Bound treelai if leafc_per_unitarea is tiny.

[mvdebolskiy]

Description:

Fixes #1487

Bound treelai to non-negative due to log(exp(-X))+X having precision ~1e-16 for 64-bit floats.

This should prevent endrun here:

fates/biogeophys/FatesPlantRespPhotosynthMod.F90

Lines 1091 to 1097 in 75a465c

	if (r_sb_leaves<bc_in(s)%rb_pa(ifp)) then
	write(fates_log(),*) 'Combined canopy resistance was somehow smaller than'
	write(fates_log(),*) 'its boundary layer resistance component'
	write(fates_log(),*) 'r_sb_leaves [s/m]: ',r_sb_leaves
	write(fates_log(),*) 'bc_in(s)%rb_pa(ifp) [s/m]: ',bc_in(s)%rb_pa(ifp)
	call endrun(msg=errMsg(sourcefile, __LINE__))
	end if

Collaborators:

@JessicaNeedham @rosiealice @glemieux

Expectation of Answer Changes:

Should be bfb, mb roundoff.

Checklist

All checklist items must be checked to enable merging this pull request:

Contributor

• The in-code documentation has been updated with descriptive comments
• The documentation has been assessed to determine if updates are necessary

Integrator

• FATES PASS/FAIL regression tests were run
• Evaluation of test results for answer changes was performed and results provided
• FATES-CLM6 Code Freeze: satellite phenology regression tests are b4b

If satellite phenology regressions are not b4b, please hold merge and notify the FATES development team.

Documentation

No update needed.

Test Results:

CTSM (or) E3SM (specify which) test hash-tag:

CTSM (or) E3SM (specify which) baseline hash-tag:

FATES baseline hash-tag:

Test Output:

[rosiealice]

This looks sensible to me...