How do you estimate the age of immigration of a specific pollen grain e.g. Betula from a pollen diagram?

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How do you estimate the age of immigration of a specific pollen grain e.g. Betula from a pollen diagram?
Pollen diagrams should have their own time scale as an axis of the diagram. The researcher who put the diagram together might have dated the soil profile by using things like charcoal that can be carbon dated. Betula will often be present in large amounts if it was growing close to the site of deposition because is Betula is a wind-pollinated plant.