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Abstract: Forests are a large carbon sink and could serve as natural climate solutions that help moderate future warming. Thus, establishing forest carbon baselines is essential for tracking climate‐mitigation targets. Western US forests are natural climate solution hotspots but are profoundly threatened by drought and altered disturbance regimes. How these factors shape spatial patterns of carbon storage and carbon change over time is poorly resolved. Here, we estimate live and dead forest carbon density in 19 forested western US ecoregions with national inventory data (2005–2019) to determine: (a) current carbon distributions, (b) underpinning drivers, and (c) recent trends. Potential drivers of current carbon included harvest, wildfire, insect and disease, topography, and climate. Using random forests, we evaluated driver importance and relationships with current live and dead carbon within ecoregions. We assessed trends using linear models. Pacific Northwest (PNW) and Southwest (SW) ecoregions were most and least carbon dense, respectively. Climate was an important carbon driver in the SW and Lower Rockies. Fire reduced live and increased dead carbon, and was most important in the Upper Rockies and California. No ecoregion was unaffected by fire. Harvest and private ownership reduced carbon, particularly in the PNW. Since 2005, live carbon declined across much of the western US, likely from drought and fire. Carbon has increased in PNW ecoregions, likely recovering from past harvest, but recent record fire years may alter trajectories. Our results provide insight into western US forest carbon function and future vulnerabilities, which is vital for effective climate change mitigation strategies.

Increasing fire severity and warmer, drier postfire conditions are making forests in the western United States (West) vulnerable to ecological transformation. Yet, the relative importance of and interactions between these drivers of forest change remain unresolved, particularly over upcoming decades. Here, we assess how the interactive impacts of changing climate and wildfire activity influenced conifer regeneration after 334 wildfires, using a dataset of postfire conifer regeneration from 10,230 field plots. Our findings highlight declining regeneration capacity across the West over the past four decades for the eight dominant conifer species studied. Postfire regeneration is sensitive to high-severity fire, which limits seed availability, and postfire climate, which influences seedling establishment. In the near-term, projected differences in recruitment probability between low- and high-severity fire scenarios were larger than projected climate change impacts for most species, suggesting that reductions in fire severity, and resultant impacts on seed availability, could partially offset expected climate-driven declines in postfire regeneration. Across 40 to 42% of the study area, we project postfire conifer regeneration to be likely following low-severity but not high-severity fire under future climate scenarios (2031 to 2050). However, increasingly warm, dry climate conditions are projected to eventually outweigh the influence of fire severity and seed availability. The percent of the study area considered unlikely to experience conifer regeneration, regardless of fire severity, increased from 5% in 1981 to 2000 to 26 to 31% by mid-century, highlighting a limited time window over which management actions that reduce fire severity may effectively support postfire conifer regeneration.

Fire is an integral component of ecosystems globally and a tool that humans have harnessed for millennia. Altered fire regimes are a fundamental cause and consequence of global change, impacting people and the biophysical systems on which they depend. As part of the newly emerging Anthropocene, marked by human-caused climate change and radical changes to ecosystems, fire danger is increasing, and fires are having increasingly devastating impacts on human health, infrastructure, and ecosystem services. Increasing fire danger is a vexing problem that requires deep transdisciplinary, trans-sector, and inclusive partnerships to address. Here, we outline barriers and opportunities in the next generation of fire science and provide guidance for investment in future research. We synthesize insights needed to better address the long-standing challenges of innovation across disciplines to (i) promote coordinated research efforts; (ii) embrace different ways of knowing and knowledge generation; (iii) promote exploration of fundamental science; (iv) capitalize on the "firehose" of data for societal benefit; and (v) integrate human and natural systems into models across multiple scales. Fire science is thus at a critical transitional moment. We need to shift from observation and modeled representations of varying components of climate, people, vegetation, and fire to more integrative and predictive approaches that support pathways toward mitigating and adapting to our increasingly flammable world, including the utilization of fire for human safety and benefit. Only through overcoming institutional silos and accessing knowledge across diverse communities can we effectively undertake research that improves outcomes in our more fiery future.

