Philippe Cousteau looks at ways scientists are measuring the health of the forests. In New England, Philippe visits a white pine forest that is being threatened by invasive plants. At the University of California - Santa Cruz, students use sonic-tomography to look inside trees in order to detect any signs of disease. At the Pine Barrens of New Jersey, discovering why the pitch pine tree is able to withstand occasional wildfires. And in Los Angeles, volunteers are planting trees.