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Managed land with grassy vegetation is treated as pasture on which the grass is harvested. The default option as described below tries to mimic optimal biomass removal whereas distinct management options are under development.

So far, removal of grass biomass on managed grassland occurred globally with only one rule: reducing the leaf biomass to 50% when the accumulated NPP since the last harvest event was more than 100 g C m^-2. Here, this option is accompanied by a second rule and allocation is changed to daily.

Activating the flag NEW_GRASS uses daily allocation for grass and allows for the use of new harvest rules.
At the moment, leaf longevity is set to 1 instead of 0.5 but this is under discussion.

For improving the ability of grass to react on the biomass loss of the leaves through mowing and grazing, the allocation now uses the biomass increment and not the whole biomass. Allocation is done per day. For a visualization of the changes in the procedures, see attachment:GrassFunctionsLPJ.pdf.
The phenology indicator phen is deactivated by setting it to 1 for grassland. The actual phenology is calculated from leaf biomass (see lai_grass.c). Functions used daily are photosynthesis before turnover and allocation.

• lmtorm is calculated with wscal_mean. This value is for grassland the actual daily wscal, for natural vegetation the long-term value. wscal_mean is here set to 0 again.
• lmtorm is constrained to minimum 0.25, i.e. not more than 75% is in the roots and at least 25% is in the leaves. Otherwise, there would not be any growth!
• biomass increment is the daily NPP
• actual biomass increment is used to achieve today’s lmtorm.
  • bm_inc > 0: none of the pools decreases, no reallocation between leaves and roots can occur (i.e. the increment which is going to the leaf must be between 0 and the total daily increment).
  • bm_inc < 0: both pools are reduced proportionally

In order to enhance the productivity of marginal areas and to preserve the productivity of areas with the landuse ‘other’, we introduce two conditions for a harvest event:

1. Leaf biomass > 100 gC m^-2 induces harvest of 75% of leaf biomass
2. Leaf biomass < 75% of maximum leaf biomass AND leaf biomass > 1 gC m^-2 induces harvest of 75% of leaf biomass

For the second condition, maximum leaf biomass is recorded in a variable Pftgrass->max_leaf (grass.h, introduced in reading the restart in fread_grass.c, set to 0 in new_grass.c).

• harvest is restricted to the time when the biomass maximum is already over and
• after harvest, the max_leaf is reset to the actual leaf biomass

This ensures

• harvest also in low productive regions and
• is also accelerating biomass decline when the vegetation period is going to an end -> keep an eye on it

The options are included in the trunk version. For the implementation of the options, the grassland stand was used and changed to daily allocation.

Jens Heinke, Susanne Rolinski

Bayesian_uncertainty_analysis, LPJmL_seminar