Germination response to fire-related factors of seeds from non-serotinous and serotinous cones

Pinus halepensis, a Mediterranean pine tree, is a partially serotinous species: individual trees of this species carry both non-serotinous and serotinous cones. Serotinous cones open mainly after fire, whereas non-serotinous cones open in absence of fire. In this study we addressed the question, whether or not this cone response is linked with the germination response of seeds to fires. Two main factors associated with fire are heating of seeds and soil pH. A combination of high heat and high pH simulates a scenario with fire, whereas low heat and low pH simulates a scenario without fire. We assessed the separate and combined effects of heat and pH on the germination rate and the percentage of germination of seeds from non-serotinous cones and two age classes of serotinous cones of P. halepensis. Heat had no effect on the percentage of germination of seeds from any of the cone types, but did positively affect the germination rates of seeds from both age-classes of serotinous cones. High pH negatively affected the germination rate of seeds from all cone types as well as the percentage of germination of seeds from non-serotinous cones. The combinations of heat and pH had different effects on the three cone types: percentage of germination and rate of germination of seeds from non-serotinous cones was higher in the combination high heat-high pH than in the combination low heat-low pH. In the combination high heat-high pH, seeds from serotinous cones germinated better than seeds from non-serotinous cones. The different germination responses of seeds from non-serotinous and serotinous cones could not be attributed to differences in cone age. Our results indicate that the cone response is linked to the germination response of the seeds in P. halepensis, with seeds from serotinous cones being more tolerant to fire related factors. This revised version was published online in June 2006 with corrections to the Cover Date.     

Microclimate Change and Effect on Fire Following Forest-Grass Conversion in Seasonally Dry Tropical Woodland1

We tested the hypothesis that, where fire has historically been infrequent, wooded areas that have been invaded by grasses and converted to grassland by fire are predisposed to future fire compared to adjacent areas that remain wooded; thus, an initial forest fire may promote future fires. We compared microclimate between a grass-dominated burned area and a nearby grass-invaded woodland that has not burned in recent history, both located in the submontane dry forest of Hawaii Volcanoes National Park. The results were used to parameterize BEHAVE, a fire behavior prediction model developed by the USDA Forest Service. The model's predictions include probability of ignition, intensity, rate of spread, and tree mortality. Contrary to expectations, daytime hourly mean temperatures were higher and relative humidity was lower in the woodland site. However, the differences in temperature and humidity were not great enough to affect spread rate or probability of ignition. Wind speeds were substantially greater in the grassland, and this was most important in driving differences in modeled fire spread. Given similar synoptic conditions, a fire started in the grassland can be expected to spread an order of magnitude faster than one started in the woodland.     

Seed size and seed germination in the Mediterranean fire-prone shrub <Emphasis Type="Italic">Cistus </Emphasis><Emphasis Type="Italic">ladanifer</Emphasis>

The germination response of different sized seeds from individuals of a Mediterranean fire-prone shrub (Cistus ladanifer) was investigated in relation to pre-germination heating. A control (no heating), a low temperature during a short exposure time (50°C during 5 min), a high temperature during a short exposure time (100°C during 5 min) and a high temperature during a long exposure time (100°C during 15 min) were applied to seeds from different individual plants with different mean seed weight. These pre-germination treatments resemble natural germination scenarios for the studied species, absence of fire, low intensity pasture fire, typical Mediterranean shrub fire, and severe fire with high fuel load. Mean seed weight only showed a marginally significant positive correlation with the proportion of germinated seeds whatever the pre-germination treatment. These results suggest that seed dormancy is unrelated to seed size and that under the experimental conditions used in this study, the effect of seed size on seed germination is low. Nevertheless, larger seeds could be favoured in natural conditions, especially under the high competition scenario which arise after wildfires. Control seeds showed a negative correlation between seed size and germination velocity suggesting that lighter seeds could take advantage from early germination in recruitment events in the absence of wildfires. Nevertheless, even the lower pre-germination heating treatment turns this correlation in not significant, suggesting a strong selection pressure (unrelated to seed size) for early germination after fire events. In our study, different sized seeds of C. ladanifer seem to perform better under different germination scenarios suggesting that seed size variation could be maintained by the alternation of recruitments without wildfires and recruitments after wildfire events.     

