Seasonality of Fire Weather Strongly Influences Fire Regimes in South Florida Savanna-Grassland Landscapes

Fire seasonality, an important characteristic of fire regimes, commonly is delineated using seasons based on single weather variables (rainfall or temperature). We used nonparametric cluster analyses of a 17-year (1993–2009) data set of weather variables that influence likelihoods and spread of fires (relative humidity, air temperature, solar radiation, wind speed, soil moisture) to explore seasonality of fire in pine savanna-grassland landscapes at the Avon Park Air Force Range in southern Florida. A four-variable, three-season model explained more variation within fire weather variables than models with more seasons. The three-season model also delineated intra-annual timing of fire more accurately than a conventional rainfall-based two-season model. Two seasons coincided roughly with dry and wet seasons based on rainfall. The third season, which we labeled the fire season, occurred between dry and wet seasons and was characterized by fire-promoting conditions present annually: drought, intense solar radiation, low humidity, and warm air temperatures. Fine fuels consisting of variable combinations of pyrogenic pine needles, abundant C4 grasses, and flammable shrubs, coupled with low soil moisture, and lightning ignitions early in the fire season facilitate natural landscape-scale wildfires that burn uplands and across wetlands. We related our three season model to fires with different ignition sources (lightning, military missions, and prescribed fires) over a 13-year period with fire records (1997–2009). Largest wildfires originate from lightning and military ignitions that occur within the early fire season substantially prior to the peak of lightning strikes in the wet season. Prescribed ignitions, in contrast, largely occur outside the fire season. Our delineation of a pronounced fire season provides insight into the extent to which different human-derived fire regimes mimic lightning fire regimes. Delineation of a fire season associated with timing of natural lightning ignitions should be useful as a basis for ecological fire management of humid savanna-grassland landscapes worldwide.     

Fire research for conservation management in tropical savannas: Introducing the Kapalga fire experiment

Abstract Fire is a dominant feature of tropical savannas throughout the world, and provides a unique opportunity for habitat management at the landscape scale. We provide the background and methodology for a landscape-scale savanna fire experiment at Kapalga, located in Kakadu National Park in the seasonal tropics of northern Australia. The experiment addresses the limitations of previous savanna fire experiments, including inappropriately small sizes of experimental units, lack of replication, consideration of a narrow range of ecological responses and an absence of detailed measurement of fire behaviour. In contrast to those elsewhere in the world, Australia's savannas are sparsely populated and largely uncleared, with fires lit primarily in a conservation, rather than pastoral, context. Fire management has played an integral role in the traditional lifestyles of Aboriginal people, who have occupied the land for perhaps 50 000 years or more. Currently the dominant fire management paradigm is one of extensive prescribed burning early in the dry season (May-June), in order to limit the extent and severity of fires occurring later in the year. The ecological effects of different fire regimes are hotly debated, but we identify geo-chemical cycling, tree demography, faunal diversity and composition, phenology, and the relative importance of fire intensity, timing and frequency, as critical issues. Experimental units (‘compartments’) at Kapalga are 15–20km² catchments, centred on seasonal creeks that drain into major rivers. Each compartment has been burnt according to one of four treatments, each replicated at least three times: ‘Early’- fires lit early in the dry season, which is the predominant management regime in the region; ‘Late’- fires lit late in the dry season, as occurs extensively in the region as unmanaged ‘wildfires’; ‘Progressive’- fires lit progressively throughout the dry season, such that different parts of the landscape are burnt as they progressively dry out (believed to approximate traditional Aboriginal burning practices); and ‘Unburnt’- no fires lit, and wildfires excluded. All burning treatments have been applied annually for 5 years, from 1990 to 1994. Six core projects have been conducted within the experimental framework, focusing on nutrients and atmospheric chemistry, temporary streams, vegetation, insects, small mammals, and vertebrate predators. Detailed measurements of fire intensity have been taken to help interpret ecological responses. The Kapalga fire experiment is multidisciplinary, treatments have been applied at a landscape scale with replication, and ecological responses can be related directly to measurements of fire intensity. We are confident that this experiment will yield important insights into the fire ecology of tropical savannas, and will make a valuable contribution to their conservation management.     

