I want to use the word demography but that is human specific (demography: the scientific study of human populations, esp with reference to their size, structure, and distribution). Is there an alternative which is not human specific, particularly it's for a biological question about the distribution of trees.
What you describe sounds to be a specific example of population ecology.
The term "population" is interpreted differently in various sciences:
In human demography a population is a set of humans in a given area.
In genetics a population is a group of interbreeding individuals of the same species, which is isolated from other groups.
In population ecology a population is a group of individuals of the same species inhabiting the same area
If you are looking for an accurate corresponding synonym it will have to be a neologism - DENDRODEMOGRAPHY; and I see nothing wrong with that term.
Otherwise plant demography as suggested is perfectly acceptable.