Increasing wildfires threaten historic carbon sink of boreal forest soils.

Boreal forest fires emit large amounts of carbon into the atmosphere primarily through the combustion of soil organic matter1-3. During each fire, a portion of this soil beneath the burned layer can escape combustion, leading to a net accumulation of carbon in forests over multiple fire events4. Climate warming and drying has led to more severe and frequent forest fires5-7, which threaten to shift the carbon balance of the boreal ecosystem from net accumulation to net loss1, resulting in a positive climate feedback8. This feedback will occur if organic-soil carbon that escaped burning in previous fires, termed 'legacy carbon', combusts. Here we use soil radiocarbon dating to quantitatively assess legacy carbon loss in the 2014 wildfires in the Northwest Territories of Canada2. We found no evidence for the combustion of legacy carbon in forests that were older than the historic fire-return interval of northwestern boreal forests9. In forests that were in dry landscapes and less than 60 years old at the time of the fire, legacy carbon that had escaped burning in the previous fire cycle was combusted. We estimate that 0.34 million hectares of young forests (<60 years) that burned in the 2014 fires could have experienced legacy carbon combustion. This implies a shift to a domain of carbon cycling in which these forests become a net source-instead of a sink-of carbon to the atmosphere over consecutive fires. As boreal wildfires continue to increase in size, frequency and intensity7, the area of young forests that experience legacy carbon combustion will probably increase and have a key role in shifting the boreal carbon balance.

North American tree migration paced by climate in the West, lagging in the East.

Tree fecundity and recruitment have not yet been quantified at scales needed to anticipate biogeographic shifts in response to climate change. By separating their responses, this study shows coherence across species and communities, offering the strongest support to date that migration is in progress with regional limitations on rates. The southeastern continent emerges as a fecundity hotspot, but it is situated south of population centers where high seed production could contribute to poleward population spread. By contrast, seedling success is highest in the West and North, serving to partially offset limited seed production near poleward frontiers. The evidence of fecundity and recruitment control on tree migration can inform conservation planning for the expected long-term disequilibrium between climate and forest distribution.

The Relationship Between Maturation Size and Maximum Tree Size From Tropical to Boreal Climates.

The fundamental trade-off between current and future reproduction has long been considered to result in a tendency for species that can grow large to begin reproduction at a larger size. Due to the prolonged time required to reach maturity, estimates of tree maturation size remain very rare and we lack a global view on the generality and the shape of this trade-off. Using seed production from five continents, we estimate tree maturation sizes for 486 tree species spanning tropical to boreal climates. Results show that a species' maturation size increases with maximum size, but in a non-proportional way: the largest species begin reproduction at smaller sizes than would be expected if maturation were simply proportional to maximum size. Furthermore, the decrease in relative maturation size is steepest in cold climates. These findings on maturation size drivers are key to accurately represent forests' responses to disturbance and climate change.

Low-severity fires in the boreal region: reproductive implications for black spruce stands in between stand-replacing fire events.

BACKGROUND AND AIMS: Stand-replacing crown fires are the most prevalent type of fire regime in boreal forests in North America. However, a substantial proportion of low-severity fires are found within fire perimeters. Here we aimed to investigate the effects of low-severity fires on the reproductive potential and seedling recruitment in boreal forests stands in between stand-replacing fire events. METHODS: We recorded site and tree characteristics from 149 trees within twelve sites dominated by mature black spruce [Picea mariana (Mill.) B.S.P.] trees in the Northwest Territories, Canada. The presence of fire-scarred trees supported classification of sites as unburned or affected by low-severity fires in recent history. We used non-parametric tests to evaluate differences in site conditions between unburned and low-severity sites, and mixed effect models to evaluate differences in tree age, size, and reproductive traits among unburned trees and trees from low-severity sites. KEY RESULTS: Results showed significantly higher density of dead black spruce trees in low-severity sites, and marginally significant higher presence of permafrost. Trees from low-severity fire sites were significantly older, exhibited significantly lower tree growth, and showed a tendency towards a higher probability of cone presence and percentage of open cones compared to trees from unburned sites. Surviving fire-scarred trees affected by more recent low-severity fires showed a tendency towards higher probability of cone presence and cone production. Density of black spruce seedlings significantly increased with recent low-severity fires. CONCLUSIONS: Trees in low-severity sites appeared to have escaped mortality from up to three fires, as indicated by fire scar records and their older ages. Shallow permafrost at low-severity sites may cause lower flammability, allowing areas to act as fire refugia. Low-severity surface fires temporarily enhanced the reproductive capacity of surviving trees and the density of seedlings, likely as a stress response to the fire event.

