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PubMed2003

Diurnal and seasonal variations in the photosynthetic performance and water relations of two co-occurring Mediterranean shrubs, Erica multiflora and Globularia alypum.

Diurnal and seasonal fluctuations in the photosynthetic performance and water relations of two co-occurring Mediterranean shrubs, Erica multiflora and Globularia alypum were monitored throughout two consecutive years at Garraf Natural Park in north-east Spain. Leaf gas exchange rates, chlorophyll fluorescence and shoot water potentials were measured once each season. Leaf nitrogen and carbon concentrations, leaf delta13C and delta15N and specific leaf area (SLA) were also measured once a year (August) on well developed mature leaves. Globularia alypum experienced seasonal fluctuations in their water potential, with the lowest values recorded in summer, whereas E. multiflora did not show significant differences in water potential among seasons. Moreover, lower water potentials were found in G. alypum than in E. multiflora throughout the entire study, suggesting that the latter behaved as a drought-avoiding species, whereas the former tolerated lower water potentials. In both species, maximum leaf gas exchange rates were observed in autumn and secondarily in spring; in contrast, photosynthetic and transpiration rates reached absolute minima in summer. The stronger fluctuations in water potential and leaf gas exchange rates found in G. alypum compared to E. multiflora, suggest that G. alypum is, sensu Levitt (1980), a water spender, whereas E. multiflora is a water conservative. This hypothesis is further supported by a higher integrated water-use efficiency (higher delta13C values) and a higher degree of sclerophylly (lower SLA) in E. multiflora in comparison with G. alypum. Globularia alypum showed higher leaf gas exchange rates and higher predawn potential photochemical efficiency (Fv/Fm) than E. multiflora during most of the study. In spring and autumn, predawn Fv/Fm values were within the optimal range, whereas chronic photoinhibition in summer and winter was detected in both species. However, whereas both species could maintain positive photosynthetic rates in winter, frequent negative values were found in summer, suggesting higher levels of stress during the drought period. These results together with the high correlations that were found between the net photosynthetic rates and several parameters of water availability (accumulated rainfall, soil moisture or midday water potential) provided further evidence of the key role of water availability in the regulation of the photosynthetic rates in these Mediterranean species. Warmer and drier conditions in future decades, as a consequence of climate change, may alter the present, slight competitive advantage of G. alypum and the fitness of both shrub species within semi-arid Mediterranean environments.

Journal Article↗
PubMed2000

Calcium channels activated by hydrogen peroxide mediate abscisic acid signalling in guard cells.

Drought is a major threat to agricultural production. Plants synthesize the hormone abscisic acid (ABA) in response to drought, triggering a signalling cascade in guard cells that results in stomatal closure, thus reducing water loss. ABA triggers an increase in cytosolic calcium in guard cells ([Ca2+]cyt) that has been proposed to include Ca2+ influx across the plasma membrane. However, direct recordings of Ca2+ currents have been limited and the upstream activation mechanisms of plasma membrane Ca2+ channels remain unknown. Here we report activation of Ca2+-permeable channels in the plasma membrane of Arabidopsis guard cells by hydrogen peroxide. The H2O2-activated Ca2+ channels mediate both influx of Ca2+ in protoplasts and increases in [Ca2+]cyt in intact guard cells. ABA induces the production of H2O2 in guard cells. If H2O2 production is blocked, ABA-induced closure of stomata is inhibited. Moreover, activation of Ca2+ channels by H2O2 and ABA- and H2O2-induced stomatal closing are disrupted in the recessive ABA-insensitive mutant gca2. These data indicate that ABA-induced H2O2 production and the H2O2-activated Ca2+ channels are important mechanisms for ABA-induced stomatal closing.

Abscisic Acid↗
PubMed2007

Seasonal characteristics of the Indian Ocean Dipole during the Holocene epoch.

