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Three biomass functional types (BFTs) were implemented in LPJmL (one fast-growing C4 grass, a temperate and a tropical tree) to allow the simulation of dedicated biomass plantations. These BFTs are generic representations of some of the most promising types of crops for the production of 2nd generation biofuels, biomaterials, or energy (possibly in combination with CCS). Their parametrization is largely identical to their natural PFT equivalents “C4 perennial grass”, “temperate broadleaved summergreen tree” and “tropical broadleaved raingreen tree” [Sitch et al., 2003]. Table 1 shows the modified and original parameter values. Increased transpiration rates (E_max) are based on recent measurements that report higher maximum rates of up to 8 mm per day during the growing season [Allen et al., 1999, Beale et al., 1999, Hall et al., 1998, Tricker et al., 2009]. The regular arrangement of crops on plantations improves the overall light availability for individual plants. Consequently, the relevant parameter ?a for CFTs in LPJmL is higher compared to PFTs in natural vegetation [Bondeau et al., 2007]. The same logic was applied to the newly implemented BFTs. Some varieties of C4 grass species used as energy crops can maintain high rates of photosynthesis at low ambient temperatures [Naidu et al., 2003] and therefore the C4 BFT has a smaller lower limit for photosynthesis than the C4 PFT.

Table 1: Overview of modified BFT parameter values and constants in model equations. Original values are in brackets.

Parameter	Description	BFT temperate tree	BFT tropical tree	BFT C4 grass
Gmin	Minimum canopy conductance	0.3 (0.5)	0.2 (0.5)
Rfire	Fire resistance (-)	0.95 (0.12)	1 (0.12)	1 (0.01)
ramp	Phenology parameter (growing degree day requirement to grow full leaf coverage)	100 (300)
LAIsapl	Leaf area index of saplings (-)	1.6 (1.5)	1.6 (1.5)
gdd5min	Minimum degree-day sum (5°C base)	300 (1200)
Emax	Maximum transpiration rate (mm/day)	5.0	7.0	7.0
	fraction of PAR assimilated at ecosystem level, relative to leaf level (-)	0.8 (0.5)	0.8 (0.5)	0.8 (0.5)
Tlim,CO2	lower and upper temperature limit for CO2 (°C)	–4 38 (–4 38)	4 55 (2 55)	4 55 (6 55)
Tlim,opt,photo	lower and upper limit of temperature optimum for photosynthesis (°C)	15 30 (20 25)	25 38 (25 30)	15 45 (20 45)
Tlim,cold,month	lower and upper coldest monthly mean temperature (°C)	–30 8 (–17 15.5)	7 1000 (15.5 1000)	–40 1000 (15.5 1000)
Fuel bulk density		22 (22)	13 (25)
Roughness length			1 (2)
Turnover	Turnover leaf, sapwood, root	1 10 1 (1 20 1)	2 10 2 (2 202)
Crownarea max	Tree maximum crown area (m2)	1.25 (15)	2 (15)
Sapwood sapling	sapling carbon per m2 (gC/m2)	2.3 (1.2)	2.2 (1.2)
Allom par 1	Allometry parameter 1	110 (100)	110 (100)
Allom par 2	Allometry parameter 2	35 (40)	35 (40)
Allom par 3	Allometry parameter 3	0.75 (0.67)	0.75 (0.67)
Establishment rate	Saplings per	0.8 (KEST)	0.5 (KEST

Parameters describing biomass plantation management are given in table 2. Woody energy crops are simulated as short rotation coppice (SRC) in which young tree stems are regularly cut down to near ground level at short intervals. An established root system at the time of harvest and nutrient storage in roots and stumps enables high yielding varieties of poplar, willow and eucalypts used for SRC to regrow vigorously in subsequent years. Several harvest cycles are possible until plantations need to replanted after 40 years. Mortality and fire occurrence on plantations are also reduced compared to natural vegetation, because it is assumed that effective pest and fire control will be established on modern biomass plantations.

Table 2: Model parameters describing biomass plantation management.

Coppicing of trees on biomass plantations is described in coppice_tree.c called from annual_biomass_tree.c when the time since the last harvest equals the rotation period defined for each BFT in pft.par. During harvest 90% of the heartwood and 65% of the sapwood are transferred to the harvest pool and the plants carbon pools reduced accordingly. Leaf carbon goes into the litter pool. Fine root pools remain unchanged and allow fast regrowth in subsequent years. Allometry is called after harvest to adjust tree height and crown area.

    #define HARVEST_EFFICIENCY 0.9
    /* Sapwood reduction needs to be lower to avoid fatal numerical instabilities in allometry */
    /* Makes sense as part (~30 %) of sapwood carbon is located underground */
    #define HARVEST_EFFICIENCY_SAP 0.65

    Real coppice_tree(Pft *pft)
    {
      Pfttree *tree;
      const Pfttreepar *treepar;
      Real harvest=0.0;
      Real leaf_old;
      tree=pft->data;
      treepar=getpftpar(pft,data);

      harvest=(((tree->ind.sapwood*HARVEST_EFFICIENCY_SAP+
                 tree->ind.heartwood*HARVEST_EFFICIENCY)-tree->ind.debt)*pft->nind);
      leaf_old=tree->ind.leaf;

      tree->ind.heartwood*=1-HARVEST_EFFICIENCY;
      tree->ind.sapwood*=1-HARVEST_EFFICIENCY_SAP;
      tree->ind.debt=0;
      tree->ind.leaf=8000*tree->ind.sapwood/(wooddens*tree->height*pft->par->sla);
      harvest+=(leaf_old-tree->ind.leaf)*pft->nind;
      /* Call allometry to adjust height and crownarea */
      allometry_tree(pft);
      fpc_tree(pft);

      return harvest;
    } /* of 'coppice_tree' */

Grass harvest on biomass plantations (harvest_stand.c) is called from daily_biomass_grass.c if bm_inc >=200. Biograss harvest follows the same harvest routine as for standard grasses, i.e. 50% of ind.leaf is harvested. This results in maximal achievable biomass yields.
If the flag #NEW_GRASS is switched on, harvest_stand.c is called from daily_biomass_grass.c if ind.leaf >=400 (or if ind.leaf>1 AND ind.leaf<(0.75*max_leaf) ). In the harvest routine 85% of ind.leaf is harvested. This leads to less annual harvest events (2-3 per year in temperate regions, 3-5 in tropical regions, and even more if irrigated).

Tim Beringer, Sibyll Schaphoff

Crop functional types