Regulation of Co-transcriptional Pre-mRNA Splicing by m6A through the Low-Complexity Protein hnRNPG.

N6-methyladenosine (m6A) modification occurs co-transcriptionally and impacts pre-mRNA processing; however, the mechanism of co-transcriptional m6A-dependent alternative splicing regulation is still poorly understood. Heterogeneous nuclear ribonucleoprotein G (hnRNPG) is an m6A reader protein that binds RNA through RRM and Arg-Gly-Gly (RGG) motifs. Here, we show that hnRNPG directly binds to the phosphorylated carboxy-terminal domain (CTD) of RNA polymerase II (RNAPII) using RGG motifs in its low-complexity region. Through interactions with the phosphorylated CTD and nascent RNA, hnRNPG associates co-transcriptionally with RNAPII and regulates alternative splicing transcriptome-wide. m6A near splice sites in nascent pre-mRNA modulates hnRNPG binding, which influences RNAPII occupancy patterns and promotes exon inclusion. Our results reveal an integrated mechanism of co-transcriptional m6A-mediated splicing regulation, in which an m6A reader protein uses RGG motifs to co-transcriptionally interact with both RNAPII and m6A-modified nascent pre-mRNA to modulate RNAPII occupancy and alternative splicing.

A modest increase in fire weather overcomes resistance to fire spread in recently burned boreal forests.

Recently burned boreal forests have lower aboveground fuel loads, generating a negative feedback to subsequent wildfires. Despite this feedback, short-interval reburns (≤20 years between fires) are possible under extreme weather conditions. Reburns have consequences for ecosystem recovery, leading to enduring vegetation change. In this study, we characterize the strength of the fire-fuel feedback in recently burned Canadian boreal forests and the weather conditions that overwhelm resistance to fire spread in recently burned areas. We used a dataset of daily fire spread for thousands of large boreal fires, interpolated from remotely sensed thermal anomalies to which we associated local weather from ERA5-Land for each day of a fire's duration. We classified days with >3 ha of fire growth as spread days and defined burned pixels overlapping a fire perimeter ≤20 years old as short-interval reburns. Results of a logistic regression showed that the odds of fire spread in recently burned areas were ~50% lower than in long-interval fires; however, all Canadian boreal ecozones experienced short-interval reburning (1981-2021), with over 100,000 ha reburning annually. As fire weather conditions intensify, the resistance to fire spread declines, allowing fire to spread in recently burned areas. The weather associated with short-interval fire spread days was more extreme than the conditions during long-interval spread, but overall differences were modest (e.g. relative humidity 2.6% lower). The frequency of fire weather conducive to short-interval fire spread has significantly increased in the western boreal forest due to climate warming and drying (1981-2021). Our results suggest an ongoing degradation of fire-fuel feedbacks, which is likely to continue with climatic warming and drying.

Genome-wide association studies with experimental validation identify a protective role for B lymphocytes against chronic post-surgical pain.

BACKGROUND: Chronic post-surgical pain (CPSP) significantly impacts patients' recovery and quality of life. Although environmental risk factors are well-established, genetic risk remains less understood. METHODS: A meta-analysis of genome-wide association studies followed by partitioned heritability was performed on 1350 individuals across five surgery types: hysterectomy, mastectomy, abdominal, hernia, and knee. In subsequent animal studies, withdrawal thresholds to evoked mechanical stimulation were measured in Rag1 null mutant and wild-type mice after plantar incision and laparotomy. Cell sorting by flow cytometry tracked recruitment of immune cell types. RESULTS: We discovered 77 genome-wide significant single-nucleotide polymorphism (SNP) hits, distributed among 24 loci and 244 genes. Meta-analysis of all cohorts estimated a SNP-based narrow-sense heritability for CPSP at ∼39%, indicating a substantial genetic contribution. Partitioned heritability analysis across a wide variety of tissues revealed enrichment of heritability in immune system-related genes, particularly those associated with B and T cells. Rag1 null mutant mice lacking both T and B cells exhibited exacerbated and prolonged allodynia up to 42 days after surgery, which was rescued by B-cell transfer. Recruitment patterns of B cells but not T cells differed significantly during the first 7 days after injury in the footpad, lymph nodes, and dorsal root ganglia. CONCLUSIONS: These findings suggest a key protective role for the adaptive immune system in the development of chronic post-surgical pain.

