one one had Japan seem to have quite bad luck with the specific tree(s) mass planted

but also on the other hand in Germany problems with allergies are very common and a pretty big deal for many people, it's just that we got used to it

but also while Germany has not-very-diverse "tree farms" for a very long time, the level of monoculture got way worse in the last 70-100 years AFIK, especially after WW2 the only way to cope with the extreme high demand was to mostly plant very fast growing trees. I.e. mostly spruce and pine.

Idk. if allergies got worse due to this and we just didn't notice because of having so much bigger problems (like many cities lying in ashes) or if Germany always had similar bad allergy problems. But this WW2 induced increase in monoculture is still a huge problem even ignoring allergies as this made German forests especially susceptible to things like pests and adding stress from climate change has lead to mass dying of trees in some regions.