Trees for Cities are a nice charity who aim to help people to plant more urban trees, which is a great cause as trees make a big difference to the urban environment.
Their website is here: http://www.treesforcities.org/
It’s also worth bearing in mind that hedges are trees too. The individual plants may be humble compared to fully grown trees but they carry all the same environmental advantages and more – they provide shelter for birds, insects and other wildlife, they help to absorb CO2, filter pollution, reduce noise, and they soften the urban environment with a touch of greenery. In the average street there are far more trees planted as part of a hedge than fully grown.
One of the blights of my part of London is people pulling up their front gardens and hedges and concreting over them to create parking spaces. It’s ugly, environmentally damaging and increases flood risk by reducing the amount of soil that will absorb water. Urban hedges aren’t afforded the same protection as their country cousins, but in their own way they are just as valuable in their contribution to biodiversity and the environment. They’re not quite our version of the rainforest, but without them we’d be a lot worse off.