Although ragweed is the main culprit for most hay fever cases in summer, some trees also contribute considerably to the pollen in the air. Here are some of the top "problem" trees for people with allergic rhinitis or pollen allergies:

Grey Alder: Cross-reactivity may also occur between this and birch or oak
White Elm: A major source of pollen and is a favorite avenue tree in cities and parks
Olive: There are 10 allergenic components identified it its pollen, and cross-reactivity between olive and Ash has been observed.
London Plane: An allergenic glycoprotein compound has been isolated from the pollen extract..
Willow: High cross-reactivity between the different species and with Cottonwood
Cottonwood: High cross-reactivity between the different species and with Willow

Read more about tree pollens and allergies in this article from Health24.