(Written in September, 2019, for Princeton University students prior to a tour of Mountain Lakes Nature Preserve)


Vera asked me to discuss succession as part of our visit to Mountain Lakes. Below are some initial thoughts. On our walk, we'll be taking paths through varied habitats, looking behind the facade of peaceful green at the dynamic economic, social and ecological forces that have shaped this land.

The woodlands we see in Princeton appear to have been here forever, but most of them have regrown from agricultural fields that were abandoned by farmers. The Great Depression of the 1930s hastened this process. Princeton’s open space movement got its start in the 1930s, when a visionary Princeton mathematics professor by the name of Oswald Veblen began acquiring land that later became the Institute Woods and Herrontown Woods.

The abandoned fields had been altered by plowing, draining, soil depletion, etc, but they did then go through a process of succession, in which one plant community displaced another. Forbs, grasses and brambles gave way to shrubs and smaller trees like eastern red cedars. Ultimately, hardwoods like oak, maple, ash, and hickory moved in, shading out many of the shrubs and cedars, creating the woodlands we see today.

But a closer look reveals that this seemingly natural succession to woodlands has not really mended the distortions caused by Princeton’s agricultural era, and more recent impacts are altering the ecology even of lands we think of as protected.

When I was studying botany and forest ecology, the process of succession was thought to be natural, inevitable, eternal, but many factors have conspired to hamper the process, or redirect it altogether.
  • A seedbank that is now dominated by non-native species. (“Seedbank” refers to the seeds that reside dormant in the soil, ready to sprout to fill a void. They can remain viable for many decades.) The seedbank constitutes a soil’s “memory” of all the plants that have grown there in the past, plus any seeds that have been brought in by birds, wind, or flood. Plowing and/or intense grazing tend to erase a soil’s “memory.”
  • An overpopulation of deer. The extirpation of predators has allowed the deer population to explode, most dramatically since the 1970s. Deer tend to prefer eating native plants, which gives the competitive advantage to non-native species that remain uneaten. Princeton began professional deer culling in 2000. It was highly controversial, but has helped compensate for missing predators, and has dramatically improved native flora while reducing auto accidents. 
  • An explosion of non-native species that behave invasively, smothering or otherwise displacing or marginalizing native plants. These could be vines like porcelainberry that have a suffocating effect like the more famous kudzu of the south, introduced diseases that killed off chestnut and elm, or more recent introduced insects that are stripping our woodlands of ash trees and Viburnum shrubs.
  • Past changes in a site’s hydrology and the exclusion of fire can also impact the course that succession will take. 
  • Climate change: The increasing intensity of storms is causing more disturbance and intensifying change in the woodlands. As trees fall, more sunlight reaches the understory, which shifts energy from native trees to (often) non-native shrubs, grasses, and forbs. 


Various natural forces can keep a particular location from becoming forested. Natural and human-caused fire has historically promoted more open habitats like prairie and savanna. These are called fire-climax communities. Wetlands are often inhospitable to trees, as are some clay soils with traits like exaggerated shrinking and swelling that discourage tree growth.


When I moved to Princeton in 2003 and began getting involved in open space work, I offered a very simple observation to those who had been so successful at preserving open space in town. I pointed out that nearly all of the trees in the nature preserves are native, but that the shrub and herbaceous understory is largely exotic.

There are many words for plant species that did not evolve here in eastern North America: exotic, alien, non-native, introduced. A subset of these has behaved invasively, which means they have escaped from people’s yards and invaded local forests. There is a very long list of these. What they have in common is that they exist outside the evolved checks and balances that otherwise maintain ecological balance. In other words, they left behind the predators or diseases that kept their populations in check where they originally evolved. They compete above and below ground. In some cases, their roots exude chemicals that alter the soil chemistry to discourage growth of other species. Thus, Japanese stiltgrass forms vast grassy expanses in our woodlands. Porcelainberry smothers native vegetation along the edges of the Princeton Battlefield. Winged euonymus, privet, and Asian Photinia form impenetrable thickets in local woodlands.

It would be great if nature would quickly evolve to bring these invasive species into balance, much like our bodies’ immune systems can respond effectively to new infections. But no species has yet evolved to effectively eat or infect the various introduced species that now dominate many areas. This means that a substantial portion of our preserved lands has been rendered inedible for wildlife—representing a substantial reduction in the functional preserved acreage.

It’s important to remember that the word “invasive” refers to behavior, not place of origin. Most non-native species do not become invasive. A native species can behave invasively if people have eliminated its predator. Deer are an obvious example. It’s simplistic but useful to say that most invasive plant species are non-native because nothing on our continent has evolved to eat them. Thus, in order to restore balance, land managers have to assume the role of herbivores, discouraging the plants that the deer and other herbivores find unpalatable.

Invasive species denial: Climate change denial has its parallel in invasive species denial. A surprising number of books have been written that try to tell us that invasive species are not a problem after all. The books tend to characterize habitat restoration as a sentimental and futile exercise. Some of my critiques of these books, opeds and articles can be found at this link.


Mountain Lakes is a beautiful nature preserve saved from development in the 1980s by the Friends of Princeton Open Space and other preservation entities. Unlike many other states, New Jersey has preservation funding hard-wired into the system. There are open space taxes at the state, county, and local level, all of which can be brought to bear to buy cherished lands threatened with development. Legal preservation is only the first step, however, as past trauma means that most of our lands are in need of restorative intervention to bring back or maintain a functional ecology.

THE LAKES: The two lakes at Mountain Lakes (they might be called ponds if they were anywhere else than Princeton) are impoundments made in the early 20th century to make ice for refrigeration. Most lands around the lakes were farmed, whether for crops or pasture, before being allowed to grow up in trees.

COMMUNITY-PARK-NORTH WOODS: This was a field long ago planted with evergreen trees—pine and spruce that are native to the eastern U.S. but not typical of central in NJ. A decade ago, this was a dark woods carpeted with needles, lacking in biodiversity but pleasant to walk through. In recent years, storms intensified by global warming devastated the forest, creating a patchwork of remnant trees and mostly non-native vegetation growing up among the downed trunks. We’ll discuss what sort of abnormal succession might take place here.

TUSCULUM MEADOWS: Portions of John Witherspoon’s old estate became preserved meadows. Annual mowing performs the ecological role that fire might once have played, to keep these meadows from succeeding to scrub and forest. We’ll discuss what might displace the meadows if the mowing was stopped.

FROG HOLLOW: A small field near Mountain Lakes House that underwent a most unusual course of succession, from floodplain to Olympic-sized swimming pool, to mowed lawn to a wet meadow.

HABITAT RESTORATION PROJECT: A large ecological restoration project is underway on the west side of Mountain Lakes. Heavy machinery is clearing invasive shrubs, and volunteers are gearing up to plant 7400 native species on the site. We’ll discuss the site’s history and factors that will influence the project’s success.

-- Steve Hiltner

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