Among the curious things people to do trees is improper crown reduction resulting in a crown that looks like a lion's tail. It is terribly heart breaking to see a tree, like the sweetgum above, that is not growing like a sweetgum (the difference between sidewalk and lawn habitat?). Also, this sweetgum has been raised improperly. The Forest Service recommends maintaining "live branches on at least two-thirds of a tree's total height" (USDA Forest Service publication NA-FR-01-95).
The sweetgum's crown
can be is described as follows:
Young sweetgum have a strong excurrent growth habit and long, conical crowns that usually prune themselves readily under forest conditions. There is a wide range in branch angle from acute to almost 90' in young trees. Depending on site quality, and at a definite stage in development, sweetgum. becomes decurrent as the trees mature, and the crown becomes rounded and wide spreading. The tops of overmature trees are usually broken or stag headed.
The horticulture program at the University of Florida describes the sweetgum's crown as follows: "symmetrical canopy with a regular (or smooth) outline, and individuals have more or less identical crown forms [and the] crown shape [is] oval; pyramidal."
Here is another example of lion tail pruning on the left with fuller crowns on the right side of the street without overhead wires. The trees on the left
right resemble begin to look like palms.
One solution to a tree whose trunk is broken: repair with wood and nails.
A common urban design element is the tree grate (more popular on the East Coast than in the SF Bay Area?). Surely it is time to remove this grate? (Disclosure: the photograph was taken in 2003.) Read a related post about infrastructure - street tree conflicts.