Friday, June 17, 2022

Letter to Radio NZ’s Country Life programme

I was interested to hear your interviews with people about carbon forestry. I’d be very keen to read the scientific basis for the comment about soil carbon and pine. I have read about some studies but I know of none that conclusively support the statement on your programme.
Factors influencing rates of carbon sequestration per hectare are species used, quality of site, and how the forests are managed (we call it silviculture). The number of trees per hectare is very important. So if people compare C sequestration for one species on a poor site at low stems/ha with C sequestration of another species on a good site at high stems/ha, then the species comparison can be highly misleading.
In the Emissions Trading Scheme (ETS) forest growers are required to use a “lookup table” provided by the Ministry for Primary Industries (MPI) to estimate C sequestration if they have a forest smaller than 100 ha, while those with forests 100 ha or larger have their forests measured. So comparing the measured forests with lookup tables is interesting. MPI recently released average figures for forests with the forest measurement approach (FMA) by species, so that we can compare their measurements with the lookup tables by species or species groups.
What we got for radiata pine on average is shown in the first image below. As you can see, the lookup tables, on average, underestimated actual average C sequestration by a considerable margin.

What we got for native C forests is shown in the second image below. In this case it's clear that the lookup table overestimated average measured C sequestration in indigenous forests by a large margin. Look also at the difference in y axis scales on these two graphs.

This result needs interpretation. We know that many natural indigenous forests contain much more CO2-e than what's shown as the asymptote in the second graph. We also don't know what the variation around the lines is, how many points there are, where the sites are in the landscape, or how they've been managed. Those drawbacks haven't stopped people from using these numbers for political purposes, however, as you can imagine.
What this does mean is that, so far indigenous C forests, on average, haven't performed as well as the indigenous forest lookup table indicates they should, while radiata pine has out-performed the average of radiata pine tables. We need to ask why. What indigenous species have been planted, where, and how have they been managed?
I am writing to you not to push any particular viewpoint, but to warn you that you will encounter a great variety of views, some well-informed and some based on fairy tales, and that the truth is reasonably complicated. It is very easy to mistakenly present an unbalanced picture.
Kind regards,
Euan Mason