Ecological Succession at the Lehigh Gap

Ecological Succession and the Lehigh Gap

| by Dan R. Kunkle |

The following article is from Summer 2004 issue of Wildlife Activist.

Succession is an important ecological concept. It refers to the gradual, predictable changes that occur in the flora and fauna of a given place over time as an ecosystem develops.  We are all familiar with succession in an abandoned farm field or lawn. In our part of the continent, the end result of succession is a Temperate Deciduous Forest – a forest consisting primarily of a variety of broad-leaved trees that lose their leaves in winter. This final stage of succession is called the climax community. In the North Woods of Maine, the climax community is a coniferous forest; in Nebraska, it is a grassland community.   When a climax forest remains undisturbed for a long period of time, we call it an “old growth” ecosystem.  Only a few small stands of old growth remain in Pennsylvania.

Ecologists often refer to two kinds of succession: primary and secondary. Primary succession occurs when new land is formed, such as a volcanic island rising out of the ocean or the shifting sand of a barrier island creating new land.  Most succession is secondary – occurring in a habitat that has been altered by humans or nature.  Examples include the abandoned farm fields noted above, and areas where natural disturbances such as fires, hurricanes, or floods have occurred.

Succession has just begun on the degraded areas of the Lehigh Gap Wildlife Refuge. While a natural forest occurred on the slopes of the Kittatinny at Lehigh Gap in the past, those forests had been altered over time by lumbering, charcoaling, and fires set to encourage the growth of huckleberries at the higher elevations. But the most severe impact was that of the air pollution from the zinc smelter.  Life on the ridge at places was so completely destroyed that the succession that is beginning is much like primary succession – and primary succession takes a long time.  Time is needed to rebuild organic soils and a community of bacteria, fungi, worms, and other invertebrates.

The leaders of the Wildlife Center have decided to take the slow, but ecologically sound approach of restoration by succession on our refuge.  We are using native warm-season grasses to start the process.  These are the same grasses that followed the glaciers north and began the soil building process tens of thousands of years ago following the ice ages. We will accelerate succession somewhat by introducing a variety of flowering plants and scattered trees into the mix over time, but eventually, natural succession will take over, and a mature, old growth forest will return to the ridge in a few centuries.

We are documenting this succession scientifically and photographically, and will be using the refuge to teach about succession and a variety of other ecological concepts to visitors and school groups.  Succession will be a core message, relating to the human and natural history of the gap, to geology and pollution, and to the flora and fauna, past present, and future.

Pioneers on the Refuge

When you visit the refuge, you will notice some of the early successional species that are pioneering the new growth on the refuge. Following are some profiles of these pioneers. We are introducing some of these species, while the seeds of others are coming in on their own via wind or animals.

Warm-Season Grasses

The major species in our reintroduction efforts are the native, warm-season grasses. These are all deeply rooted perennial grasses that tend to grow in bunches rather than forming sod, especially in poor growing conditions such as those on the refuge.  Their deep roots allow them to grow all summer and some species reach heights up to 8 or 9 feet. They flower and produce seeds in summer or fall.  Their ability to colonize barren, mineral soils and avoid taking up heavy metals make them the plants of choice for our restoration efforts.  While eight different species are contained in our experimental planting mix, we feature three of these grasses that have pioneered the refuge on their own.

Little Bluestem (Andropogon scoparius)

Once the most common grass in the mixed grass prairies of the Great Plains, this grass is a common invader of abandoned farm fields in our region. Both bluestems are blue only in the early stages of growth each year.  Little bluestem colonized long sections along the side of the LNE (upper) railroad bed. Some of this was lost during construction to allow truck access to the test plot area.  It turns a beautiful amber color (a mixture of tan and wine coloring) in the fall.  Its seeds are preferred by finches and sparrows which cling to a stalk in fall or winter, pin the grass to the ground, and strip it of seeds.  The seeds are “bearded,” meaning they have a feathery attachment that allows them to blow in the wind.  A similar species, Broomsedge (Andropogon virginicus) is also found on the refuge.

Big Bluestem (Andropogon gerardi)

This species tends to grow in large clumps up to two feet in diameter around the Osprey House area and along the lower railroad bed (D&L Trail).  This was a major species in the tallgrass prairie and produces clusters of seeds on finger-like seed heads that gave it the common name “turkey foot.”  Big bluestem is one of the species we are counting on most highly to create valuable wildlife habitat on our restoration area.  Its seeds are sought out by many birds and small mammals.  Look for this 4 to 6-foot species on the bank at the Osprey House, where clumps are easy to spot.

