Or, to be more specific: do trees die of old-age, and if so, why?
I can understand a tree getting diseased, but I don’t follow a tree dying because of its age. It doesn’t face the same issues as animal life where organs simply wear out, and if a branch got the tree equivalent of cancer (is there such a thing?) then it could just let that bit die and drop off. As a worse-case scenario, I don’t see why a tree couldn’t let everything die off from the roots up and start anew with a brand new trunk.
Is it the case that if a tree avoided disease, didn’t outgrow the ability for its surroundings to support it and never had anything else damage it; it could, quite literally, live forever?
Possibly, BUT those conditions simply do not occur in nature; the longer something has been around, the more chance it has had to be attacked by insect or disease, struck by lightning or otherwise physically damamged; the bigger a tree gets, the more heartwood it has and this is an ideal growth medium for fungi (in some cases, the fungal decay of the heartwood can release nutrients and the tree will sometimes grow roots into the central cavity to reclaim them).
There’s got to be a physical limit to size though, hasn’t there? (and once the tree has produced a few million offspring, evolution is finished with it).
In addition to that, there are manmade physical hazards. For example, If you start parking a car over the roots, the compression will kill the roots. If you add 6 or 8 inches of soil to the ground, you’ll suffocate the roots. If you dig a ditch to repair a sewer, you can wipe out half the roots. Some kinds of weed killer, applied to a gravel driveway over roots, can stress the tree. When a tree is stressed, it is vulnerable to bugs and fungus.