Trees don't live forever. A fully grown cottonwood tree (I'm picking on example I know of, others are similar, but different in details) at 100 years old is impressive, but that is near the typical lifespan and so at 120 years old that tree is dead of old age (in ideal conditions they live to 400 years, but 100 years is above average in nature). Forests are a constant cycle of new trees growing.

Forest fires are a very important part of the forest cycle in many places. So be careful about funding efforts to fight forest fires - often the correct answer is to light more of them (see a forester for details of your local forest).