Add / calculate ∆C_agb data

[teixeirak]

It would be very nice to report statistics on changes in live aboveground biomass- ∆C_agb (delta.agb_OM, delta.agb_C). This is an important variable that links to the global C cycle, and therefore is of particular interest for this review.

Unfortunately, we started recording ∆C_agb late in the process of assembling ForC, so we have very few records. Our original philosophy was that this could be easily calculated based on biomass measurements at two points in time, but in reality this variable is often reported on its own.

The following is the start of a list of sources that could be relatively easy sources that could be added:

• [ ] References in table 14.1 of Muller-Landau et al. 2014. I assume Helene Muller-Landau (@hmullerlandau) has all these values in a spreadsheet.
• [ ] Chave et al. 2008
• [ ] https://www.nature.com/articles/s41467-017-01997-0.pdf We can also obtain this variable based on data that is already in ForC by calculating change in biomass_ag (see ForC issue #67). There should be a fair number of records for secondary forests.

[teixeirak]

Helene has provided these data. I have loaded them to the ForC private repository, here.

[teixeirak]

I have moved this issue to the main ForC repository, here.

[teixeirak]

This issue is replicated here: https://github.com/forc-db/ForC/issues/79. Closing this copy.