I believe a dendrologist is his position at NRCan, I'm not 100% sure, but he's also a registered professional forester. That's a different qualification. A certified arborist is somebody who practises in the field and has certification offered by an organization called the International Society of Arboriculture. You write an exam to enter, and then you have to maintain your certification with continuing education credits.
On April 2nd, 2019. See this statement in context.