Sapwood area allometry for broadleaf evergreen tropical trees

[jennykowalcz]

Hi all! I wanted to start a discussion about sapwood area allometry now that many of us are working on comparing FATES modeled sapflow to observation.

FATES outputs sapflow in units of kg per m2 ground area per second. This needs to be divided by sapwood area per unit ground area in post-processing to compare to measured values. Sapwood area is calculated in FATES from an allometric relationship with target leaf biomass:

fates/biogeochem/FatesAllometryMod.F90

Line 1406 in 626f5e2

sapw_area = slatop * bleaf * g_per_kg / (la_per_sa*cm2_per_m2 )

This appears to be happening as intended:

However, for broadleaf evergreen tropical trees, the resulting sapwood areas are much smaller than observed in Manaus. Empirically correcting the sapwood area by a factor of 5 (by dividing la_per_sa_int by 5) brings the sapwood areas closer to observed:

In these figures, the blue curves on the FATES plots on the right are the same as the black curve shown in the figure from Aparecido et al (2019) on the left.

Below, I also compare to a function for sapwood area derived by Christoffersen et al (2016):

I'm wondering if we might want to add some other choices for sapwood area allometry to FatesAllometryMod.F90 at some point. I'd be happy to work on this.

References:
Aparecido, L.M.T., Santos, J.D., Higuchi, N. and Kunert, N., 2019. Relevance of wood anatomy and size of Amazonian trees in the determination and allometry of sapwood area. Acta Amazonica, 49, 1-10.
https://doi.org/10.1590/1809-4392201800961

Christoffersen, B.O., Gloor, M., Fauset, S., Fyllas, N.M., Galbraith, D.R., Baker, T.R., Kruijt, B., Rowland, L., Fisher, R.A., Binks, O.J. and Sevanto, S., 2016. Linking hydraulic traits to tropical forest function in a size-structured and trait-driven model (TFS v. 1-Hydro). Geoscientific Model Development, 9, 4227-4255.