Moisture, a few warm days, and +15hrs of sunlight. Trees went from budding to massive dense foliage.  My relatively subjective signal penetration, strength test is a flight at ~1.5M altitude down the driveway ~300 meters to the mailbox. Solid wall of trees from launch pad to mailbox.  Have never made it down to the mailbox with the analog hardware at various vtx power without significant breakup or total loss of signal.  Even when the trees are bare.  Avata2.  4 bars for both video and rc.  Yesterday, not so.  Video, yellow alert. Rx, 3rd bar flickering. At the mailbox needed to climb above the tree line to regain a strong signal.  Always has amazed me the effect that just leafs have on signal strength.  

On the positive side or maybe just a one off.  There has been a "wall" over my neighbor's orchard directly to the W of my 40.  He has given me permission to fly over his orchard.    Hitting the "wall" and need to turn around. Video and RC goes from 4 bars down to 1 in an instant.  Yesterday flying a few meters above the surrounding tree tops. Instead of heading due W.  Zig-zagged heading N and S with some W bias.  No "wall".  Made it over to the W side of the orchard. Flew straight E back without issue. Had me guessing.  Maybe the foliage is blocking the maybe interference from the cell/wifi tower located uphill?  Next test flight.  At altitude to be in direct line with that tower and the one S.

It's all about the moisture,  the water sucks up the RF signal strength better than most other items.