There are 2 allergens associated with alder tree pollen, Aln g 1 and Aln g 2. Both are allergens associated with airways.
Alder pollen occurs in the UK between January and April, peaking in February. This varies in different countries. It is considered to have medium to high allergenicity.
I have put together some more comprehensive resources at http://www.allergyresources.co.uk/Alder.php
The allergy resources page for alder pollen covers the key allergens, which allergic syndromes are linked to this allergy and other foods which are commonly cross reactive with alder pollen.
There is also regularly updated links to useful websites specifically for alder 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.