How root biomass and root traits vary according to fire in Brazilian savanna

Soizig Le Stradic discusses her recent article: ‘Variation in biomass allocation and root functional parameters in response to fire history in Brazilian savannas‘. Find out more about their new research into how below-ground plant components respond to fire in the open savannas of the Cerrado.

Fire is a major factor in the structure of plant communities, especially in tropical open ecosystems like savannas. Recurrent surface fires shape the landscape, favouring the dominance of grassy and herbaceous species and limiting the establishment of woody species. Evidence of savanna species’ adaptation to fire is clearly visible aboveground; like trees with thick bark, the important resprouting capacity of many species thanks to well-protected buds, or massive flowering events following fire. By contrast, the effect of fire on the belowground compartments is not visible and remains poorly understood, even though a large share of biomass is located belowground in savannas. This is why we decided to look at the impacts of fire on savanna communities below soil surface.

In such ecosystems, the vegetation resprouts rapidly after a fire. To recover, plant species present various strategies that rely primarily on the presence of fire-protected belowground buds associated with diverse storage organs that ensure fast vegetation recovery. The biomass is then preferentially allocated belowground in savannas to cope with recurrent fires. That is especially true in mesic environments like Brazilian savannas, commonly called Cerrado.

Picture of a campo sujo located at the Ecological Station of Santa Barbara (São Paulo state, Brazil) (Picture: Soizig Le Stradic)

Within the belowground compartment, roots (and especially fine roots) play a major role in nutrient acquisition. In addition to storage organs, roots may therefore be of vital importance in facilitating rapid resource uptake and ensuring regeneration after fire. The increase in nutrient acquisition can be improved by increasing fine root biomass at soil surface, as already observed on some frequently burnt savannas. The morphology of fine roots may also favour post-fire regeneration: fine roots with thin diameters, low tissue densities and high specific root length present high capacities for nutrient uptake.

Fire might also affect root trait and root distribution in several indirect ways, including through changes in vegetation structure or soil properties. For example, a long period without fire may lead to changes in grass composition or woody encroachment, whereas frequent fire will often favour the dominance of grass species. Soil properties may also be modified by fire history (i.e. increasing or decreasing soil fertility). This will constrain both root traits and belowground biomass distribution.

Because roots are related to several ecosystem processes, like decomposition or nutrient cycling, understanding how root traits and root biomass vary in response to fire is of critical importance and was the core of our study. The understanding of belowground biomass allocation and variation in root traits in savannas has been primarily focused around tree species and coarse roots. Research into the response of fine roots to fire is still lacking, particularly for open communities. We therefore focused our study on open savannas dominated by grass and herbaceous species with very low woody cover. Within the Cerrado, such vegetation is named campo sujo.

We selected eight different campos sujos along a gradient of time since the last fire and fire frequency to understand how above and below-ground plant components responded to fire and whether changes in vegetation and soil properties are related to modification in belowground biomass and root traits. To do so, we measured above- and below-ground biomass, root depth distribution, root functional parameters and nutrient content in the upper soil layer, in the different sampling areas.

Root sampling in an area of campo sujo located at the Ecological Station of Itirapina (São Paulo state, Brazil) (Picture: Soizig Le Stradic)

We found that, at the community level, absorptive root biomass increased after fire in the upper soil layer, when vegetation recovered and the demand for belowground resources was higher. In the deeper soil layer however, the root biomass remained unaffected by fire. Soil texture also impacted root biomass, as we observed a higher proportion of absorptive roots in soils with fine sand. Whereas root traits did not vary with fire history, we showed that variation in vegetation structure and soil properties lead to some changes in root traits of the communities. Root tissue density (RTD) for example was positively associated with graminoid biomass, and the specific root length (SRL) increased with soil P but decreased with K, Mg, Al, N and C concentration.

We showed that an increase of absorptive root production is part of the regeneration strategy of plant species in savanna after fire. However, if fire history affected root biomass, it did not impact directly morphological root traits. Modifications in soil chemical composition or vegetation structure will more likely lead to variations in root morphological traits. Because modifications of root traits may have consequences on ecosystem functioning, including nutrient cycling or carbon storage, we encourage future research to disentangle how variations in edaphic conditions or vegetation structure modify root traits in tropical open ecosystem like savannas.

Soizig Le Stradic Technical University of Munich, Germany & UNESP, Brazil

Read the full article online: Variation in biomass allocation and root functional parameters in response to fire history in Brazilian savannas‘.

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