In case anyone else is wondering what “Carbon Storage Value” is, in relation to the $50 billion worth of trees in American cities, here is the explanation by Nowak et al. (2013):
To estimate monetary value associated with urban tree carbon storage and sequestration, carbon values are multiplied by $78.5 per tonne of carbon (range = $17.2-128.7 tC-1) based on the estimated social costs of carbon for 2010 with a 3% discount rate (Interagency Working Group, 2010).
I’ll have to dig a little more into the “estimated social costs of carbon,” but the detail of what that $50 billion number actually means seems to have been lost on many media outlets.