I also heard that there was a C02 spike during Autumn because of the all the carbon released from the decaying leaves.
But going with the 48lbs per tree per year. A bit of Googling suggests that the UK contains about 2bn trees, enough woodland and forest to 'absorb' 17m cars per year. In more general terms, we need 6bn trees for all transport related emissions and 8bn for all emissions.
Not that I believe the is a GW threat, just in case anybody gets the wrong idea!