Effects of experimental fuel additions on fire intensity and severity: unexpected carbon resilience of a neotropical forest

Global changes and associated droughts, heat waves, logging activities, and forest fragmentation may intensify fires in Amazonia by altering forest microclimate and fuel dynamics. To isolate the effects of fuel loads on fire behavior and fire‐induced changes in forest carbon cycling, we manipulated fine fuel loads in a fire experiment located in southeast Amazonia. We predicted that a 50% increase in fine fuel loads would disproportionally increase fire intensity and severity (i.e., tree mortality and losses in carbon stocks) due to multiplicative effects of fine fuel loads on the rate of fire spread, fuel consumption, and burned area. The experiment followed a fully replicated randomized block design (N = 6) comprised of unburned control plots and burned plots that were treated with and without fine fuel additions. The fuel addition treatment significantly increased burned area (+22%) and consequently canopy openness (+10%), fine fuel combustion (+5%), and mortality of individuals ≥5 cm in diameter at breast height (dbh; +37%). Surprisingly, we observed nonsignificant effects of the fuel addition treatment on fireline intensity, and no significant differences among the three treatments for (i) mortality of large trees (≥30 cm dbh), (ii) aboveground forest carbon stocks, and (iii) soil respiration. It was also surprising that postfire tree growth and wood increment were higher in the burned plots treated with fuels than in the unburned control. These results suggest that (i) fine fuel load accumulation increases the likelihood of larger understory fires and (ii) single, low‐intensity fires weakly influence carbon cycling of this primary neotropical forest, although delayed postfire mortality of large trees may lower carbon stocks over the long term. Overall, our findings indicate that increased fine fuel loads alone are unlikely to create threshold conditions for high‐intensity, catastrophic fires during nondrought years.     

Scaling up flammability from individual leaves to fuel beds

Wildfires play an important role in vegetation composition and structure, nutrient fluxes, human health and wealth, and are interlinked with climate change. Plants have an influence on wildfire behaviour and predicting this feedback is a high research priority. For upscaling from leaf traits to wildfire behaviour we need to know if the same leaf traits are important for the flammability of 1) individual leaves, and 2) multiple leaves packed in fuel beds. Based on a conceptual framework, we hypothesised that fuel packing properties, through airflow limitation, would overrule the effects of individual leaf morphology and chemistry. To test this hypothesis we compared the results of two experiments, respectively addressing individual leaf flammability and monospecific fuel bed flammability of 25 perennial species from eastern Australia. Across species, fuel bed packing ratio and bulk density scaled negatively with fire spread and positively with maximum temperature and burning time. Species with ‘curlier’ leaves, higher specific leaf area, lower tannin concentrations and lower tissue density promoted faster fire spread through fuel beds. We found that species with shorter individual leaf ignition times had a faster fire spread, shorter burning times and lower temperatures in fuel beds. Leaf traits that affect the flammability of individual leaves (e.g. specific leaf area), continue to do so even when packed in fuel beds. While previous studies have focused on either flammability of individual plant particles or fire behaviour in fuel beds, this is the first time that an overarching combination of the two approaches was made for a wide range of species. Our findings provide a better understanding of fuel bed flammability based on interspecific variation in morphological and some chemical leaf traits. This can be a first step in linking leaf traits to fire behaviour in the field.     

Do biological invasions by <Emphasis Type="Italic">Eupatorium adenophorum</Emphasis> increase forest fire severity?

