Sandplain Grasslands of the Vineyard

by Matt Pelikan of the Nature Conservancy
Restoration Ecologist, with the Nature Conservancy’s
Massachusetts Island Office.

For several decades now, one of the most important objectives for conservation on Martha’s Vineyard has been the preservation and restoration of a natural community known as Sandplain Grassland. Occuring on flat, sandy soils, Sandplain Grassland supports a wide range of rare or highly specialized wildlife, and the distinctive mix of grasses, wildflowers, and low shrubs that characterize this community type only occurs under specific conditions. Because of the rarity of the conditions that produce it, Sandplain Grassland was never a common community type. And a high percentage of what Sandplain Grassland once existed has been lost to development, agriculture, erosion, and ecological succession into woodland. For these reasons, then, good examples of sandplain grassland are among the most important conserved sites on the Vineyard.

The ecological importance of Sandplain Grassland reflects the surprisingly harsh physical conditions that characterize this community. In particular, the soil characterisitics of sandplain limit what species will grow there. Composed of “outwash deposits” – sand, clay and fine gravel swept downhill from the terminal moraine as the last ice age waned – sandplains are characterized by soils that do not retain water well and contain few nutrients. For this reason, sandplains are dominated by vegetation that tolerates drought and doesn’t require much fertility to flourish; grassy or shrubby habitat covers much of a typical sandplain, and even the wooded portions of a sandplain are generally stunted relative to similar woodland on more fertile soils.

Drought-prone and generally covered by scrubby vegetation, sandplains are fire-prone, as well. The archaeological record firmly demonstrates a very active fire history on the Vineyard’s sandplain, with evidence of frequent fire going back thousands of years. (The hilly morainal sections of the Vineyard, with somewhat wetter and more fertile soils, evidently burned much less frequently.) Most of the fire was probably of human origin: both Native Americans and European settlers used fire deliberately as way to clear land, rejuvenate agricultural sites, or stimulate blueberry production. Of course accidental fires were also frequent, because open fires were an essential energy source for both of these cultures. As recently as the first half of the 20^th century, wildfires of 1000 acres or so were occurring roughly every other year on the Vineyard sandplain, and the largest fires, like one in May 1916 that raged across about 16,000 acres in a single day, burned significant fractions of the Island’s dry-land area.

Fire has not been the only form of disturbance to affect our sandplain. Settlers cleared vast areas for firewood or to use as pasture or for growing crops. Countless storms have knocked over tall vegetation and doused what remained with hostile salt spray. Like fire, these forces acted as powerful ecological forces shaping the landscape.

The Vineyard’s sandplain grassland, then, results from a rare convergence of geological history, location, and persistent human intervention. Glacial outwash soils, a coastal setting, a history of fire, and sustained human management over many centuries, by favoring some plants and animals over others, have shaped a distinctive natural community composed of highly specialized, highly resilient speices. In areas subject to relatively infrequent disturbance, shrubland or even oak woodland have managed to grow. But where fire and other disturbances occurred most often, woody vegetation had difficulty getting established, leaving grasses and herbaceous plants to dominate. There areas feature what we now call Sandplain Grassland.

Native vegetation found in Sandplain Grassland generally displays one or more of these characteristics:

• -Adaptation to growth in low-nutrient conditions. (Sandplain plants either don’t require many nutrients or have evolved ecological partnerships that allow them to obtain nutrients from some source other than the soil).
• Resistance to the effects of drought. (Waxy leaf coatings that retain water, narrow leaves with limited surface area through which water could transpire, deep root systems, etc.)
• Tolerance for or even requirement for full sun. (Optimized for life in a frequently disturbed habitat, most sandplain plants are short but adapted to bright light.)
• Ability to disperse into new locations. (Seeds may disperse on the wind, be transported by ants, or be carried by birds in order to colonize new locations opened up by fire or other disturbance.)
• Adaptation to disturbance, especially fire. (Some plants, like thick-barked oaks, simply resist fire.  Other plants resprout readily from their root crowns following a fire. For many sandplain plants, disturbance can even be a virtual necessity: low-intensity fire, for example, prompts germination in many Sandplain Grassland plants, so they tend to sprout right after a fire has freed up nutrients and suppressed competitors. And for many plants, frequent fire helps keep populations of harmful insects at tolerable levels.)