Factors limiting the potential range expansion of lodgepole pine in Interior Alaska.

Understanding the factors influencing species range limits is increasingly crucial in anticipating migrations due to human-caused climate change. In the boreal biome, ongoing climate change and the associated increases in the rate, size, and severity of disturbances may alter the distributions of boreal tree species. Notably, Interior Alaska lacks native pine, a biogeographical anomaly that carries implications for ecosystem structure and function. The current range of lodgepole pine (Pinus contorta var. latifolia) in the adjacent Yukon Territory may expand into Interior Alaska, particularly with human assistance. Evaluating the potential for pine expansion in Alaska requires testing constraints on range limits such as dispersal limitations, environmental tolerance limits, and positive or negative biotic interactions. In this study, we used field experiments with pine seeds and transplanted seedlings, complemented by model simulations, to assess the abiotic and biotic factors influencing lodgepole pine seedling establishment and growth after fire in Interior Alaska. We found that pine could successfully recruit, survive, grow, and reproduce across our broadly distributed network of experimental sites. Our results show that both mammalian herbivory and competition from native tree species are unlikely to constrain pine growth and that environmental conditions commonly found in Interior Alaska fall well within the tolerance limits for pine. If dispersal constraints are released, lodgepole pine could have a geographically expansive range in Alaska, and once established, its growth is sufficient to support pine-dominated stands. Given the impacts of lodgepole pine on ecosystem processes such as increases in timber production, carbon sequestration, landscape flammability, and reduced forage quality, natural or human-assisted migration of this species is likely to substantially alter responses of Alaskan forest ecosystems to climate change.

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.

Temporal coexistence mechanisms contribute to the latitudinal gradient in forest diversity.

The tropical forests of Borneo and Amazonia may each contain more tree species diversity in half a square kilometre than do all the temperate forests of Europe, North America, and Asia combined. Biologists have long been fascinated by this disparity, using it to investigate potential drivers of biodiversity. Latitudinal variation in many of these drivers is expected to create geographic differences in ecological and evolutionary processes, and evidence increasingly shows that tropical ecosystems have higher rates of diversification, clade origination, and clade dispersal. However, there is currently no evidence to link gradients in ecological processes within communities at a local scale directly to the geographic gradient in biodiversity. Here, we show geographic variation in the storage effect, an ecological mechanism that reduces the potential for competitive exclusion more strongly in the tropics than it does in temperate and boreal zones, decreasing the ratio of interspecific-to-intraspecific competition by 0.25% for each degree of latitude that an ecosystem is located closer to the Equator. Additionally, we find evidence that latitudinal variation in climate underpins these differences; longer growing seasons in the tropics reduce constraints on the seasonal timing of reproduction, permitting lower recruitment synchrony between species and thereby enhancing niche partitioning through the storage effect. Our results demonstrate that the strength of the storage effect, and therefore its impact on diversity within communities, varies latitudinally in association with climate. This finding highlights the importance of biotic interactions in shaping geographic diversity patterns, and emphasizes the need to understand the mechanisms underpinning ecological processes in greater detail than has previously been appreciated.

Globally, tree fecundity exceeds productivity gradients.

