This of course depends on your research question. If you search the literature, you will find all kinds of measures of all kinds of different ecosystem services.
You can quantify ecosystem services of trees in a couple of ways:
- You select an indicator variable for the ecosystem service you want to study. Examples are: soil organic carbon content for carbon sequestration, standing biomass for wood production, Edible fruits for food supply, ...
- You use an expert panel to assign ecosystem service value to your tree, as we did in this paper: Article Effects of biodiversity loss and restoration scenarios on tr...