The maritime holly forest of Fire Island is dominated by American holly (Ilex opaca), some trees estimated to be up to 300 years old. Other trees in this forest canopy include the sassafras (Sassafras albidum), shadbush or serviceberry (Amelanchier canadensis), and other hardwoods that are able to develop behind tall secondary dunes. The black oak (Quercus velutina), post oak (Quercus stellata), black cherry (Prunus serotina), and pitch pine (Pinus rigida) may be seen in the Sunken Forest, although none of these trees will grow higher than the dunes that protect them. A fully mature oak may be no more than 25 feet high when exposed to harsh maritime conditions.
In low depressions where fresh water can accumulate, bog species may grow. Red maple (Acer rubrum), blackgum (Nyssa sylvatica), blueberry (Vaccinium sp.) and other shrubs, and sphagnum moss and ferns are also found in the Sunken Forest.
Vines are particularly abundant and add to the mystique of the Sunken Forest. Wild grapes (Vitis spp.), greenbriar (Smilax rotundifolia), Virginia creeper (Parthenocissus quinquefolia), and poison ivy (Toxicodendron radicans) are common. In addition to providing cover, their fruits provide food for a variety of birds and other wildlife that may be seen in the Sunken Forest.