Thanks for firming up my initial impression. Looked into Albizia julibrissin seeds after hearing of Caroltropicals' experience and found they're known to remain viable in dry storage for nearly 150 years.... so imagine a mere 10 or 20 in well draining soil is not unreasonable. The spot is in a corner of a long alley that has always collected a lot of windblown tree debris, such as leaves...... or seedpods. Am guessing I disturbed a seedbank when I removed the mulch and turned the soil in a multi-season effort to eradicate running bamboo that finally found its way in from the adjacent yard. Thanks again for the inputs.

Oh it's just the same old 'invasive weedy tree' thing, that is being discussed in a thread on the Tree Forum. http://forums2.gardenweb.com/forums/load/trees/msg0700432818752.html?15

Yellowing or stunting, followed by early defoliation, are typically symptoms of Fusarium wilt, common to silk trees. Unfortunately, the disease is eventually fatal and there is nothing you can do with regards to treatment. Since this tree is a listed invasive species for your area, removal should be considered a good thing and an opportunity to replant with a more desirable tree :-)

If you're interested, I've got a couple of these in my yard and it produces LOTS of seed pods. I can send a bunch of seeds for you to try and germinate if you'd like... I also get lots of volunteers in the yard too - one sprouted a about 3 years ago next to the garage in my North side yard where there's NO supplemental water and little direct sun and it's 15ft tall now. They grow very fast from seed. My "main" tree is on the south side of my house and gets full sun and some supplemental water (not much). It has been there since I've owned the house (7 years or so) and it has a great umbrella canopy about 20-25ft tall. It's just about to start blooming, the hummingbirds love it!