It is a critical time to reflect on the National Ecological Observatory Network (NEON) science to date as well as envision what research can be done right now with NEON (and other) data and what training is needed to enable a diverse user community. NEON became fully operational in May 2019 and has pivoted from planning and construction to operation and maintenance. In this overview, the history of and foundational thinking around NEON are discussed. A framework of open science is described with a discussion of how NEON can be situated as part of a larger data constellation—across existing networks and different suites of ecological measurements and sensors. Next, a synthesis of early NEON science, based on >100 existing publications, funded proposal efforts, and emergent science at the very first NEON Science Summit (hosted by Earth Lab at the University of Colorado Boulder in October 2019) is provided. Key questions that the ecology community will address with NEON data in the next 10 yr are outlined, from understanding drivers of biodiversity across spatial and temporal scales to defining complex feedback mechanisms in human–environmental systems. Last, the essential elements needed to engage and support a diverse and inclusive NEON user community are highlighted: training resources and tools that are openly available, funding for broad community engagement initiatives, and a mechanism to share and advertise those opportunities. NEON users require both the skills to work with NEON data and the ecological or environmental science domain knowledge to understand and interpret them. This paper synthesizes early directions in the community’s use of NEON data, and opportunities for the next 10 yr of NEON operations in emergent science themes, open science best practices, education and training, and community building.

Escalating burned area in western US forests punctuated by the 2020 fire season has heightened the need to explore near-term macroscale forest-fire area trajectories. As fires remove fuels for subsequent fires, feedbacks may impose constraints on the otherwise climate-driven trend of increasing forest-fire area. Here, we test how fire-fuel feedbacks moderate near-term (2021–2050) climate-driven increases in forest-fire area across the western US. Assuming constant fuels, climate–fire models project a doubling of forest-fire area compared to 1991–2020. Fire-fuel feedbacks only modestly attenuate the projected increase in forest-fire area. Even models with strong feedbacks project increasing interannual variability in forest-fire area and more than a two-fold increase in the likelihood of years exceeding the 2020 fire season. Fuel limitations from fire-fuel feedbacks are unlikely to strongly constrain the profound climate-driven broad-scale increases in forest-fire area by the mid-21st century, highlighting the need for proactive adaptation to increased western US forest-fire impacts.

Wildland fires have a multitude of ecological effects in forests, woodlands, and savannas across the globe. A major focus of past research has been on tree mortality from fire, as trees provide a vast range of biological services. We assembled a database of individual-tree records from prescribed fires and wildfires in the United States. The Fire and Tree Mortality (FTM) database includes records from 164,293 individual trees with records of fire injury (crown scorch, bole char, etc.), tree diameter, and either mortality or top-kill up to ten years post-fire. Data span 142 species and 62 genera, from 409 fires occurring from 1981-2016. Additional variables such as insect attack are included when available. The FTM database can be used to evaluate individual fire-caused mortality models for pre-fire planning and post-fire decision support, to develop improved models, and to explore general patterns of individual fire-induced tree death. The database can also be used to identify knowledge gaps that could be addressed in future research.

