[Pollinator] Maples and Willows

[Kirk Wattles]

Hi,

I'm exploring topics relating to bee-friendly forage plants, particularly trees and shrubs.  A friend suggested I post my question to this group:

"Difficult as the problem once seemed, there is no longer any mystery why the flowers of many forest-trees appear before their leaves. They are or were in time past pollinated by the wind, although the willows and maples have in comparatively recent years changed over to insect-pollination." 

— p. 21 in John H. Lovell, "The Flower and the Bee," first published in 1918, reprint edition Wicwas Press, 2015.

The book is also online, at:
https://archive.org/details/americanhoney00pell

It's a great book.  John Lovell was deeply immersed in the botanical literature of his day, and he did a lot of fascinating hands-on research with flowers and bees, which is detailed in his book.  The quote above tells a nice story that I'd like to retell in various contexts, if only it were true, but I need to verify it.

The closest I've gotten so far relates to Acer negundo (Box Elder, Ash-leaved Maple), a species which supposedly has been sliding the other way, from insect-pollinated to wind-pollinated.  And supposedly the other maples are following in the same direction.  (D. M. van.Gelderen, Maples of the world.)

So specifically, with willows and maples, what does the evolutionary record show?

What's the time scale?  In what periods have the individual species slipped significantly in one direction and/or the other?  How quickly do they slide?

Are there specific citations that would help me nail down these questions? 

I have other questions, too, which may be on-topic here if this one is.  But this is the one I'm most curious about.

Thanks.

--
Kirk Wattles
'Bees and Trees' project committee
Philadelphia Beekeepers Guild