SPRING

Photo by I. W. Cahalan

BULLETIN OF THE RHODE ISLAND WILD PLANT SOCIETY
VOL. 16, No. 2

In This Issue:

The Role of Disturbance in the Ecology of Natural Communities

Included With This Issue:

All in the Family: The Evening Primroses, Onagraceae family

 THE ROLE OF DISTURBANCE IN THE ECOLOGY OF NATURAL COMMUNITIES
by Richard Enser

Conservation of biodiversity has traditionally focused on protecting individual species of plants and animals, usually those identified on federal and state lists as endangered, threatened, or rare. This approach is often referred to as the "fine-filter" method of conservation. However, individual species occur in discrete communities along with other species that share similar environmental requirements. Identifying and protecting entire communities facilitates conservation by a "coarse-filter approach. Protecting the best examples of natural communities automatically ensures the perpetuation of the species inhabiting them.

Natural communities create a mosaic over the landscape. Where each type occurs is determined by local conditions of bedrock, soil type, elevation, topography, hydrology, and microclimate. For example, the community known as `bog' is characterized by water-saturated, peaty soils, generally occurring in a topographical basin, possibly underlain by impermeable bedrock that prevents drainage. In an upland situation, the community identified as 'maritime shrubland' occurs on bluffs and headlands that are exposed to ocean winds and salt spray. Only a few woody species, such as Bayberry (Myrica pensylvanica), Beach Plum (Prunus maritima), and Poison Ivy (Toxicodendron radicans ) are capable of withstanding these extreme conditions.

Although communities are primarily governed by the local physical environment, all of them are constantly changing through the process of natural succession. Over time, species alter their environment, allowing new species to become established so that the structure of the community changes noticeably. Continuing with the bog example, the open water of a young bog gradually disappears as sphagnum peat accumulates, creating a floating mat that eventually becomes dense enough to support woody plants.

Understanding the processes that govern a community's appearance and composition is essential to preserving it. It is easier to perpetuate a community at a particular successional stage when dealing with those that undergo change imperceptibly over time, such as bogs. On the other hand, some communities are defined by their inconstancy; they are maintained by periodic and sometimes catastrophic disturbances. In such cases, the resident plant community consists of species that have evolved adaptations to cope with the impacts of a particular disturbance, and when the disturbance is eliminated, these specialists are outcompeted and replaced. Disturbances can vary in their periodicity and level of destruction, but all generally recur with some regularity. These include catastrophic disturbances such as fire, floods, and storms (especially hurricanes) as well as less drastic but equally defining ones such as tidal flows.

Disastrous as the aftermath of fire in the Pitch Pine forest appears. The destruction is essential to regenerate and sustain this plant community

Several Rhode Island plant communities are maintained by fire, the primary example being Pitch Pine (Pinus rigida) barrens. Woodlands dominated by Pitch Pine tend to be found on extremely well-drained sandy soils. Historically, these somewhat austere communities were called barrens because of the infertile nature of the soils, which were considered too poor, or "barren," to support agriculture. These soils are not only extremely dry but also strongly acidic, which limits the decomposition of organic matter and results in a heavy accumulation of duff (pine needles, leaves, and sticks) on the surface. The combination of dry soils and duff creates the volatile conditions that make pine barrens particularly susceptible to burning. Although it is well documented that Native Americans regularly set fires to manage the environment, certain fire-prone communities, such as Pitch Pine Barrens, were probably burned periodically by natural wildfires. Rhode Island barrens have likely been subjected to fires for several thousand years. Most recently, research by Gordon Tucker at the Kingston Pine Barrens south of Worden Pond determined that the area had severely burned twice in the last century, in 1905 and 1910, and smaller portions had been burned between 1930 and 1968_the last recorded fire in that area.