Effect of season of burning and removal of herbaceous cover on seedling emergence in a eucalypt savanna of north‐eastern Australia

Abstract Seedling emergence in a eucalypt savanna of north‐eastern Australia was documented over a 12‐month period, between May 1999 and May 2000. Seedling emergence for grasses, forbs and subshrubs was found to mainly occur in a brief pulse at the start of the wet season following fire or the removal of grass biomass. Only a minor number of tree and shrub seedlings were detected overall. Burning, or cutting away the grass layer in unburnt savanna, in both the early (i.e. May) and the late (i.e. October) dry seasons significantly increased seedling emergence over undisturbed savanna that had been unburnt for 3 years. Removing the grass layer in unburnt savanna, during either the early or the late dry season, triggered similar seedling densities to savanna burnt in the early dry season. Late dry season fires promoted the greatest seedling density. We attribute this to the higher intensity, late dry season fires releasing a greater proportion of seed from dormancy, coupled with the higher density of soil seed reserves present in the late dry season.     

Arthropod responses to experimental fire regimes in an Australian tropical savannah: ordinal‐level analysis

Fire is widely used for conservation management in the savannah landscapes of northern Australia, yet there is considerable uncertainty over the ecological effects of different fire regimes. The responses of insects and other arthropods to fire are especially poorly known, despite their dominant roles in the functioning of savannah ecosystems. Fire often appears to have little long‐term effect on ordinal‐level abundance of arthropods in temperate woodlands and open forests of southern Australia, and this paper addresses the extent to which such ordinal‐level resilience also occurs in Australia’s tropical savannahs. The data are from a multidisciplinary, landscape‐scale fire experiment at Kapalga in Kakadu National Park. Arthropods were sampled in the two major savannah habitats (woodland and open forest) using pitfall traps and sweep nets, in 15–20 km² compartments subjected to one of three fire regimes, each with three replicates: ‘early’ (annual fires lit early in the dry season), ‘late’ (annual fires lit late in the dry season), and ‘unburnt’ (fires absent during the five‐year experimental period 1990–94). Floristic cover, richness and composition were also measured in each sampling plot, using point quadrats. There were substantial habitat differences in floristic composition, but fire had no measured effect on plant richness, overall composition, or cover of three of the four dominant species. Of the 11 ordinal arthropod taxa considered from pitfall traps, only four were significantly affected by fire according to repeated‐measures ANOVA . There was a marked reduction in ant abundance in the absence of fire, and declines in spiders, homopterans and silverfish under late fires. Similarly, the abundances of only four of the 10 ordinal taxa from sweep catches were affected by fire, with crickets and beetles declining in the absence of fire, and caterpillars declining under late fires. Therefore, most of the ordinal taxa from the ground and grass‐layer were unaffected by the fire treatments, despite the treatments representing the most extreme fire regimes possible in the region. This indicates that the considerable ordinal‐level resilience to fire of arthropod assemblages that has previously been demonstrated in temperate woodlands and open forests of southern Australia, also occurs in tropical savannah woodlands and open forests of northern Australia.     

Effect of fires on soil nutrient availability in an open savanna in Central Brazil

Fire is common in savannas but its effects on soil are poorly understood. We analyzed long-term effects of fire on surface soil of an open Brazilian savanna (campo sujo) in plots submitted to different fire regimes during 18 years. The five fire regimes were: unburned, quadrennial fires in middle dry season, and biennial fires in early, middle or late dry season. Soil was collected during the wet and the middle dry season of 2008, and analyzed for pH, organic matter, total N, potential acidity, exchangeable cations and available P, S, Mn, Cu, Zn and Fe. We applied multivariate analysis to search for patterns related to fire regimes, and to local climate, fuel, and fire behavior. Spearman test was used to establish correlations between soil variables and the multivariate analysis gradient structure. Seasonal differences were tested using t-test. We found evidence of long-term fire effects: the unburned plot was segregated mainly by lower soil pH; the quadrennial plot was also segregated by lower soil pH and higher amount of exchangeable cations; the time of burning during the dry season in biennial plots did not significantly affect soil availability of nutrients. Differences in elements amounts due to the season of soil sampling (wet or dry) were higher than due to the effect of fires. Higher availability of nutrients in the soil during the wet season was probably related to higher nutrient inputs via rainfall and higher microbial activity.     

