Fire severity influences the post-fire habitat structure and abundance of a cool climate lizard

In the spring and summer of 2019–2020, the ‘Black Summer’ bushfires burned more than 97 000 km² of predominantly Eucalyptus dominated forest habitat in eastern Australia. The Black Summer bushfires prompted great concern that many species had been imperilled by the fires. Here, we investigate the effects that fire severity had on the habitat and abundance of a cool climate lizard Eulamprus tympanum that was identified as a species of concern because 37% of its habitat was burnt in the Black Summer bushfires. We quantified habitat structure and the abundance of E. tympanum at sites which were unburnt, burnt at low severity and at high severity 10, 15 and 23 months after the fires. Our classification of fire severity based on scorch height and canopy status corresponded well with the Australian Government Google Earth Engine Burnt Area Map (AUS GEEBAM) fire severity layer. Ten months after the fires, sites burnt at high severity had less canopy cover, more bare ground and less fine fuel than sites burnt at low severity or unburnt sites. The abundance of E. tympanum varied with survey occasion and was greatest during the warmest sampling period and lowest during the coolest sampling period. The abundance of E. tympanum was consistently lower on sites burnt at high severity than sites burnt at low severity or unburnt sites. Our findings show that higher severity fires had a greater effect on E. tympanum than low severity fires. Our results suggest that E. tympanum were likely to have persisted in burnt sites, with populations in low severity and unburnt sites facilitating population recovery in areas burnt at high severity. Our results also suggest that wildfire impacts on E. tympanum populations will increase because the frequency and extent of severe fires are expected to increase due to climate change.     

Abrupt fire regime change may cause landscape‐wide loss of mature obligate seeder forests

Obligate seeder trees requiring high‐severity fires to regenerate may be vulnerable to population collapse if fire frequency increases abruptly. We tested this proposition using a long‐lived obligate seeding forest tree, alpine ash (Eucalyptus delegatensis), in the Australian Alps. Since 2002, 85% of the Alps bioregion has been burnt by several very large fires, tracking the regional trend of more frequent extreme fire weather. High‐severity fires removed 25% of aboveground tree biomass, and switched fuel arrays from low loads of herbaceous and litter fuels to high loads of flammable shrubs and juvenile trees, priming regenerating stands for subsequent fires. Single high‐severity fires caused adult mortality and triggered mass regeneration, but a second fire in quick succession killed 97% of the regenerating alpine ash. Our results indicate that without interventions to reduce fire severity, interactions between flammability of regenerating stands and increased extreme fire weather will eliminate much of the remaining mature alpine ash forest.     

Modeling organic matter dynamics in conifer-broadleaf forests in different site types upon fires: A computational experiment

The effect of forest fires differing in intensity on organic matter dynamics in forest soils has been assessed in different types of forest sites using the EFIMOD system of models. Differences between the patterns of organic matter dynamics according to scenarios of forest ecosystem development under normal conditions and upon forest fires have been analyzed. Recovery rates of soil organic matter pools after fires depend on their intensity and frequency. The most profound changes take place upon high-intensity crown fires, which may even result in ecosystem destruction.     

火烧对森林土壤有机碳的影响研究进展

对国内外火烧影响森林土壤有机碳动态的研究成果进行了综合述评。较多研究表明低强度火烧不会造成土壤有机碳贮量的明显变化,但火烧非常强烈而彻底,土壤有机碳明显减少。有限研究表明火烧对森林土壤呼吸的影响结果有增加、降低或无影响,因火烧强度、火后观测时间、森林类型、火烧迹地上植被恢复进程和气候条件等而异。同时,火烧对土壤有机碳组分(活性有机碳和黑碳)也具有不同程度的影响。随着全球变化研究的深入,火烧作为森林主要管理措施对大气CO2浓度影响亦愈来愈受重视,今后应着重开展以下几方面研究:(1)扩大气候和经营管理的变化对森林土壤有机碳贮量时空动态影响研究;(2)深入探讨火烧影响土壤CO2释放的过程及机理;(3)加强火烧历史和频率对黑碳影响的研究;(4)从广度和深度上加强火烧等经营措施对亚热带森林土壤碳动态影响的研究。     

Fire Severity and Long-term Ecosystem Biomass Dynamics in Coniferous Boreal Forests of Eastern Canada

