Thinking about trees in cities here's an experiment that you might like to try:

If you happen to live in a largeish city in the UK pick any two points, about two miles apart, and connected by a bus route. Take the bus ride between the two points (preferably on the top deck - if you have double-deckers in your area) and try and count the trees that you can see. I suspect that you'll probably give up after the first quarter of a mile!

If nothing else, this experiment might encourage you to really look at the world around you. And if you have convinced yourself that tree planting is a good, and invariably virtuous, activity it might encourage you to think about how many trees we've already got, how many we actually need and whether we really need to plant any more.