How are stress fractures of the lower back usually treated?

Mostly non surgical. Lifestyle modifications physical therapy orthotic devices, such as corsets and braces not often surgery?
Conservative. Lower back stress fractures, usually called spondylolysis, are usually treated with non-operative means. Some docs recommend bracing, but others recommend pain-free core lumbar stabilization protocols. These are notorious for not healing, but most patients become non-symptomatic with spondylolysis. The condition may progress to instability (spondylolisthesis) over years or decades.