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Decadal scale lake drying in interior Alaska results in lake margin colonization by willow shrub and graminoid vegetation, but the effects of these changes on plant production, biodiversity, soil properties, and soil microbial communities are not well known. We studied changes in soil organic carbon (SOC) and nitrogen (N) storage, plant and microbial community composition, and soil microbial activities in drying and non‐drying lakes in the Yukon Flats National Wildlife Refuge. Historic changes in lake area were determined using Landsat imagery. Results showed that SOC storage in drying lake margins declined by 0.13 kg C m−2 yr−1 over 30 years of exposure of lake sediments, with no significant change in soil N. Lake drying resulted in an increase in graminoid and shrub aboveground net primary production (ANPP, +3% yr−1) with little change in plant functional composition. Increases in ANPP were similar in magnitude (but opposite in sign) to losses in SOC over a 30‐year drying trend. Potential decomposition rates and soil enzyme activities were lower in drying lake margins compared to stable lake margins, possibly due to high salinities in drying lake margin soils. Microbial communities shifted in response to changing plant communities, although they still retained a legacy of the previous plant community. Understanding how changing lake hydrology impacts the ecology and biogeochemistry of lake margin terrestrial ecosystems is an underexamined phenomenon with large impacts to landscape processes. Plain Language Summary: Lake dominated ecosystems in Alaska have experienced periods of widespread lake drying in the past. We understand little of what occurs on these new nearshore habitats following lake drying, especially changes in soil properties and soil microbial communities. Using a combination of remote sensing and field sampling on hundreds of lakes in the Yukon Flats National Wildlife Refuge, we learned that lake margin soils lose significant quantities of soil C in the decades following lake drying, and at the same time soils become increasingly saline and soil microbial activites are reduced. Overall, lake drying has large impacts on the surrounding ecosystems and it is important to understand these changes for management. Key Points: Lake drying in the Yukon Flats National Wildlife Refuge triggers a succession of plant and microbial communities and losses of lake margin soil organic carbon (SOC)Lake drying also results in increases in the salinity of lake margin soils, slowing rates of microbial activitiesAlthough lake drying resulted in losses of SOC, increases in Aboveground Net Primary Production (ANPP) compensated for these losses [ABSTRACT FROM AUTHOR]

Studies in recent decades have shown strong evidence of physical and biological changes in the Arctic tundra, largely in response to rapid rates of warming. Given the important implications of these changes for ecosystem services, hydrology, surface energy balance, carbon budgets, and climate feedbacks, research on the trends and patterns of these changes is becoming increasingly important and can help better constrain estimates of local, regional, and global impacts as well as inform mitigation and adaptation strategies. Despite this great need, scientific understanding of tundra ecology and change remains limited, largely due to the inaccessibility of this region and less intensive studies compared to other terrestrial biomes. A synthesis of existing datasets from past field studies can make field data more accessible and open up possibilities for collaborative research as well as for investigating and informing future studies. Here, we synthesize field datasets of vegetation and active-layer properties from the Alaskan tundra, one of the most well-studied tundra regions. Given the potentially increasing intensive fire regimes in the tundra, fire history and severity attributes have been added to data points where available. The resulting database is a resource that future investigators can employ to analyze spatial and temporal patterns in soil, vegetation, and fire disturbance-related environmental variables across the Alaskan tundra. This database, titled the Synthesized Alaskan Tundra Field Database (SATFiD), can be accessed at the Oak Ridge National Laboratory Distributed Active Archive Center (ORNL DAAC) for Biogeochemical Dynamics (Chen et al., 2023: 10.3334/ORNLDAAC/2177). [ABSTRACT FROM AUTHOR]

Arctic tundra largely in response to exceptionally rapid rates of warming. Given the important implications of these changes on ecosystem services, hydrology, surface energy balance, carbon budgets, and climate feedbacks, research on the trends and patterns of these changes is becoming increasingly important and can help better constrain estimates of local, regional, and global impacts as well as inform mitigation and adaptation strategies. Despite this high need, scientific understanding of tundra ecology and change remains limited largely due to the inaccessibility of this region and less intensive study compared to other terrestrial biomes. A synthesis of existing datasets from past field studies can make field data more accessible and open up possibilities for collaborative research as well as for investigating and informing future studies. Here, we synthesize field datasets of vegetation, and active layer properties from the Alaskan tundra, one of the most well-studied tundra regions. Given the potential increasingly intensive fire regimes in the tundra, fire history and severity attributes have been added to data points where available. The resulting database is a resource that future investigators can employ to analyze spatial and temporal patterns in soil, vegetation, and fire disturbance-related environmental variables across the Alaskan tundra. This database, titled Synthesized Alaskan Tundra Field Database (SATFiD), can be accessed at the Oak Ridge National Laboratory Distributed Active Archive Center (ORNL DAAC) for Biogeochemical Dynamics (Chen et al., 2023: https://doi.org/10.3334/ORNLDAAC/2177). [ABSTRACT FROM AUTHOR]