The objective of this study was to characterize the effects of soil burn severity and initial tree composition on long-term forest floor dynamics and ecosystem biomass partitioning within the Picea mariana [Mill.] BSP-feathermoss bioclimatic domain of northwestern Quebec. Changes in forest floor organic matter and ecosystem biomass partitioning were evaluated along a 2,355-year chronosequence of extant stands. Dendroecological and paleoecological methods were used to determine the time since the last fire, the soil burn severity of the last fire (high vs. low severity), and the post-fire tree composition of each stand (P. mariana vs. Pinus banksiana Lamb). In this paper, soil burn severity refers to the thickness of the organic matter layer accumulated above the mineral soil that was not burned by the last fire. In stands originating from high severity fires, the post-fire dominance by Pinus banksiana or P. mariana had little effect on the change in forest floor thickness and tree biomass. In contrast, stands established after low severity fires accumulated during the first century after fire 73% thicker forest floors and produced 50% less tree biomass than stands established after high severity fires. Standing tree biomass increased until approximately 100 years after high severity fires, and then decreased at a logarithmic rate in the millennial absence of fire. Forest floor thickness also showed a rapid initial accumulation rate, and continued to increase in the millennial absence of fire at a much slower rate. However, because forest floor density increased through time, the overall rate of increase in forest floor biomass (58 g m⁻² y⁻¹) remained constant for numerous centuries after fire (700 years). Although young stands (< 200 years) have more than 60% of ecosystem biomass locked-up in living biomass, older stands (> 200 years) sequester the majority (> 80%) of it in their forest floor. The results from this study illustrate that, under similar edaphic conditions, a single gradient related to time since disturbance is insufficient to account for the full spectrum of ecosystem biomass dynamics occurring in eastern boreal forests and highlights the importance of considering soil burn severity. Although fire severity induces diverging ecosystem biomass dynamics in the short term, the extended absence of fire brings about a convergence in terms of ecosystem biomass accumulation and partitioning.     

The relative importance of intrinsic and extrinsic factors in the decline of obligate seeder forests

Forests that regenerate exclusively from seed following high‐severity fire are particularly vulnerable to local extinction if fire frequency leaves insufficient time for regenerating plants to reach sexual maturity. We evaluate the relative importance of extrinsic (such as fire weather and climate cycles) and intrinsic (such as proneness to fire due to stand age and structural development) factors in driving the decline of obligate seeder forests. We illustrate this using obligate seeding alpine ash (Eucalyptus delegatensis) forests in the montane regions of Victoria, Australia, that were burnt by megafires in 2003 (142,256 ha) or 2007 (79,902 ha), including some twice‐burnt areas (11,599 ha). Geospatial analyses showed only a small effect of stand age on the remote sensing estimates of crown defoliation, but a substantial effect of forest fire weather, as measured by forest fire danger index (FFDI). Analysis of meteorological data over the last century showed that 5‐year increases in FFDI precede cycle major fires in the E. delegatensis forests. Such strong extrinsic climate/weather driving of high‐severity fires is consistent with the ‘interval squeeze model’ that postulates the vulnerability of obligate seeder forests to landscape‐scale demographic collapse in response to worsening fire weather under climate change.     

Effects of high- and low-intensity fires on soil properties and plant growth in a Bolivian dry forest

We compared soil nutrient availability and soil physical properties among four treatments (high-intensity fire, low-intensity fire, plant removal, and harvesting gap) and a control (intact forest understory) over a period of 18 months in a tropical dry forest in Bolivia. The effect of treatments on plant growth was tested using a shade intolerant tree species (Anadenanthera colubrina Vell. Conc.) as a bioassay. Surface soils in high-intensity fire treatments had significantly greater pH values, concentrations of extractable calcium (Ca), potassium (K), magnesium (Mg), and phosphorus (P), and amounts of resin-available P and nitrogen (N) than other treatments; however, a loss of soil organic matter during high-intensity fires likely resulted in increased bulk density and strength, and decreased water infiltration rates. Low intensity fires also significantly increased soil pH, concentrations of extractable Ca, K, Mg, and P, and amounts of resin-available P and N, although to a lesser degree than high-intensity fires. Low-intensity fires did not lower soil organic matter contents or alter soil physical properties. Plant removal and harvesting gap treatments had little effect on soil chemical and physical properties. Despite the potentially negative effects of degraded soil structure on plant growth, growth of A. colubrina seedlings were greater following high-intensity fires. Evidently, the increase in nutrient availability caused by high-intensity fires was not offset by degraded soil structure in its effects on seedling growth. Long-term effects of high intensity fires require further research.     

