Forests, in their role as massive carbon stores

Besides being an integral part to the climate system, Forests (especially thick tropical rainforests) are a large storage of organic carbon. The carbon is taken up by terrestrial “sinks” through plant growth/primary productivity and stored within their structures, as well as in the soils underneath. Looking at the world scale, out of all carbon stored in terrestrial ecosystems, about 55% is stored in tropical forests which is at a density of 242 MG C per hectare, most of which is stored in the biomass. Lands are said to become net emitters when ecological disturbances or deforestation cause a reduction in primary productivity. Deforestation or land-cover conversion to less productive ecosystem will result in the emission of CO2 to the atmosphere via forest burning (prominent in the Amazon in the land conversion to pasture), decomposition of harvested matter, fuel-wood burning and/or soil respiration (IPCC.www).

Here I want to present a map that shows in overview the carbon sinks and sources (Pg C per year) over two time periods arising from global forests - and I will focus on South America. 
(Downward facing bars indicate storage, upward facing indicate emission)

Source: click here

We can see that the South American continent plays a large role in terrestrial carbon fluxes. What I found interesting here is that in the period 2000-2007 the net flux is still a sink but at a lower value compared to the previous period 1990-1999 (-23%) despite reduced emissions from gross deforestation. And to understand this, my topic of land-cover change is highly important. Deforestation emits CO2 almost directly as described above, but it doesn't stop there: the area left as pasture (for example) starts to act as a source again as the grasses grow, but at a much lower productivity. Therefore, the fact that the South American total area of intact tropical forest is reduced over time is in itself a loss in 'potential carbon sink' and contributed to weaken the continents total "sink" characteristic.

It is estimated that about half of anthropogenic CO2 emissions are taken up by natural sinks like the oceans and terrestrial ecosystems, but this hasn't been stable through time. A widely cited paper by Schimel et al (2001) shows that terrestrial systems have only been a net sink since 1990, powered by land use changes that supported regrowth and nutrient fertilization that enhanced primary productivity. Can these changes power a further increase in carbon uptake in the future? I quickly became aware through my reading that scientific opinions and modelling studies on the future of forests as carbon sinks is highly divided and uncertain. The straight-forward conceptualisation of "more forest- more carbon stored" is not completely accurate. It surely depends on the health of the forest (relates to my posts on forest degradation and edge effects), climate-vegetation feedbacks, and ecological changes that occur within the forests in response to climate pressure (e.g. Amazon drought 2005 shows that this is NOT negligible by turning Amazon from sink to a source through the loss of biomass (Philipps et al 2009)).