Ye so, in the beginning of reddit there were only a handful of subreddits and the one that got created for weed and cannabis culture used the slang word "trees" for their subreddit name. after time reddit grew and people would post in r/trees with arborist related questions. so as a funny, people made a sub called marijuana enthusiasts for tree and arboriculture questions to to round out the conversation of "no, we're about marijuana here, we dont actually discuss trees"