Lack of tree fecundity data across climatic gradients precludes the analysis of how seed supply contributes to global variation in forest regeneration and biotic interactions responsible for biodiversity. A global synthesis of raw seedproduction data shows a 250-fold increase in seed abundance from cold-dry to warm-wet climates, driven primarily by a 100-fold increase in seed production for a given tree size. The modest (threefold) increase in forest productivity across the same climate gradient cannot explain the magnitudes of these trends. The increase in seeds per tree can arise from adaptive evolution driven by intense species interactions or from the direct effects of a warm, moist climate on tree fecundity. Either way, the massive differences in seed supply ramify through food webs potentially explaining a disproportionate role for species interactions in the wet tropics.

MASTREE+: Time-series of plant reproductive effort from six continents.

Significant gaps remain in understanding the response of plant reproduction to environmental change. This is partly because measuring reproduction in long-lived plants requires direct observation over many years and such datasets have rarely been made publicly available. Here we introduce MASTREE+, a data set that collates reproductive time-series data from across the globe and makes these data freely available to the community. MASTREE+ includes 73,828 georeferenced observations of annual reproduction (e.g. seed and fruit counts) in perennial plant populations worldwide. These observations consist of 5971 population-level time-series from 974 species in 66 countries. The mean and median time-series length is 12.4 and 10 years respectively, and the data set includes 1122 series that extend over at least two decades (≥20 years of observations). For a subset of well-studied species, MASTREE+ includes extensive replication of time-series across geographical and climatic gradients. Here we describe the open-access data set, available as a.csv file, and we introduce an associated web-based app for data exploration. MASTREE+ will provide the basis for improved understanding of the response of long-lived plant reproduction to environmental change. Additionally, MASTREE+ will enable investigation of the ecology and evolution of reproductive strategies in perennial plants, and the role of plant reproduction as a driver of ecosystem dynamics.

Aún existen importantes vacíos en la comprensión de la respuesta reproductiva de las plantas al cambio medioambiental, en parte, porque su monitoreo en especies de plantas longevas requiere una observación directa durante muchos años, y estos conjuntos de datos rara vez han estado disponibles. Aquí presentamos a MASTREE +, una base de datos que recopila series de tiempo de la reproducción de las plantas de todo el planeta, poniendo a disposición estos datos de libre acceso para la comunidad científica. MASTREE + incluye 73.828 puntos de observación de la reproducción anual georreferenciados (ej. conteos de semillas y frutos) en poblaciones de plantas perennes en todo el mundo. Estas observaciones consisten en 5971 series temporales a nivel de población provenientes de 974 especies en 66 países. La mediana de la duración de las series de tiempo es de 10 años (media = 12.4 años) y el conjunto de datos incluye 1.122 series de al menos dos décadas (≥20 años de observaciones). Para un subconjunto de especies bien estudiadas, MASTREE +incluye un amplio conjunto de series temporales replicadas en gradientes geográficos y climáticos. Describimos el conjunto de datos de acceso abierto disponible como un archivo.csv y presentamos una aplicación web asociada para la exploración de datos. MASTREE+ proporcionará la base para mejorar la comprensión sobre la respuesta reproductiva de plantas longevas al cambio medioambiental. Además, MASTREE+ facilitará los avances en la investigación de la ecología y la evolución de las estrategias reproductivas en plantas perennes y el papel de la reproducción vegetal como determinante de la dinámica de ecosistemas.

Experimental assessment of tree canopy and leaf litter controls on the microbiome and nitrogen fixation rates of two boreal mosses.

Nitrogen (N2 )-fixing moss microbial communities play key roles in nitrogen cycling of boreal forests. Forest type and leaf litter inputs regulate moss abundance, but how they control moss microbiomes and N2 -fixation remains understudied. We examined the impacts of forest type and broadleaf litter on microbial community composition and N2 -fixation rates of Hylocomium splendens and Pleurozium schreberi. We conducted a moss transplant and leaf litter manipulation experiment at three sites with paired paper birch (Betula neoalaskana) and black spruce (Picea mariana) stands in Alaska. We characterized bacterial communities using marker gene sequencing, determined N2 -fixation rates using stable isotopes (15 N2 ) and measured environmental covariates. Mosses native to and transplanted into spruce stands supported generally higher N2 -fixation and distinct microbial communities compared to similar treatments in birch stands. High leaf litter inputs shifted microbial community composition for both moss species and reduced N2 -fixation rates for H. splendens, which had the highest rates. N2 -fixation was positively associated with several bacterial taxa, including cyanobacteria. The moss microbiome and environmental conditions controlled N2 -fixation at the stand and transplant scales. Predicted shifts from spruce- to deciduous-dominated stands will interact with the relative abundances of mosses supporting different microbiomes and N2 -fixation rates, which could affect stand-level N inputs.

