What is the significance that Saul mentions there are no trees around the building but only shrubs? Think of the role nature has played this far.

I think this is Saul referring to St. Jeromes School. It is the antithesis of nature. The school is an ugly building, around which there are no trees, only shrubs. This represents the lack of life and spirit of the place. Saul misses the dense forests, tall trees, and the smells of nature. All Saul smells at the school is disinfectant.