As with most longer-form sentences, the difference is in tone.
" Trees and grass and shade are clam and pleasant": This is a statement of fact. These things ARE pleasant. End of story.
"There is something ____ about ___": Is a form which implies a sense of wonder, mystery, or the unknown. In other words "I don't know WHY trees and grass and shade are so pleasant, but they are". The author can't tell you specifically why, or what it is that makes it so... they just feel this way.
Some other examples which might help:
- "Crowds infuriate me" vs "There is something infuriating about crowds": The first one is a simple fact, the second one is saying "This is how I feel in crowds, but I can't tell you exactly why". Maybe the author has a few ideas as to why, but can't tell you exactly. For example: "There is something infuriating about crowds. I don't mind loud parties with my friends, but when there's a large crowd in public I just feel so angry!"
- "I like running" vs "There is something about running that I like": Again, the first states a fact, and the second is saying "When I run I really enjoy it, but I don't know if it's the fresh air, the blood pumping through my veins, the sunshine - maybe it's all of them!"
In other words, the author here just wanted to add a sense of wonder about nature, rather than making the language dry and boring, they want to make the reader interested in the leaves, grass etc.