Published in the February 2021 Issue of Memento Mori
By Sara Evans
The Green-Wood Cemetery in Brooklyn, NY, was founded in 1838 and opened to the public in 1842 as a private, rural cemetery, designed to offer visitors a sanctuary for solace, reflection, and escape from the surrounding cityscape. Recognized for its art, architecture, history, and landscape, Green-Wood was designated a National Historic Landmark in 2006. As of 2015, Green-Wood became an accredited arboretum. Its 478 acres supports over 7,000 trees and shrubs, a landscape that closely reflects the original rolling topography of the region. Today, Green-Wood continues to function as an urban oasis, a contrast from the relatively flat, illuminated streets of the surrounding built environment.
There are two phenology sites within Green-Wood—Valley Water and Vista Grove. Valley Water is located close to the perimeter of the cemetery, where the plant life is exposed to more artificial light at night. The Vista Grove site is located in a valley between sloping topography, with a higher density of surrounding canopy, and is over 700 meters away from an artificial light source. In our efforts to understand how climate change and urbanization affect the ecological dynamics of our city, our sites include species that are common street and park trees. We seek to examine whether the quantity of light pollution influences the timing of life-cycle events.
What Is Phenology?
Phenology is the study of the annual lifecycle events of all plant and animal species. These are usually marked by the changing of seasons. Recording day-to-day observations allows us to measure phenological trends over long periods of time. Observing the lifecycle events of plants, animals, and insects is a long-standing human tradition. It has historically facilitated us becoming effective hunters, gatherers, and farmers.
Why Is Phenology Important?
Recording lifecycle events of plants and pollinators helps us to more effectively predict environmental responses to climate change. This enables us to implement better environmental, agricultural, resource, and public health management practices to mitigate or adapt to these changes.
In many plants and animals, phenological transitions—especially those that occur in the spring—happen when enough warmth has accumulated. This warmth is often measured using growing degree days (GDDs). Growing degree days are defined as those in which the average daily temperature exceeds the base temperature of 32°F, i.e., the temperature below which the plant will remain dormant.
Meet the Trees
Since 2015, Green-Wood has been an accredited arboretum through Morton Arboretum’s ArbNet international accreditation program. This accreditation is a public acknowledgment of the importance of Green-Wood’s collection of trees. You may be familiar with some of these species, as they are prominent street trees in New York City. The less familiar species are native to New York forests and are being observed for
The New York Phenology Project, which aims to understand the effects of climate change and urbanization on plants and pollinators:
- Acer saccharum—Sugar maple
- Asimina triloba—Pawpaw
- Cornus florida—Flowering dogwood
- Fagus grandifolia—American beech
- Ginkgo biloba—Gingko
- Liquidambar styraciflua—Sweetgum
- Liriodendron tulipifera—Tulip tree
- Magnolia virginiana—Sweetbay magnolia
- Quercus alba—White oak
- Quercus rubra—Red oak
- Quercus velutina—Black oak
- Querus virginiana—Live oak
- Sassafras albidum—Sassafras
Below is a sample of the trees on the Green-Wood property that are being studied.
Maple syrup is made from the sap of the sugar maple! Native to eastern North America, the sugar maple is a deciduous tree that matures between 50 to 130 feet tall. For the most part, they are absent from the urban street environment because they are highly sensitive to salt and soil compaction. Leaves are simple and palmately veined, with five lobes: two small lobes at the base and three larger lobes, each marked with three pointed lobules. The margins are smooth. The leaves are green in the summer and turn brilliant colors of red, orange, and yellow in the autumn. Its flowers function as either male (delivering pollen) or female (producing fruits/seeds) despite appearing to have both parts within each flower.
Most trees are dioecious (meaning they have either female or male reproductive structures), but some individuals will have both male and female flowers that grow on separate limbs of the same tree. The elongated, yellow-green pedicellate flowers emerge with new leaves in the spring and are wind pollinated. Some pollination may occur as bees visit the flowers in early spring.
Female flowers that are pollinated develop into fruits that are paired winged seeds (samaras). The globular seeds attach at the penducle, with the wings dropping down off the outer sides of the seeds, creating a “V” shape. They are borne bright green and turn yellowish-tan when ripe.
The wings of the samara aid in wider dispersal ranges by spinning like a helicopter in the wind, flying farther from the parent tree. Sugar maple is a USA-NPN regional species—they are ecologically or economically important. It is the only tree used for commercial sugar production today. Sugar maple is also an allergen, and phenology observations will provide valuable data to benefit people with allergies and the public health community.
The pawpaw is a perennial deciduous tree or shrub that can grow 10–40 feet tall. Wild pawpaws are found in the understory of hardwood forests. The drooping, oblong, teardrop-shaped leaves are alternately arranged on the stems, from four to 12 inches in length, with smooth margins. The leaves are coated with fine whitish hairs on the upper surface and with rusty-colored hairs on the under-side. Leaves are aromatic, emitting a smell similar to bell pepper when crushed.
The flowers are inconspicuous, growing up to two inches in diameter. The flowers emerge green in color and mature to a dark purple or maroon. Each flower has six petals that are stiff and curl backwards, toward the pedicel, and three sepals. The flowers smell faintly of rotting meat to attract carrion flies and nitidulid beetles for their pollination services.
