I have used poplar for timber framing, and so have many framers in the midwest for over a century. The tree groes very straight and tall, and has some very predictable design values. It's actually quite strong, flexible and rot resistant, if it's true "yellow" poplar.
I recently had the opportunity to survey and consult on the proposed repairs to a covered bridge in southern Ohio. The bridge has very tall trusses and was built entirely of yellow polar. During the process of developing the documentation I was asked to provide a method of specifying yellow poplar to ensure it was the correct timber. My research showed that there is actually only one species of poplar indigenous to the United States. That is liriodendron tulipifera, or tulip tree.
The tulip is common to many areas of the midwest, and was popular for both timber frame and log cabin construction. This fact lead to it's being harvested vigorously. The regrowth poplar has all too often grown under conditions that do not produce true "yellow" poplar. This due to the fact the rapid growth rate prodces wide growth rings with very thick sapwood. The wood that is harvested from these regrowth forests is often called white or "hickory" poplar (because of the purple and grey coloration). It has significantly lower design values, and is not nearly as rot resistant.
The poplar I have worked with was white poplar. It served the limited needs I required, worked very nicely with a sharp chisel, and checked pretty heavily. Had it been more dense heartwood, and less pithy sapwood, I'm confident it would have been stronger and more stable.
