I confess, I did not know that trees were the MAJOR source of honey.
I would much rather let the bees do the work instead of me, tapping trees for sap and boiling it down to syrup.
If you don't watch the video (55:40 min.), I heard these important points:
1) Trees produce more per acre than ground crops like clover or flower corps
2) Trees start their pollen set WELL BEFORE flowering ground crops.
3) "Hybrid" willow are NOT popular with Bees. Native willow is VERY popular.
4) Nectar "flow" is not constant, with periods DURING THE YEAR when the supply is low. This is why you plant flowing ground crops, to cover the dips in nectar flow.
5) Planting crops which flow at different times of the year makes honey production more consistent over the growing season. It also keeps your bees from eating their stores between nectar flows.
The video gives many suggestions on which trees are the best pollen producers. Sorry, I didn't keep a list, so you WILL want to watch/listen to the video. Take Notes if you plan on having trees for pollen production.
Elms WERE (past tense) a major nectar and pollen source, but most Elms were lost to Dutch Elm Disease. New Elms are available, which are disease resistant, but they are not yet as wide spread the original trees.
Alders are also good bee trees, which grow fast and in poor soils.
Watch this video if you keep bees, or if you want to support bees in your region...