I love brush piles. On our land there are lots of them. For years our land was dry-farmed for almonds, and after that, it has been left in a natural state for over twenty years. In the riparian areas, the larger willow trees often have their soft branches broken in the wind. In the savanna areas, the scattered almond trees need pruning. I seldom use my prunings for firewood. I use them for brush piles. Even when a large oak dies, its dead body becomes a critter haven and its branches are protective covering for its now-growing progeny, while most of it dissolves into apparent nothing.
To the south and up the hill from our fruit orchard, there are two large brush piles. Both have a couple of ten to twelve-foot-tall vigorous ailanthus (trees of heaven) growing from the middle of them, and several smaller toyon bushes. The toyon produces summer flowers that the bees like, and the ailanthus is symmetrical and vigorous looking. Without the brush piles, probably none of these trees would have occurred or survived. Certainly, they would not have thrived.
What does a brush pile do? It provides a place for birds to perch and drop seeds on which they are feeding, some of the seeds first going through their gut. The shade from the brush pile stops growth of the abundant star thistle, which would otherwise out-compete the trees and the bushes. Brush piles also provide safety from deer and rabbit browsing.
When I think of their storage capacity for my pruning, the nursery haven for new trees and bushes, the shelter offered to quail and many other creatures, I figure the brush pile is one heck of a good ecological deal. Too bad most people simply feed their brush to a fireplace!