Forests of Carbon or Fields of Food?

 As we saw from the last post deforestation for agriculture is most likely going to occur more in the tropical regions. So if we want to quantify the effects of deforestation in these regions on carbon we need comphrensive data on carbon stocks. This is what Saatchi et al., (2010) set out to do, and through the use of Lidar, tree allometry, and spatial modelling they created a benchmark map of carbon stocks for tropical regions for the year 2000.

Figure 1 below is a map of the above ground biomass. Which is defined by the IPCC (2006) as all living biomass above the soil including stems, stumps, branches, leaves, bark, and seeds.

Figure 1

Figure 2 below shows the total carbon biomass (so this is the above ground biomass plus below ground biomass which is the roots) and, most importantly in my opinion, the uncertainty in these estimates. Why the uncertainty? Due to errors with estimating spatial distribution of above ground biomass from Lidar canopy height measurements, errors with estimating below ground biomass from above ground biomass, a 1km resolution might not fully capture the spatial variability in above ground biomass, and errors associated with the satellite imagery used (Saatchi et al., 2010). The supporting information accessible through their paper as 'Fig. S3' gives a good detailed breakdown.

Figure 2

We can see that these regions hold a lot of carbon! And that’s without us including the carbon in the soil as soil organic carbon! If we break this carbon storage down across the regions South America potentially stores a whopping 49% of the total stock for these regions (Saatchi et al., 2010). So how is changing land use for agriculture going affecting the carbon in these forests....  

One argument is: "weeellll we are replacing forests with crops that also use CO₂ for growth and hence also store carbon as biomass, as opposed to deforesting to graze animals or build houses; so it shouldn’t make too much of a difference in terms of carbon storage what plant is growing there".  

Well, Conti ­et al., (2014) undertook a study to assess carbon storage under different land uses using biomass models* and it is an excellent example of why the above statement is really not the case! I chose this study by Conti et al., (2014) as they looked at the Chaco Forest, which is a subtropical seasonally dry forest, in South America which ties in with our carbon findings and agricultural expansion predications so far. It is also experiencing some of the highest deforestation rates for agriculture crops in the world (Conti et al., 2014).

*where you use allometic models / species specific equations to estimate biomass, then combine accordingly across a plot to obtain total biomass per hectare.

This is what they found:

Plant and soil carbon pools in the different ecosystem types.  

As we can see in the graphs, above ground biomass is significantly higher in primary forest, secondary forest, closed shrub-land and open-shrubland compared to potato crops. And the same pattern is observed for above ground dead biomass of which a potato crop has none. Looking at the amounts of organic carbon and inorganic carbon (which is the carbonate content) in the soil, we have to look at the individual layers: for the surface (0-10cm) and subsurface (10-30cm) soil layers, soil organic carbon is much higher in primary and secondary forest soils than potato crop soils. Analysing the soil inorganic carbon the authors found soils undergoing potato cultivation had the lowest amount of inorganic carbon.

Already we can see a difference in the carbon sequestration between forests and crops. Combining these results and looking at total organic amount of carbon each ecosystem has:

.....we can see the carbon lost though turning a forest into cropland: which is 31.6%! (Conti ­et al., 2014).

By switching forests of carbon for fields of food in South America we are causing less carbon to be stored on earth and more in the atmosphere, which we know as CO₂ will increase earth’s global temperature. Deforestation for agricultural land is one of the reasons we have been seeing an increase in the average net CO₂ emissions from land to the atmosphere over the last 10 years (IPCC 2013).  

Aggghh - so what do we do?!?!! We need to store the carbon, but we need to eat, but we are putting loads more carbon into the atmosphere, but we can’t say to developing regions: no you can’t have any more land for food stop what you are doing now.

Well that is what we will tackle in the next post: Carbon or food? What to choose.......