From burned vegetation to streams water: fire effects on vegetation resilience and nutrient fluxes

Wildfire effects on riparian zones and stream water can be significant, particularly in the vegetation recovery and flow of nutrients between the terrestrial and aquatic ecosystem. However, the integrated knowledge about the impacts of fire on terrestrial and aquatic ecosystems in the Brazilian Cerrado is poorly known. In Brazilian Cerrado, wildfire is one of the main vectors of degradation of riparian vegetation, because the forest formation in riparian zones can be more sensitive to fire than the other savanna formation due to a less evident vegetation fire-adaptations. Our main objective was to understand the effects of fire on the resilience of riparian vegetation and their consequences to nutrient fluxes between terrestrial and aquatic ecosystem. This study was conducted in the Environmental Protection Area (APA) located in the Federal District - Brazil, which is one of Brazil’s Long-Term Ecological Research Site, after a wildfire (September 2011) that burned an area of about 140 km². We analyzed the riparian vegetation resilience (for forests and surrounding savannas formations) and nutrients fluxes (in surface runoff and stream water) in five streams. We estimated the fire severity with Delta Normalized Burn Ratio index and the riparian vegetation resilience with the Normalized Vegetation Index and evaluated the changes in nutrient concentrations for nitrite + nitrate ([NO^2- + NO^3-]), ammonium (NH₄⁺), and phosphate (PO₄^3-) during 16 months on stream water and surface runoff solution in burned and unburned areas using the Generalized Linear Models. Our results show that fire severity was similar between forests and savannas formations, but in savannas we observed higher vegetation resilience, with faster vegetation regrowth and recovery after three weeks. The concentration of nutrients on both surface runoff and inside the stream have changed in burned areas regarding unburned areas, with an increase of PO₄^3- and [NO^2- + NO^3-] and a decreased of NH₄⁺. After 16 months of the fire event, the concentration of PO₄^3-, [NO^2- + NO^3-] and NH₄⁺ increased in surface runoff, while [NO^2- + NO^3-] decreased inside the streams in burned areas. Precipitation was a factor that caused the increase of concentrations of [NO^2- + NO^3-] and NH₄⁺ and, the high precipitation on rainy season (October – March), that started after the fire, could have contributed to the input of these nutrients and particulate materials from ashes to streams. Our results showed that the occurrence of fire in riparian environments reduces the biomass of riparian forests and increases the concentration of nutrients on streams.  These elevated postfire nitrogen and phosphate loading can influence streams ecosystem health, especially in oligotrophic streams like those found in Brazilian Cerrado. It is known that phosphorus and nitrogen are limited nutrients for algal and cyanobacterial growth in freshwater ecosystem and an increase of these organisms can disrupt the ecosystem integrity. Fire is a pulsed disturbance and its effect on freshwater ecosystem depends on terrestrial ecosystem recovery, in this way, it is necessary to integrate the knowledge about the impacts of fire on terrestrial and aquatic ecosystems to better understand the effects on the entire ecosystem.