vegetation_dynamics - PIK-LPJmL/LPJmL GitHub Wiki

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Plants interact by competing for resources, mainly water and energy (light via space) [and also for nutrients, which are included in LPJmL5].
The competitive advantage is determined by plant traits such as phenology, mortality, turnover, fire_resistance and establishment.

For PFTs within their bioclimatic limits (T_c,min), each year, new woody PFT individuals and herbaceous PFTs can establish depending on available space. Woody PFTs have a maximum establishment rate k_est of 0.12 (saplings m^-2 a^-1), which is a medium value of tree density for all biomes (Luyssaert et al. 2007). New saplings can establish on bare ground in the grid cell that is not occupied by woody PFTs. Establishment rate of tree individuals is calculated:

The number of new saplings per unit area (EST_TREE in ind m^-2 a^-1 is proportional to k_est and to the FPC of each PFT present in the grid cell (FPC_TREE resp. FPC_GRASS). It declines in proportion to canopy light attenuation when the sum of woody FPCs exceeds 0.95, thus simulating a decline in establishment success with canopy closure (Prentice et al., 1993). The parameter n_estTREE gives the number of tree PFTs present in the grid cell. Establishment increases the population density P. Herbaceous PFTs can establish if the sum of all FPCs is less than 1. If the accumulated growing degree days (GDD) reach a PFT-specific threshold GDD_min the respective PFT is established.

Mortality is modelled by a fractional reduction of P. Mortality always leads to a reduction in biomass per unit area. Similar as in Sitch et al. (2003), a background mortality rate (mort_greff in ind/m^-2a^-1, the inverse of mean PFT longevity, is applied from the yearly growth efficiency (GREFF=_inc/(C_leaf,ind · SLA)) (Waring et al. 1983) expressed as the ratio of net biomass increment (bm_inc) to leaf area:

where k_mort1 is an asymptotic maximum mortality rate, and k_mort2 is a parameter governing the slope of the relationship between growth efficiency and mortality.

Mortality from competition occurs when tree growth leads to too high tree densities (FPC of all trees exceeds > 95%). In this case, all tree PFTs are reduced proportionally to their expansion. Herbaceous PFTs are outcompeted by expanding trees until these reach their maximum FPC of 95% or by light competition between herbaceous PFTs. Dead biomass is transferred to the litter pools.

Boreal trees can die from heat stress (mort_heat in ind m^-2 a^-1) \citep{allen_global_2010}. It occurs in LPJmL4 when a temperature threshold (T_mort,min in °C) is exceeded, but only for boreal trees (Sitch et al., 2003). Temperatures above this threshold are accumulated over the year (gdd_tw) and this is related to a parameter value of the heat damage function (tw_PFT), which is set to 400:

P is reduced for both mort_heat and mort_greff.

Two different fire modules can be applied in the LPJmL4 model: the simple Glob-FIRM model (Thonicke et al., 2001) and the process-based SPITFIRE model (Thonicke et al., 2010). In Glob-FIRM, fire disturbance is calculated as an exponential probability function dependent on soil moisture in the top 50 cm and a fuel load threshold. The sum of the daily probability determines the length of the fire season. Burnt area is assumed to increase nonlinearly with increasing length of fire season. The fraction of trees killed within the burnt area depends on a PFT-specific fire resistance parameter for woody plants, while all litter and live grasses are consumed by fire. Glob-FIRM does not specify fire ignition sources and assumes a constant relationship between fire season length and resulting burnt area. The PFT-specific fire resistance parameter implies that fire severity is always the same, an approach suitable for model applications to multi-century time scales or paleo-climate conditions. The SPITFIRE module is more complex and described on its own page.

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