Seasonal distribution of pollen in the atmosphere of Darwin,tropical Australia: Preliminary results

Pollen loads in the atmosphere of Darwin, a city located in the wet‐dry tropics of Australia, have been monitored for the period March 2004 to November 2005 as part of a large research program looking at atmospheric particles and human health. Seven pollen types dominate the pollen spectrum, the herbaceous families of Poaceae (grasses) and Cyperaceae (sedges), as well as several native tree and shrub taxa, Acacia, Callitris, Casuarina, Arecaceae and Myrtaceae. The pollen loads were found to have a strong seasonal component associated with the alternating wet (November to March) and dry (April to October) seasons of the region. Seventy percent of the yearly pollen load is captured during the dry season, with the peak pollen period occurring at the onset of the dry season (April–May) when most grasses are in flower. The daily pollen concentration decreases as the dry season progresses, accompanied by a change in composition; fewer herbaceous but increasing woody taxa. Preliminary health outcomes reveal a positive association between hay fever, Poaceae and Acacia pollen, as well as a significant association between total fungal spore concentrations and asthma. The Darwin record contrasts significantly with surveys conducted in the subtropical and temperate cities of Australia where temperature as opposed to rainfall and the prevalence of northern hemisphere exotic tree species have a greater influence over the seasonality and composition of the pollen loads.     

Role of weather and fuel in stopping fire spread in tropical savannas

Analysis of wildfire extinguishment can help to identify the relative contribution of weather and management to the prevention of fire spread. Here we examine the role of weather, previous fire scars and other fuel interruptions at stopping the spread of nine large (mean 90 000 ha) late dry season fires in Arnhem Land, in the tropical savannas of northern Australia. Daily spread was mapped using Moderate‐resolution Imaging Spectroradiometer (MODIS) satellite imagery with a resolution of 250 m. We sampled points along the boundary of the fires and 1 km inside the boundary and compared conditions between the two sets. Using a combination of binomial regression and regression tree analysis, we found that recent burn scars (from the same year) were very effective at stopping fires. Where there was any recent burning within 500 m of a point, there was a 92% likelihood that it was a boundary. Interruptions such as roads, rivers and topography had small but significant effects. Vegetation type and vegetation greenness also had minor effects. Weather had a small effect via wind speed. This minor role of weather was reinforced by the fact that on most days the fires were both spreading and stopping at different parts of their perimeter. In these savannas, the weather in the late dry season is relatively invariant and is probably always conducive to some degree of fire spread. Here, interruptions to the fuel are critical to stopping fires. Nevertheless, for approximately half of boundary cases, the cause of stopping was not clear. This is probably due to the coarse scale of the analysis that does not reflect fine patterns of fuel arrangements.     

Effect of fire regime on plant abundance in a tropical eucalypt savanna of north‐eastern Australia

Abstract Changes in plant abundance within a eucalypt savanna of north‐eastern Australia were studied using a manipulative fire experiment. Three fire regimes were compared between 1997 and 2001: (i) control, savanna burnt in the mid‐dry season (July) 1997 only; (ii) early burnt, savanna burnt in the mid‐dry season 1997 and early dry season (May) 1999; and (iii) late burnt, savanna burnt in the mid‐dry season 1997 and late dry season (October) 1999. Five annual surveys of permanent plots detected stability in the abundance of most species, irrespective of fire regime. However, a significant increase in the abundance of several subshrubs, ephemeral and twining perennial forbs, and grasses occurred in the first year after fire, particularly after late dry season fires. The abundance of these species declined toward prefire levels in the second year after fire. The dominant grass Heteropogon triticeus significantly declined in abundance with fire intervals of 4 years. The density of trees (>2 m tall) significantly increased in the absence of fire for 4 years, because of the growth of saplings; and the basal area of the dominant tree Corymbia clarksoniana significantly increased over the 5‐year study, irrespective of fire regime. Conservation management of these savannas will need to balance the role of regular fires in maintaining the diversity of herbaceous species with the requirement of fire intervals of at least 4‐years for allowing the growth of saplings >2 m in height. Whereas late dry season fires may cause some tree mortality, the use of occasional late fires may help maintain sustainable populations of many grasses and forbs.     

