Overview

Purpose

The MEDFIRE model aims to examine the spatial interaction between wildfires and vegetation dynamics in heterogeneous landscapes. It has been designed to model different fire regime drivers allowing the investigation of their relative effects on the resulting annual area burn distribution, fire size distribution and landscape composition at short- and medium-term time scales in a Mediterranean context. The model permits the characterization of the spatial variability in burning and land cover change probability under different climatic scenarios and fire suppression strategies. The MEDFIRE model assumes that the main driver of fire regime is climate and that the key elements of fire regime such as fire size can be modulated by active fire suppression.

State variables

State variables in the MEDFIRE model are spatial variables that describe the landscape context and conditions. They are represented in raster format and cover the full extent of the study area at 100 m resolution. The temporal scale is fixed and one time step represents one year; simulations are normally run for several decennia. The state variables whose values change as a result of spatial processes are land cover type (LCT) and time since last fire. LCT is a categorical variable whose states are divided into those land covers that can be affected by fire disturbance because of their burnable condition (e.g. forests, croplands, grasslands), and those that cannot (e.g. urban, water, rock). Whereas several LCTs are burnable, and therefore contribute to spread of fires, shrublands and forests are the only LTCs that can undergo land cover changes. Forested cells include information of the dominant tree species in the canopy. The list of tree species considered (Pinus halepensis, Pinus nigra, Pinus pinea, Pinus sylvestris, Quercus ilex and Quercus suber) is representative of the Mediterranean landscapes dominant in Catalonia and other areas of Western-Mediterranean. Three additional forest categories are considered to complete the classification of forests in the study area: other conifers, other Quercus species and other trees. Time since fire is a discrete count variable that is used as a surrogate of vegetation age and stand volume for cells belonging to shrubland and forest cover types only. Other spatial state variables describe additional landscape features but are static in the current version of the model: elevation, aspect, ignition probability, climatic region, spread type and main wind direction. The ignition probability layer is used to determine spatial location of new fires. A climatic region layer was introduced to allow for regionalization in post-fire vegetation transitions. The spread type layer describes the proportion between wind-driven versus topographic fire spread patterns in each cell. Main wind direction accounts for the spread speed in wind driven fires, whereas elevation and aspect are used to determine spread rate in fires driven by relief.

Process overview and scheduling

Fire disturbance and vegetation change processes are designed as two separate sub-models whose action needs to be completed before the next one starts (Figure 1). Each time step (a year) the fire disturbance sub-model is scheduled first and then the sub-module responsible for vegetation changes follows. The fire sub-model begins by setting the potential total area to be burned. The sub-model will simulate as many fires as necessary until the potential annual area to be burnt is reached. For each fire, the model first chooses a potential size and an ignition location. The location chosen for ignition is used to determine the spread type (relief- or wind-driven). If fire suppression is not active, the fire is allowed to spread until the potential fire size is attained. However, if fire suppression is active not all the cells potentially affected by a fire will be effectively burned. The fire sub-model resets the value of time since fire to zero each time a given cell is effectively burned. The vegetation dynamics sub-model goes through all cells of the grid and updates the LCT of a given cell in the following two cases: (1) if the cell was burned by the fire sub-module, its LCT may change according to a post-fire vegetation transition; (2) if the cell was not burned but its LCT is shrubland, then natural succession from shrubland to forest may occur. Reporting tasks are carried out at the end of each time step summarizing the results by year and by fire simulated.

Figure 1: Conceptual design of the MEDFIRE model

(from Brotons et al. 2013)

Brotons, L., N. Aquilué, M. de Cáceres, M.-J. Fortin, and A. Fall. 2013. How fire history, fire suppression practices and climate change affect wildfire regimes in mediterranean landscapes. PloS one 8:1–12.