The Indian Ocean Dipole (IOD)--an oscillatory mode of coupled ocean-atmosphere variability--causes climatic extremes and socio-economic hardship throughout the tropical Indian Ocean region. There is much debate about how the IOD interacts with the El Niño/Southern Oscillation (ENSO) and the Asian monsoon, and recent changes in the historic ENSO-monsoon relationship raise the possibility that the properties of the IOD may also be evolving. Improving our understanding of IOD events and their climatic impacts thus requires the development of records defining IOD activity in different climatic settings, including prehistoric times when ENSO and the Asian monsoon behaved differently from the present day. Here we use coral geochemical records from the equatorial eastern Indian Ocean to reconstruct surface-ocean cooling and drought during individual IOD events over the past approximately 6,500 years. We find that IOD events during the middle Holocene were characterized by a longer duration of strong surface ocean cooling, together with droughts that peaked later than those expected by El Niño forcing alone. Climate model simulations suggest that this enhanced cooling and drying was the result of strong cross-equatorial winds driven by the strengthened Asian monsoon of the middle Holocene. These IOD-monsoon connections imply that the socioeconomic impacts of projected future changes in Asian monsoon strength may extend throughout Australasia.

Animals↗
PubMed2006

Expression and functional analysis of the rice plasma-membrane intrinsic protein gene family.

Plasma membrane intrinsic proteins (PIPs) are a subfamily of aquaporins that enable fast and controlled translocation of water across the membrane. In this study, we systematically identified and cloned ten PIP genes from rice. Based on the similarity of the amino acid sequences they encoded, these rice PIP genes were classified into two groups and designated as OsPIP1-1 to OsPIP1-3 and OsPIP2-1 to OsPIP2-7 following the nomenclature of PIP genes in maize. Quantitative RT-PCR analysis identified three root-specific and one leaf-specific OsPIP genes. Furthermore, the expression profile of each OsPIP gene in response to salt, drought and ABA treatment was examined in detail. Analysis on transgenic plants over-expressing of either OsPIP1 (OsPIP1-1) or OsPIP2 (OsPIP2-2) in wild-type Arabidopsis, showed enhanced tolerance to salt (100 mM of NaCl) and drought (200 mM of mannitol), but not to salt treatment of higher concentration (150 mM of NaCl). Taken together, these data suggest a distinct role of each OsPIP gene in response to different stresses, and should add a new layer to the understanding of the physiological function of rice PIP genes.

Abscisic Acid↗
PubMed2002

The distribution and possible origin of blue-green fluorescence in control and stressed barley leaves.

Apart from red (chlorophyll a) fluorescence, ultraviolet (UV) excitation of plant leaves results in blue-green intrinsic light emission. In order to analyse the potentially modifying influence of stress on this blue-green fluorescence (BGF), and to determine its origin, the fluorescence was characterised by spectroscopy, and two and three dimensional microscopy in leaves of Hordeum vulgare L. under UV and drought stress. Leaves of wild-type and proanthocyanidin-free mutant barley were compared. In general, the BGF was enhanced by stress. The analysis of the distribution of BGF pointed to differences in response to the different stress conditions and served to indicate possible sources of the fluorescence. This study showed that (1) the selective enhancement of BGF by supplementary UV radiation (280-360 nm) in upper leaf tissues of wild-type but not in proanthocyanidin-free leaves, pointed to flavonoids as possible emitters; (2) drought-inducible BGF emission was found in cell walls and to a smaller extent in upper leaf tissue regardless of genotype, thus independent of impairment in flavonoid biosynthesis and (3) intracellular BGF, which co-localised with red chlorophyll fluorescence, was inducible in mesophyll cells of both genotypes by short-term application of high levels of UV radiation.

Anthocyanins↗
PubMed2002

Water deficits and hydraulic limits to leaf water supply.

