Please answer a question about the following article about The Bronx:

The northern side of the borough includes the largest park in New York City—Pelham Bay Park, which includes Orchard Beach—and the fourth largest, Van Cortlandt Park, which is west of Woodlawn Cemetery and borders Yonkers. Also in the northern Bronx, Wave Hill, the former estate of George W. Perkins—known for a historic house, gardens, changing site-specific art installations and concerts—overlooks the New Jersey Palisades from a promontory on the Hudson in Riverdale. Nearer the borough's center, and along the Bronx River, is Bronx Park; its northern end houses the New York Botanical Gardens, which preserve the last patch of the original hemlock forest that once covered the entire county, and its southern end the Bronx Zoo, the largest urban zoological gardens in the United States. Just south of Van Cortlandt Park is the Jerome Park Reservoir, surrounded by 2 miles (3 km) of stone walls and bordering several small parks in the Bedford Park neighborhood; the reservoir was built in the 1890s on the site of the former Jerome Park Racetrack. Further south is Crotona Park, home to a 3.3-acre (1.3 ha) lake, 28 species of trees, and a large swimming pool. The land for these parks, and many others, was bought by New York City in 1888, while land was still open and inexpensive, in anticipation of future needs and future pressures for development.

What do the NY Botanical Gardens preserve?
the last patch of the original hemlock forest that once covered the entire county