Mapping fire severity and fire extent in forest in Victoria for ecological and fuel outcomes

This study shows how high‐resolution (~15 cm) simultaneous colour and infra‐red digital aerial photography can be used to map both fire severity and, particularly, fire extent, in forest in south‐eastern Australia. The results show that this methodology is capable of detecting and mapping burnt and unburnt edges under unaffected forest canopy (i.e. still green) – that is, revealing the mosaic of burnt and unburnt areas that often result from planned landscape burning under mild weather conditions (i.e. with little of the brownish canopy scorch that results from more intense bushfires). This has important implications for both fuel management and ecology. It can answer the basic questions of fire and biodiversity managers following planned burning –’how much of the planned area burnt, and, within the burnt area, what aspects were burnt, and how hot did they burn?’ The analysis of fire extent by aspect showed that about 80% of southern and eastern aspects remained unburnt during broadscale autumn prescribed burning, with many of these moister aspects potentially providing longer unburnt refuges over multiple burn rotations. The fire severity and extent mapping products, produced using the methodology outlined in this study, have the potential to substantially increase the understanding of the ecological and fuel outcomes of landscape‐scale autumn prescribed burning.     

Origin of the lichen–spruce woodland in the closed-crown forest zone of eastern Canada

Aim We investigate the timing and factors responsible for the transformation of closed‐crown forests into lichen–spruce woodlands. Location The study area extends between 70° and 72° W in the closed‐crown forest zone from its southern limit near 47°30′ N to its northern limit at the contact with the lichen–spruce woodland zone around 52°10′ N. A total of 24 lichen–spruce woodlands were selected. Methods Radiocarbon dating of charcoals at mineral soil contact and within the organic horizons allowed the principal factors causing the degradation of the closed‐crown forest to be identified, i.e. light fires, successive fires and the occurrence of a spruce budworm epidemic followed by a fire. Results Charcoals dated in the organic horizon were less than 200 years old, suggesting a recent transformation of the closed‐crown forest following surface fires. Before their transformation into lichen–spruce woodlands, stands were occupied by old, dense forests that originated from fires dating back to 1000 yr bp . The radiocarbon dating of charcoals in the organic horizon indicated that several stands burned twice in less than 50 years, while others burned shortly after a spruce budworm epidemic. Light fires are frequent within the lichen–spruce woodlands according to multiple charcoal layers found within the organic matter horizon. Main conclusions While closed‐crown forests are predicted to expand under climate warming, compound disturbances diminish the natural regeneration of the closed‐crown forests in the south and favour the expansion of lichen–spruce woodlands. As black spruce germinates on mineral soils, surface fires accentuate the expansion of the lichen–spruce woodlands southward. Under global warming, warmer springs will lead to earlier low‐intensity fires that do not remove as much organic matter, and hence prevent conditions suitable for black spruce regeneration. Also, spruce budworm reduces seed production for a certain time. The occurrence of fire during this period is critical for regeneration of black spruce.     

Vegetation-soil relations in the lowlands of south-west Tasmania

A geographic survey of 14 south-west Tasmanian sedgeland-heaths revealed that soil organic matter is related to: water content: total nitrogen (N): total and exchangeable sodium (Na), calcium (Ca) and magnesium (Mg); exchangeable potassium (K) cation exchange capacity; and total exchangeable bases. However, total and available phosphorus (P), total K and Iron (Fe). pH level and percentage base saturation were found to be Independent of organic content. Most of the soil nutrient capital is contained In the A₀ horizon, the depth of which was found to be positively related to the time elapsed since the last fire. There is no clear relationship between rock type and soil fertility, but there is evidence of soil-vegetation interaction. The sedgeland-heath species have lower concentrations of P, Ca and Mg in their foliage and are more efficient In the withdrawal of P and K upon tissue senescence than the surrounding scrub and forest species. Over a vegetation transition from sedgeland-heath to forest on uniform geology there was a change in soil type. The forest was found to have more fertile soils and a higher concentration of nutrients in the above-ground biomass than the adjacent sedgeland-heath. The ecotone was burnt between 20–30 years prior to sampling, but the fire did not kill all the forest trees, and the structural differences suggest a mare rapid recovery of forest species. Soil fertility appears to be an important factor in controlling the rate of recovery and succession following a fire, especially if the nutrient-rich organic layer is burnt.     