Northern peatlands represent a long-term net sink for atmospheric C[O.sub.2], but these ecosystems can shift from net carbon (C) sinks to sources based on changing climate and environmental conditions. In particular, changes in water availability associated with climate control peatland vegetation and carbon uptake processes. We examined the influence of changing hydrology on plant species abundance and ecosystem primary production in an Alaskan fen by manipulating the water table in field treatments to mimic either sustained flooding (raised water table) or drought (lowered water table) conditions for 6 years. We found that water table treatments altered plant species abundance by increasing sedge and grass cover in the raised water table treatment and reducing moss cover while increasing vascular green area in the lowered water table treatment. Gross primary productivity was lower in the lowered treatment than in the other plots, although there were no differences in total biomass or vascular net primary productivity among the treatments. Overall, our results indicate that vegetation abundance was more sensitive to variation in water table than total biomass and vascular biomass accrual. Finally, in our experimental peatland, drought had stronger consequences for change in vegetation abundance and ecosystem function than sustained flooding. Key words: primary productivity, peatland, hydrologic manipulation, drying, flooding. A long terme les tourbieres nordiques constituent un puits net pour le C[O.sub.2] atmospherique mais ces ecosystemes peuvent passer d'un puits a une source nette de carbone (C) a cause du changement climatique et des conditions environnementales. Plus particulierement, les modifications de la disponibilite en eau associees a l'effet du climat influencent la vegetation des tourbieres et les processus d'absorption de C. Nous avons etudie l'influence de la modification du regime hydrique sur l'abondance des especes vegetales et la production primaire de l'ecosysteme dans une tourbiere basse en Alaska. Le regime hydrique a ete modifie en manipulant la nappe phreatique avec des traitements sur le terrain visant a reproduire les conditions d'une inondation (elevation de la nappe phreatique) ou d'une secheresse (abaissement de la nappe phreatique) qui ont persiste pendant 6 ans. Nous avons observe que les modifications de la nappe phreatique ont influence l'abondance des especes vegetales : la couverture de carex et de graminees a augmente alors que la couverture de mousses a diminue ou la nappe phreatique avait ete elevee tandis que la superficie occupee par les plantes vasculaires a augmente ou la nappe phreatique avait ete abaissee. La production primaire brute etait plus faible ou la nappe phreatique avait ete abaissee que dans les autres parcelles mais il n'y avait pas de differences entre les traitements quant a la biomasse totale ou la productivite primaire nette des plantes vasculaires. Globalement, nos resultats indiquent que l'abondance de la vegetation est plus sensible a la variation de la nappe phreatique que la biomasse totale et l'accroissement de la biomasse des plantes vasculaires. Finalement, dans notre tourbiere experimentale la secheresse a davantage contribue a modifier l'abondance de la vegetation et la fonction de l'ecosysteme que l'inondation persistante. [Traduit par la Redaction] Mots-cles: productivite primaire, tourbiere, manipulation hydrologique, secheresse, inondation., Introduction Carbon (C) sequestration in the biosphere plays a substantial role in offsetting increases of atmospheric C from anthropogenic sources (Esser et al. 2011). Despite a small global land area, [...]

