I must point out that, while this yard looks very barren, it is much nicer in the summer. THis was taken before many of the trees got their leaves. This zoo looks 100x better in the summer.
I've seen all of America's great zoos in the summer (2008 and 2010 road trips) but you bring up a good point. Many northern establishments can be disappointing in the winter, with cafes and shops closed and barren, dull yards with bare trees. It is tough for such zoos to be compared to southern collections in terms of green, lush surroundings in places such as San Diego.