Riverdale Park - the Forests of New York City
Riverdale Park is known as a haven for birds. Twenty-seven species, including the screech owl (Otus asio), have been confirmed as breeding in the park. Riverdale contains about 50 forested acres, including mature oak-hickory forest, a forest that occurs on well-drained sites, often on ridge tops, upper slopes, or south and west facing slopes. The soils are usually loams (mixtures of clay, sand, silt and organic matter). The rest of Riverdale is successional forest. This type of forest occurs on sites that have been cleared or otherwise disturbed, with the result being that the canopy layer of trees – the highest level providing leafy coverage – has not endured. These forests are dominated by particular species depending on when they arrive.
There were no forests in New York City 15,000 years ago, as a glacier dominated the landscape. As the global climate warmed, the glacier receded and torrents of meltwater carried boulders, gravel, and sand, which were deposited in a high ridge that marked the southern limit of the glacier’s advance, called a terminal moraine. The dunes and drifts left behind by storms coming off the glacier were early sites of soil formation, providing an opportunity for plant growth. As grasses, sedges, mosses, lichens, and shrubs began to colonize the area, their roots held the sand in place. Successive generations of dead plant material added organic matter and nutrients to the developing soil.
The accumulating soil, with continued warming trends, allowed for the growth of trees, but not yet forests. These trees were widely spaced, and grasses and sedges remained a major component of the vegetation. About 8,000 years ago, forests of spruce, fir, and pine (species now restricted to more northern latitudes) began to dominate the landscape. Over time, the gradually warming climate ushered in dramatically different fauna, and forests full of oaks, hickories, maples, sweetgum (Liquidambar styraciflua), and hemlock (Tsuga canadensis) took hold of the land.
There are four main layers to a forest: the canopy layer, the understory, the shrub layer, and the ground layer. The canopy layer is made of the tallest whose dense, leafy crowns absorb almost all of the sun’s light; sometimes only one percent filters through to the ground. Consequently, other forest plants have to adapt their structure, leaf emergence, and growth behaviors to get the nourishment they need from such a scarce resource. The understory is composed of small trees, which flower and leaf out at the same time as shrubs and can tolerate the lower light levels. Many birds reside here and the flora provides a space between the forest floor and the canopy for such animals as squirrels (Sciuridae), raccoons (Procyon lotor), and opossums (Didelphis marsupialis). The shrub layer is made up of woodland shrubs that flower first, then leaf-out. Shrubs can range in height anywhere from knee-high plants to bushes that reach up ten feet or more into the understory. The ground layer is composed of the lowest growing forest plants, the wildflowers and ferns. These plants, which can be as low as a carpeting layer or weeds up to six feet tall, are the first plants to emerge in the spring.