Wildfire combustion and carbon stocks in the southern Canadian boreal forest: Implications for a warming world.

Boreal wildfires are increasing in intensity, extent, and frequency, potentially intensifying carbon emissions and transitioning the region from a globally significant carbon sink to a source. The productive southern boreal forests of central Canada already experience relatively high frequencies of fire, and as such may serve as an analog of future carbon dynamics for more northern forests. Fire-carbon dynamics in southern boreal systems are relatively understudied, with limited investigation into the drivers of pre-fire carbon stocks or subsequent combustion. As part of NASA's Arctic-Boreal Vulnerability Experiment, we sampled 79 stands (47 burned, 32 unburned) throughout central Saskatchewan to characterize above- and belowground carbon stocks and combustion rates in relation to historical land use, vegetation characteristics, and geophysical attributes. We found southern boreal forests emitted an average of 3.3 ± 1.1 kg C/m2 from field sites. The emissions from southern boreal stands varied as a function of stand age, fire weather conditions, ecozone, and soil moisture class. Sites affected by historical timber harvesting had greater combustion rates due to faster carbon stock recovery rates than sites recovering from wildfire events, indicating that different boreal forest land use practices can generate divergent carbon legacy effects. We estimate the 2015 fire season in Saskatchewan emitted a total of 36.3 ± 15.0 Tg C, emphasizing the importance of southern boreal fires for regional carbon budgets. Using the southern boreal as an analog, the northern boreal may undergo fundamental shifts in forest structure and carbon dynamics, becoming dominated by stands <70 years old that hold 2-7 kg C/m2 less than current mature northern boreal stands. Our latitudinal approach reinforces previous studies showing that northern boreal stands are at a high risk of holding less carbon under changing disturbance conditions.

Climate change decreases the cooling effect from postfire albedo in boreal North America.

Fire is a primary disturbance in boreal forests and generates both positive and negative climate forcings. The influence of fire on surface albedo is a predominantly negative forcing in boreal forests, and one of the strongest overall, due to increased snow exposure in the winter and spring months. Albedo forcings are spatially and temporally heterogeneous and depend on a variety of factors related to soils, topography, climate, land cover/vegetation type, successional dynamics, time since fire, season, and fire severity. However, how these variables interact to influence albedo is not well understood, and quantifying these relationships and predicting postfire albedo becomes increasingly important as the climate changes and management frameworks evolve to consider climate impacts. Here we developed a MODIS-derived 'blue sky' albedo product and a novel machine learning modeling framework to predict fire-driven changes in albedo under historical and future climate scenarios across boreal North America. Converted to radiative forcing (RF), we estimated that fires generate an annual mean cooling of -1.77 ± 1.35 W/m2 from albedo under historical climate conditions (1971-2000) integrated over 70 years postfire. Increasing postfire albedo along a south-north climatic gradient was offset by a nearly opposite gradient in solar insolation, such that large-scale spatial patterns in RF were minimal. Our models suggest that climate change will lead to decreases in mean annual postfire albedo, and hence a decreasing strength of the negative RF, a trend dominated by decreased snow cover in spring months. Considering the range of future climate scenarios and model uncertainties, we estimate that for fires burning in the current era (2016) the cooling effect from long-term postfire albedo will be reduced by 15%-28% due to climate change.

Perioperative Outcomes After Use of Iliac Branch Devices Compared With Hypogastric Occlusion or Open Surgery for Elective Treatment of Aortoiliac Aneurysms in the NSQIP Database.