We used a time series of 1986-2009 Landsat sensor data to compute the Normalized Difference Vegetation Index (NDVI) for 30 m pixels within the Bonanza Creek Experimental Forest of interior Alaska. Based on simple linear regression, we found significant (p < 0.05) declining trends in mean NDVI of three dominant landscape types of floodplains, lowlands, and uplands. At smaller patch sizes, similar declining trends occurred among topographic classes of north-and south-facing slopes and valley bottoms and among forest classes, including black spruce (Picea mariana (Mill.) B.S.P.). Significant positive trends in mean NDVI occurred only in areas that were recently burned, whereas wetlands had no significant trend. The greatest departure from the NDVI trend line occurred following the 2004 drought for all forest classes except black spruce, which dominates the coldest sites, and balsam poplar (Populus balsamifera L.), which occurs on low, moist terraces within the Tanana River floodplain. The consistent long-term declining trend at several spatial scales may be due to a regional climatic regime shift that occurred in the mid-1970s. Resume: Dans une perspective circumpolaire boreale, l'interieur de l'Alaska est un point chaud de la baisse de productivite de la vegetation et de la tendance au brunissement selon l'indice de vegetation par difference normalisee (IVDN) obtenu par la teledetection. Du point de vue de l'Alaska, plusieurs etudes ont montre que la tendance au brunissement etait plus pro-noncee dans la region boreale orientale, ce qui en Alaska est coherant avec un gradient climatique d'ouest en est. Cepen-dant, on possede peu d'information au sujet de la variation a l'echelle du paysage de la baisse de l'IVDN a des echelles spatiales plus fines (c.-a-d., 1 ha ou moins). Nous avons utilise des series temporelles du capteur de l'IVDN de Landsat de 1986 a 2009 pour etudier l'evolution de l'IVDN parmi une hierarchie spatiale de classes de paysage, de topographie et de vegetation a la foret experimentale de Bonanza Creek a l'interieur de l'Alaska. En utilisant une regression lineaire simple, nous avons trouve des tendances a la baisse significatives (p < 0,05) de l'IVDN moyen de trois types dominants de paysage de plaine inondable, de basses terres et de hautes terres. A l'echelle de plus petites parcelles de terrain, des tendances similaires a la baisse sont apparues parmi les classes topographiques des pentes exposees au nord et au sud et du fond des val-lees, ainsi que des classes de foret incluant l'epicea noir (Picea mariana (Mill.) Britton, Sterns, Poggenb.). Des tendances positives significatives de l'IVDN moyen ont ete observees seulement dans les zones recemment brulees tandis que les zo-nes humides n'etaient associees a aucune tendance significative. L'ecart le plus important de la courbe d'evolution de l'IVDN est survenu a la suite de la secheresse de 2004 dans toutes les classes de foret a l'exception de l'epicea noir qui do-mine dans les stations les plus froides et du peuplier baumier (Populus balsamifera L.) qui croit sur les terrasses basses et humides dans la plaine inondable de la riviere Tanana. La tendance a la baisse constante a long terme a plusieurs echelles spatiales pourrait etre due a un changement dans le regime du climat regional qui est survenu au milieu des annees 1970. [Traduit par la Redaction], Introduction High-latitude biomes are currently experiencing warming faster than most locations on earth (Arctic Council and the International Arctic Science Committee (IASC) 2005). In boreal Alaska, warming since the 1950s [...]

Arctic tundra soils store large amounts of carbon (C) in organic soil layers hundreds to thousands of years old that insulate, and in some cases maintain, permafrost soils (1,2). Fire has been largely absent from most of this biome since the early Holocene epoch (3), but its frequency and extent are increasing, probably in response to climate warming (4). The effect of fires on the C balance of tundra landscapes, however, remains largely unknown. The Anaktuvuk River fire in 2007 burned 1,039 square kilometres of Alaska's Arctic slope, making it the largest fire on record for the tundra biome and doubling the cumulative area burned since 1950 (ref. 5). Here we report that tundra ecosystems lost 2,016 ± 435 g C [m.sup.-2] in the fire, an amount two orders of magnitude larger than annual net C exchange in undisturbed tundra (6). Sixty per cent of this C loss was from soil organic matter, and radiocarbon dating of residual soil layers revealed that the maximum age of soil C lost was 50 years. Scaled to the entire burned area, the fire released approximately 2.1 teragrams of C to the atmosphere, an amount similar in magnitude to the annual net C sink for the entire Arctic tundra biome averaged over the last quarter of the twentieth century (7). The magnitude of ecosystem C lost by fire, relative to both ecosystem and biome-scale fluxes, demonstrates that a climate-driven increase in tundra fire disturbance may represent a positive feedback, potentially offsetting Arctic greening (8) and influencing the net C balance of the tundra biome., The Arctic terrestrial C cycle has the potential to influence global climate through feedbacks to recent warming, but the net sign, magnitude and pace of these feedbacks remains uncertain (7,9,10). [...]

