1.   The trees are being killed by acid rain and other industrial emissions.

2.   A virulent and incurable fungus known as the pitch canker is sweeping through the last stands here, killing healthy trees and threatening to turn the verdant hills brown.

3.   A very small fire completely kills paloverde trees, for instance.

4.   But constantly soggy soil can rot roots and make leaves fall, and can even kill the tree.

5.   But even killing that tree requires a permit, and the county feared that Brown would go after protected trees next.

6.   But if the flooding continues to kill trees on a widespread basis, the recovery could take decades.

7.   But maple trees throughout the region suffered a staggering beating during the freakish January storm, which stripped trees of branches, uprooting and killing many trees outright.

8.   Cutting tree roots to put in a sprinkler system can damage or even kill the trees.

9.   Did the beetles kill the tree, or did something kill the tree and the beetles came in?

10.   Even though trees were killed or damaged by the recent fire, there remains a potential for a new kind of threat as they fall to the ground.