The invasive species Eupatorium adenophorum is known to influence stand structure and wildfire the hazard in forests. In the current work, we quantitatively examined fire effects in invaded and uninvaded plots in southwestern Sichuan Province, China, with five different forest sites that had different types of dominant species: Pinus yunnanensis, P. yunnanensis–Quercus spp., Keteleeria fortunei, K. fortunei–Quercus spp., and Eucalyptus robusta. We compared the fuel chemistry (moisture, ash, heat value, and ignition point) and fire severity (flame length, fire intensity) under three burning conditions between the invaded and uninvaded plots in each forest sites, and then analyzed the results using multivariate response permutation procedures (MRPP). The burning conditions included: low (fine fuel moisture of 15 % and 5 km/h windspeed), moderate (fine fuel moisture of 10 % and 15 km/h windspeed), and extreme (fine fuel moisture of 5 % and 30 km/h windspeed). With all five sites, the fire severity and fuel loads were clearly significantly higher at the invaded sites. Fire severity was also intensified in the invaded coniferous sites compared to their mixed forest sites. These results indicate that biological invasions may increase the surface fire severity, perhaps through an increase in the heat value, and fuel loads, while reducing the moisture, ash, and ignition point of the understory herbaceous.     

Managing road corridors to limit fire hazard. A simulation approach in southern France

Road corridors are sources of fire ignition and fire spread in French Mediterranean areas, but little is known about the flammability of vegetation and the probability of fire ignition and spread to neighbouring forests. This study simulated fire propagation in road corridors with the help of a cellular automata (CA). We assessed the relative importance of vegetation type, fuel treatment and the spatial patterning of vegetation on the probability of fire spreading to forests. The cellular automaton simulator was implemented with different types of corridors (30 m × 40 m), on the basis of an extensive field survey of vegetation. We used data from laboratory flammability experiments to determine the probability of ignition and propagation for each of the 20 cm × 20 cm cells of the CA. The probability of a fire reaching the neighbouring forest (P_FR) indicated that certain types of road corridors represent a very high risk owing to a combination of highly flammable vegetation and high spatial connectivity. The lowest P_FR values correspond to corridors with a decreasing vegetation flammability gradient in the vicinity of the forest. A several meter-wide embankment with low-flammable and/or managed vegetation can substantially reduce P_FR. These results suggest that firewise landscaping and local vegetation management can reduce fire risk in road corridors. Each corridor type should be subjected to specific vegetation management to account for flammability, growth pattern and lifetime.     

不同烈度林火对油松林潜在地表火行为的影响

油松是我国华北地区代表性树种之一,含有丰富油脂,容易引发大面积高烈度森林火灾。阐明不同烈度林火对油松林地表可燃物负荷量和潜在地表火行为的影响,对于油松林林火管理具有重要意义。以辽河源自然保护区2014年不同烈度林火干扰后油松林分为研究对象,根据不同烈度(重度、中度、轻度)和对照(未过火)分别设置3块20 m×20 m样地,共12块样地,调查地表可燃物和林分结构指标,结合室内实验,利用BehavePlus 5.0软件进行潜在火行为模拟,探讨不同烈度林火5年后油松林地表可燃物负荷量和潜在地表火行为特点,并分析影响潜在地表火行为的主要因素。研究结果表明：(1)不同烈度林火之间,细小可燃物负荷量和地表可燃物总负荷量均不存在显著性差异(P>0.05)。(2)不同烈度林火后,在不同风速和可燃物含水率条件下,油松林潜在地表火蔓延速度、火线强度不存在显著性差异(P>0.05),单位面积发热量、火焰高度、反应强度存在显著性差异(P<0.05)。(3)不同烈度林火后油松林潜在地表火行为主要受油松更新幼苗基径、灌木负荷量、油松平均冠幅、上层枯叶负荷量、油松更新幼苗密度的影响。研究结果表明不...     

Anthropogenic modifications to fire regimes in the wider Serengeti‐Mara ecosystem

