The fire model
We developed a new meso-scale fire model, which considers only a few factors of the fire regime, following the broader concept of modelling fire regimes. This fire model is a compromise between the fire history concept (the spatio-temporal aspect of the fire regime) and a process-oriented methodology ( Thonicke et al. 2001 ). The model was then incorporated into the LPJ-DGVM in order to simulate fire over the historic time period in conjunction with vegetation dynamics.
The concept of the fire model is the following: the burning condition that lets a fire ignite and continue to spread are determined by the litter moisture and fuel load. The probability of at least one fire occurring in a day increases with litter dryness (Fig. 1). If there is not enough fuel to carry a fire (<200 gC*m -2), no spread will be allowed.  
 
Fig. 1. The probability of at least one fire occurring on a day
 
 Fig. 1. The probability of at least one fire occurring on a day with a given litter moisture content, simulated by the LPJ-DGVM. Dots represent experimental data from Viegas (1992), the dashed line the function (p(m)) used.
 
The longer these burning condition persist, the longer the fire size can grow. Given the length of the fire season, which is the relative sum of the fire probabilities over the year, the fraction of the grid cell burnt in a particular year can be calculated. We assume, the same length of fire season gives the same fractional annual area burnt (affected fraction of the grid cell) for all ecosystems across the globe. A small length would result in small area burnt, increasing rather rapidly when the fire season approaches the entire year (Fig. 2).   
 
Fig. 2. Length of fire season in relation to fractional area burnt

                             

Fig. 2. Length of the fire season in relation to fractional area burnt; the dashed line representing the function, dots experimental data, which contained both the length of the fire season and the annual area burnt.

 
The resulting fire effects on vegetation depend on the plants resistance to fire, here taken as the fraction of plant individuals that survive fire. A fire resistance is defined for each vegetation type (in the LPJ-DGVM called Plant Functional Type ). In the following year, the increased area of bare ground enhances establishment of plant individuals, a key part in vegetation regeneration.   
 
Main