Effects of prescribed fire frequency on wildfire emissions and carbon sequestration in a fire adapted ecosystem using a comprehensive carbon model

Contrasting immediate impact of prescribed fires and experimental summer fires on soil organic matter quality and microbial properties in the forest floor and mineral soil in Mediterranean black pine forest

Prescribed fire (PB) is used to achieve ecological objectives and to reduce fuel hazard thus limiting detrimental impacts of wildfire and appropriate selection of prescription window is critical for these goals. Operational use of PB in the Mediterranean forest is scarce and information about its effects on soil remains incomplete. This study for the first time i) compared the immediate impact of spring and autumn PB and experimental summer fire on key properties of forest floor and mineral topsoil in Mediterranean black pine forest, and ii) assessed the capacity of PB to reduce fuel, with limited immediate impacts on soil. PB significantly reduced the 32.5 % of pre-fire forest floor depth, while summer fire consumed 88.5 % and exposed about 30 % of the mineral soil surface. Mean maximum temperature during fire at the mineral soil surface was 23 °C in PB, in contrast to 128 °C in summer fire, while soil heating at 2 cm depth was negligible in both cases. PB did not cause immediate changes in OM quality parameters, and chemical (C and N concentrations, C/N and pH) and microbiological properties (Cmic, Cmic/C, and β-glucosidase, acid phosphatase and alkaline phosphatase activities) in forest floor or mineral topsoil (0-2 cm). By contrast, summer fire greatly increased OM recalcitrance and reduced Cmic, Cmic/C and enzyme activities in forest floor immediately after fire. In the mineral topsoil, only microbial properties were significantly reduced. The maximum temperature reached during fire in forest floor and topsoil was associated with most of the overall changes in properties in both layers. The findings suggest that prescribed fire can significantly reduce fuel with limited initial impacts on soil. Although these findings are encouraging for operational use of prescribed burning in the ecosystem under study, long-term monitoring of repeated application of the technique on soil properties and other ecosystem components is necessary.

Spatio-temporal assessment of soil properties immediately and eight months after a high intensity-controlled burn in the south of Spain

In recent years, the use of fire as a means by which to manage forest ecosystems has become more frequent in Europe. Fire has a significant impact on the soil, and it is therefore necessary to understand how controlled burns affect this invaluable resource. The purpose of this study was to evaluate the main alterations in the physical-chemical and biological properties of the soil because of a high intensity-controlled burn in "Los Boquerones" area (Villaviciosa de Córdoba, Spain). Additionally, we assessed the spatial heterogeneity of the alterations of different soil properties. A grid of 12 points was established on a hillside in Sierra Morena (Córdoba). Thermocouples were placed at each point, and soil samples were collected at two depths (0-2 cm and 2-5 cm) before burning, immediately after burning and eight months later. Soil pH, electrical conductivity, nutrient content and/or availability, among others, and their spatio-temporal variations were analysed. Soil pH, increased in the first centimetres of the soil (0-2 cm) immediately after burning up to >2 units, and the increase was maintained eight months following the burn. Additionally, the high-intensity burn had a positive short-term effect on some of the soil properties, such as nutrient availability for plants, which was considerably increased. The magnitude of the alterations in the soil indicators assessed was spatially explained by the behaviour of the fire during the controlled burning. The burn also had both direct and indirect effects on soil microorganisms. In conclusion, the possible immediate and short-term effects of burning on the soil resource should be considered for a more holistic management of fire in forest ecosystems, as its functionality and capacity to provide ecosystem services is largely altered by these events as a function of their intensity.

Tree mortality and carbon emission as a function of wildfire severity in south-eastern Australian temperate forests

Disturbance trends over recent decades indicate that climate change is resulting in increased fire severity and extent in Australia's temperate Eucalyptus forests. As disturbance cycles become shorter and more severe, empirical measurements are required to identify potential change in forest carbon (C) stock and emissions. However, such estimates are rare in the literature. The 2019-2020 wildfires burnt through 6 to 7 million ha of mainly temperate open Eucalyptus forest in south-east Australia, with top down emission estimates ranging from 97 to 130 tonnes CO₂ ha^-1. Study sites that had been assessed for all aboveground C pools prior to the wildfires, were burnt in January 2020 by wildfire that varied in severity. Here we quantify the impact of high and low/moderate fire severities on tree mortality, C loss and C redistribution and assess implications for future C storage in these temperate Eucalyptus forests. Higher fire severity resulted in greater overstorey tree mortality but not understorey or loss of dead standing trees than in low/moderate severity fires. High severity fires combusted almost twice as much C from live trees (42 Mg C ha^-1) as low/moderate severity fires (25 Mg C ha^-1), while C loss from dead standing trees was similar among fire severity classes (average 17 Mg C ha^-1). Total aboveground C lost across study sites was 42 Mg C ha^-1 for high and 47 Mg C ha^-1 for low/moderate severity, with an average of 45 Mg C ha^-1 equivalent to 15 % (high severity) and 14 % (low/moderate severity) of AGC. Extrapolating our findings to other tall to medium open Eucalyptus forests across Victoria revealed that 37.33 ± 12.25 Tg C (mean ± s.e.) or 152 ± 50 Mg CO₂ ha^-1 was lost to the atmosphere from the 0.9 million ha of these productive forests, equating to about 20 % of Australia's total net annual emissions.

The Effect of Repeated Prescribed Burning on Soil Properties: A Review

Prescribed burning is a tool that is frequently used for various land management objectives, mainly related to reduction of hazardous forest fuels, habitat management and ecological restoration. Given the crucial role of soil in forest ecosystem processes and functions, assessing the effects of prescribed burning on soil is particularly relevant. This study reviews research on the impacts of repeated prescribed burning on the physical, chemical and biological properties of soil. The available information shows that the effects are highly variable, rather inconsistent and generally minor for most of the soil characteristics studied, while a number of soil properties show contrasting responses. On the other hand, ecosystem characteristics, differences in fire severity, frequency of application and the cumulative effect of treatment repetition over time, have possibly made it more difficult to find a more common response in soil attributes. Our study has also revealed some limitations of previous research that may have contributed to this result, including a limited number of long-term studies, conducted at a few experimental sites, and in a limited number of forest ecosystems. Research issues concerning the effects of prescribed fire on soil are presented. The need to integrate such research into a broader interdisciplinary framework, encompassing the role of the fire regime on ecosystem functions and processes, is also highlighted.