If you have a specific question about the High Line's plantings or its ecosystem, gardeners (Andi, Johnny , Kaspar, Kyla, Maeve , and Maryanne) are a great resource. They regularly answer questions sent to the Ask a Gardener  page on our web site.
Here's one of our recent favorites:
QUESTION (via Jean, Hopewell, New Jersey):
- I have not known any indigenous birch trees anywhere in NYC. Was there ever a time when they were native to the City?
ANSWER (via High Line Gardener Andi Lawton):
- Betula populifolia, or grey birch, which grows on the High Line, is native to the north eastern United States along with several other species of birch. Researchers with Brooklyn Botanic Garden's New York Metropolitan Flora Project , have sighted Betula populifolia growing in King's County before and since 1980, though not in large populations. I also checked the Mannahatta Project  web site. According to their research, birch trees, including Betula populifolia, were probably growing on the island of Manhattan in 1609, before Europeans ever set foot here.
Other birch trees native to the area include Betula lenta (sweet birch) and Betula papyrifera (paper birch).
Thanks for your question. I always love an excuse to do some plant-related research!