Sandplain Grassland shares many species with other sandplain community types, differing from related communities mainly in the frequency of disturbance it experiences and in the mix of vegetation that results from that disturbance. Indeed, there are few plants that are found only in Sandplain Grassland: many of the most common species in this community (including grasses like little bluestem, goldenrods and asters, and shrubs such as lowbush blueberry) are common in open habitats of all kinds in our region. But the unusual conditions of a Sandplain Grassland result in combinations of species that don’t generally occur in other settings, or in particular plants reaching levels of abundance that they don’t seem capable of obtaining elsewhere. (For example, wild indigo occurs quite widely in southern New England, but on sandplains subjected to frequent fire, this plant, which is usually a minor component of a habitat, can become quite dominant.) And there are some species, such as New England blazing-star, sandplain gerardia, sandplain flax, and sandplain blue-eyed grass, that really do seem to require the conditions found in Sandplain Grassland in order to persist.

A good quality Sandplain Grassland (meaning one that is large, has been in existence for a long time, and has been spared much invasion by non-native species) features a high diversity of plants (numbers vary from example to example, but hundreds of plant species may be present). This partly reflects the availability of “micro-niches” within a sandplain.  Because sandplain soils vary somewhat in composition on a fairly small scale, and because most forms of disturbance are patchy in their effects, a large number of plants manage to find the conditions that they’re best adapted to. Since insects often have close associations with particular plant species (many caterpillars, for example, confine their feeding to plants of a single genus or even species), a high diversity of plants tends to translate into a high diversity of insects life as well. High insect diversity, in turn, means that a variety of insects are available during a long season to other animals, like birds, that prey on them. So Sandplain Grasslands, despite their austere appearance, tend to be highly productive biological communities.

Moreover, the diversity of plants on a Sandplain Grassland means that at least a few species are actively growing over a very long season – from mid-April on the Vineyard, when the first sandplain plants break dormancy, until well into October, when plants like stiff aster are still maturing their seeds. In a time when nitrogen falling in the form of acid rain poses a major threat to groundwater quality (and hence to water quality in groundwater-fed ponds), the diversity and long growing season of a sandplain grassland has a useful ecological effect. For an unusually long portion of the year, actively growing plants are present to absorb nitrogen from sandplain soils. The effectiveness of Sandplain Grassland in protecting water quality is enhanced by the dense, massive root systems that some grassland plants have evolved as a means to resist drought: for example, a mature clump of little bluestem grass has appreciably more mass below the ground than above it, with a dense, fibrous root system extending as much as six feet underground.

Grassland is not the only habitat type found on sandplain, and the various natural communities found on sandplains convert quite readily into other community types, depending on how they are managed. If you prevent disturbance of a Vineyard sandplain grassland, progressively taller and woodier vegetation will gradually establish itself until finally the site turns into oak woodland (which represents the closest thing to “climax forest” on the Vineyard). Conversely, if you remove the trees from a sandplain woodland and introduce regular disturbance (mowing, fire, or grazing), the mix of vegetation will become grassier and more herbaceous; eventually turning into Sandplain Grassland. Human activity and natural disturbance have caused constant change on the Vineyard sandplain, and any given point may have gone through many cycles of growth and disturbance. For this reason, ecologists generally envision the Island’s past as a “shifting mosaic” of habitat types, changing over time and never fully stable.

Sandplain Grassland, then, and indeed all of the sandplain community types, are highly evolved associations of plants and animals, many of which are themselves specialized. Diverse and biologically productive, these communities are ideally suited to the conditions that shape them. But the flip side of this specialization is that sandplain vegetation loses its competitive edge if conditions change. Deprived of disturbance, Sandplain Grassland plants will be out-competed by taller species. If more nutrients become available (due to fertilizer application, for example), or if supplemental water is applied, sandplain vegetation is not able to take advantage of the “improved” conditions and will be out-competed by other species (often by weedy species associated with agricultural land).

Preservation of sandplain communities, then – and of the specialized species that occur within them – requires protection of significant tracts of land, large enough to hold viable populations of species that may occupy only a very specific “micro-niche” on the landscape. And to maintain the full suite of sandplain community types, active management (the application of disturbance such as mowing, grazing, or prescribed fire) is necessary. But maintenance of these unique natural systems makes good sense on Martha’s Vineyard: thousands of years of natural selection have produced associations of plants and animals that are ideally suited to this unique corner of the world.