BACKGROUND: Iliac branch devices (IBDs) can treat iliac and aortoiliac aneurysms (AIAs) less invasively than open surgery (OS) and preserve pelvic perfusion. Our hypothesis was that the rates of perioperative complications after treatment for AIAs are similar between IBDs and hypogastric occlusion with coil and cover (C&C), and lower than OS. METHODS: We identified patients undergoing elective AIA repair by IBD, C&C, and OS (all with infrarenal clamps) within the National Surgical Quality Improvement Program (NSQIP) vascular aneurysm specific Participant User Files (2012-2016). Baseline characteristics, procedural variables, and 30-day outcomes were compared. The primary outcomes were any major complication or death. Secondary outcomes included minor complications, total operative time, total and intensive care unit length of stay (LOS), and reinterventions. Multivariable logistic regression assessed differences in major complications between IBD and C&C/OS after adjusting for patient and procedural variables. RESULTS: We identified 593 patients (83% men, mean age 71.6 ± 9 years) undergoing elective AIA repair (IBD = 283, C&C = 118, and OS = 192). Patient age and American Society of Anesthesiology (ASA) classification varied significantly between groups. Mean aneurysm diameter was higher for OS and similar between IBD and C&C (5.9 cm vs. 5.5 cm and 5.2 cm, respectively, P < 0.001). OS was associated with higher rate of major complications (65.5% vs. IBD: 8.8% and C&C: 13.6%, P=<0.001) and higher mortality (3.6% vs. IBD: 0.7% and C&C: 0%, P = 0.017). Minor complications and reinterventions were similar. IBD patients had significantly shorter total operative time and total and intensive care unit LOS. After adjustment, OS was associated with higher major complications compared with IBD (Odds ratio [OR]: 11.3, 95% confidence interval [CI]: 5.8-21.9, P < 0.001), primarily because of the use of transfusions (major complications excluding transfusions OR: 1.3, 95% CI: 0.6-2.8, P = 0.52). Major complications between IBD and C&C were similar (OR: 1.6, 95% CI: 0.8-3.4, P = 0.23). CONCLUSIONS: The use of IBDs for elective treatment of AIAs is associated with favorable perioperative outcomes and a lower rate of major complications compared with OS, primarily because of fewer transfusions. IBDs use has perioperative outcomes similar to C&C with the associated benefit of preserving pelvic perfusion. Pending long-term durability results for this technique, IBDs appear to be associated with several perioperative advantages in patients with AIAs compared with OS and C&C.

A Goodness-of-fit Test for Zero-Inflated Poisson Mixed Effects Models in Tree Abundance Studies.

Field studies in ecology often make use of data collected in a hierarchical fashion, and may combine studies that vary in sampling design. For example, studies of tree recruitment after disturbance may use counts of individual seedlings from plots that vary in spatial arrangement and sampling density. To account for the multi-level design and the fact that more than a few plots usually yield no individuals, a mixed effects zero inflated Poisson model is often adopted. Although it is a convenient modeling strategy, various aspects of the model could be misspecified. A comprehensive test procedure, based on the cumulative sum of the residuals, is proposed. The test is proven to be consistent, and its convergence properties are established as well. The application of the proposed test is illustrated by a real data example and simulation studies.

Performance of current claims-based approaches to identify aortic dissection hospitalizations.

