Variation in biomass allocation and root functional parameters in response to fire history in Brazilian savannas

Abstract

Fire is a fundamental ecological factor in savannas because it affects vegetation dynamics and ecosystem functioning. However, the effects of fire on below-ground compartments, including biomass and root traits, and their regeneration remain poorly understood. In this study, we assess the variation of above- and below-ground plant components along fire-history gradients in Brazilian open savannas and investigate whether changes in vegetation and soil properties are associated with the responses of below-ground biomass and root traits. The study was conducted in eight sampling areas of open savanna (campo sujo, i.e. vegetation having low woody cover) within the Cerrado (Brazilian savannas), located along a gradient of time since the last fire (1–34 years); the number of fires that occurred within the past 34 years (0–9 fires) varied by sampling area. In each sampling area, we measured above- and below-ground biomass, root depth distribution, root functional parameters and nutrient levels in the upper soil layers (0–10 cm). Rapid recovery of above-ground live biomass after a fire was primarily due to resprouting of graminoids. This recovery was associated with an increase in absorptive root biomass in the upper soil layer in the most recently burnt sites, whereas root biomass was unaffected in deeper layers. Root parameters remained constant regardless of fire history but responded to variations in vegetation structure and soil properties. Specific root length (SRL) decreased with K, Mg2+, Al3+, N and C and increased with P concentration. In contrast, root tissue density (RTD) and absorptive root proportion were negatively correlated with soil P. RTD was strongly associated with the above-ground biomass of graminoids. Soil texture impacted the root system: the proportion of absorptive roots increased with fine sand content in the soil, inversely to transport root biomass. The relationship between fire and soil properties was insignificant. Synthesis. In savannas, fire stimulates absorptive root biomass in response to the higher demand for below-ground resources. This response is correlated with shoot regrowth after a fire. Variations in morphological root parameters are not directly associated with fire history; instead, they reflect differences in soil chemistry, especially soil P and graminoid biomass changes.