Probably we can blame higher hygiene standards, or some other environmental factor for it. Forests haven't changed much in past decades.
Here in Finland I've never been affected by any kind of tree pollen at all, but somehow timothy grass pollen gives me horrible symptoms, forcing me to take antihistamine most of the summer. I lived my childhood near farmland and forests, so definitely got exposed to both forms of pollen at early age.
food is full of histamine, especially fish and fermented food which is considered healthy but some (MCAS, HIT) people are sensitive.
Perhaps we can blame exposure to actual pathogens which resemble the pollen in some way, triggering a misdirected immune system response.
Trees react to climate change.
After droughts some release more pollen as a survival reaction
Yes timotej is my allergy as well.
And I got it as an adult, in 2009. So 26 years without any allergies, then suddenly, one summer in Helsingborg, the air was thick with pollen. I remember the smell was like cheese doodles in the air, musty.
Once I got back from an errand in the city my face was leaking, I walked to the pharmacy with blurry vision to get my first antihistamines. Ever since then every year june is a nightmare. It affects your sleep, so it affects every part of life.
And since then I've observed more and more pollen allergies around me, friends, co-workers, strangers on the bus. It's very prevalent.
I would not be surprised if humans caused this somehow with our modern city planning.