Glencarlyn Park is in a highly urbanized area, yet it includes examples of mature native forests, unique water resources, and a great diversity of native flora. In addition to high native plant diversity, the park includes a number of Champion and Significant Tree specimens. The active floodplain and lower slopes are documented as Mesic Mixed Hardwood Forest, with Oak Hickory and Oak Heath forests occupying the drier slopes. The estimated age of the community is over 100 years. It is most likely the second-oldest extant growth of natural forest remaining in Arlington, and appears to pre-date the large-scale forest removal associated with the Civil War (the oldest forest remnant is most likely Arlington Woods within Arlington Cemetery). Invasive plant species are a concern, but there are management plans in place to deal with them.