With its branches poking out of a drawstring bag, a potted plum tree accompanied Tristan Shaw to a recent nonprofit luncheon. For the last three years, he has carried a sapling for “emotional support” out to dinner, family parties and even canoeing and camping. Buddy, Pawpaw and now Professor Plum — his three woody companions of the last few years — were collected as seedlings and grown during the summer. Professor Plum is destined to join his predecessors somewhere in Shaw’s yard this fall. But what began as an experiment to take care of himself during stressful pandemic times soon became a w…