Notices like that one are posted on some of the trees along 16th Avenue SW by South Seattle College (WSB sponsor), particularly on the west side. This note we received from SDOT explains why:
The Sidewalk Safety Repair Program is planning to make repairs to the sidewalks damaged by tree roots on both sides of 16th Ave SW between SW Findlay St and South Seattle College Entry #2. Our tentative start date is October 5 and will likely take a few weeks once the work begins. The schedule depends on how much root pruning work there will be, especially on the west sidewalk. Our goal is to preserve all the trees, but there are a few trees at risk for removal because of the size and amount of roots that will need to be pruned to bring the sidewalk and driveways back down for ADA accessibility. The trees that are at higher risk for removal have been posted with SDOT Urban Forestry’s contact information. The sidewalk team and Urban Forestry will work closely together during construction to achieve both pedestrian safety and tree preservation to the extent feasible.