OBJECTIVE: To describe index visits for acute aortic dissection (AD) to an academic center and validate the prevailing claims-based methodology to identify and stratify them. METHODS: Inpatient hospitalizations at a single center assigned an International Classification of Diseases, Ninth Revision (ICD-9) diagnosis code for AD from January 2005 to September 2015 were identified. Diagnoses were verified by review of medical records and imaging studies. All visits were secondarily stratified with the algorithm based on ICD-9 codes. Sensitivity and specificity analyses were conducted to evaluate the ability of the algorithm to correctly identify acute AD by Stanford class and treatment modality (type A open repair [TAOR], type B open repair [TBOR], thoracic endovascular repair [TEVAR], medical management [MM]). RESULTS: In the study interval, there were 1245 visits coded for AD attributed to 968 unique patients. Chart review verification demonstrated that the majority of visits were for AD (79%; n = 981), of which 32% (n = 310) were for an index acute AD event. The true distribution of acute AD visit classifications was TAOR (46.1%; n = 143), TBOR (5.2%; n = 16), TEVAR (7.7%; n = 24), and MM (39.4%; n = 122). The algorithm, which used ICD-9 codes, identified 631 acute visits and stratified them as TAOR (27.1%; n = 171), TBOR (4.1%; n = 26), TEVAR (4.9%; n = 31), and MM (63.9%; n = 403). Analyses demonstrated high specificities, but generally low sensitivities of the algorithm (TAOR: sensitivity, 58%, specificity, 92%; TBOR: sensitivity, 13%, specificity, 98%; TEVAR: sensitivity, 17%, specificity, 98%; MM: sensitivity, 73%, specificity, 72%). CONCLUSIONS: The prevalent claims-based strategy to identify hospitalizations with acute AD is specific, but lacks sensitivity. Caution should be exercised when studying AD with ICD-9 codes and improvements to existing claims-based methodologies are necessary to support future study of acute AD.

Outcomes of an iliac branch endoprosthesis using an "up-and-over" technique for endovascular repair of failed bifurcated grafts.

OBJECTIVE: Type IB endoleak after endovascular aneurysm repair may be treated by an iliac branch endoprosthesis (IBE) through brachial access for internal iliac artery (IIA) stenting. The aim of this study was to evaluate outcomes of the IBE using an "up-and-over" transfemoral technique in patients with prior aortic repair compared with the standard technique in patients with de novo iliac aneurysms. METHODS: We reviewed the clinical data of patients treated for aortoiliac aneurysms using Gore IBE (W. L. Gore & Associates, Flagstaff, Ariz) between 2014 and 2017. The up-and-over technique was indicated in patients with type IB endoleak or common iliac aneurysms after prior aortic repair with bifurcated endografts or surgical grafts. End points were technical success, mortality, major adverse events, IIA patency, freedom from IIA branch instability (composite end point of any IIA branch-related complication leading to aneurysm rupture, death, occlusion, component separation, or reintervention to maintain branch patency or to treat a branch-related separation or endoleak), and freedom from secondary interventions or new-onset buttock claudication. RESULTS: There were 53 patients (51 male; 74 ± 8 years old) treated by 62 IBEs (9 bilateral). Standard technique was used in 36 patients (43 IBEs) and up-and-over technique in 17 (19 IBEs). Three patients had contralateral IIA embolization. Total procedure time, contrast material volume, and radiation dose averaged 168 ± 98 minutes, 140 ± 50 mL, and 1096 ± 1009 mGy, with no difference between techniques. Technical success was achieved in 98% of patients. Eleven patients had extension of IIA bridging stent into the posterior branch (eight standard, three up-and-over). Four patients (8%) had major adverse events due to estimated blood loss >1000 mL in all patients. There was no 30-day mortality after a median follow-up of 7 months (interquartile range, 3-12 months). There were two IIA stent occlusions (all standard), three iliac-related type I endoleaks (one standard, two up-and-over), and four secondary interventions (three standard, one up-and-over). At 1 year, patients treated by standard or up-and-over technique had similar primary patency (94% ± 4% vs 100%; P = .38) and secondary patency (97% ± 3% vs 100%; P = .54) and freedom from IIA branch instability (90% ± 6% vs 93% ± 7%; P = .48), secondary intervention (84% ± 8% vs 90% ± 9%; P = .63), and new-onset buttock claudication (90% ± 6% vs 100%; P = .35). CONCLUSIONS: Endovascular repair using IBE was associated with high technical success, no mortality, and low rate of complications using either the standard technique for de novo aneurysms or an up-and-over technique for patients with failed bifurcated endografts or grafts. The up-and-over technique should be considered a suitable alternative to brachial access in patients who require distal extension using IBEs.

Examining forest resilience to changing fire frequency in a fire-prone region of boreal forest.

