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Each living tissue of trees and grass (leafs, fine roots, sapwood) is assigned a PFT specific tissue turnover time (Schoettle Fahey, 1994; to find in pft.par. The reciprocal of these values gives the fraction of tissue that dies each year. This fraction is either transferred into the above/below ground litter pools (dead leafs and fine root) or from living sapwood into dead heartwood. The following equations apply for the calculation of litter pools, heartwood pools and living tissue pools:

litter_ag_new = litter_ag_old + sum[(lm_ind_old/t_leaf)*nind]

litter_bg_new = litter_bg_old + sum[(rm_ind_old/t_fineroot)*nind]

lm_ind_new = lm_ind_old*(1-1/t_leaf)

sm_ind_new = sm_ind_old*(1-1/t_sapwood)

hm_ind_new = hm_ind_old +(sm_ind_old/t_sapwood)

rm_ind_new = rm_ind_old * (1-1/t_fineroot)

litter_ag_new = new above ground litter pool for current simulation year litter_ag_old = old below ground litter pool of last simulation year
lm_ind_new = new leaf mass pool for current simulation year
lm_ind_old = old leaf mass pool of last simulation year
sm_ind_new = new sapwood mass pool for current simulation year
sm_ind_old = old sapwood mass pool of last simulation year
hm_ind_new = new heartwood mass pool for current simulation year
hm_ind_old = old heartwood mass pool of last simulation year
rm_ind_new = new fine root mass pool for current simulation year
rm_ind_old = old fine root mass pool of last simulation year
t_leaf = leaf turnover time; in pft.par t_sapwood = sapwood turnover time; in pft.par t_fineroot = fine root turnover time; in pft.par nind = individual density of PFT

t_leaf in LPJmL can’t be smaller than 1, because turnover and carbon balances are calculated on a yearly basis. t_leaf=1 means all leaf mass established in the simulation year is transferred into the litter pool at the end of the simulation year. Natural leaf longevities can be smaller than 1. A t_leaf<1 in LPJmL would imply loosing more leafs than actually established in the whole simulation year. In LPJmL exists another PFT specific parameter called leaf longevity (LL). LL is used to calculate the specific leaf area (SLA) of PFTs and to calculate a excess of biomass (cmass_excess). This biomass excess is calculated as:

Cmass_excess = aphen/(LL*365)*Cmass_l_r - Cmass_l_r

Where aphen is the annual phenology or number of green days of a PFT in a year and Cmass_l_r is the leaf and root carbon content of the PFT in the simulation year. All LLs >=1 yield a Cmass_excess < 0. In this case the biomass increment is not affected. The constellation LL < 1 and aphen/365 > LL, yields a positive Cmass_excess which is then subtracted from the annual biomass increment of the PFT to adjust the turnover to the actual length of the vegetation productivity. For example the PFT “tropical rain green tree” with a LL=0.5 has a disadvantage in regions with constant and high precipitation, since the possible biomass is lowered according to its LL. In drier regions, annual phenology of the rain green PFT is decreased due to water stress, so the probability of a Cmass_excess > 0 is decreased as well. (Moreover, maintenance respiration costs over dry periods are reduced due to reduced biomass. This is an important advantage for the rain green PFT in drier regions).

LPJmL uses a pipe model to make sure a certain amount of leaf mass is always supported by a certain amount of transport tissue i.e. sapwood mass. In simulation years with prolonged stress (e.g. water stress) the biomass increment may not be adequate for sufficient resource allocation to leafs in order to fully utilize all the sapwood. In this case biomass increment is allocated to leaves and roots only and the excess of sapwood mass is transferred to the heartwood pool to satisfy the pipe model. I years where both sapwood mass and leaf mass can’t be maintained all the biomass increment is allocated to the fine roots and excess sapwood mass and leaf mass enters the heartwood and above-ground litter pool. !!!This kind of turnover has to be handled with caution when you focus on climate extremes, since the PFTs can adapt their sapwood in a very short time!!!

turnover_grass.c and turnover_tree.c

Parameters are defined in pft.par

Mortality, Fire, Spitfire, Light Competition, Establishment