Indian Grass (Sorghastrum nutans)

This relative of the crop plant sorghum can grow up to 9 feet tall and produces large, golden brown flag-like seed heads.  It is found growing primarily along the lower railroad bed (D&L Trail) in the center of our refuge. The beautiful seed heads are often seen waving amidst those of Big bluestem in this area of the refuge where both plants grow to be 6 or 7 feet tall.  This was another major species of the tallgrass prairies of the Midwest.   It provides excellent food for wildlife, both as forage and from its abundant seeds.


Gray Birch (Betula populifolia)

The 2 to 3.5 inch triangular shaped leaves and chalky white bark with black markings of this small (up to 30 foot) tree are familiar to many people.  It is one of the first tree species to colonize abandoned farmland or burn areas in the Northeastern U.S., and in Lehigh Gap it is re-colonizing some of the steep slopes below the Devil’s Pulpit area and around the Osprey House.  Their 2-3 inch reproductive structures called catkins hang like tassels from the twigs and produce seeds that are eaten in winter by a variety of birds.  Of concern to the agencies overseeing the Superfund process is the fact that these trees take up the heavy metals, especially zinc, and deposit them in their leaves.  This mobilization of metals is of concern because the toxic metals are now available to the food chain through leaf-eating caterpillars.  Also, fallen leaves bring the metals to the surface of the ground in an organic layer fed upon by worms and arthropods.  For this reason, we will not encourage the colonization of test plot areas with gray birch seedlings.

Sassafras (Sassafras albidum)

Sassafras’s aromatic bark and roots were once used to brew a tea.  Typically growing to about 20 to 30 feet, Sassafras is easily recognizable by the three different shapes of leaves present on the same tree. It produces purple berries that are relished by birds.  This tree colonizes when birds spread the seeds in their droppings, and then can form clusters of trees by new trunks growing up from roots.  Groups of Sassafras (and also Black Gum) are found in the most degraded areas of the refuge and stand out as islands among the barren areas.

Aspens (Populus sp.)

Bigtooth and Quaking Aspen are two different species that have appeared on the refuge.  Seen mostly along the D&L Trail and at the Osprey House, we have also spotted some seedlings in the test plot areas.  Quaking Aspens are noticeable when there is a gentle breeze as their leaves quiver (hence the name) in the slightest breeze.  These softwood trees grow rapidly, and can reach 60 feet, but often die quickly or prematurely.

River Birch (Betula nigra)

In the riparian zone along the Lehigh River, many young River Birches are growing. Their leaves are similar to the Gray Birch, though are more rounded, but the pinkish to reddish-brown bark that is peeling is the easiest field mark.  These trees grow along stream banks and flood plains throughout the eastern U.S.

Invasive Alien Species

When a species is introduced to a new habitat, it frequently dies, but occasionally a species finds its new habitat free of disease and predators and it flourishes.  These aliens often crowd out the native species, forming large, dense stands of only the alien.  Such an area is useless to most native species of animals. Invasive aliens are a major cause of species becoming endangered in the U.S., second only to habitat loss. While some invasive aliens, such as Zebra Mussels or Gypsy Moths are animals, many are plants such as Kudzu and Purple Loosestrife.  Here are three alien invasive plants that have gained a foothold on the refuge and need to be controlled.  Japanese Barberry and Japanese Knotweed are two more that we have found in smaller numbers.

Butterfly Bush (Buddleia davidii)

This ornamental is often planted by gardeners to attract butterflies, which it does, but at Lehigh Gap it has become an invasive species.  For nearly two miles, the abandoned rail line that will be the D&L Trail was a solid corridor of butterfly bush. It is also prominent around the Osprey House and is moving up the slopes and into the test plots.  It produces lilac-like clusters of flowers that produce huge numbers of seeds that allow the plant to spread easily.  We fear it could take over at the test plots and outcompete the native grasses, so it must be controlled, which is easily done by pulling out the young plants.

Common Reed (Phragmites australis)

A dense stand of Phragmites greets visitors along the entry road to the Osprey House. Stands of this 8-12 foot tall plant with their large plumy flower heads are picturesque, but as the reed overtakes an area, biodiversity is eliminated.  Few species survive in or make use of a dense stand of Phragmites.  This is one of the more difficult to control species. Small stands of Phragmites also are cropping up at springs and seeps along the D&L Trail and near the Osprey House.

Ailanthus (Ailanthus altissima)

This tree, imported form eastern Asia, has pinnately compound leaves like the various sumacs, and grows in profusion at the Osprey House and along the railroad bed near the ponds. This tenacious tree spreads easily from root suckers and responds to cutting by sending up new stems that grow as much as ten feet in a year. It is native to eastern Asia and was brought to America as an ornamental tree, where it is often called Tree-of-Heaven or Chinese Sumac.

On your next hike through the refuge, take a look at the pioneering species that are returning the barren landscape. Understanding their ecological role will enhance your experience.