Fire is a key driver in savannah systems and widely used as a land management tool. Intensifying human land uses are leading to rapid changes in the fire regimes, with consequences for ecosystem functioning and composition. We undertake a novel analysis describing spatial patterns in the fire regime of the Serengeti‐Mara ecosystem, document multidecadal temporal changes and investigate the factors underlying these patterns. We used MODIS active fire and burned area products from 2001 to 2014 to identify individual fires; summarizing four characteristics for each detected fire: size, ignition date, time since last fire and radiative power. Using satellite imagery, we estimated the rate of change in the density of livestock bomas as a proxy for livestock density. We used these metrics to model drivers of variation in the four fire characteristics, as well as total number of fires and total area burned. Fires in the Serengeti‐Mara show high spatial variability—with number of fires and ignition date mirroring mean annual precipitation. The short‐term effect of rainfall decreases fire size and intensity but cumulative rainfall over several years leads to increased standing grass biomass and fuel loads, and, therefore, in larger and hotter fires. Our study reveals dramatic changes over time, with a reduction in total number of fires and total area burned, to the point where some areas now experience virtually no fire. We suggest that increasing livestock numbers are driving this decline, presumably by inhibiting fire spread. These temporal patterns are part of a global decline in total area burned, especially in savannahs, and we caution that ecosystem functioning may have been compromised. Land managers and policy formulators need to factor in rapid fire regime modifications to achieve management objectives and maintain the ecological function of savannah ecosystems.     

Influence of nutrient availability and tree wildling density on nutrient uptake by Oxalis acetosella and Acer saccharum

The different weight-number strategies of seed production displayed by individuals of a Mediterranean fire-prone plant species (Cistus ladanifer) were investigated in relation to seed germination responses to pre-germination heating. A control (no heating), a high temperature during a short exposure time (100 degrees C during 5 min) and a high temperature during a long exposure time (100 degrees C during 15 min) were applied to seeds from different individual plants with different mean seed weight. These pre-germination treatments resemble natural germination scenarios for the studied species, absence of fire, typical Mediterranean shrub fire, and severe fire with high fuel load. Seed germination was related to heat treatments and seed mass. Seed heating increased the proportion of seeds germinating compared with the control treatment. Mean seed weight was positively correlated to the proportion of germinated seeds but only within heat treatments. These results suggest that in periods without fire, the relative contributions to the population dynamics are equal for all seeds, regardless of their mass, whereas heavier seeds would be the main contribution after wildfire events. Since lighter seeds can be produced in higher quantities than heavier ones within a given fruit, the number of seedlings produced per fruit depended strongly on the germination conditions. In the absence of wildfire, fruits producing lighter seeds gave rise to more seedlings; nevertheless, they were numerically exceeded by those producing heavy seeds after a wildfire. The implications of these results are discussed in relation to their consequences on the population dynamics of this species, considering also additional information on stand flammability and changes in seed mass with plant age.     

Seasonality of vegetation fires as modified by human action: observing the deviation from eco‐climatic fire regimes

Aim In any region affected, fires exhibit a strong seasonal cycle driven by the dynamic of fuel moisture and ignition sources throughout the year. In this paper we investigate the global patterns of fire seasonality, which we relate to climatic, anthropogenic, land‐cover and land‐use variables. Location Global, with detailed analyses from single 1°× 1° grid cells. Methods We use a fire risk index, the Chandler burning index (CBI), as an indicator of the ‘natural’, eco‐climatic fire seasonality, across all types of ecosystems. A simple metric, the middle of the fire season, is computed from both gridded CBI data and satellite‐derived fire detections. We then interpret the difference between the eco‐climatic and observed metrics as an indicator of the human footprint on fire seasonality. Results Deforestation, shifting cultivation, cropland production or tropical savanna fires are associated with specific timings due to land‐use practices, sometimes largely decoupled from the CBI dynamics. Detailed time series from relevant locations provide comprehensive information about these practices and how they are adapted to eco‐climatic conditions. Main conclusions We find a great influence of anthropogenic activities on global patterns of fire seasonality. The specificity of the main fire practices and their easy identification from global observation is a potential tool to support land‐use monitoring efforts. Our results should also prove valuable in the development of a methodological approach for improving the representation of anthropogenic fire practices in dynamic global vegetation models.     

