That also depends on the municipality. I'm going for my ISA certification and I'm on the scientific side of arboriculture where phytopathology/entomology/physics are all advanced elements requiring more than just that certification. But here every single person who works on a tree needs to be an ISA arborist. Even the shittiest tree trimmers are either certified or are hired with the possibility of certification in like 6mo, and the only professional difference between them and I is that I'm testing for climbing/risk assessment/municipal specialist certs which make me a more expensive arborist.