Jason Mraz isn’t one of those rock stars who holes up in the hotel room while on the road. Before his gig at the Comcast Center Friday, Mraz spent the afternoon with a few dozen fans on Castle Island. Together, they tended to the trees around Fort Independence, which Mraz told us are “a bit stressed.” “We did some deep-root fertilization,” said the environmentalist and amateur arborist. “I’ve been doing this in my own backyard for quite some time, and I realized I could do it when I’m on tour.” Mraz was joined by the gang from MIX 104.1 and a crew from Southie Trees, an environmental group trying to protect and enhance tree cover in South Boston. “I like to do something that will have a lasting impact,” the singer told us. “I’ll be able to come back and visit these trees.”