Biological sex differences exist for many airway diseases in which females have either worse or better health outcomes. Inflammatory airway diseases such as cystic fibrosis (CF) and asthma display a clear male advantage in post-puberty while a female benefit is observed in asthma during the pre-puberty years. The influence of menstrual cycle stage and pregnancy on the frequency and severity of pulmonary exacerbations in CF and asthma point to a role for sex steroid hormones, particularly estrogen, in underpinning biological sex differences in these diseases. There are many ways by which estrogen may aggravate asthma and CF involving disturbances in airway surface liquid (ASL) dynamics, inappropriate hyper-immune and allergenic responses, as well as exacerbation of pathogen virulence. The deleterious effect of estrogen on pulmonary function in CF and asthma contrasts with the female advantage observed in airway diseases characterised by pulmonary edema such as pneumonia, acute respiratory distress syndrome (ARDS) and COVID-19. Airway surface liquid hypersecretion and alveolar flooding are hallmarks of ARDS and COVID-19, and contribute to the morbidity and mortality of severe forms of these diseases. ASL dynamics encompasses the intrinsic features of the thin lining of fluid covering the airway epithelium which regulate mucociliary clearance (ciliary beat, ASL height, volume, pH, viscosity, mucins, and channel activating proteases) in addition to innate defence mechanisms (pathogen virulence, cytokines, defensins, specialised pro-resolution lipid mediators, and metabolism). Estrogen regulation of ASL dynamics contributing to biological sex differences in CF, asthma and COVID-19 is a major focus of this review. Plain Language Summary: Sex differences exist in many airway diseases in which females have either worse or better health outcomes. These include cystic fibrosis (CF) and asthma where females post-puberty have more frequent lung exacerbations and higher mortality. Lung infections and difficulty in breathing become worse in post-puberty in females and during the ovulation period of the menstrual cycle, and in pregnancy, indicating a role for sex hormones in underpinning sex differences in CF and asthma. Evidence also exists for sex differences with a female advantage in airway diseases which are characterised by flooding of the airways, as in pneumonia and COVID-19, where females have better lung function and lower risk of death than males. The female sex hormone estrogen plays an important role in determining the role of sex biology in airway disease severity and mortality. Estrogen can control the movement of salt and water in and out of the lung airway tubes and dehydrate the lung surface to make it more sticky with mucus, as observed in CF and asthma, thus worsening the condition. In contrast, estrogen can have beneficial effects in lowering the volume of water in the lungs in COVID-19 thus alleviating flooding of the airways. This review focusses on the biology of sex differences in CF, asthma and COVID-19, and the cellular mechanisms by which estrogen can have either detrimental or beneficial effects in these diseases. Highlights: Airway diseases such as cystic fibrosis (CF), asthma and COVID-19 show marked sex differences in their morbidity and mortality Estrogen and female sex are associated with exacerbated CF and asthma Estrogen aggravates CF and asthma by compromising the optimal airway surface liquid dynamics for effective mucociliary clearance and by exacerbating bacterial virulence Estrogen confers a female advantage in COVID-19 by reducing airway flooding and reinforcing immunity. [ABSTRACT FROM AUTHOR]

Climate-induced increases in tree mortality are reported for many forests worldwide. Understanding the potential effects on carbon pools requires long-term monitoring of changes in forest biomass. We measured aboveground biomass (AGB) of living trees over a 34-year period (1982-2016) in permanent plots with varying stand ages, species compositions, and topographic settings in a subalpine forest in the Colorado Front Range. Stand-level and species-level AGB varied spatially and temporally in relation to stand age, successional processes, and site moisture classification. Young (ca. 122 years) postfire stands composed of lodgepole pine (Pinus contorta var. latifolia Engelm. ex S. Watson) had lower mean AGB than older (>250 years) mixed-species stands. Mesic stands had higher AGB than xeric or hydric stands of similar age. At the level of individual species, significant shifts in AGB among species were primarily explained by successional replacement of shade-intolerant pines by shade-tolerant Engelmann spruce (Picea engelmannii Parry ex Engelm.) and subalpine fir (Abies lasiocarpa (Hook.) Nutt.). The permanent plot network recorded significant shifts in species dominance and tree densities between 1982 and 2016, reflecting successional patterns developing over several centuries and the effects of recent localized windthrow, insects, and pathogens. Despite increases in tree mortality, there was a general pattern of increasing AGB across the forest. Key words: forest biomass, subalpine forest, permanent plots, forest dynamics, Colorado Front Range. On rapporte une augmentation de la mortalite des arbres en lien avec le climat dans plusieurs forets un peu partout dans le monde. Un suivi a long terme des changements dans la biomasse forestiere est necessaire pour comprendre les effets potentiels des reservoirs de carbone. Nous avons mesure la biomasse aerienne (BA) des arbres vivants pendant une periode de 34 ans (1982-2016) dans des placettes d'echantillonnage permanent representatives d'une variete d'ages, de compositions en especes et de configurations topographiques des peuplements forestiers dans une foret subalpine situee dans le Front Range du Colorado. La BA a l'echelle du peuplement et de l'espece variait dans l'espace et dans le temps en relation avec l'age du peuplement, les processus successionnels et la classification des stations basee sur l'humidite du sol. Les jeunes (~122 ans) peuplements issus de feux et composes de pin tordu latifolie (Pinus contorta var. latifolia Engelm. ex S. Watson) avaient une BA plus faible que les peuplements mixtes plus vieux (>250 ans). Les peuplements etablis sur des stations mesiques avaient une plus grande BA que les peuplements du meme age etablis sur des stations xeriques ou hydriques. A l'echelle de l'espece, d'importantes variations interspecifiques de la BA s'expliquaient principalement par le remplacement successionnel des pins intolerants a l'ombre par l'epicea d'Engelmann (Picea engelmannii Parry ex Engelm.) et le sapin subalpin (Abies lasiocarpa (Hook.) Nutt.), des essences tolerantes a l'ombre. Le reseau de placettes d'echantillonnage permanent a connu des variations importantes dans la dominance des especes et la densite des arbres de 1982 a 2016. Ces variations refletent des patrons successionnels evoluant sur plusieurs siecles ainsi que les effets recents et localises des chablis, des insectes et des pathogenes. Malgre l'augmentation de la mortalite des arbres, la tendance generale allait dans le sens d'un accroissement de la BA a travers la foret. [Traduit par la Redaction] Mots-cles : biomasse forestiere, foret subalpine, placettes d'echantillonnage permanent, dynamique forestiere, Front Range du Colorado., Introduction Recent climate-induced increases in tree mortality have been documented in forests worldwide and are likely to profoundly affect aboveground forest biomass and, therefore, carbon storage (van Mantgem et al. [...]

