The Tree event instructions are in the intro to the mini-event. Admittedly they are not comprehensive. Trees can only sustain 4 attacks before they bubble, then they hang around for 15 mins before disappearing.
It may be possible that someone added the 10 seeds and summoned the trees without anyone around and then donated 14 more. It will be hard to find out as a lot of alliance members don't talk or respond to questions.