The fruits are large, oblong, fleshy, and mango-shaped and grow solitary or in clusters of up to four. The fruit has thin skin and a yellow custard-like flesh that tastes similar to papaya. As the fruit ripens, it turns from yellow-green to brown.
Research has found that the rate of fertilization in pawpaws is low relative to the ratio of flower production and fruit development, suggesting that pollination is low. Pawpaws then depend on clonal reproduction through root sprouting. The pawpaw is the only host plant to the larvae of zebra swallowtail butterflies; they cannot survive without it. The caterpillars feed on the young leaves of the pawpaw, which are rich in acerogenins, chemicals that make the leaves unpalatable to other herbivores—which, in turn, make the larvae unappealing to predators.
Native to the central and eastern United States, the flowering dogwood is a small deciduous tree or shrub, reaching between 10 and 55 feet in height. In the wild, flowering dogwoods are found growing in the under story of moist deciduous and evergreen forests, in ravines, along streams on flood plains, on lower to middle slopes, and on bluffs.
The simple, ovate-shaped leaves are three to six inches in length, with smooth, wavy margins, and lateral veins that alternately extend from the midrib to the margin, curving toward the leaf tip. Foliage is dark green in the summer and turns red to maroon in the autumn.
The small yellow flowers are clustered in the center of four showy white or whitish-pink bracts—these modified leaves attract pollinators to the inconspicuous yellow-green flowers. Bracts are not petals and will open while the real flower buds remain closed at the center, so checking for open flowers requires closer inspection. The fruit is a glossy-red, ovoid drupe that measures ½-inch long, born singularly or in clusters of up to ten.
The fruit is a valuable resource for wildlife; it is particularly a significant source of food for the American robin. Historically, flowering dogwoods were sourced for various medicinal and therapeutic uses by native North Americans.
Native to eastern North America, the American beech is a large deciduous tree that can reach heights of up to 120 feet. The American beech grows in moderately moist deciduous forests with deep, rich, well-drained soils, and is very tolerant of shade. It is common in ravines and small valleys, on slopes, and bordering streams or springs. The leaves are ovate, tapering to a point at the apex of the leaf, measuring between two and four inches in length. The margins are entire and slightly wavy.
European and American beech are most notably distinguished by the number of vein pairs on the leaves; European beech leaves have between eight and 10, whereas American beech leaves have between 11 and 14 vein pairs. Leaves are a deep, glossy green with a crimson hue in the summer and change to a copper-bronze color in the autumn. Beech trees usually retain their dead leaves throughout the winter. The American beech is monoecious, with inconspicuous greenish-yellow unisexual flowers. Male flowers appear as small, globular clusters at the end of a drooping, hairy two-inch penduncle. Female flowers grow in pairs on short spikes—they are encased in a covering of scales, surrounded by pinkish-red bracts. Flowers bloom for about one week in the spring and are wind pollinated.
American beech was the tree most associated with the extinct passenger pigeon, which fed on its nuts and roosted in its branches. It is also the host plant to the early hairstreak butterfly (Erora laeta), one of the rare butterflies of its range, and listed as endangered in the state of Maryland, threatened in Massachusetts, and a species of concern in Michigan.
Maidenhair Tree (or Ginkgo Biloba)
Native to China and introduced in the United States in the late 18th century, ginkgo biloba is a deciduous tree that can reach 100 feet in height. It is the sole extant species of its genus, considered a “living fossil” with its form remaining unchanged for more than 200 million years. Like conifers, ginkgo trees are true gymnosperms, their fruit is a naked seed that is unprotected by a fruit or ovary.
The Latin word “biloba” translates to “two-lobed,” referencing the fan-shaped leaves of the ginkgo tree. They are simple, leathery, green leaves that reach two to three inches in width and length. The margins of the leaf are smooth and irregular, with palmate veins fanning from the petiole and running parallel to the leaf margin. In the autumn, the leaves turn bright yellow.
Ginkgo trees are dioecious—an individual tree has either female or male reproductive structures. The male flowers are inconspicuous, drooping, long-stemmed, yellow-green catkins, and the female flowers are inconspicuous two-inch peduncles that lack sepals or petals. Like conifers, ginkgo trees are wind pollinated. The small plum-like fruits are actually naked seeds covered in a fleshy coating. The whitish-green flesh of the seeds ripens to a bright orange. Only female ginkgo trees bear seeds. The seeds give off an unpleasant odor when ripe.
Ginkgo biloba are highly resilient and long-living trees. They are highly tolerant of salt, pollution, and soil compaction, making them a popular street tree. The oldest tree is still thriving near the Zempuku-ji Temple in Tokyo, believed to be planted in 1200 A.D. Despite widespread cultivation, maidenhair trees are endangered due to the public’s preference for male trees and propagation methods.
Coordinated by Sara Evans, project manager for the Department of Design & Landscape, www.nyphenologyproject.org/green-wood-cemetery. Community Greenways Collaborative manages the New York Phenology Project, utilizing the USA-National Phenology Network database and Nature’s Notebook observation platform.