Source: patkgreen fom a reddit post 14 years ago answering the question "do trees die of old age?"

The answer, of course, is sort of. Like another poster said, old age is technically a secondary cause. What happens when a tree gets old is that it can't protect most of its inner heartwood or exterior. It certainly does not fight damage as well.

Woody plants heal through CODIT (http://en.wikipedia.org/wiki/Compartmentalization_of_decay_in_trees) in four steps, called walls. This is originally from a study where a guy named Shigo axed a bunch of trees and watched them heal.

The four walls are explained in the wiki, but I'll simplify it.

Wall 1: the tree prevents the wound/infection from spreading up and down the tree by blocking the transport tubes on either side of the wound.

Wall 2: the tree prevents the wound/infection from spreading inward by building up a ring to act as a barrier

Wall 3: the tree prevents the wound/infection from spreading circularly by sectioning the wound laterally. This basically means the wound/infection can't spread left or right. After wall 3, the would should look like a piece of pie.

Wall 4: the new growth rings seal off the wound/infection so it doesn't spread outward, as the trunk grows.

So in old age, the trees lose the ability to compartmentalize their wounds and eventually die. When wall 4 fails, you'll find a standing tree with a hollow inside- it's not uncommon to see them with leaves still. But unless wall 4 has succeeded just in the nick of time, the tree will die soon.

Another way trees die is they girdle themselves because they get too big or something that is sucked up by the roots get caught in the xylem and phloem. Think of it as getting a piece of food caught in your throat. It slowly chokes and dies.

Girdling is also a cause of death from insects and beavers. Most animals and insects only eat the newest part of the tree, where the xylem and phloem channels are active. This area, just under the bark, is called the cambium. When this cambium is eaten, the tree has no way to pass water/food/nutrients up and down the tree, essentially starving.

When a tree is old enough for the bark to start falling off simple weathering is enough to damage the flow of food/water/nutrients and that is as close to dying of old age as a tree can get, in my experience.

Lastly, some trees literally have no ceiling of growth (redwood, white pine) except for when the atmospheric pressure prevents the tree from getting its food/nutrients/water to the roots or leaves. In this case, they eventually die because they weaken and can no longer transport at a functional rate.

As an edit: when trees get unhealthy, you'll see weird sucker sprouts along a normally bare stem, or lots of leaves in clusters in weird spots.

source: professional ecologist

We aren't killing the host. The host will go on, we're just destroying the growth medium for ourselves and other creatures.

Something will go one even after we manage to wipe ourselves out.

Every day we stray closer to The Matrix

how exactly are we killing the planet? last i checked we struggle to drill 5km down

Drunk + target fixation.

On accident? Seems unlikely.

We do and we might get back there eventually if we keep up with water harvesting techniques and permaculture designs in the region, which are aimed at greening the edge of Sahara

Andrew Millison has a few episodes on this on his channel. For those interested, non YT links:

How Africa's Great Green Wall is Solving an Ancient Conflict which shows projects in Chad

How Refugees are Greening the Edge of the Sahara near the border with Sudan in Chad

Inside Africa's Food Forest Mega-Project with projects in Niger

New trees haven't been able to grow there for centuries. You'd have to spend a century giving the new tree progressively deeper irrigation until the roots reach the current water table, 40+m deep.