Soil temperature and depth of legume germination during early and late dry season fires in a tropical eucalypt savanna of north‐eastern Australia

Abstract Temperatures that significantly increase seed germination of some tropical legumes (i.e. 80–100°C) were documented in the topsoil during the passage of early (May) and late (October) dry season fires in a tropical eucalypt savanna of north‐eastern Australia. Elevated temperatures penetrated at least 30 mm into the soil during the higher‐intensity, late dry season fires, but were only detected at 10 mm during the early dry season fires. The depth from which germination of two native legume forbs Galactia tenuiflora and Indigofera hirsuta occurred was positively related to the temperature elevation in the topsoil and was greater after late compared with early dry season fires. A broader range in germination depth, resulting in higher seedling densities, was recorded for I hirsuta after late dry season fires. These results suggest that seedling emergence of native leguminous forbs is likely to occur at a greater density after late rather than early dry season fires in tropical eucalypt savannas of north‐eastern Australia. Therefore, the season of burning, as a result of its relationship to fire intensity, can influence species composition through its effect on seed germination.     

Modelling the recovery of an annual savanna grass following a fire-induced crash

Abstract The native annual Sorghum populations of the Australian wet-dry tropics are highly resilient to dry season fires. During the early wet season, however, fires that occur after the new grass population has emerged can cause catastrophic population crashes. We examined savanna plots that had been burnt in this way, and compared them with adjacent unburnt plots. We found that Sorghum densities in the burnt plots were lower on average by a factor of 10, but that some fires had reduced the density only to one-third of the unburnt plots. It is not clear whether these differences relate directly to site or seasonal factors, or to differences in the way the burning was carried out. Other vegetation components responded to the fires differently: forbs (dicotyledonous herbs) increased in cover, while perennial grasses, woody plants, and overall species richness, were not significantly affected. The amount of leaf litter declined. A population model for Sorghum based on the demography of unburnt populations predicted that they should recover from a wet season burn, taking 7–16 years to return to normal densities. However, the actual field populations did not seem to be recovering, suggesting that wet season fires not only lower densities, but may also fundamentally change population processes in these annual grasses.     

The impact of experimental fire regimes on seed production in two tropical eucalypt species in northern Australia

Abstract This study investigated the effect of three experimental fire regimes on the fecundity, ovule development and seedfall of two common wet-dry tropical savanna eucalypts, Eucalyptus minima and Eucalyptus tetrodonta, in northern Australia. Both species flower early in the dry season and ovule development occurs during the dry season. This coincides with a period of frequent fires. The three fire regimes considered were applied for four years between 1990 and 1994. These regimes were (i) Unburnt, (ii) Early, fires lit early in the dry season, and (iii) Late, fires lit late in the dry season. The treatments were applied to nine catchments (15–20 km²) with each fire regime replicated three times. Fire intensity typically increases as the dry season proceeds. Therefore, early dry season fires generally differ from late dry season fires in both their intensity and their timing in relation to the reproductive phenology of the eucalypts. Late dry season burning significantly reduced the fecundity of both species, whereas Early burning had no significant effect. Ovule success was significantly reduced by the Early burning for both species. The Late burning significantly reduced ovule success in E. tetrodonta, but not in E. miniata. The results suggest that fire intensity and fire timing may both be important determinants of seed supply. Fire intensity may be a determinant of fecundity, whereas fire timing in relation to the reproduction phenology may have a significant impact on ovule survival. Both fire regimes resulted in a substantial reduction in seed supply compared with the Unburnt treatment. This may have a significant impact on seedling regeneration of these tropical savanna eucalypts.     

Soil seed densities and emergence patterns in pastures in the seasonally dry tropics of northeastern Australia

Germinable seed densities in the surface (0–10 cm) soil of pasture communities growing at Lansdown, near Townsville, were measured during the late dry season before the first germinating rain and again during the following wet season after germination but before the input of new seed. Seedlings emerging in the field were counted at approximately weekly intervals during this period to determine emergence patterns. Twelve communities were sampled in 1980–81 and six were re-sampled in 1981–82. During the late dry season germinable seed densities ranged from 5000 to 40 000 seeds m^-2. Seeds of the introduced legume, Stylosanthes hamata, were present in all pastures. There were many seeds of annual grasses (Digitaria ciliaris and Brachiaria miliiformis) and sedges (Cyperus and Fimbristylis spp.) but only few seeds of perennial grasses (both native and introduced). Soil seed densities were much lower during the wet season than during the preceding dry season, particularly for the grasses. Emergence commenced and approximately 70% of all seedlings emerged on the first major rainfall of the wet season. The subsequent emergence pattern varied between years. In 1980–81 there was a gradual and continuous increase in seedling numbers under the continuously moist conditions which prevailed. In 1981–82 further emergence occurred in discrete events related to rainfall and intervening dry periods. Maximum seedling densities exceeded 34 000 seedlings m^-2 including 29 000 grass seedlings (mainly annual species). The implications of these results for species survival and pasture composition are discussed.     