Increasing fire and the decline of fire adapted black spruce in the boreal forest.

Intensifying wildfire activity and climate change can drive rapid forest compositional shifts. In boreal North America, black spruce shapes forest flammability and depends on fire for regeneration. This relationship has helped black spruce maintain its dominance through much of the Holocene. However, with climate change and more frequent and severe fires, shifts away from black spruce dominance to broadleaf or pine species are emerging, with implications for ecosystem functions including carbon sequestration, water and energy fluxes, and wildlife habitat. Here, we predict that such reductions in black spruce after fire may already be widespread given current trends in climate and fire. To test this, we synthesize data from 1,538 field sites across boreal North America to evaluate compositional changes in tree species following 58 recent fires (1989 to 2014). While black spruce was resilient following most fires (62%), loss of resilience was common, and spruce regeneration failed completely in 18% of 1,140 black spruce sites. In contrast, postfire regeneration never failed in forests dominated by jack pine, which also possesses an aerial seed bank, or broad-leaved trees. More complete combustion of the soil organic layer, which often occurs in better-drained landscape positions and in dryer duff, promoted compositional changes throughout boreal North America. Forests in western North America, however, were more vulnerable to change due to greater long-term climate moisture deficits. While we find considerable remaining resilience in black spruce forests, predicted increases in climate moisture deficits and fire activity will erode this resilience, pushing the system toward a tipping point that has not been crossed in several thousand years.

Broadleaf tree phenology and springtime wildfire occurrence in boreal Canada.

Although broadleaf tree species of the boreal biome have a lower flammability compared to conifers, there is a period following snow melt and prior to leaf flush (i.e., greenup), termed the "spring window" by fire managers, when these forests are relatively conducive to wildfire ignition and spread. The goal of this study was to characterize the duration, timing, and fire proneness of the spring window across boreal Canada and assess the link between these phenological variables and the incidence of springtime wildfires. We used remotely sensed snow cover and greenup data to identify the annual spring window for five boreal ecozones from 2001 to 2021 and then compared seasonality of wildfire starts (by cause) and fire-conducive weather in relation to this window, averaged over the 21-year period. We conducted a path analysis to concomitantly evaluate the influence of the spring window's duration, the timing of greenup, and fire-conducive weather on the annual number and the seasonality of spring wildfires. Results show that the characteristics of spring windows vary substantially from year to year and among geographic zones, with the interior west of Canada having the longest and most fire-conducive spread window and, accordingly, the greatest springtime wildfire activity. We also provide support for the belief that springtime weather generally promotes wind-driven, rather than drought-driven wildfires. The path analyses show idiosyncratic behavior among ecozones, but, in general, the seasonality of the wildfire season is mainly driven by the timing of the greenup, whereas the number of spring wildfires mostly responds to the duration of the spring window and the frequency of fire-conducive weather. The results of this study allows us to better understand and anticipate the biome-wide changes projected for the northern forests of North America.

Genome-wide analysis identifies impaired axonogenesis in chronic overlapping pain conditions.

Chronic pain is often present at more than one anatomical location, leading to chronic overlapping pain conditions. Whether chronic overlapping pain conditions represent a distinct pathophysiology from the occurrence of pain at only one site is unknown. Using genome-wide approaches, we compared genetic determinants of chronic single-site versus multisite pain in the UK Biobank. We found that different genetic signals underlie chronic single-site and multisite pain with much stronger genetic contributions for the latter. Among 23 loci associated with multisite pain, nine loci replicated in the HUNT cohort, with the DCC netrin 1 receptor (DCC) as the top gene. Functional genomics identified axonogenesis in brain tissues as the major contributing pathway to chronic multisite pain. Finally, multimodal structural brain imaging analysis showed that DCC is most strongly expressed in subcortical limbic regions and is associated with alterations in the uncinate fasciculus microstructure, suggesting that DCC-dependent axonogenesis may contribute to chronic overlapping pain conditions via corticolimbic circuits.