Future changes in climate are widely anticipated to increase fire frequency, particularly in boreal forests where extreme warming is expected to occur. Feedbacks between vegetation and fire may modify the direct effects of warming on fire activity and shape ecological responses to changing fire frequency. We investigate these interactions using extensive field data from the Boreal Shield of Saskatchewan, Canada, a region where >40% of the forest has burned in the past 30 years. We use geospatial and field data to assess the resistance and resilience of eight common vegetation states to frequent fire by quantifying the occurrence of short-interval fires and their effect on recovery to a similar vegetation state. These empirical relationships are combined with data from published literature to parameterize a spatially explicit, state-and-transition simulation model of fire and forest succession. We use this model to ask if and how: (a) feedbacks between vegetation and wildfire may modify fire activity on the landscape, and (b) more frequent fire may affect landscape forest composition and age structure. Both field and GIS data suggest the probability of fire is low in the initial decades after fire, supporting the hypothesis that fuel accumulation may exert a negative feedback on fire frequency. Field observations of pre- and postfire composition indicate that switches in forest state are more likely in conifer stands that burn at a young age, supporting the hypothesis that resilience is lower in immature stands. Stands dominated by deciduous trees or jack pine were generally resilient to fire, while mixed conifer and well-drained spruce forests were less resilient. However, simulation modeling suggests increased fire activity may result in large changes in forest age structure and composition, despite the feedbacks between vegetation-fire likely to occur with increased fire activity.

Wildfire severity reduces richness and alters composition of soil fungal communities in boreal forests of western Canada.

Wildfire is the dominant disturbance in boreal forests and fire activity is increasing in these regions. Soil fungal communities are important for plant growth and nutrient cycling postfire but there is little understanding of how fires impact fungal communities across landscapes, fire severity gradients, and stand types in boreal forests. Understanding relationships between fungal community composition, particularly mycorrhizas, and understory plant composition is therefore important in predicting how future fire regimes may affect vegetation. We used an extreme wildfire event in boreal forests of Canada's Northwest Territories to test drivers of fungal communities and assess relationships with plant communities. We sampled soils from 39 plots 1 year after fire and 8 unburned plots. High-throughput sequencing (MiSeq, ITS) revealed 2,034 fungal operational taxonomic units. We found soil pH and fire severity (proportion soil organic layer combusted), and interactions between these drivers were important for fungal community structure (composition, richness, diversity, functional groups). Where fire severity was low, samples with low pH had higher total fungal, mycorrhizal, and saprotroph richness compared to where severity was high. Increased fire severity caused declines in richness of total fungi, mycorrhizas, and saprotrophs, and declines in diversity of total fungi and mycorrhizas. The importance of stand age (a surrogate for fire return interval) for fungal composition suggests we could detect long-term successional patterns even after fire. Mycorrhizal and plant community composition, richness, and diversity were weakly but significantly correlated. These weak relationships and the distribution of fungi across plots suggest that the underlying driver of fungal community structure is pH, which is modified by fire severity. This study shows the importance of edaphic factors in determining fungal community structure at large scales, but suggests these patterns are mediated by interactions between fire and forest stand composition.

Comparison of Perioperative Outcomes of Patients with Iliac Aneurysms Treated by Open Surgery or Endovascular Repair with Iliac Branch Endoprosthesis.