Potential effects of prescribed savannah burning on the diet selection of forest buffalo (Syncerus caffer nanus) in Lopé National Park,Gabon

Seasonality and management are factors that may affect the diet selection of the forest buffalo (Syncerus caffer nanus). Fire is considered a major driving force in savannah systems and prescribed burning is a commonly applied conservation tool in protected areas such as Lopé National Park, Gabon. Prescribed annual fires contribute to the maintenance of open areas and provide high‐quality forage for forest buffalo, a major herbivore in the park. We used microhistological faecal analysis to determine the diet selection of forest buffalo and measured the extent of variation between a dry season, preburn and a wet season, postburn sampling period. The buffalo diet comprised mainly of monocotyledons, primarily grasses (Poaceae) and sedges (Cyperaceae). Intake of open‐area‐associated plant species was higher in the wet season, postburn treatment sampling period (97%) than the dry season, preburn sampling period (87%), which corresponded conversely to a reduction in forest‐associated Marantaceae plants (10% versus 1%). High proportions of grasses and sedges in the diet signify the importance of open areas for forest buffalo. Controlled burning as tool for maintenance of open areas may play a key role in the meta‐population management of the forest buffalo.     

Effects of Differences in Prescribed Fire Regimes on Patchiness and Intensity of Fires in Subtropical Savannas of Everglades National Park, Florida

Abstract We investigated effects of fire frequency, seasonal timing, and plant community on patchiness and intensity of prescribed fires in subtropical savannas in the Long Pine Key region of Everglades National Park, Florida (U.S.A.). We measured patchiness and intensity in different plant communities along elevation gradients in “fire blocks.” These blocks were prescribed burned at varying times during the lightning season and at different frequencies between 1995 and 2000. Fire frequency, seasonal timing, and plant community all influenced the patchiness and intensity of prescribed fires. Fires were less patchy and more intense, probably because of drier conditions and pyrogenic fuels, in higher elevation plant communities (e.g., high pine savannas) than in lower elevation communities (e.g., long‐hydroperiod prairies). In all plant communities fires became increasingly patchy and less intense as the wet season progressed and moisture accumulated in fuels. Frequent prescribed fire resulted in increased patchiness but a wider range of intensities; higher intensities appeared to result from regrowth of more flammable vegetation. Our study suggests that frequent early lightning season prescribed fires produce a wider range of post‐fire conditions than less frequent late lightning season prescribed fires. Our study also suggests that natural early lightning season fires readily carried through pine savannas and short‐hydroperiod prairies, but lower elevation long‐hydroperiod prairies functioned as firebreaks. Natural fires probably crossed these firebreaks only during drier years, potentially producing large landscape‐level fires. Knowledge of how patchily and intensely fires burn across a savanna landscape should be useful for developing landscape‐level fire management.     

Germinable soil seed banks in a tropical savanna: seasonal dynamics and effects of fire.

Abstract The germinable soil seed bank of a tropical eucalypt savanna of north‐eastern Australia was found to be dominated by grasses and forbs, with seed bank density ranging from 58 to 792 seeds per square metre, from a total of 53 species. Late dry season fires and the fire‐related cues, heat shock and smoke, broke the seed dormancy of a range of tropical savanna species. Heat shock promoted the germination of the species groups natives, exotics, subshrubs, ephemeral and twining perennial forbs, and the common species Indigofera hirsuta, Pycnospora lutescens and Triumfetta rhomboidea. Exposure to smoke at ambient temperature promoted germination from the soil seed bank of the species groups combined natives, upright perennial forbs and grasses, as well as the common grasses Digitaria breviglumis and Heteropogon triticeus. The germinable soil seed bank varied seasonally, increasing from the mid wet season (February) and early dry season (May) to a maximum in the late dry season (October). The effect of recent fire history on soil seed bank dynamics was limited to the immediate release of some seed from dormancy; a reduction in seed densities of subshrubs and monocots, other than grasses, in recently burnt savanna; and enhanced seed density of the ephemeral I. hirsuta in the year following fire. The seed banks of most savanna species were replenished in the year following burning.     