Studies of forest dynamics commonly assume that species composition of the seedling bank reflects the composition of the future forest canopy. However, many forest types exhibit persistent differences in relative dominance of species in the seedling bank versus the forest canopy. Species-specific differences in tree vital rates (e.g., in-growth, mortality, height growth, canopy residence time) across canopy positions may explain this discord in dominance between seedling banks and forest canopies. We tested for differences in tree vital rates for two widely distributed, coexisting species in subalpine forests of the Rocky Mountains, North America. We quantified seedling bank dynamics (>950 aged seedlings) and vital rates in permanent plots (>2500 trees) from 1982 to 2017 to determine if differences in vital rates explained the shift from seedling bank dominance by subalpine fir (Abies lasiocarpa (Hook.) Nutt.) to codominance of the main canopy by subalpine fir and Engelmann spruce (Picea engelmannii Parry ex Engelm.). Higher rates of fir recruitment into the main canopy were balanced by equally high rates of mortality, whereas spruce exhibited higher rates of net population increase and longer residence time in the main canopy. Projections of future forest trajectories from seedling bank composition can be improved by considering species-specific differences in vital rates.Key words: permanent forest plots, long-term study, forest demography, seedling bank, forest dynamics, Colorado Front Range.Les etudes portant sur la dynamique forestiere supposent generalement que la composition en especes de la banque de semis est le reflet de la composition du futur couvert forestier. Cependant, de nombreux types forestiers comportent des differences persistantes de dominance relative des especes entre la banque de semis et le couvert forestier. Les differences de taux vitaux des arbres (par exemple, le recrutement, la mortalite, la croissance en hauteur, le temps de sejour dans le couvert) entre les especes en fonction de la position dans le couvert peuvent expliquer ce desaccord de dominance entre la banque de semis et le couvert forestier. Nous avons examine les differences de taux vitaux des arbres entre deux especes coexistantes tres repandues dans les forets subalpines des montagnes Rocheuses, en Amerique du Nord. Nous avons quantifie la dynamique des banques de semis (>950 semis ages) et les taux vitaux dans des placettes permanentes (>2500 arbres) de 1982 a 2017 pour determiner si des differences de taux vitaux expliquaient le passage d'une banque de semis dominee par le sapin subalpin (Abies lasiocarpa (Hook.) Nutt.) a une codominance du couvert principal par le sapin subalpin et l'epinette d'Engelmann (Picea engelmannii Parry ex Engelm.). Le taux de recrutement plus eleve du sapin dans le couvert principal etait compense par un taux de mortalite tout aussi eleve, alors que l'epinette avait un taux plus eleve d'augmentation nette de la population et une plus longue duree de sejour dans le couvert principal. La projection des futures trajectoires forestieres a partir de la composition des banques de semis peut etre amelioree en tenant compte des differences de taux vitaux entre les especes. [Traduit par la Redaction]Mots-cles : placettes forestieres permanentes, etude a long terme, demographie forestiere, banque de semis, dynamique forestiere, Front Range du Colorado., IntroductionTree seedlings are the source of the future forest canopy. Studies of forest dynamics commonly assume that species composition of the seedling bank reflects the composition of the future forest [...]