BACKGROUND: Treatment of common and internal iliac aneurysms is usually done by open surgery. A novel iliac branch endoprosthesis (IBE) is commercially available with encouraging initial results. Our objective is to compare perioperative outcomes of patients with iliac aneurysms treated by open surgery (OS) versus endovascular repair with IBE. METHODS: The study was a retrospective, single-center review of patients who were treated for aortoiliac or isolated common and/or internal iliac artery aneurysms from 2014 to 2017. Patients with connective tissue disorders, infected grafts, or thoracoabdominal aneurysms were excluded. Primary outcomes were perioperative mortality, length of hospital (LOS) and intensive care unit (ICU) stay, estimated blood loss, need for red blood cell transfusion (RBC), and perioperative reinterventions. RESULTS: Sixty-seven patients (96% male) were treated with OS (n = 25, mean age 68 ± 8 years) or IBE (n = 42, mean age 73 ± 8 years; P = 0.02) with 1 symptomatic patient in each group. Perioperative mortality occurred in 1 patient in the OS group (4%), with no mortality in the IBE group (P = 0.37) Total LOS and ICU stay was higher for OS compared to IBE (total stay 7.5 ± 3.4 vs. 1.7 ± 1.4 days for IBE, P < 0.0001 and ICU LOS 3.3 ± 2.1 vs. 0.1 ± 0.4 days, P < 0.0001). Estimated blood loss was higher for patients undergoing OS (4,732 ± 2,540 mL) compared to patients treated with IBE (263 ± 451 mL, P < 0.0001), resulting in higher RBC transfusion requirements (1.5 ± 2.4 vs. 0.2 ± 0.8 units, P = 0.001). Five patients in the OS group had early procedure-related reinterventions, while 2 patients in the IBE group required reintervention for access site complications (20% vs. 4.7%, P = 0.09). CONCLUSIONS: Endovascular repair of iliac aneurysms with IBE is feasible and is associated with lower blood loss, LOS and ICU stay, and had lower RBC transfusion requirements. Cost analysis and long-term follow-up will be needed to define the value of this modality for iliac artery aneurysm repair.

Superior mesenteric artery stenting using embolic protection device for treatment of acute or chronic mesenteric ischemia.

OBJECTIVE: The objective of the study was to report the feasibility and results of superior mesenteric artery (SMA) stenting using embolic protection devices (EPDs) to treat acute mesenteric ischemia (AMI) and chronic mesenteric ischemia (CMI). METHODS: A retrospective review was conducted of consecutive patients who underwent SMA stenting with EPDs from 2007 to 2016. EPDs were used selectively in patients with occlusions, severe calcification, or acute thrombus. A two-wire technique with SpiderFX 0.014-inch filter wire (Medtronic, Minneapolis, Minn) combined with a 0.018-inch wire was used to provide support and to facilitate stenting and EPD retrieval. Presence of macroscopic debris in the EPD was recorded and graded as minor (minimal debris) or major (large thrombus or plaque). End points were technical success, presence of EPD debris, embolization, early morbidity, and mortality. RESULTS: SMA stenting was performed in 179 patients, of whom 65 (36%) had EPDs. The mean age was 73 ± 11 years, and 49 were female (75%). Clinical presentation was CMI in 48 patients (74%) and AMI or acute-on-CMI in 17 (26%). Indications for EPD were severe calcification in 22 patients (34%), acute thrombus in 18 (28%), and total occlusion in 16 (25%). Bare-metal stents were used in 33 patients, covered stents in 26, and both types in 6. Adjunctive therapy included thrombolysis in seven patients, thrombectomy in four, and atherectomy in three. Technical success was 100%. There were no instances of filter retention or arterial trauma due to filter manipulation. Distal embolization was noted in four patients (6%), of whom two had AMI. All large emboli were retrieved using catheter aspiration devices, but one small distal embolus was left untreated with no clinical consequences. Two patients had vessel spasm treated by nitroglycerin. Macroscopic debris was noted in 43 patients (66%) and was major in 21 (49%) or minor in 22 (51%). Of the patients with AMI, five (29%) required exploratory laparotomy and four (23%) had bowel resection. Eight additional patients (12%) had early complications (five CMI, three AMI), including cardiac complications, brachial hematoma, acute cholecystitis, and acute respiratory distress syndrome in two patients each. There were no deaths among CMI patients and two early deaths (12%) among those who had AMI. CONCLUSIONS: Use of EPDs during SMA stenting is safe and feasible with a two-wire technique. Large macroscopic debris was noted in one-third of the patients when the filter was applied selectively in patients with acute symptoms, occlusions, or severely calcified lesions. Despite the use of EPD, distal embolization occurred in 6% of patients and was successfully treated using catheter aspiration devices.