Experimental warming and burn severity alter soil CO2 flux and soil functional groups in a recently burned boreal forest

Global warming is projected to be greatest in northern regions, where forest fires are also increasing in frequency. Thus, interactions between fire and temperature on soil respiration at high latitudes should be considered in determining feedbacks to climate. We tested the hypothesis that experimental warming will augment soil CO₂ flux in a recently burned boreal forest by promoting microbial and root growth, but that this increase will be less apparent in more severely burned areas. We used open‐top chambers to raise temperatures 0.4–0.9°C across two levels of burn severity in a fire scar in Alaskan black spruce forest. After 3 consecutive years of warming, soil respiration was measured through a portable gas exchange system. Abundance of active microbes was determined by using Biolog EcoPlates^? for bacteria and ergosterol analysis for fungi. Elevated temperatures increased soil CO₂ flux by 20% and reduced root biomass, but had no effect on bacterial or fungal abundance or soil organic matter (SOM) content. Soil respiration, fungal abundance, SOM, and root biomass decreased with increasing burn severity. There were no significant interactions between temperature and burn severity with respect to any measurement. Higher soil respiration rates in the warmed plots may be because of higher metabolic activity of microbes or roots. All together, we found that postfire soils are a greater source of CO₂ to the atmosphere under elevated temperatures even in severely burned areas, suggesting that global warming may produce a positive feedback to atmospheric CO₂, even in young boreal ecosystems.     

The effect of high temperatures on seed germination of one native and two introduced conifers in Patagonia

We examined the effect of thermal shock on the germination of seeds of three conifers, two introduced (Pseudotsuga menziesii and Pinus ponderosa), and one native to Patagonia (Araucaria araucana). Previous research has suggested increased susceptibility to invasions in burnt areas, and therefore, the effect of simulated fire (heat) on seed germination in these native and introduced species was compared. Seeds were heated to two different heat intensities (50°C and 100°C) for 1 or 5 min, which is within the temperature range reached in the upper soil layers during forest fires. Germination tests were then carried out in a growth chamber. The heat treatments had a negative effect on the germination of P. menziesii at temperatures of 100°C, and a negative effect on the germination of P. ponderosa at the temperature of 100°C and the exposure of 5 min. The heat treatments had no affect at all on A. araucana. The species with larger seeds (A. araucana) had higher survival rates after the thermal shocks. Also intraspecific differences in seed sizes possibly point at larger seeds surviving thermal shocks better than smaller seeds. In addition, thermal shock caused a delay in the onset of germination in the two introduced species, while it did not change the time for germination in A. araucana.     

Historic distribution and driving factors of human-caused fires in the Chinese boreal forest between 1972 and 2005

Aims The pattern and driving factors of forest fires are of interest for fire occurrence prediction and forest fire management. The aims of the study were: (i) to describe the history of human-caused fires by season and size of burned area over time; (ii) to identify the spatial patterns of human-caused fires and test for the existence of 'hotspots' to determine their exact locations in the Daxing'an Mountains; (iii) to determine the driving factors that determine the spatial distribution and the possibility of human-caused fire occurrence.Methods In this study, K -function and Kernel density estimation were used to analyze the spatial pattern of human-caused fires. The analysis was conducted in S-plus and ArcGIS environments, respectively. The analysis of driving factors was performed in SPSS 19.0 based on a logistic regression model. The variables used to identify factors that influence fire occurrence included vegetation types, meteorological conditions, socioeconomic factors, topography and infrastructure factors, which were extracted and collected through the spatial analysis mode of ArcGIS and from official statistics, respectively.Important findings The annual number of human-caused fires and the area burnt have declined since 1987 due to the implementation of a forest fire protection act. There were significant spatial heterogeneity and seasonal variations in the distribution of human-caused fires in the Daxing'an Mountains. The heterogeneity was caused by elevation, distance to the nearest railway, forest type and temperature. A logistic regression model was developed to predict the likelihood of human-caused fire occurrence in the Daxing'an Mountains; its global accuracy attained 64.8%. The model was thus comparable to other relevant studies.     

