If you're asking for a list of reasons, 1) environmental damage like fire, drought, flooding, lack of nutrients, lack of sunlight (or oddly enough, for some plants too much sunlight), too hot or too cold 2) diseases, fungi, pasasitic growths 3) insects, animals & the damage they cause 4) age 5) chemical poisoning (including the effects of acid rain) And those are just the simple reasons! If you're asking for the technical reason why they stop growing and die around a certain age, then the answer is not far off from the reason why we live a certain number of years, aging as we go, then die. There is a genetic 'clock' in the cells of the trees which determine, more or less, what the possible life span of a plant will be. The plant begins to create fewer and fewer new cells, while water and nutrients are transported slower and slower until the tree stops functioning and dies. Some life spans are short and some are long. The oldest living tree is over 4500 years old! http://biology.about.com/library/bldyknow102199.htm This link will give you info on it. Meanwhile, there are lots of scientists working on the question of how to locate and manipulate the genetic lock on aging with the hopes of significantly improving both our life spans and our health over time.

You are right and some bristle cone pines are over 5000 years old. All things have a life span.