Controls on carbon consumption during Alaskan wildland fires

A method was developed to estimate carbon consumed during wildland fires in interior Alaska based on medium‐spatial scale data (60 m cell size) generated on a daily basis. Carbon consumption estimates were developed for 41 fire events in the large fire year of 2004 and 34 fire events from the small fire years of 2006–2008. Total carbon consumed during the large fire year (2.72 × 10⁶ ha burned) was 64.7 Tg C, and the average carbon consumption during the small fire years (0.09 × 10⁶ ha burned) was 1.3 Tg C. Uncertainties for the annual carbon emissions ranged from 13% to 21%. Carbon consumed from burning of black spruce forests represented 76% of the total during large fire years and 57% during small fire years. This was the result of the widespread distribution of black spruce forests across the landscape and the deep burning of the surface organic layers common to these ecosystems. Average carbon consumed was 3.01 kg m^?2 during the large fire year and 1.69 kg m^?2 during the small fire years. Most of the carbon consumption was from burning of ground layer fuels (85% in the large fire year and 78% in small fire years). Most of the difference in average carbon consumption between large and small fire years was in the consumption of ground layer fuels (2.60 vs. 1.31 kg m^?2 during large and small fire years, respectively). There was great variation in average fuel consumption between individual fire events (0.56–5.06 kg m^?2) controlled by variations in fuel types and topography, timing of the fires during the fire season, and variations in fuel moisture at the time of burning.     

城市化对区域生态足迹供需的影响

基于1994-2003年吉林省生态足迹时间序列,建立了城市化水平与生态足迹、生态盈亏、生态足迹强度的关系模型.结果表明:吉林省生态足迹、生态盈亏、生态足迹强度与城市化率呈显著相关;生态足迹随城市化的发展由1994年的每人1.59 hm2增至2003年的每人2.23 hm2,主要受城市化率和第三产业比例的影响;建筑用地、草地和化石燃料用地生态足迹的变化较显著,建筑用地生态足迹变化的驱动因素以人均GDP和第三产业比例为主,草地和化石燃料用地生态足迹主要受居民消费水平的影响;源于居民消费水平的驱动作用,吉林省生态赤字渐趋恶化,从1994年的每人0.319 hm2增至2003年的每人0.923 hm2;草地和化石燃料用地生态盈亏的变化最显著;在经济结构和消费结构优化的驱动下,研究期间生态足迹强度从每万元4.14 hm2降至2.35 hm2,而且仍具有较大的降低空间.通过调整经济结构和消费结构,吉林省可以消除生态赤字、实现自然资源的供需平衡.     

森林可燃物及其管理的研究进展与展望

森林可燃物是森林生态系统的基本组成部分, 是影响林火发生及火烧强度的重要因素之一, 因此, 受到国内外学者的广泛关注。该文从以下4个方面综述了国内外可燃物研究的最新进展: 森林可燃物特性, 森林可燃物类型与火行为, 森林可燃物类型、载量的调查与制图, 森林可燃物管理。同时提出了我国森林可燃物今后的研究方向: 开展多尺度可燃物研究; 可燃物类型与火行为的研究; 把以试验观测为基础的静态研究与以空间技术和生态模型为基础的动态预测相结合, 研究可燃物处理效果; 全球气候变化背景下可燃物处理与碳收支。     

Climatic stress increases forest fire severity across the western United States

Pervasive warming can lead to chronic stress on forest trees, which may contribute to mortality resulting from fire‐caused injuries. Longitudinal analyses of forest plots from across the western US show that high pre‐fire climatic water deficit was related to increased post‐fire tree mortality probabilities. This relationship between climate and fire was present after accounting for fire defences and injuries, and appeared to influence the effects of crown and stem injuries. Climate and fire interactions did not vary substantially across geographical regions, major genera and tree sizes. Our findings support recent physiological evidence showing that both drought and heating from fire can impair xylem conductivity. Warming trends have been linked to increasing probabilities of severe fire weather and fire spread; our results suggest that warming may also increase forest fire severity (the number of trees killed) independent of fire intensity (the amount of heat released during a fire).     