Seeds of the annual grasses Schizachyrium spp. as a food resource for tropical granivorous birds

Seed dynamics of the annual tropical grasses Schizachyrium fragile (R. Br.) A. Camus, S. pachyarthron C. Gardner and S. pseudeulalia (Hosok.) S.T. Blake were studied with the aim of documenting fluxes in granivore food resources. In S. fragile, seed production began in the early dry season, and seed output was primarily influenced by seedling survival. Following seed-fall, there were 651 S. fragile seeds/m² (393 kernels/m²⁾ and 1014 S. pachyarthron seeds/m² (593 kernels/m²) across the study area, with a combined kernel biomass of 14.1 × 10³ g/ha. Seed densities remained stable through the dry season, then declined rapidly after wet season rain. Initial wet season rains of up to 25 mm, 40 mm and 50 mm resulted in seed-bank germination of < 5%, 57%, and 93%, respectively. Some seeds were buried by rain and resurfaced or germinated later, but about 30% was lost. Widespread and abrupt depletion of the seed-bank is likely to occur at the start of about 8% of wet seasons on central Cape York Peninsula, leaving little seed, either for subsequent germination or as food for granivores. Burning early in the dry season, when most seeds were still on the plants, reduced seed densities by 85%. The proportion of seeds with sound kernels was reduced in areas burnt by dry season fires, both directly after the fires and, subsequently, as a result of preferential granivore activity. Areas burnt in the dry season were thus depleted of seed earlier than were unburnt areas. Food availability for granivores should therefore be optimized by fire regimes that include a range of burning histories, including fires in both early dry and early wet seasons as well as keeping other areas unburnt.     

1957-2007年云南省森林火险变化

当前以变暖为主要特征的气候变化已对全球林火的发生产生重要影响.本文基于日气象数据(气温、降水量、空气相对湿度、风速),按照加拿大火险天气指数的计算方法,计算了云南省1957-2007年间每日的森林火险天气指数,分析了该省50年来森林火险的变化趋势.结果表明:云南省的火险期为上年11月至当年6月,持续期约8个月;林火数据(林火数量、过火面积和受害森林面积)与火险天气指数的相关性显著,半腐层湿度码(DMC)的火险期平均值和火险期严峻度(SSR)可作为不同火险期火险状况比较的良好指标.1957-2007年云南省森林火险状况呈现2种变化趋势:1)总体上周期性变化趋势明显,在周期性变化的同时呈上升趋势,表现为1991-2007年的火险状况比1961-1990年的火险状况略有升高;2)1991-2007年各火险期火险状况的波动性下降,火险状况异常严峻的火险期数量比1961-1990年减少.

Abstract：

Climate warming has already made great impact on forest fires. Based on the daily me-teorological data (temperature, precipitation, relative humidity, and wind speed), and by using the Canadian Fire Weather Index calculation formula, the daily forest fire weather indices (FWIs) of Yunnan Province in 1957-2007 were calculated, and through the statistical analysis of FWIs, the forest fire trends in this province over the past 50 years were studied. In the past 50 years, the forest fire season in Yunnan Province was from previous year November to current year June, lasting 8 months. Fire data (fire numbers, burned area, and burned forest area) had sig-nificant relationships with fire weather indices. The average daily duff moisture code (DMC) in whole fire season and the seasonal severity rating (SSR) were the good indices to evaluate the fire danger conditions among different fire seasons. The forest fire danger in Yunnan Province in 1957-2007 showed two change trends. One showed a clear cyclical change and a weak upward trend, i. e. , the fire danger conditions in 1991-2007 was slightly severer than that in 1961-1990 ; and the another was that the fluctuation of forest fire danger conditions among different fire seasons decreased in 1991-2007, and the number of abnormal severe fire seasons was less than that in 1961-1990.     