Short- and long-term wildfire threat when adapting infrastructure for wildlife conservation in the boreal forest.

Managers designing infrastructure in fire-prone wildland areas require assessments of wildfire threat to quantify uncertainty due to future vegetation and climatic conditions. In this study, we combine wildfire simulation and forest landscape composition modeling to identify areas that would be highly susceptible to wildfire around a proposed conservation corridor in Québec, Canada. In this measure, managers have proposed raising the conductors of a new 735-kV hydroelectric powerline above the forest canopy within a wildlife connectivity corridor to mitigate the impacts to threatened boreal woodland caribou (Rangifer tarandus). Retention of coniferous vegetation, however, can increase the likelihood of an intense wildfire damaging powerline infrastructure. To assess the likelihood of high-intensity wildfires for the next 100 years, we evaluated three time periods (2020, 2070, 2120), three climate scenarios (observed, RCP 4.5, RCP 8.5), and four vegetation projections (static, no harvest, extensive harvesting, harvesting excluded in protected areas). Under present-day conditions, we found a lower probability of high-intensity wildfire within the corridor than in other parts of the study area, due to the protective influence of a nearby, poorly regenerated burned area. Wildfire probability will increase into the future, with strong, weather-induced inflation in the number of annual ignitions and wildfire spread potential. However, a conversion to less-flammable vegetation triggered by interactions between climate change and disturbance may attenuate this trend. By addressing the range of uncertainty of future conditions, we present a robust strategy to assist in decision-making about long-term risk management for both the proposed conservation measure and the powerline.

Discovery of a Redox Thiol Switch: Implications for Cellular Energy Metabolism.

The redox-based modifications of cysteine residues in proteins regulate their function in many biological processes. The gas molecule H2S has been shown to persulfidate redox sensitive cysteine residues resulting in an H2S-modified proteome known as the sulfhydrome. Tandem Mass Tags (TMT) multiplexing strategies for large-scale proteomic analyses have become increasingly prevalent in detecting cysteine modifications. Here we developed a TMT-based proteomics approach for selectively trapping and tagging cysteine persulfides in the cellular proteomes. We revealed the natural protein sulfhydrome of two human cell lines, and identified insulin as a novel substrate in pancreatic beta cells. Moreover, we showed that under oxidative stress conditions, increased H2S can target enzymes involved in energy metabolism by switching specific cysteine modifications to persulfides. Specifically, we discovered a Redox Thiol Switch, from protein S-glutathioinylation to S-persulfidation (RTSGS). We propose that the RTSGS from S-glutathioinylation to S-persulfidation is a potential mechanism to fine tune cellular energy metabolism in response to different levels of oxidative stress.

Profound analgesia is associated with a truncated peptide resulting from tissue specific alternative splicing of DRG CA8-204 regulated by an exon-level cis-eQTL.

Carbonic anhydrase-8 (CA8) is an intracellular protein that functions as an allosteric inhibitor of inositol trisphosphate receptor-1 (ITPR1) critical to intracellular Ca++ release, synaptic functions and neuronal excitability. We showed previously that murine nociception and analgesic responses are regulated by the expression of this gene in dorsal root ganglion (DRG) associated with a cis-eQTL. In this report, we identify an exon-level cis-eQTL (rs6471859) that regulates human DRG CA8 alternative splicing, producing a truncated 1,697bp transcript (e.g., CA8-204). Our functional genomic studies show the "G" allele at rs6471859 produces a cryptic 3'UTR splice site regulating expression of CA8-204. We developed constructs to study the expression and function of the naturally occurring CA8-204G transcript (G allele at rs6471859), CA8-204C (C allele at rs6471859 reversion mutation) and CA8-201 (full length transcript). CA8-204G transcript expression occurred predominantly in non-neuronal cells (HEK293), while CA8-204C expression was restricted to neuronal derived cells (NBL) in vitro. CA8-204G produced a stable truncated transcript in HEK293 cells that was barely detectable in NBL cells. We also show CA8-204 produces a stable peptide that inhibits pITPR1 and Ca++ release in HEK293 cells. These results imply homozygous G/G individuals at rs6471859, which are common in the general population, produce exclusively CA8-204G that is barely detectable in neuronal cells. CA8 null mutations that greatly impact neuronal functions are associated with severe forms of spinal cerebellar ataxia, and our data suggest G/G homozygotes should display a similar phenotype. To address this question, we show in vivo using AAV8-FLAG-CA8-204G and AAV8-V5-CA8-201 gene transfer delivered via intra-neural sciatic nerve injection (SN), that these viral constructs are able to transduce DRG cells and produce similar analgesic and anti-hyperalgesic responses to inflammatory pain. Immunohistochemistry (IHC) examinations of DRG tissues further show CA8-204G peptide is expressed in advillin expressing neuronal cells, but to a lesser extent compared to glial cells. These findings explain why G/G homozygotes that exclusively produce this truncated functional peptide in DRG evade a severe phenotype. These genomic studies significantly advance the literature regarding structure-function studies on CA8-ITPR1 critical to calcium signaling pathways, synaptic functioning, neuronal excitability and analgesic responses.

A study in scarlet: MC1R as the main predictor of red hair and exemplar of the flip-flop effect.

Genetic variation in melanocortin-1 receptor (MC1R) is a known contributor to disease-free red hair in humans. Three loss-of-function single-nucleotide variants (rs1805007, rs1805008 and rs1805009) have been established as strongly correlated with red hair. The contribution of other loss-of-function MC1R variants (in particular rs1805005, rs2228479 and rs885479) and the extent to which other genetic loci are involved in red hair colour is less well understood. Here, we used the UK Biobank cohort to capture a comprehensive list of MC1R variants contributing to red hair colour. We report a correlation with red hair for both strong-effect variants (rs1805007, rs1805008 and rs1805009) and weak-effect variants (rs1805005, rs2228479 and rs885479) and show that their coefficients differ by two orders of magnitude. On the haplotype level, both strong- and weak-effect variants contribute to the red hair phenotype, but when considered individually, weak-effect variants show a reverse, negative association with red hair. The reversal of association direction in the single-variant analysis is facilitated by a distinguishing structure of MC1R, in which loss-of-function variants are never found to co-occur on the same haplotype. The other previously reported hair colour genes' variants do not substantially improve the MC1R red hair colour predictive model. Our best model for predicting red versus other hair colours yields an unparalleled area under the receiver operating characteristic of 0.96 using only MC1R variants. In summary, we present a comprehensive statistically derived characterization of the role of MC1R variants in red hair colour and offer a powerful, economical and parsimonious model that achieves unsurpassed performance.

Alternative Splicing of Opioid Receptor Genes Shows a Conserved Pattern for 6TM Receptor Variants.

The opioid receptor (OPR) family comprises the mu-, delta-, and kappa-opioid, and nociceptin receptors that belong to the superfamily of 7-transmembrane spanning G protein-coupled receptors (GPCRs). The mu-opioid receptor is the main target for clinically used opioid analgesics, and its biology has been extensively studied. The N-terminally truncated 6TM receptors isoform produced through alternative splicing of the OPRM1 gene displays unique signaling and analgesic properties, but it is unclear if other OPRs have the same ability. In this study, we have built a comprehensive map of alternative splicing events that produce 6TM receptor variants in all the OPRs and demonstrated their evolutionary conservation. We then obtained evidence for their translation through ribosomal footprint analysis. We discovered that N-terminally truncated 6TM GPCRs are rare in the human genome and OPRs are overrepresented in this group. Finally, we also observed a significant enrichment of 6TM GPCR genes among genes associated with pain, psychiatric disorders, and addiction. Understanding the biology of 6TM receptors and leveraging this knowledge for drug development should pave the way for novel therapies.

N(6)-methyladenosine-dependent RNA structural switches regulate RNA-protein interactions.