Periglacial fires and trees in a continental setting of Central Canada,Upper Pleistocene

Fire is a key factor controlling global vegetation patterns and carbon cycling. It mostly occurs under warm periods during which fuel builds up with sufficient moisture, whereas such conditions stimulate fire ignition and spread. Biomass burning increased globally with warming periods since the last glacial era. Data confirming periglacial fires during glacial periods are very sparse because such climates are likely too cold to favour fires. Here, tree occurrence and fires during the Upper Pleistocene glacial periods in Central Canada are inferred from botanical identification and calibrated radiocarbon dates of charcoal fragments. Charcoal fragments were archived in sandy dunes of central Saskatchewan and were dated >50 000–26 600 cal BP. Fragments were mostly gymnosperms. Parallels between radiocarbon dates and GISP2‐δ¹⁸O records deciphered relationships between fire and climate. Fires occurred either hundreds to thousands of years after Dansgaard–Oeschger (DO) interstadial warming events (i.e., the time needed to build enough fuel for fire ignition and spread) or at the onset of the DO event. The chronological uncertainties result from the dated material not precisely matching the fires and from the low residual ¹⁴C associated with old sample material. Dominance of high‐pressure systems and low effective moisture during post‐DO coolings likely triggered flammable periglacial ecosystems, while lower moisture and the relative abundance of fuel overshadowed lower temperatures for fire spread. Laurentide ice sheet (LIS) limits during DO events are difficult to assess in Central Canada due to sparse radiocarbon dates. Our radiocarbon data set constrains the extent of LIS. Central Saskatchewan was not covered by LIS throughout the Upper Pleistocene and was not a continental desert. Instead, our results suggest long‐lasting periods where fluctuations of the northern tree limits and fires after interstadials occurred persistently.     

Modelling the Meteorological Forest Fire Niche in Heterogeneous Pyrologic Conditions

Fire regimes are strongly related to weather conditions that directly and indirectly influence fire ignition and propagation. Identifying the most important meteorological fire drivers is thus fundamental for daily fire risk forecasting. In this context, several fire weather indices have been developed focussing mainly on fire-related local weather conditions and fuel characteristics. The specificity of the conditions for which fire danger indices are developed makes its direct transfer and applicability problematic in different areas or with other fuel types. In this paper we used the low-to-intermediate fire-prone region of Canton Ticino as a case study to develop a new daily fire danger index by implementing a niche modelling approach (Maxent). In order to identify the most suitable weather conditions for fires, different combinations of input variables were tested (meteorological variables, existing fire danger indices or a combination of both). Our findings demonstrate that such combinations of input variables increase the predictive power of the resulting index and surprisingly even using meteorological variables only allows similar or better performances than using the complex Canadian Fire Weather Index (FWI). Furthermore, the niche modelling approach based on Maxent resulted in slightly improved model performance and in a reduced number of selected variables with respect to the classical logistic approach. Factors influencing final model robustness were the number of fire events considered and the specificity of the meteorological conditions leading to fire ignition.     

Stand Age Influence on Potential Wildfire Ignition and Spread in the Boreal Forest of Northeastern Canada

Although it has long been assumed that wildfire occurrence is independent of stand age in the North American boreal forest, recent studies indicate that young forests may influence burn rates by limiting the ignition and spread of fires for several years. Wildfires not only structure the stand-age mosaic of boreal landscapes, but also alter the likelihood and behavior of subsequent fires. Using a fire simulation model, we evaluated the effect of stand age on the magnitude and spatial patterns of burn probability (BP) in the boreal forest of northeastern Canada. Specifically, we assessed the stand age effect on the two processes driving fire likelihood, ignition and spread, by simulating tens of thousands of fires under three fire regime scenarios that vary in terms of mean fire size and number of burned patches. Assuming minimal resistance to fire ignition and spread, where only the youngest stands (≤ 10 years) are resistant to burning, mean BP is reduced by 10%; in contrast, assuming maximum resistance, where stands up to 90 years old impede wildfires, mean BP can be reduced up to 85%. Although the resistance to ignition on BP is almost identical in magnitude to that of spread, it yields substantially different spatial arrangements of BP. Furthermore, stand age resistance reduces subsequent fire activity not only within but also outside the perimeter of burned patches through a shadow effect. Our results help to untangle the role of factors contributing to stand age resistance on wildfires and offer new insights for improving the spatial mapping of fire likelihood.