Fire risk and vegetation structural dynamics in Mediterranean shrubland

Phytomass structural characteristics are highly related to vegetation flammability. In fire-prone species like Mediterranean gorse, which accumulate standing dead fuel, susceptibility to fire is a function of fuel load, vegetation composition and fuel cover, and these characteristics change with time. Thus, for effective fuel control management, knowledge of the vegetation structural dynamics related to fire risk is crucial for preventing future fires. This study analyses structural dynamics in the above-ground phytomass of Ulex parviflorus shrublands in relation to different stages of flammability, i.e., the amount of time elapsed since the last fire. For this, 152 plants were cut from shrublands at different stages of development (young, mature and senescent), and various dimensional measurements were taken on each. The phytomass was separated into living or dead fuel fractions as well as into twigs or branches depending on the stem diameter. Basal diameter is the variable that best predicted Ulex parviflorus total phytomass as well as that of the different fractions. Both dimensional and phytomass variables increased with plant development. In the young shrublands Ulex parviflorus constitutes 54% of total phytomass, and Ulex parviflorus's dead twigs fraction accounts for 5% of total phytomass. In the mature and senescent shrublands, this species represents 80% of total shrubland phytomass, and dead twigs reach values greater than 40%. Our results show that structural changes in the fuel over short periods of time (young and mature) reveal critical periods in shrub development. Identification of these stages is a necessary tool for planning fuel control programmes.