Model selection for basal area trends

I'm currently working analizing basal area trends contemplating three different temporal points using national forest inventories from two different countries in Europe (Spain and Sweden). Time between censuses varies from countries and I think it's something I should have into consideration.

The main idea is analize how extreme climatic events have affected these basal area trends over time. So I'm using the periods between inventories to calculate the frequency and intensity of these extreme climatic events.

My problem is that I'm not sure what model should be used to analize the effects of these events in basal area trends. Something I don't know how to manage is that basal area trends between de first and the second period are no independent each other. Also to analize the effect of extreme climatic episodes I was calculating annual mean climatic variables, and they are highly correlated with annual mean extreme climatic variables (droughts, heatwaves...). I'd like to have some suggestion about how to start dealing with this problema.

If anybody could suggest me ideas or factible ways to deal with this problem, I would be highly thankfull.

Thanks in advance for your time.
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