I love this post. 
I live in the desert in northern arizona by the utah border. I have no pesticide problems and varroa, although present, has not been a problem yet. 
Figuring out which plants will provide the nectar flow is my main problem. So the question of when a plant provides nectar or not is forefront in my mind. We had very little rain this spring (it rained once for about 10 minutes) and so we had a rather weak bloom. There were still flowers present but for the second year in a row I was forced to feed. I finally realized that i just had to move my bees. Which I did last week to a higher elevation which has a lot of snow melt and consequently the meadows bloom with a plethora of plants unavailable in the desert just 20 miles away. Should have done that from the beginning but I'm a newbie and the only beekeeper for 2 hours in any direction so it has been touch and go for the last 2 years. Many of the listed Arizona honey plants are just not present in my region. 
Sorry for the personal tangent. 
My obvious first question would be to ask you how much snow melt there is in your area and how much spring rain you have had before the trees bloom? And how it has differed in the years before and after the bees used these plants. 
Second, has it been hotter at the time of year when the trees bloom? I am wondering if it is too hot during the bloom if either the nectar available in the flower can evaporate, or if the tree "chooses" not to provide the nectar and instead retains its water for its more immediate needs. 
If anyone else knows more about what effects nectar availability i would eat your post up. 
J Nadal
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