If it’s just a reply to your self, it’s an ordinary https://dev.twitter.com/rest/reference/post/statuses/update with
in_reply_to_status_id set to the id of your first tweet.
Mentioning lots of users in a single tweet is a bit annoying though. How about:
Post your Tweet asking for RT = 1 Tree,
Upload photos of your planted trees as a reply to your original tweet, and Tag https://blog.twitter.com/2014/photos-just-got-more-social people (up to 10 per tweet) manually instead.
Tagging people instead of @ mentioning them might be better because: The “Thanks” tweet will still send a notification to users, but you won’t be able to tag people unless they have “allow anyone to tag me in photos” or “Only allow people I follow to tag me in photos” & they follow your account - these people likely won’t mind the followup, but you won’t be able to tag anyone that doesn’t want to be tagged.