Adds a lower limit to stem allocation to avoid negative states

[thorntonpe]

The stem:leaf allocation variable can go negative without this limiter, which
can cause negative stem carbon pools.

Fixes #6591
[non-BFB]

[rljacob]

Can you say more about why this is non-BFB ?

[thorntonpe]

A condition can occur where the climate in a given gridcell is not supporting the growth of a woody vegetation type, and the carbon stock for that type as well as its productivity is declining over time. In some very rare instances this can lead to a negative value for a calculated parameter controlling how much new photosynthesis is put toward production of new wood. In even more rare circumstances this can lead to a calculation of unrealistically tall vegetation height, which can cause unrealistically high surface temperatures, which can trigger a model stop if the temperature exceeds a critical threshold. With this bug fix in place the calculated vegetation parameter for allocation of new wood is not allowed to go negative, and so there will be non-BFB results in a small number of gridcells for a small number of timesteps. The bug-fix will also prevent the occurrence of the even more rare canopy height and temperature threshold problems, and so runs that previously stopped will now continue.

[rljacob]

Can you open an issue for the bug that this fixes?

[thorntonpe]

Issue created (#6591 ) and PR description updated.

[thorntonpe]

I see that this is connected to the v3.1beta milestone. Can it also be added to the v3.0.1 milestone, for integration with the V3 HES simulations?

[rljacob]

v3 HES is going to use 3.1. It changes answers compared to 3.0.0 for watercycle cases so can't go in a 3.0.x tag.

[rljacob]

@bishtgautam is this ready?

[bishtgautam]

Yes, this is ready to be merged.

[rljacob]

Go ahead and merge this to next today.