The next time you take a walk, encourage your children to "adopt" a tree in a park or other public place. This makes for a personal connection between your child and his or her environment, and also builds a sense of local knowledge. Explain that no one can really own a tree, but that a person can take on the role of the tree's special friend and caretaker.
Help get to know your tree together. First get to know the tree by do bark rubbings with crayons and paper in the winter, look for flowers and fruits in the spring, and do leaf rubbings in the summer.
One walks, your child should bring along a drink of water for the tree. Older kids might also keep a little "log book", noting the tree's characteristics: height, leaf color, flowering time, fragrance, and so on. In addition, you might show your child how to use a tape measure - and note the tree's girth on a chart.
Finally, be sure to take pictures of your child standing next to the adopted tree once or twice a year. As your child gets older, the three will serve as a reminder of the patterns of growth common to all the residents of your community.
I would also recommend reading the book, "The Giving Tree" with your children!