Many aspects of plant water use -- particularly in response to soil drought -- may have as their basis the alteration of hydraulic conductance from soil to canopy. The regulation of plant water potential (Psi) by stomatal control and leaf area adjustment may be necessary to maximize water uptake on the one hand, while avoiding loss of hydraulic contact with the soil water on the other. Modelling the changes in hydraulic conductance with pressure gradients in the continuum allows the prediction of water use as a function of soil environment and plant architectural and xylem traits. Large differences in water use between species can be attributed in part to differences in their 'hydraulic equipment' that is presumably optimized for drawing water from a particular temporal and spatial niche in the soil environment. A number of studies have identified hydraulic limits as the cause of partial or complete foliar dieback in response to drought. The interactions between root:shoot ratio, rooting depth, xylem properties, and soil properties in influencing the limits to canopy water supply can be used to predict which combinations should optimize water use in a given circumstance. The hydraulic approach can improve our understanding of the coupling of canopy processes to soil environment, and the adaptive significance of stomatal behaviour.

Journal Article↗
PubMed1994

Characterization of an Arabidopsis cDNA for a soluble epoxide hydrolase gene that is inducible by auxin and water stress.

A cDNA (1122 bp) was isolated from a cDNA library prepared from Arabidopsis thaliana L. that had been subjected to drought stress for 1 h. The sequencing of a genomic clone corresponding to the cDNA and S1 mapping analysis revealed that the cDNA lacked the first 6 bp from its translational start (ATG). The resulting open reading frame encodes a polypeptide of 321 amino acids, and the calculated molecular weight of this polypeptide is 36,423 Da. The deduced amino acid sequence shows a high degree of similarity to C terminal halves of those of soluble epoxide hydrolases (sEHs) of human, mouse and rat, 35.5%, 34.1% and 33.1%, respectively. The cDNA was expressed in Escherichia coli cells, and the expressed protein migrates at 40 kDa when analyzed by SDS-PAGE. The recombinant protein at 40 kDa is much smaller than the mammalian sEH (58 kDa) but has characteristics of activity and inhibition similar to the mammalian sEHs when assayed with the substrate trans-stilbene oxide and the inhibitors 4-fluorochalcone oxide (4FCO), (2R,3R)-3-(4-nitrophenyl) glycidol (RRNPG), and (2S,3S)-3-(4-nitrophenyl)glycidol (SSNPG), which indicates that the cDNA did encode a soluble epoxide hydrolase of A. thaliana (AtsEH). Drought stress, but not heat or cold stress, slightly increased the accumulation of the mRNAs for AtsEH. The level of AtsEH transcripts increased strongly after treatment with a plant hormone, auxin (2,4-dichlorophenoxyacetic acid, 2,4-D; naphthalene-acetic acid, NAA; and indole-3-acetic acid, IAA) in young, pre-bolting plants. Treatment with cytokinin (6-benzylaminopurine, BA), abscisic acid (ABA) or gibberellin (GA3) had no detectable effect on AtsEH transcript levels. The transcripts for AtsEH gene were detected in the aerial vegetative organs of bolting plants (i.e. stems and leaves), but not in roots, flowers and seeds. The possible function of AtsEH is discussed. A similar sEH cDNA has recently been characterized in potato (Stapleton et al., 1994).

Amino Acid Sequence↗
PubMed1997

Regulatory elements in vivo in the promoter of the abscisic acid responsive gene rab17 from maize.

