Add updated conversion from aboveground biomass to carbon

[Kah5]

Right now we are using a basic conversion of 50% carbon fraction to convert biomass to carbon. We should update this based on updated values of carbon fractions for deadwood in Martin et al. 2021 (https://www.nature.com/articles/s41467-021-21149-9/tables/1) and Martin et al. 2018 (https://www.nature.com/articles/s41561-018-0246-x/tables/1). For temperate conifers, the live carbon fraction is vary close to 50%, at 50.1% (+/- 0.4 SE). But for confer standing dead wood carbon fraction, the live carbon fraction is 48.22 +/- 1.06 SE.

Some things that still need to be addressed:

• [ ] How/if we should include the uncertainty in the C fraction into the estimates?
• [ ] Do we need to break up dead wood into standing dead/downed dead, and downed dead decay classes as these have different C fractions?

Initial thoughts on this:

Including uncertainty seems easy to do, it will just increase uncertainty a bit, but we can separate this uncertainty from uncertainty in biomass if need be.

We already include a lag in downed wood when calculating fire risk parameters, so estimating C could easily include the different C fractions for standing vs downed wood. I don't think we should include decay classes, because of uncertainty about decay rates.

[Kah5]

We are not including the uncertainty in the scaling up to C currently. We have updated carbon fractions for deadwood and live trees.