologists and others who study forests are always curious about the past and the future. They ask questions like, “What was the forest like at Avondale Park before settlers arrived?” and “What will the forest at Avondale Park be like in a century?” One trick scientists use for deciphering the past or predicting the future of a forest is to apply the principle of ecological succession. You, too, can do this by learning this principle and some of the tree species in your area.
Ecological succession is the process by which some species more or less predictably replace others as an ecosystem gets older. A young forest is usually populated by trees that grow quickly and need exposure to full sunlight throughout the day to survive and grow. Such species are known as early successional species. Here in the Southeast, Yellow Poplar, Sweetgum, and most pines are good examples. By the time these trees mature and produce seeds, the forest understory is shaded, and the seedlings from these trees cannot survive under the mature tree canopy. Mature early successional trees developed a survival strategy by producing small seeds that disperse long distances to get their offspring to new sunny habitats.
Another group of trees, known as late-successional species, have offspring that can survive in the shady understory beneath a canopy of Poplars and Sweetgums. These species grow slowly but can out-compete the early successional species and eventually take over the canopy (note that some tree species, such as many oaks, fall somewhere in between and are known as mid-successional species). Late-successional species include many of the South’s hickory and magnolia trees. In contrast to the early successional species, seedlings of these late-successional tree species can survive in the shade of their parents. Thus, late-successional species have the capacity to populate a forest indefinitely once they are established. But, thanks to ecological disturbances, these species don’t dominate forever.
An ecological disturbance is an event that significantly alters the composition and structure of an ecosystem. Severe disturbances can cause trees to die and forest succession to begin again. Sometimes this occurs in a small patch of forest, such as when an individual tree is killed by disease or lightning strike or is tipped over by strong winds. Sometimes entire swaths of trees die due to drought, fire, or big storms, such as hurricanes or tornados. In all these instances, the break in the tree canopy allows sunlight to reach the forest floor, and seedlings of early successional tree species get another chance to establish and grow.
With an understanding of ecological succession, plus the ability to identify a few of the trees in our area, you can begin deciphering the past of a forest – or predicting its future. Let’s try this at Avondale Park. Above the amphitheater and the ball fields are woodlands of large trees, with no seedlings or mid-sized trees. This habitat is kept artificially open by the periodic mowing and cutting of small trees. The large trees in the canopy are mostly mid- to late-successional oaks, including White, Post, Southern Red, and Northern Red Oak. Because these species can grow in partial to full shade, this suggests that there was a mature or maturing forest here when these tall trees were young. When the forest was cleared, these trees were probably left to provide shade for picnickers and zoo animals. The periodic mowing and clearing maintains these zones as artificial woodland where only large trees can be found. The woodlands above the amphitheater and above the ball fields will gradually lose the large oaks over the next few decades as they succumb to storms and rot. Unless park managers create opportunities for young trees to establish, these areas will become increasingly open, sunny, and barren.
Above the spring is a dense forest of many species. The largest of the trees here are mid- and late-successional oaks similar in size to the oaks in the adjacent woodlands. But surrounding them are smaller trees of early successional species such as Black Cherry and Sweetgum. What could cause this unusual combination of early and late-successional species? It’s likely that the forest patch above the spring was once cleared of most trees. Perhaps this was a single event of forest clearance, or perhaps this area was maintained as an artificial woodland for a while. The few oaks that were not cut have grown to large sizes and are now the biggest trees in the forest. Then, sometime in the second half of last century, park managers stopped clearing the area between these oaks. This allowed young trees to establish and begin a race for the canopy in the spaces between the large oaks. Early successional species were able to move in quickly and beat most of the competition in the race to the canopy level. Many succeeded, but they are skinny and have narrow canopies due to competition with the large oaks for water, sunlight, and soil nutrients. The presence of a few young mid-successional oak and late-successional hickory trees in the midstory of the forest shows that succession continues. Over time, these shade-tolerant species will overtake and exclude the cherries and gums that currently fill the gaps between the big oaks. And they will eventually take over the spaces vacated by the large, old oaks as they die off.
The above interpretations are the first impressions of the author during a quick survey of the forest and are not to be taken too seriously. Scientists could test these or other hypotheses about the history of the park’s forest by using tricks such as dendrochronology, which is the study of tree rings. This can reveal the age of trees, which can be used to help determine the history of a forest much more precisely. On a final note, remember that trees and forests do many good things for us. They beautify our landscape, provide shade and keep the surrounding area cooler in summer, filter the air of particulate matter, and provide habitats for wildlife. They also help rainwater infiltrate the soil and reduce erosion by intercepting raindrops. Whenever possible, it is in our best interest to promote the growth of native trees in our yards, neighborhoods, and parks.