Selective burning of forest vegetation in Canton Ticino (southern Switzerland)

Abstract

Detailed knowledge of factors controlling fire regime is a prerequisite for efficient fire management. We analyzed the fire selectivity of given forest vegetation classes both in terms of fire frequency and fire size for the present fire regime (1982–2005) in Canton Ticino (southern Switzerland). To this end, we investigated the dataset in four categories (all fires, anthropogenic winter fires, anthropogenic summer fires, and natural summer fires) and performed 1000 random Monte Carlo simulations on frequency and size. Anthropogenic winter and summer fires have a similar selectivity, occurring mostly at low elevations in chestnut stands, broadleaved forests, and in the first 50 m from the forest edge. In winter half of the fires in chestnut stands are significantly larger than 1.0 ha and the average burnt area in some coniferous forests tends to be high. Lightning fires seem to occur more frequently in spruce stands and less often in the summer‐humid chestnut and beech stands and the 50–100 m buffer area. In beech forests, in mixed forests, and in the spruce stands affected by natural fire in summer, the fires tend to be small in size. The selectivity observed, especially the selectivity of anthropogenic fires in terms of fire frequency, seems to be also related to geographical parameters such as altitude and aspect, and to anthropogenic characteristics such as closeness to roads or buildings.     

Fire frequency has a contrasting effect on vegetation and topsoil in subcoastal heathland,woodland and forest ecosystems,south-east Queensland,Australia

Ecosystems managed with contrasting fire regimes provide insight into the responses of vegetation and soil. Heathland, woodland and forest ecosystems along a gradient of resource availability were burnt over four decades in approximately 3- or 5-year intervals or were unburnt for 45–47 years (heathland, woodland), or experienced infrequent wildfires (forest: 14 years since the last fire). We hypothesized that, relative to unburnt or infrequent fires, frequent burning would favour herbaceous species over woody species and resprouting over obligate seeder species, and reduce understorey vegetation height, and topsoil carbon and nitrogen content. Our hypothesis was partially supported in that herbaceous plant density was higher in frequently burnt vegetation; however, woody plant density was also higher in frequently burnt areas relative to unburnt/infrequently burnt areas, across all ecosystems. In heathland, omission of frequent fire resulted in the dominance of fern Gleichenia dicarpa and subsequent competitive exclusion of understorey species and lower species diversity. As hypothesized, frequent burning in woodland and forest increased the density of facultative resprouters and significantly reduced soil organic carbon levels relative to unburnt sites. Our findings confirm that regular burning conserves understorey diversity and maintains an understorey of lower statured herbaceous plants, although demonstrates the potential trade-off of frequent burning with lower topsoil carbon levels in the woodland and forest. Some ecosystem specific responses to varied fire frequencies were observed, reflecting differences in species composition and fire response traits between ecosystems. Overall, unburnt vegetation resulted in the dominance of some species over others and the different vegetation types were able to withstand relatively high-frequency fire without the loss of biodiversity, mainly due to high environmental productivity and short juvenile periods.     

Low‐severity fire as a mechanism of organic matter protection in global peatlands: Thermal alteration slows decomposition

Worldwide, regularly recurring wildfires shape many peatland ecosystems to the extent that fire‐adapted species often dominate plant communities, suggesting that wildfire is an integral part of peatland ecology rather than an anomaly. The most destructive blazes are smoldering fires that are usually initiated in periods of drought and can combust entire peatland carbon stores. However, peatland wildfires more typically occur as low‐severity surface burns that arise in the dormant season when vegetation is desiccated, and soil moisture is high. In such low‐severity fires, surface layers experience flash heating, but there is little loss of underlying peat to combustion. This study examines the potential importance of such processes in several peatlands that span a gradient from hemiboreal to tropical ecozones and experience a wide range of fire return intervals. We show that low‐severity fires can increase the pool of stable soil carbon by thermally altering the chemistry of soil organic matter (SOM), thereby reducing rates of microbial respiration. Using X‐ray photoelectron spectroscopy and Fourier transform infrared, we demonstrate that low‐severity fires significantly increase the degree of carbon condensation and aromatization of SOM functional groups, particularly on the surface of peat aggregates. Laboratory incubations show lower CO₂ emissions from peat subjected to low‐severity fire and predict lower cumulative CO₂ emissions from burned peat after 1–3 years. Also, low‐severity fires reduce the temperature sensitivity (Q₁₀) of peat, indicating that these fires can inhibit microbial access to SOM. The increased stability of thermally altered SOM may allow a greater proportion of organic matter to survive vertical migration into saturated and anaerobic zones of peatlands where environmental conditions physiochemically protect carbon stores from decomposition for thousands of years. Thus, across latitudes, low‐severity fire is an overlooked factor influencing carbon cycling in peatlands, which is relevant to global carbon budgets as climate change alters fire regimes worldwide.     

