A tree is nourished by the sun, by nutrients in the soil and the air and the refreshing rain, among other ideal conditions for its growth. So is our being nourished in different ways. We are nourished by external factors such as movement, fresh air, nutrituous food and so on. There are also internal factors that give us nourishment such as healthy thoughts and beliefs, receiving and expressing love and a life of purpose and contribution. To me, 'the green tree in your heart' refers to keeping your heart and mind and body open and nourished.