Biomass CO2-effect in trees: no increase in total biomass?

[dschlaep]

Commit 69da0d9:

The function SW_VPD_init applies biomass CO2-effects for grasses, forbs, and shrubs to total biomass and keeps percent-live constant to calculate live biomass. Thus, total and live biomass increase with higher CO2 concentrations.

For trees, the biomass CO2-effect is applied to percent-live while total biomass is kept constant. Thus, only live but not total biomass increases with higher CO2 concentrations.

@jbbradford Is this really want we want? Shouldn't total tree biomass be incremented by the gain in live biomass?

	CO2 (ppm)	Biomass CO2-effect	WUE CO2-effect	Tree biomass (g/m2)	Tree live biomass (g/m2)	Tree live (%)	Grass biomass (g/m2)	Grass live biomass (g/m2)	Grass live (%)
1980	338	0.976	1.019	15000	1406	0.094	179	50	0.281
1981	340	0.977	1.017	15000	1409	0.094	179	50	0.281
...
2009	387	1.023	0.949	15000	1474	0.098	188	53	0.281
2010	389	1.025	0.946	15000	1477	0.098	188	53	0.281

[dschlaep]

John:

My impression from reading the CO2 literature was that it was foliage biomass (well, really LAI) that reliably responded to CO2. So we did intend for this effect to only influence the live biomass (how we represent foliage biomass in trees) for that functional group. While it may be that total biomass also responds, I'm not sure we have CO2 enrichment studies long enough to quantify those effects in long-lived woody plants.

[dschlaep]

see answer in #330