These are galls. An insect lays an egg and the plant reacts to it, kinda like a wart...It is caused by the Poplar Twiggall Fly. If there are just a few the plant can grow through it but when there are so many, it weakens the stems.  See handout for life cycle and controls.  http://static.colostate.edu/client-files/csfs/pdfs/poplar_twig_gall_fly.pdf

We purchased this property last July, and know that the previous owners allowed the ground to be continually water-saturated, so this makes sense.  We also have major issues with nipple galls on our hackberry trees.  UGH...this is really a frustrating problem, as the trees are ugly, and don't seem at all healthy.  I understand it is not supposed to harm them, but it seems that with this kind of infestation, the trees can't help but suffer. 
Thank you for sharing the article; and if there is any  further advice, I'd be grateful!

Sharon

You can use a systemic for the Hackberry Nipple gall if it is a continual problem and heavy infestations. Since it was a very mild winter, we expect insect populations to be high this year, unless we have a late freeze...http://extension.colostate.edu/topic-areas/insects/insect-and-mite-galls-5-577/