CHAPTER 8

Opportunistic Natives

The kinds of changes that have occurred in the landscapes around us have significantly altered interrelationships between native plants and animals, even where no exotic species are involved. Today, woodland managers are concerned about the rapid spread of several native species, among them the black cherry. The 1982 tree inventory in Central Park, for example, revealed that almost 20 percent of the trees in the park greater than 6 inches in diameter were black cherry. That ratio was even higher when sapling-size trees were included in the count. This species is clearly proliferating in the park, and some argue that, in the interest of maintaining diversity, it should be treated like an exotic and removed. Black locust engenders even greater concern because it is extending its range at a rapid rate. It is both prolific and invasive. Management plans frequently recommend removing locust.

Cherries and other aggressive native species often take advantage of the opportunities provided when once-tended areas are released from management or woodlands are partially cleared for security reasons. Where disturbance limits the variety of native communities in the vicinity, only a few natives, all easily dispersing propagators, may account for a disproportionate amount of new growth. They also represent a problem in horticultural landscapes, where they grow like weeds. These species of seminatural areas thrive in the kind of simplified environments we make of the landscapes around us. Like white-tailed deer and Canada geese, however, they are not well adapted to the habitat of continuous mature forest, and their reign in the forest may well be short-lived. But a maturing landscape does not necessarily daunt exotics such as Norway or sycamore maple. We need far more research and documentation in order to define effective management strategies. The real conditions and trends over time on each site must be the actual basis for any good proposed management plan.

A general trend is that species native to the midwestern part of the United States, including many common midwestern plants such as the catalpa, osage orange, and western red clover, are spreading into the East. Many of these plants, along with other exotics, spread quickly along the early linear corridors created by trails, roadways, railroads, highways, and seams of disturbance (Brothers 1992). In addition to the original creation of these corridors, humans have further helped spread the species through direct planting and spreading of seed. This trend continues even more intensively today with the popularity of mixed meadow seeds grown in the Midwest or California. The East currently has no major commercial native-seed propagators, so no regionally appropriate mixes of locally indigenous seed are currently available. We have also helped some midwestern natives in their settlement of the East by managing the landscape to replicate the drier native environments of their origins, rather than managing to replicate the actual environment of the East, the result of conventional engineering practices designed to provide drainage and flood protection.

Several considerations are important in evaluating the invasiveness of an exotic species, such as persistence. An important characteristic of an invasive plant is that it replaces indigenous species and often whole native communities, preempting the return to native forest. Aggressive invasive native species are sometimes, but not always, so persistent. Where Norway maples are established, for example, there is no evidence that native communities will gradually colonize the site, even if aggressive natives such as black cherry are kept at bay. Where cherries are established, however, native forest species usually appear within a few years, at least woody species. If proliferation of Norway maple and other exotic invasives could be checked and adequate seed sources for indigenous plants were available, natural succession would gradually replace the cherries with more diverse native vegetation. Nevertheless, the abundant establishment of cherries may retard this process, and management may be warranted in some cases. Black locust, although still expanding its range, may be more easily controlled naturally once more forestlike conditions prevail.

The Supercompetitor

Far more complex questions come into play when we look at the example of the common reed. In this century, common reed has moved from the very fringes of the landscape at the margins of the sea up through the estuaries and river valleys to occupy thousands of acres of once-forested swamp. This invasion includes many acres of marshland formerly occupied by other species. You can also find common reed on landfills, on roadside edges, and in vacant lots, as well as in woodland seeps and along shorelines as it moves farther and farther inland and upland. The pattern of its spread seems much more like that of an invasive exotic than a native and raises questions about what is native and how natural the resultant landscape is (Figure 8.1).

Until recently, two species of common reed were generally recognized, Phragmites communis of the New World and P. australis of the Old World. Scientists later reclassified common reed and combined these two species under the same scientific name, P. australis. The confusion this change created seems quite apt given the already strange behavior of common reed in our landscape. Some people refer to the plant as nonnative, and others recognize it as native. In some ways both are right. The plant that has swept across the Delaware estuary may be less the “native” of the last century and more the “nonnative” of the Nile, the Danube, and the Thames, a new supercompetitor that is a genetic concoction of our own making that combines these international strains. Plants of shorelines are typically very mobile, and their populations are linked rather than isolated by water. This natural mobility, however, has increased exponentially with the transglobal journeys of migrating and trading humans. Seeds carried in grain sacks on wagon wheels, and in ship ballast spread rapidly carried seed from several continents across exceptionally large distances over and over again, and very swiftly. Modern transport is an even faster distributor.

The introduction of distant strains into local plant stock is documented for other species, such as rice cutgrass. Eurasian stock, sold as seed and planted widely, has invaded natural wetlands and replaced native subspecies. Even introduction of native species in distant locales has impacts. The characteristics of saltwater cordgrass from New England sources planted in mid-Atlantic locales are noticeably distinct from those of local stands, which may be replaced.

Figure 8.1. Common reed, growing in monospecific stands, now dominates many once-diverse wetlands and is making inroads on many upland sites as well. (Photo by Ann F. Rhoads/Morris Arboretum)

Even minor geographic distances may be enough to account for important variations that maintain subspecies identity. Transplants of hairgrass rescued from a construction site that were planted amid local stands were notably different in appearance from the original plants that they gradually expanded to replace (Gehring and Linhart 1992). Some of the behavior of common reed, such as its new affinity for roadside slopes, may, in fact, be due to the same kind of subspecies mixing. The expansion of common reed has been repeated in many eastern estuaries from Maine to Florida. In the Delaware River estuary, this explosion began quite suddenly in the middle of this century for reasons that are not known but may involve many of the factors raised above as well as significant changes in the hydrologic regime (Kraft, Yi, and Khalequzzaman 1992).

There is also some question about when common reed arrived in the New World, because it provides habitat to so few indigenous wildlife species here, while in the vast reed marshes of Europe and Asia complex ecosystems revolve around the plant. Some have suggested that the earliest human travelers from Eurasia, indigenous people, may have introduced the reed, using it like tobacco for rituals well before Columbus and well before the first botanist arrived to find it already established.

Clearly, the horticultural practice of propagating and moving plants to any location where they will survive creates problems for restoration efforts. Among the environmental problems it poses is to threaten subspecies diversity by eliminating the level of isolation between populations. The subspecies level of biodiversity is the least understood and the most endangered. So to retain these often very localized plants, the restorationist will need a new kind of nursery centered on local diversity. Monitoring of changes in naturalizing populations of both native as well as nonnative plants also will need to become an integral part of the growing and transporting of plants. And, last, we must expect the unexpected, especially in this age of supersonic travel. As the world gets smaller, the diversity that was dependent on isolation is lost. In the years to come, new permutations of the problems we are already observing are likely to emerge.