The videos for my LambdaConf Tree Manipulation workshop are now online. I was moving through the workshop pretty quickly, and we cover a lot of different topics across a wide range of topics related to APL, so don't expect a very focused lecture or anything like that. These videos are more along the lines of an extended Q&A session with some demonstrations and considerations of compiler design, tree manipulation, foundational structures, and so forth.
Part 1: https://youtu.be/lc4IjR1iJTg
Part 2: https://youtu.be/X5_5MtOYNos
If you're looking for a more focused treatment of tree manipulation, please see my talk on tree manipulation at Dyalog '18: