Reddit mentions of Stone by Stone: The Magnificent History in New England's Stone Walls

I just bought it 45 seconds ago because it's been in my Amazon cart for over a year, but he may enjoy it as well. Or he may ask why you got him a book about rocks for Christmas.

https://www.amazon.com/Stone-Magnificent-History-Englands-Walls/dp/0802776876

I believe the consensus answer is that the western US states are dry due to the rain shadow of all the mountains between the Pacific coast and the Rocky Mountains...

But you do have a point. The presence or absence of forests and woodlands do change regional climates. Woodlands, somewhat like large bodies of water do reduce temperature extremes. Read this book -- it tells the story of how the deforestation of New England between 1620 and 1820 MADE the stones rise to the surface (cleared land freezes, so stones rise from frost-heave, woodland soil doesn't usually freeze). No Europeans in New England: few stones, lots of trees. Europeans move to New England: clearcut all the trees, farm intensively, more and more stones appear at surface, 200 years later, farmers create 250,000 miles of stone walls in New England. Then the farmers packed up in the 1820's and move to points west (mostly towards the Ohio River Valley by riding the Erie Canal). Remaining Yankees get jobs in mills, or fishing or whaling...

Today New England is again full of trees (they do come back if allowed). But the all the stones and stone-walls make the whole region unsuitable for large-scale agriculture.