How do I continue to progress after completing the tree?
Continue practicing with the "strengthen skills" button until you're level 25. Then you could do the reverse Spanish tree (English for Spanish speakers). Also, there are many resources that teach more advanced Spanish than Duolingo. https://www.reddit.com/r/languagelearning/wiki/index
After finishing the tree, keep the tree gold for a while, to let Duolingo's spaced repetition algorithms help you revise the stuff you've learned. At the same time, start reading Spanish children's books, and listening to children's radio programs. If you can get a book and audiobook of the same thing, that can really help to improve both your reading and your listening skills.
A lot of people on these forums recommend using Memrise courses to expand your vocabulary, and Lang-8 for improving your writing skills.
You'll probably also want to find someone to speak Spanish with. Have a look at this post about how to find language exchange partners: https://www.duolingo.com/comment/1444346. There are links to some useful websites on there. Don't miss DamedesLangues's comment, which has links to more sites.
It's also worth looking on the Multilingual learning resources page on the Duolingo Wiki.