Plants don't usually store CO2 long term. You have to do something with them after they grow, or else they decompose and put the CO2 right back into the air.
That's not true, a stable mature forest is a long-term carbon store, the release from decomposition being offset by capture by new growth.
And depending on the underlying soils it may be a net sink until it's clear cut and burned away (e.g. peat swamps as in south-east asian islands).
Growing forests are carbon sinks, until the forest stops growing (and it becomes a carbon store)...
http://www.wri.org/blog/2013/11/carbon-dioxide-emissions-fos...
Total carbon stored worldwide in biomass, dead wood, and soil is about 640 Gt.
ftp://ftp.fao.org/docrep/fao/010/i0105e/i0105e04.pdf
Of the 550 Gt of carbon humans induced into the atmosphere since 1870, 390 Gt was emitted directly (through burning, cement production, etc), and only 160 Gt was through deforestation and other land use change.
That's a nonsensical argument and quite literally missing the forest for the tree.
[1] http://www.ipcc.ie/a-to-z-peatlands/peatland-action-plan/cli...
It's more meaningful to reason about co2 storage in terms of area dedicated to plants, than the plants themselves.