RNA-binding proteins control many aspects of cellular biology through binding single-stranded RNA binding motifs (RBMs). However, RBMs can be buried within their local RNA structures, thus inhibiting RNA-protein interactions. N(6)-methyladenosine (m(6)A), the most abundant and dynamic internal modification in eukaryotic messenger RNA, can be selectively recognized by the YTHDF2 protein to affect the stability of cytoplasmic mRNAs, but how m(6)A achieves its wide-ranging physiological role needs further exploration. Here we show in human cells that m(6)A controls the RNA-structure-dependent accessibility of RBMs to affect RNA-protein interactions for biological regulation; we term this mechanism 'the m(6)A-switch'. We found that m(6)A alters the local structure in mRNA and long non-coding RNA (lncRNA) to facilitate binding of heterogeneous nuclear ribonucleoprotein C (HNRNPC), an abundant nuclear RNA-binding protein responsible for pre-mRNA processing. Combining photoactivatable-ribonucleoside-enhanced crosslinking and immunoprecipitation (PAR-CLIP) and anti-m(6)A immunoprecipitation (MeRIP) approaches enabled us to identify 39,060 m(6)A-switches among HNRNPC-binding sites; and global m(6)A reduction decreased HNRNPC binding at 2,798 high-confidence m(6)A-switches. We determined that these m(6)A-switch-regulated HNRNPC-binding activities affect the abundance as well as alternative splicing of target mRNAs, demonstrating the regulatory role of m(6)A-switches on gene expression and RNA maturation. Our results illustrate how RNA-binding proteins gain regulated access to their RBMs through m(6)A-dependent RNA structural remodelling, and provide a new direction for investigating RNA-modification-coded cellular biology.

Reversion mutation of cDNA CA8-204 minigene construct produces a truncated functional peptide that regulates calcium release in vitro and produces profound analgesia in vivo.

Intracellular calcium is critical in orchestrating neuronal excitability and analgesia. Carbonic anhydrase-8 (CA8) regulates intracellular calcium signaling through allosteric inhibition of neuronal inositol trisphosphate receptor 1 (ITPR1) to produce profound analgesia. Recently, we reported the "G" allele at rs6471859 represents cis-eQTL regulating alternative splicing of a 1697 bp transcript (CA8-204G) with a retained intron, alternative polyadenylation site and a new stop codon producing a functional 26 kDa peptide with an extended exon 3. In this study we show the reversion mutation (G to C) at rs6471859 within the CA8-204G expression vector also produced a stable 1697 bp transcript (CA8-204C) coding for a smaller peptide (~ 22 kDa) containing only the first three CA8 exons. Surprisingly, this peptide inhibited ITPR1 (pITPR1) activation, ITPR1-mediated calcium release in vitro; and produced profound analgesia in vivo. This is the first report showing CA8-204C codes for a functional peptide sufficient to regulate calcium signaling and produce profound analgesia.

Queuosine modification protects cognate tRNAs against ribonuclease cleavage.

Eukaryotic transfer RNAs (tRNA) contain on average 13 modifications that perform a wide range of roles in translation and in the generation of tRNA fragments that regulate gene expression. Queuosine (Q) modification occurs in the wobble anticodon position of tRNAs for amino acids His, Asn, Tyr, and Asp. In eukaryotes, Q modification is fully dependent on diet or on gut microbiome in multicellular organisms. Despite decades of study, cellular roles of Q modification remain to be fully elucidated. Here we show that in human cells, Q modification specifically protects its cognate tRNAHis and tRNAAsn against cleavage by ribonucleases. We generated cell lines that contain completely depleted or fully Q-modified tRNAs. Using these resources, we found that Q modification significantly reduces angiogenin cleavage of its cognate tRNAs in vitro. Q modification does not change the cellular abundance of the cognate full-length tRNAs, but alters the cellular content of their fragments in vivo in the absence and presence of stress. Our results provide a new biological aspect of Q modification and a mechanism of how Q modification alters small RNA pools in human cells.

Wildfire-Driven Forest Conversion in Western North American Landscapes.