The rab17 gene from maize is transcribed in late embryonic development and is responsive to abscisic acid and water stress in embryo and vegetative tissues. In vivo footprinting and transient transformation of rab17 were performed in embryos and vegetative tissues to characterize the cis-elements involved in regulation of the gene. By in vivo footprinting, protein binding was observed to nine elements in the promoter, which correspond to five putative ABREs (abscisic acid responsive elements) and four other sequences. The footprints indicated that distinct proteins interact with these elements in the two developmental stages. In transient transformation, six of the elements were important for high level expression of the rab17 promoter in embryos, whereas only three elements were important in leaves. The cis-acting sequences can be divided in embryo-specific, ABA-specific and leaf-specific elements on the basis of protein binding and the ability to confer expression of rab17. We found one positive, new element, called GRA, with the sequence CACTGGCCGCCC. This element was important for transcription in leaves but not in embryos. Two other non-ABRE elements that stimulated transcription from the rab17 promoter resemble previously described abscisic acid and drought-inducible elements. There were differences in protein binding and function of the five ABREs in the rab17 promoter. The possible reasons for these differences are discussed. The in vivo data obtained suggest that an embryo-specific pathway regulates transcription of the rab genes during development, whereas another pathway is responsible for induction in response to ABA and drought in vegetative tissues.

Abscisic Acid↗
PubMed2002

Light signalling mediated by phytochrome plays an important role in cold-induced gene expression through the C-repeat/dehydration responsive element (C/DRE) in Arabidopsis thaliana.

Low temperature induces a number of genes that encode the proteins promoting tolerance to freezing, mediated by ABA-dependent and ABA-independent pathways in plants. The cis-acting element called C/DRE is known to respond to low temperature independently of ABA action. To investigate the signalling and network of ABA-independent pathways, the transgenic Arabidopsis plants were generated containing several copies of the C/DRE derived from cor15a gene with a minimal promoter fused to a GUS reporter gene. The transgenic plants containing four copies of the C/DRE (4C/DRE-GUS) showed responsiveness to cold and drought treatments and were used for characterization of cold signalling and cross-talk. Cold-induced GUS expression was inhibited by okadaic acid at 1 nM, indicating that protein phosphatase 2A might act as a positive regulator. Light was shown to activate cold- and drought-induced GUS expression. Photo-reversibility of the GUS mRNA by red and far-red light with concomitant cold treatment suggests a role of phytochrome as a photoreceptor in mediating light signalling to activate the cold-induced gene expression through the C/DRE. Furthermore, GUS expression analysis in phyA or phyB or phyAphyB mutant backgrounds showed that phytochrome B is a primary photoreceptor responsible for the activation of cold-stress signalling in response to light. Light enhanced the induction kinetics of CBF1, 2, and 3 encoding the cognate transcription factors, and cor15a, in a consecutive manner compared to the dark condition in the cold, suggesting that the connection point between cold and light signalling mediated by phytochrome is at a higher step than the expression of CBF genes.

Abscisic Acid↗
PubMed2002

Use of infrared thermal imaging to isolate Arabidopsis mutants defective in stomatal regulation.

In response to drought, plants synthesise the hormone abscisic acid (ABA), which triggers closure of the stomatal pores. This process is vital for plants to conserve water by reducing transpirational water loss. Moreover, recent studies have demonstrated the advantages of the Arabidopsis stomatal guard cell for combining genetic, molecular and biophysical approaches to characterise ABA action. However, genetic dissection of stomatal regulation has been limited by the difficulty of identifying a reliable phenotype for mutant screening. Leaf temperature can be used as an indicator to detect mutants with altered stomatal control, since transpiration causes leaf cooling. In this study, we optimised experimental conditions under which individual Arabidopsis plants with altered stomatal responses to drought can be identified by infrared thermography. These conditions were then used to perform a pilot screen for mutants that displayed a reduced ability to close their stomata and hence appeared colder than the wild type. Some of the mutants recovered were deficient in ABA accumulation, and corresponded to alleles of the ABA biosynthesis loci ABA1, ABA2 and ABA3. Interestingly, two of these novel aba2 alleles were able to intragenically complement the aba2-1 mutation. The remaining mutants showed reduced ABA responsiveness in guard cells. In addition to the previously known abi1-1 mutation, we isolated mutations at two novel loci designated as OST1 (OPEN STOMATA 1) and OST2. Remarkably, ost1 and ost2 represent, to our knowledge, the first Arabidopsis mutations altering ABA responsiveness in stomata and not in seeds.

