I just learned of this GREAT group, Plant with Purpose. I'll let their site do the explaining - they can do it better than me!!!
I'd start with the "Trees Please" icon, for a great explanation of who they really are.
To learn more about them, explore the "Our Purpose" link, and to take action, click on "Get Involved" (from here you can click the sublinks depending on what you are interested in).
I know there are many social/environmental groups out there, and we can't support them all. My hope is that you'll find one that speaks to you, and support THAT one!
Remember - "It is not enough to be compassionate. You must act." ~ Tenzin Gyatso