The planet's going to be just fine and that letter is why Norman Gray is an IT manager for the Astronomy and physics dept.
'A plant is however only a temporary store of carbon, since that carbon will be released directly or indirectly into the atmosphere when the plant is consumed by animals, or dies and decays, or is otherwise disposed of. A forest is a large, temporary carbon store, in a dynamic equilibrium between growing trees and decaying or harvested trees'
Yes, but that's not the whole story. Norm seems to forget - or just never knew - that trees and plants don't grow in the vacuum of a computer program, they grow in a medium called 'soil'. The organic matter content of soil is a smorgasbord of interacting parts, including bacteria, fungi, decomposing organic matter and more. The level of organic carbon contained in soil organic matter is in direct relation to the 'use' of the soil or, if you prefer, 'environmental conditions'. It's kind of ironic that he talks about burying trees (what a stupid notion), but never thinks about *where* they might be interred. There's a clue in the wording.
So photosynthesis is the fixation of inorganic atmospheric CO2 into organic plant biomass, which is primarily carbon. Soil organic matter holds around 3x the amount of sequestered carbon than do trees and plants. What?! How did that happen? Where did it come from? Norm asks. Well, Norm, as stated above, this directly relates to environmental conditions; one of those conditions might be called 'forest' (where the soil is richer in organic matter and therefore sequestering more carbon), another might be called 'agriculture' (with all its attendant practices, like tilling, which destroys the structure of the soil and kills many of the life forms therein, thus releasing its cache of CO2 in short order). The soil carbon content is directly (and for the most part) related to the activities taking place in the rootzone (from growth and death of plant roots, although leaf litter also plays a lesser part) and indirectly by interactions and exchanges taking place through the transfer of carbon rich compounds from roots to soil microbes, such as the symbiotic relationships many plants have with fungi and bacteria.
Decomposition of biomass by soil microbes does result in CO2 release, while a proportion of the original carbon is retained in the soil by forming humus. Different forms of organic carbon in soil have variable resistance to decomposition. Humus is highly resistant, so carbon remains in the soil far longer than say that which originated from leaf litter etc. Erosion is yet another cause of release of CO2 from soil - and yet another environmental factor in which humans can have a say.
It's a slightly more complex picture than Norm paints, but that's probably why he's in IT and not ecology.