Abscisic Acid↗
PubMed2002

Evidence for the presence and activity of a complete antioxidant defence system in mature sieve tubes.

The phloem is the major route for the transport of solutes and nutrients from source to sink organs in plants. The functional transport phloem consists of parenchymal tissue, enucleate sieve elements, and the intimately connected companion cells. The general absence of a nucleus and functional ribosomes in sieve tubes poses problems especially for damage avoidance and repair of sieve element components. To examine how sieve tubes can remain functional during oxidative stress, we analysed phloem sap of cucumber and pumpkin plants with respect to the presence of antioxidant defence enzymes, their enzymatic activity, and activity changes after exposure to drought stress. Using 1D SDS-PAGE and nano ESI MS/MS, the presence of proteins such as cytosolic Cu/Zn superoxide dismutase, monodehydroascorbate reductase, and peroxidase could be shown. Moreover, activities for several antioxidant enzymes (superoxide dismutase, dehydroascorbate reductase, peroxidase) in phloem exudate could be demonstrated. The activity of these enzymes in phloem sap from cucumber and pumpkin plants increased in response to drought stress. The presented results together with earlier findings provide evidence supporting the presence of a complete machinery of antioxidant defence enzymes and detoxifying metabolites important for avoiding damage to essential components of the sieve elements due to oxidative stress.

Amino Acid Sequence↗
PubMed2003

Monitoring expression profiles of Arabidopsis gene expression during rehydration process after dehydration using ca 7000 full-length cDNA microarray.

Plants respond and adapt to drought stress in order to survive under stress conditions. Several genes that respond to drought at the transcriptional level have been described, but there are few reports on genes involved in the recovery from dehydration. Analysis of rehydration-inducible genes should help not only to understand the molecular mechanisms of stress responses in higher plants, but also to improve the stress tolerance of crops by gene manipulation. We used a full-length cDNA microarray containing ca. 7000 Arabidopsis full-length cDNAs and identified 152 rehydration-inducible genes. Venn diagram analysis showed relationship of the rehydration-inducible genes to proline-inducible and water-treatment-inducible genes. Among the 152 rehydration-inducible genes, 58 genes contained the ACTCAT sequence involved in proline- and hypoosmolarity-inducible gene expression in their promoter regions, suggesting that ACTCAT sequence is a major cis-acting element involved in rehydration-inducible gene expression, and that some novel cis-acting elements are involved in rehydration-inducible gene expression. Functional analysis of rehydration-inducible and rehydration-repressed genes revealed their functions not only in the release from a stressed status but also in the recovery of growth in plants.

Arabidopsis↗
PubMed2004

HvDRF1 is involved in abscisic acid-mediated gene regulation in barley and produces two forms of AP2 transcriptional activators, interacting preferably with a CT-rich element.

Apetala2/ethylene-responsive factor (AP2/ERF) proteins are AP2 domain-containing transcription factors and form the second largest transcription factor family in plants. Biological functions of many of these AP2 proteins are still unknown. Here, we report the characterisation of a novel member of the AP2/ERF superfamily, dehydration-responsive factor 1 (HvDRF1) from barley, and its role in abscisic acid (ABA)-mediated gene regulation. The expression of HvDRF1 was upregulated in barley leaves and roots under drought, salt or ABA treatment, and in embryos during seed maturation. Three forms of HvDRF1 transcripts were produced through alternative splicing, two of which encoded AP2 proteins. This alternative splicing pattern was also observed in a wheat homologue gene, TaDRF1. Both of HvDRF1 AP2 proteins acted as transcriptional activators, capable of activating the promoter activity of an ABA-inducible HVA1s in barley. In vitro DNA-binding analysis using synthetic oligonucleotides revealed that HvDRF1 AP2 protein bound preferably to a CT-rich element (T(T/A)ACCGCCTT). HvDRF1 activity on the activation of HVA1s expression in barley leaves was markedly enhanced by HvABI5 (a bZIP transcription factor), ABA or drought treatment. These results indicate that the HvDRF1 transcriptional activator co-operates with other ABA-responsive factors in the upregulation of stress gene expression through an ABA-dependent pathway.

Abscisic Acid↗
PubMed2002

Stratification and seasonal stability of diverse bacterial communities in a Pinus merkusii (pine) forest soil in central Java, Indonesia.

In Java, Indonesia, many nutrient-poor soils are intensively reforested with Pinus merkusii (pine). Information on nutrient cycles and microorganisms involved in these cycles will benefit the management of these important forests. Here, seasonal effects on the stratification of bacterial community structure in the soil profile of a tropical pine forest are described, and differences in bacterial communities are related to chemical and physical soil parameters. Culture-independent community profiles of litter, fragmented litter and mineral soil layers were made by denaturing gradient gel electrophoresis (DGGE) of 16S rDNA-specific polymerase chain reaction (PCR) fragments. The community profiles of the different soil layers clustered separately, correlating with significant differences in organic matter content between the three layers. The bacterial communities appeared to be stable during the wet season of 1998. The drought in 1997, caused by the El Niño climatic effect, did not influence the bacterial communities in fragmentation and mineral soil, although moisture content and other soil parameters were markedly lower than in the wet season. However, communities in litter were influenced by drought. In the litter layer, the moisture content was significantly lower than in the fragmentation and mineral layers during the dry season. A clone library was made from a litter sample taken during the wet season. Partial sequencing of 74 clones and linking the DGGE banding positions of these clones to bands in the DGGE profile of the sample from which the clone library was derived showed considerable bacterial diversity. Alpha-proteobacteria (40.5% of the clones, of which 57% belonged to the Rhizobium-Agrobacterium group) and high-G+C content, Gram-positive bacteria (36.5%) dominated the clone library.

Bacteria↗
PubMed2004

Water shortage affects the water and nitrogen balance in Central European beech forests.

Whilst forest policy promotes cultivation and regeneration of beech dominated forest ecosystems, beech itself is a highly drought sensitive tree species likely to suffer from the climatic conditions prognosticated for the current century. Taking advantage of model ecosystems with cool-moist and warm-dry local climate, the latter assumed to be representative for future climatic conditions, the effects of climate and silvicultural treatment (different thinning regimes) on water status, nitrogen balance and growth parameters of adult beech trees and beech regeneration in the understorey were assessed. In addition, validation experiments with beech seedlings were carried out under controlled conditions, mainly in order to assess the effect of drought on the competitive abilities of beech. As measures of water availability xylem flow, shoot water potential, stomatal conductance as well as delta (13)C and delta (18)O in different tissues (leaves, phloem, wood) were analysed. For the assessment of nitrogen balance we determined the uptake of inorganic nitrogen by the roots as well as total N content and soluble N compounds in different tissues of adult and young trees. Retrospective and current analysis of delta (13)C, growth and meteorological parameters revealed that beech growing under warm-dry climatic conditions were impaired in growth and water balance during periods with low rain-fall. Thinning affected water, N balance and growth mostly of young beech, but in a different way under different local climatic conditions. Under cool, moist conditions, representative for the current climatic and edaphic conditions in beech forests of Central Europe, thinning improves nutrient and water status consistent to published literature and long-term experience of forest practitioners. However, beech regeneration was impaired as a result of thinning at higher temperatures and under reduced water availability, as expected in future climate.

Carbon Isotopes↗
PubMed2006

Physiological performance of beech (Fagus sylvatica L.) at its southeastern distribution limit in Europe: seasonal changes in nitrogen, carbon and water balance.

To assess the physiological performance of drought-sensitive European beech ( Fagus sylvatica L.) under the dry Mediterranean climate prevailing at its southeastern distribution limit in Europe, we analyzed seasonal changes in carbon, nitrogen and water balance of naturally grown adult trees. We determined the foliar C and N contents, delta13C and delta18O signatures, total soluble non-protein nitrogen compounds (TSNN) in xylem, leaves, and phloem, as well as leaf water potential and photosynthetic quantum yield in northern Greece during 2003. Tissue sampling was performed in May, July, and September, while field measurements were conducted regularly. Climatic conditions for the 2003 growing season fall within the typical range of the studied area. The N- and C-related parameters displayed distinct seasonal courses. TSNN was highest in May in all tissues, and asparagine (Asn) was then the most abundant compound. Thereafter, TSNN decreased significantly in all tissues and both its concentration and composition remained constant in July and September. In both months, glutamate (Glu) prevailed in leaves, gamma-aminobutyric acid (GABA) in phloem exudates from twigs and trunks, and arginine (Arg) in the xylem sap, where loading with amino acids was rather low during that period, amounting to only 0.8 micromol N ml-1 in September. Highest total foliar N and C contents were detected in May, and the elevated abundance of nutrients as well as an increased foliar delta13C signature at the beginning of the growing season is attributed to remobilization processes. The signatures of delta18O, quantum yield and leaf water potentials varied only slightly throughout the growing season. Although summer precipitation at the study site was considerably lower compared to what is usual for typical central European beech forests, no intensive drought responses of the physiological apparatus were detected in the studied beech trees. This suggests efficient internal regulation mechanisms, constantly ensuring a favourable physiological status under the relatively dry Mediterranean climate.

Carbon↗
PubMed2006

An impending water crisis in Canada's western prairie provinces.

Canada is usually considered to be a country with abundant freshwater, but in its western prairie provinces (WPP), an area 1/5 the size of Europe, freshwater is scarce. European settlement of the WPP did not begin until the late 19th and early 20th centuries. Fortuitously, the period since European settlement appears to have been the wettest century of the past two millennia. The frequent, long periods of drought that characterized earlier centuries of the past two millennia were largely absent in the 20th century. Here, we show that climate warming and human modifications to catchments have already significantly reduced the flows of major rivers of the WPP during the summer months, when human demand and in-stream flow needs are greatest. We predict that in the near future climate warming, via its effects on glaciers, snowpacks, and evaporation, will combine with cyclic drought and rapidly increasing human activity in the WPP to cause a crisis in water quantity and quality with far-reaching implications.

Air↗
PubMed2006

Contingent Pacific-Atlantic Ocean influence on multicentury wildfire synchrony over western North America.

Widespread synchronous wildfires driven by climatic variation, such as those that swept western North America during 1996, 2000, and 2002, can result in major environmental and societal impacts. Understanding relationships between continental-scale patterns of drought and modes of sea surface temperatures (SSTs) such as El Niño-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO) may explain how interannual to multidecadal variability in SSTs drives fire at continental scales. We used local wildfire chronologies reconstructed from fire scars on tree rings across western North America and independent reconstructions of SST developed from tree-ring widths at other sites to examine the relationships of multicentury patterns of climate and fire synchrony. From 33,039 annually resolved fire-scar dates at 238 sites (the largest paleofire record yet assembled), we examined forest fires at regional and subcontinental scales. Since 1550 CE, drought and forest fires covaried across the West, but in a manner contingent on SST modes. During certain phases of ENSO and PDO, fire was synchronous within broad subregions and sometimes asynchronous among those regions. In contrast, fires were most commonly synchronous across the West during warm phases of the AMO. ENSO and PDO were the main drivers of high-frequency variation in fire (interannual to decadal), whereas the AMO conditionally changed the strength and spatial influence of ENSO and PDO on wildfire occurrence at multidecadal scales. A current warming trend in AMO suggests that we may expect an increase in widespread, synchronous fires across the western U.S. in coming decades.

Journal Article↗