Changing disturbance regimes and climate can overcome forest ecosystem resilience. Following high-severity fire, forest recovery may be compromised by lack of tree seed sources, warmer and drier postfire climate, or short-interval reburning. A potential outcome of the loss of resilience is the conversion of the prefire forest to a different forest type or nonforest vegetation. Conversion implies major, extensive, and enduring changes in dominant species, life forms, or functions, with impacts on ecosystem services. In the present article, we synthesize a growing body of evidence of fire-driven conversion and our understanding of its causes across western North America. We assess our capacity to predict conversion and highlight important uncertainties. Increasing forest vulnerability to changing fire activity and climate compels shifts in management approaches, and we propose key themes for applied research coproduced by scientists and managers to support decision-making in an era when the prefire forest may not return.

N6-methyladenosine-dependent regulation of messenger RNA stability.

N(6)-methyladenosine (m(6)A) is the most prevalent internal (non-cap) modification present in the messenger RNA of all higher eukaryotes. Although essential to cell viability and development, the exact role of m(6)A modification remains to be determined. The recent discovery of two m(6)A demethylases in mammalian cells highlighted the importance of m(6)A in basic biological functions and disease. Here we show that m(6)A is selectively recognized by the human YTH domain family 2 (YTHDF2) 'reader' protein to regulate mRNA degradation. We identified over 3,000 cellular RNA targets of YTHDF2, most of which are mRNAs, but which also include non-coding RNAs, with a conserved core motif of G(m(6)A)C. We further establish the role of YTHDF2 in RNA metabolism, showing that binding of YTHDF2 results in the localization of bound mRNA from the translatable pool to mRNA decay sites, such as processing bodies. The carboxy-terminal domain of YTHDF2 selectively binds to m(6)A-containing mRNA, whereas the amino-terminal domain is responsible for the localization of the YTHDF2-mRNA complex to cellular RNA decay sites. Our results indicate that the dynamic m(6)A modification is recognized by selectively binding proteins to affect the translation status and lifetime of mRNA.

N6-methyladenosine alters RNA structure to regulate binding of a low-complexity protein.

N6-methyladenosine (m6A) is the most abundant internal modification in eukaryotic messenger RNA (mRNA), and affects almost every stage of the mRNA life cycle. The YTH-domain proteins can specifically recognize m6A modification to control mRNA maturation, translation and decay. m6A can also alter RNA structures to affect RNA-protein interactions in cells. Here, we show that m6A increases the accessibility of its surrounding RNA sequence to bind heterogeneous nuclear ribonucleoprotein G (HNRNPG). Furthermore, HNRNPG binds m6A-methylated RNAs through its C-terminal low-complexity region, which self-assembles into large particles in vitro. The Arg-Gly-Gly repeats within the low-complexity region are required for binding to the RNA motif exposed by m6A methylation. We identified 13,191 m6A sites in the transcriptome that regulate RNA-HNRNPG interaction and thereby alter the expression and alternative splicing pattern of target mRNAs. Low-complexity regions are pervasive among mRNA binding proteins. Our results show that m6A-dependent RNA structural alterations can promote direct binding of m6A-modified RNAs to low-complexity regions in RNA binding proteins.

Examining management scenarios to mitigate wildfire hazard to caribou conservation projects using burn probability modeling.

The boreal forests of Alberta have extensive networks of legacy seismic exploration lines that have been linked to the decline of boreal woodland caribou (Rangifer tarandus caribou) populations throughout the region. In order to improve habitat quality for caribou, energy companies are investing significant resources in the restoration of many of these seismic lines in key areas, however, frequent large and intense wildfires may compromise the effectiveness of these conservation measures. To minimize the wildfire risk, managers need to know the likelihood of wildfire and the effectiveness of mitigation measures. We undertook a wildfire risk assessment across the Cold Lake caribou range where we used the Burn-P3 model to determine: a) burn probability; b) wildfire risk to restored seismic line areas; and c) the effectiveness of mitigation measures. The burn probability of the landscape was highly heterogeneous, and recent large burns and some waterbodies provided "shields" that reduced burn probability on their leeward sides. We designed mitigation scenarios to mimic the shielding effect of waterbodies and large recent burns by modeling the effects of increase suppression activity and fuel conversion within intensive management zones upwind of the resources to be protected. We found that these intensive management zones reduced the burn probability and wildfire hazard in the restored habitat areas but the effect declined rapidly as distance from the treatment zones increased. If land managers want to minimize the risk of losing their investments in caribou conservation to wildfire, it would be preferable to have mitigation measures spatially targeted closer to the conservation areas. Furthermore, it would be advisable to have redundancy in any conservation measures and wildfire-risk mitigations to ensure that losses due to wildfire on one area do not jeopardize all conservation projects within the landscape.