Fire maintains these barren communities by arresting ecological succession. It eliminates plants that are intolerant of burning, and at the same time provides optimal conditions for the relatively few species that have developed adaptations to cope with this catastrophic disturbance. Older Pitch Pines acquire a thick "fireproof" bark, and some of their cones open and release their seed only under the extreme heat of fire. In the understory, Scrub Oak (Quercus ilicifolia), a common barrens shrub, resprouts readily from a still viable root system. The seeds of some species, including Wild Lupine (Lupinus perennis) and Sand Cherry (Prunus pumila), remain viable in the soil seed bank for many years and germinate quickly after fire. Consequently, after a destructive conflagration has eliminated all competing vegetation, a small group of highly adapted species readily recolonizes the burned site and perpetuates the pine barrens.

Pitch Pine communities throughout New England have significantly decreased in acreage during the past several decades because of fire suppression. Today fires are controlled and extinguished as soon as they are discovered because of the danger they pose to nearby residential communities. Without burning, Pitch Pine Barrens change quickly as hardwood trees and other woody species in the understory become established. Sandy openings close in and become unsuitable for Wild Lupine and other typical plants such as Golden Heather (Hudsonia ericoides) and Bearberry (Arctostaphylos uva-ursi). Although some land managers have attempted controlled burns to replace those that once occurred naturally, this option is limited in most areas because of the proximity of housing and other development.

Another community maintained by fire is the grassland, or plain, as it has sometimes been called. Although the existence of some grasslands in this region is well-documented, such as the former Hempstead Plains of Long Island, the historic extent of grassland in Rhode Island is poorly understood. It is likely that European settlers quickly adopted any naturally open lands for agricultural purposes, and most fields that exist today are dominated by introduced grasses and bear little resemblance to the native grassland community.

A type of disturbance that affects plant communities on a daily basis is the ebb and flow of the tides, a condition not typically thought of in the same context as a catastrophic fire or storm. Tidal maintenance of plant communities is a familiar concept as applied to the saline and brackish marshes along the Rhode Island coast. The twice daily alteration of water levels has a significant control on plant communities, and eliminating the tidal influence can markedly change species composition. This dilemma has been most evident in the so-called freshwater tidal marshes. These fairly small communities are found along rivers that drain directly into marine waters. During the flood tide, heavy salt water pushes inland underneath lighter fresh water, forcing it up and over the river's shore. As the tide ebbs, water recedes and the river resumes its flow to the sea. Freshwater tidal marshes are naturally restricted to the discrete segments of rivers where the water level rises and falls daily, but salinity levels remain minimal. Under such conditions the plants inhabiting these communities, which are not suited to saline or brackish waters, must still compensate for the daily alteration of inundation and exposure.

A variety of adaptations have been identified in plants of the freshwater tidal marsh. Some grow as rosettes of flat leaves that hug the substrate, making them less susceptible to being washed away by tidal flow. Some have developed mechanisms to insure that their seeds, once disseminated, are not washed away by the next high tide. Seeds of Wild Rice (Zizania aquatica), for example, have a long, pointed appendage (awn) that directs the falling seed down through the water and into the substrate. Arrowhead (Sagittaria subulata) has flowering stalks that bend over and directly "sow" their seeds onto the sediment.

Unfortunately, freshwater tidal marshes and the specialized species they support are among the rarest elements of Rhode Island's biota. The primary reason is that early in the state's history, at the onset of the Industrial Revolution, most of the state's rivers were dammed near their mouths, eliminating most examples of this community before they were documented. We have no way of knowing how extensive freshwater tidal marshes were before European settlement, but there are intriguing snippets of information. In 1900, Brown University botanist William W. Bailey outlined some of the botanical heritage of Providence, as reported by his father and others, and noted how the cove of the upper Providence River (now the site of Waterplace Park and Providence Place Mall) was once abundant in Wild Rice. Today, many of the plants associated with freshwater tidal marshes are considered extremely rare or even extirpated from Rhode Island. The only way to restore this community would be to physically remove the dams that currently block tidal flow.

Dams, such as this one near the mouth of the Blackstone River, are responsible for eliminating most freshwater tidal marshes and the specialized species that depend on them for survival.

(Photos by Richard Enser)