I work in a large suburban office park (southwest Ohio, USA) adjacent to some overgrown, secondary growth woodland on a steep hillside. I noticed today (late April) that the undergrowth (I recognized Osage Orange, as well as other shrubs and some herbaceous plants) were all fully leafed out and green as though it were midsummer. But the taller trees were only just now beginning to put out leaves. Of course, I thought, there'd be selective pressure on the understory plants to put out leaves as early as possible before they are shaded by tallest trees.
But then I noticed that the tallest mature, landscaped trees in the office park, with only lawn beneath them, were all fully leafed out.
Is it possible that the undergrowth plants are doing something (chemicals, maybe?) to delay leaf development in the taller trees? Or is there a difference between the "wild" (well, feral is probably closer) tall trees in the woodland versus the domesticated tall trees in the landscaped areas?
(I wish I could identify some of the taller trees for you, but it was far away, and I can generally only recognize a few species by leaf shape.)