Fine-scale spatial climate variation and drought mediate the likelihood of reburning.

In many forested ecosystems, it is increasingly recognized that the probability of burning is substantially reduced within the footprint of previously burned areas. This self-limiting effect of wildland fire is considered a fundamental emergent property of ecosystems and is partly responsible for structuring landscape heterogeneity (i.e., mosaics of different age classes), thereby reducing the likelihood of uncharacteristically large fires in regions with active fire regimes. However, the strength and longevity of this self-limiting phenomenon is not well understood in most fire-prone ecosystems. In this study, we quantify the self-limiting effect in terms of its strength and longevity for five fire-prone study areas in western North America and investigate how each measure varies along a spatial climatic gradient and according to temporal (i.e., annual) climatic variation. Results indicate that the longevity (i.e., number of years) of the self-limiting effect ranges between 15 yr in the warm and dry study area in the southwestern United States to 33 yr in the cold, northern study areas in located in northwestern Montana and the boreal forest of Canada. We also found that spatial climatic variation has a strong influence on wildland fire's self-limiting capacity. Specifically, the self-limiting effect within each study area was stronger and lasted longer in areas with low mean moisture deficit (i.e., wetter and cooler settings) compared to areas with high mean moisture deficit (warmer and drier settings). Last, our findings show that annual climatic variation influences wildland fire's self-limiting effect: drought conditions weakened the strength and longevity of the self-limiting effect in all study areas, albeit at varying magnitudes. Overall, our study provides support for the idea that wildland fire contributes to spatial heterogeneity in fuel ages that subsequently mediate future fire sizes and effects. However, our findings show that the strength and longevity of the self-limiting effect varies considerably according to spatial and temporal climatic variation, providing land and fire managers relevant information for effective planning and management of fire and highlighting that fire itself is an important factor contributing to fire-free intervals.

Spatial and temporal dimensions of fire activity in the fire-prone eastern Canadian taiga.

The forest age mosaic is a fundamental attribute of the North American boreal forest. Given that fires are generally lethal to trees, the time since last fire largely determines the composition and structure of forest stands and landscapes. Although the spatiotemporal dynamics of such mosaics has long been assumed to be random under the overwhelming influence of severe fire weather, no long-term reconstruction of mosaic dynamics has been performed from direct field evidence. In this study, we use fire length as a proxy for fire extent across the fire-prone eastern Canadian taiga and systematically reconstruct the spatiotemporal variability of fire extent and fire intervals, as well as the resulting forest age along a 340-km transect for the 1840-2013 time period. Our results indicate an extremely active fire regime over the last two centuries, with an overall burn rate of 2.1% of the land area yr-1 , mainly triggered by seasonal anomalies of high temperature and severe drought. However, the rejuvenation of the age mosaic was strongly patterned in space and time due to the intrinsically lower burn rates in wetland-dominated areas and, more importantly, to the much-reduced likelihood of burning of stands up to 50 years postfire. An extremely high burn rate of ~5% yr-1 would have characterized our study region during the last century in the absence of such fuel age effect. Although recent burn rates and fire sizes are within their range of variability of the last 175 years, a particularly severe weather event allowed a 2013 fire to spread across a large fire refuge, thus shifting the abundance of mature and old forest to a historic low. These results provide reference conditions to evaluate the significance and predict the spatiotemporal dynamics and impacts of the currently strengthening fire activity in the North American boreal forest.