I have an Acuba (Aucuba japonica Thunb.) bush http://en.wikipedia.org/wiki/File:Aucub … ica_01.jpg and last year I trimmed it and put the cuttings in my compose pile. The cuttings from the Acuba still look the same as the day I cut them, which got me wondering that maybe evergreens, because of their slow decay have a smaller carbon footprint then deciduous plants.
Obviously plants are carbon sinks, but unless the carbon that they remove form the atmosphere is locked up indefinitely in their tissue, it will find its way back into the atmosphere through respiration.
Leaves are generally poor long term sinks of carbon as they have relatively quick rates of decomposition when compared to wood. Although there is significant differences in the decomposition rates of leaves from different species, I don’t know if I would draw the line between evergreen and deciduous species. There are many evergreen species that have large soft leaves which are easily decomposed but the climate that they evolved in had long enough growing seasons that is wasn’t worth dropping their leaves over winter.