You can know the age of a tree by counting its rings.

[keannu]

What do you think is the difference between 1 and 2 in terms of "a tree" and "trees"? Does "a tree" denote "an unspecific tree" or "general trees, generalization"? It sounds either way, ambiguous.

1. You can know the age of a tree by counting its rings. This is because each ring shows one year of wood growth...
2. You can know the age of trees by counting their rings.

 

[tedtmc]

Both are general statements. They mean the same thing.
A general statement can be singular or plural.

not a teacher

 

[probus]

I don't see any difference. In 1, a tree implies any tree. In 2, trees implies all trees. So logically they are equivalent. My preference is 1, but I wouldn't criticize someone who used 2.

BTW determine would be better than know in both cases.