Trees are social creatures" that communicate with each other in cooperative ways that hold lessons for humans, too, ecologist Suzanne Simard says. Trees are linked to neighboring trees by an underground network of fungi that resembles the neural networks in the brain, she explains. In one study, Simard watched as a Douglas fir that had been injured by insects appeared to send chemical warning signals to a ponderosa pine growing nearby. The pine tree then produced defense enzymes to protect against the insect. "This was a breakthrough," Simard says. The trees were sharing "information that actually is important to the health of the whole forest." In addition to warning each other of danger, Simard says that trees have been known to share nutrients at critical times to keep each other healthy. She says the trees in a forest are often linked to each other via an older tree she calls a "mother" or "hub" tree. The study of trees took on a new resonance for Simard when she was diagnosed with breast cancer. During the course of her treatment, she learned that one of the chemotherapy medicines she relied on was actually derived from a substance some trees make for their own mutual defense. "One of the main chemotherapy medicines that was administered to me was paclitaxel [also called Taxol]," [she said]. "Paclitaxel is a defense agent " actually a defense chemical " that is produced by the Pacific yew tree, or all yews around the world, actually. It was essential to my recovery."