Climatic change and fire potential in South-Central British Columbia, Canada

The incidence and severity of forest fires are linked to the interaction between climate, fuel and topography. Increased warming and drying in the future is expected to have a significant impact on the risk of forest fire occurrence. An increase in fire risk is linked to the synchronous relationship between climate and fuel moisture conditions. A warmer, drier climate will lead to drier forest fuels that will in turn increase the chance of successful fire ignition and propagation. This interaction will increase the severity of fire weather, which, in turn, will increase the risk of extreme fire behaviour. A warmer climate will also extend fire season length, which will increase the likelihood of fires occurring over a greater proportion of the year. In this study of the North Okanagan area of British Columbia, Canada, the impacts of climate change of fire potential were evaluated using the Canadian Forest Fire Danger Rating System and multiple climate scenario analysis. Utilizing this approach, a 30% increase in fire season length was modelled to occur by 2070. In addition, statistically significant increases in fire severity and fire behaviour were also modelled. Fire weather severity was predicted to increase by 95% during the summer months by 2070 while fire behaviour was predicted to shift from surface fire‐intermittent crown fire regimes to a predominantly intermittent‐full crown fire regime by 2070 onwards. An increase in fire season length, fire weather severity and fire behaviour will increase the costs of fire suppression and the risk of property and resource loss while limiting human‐use within vulnerable forest landscapes. An increase in fire weather severity and fire behaviour over a greater proportion of the season will increase the risks faced by ecosystems and biodiversity to climatic change and increase the costs and difficulty of achieving sustainable forest management.     

Climatic influences on fire regimes along a rain forest-to-xeric woodland gradient in northern Patagonia, Argentina

ABSTRACT. Influences of annual climatic variation on fire occurrence were examined along a rainfall gradient from temperate rainforest to xeric woodlands in northern Patagonia, Argentina. Fire chronologies were derived from fire scars on trees and related to tree-ring proxy records of climate over the period 1820–1974. Similarly, fire records of four Patagonian national parks for the period 1940–1988 were compared to instrumental weather data. Finally, the influences of broad-scale synoptic weather patterns on fire occurrence in northern Patagonia were explored.
Fire in Nothofagus rainforests is highly dependent on drought during the spring and summer of the same year in which fires occur and is less strongly favoured by drought during the spring of the previous year. The occurrence of fire in dry vegetation types near the steppe ecotone is less dependent on drought because even during years of normal weather fuels are thoroughly desiccated during the dry summer. In xeric Austrocedrus woodlands, fire occurrence and spread are promoted by droughts during the fire season and also appear to be favoured by above-average moisture conditions during the preceding 1 to 2 growing seasons which enhances fuel production. Thus, in the xeric woodlands fire is not simply dependent on drought but is favoured by greater climatic variability over time scales of several years.
Fire activity in northern Patagonia is greatly influenced by the intensity and latitudinal position of the subtropical high pressure cell of the southeast Pacific. Greater fire activity is associated with a more intense and more southerly located high pressure cell which blocks the influx of Pacific moisture into the continent. Although long-term changes in fire occurrence along the rainforest-to-xeric woodland gradient have been greatly influenced by human activities, annual variation in fire frequency and extent is also strongly influenced by annual climatic variation.     

Monsoonal influences on evapotranspiration of savanna vegetation of northern Australia

Data from savannas of northern Australia are presented for net radiation, latent and sensible heat, ecosystem surface conductance (G_s) and stand water use for sites covering a latitudinal range of 5° or 700 km. Measurements were made at three locations of increasing distance from the northern coastline and represent high- (1,750 mm), medium- (890 mm) and low- (520 mm) rainfall sites. This rainfall gradient arises from the weakened monsoonal influence with distance inland. Data were coupled to seasonal estimates of leaf area index (LAI) for the tree and understorey strata. All parameters were measured at the seasonal extremes of late wet and dry seasons. During the wet season, daily rates of evapotranspiration were 3.1-3.6 mm day^-1 and were similar for all sites along the rainfall gradient and did not reflect site differences in annual rainfall. During the dry season, site differences were very apparent with evapotranspiration 2-18 times lower than wet season rates, the seasonal differences increasing with distance from coast and reduced annual rainfall. Due to low overstorey LAI, more than 80% of water vapour flux was attributed to the understorey. Seasonal differences in evapotranspiration were mostly due to reductions in understorey leaf area during the dry season. Water use of individual trees did not differ between the wet and dry seasons at any of the sites and stand water use was a simple function of tree density. G_s declined markedly during the dry season at all sites, and we conclude that the savanna water (and carbon) balance is largely determined by G_s and its response to atmospheric and soil water content and by seasonal adjustments to canopy leaf area.