Sorry, just seeing this. Generally we only remove trees that are dead, diseased, dying or structurally unsound. Where we permit removals that do not meet these standards the requesting body must replace the trees with the equivalent of the same size tree - eg a tree that is 30" around would be replaced by 10 3" trees. For our projects like the skate park expansion we have actually designed around trees that are healthy.

In some instances trees have full canopies may seem healthy but often root defects, cracks in large limbs or other structural defects exist that can not be remedied by trimming and require that the tree be removed. Our arborist assesses each tree to determine its health and mitigate any hazards.

As to why right now we are seeing more removals there are several factors:
1) Emerald Ash Borer is a bug that eats Ash trees from the inside slowly killing the tree - about 5% of our trees are Ash
2) While we are finally out of drought, the last few years of the stress of drought have taken a toll on many of our trees
3) Storms - we have had a number of very strong storms - these high winds and winter snow weights have caused cracks and other defects to occur or be exacerbated