There is only one pollen allergen associated with poplar pollen, it is an allergen associated with airways. Poplars are in the same genus of trees as cottonwood.
Pop n 2 is a profilin protein, these are panallergens and can cause allergies across multiple species of plants and foods.
Propolis is the glue made by bees to build and repair their hives. In Europe the most common tree buds used by bees to make propolis are from the Poplar tree. Propolis is a mixture of tree waxes and resins, balsam, essential oils, pollen and other vitamins and minerals.
I have put together some more comprehensive resources at http://www.allergyresources.co.uk/Poplar.php
The allergy resources page for poplar pollen covers the key allergens, which allergic syndromes are linked to this allergy and other foods which are commonly cross reactive with poplar pollen.
There is also regularly updated links to useful websites specifically for poplar pollen allergies and the latest research papers on the topic.
If you have multiple allergies and are interested in how they are linked or want to find out more about food allergies outside the usual top 14 then you can follow me on Facebook or Twitter, where I share a food allergy card every day of the week.