Get a Camera
Documenting your photographs of your favorite trees is a good way to keep up with small changes that you would probably miss. Ideally, one would want to photograph the tree to try to spot seasonal differences. For instance, in the spring if the leaves are coming out slower than a neighboring tree of the same species, this might indicate that the tree is weak. Likewise, if the tree loses its leaves soon then manybe that is a sign of weakness and you can plan accordingly.
As trees get older they slow down in growth. The tree rings on a tree that you can see after the tree is removed allows us to look at the space between growth rings. The cells of winter wood are darker than spring wood becase the cells grow slower and show up as tree rings.
Typically trees such as our willow oaks grow fastest the first 60 years and then start to slow down,
Here is a photograph of a large willow oak that eventually died from old age. There were factors such as being crowded in between the curb and the sidewalk leaving little room for tree roots to grow and take in moisture and nutrients.