Sycamore chemical warfare ?

[VA-Sawyer]

I have a customer with a problem that I can't figure out. He has a number of trees in the front and back yards. Oaks, Mulberry, Black Walnut, Hemlock etc. Most are growing pretty well except for three Sycamores in the front yard. They form a triangle about 15-20 ft on each side. They are about 5-6" DBH and near 40' tall. Problem is most of the branches on the sides facing the other two sycamores are dead/dying. Few branches on the lower levels at all. Branches on the sides facing away from the other Sycamores look ok except maybe a little long for their diameter. No signs of recent pruning. The grass and plants at ground level inside the triangle are doing fine, so I don't suspect a chemical spill problem.
I do know that some trees can poison the ground around them to prevent competition. I have never heard of this in Sycamores, and haven't found info on such via the internet. Wondering if this could be the case here. The trees slope slightly away from each other as you go up in height, and I don't see evidence of them bumping into each other due to wind. There are other trees, not Sycamores, around these three trees on the outside of the triangle. No trees on the inside. Some are closer to the Sycamores than the Sycamores are to each other. None of them seem to have dead/dying branches like the Sycamores.
Cause ??? Solutions ???

Thanks,
Rick

 

[BC WetCoast]

Sounds like light competition to me. Got any pics?

 

[Sunrise Guy]

From science article: "Several growth inhibitors, chiefly phenolics, were isolated and identified from sycamore, hackberry, red oak, and white oak leaves and the soils under them. Thus it appears that the low productivity rate and destitute growth under test tree species are due to allelopathy. Ecological significance of allelopathy in a community is discussed."

Thing is, a given species will not inhibit growth of its own species, as those of us who deal with dense stands of hacks can painfully testify.
I tend to think, as another poster wrote, that it may be a light competition issue. The "little long for their diameter" points strongly towards that diagnosis. Pics would be helpful.