Fire, fuels and restoration of ponderosa pine–Douglas fir forests in the Rocky Mountains, USA

Aim Forest restoration in ponderosa pine and mixed ponderosa pine–Douglas fir forests in the US Rocky Mountains has been highly influenced by a historical model of frequent, low‐severity surface fires developed for the ponderosa pine forests of the Southwestern USA. A restoration model, based on this low‐severity fire model, focuses on thinning and prescribed burning to restore historical forest structure. However, in the US Rocky Mountains, research on fire history and forest structure, and early historical reports, suggest the low‐severity model may only apply in limited geographical areas. The aim of this article is to elaborate a new variable‐severity fire model and evaluate the applicability of this model, along with the low‐severity model, for the ponderosa pine–Douglas fir forests of the Rocky Mountains. Location Rocky Mountains, USA. Methods The geographical applicability of the two fire models is evaluated using historical records, fire histories and forest age‐structure analyses. Results Historical sources and tree‐ring reconstructions document that, near or before ad 1900, the low‐severity model may apply in dry, low‐elevation settings, but that fires naturally varied in severity in most of these forests. Low‐severity fires were common, but high‐severity fires also burned thousands of hectares. Tree regeneration increased after these high‐severity fires, and often attained densities much greater than those reconstructed for Southwestern ponderosa pine forests. Main conclusions Exclusion of fire has not clearly and uniformly increased fuels or shifted the fire type from low‐ to high‐severity fires. However, logging and livestock grazing have increased tree densities and risk of high‐severity fires in some areas. Restoration is likely to be most effective which seeks to (1) restore variability of fire, (2) reverse changes brought about by livestock grazing and logging, and (3) modify these land uses so that degradation is not repeated.     

基于气象因子的森林火灾面积预测模型

通过统计分析理论研究了黑龙江省林火发生规律,并建立了基于气象因子的森林火灾面积预测模型．结果表明：兴安落叶松林区林火主要发生在4-6月和10月,阔叶红松林区林火主要发生在3-6月和10月;利用林火发生当日的平均风速、相对湿度和平均温度的取值范围可知,兴安落叶松林区发生高等级林火概率较大的月份依次为4月、5月和6月,阔叶红松林区则依次为5月、4月和3月．所建模型的平均精度达到63．3％,能够较精确地预测林火发生后林地可能的过火面积．     

Simulating fire regimes in human-dominated ecosystems: Iberian Peninsula case study

A new fire model is proposed which estimates areas burnt on a macro‐scale (10–100 km). It consists of three parts: evaluation of fire danger due to climatic conditions, estimation of the number of fires and the extent of the area burnt. The model can operate on three time steps, daily, monthly and yearly, and interacts with a Dynamic Global Vegetation Model (DGVM), thereby providing an important forcing for natural competition. Fire danger is related to number of dry days and amplitude of daily temperature during these days. The number of fires during fire days varies with human population density. Areas burnt are calculated based on average wind speed, available fuel and fire duration. The model has been incorporated into the Lund‐Potsdam‐Jena Dynamic Global Vegetation Model (LPJ‐DGVM) and has been tested for peninsular Spain. LPJ‐DGVM was modified to allow bi‐directional feedback between fire disturbance and vegetation dynamics. The number of fires and areas burnt were simulated for the period 1974–94 and compared against observations. The model produced realistic results, which are well correlated, both spatially and temporally, with the fire statistics. Therefore, a relatively simple mechanistic fire model can be used to reproduce fire regime patterns in human‐ dominated ecosystems over a large region and a long time period.