By the way, doesn't Lillie P. Bliss sounds almost like a pseudonym someone would daftly invent, were one in a screwball comedy? The real Miss Bliss was a key figure to MoMA due to the terms in her will. She left a large art collection to MoMA but also allowed MoMA to sell art from that collection in order to acquire better pieces. For example, Les Demoiselles d'Avignon was partially bought with funds raised by selling a Degas in Bliss's collection.
Back to the garden:
When MoMA was redesigned by architect Yoshio Taniguchi, Taniguchi preserved Philip Johnson's original design (1953) for the garden. Some expansions were made, but for the most part the garden was untouched.
Note that the bridge is comprised of only three slabs of marble. In the background, there are the Bertoia chairs we last saw in Paley Park.
The weeping European beech trees, or Fagus sylvatica, (so sensitive to compaction due to their shallow roots) were preserved, though I'm skeptical as to whether I could say the same of the birches (Betula populifolia); they look a bit small for the originals.
Birch trees seem to be a touch ubiquitous outside museums. The Tate Modern in London opened 53 years after MoMA, and birch trees were used once again. Of course, birches have an austere quality that mirrors the sense of peace and order you achieve in a museum, and they're vertical, compact plants which work in the urban environments of most museums.