In the boreal forests of interior Alaska, feedbacks that link forest soils, fire characteristics, and plant traits have supported stable cycles of forest succession for the past 6000 years. This high resilience of forest stands to fire disturbance is supported by two interrelated feedback cycles: (i) interactions among disturbance regime and plant--soil--microbial feedbacks that regulate soil organic layer thickness and the cycling of energy and materials, and (ii) interactions among soil conditions, plant regeneration traits, and plant effects on the environment that maintain stable cycles of forest community composition. Unusual fire events can disrupt these cycles and trigger a regime shift of forest stands from one stability domain to another (e.g., from conifer to deciduous forest dominance). This may lead to abrupt shifts in forest cover in response to changing climate and fire regime, particularly at sites with intermediate levels of moisture availability where stand-scale feedback cycles are only weakly constrained by environmental conditions. However, the loss of resilience in individual stands may foster resilience at the landscape scale, if changes in the landscape configuration of forest cover types feedback to stabilize regional patterns of fire behavior and climate conditions. Resume: Dans la foret boreale de l'interieur de l'Alaska, les retroactions qui relient les sols forestiers, les caracteristiques des incendies et celles de la vegetation ont assure le maintien des cycles de succession forestiere pendant les 6000 dernieres annees. Cette grande resilience des peuplements forestiers face aux perturbations causees par le feu est le resultat de deux cycles interrelies de retroactions: (i) les interactions entre le regime de perturbations et les retroactions entre les plantes, le sol et les microbes qui controlent l'epaisseur de l'horizon organique du sol et le recyclage de l'energie et des materiaux et (ii) les interactions entre les conditions du sol, les caracteristiques de la regeneration des plantes et les effets des plantes sur l'environnement qui maintiennent la stabilite des cycles de composition des communautes forestieres. Les incendies inhabituels peuvent perturber ces cycles et declencher un changement de regime des peuplements forestiers d'un domaine de stabilite a un autre (p. ex., d'une foret dominee par des coniferes a une foret dominee par des feuillus). Cela peut entrainer des changements abrupts dans le couvert forestier en reponse aux changements climatiques et a la modification du regime de feux, particulierement dans les stations avec un niveau intermediaire de disponibilite en eau ou les cycles de retroaction a l'echelle du peuplement sont assujettis seulement faiblement aux conditions environnementales. Cependant, la perte de resilience de peuplements particuliers peut favoriser la resilience a l'echelle du paysage si les changements dans la configuration des types de couvert forestier a l'echelle du paysage ont un effet de retroaction qui stabilise les patrons regionaux du comportement du feu et des conditions climatiques. [Traduit par la Redaction], Introduction Earth is undergoing pronounced environmental changes that are strongly influenced by human activities. Climate warming in recent decades is particularly pronounced at high latitudes, and past carbon emissions from [...]

Shifts in moss communities may affect the resilience of boreal ecosystems to a changing climate because of the role of moss species in regulating soil climate and biogeochemical cycling. Here, we use long-term data analysis and literature synthesis to examine the role of moss in ecosystem succession, productivity, and decomposition. In Alaskan forests, moss abundance showed a unimodal distribution with time since fire, peaking 30-70 years post-fire. We found no evidence of mosses compensating for low vascular productivity in low-fertility sites at large scales, although a trade-off between moss and vascular productivity was evident in intermediate-productivity sites. Mosses contributed 48% and 20% of wetland and upland productivity, respectively, but produced tissue that decomposed more slowly than both nonwoody and woody vascular tissues. Increasing fire frequency in Alaska is likely to favor feather moss proliferation and decrease Sphagnum abundance, which will reduce soil moisture retention and decrease peat accumulation, likely leading to deeper burning during wildfire and accelerated permafrost thaw. The roles of moss traits in regulating key aspects of boreal performance (ecosystem N supply, C sequestration, permafrost stability, and fire severity) represent critical areas for understanding the resilience of Alaska's boreal forest region under changing climate and disturbance regimes. Resume: Des changements dans les communautes de mousses peuvent alterer la resistance des ecosystemes boreaux aux changements climatiques a cause du role des especes de mousses dans la regulation du pedoclimat et le recyclage biogeochimique. Dans cette etude, nous avons eu recours a l'analyse de donnees a long terme et a une synthese de la litterature pour etudier le role des mousses dans la succession, la productivite et la decomposition dans les ecosystemes. Dans les fore ts de l'Alaska, l'abondance des mousses a une distribution unimodale dans le temps avec un maximum qui survient 30-70 ans apres un feu. Nous n'avons pas trouve d'indice a grande echelle demontrant que les mousses compensent la faible productivite des plantes vasculaires dans les stations a faible productivite. Par contre, un compromis entre la productivite des mousses et celle des plantes vasculaires etait evident dans les stations a productivite intermediaire. Les mousses etaient responsables de respectivement 48% et 20% de la productivite des zones humides et seches, mais elles produisaient des tissus qui se decomposent plus lentement que les tissus vasculaires photosynthetiques et ligneux. L'augmentation de la frequence des feux en Alaska va probablement favoriser la proliferation des mousses hypnacees et diminuer l'abondance des Sphagnum. Cela va diminuer la retention de l'humidite dans le sol et reduire l'accumulation de tourbe, ce qui entrainera probablement un brulage plus en profondeur lors de feux de foret et la fonte acceleree du pergelisol. Les roles associe aux caracteristiques des mousses dans la regulation des aspects cles de la performance des ecosystemes boreaux (l'apport de N dans l'ecosysteme, la sequestration de C, la stabilite du pergelisol et la severite du feu) representent des domaines cruciaux pour comprendre la resilience de la region de la foret boreale de l'Alaska face aux changements du climat et des regimes de perturbation. [Traduit par la Redaction], Introduction Mosses, one of the major groups of bryophytes, are ubiquitous and dominant components of ground-layer vegetation in both upland forests and peatlands across the boreal biome. These plants have [...]

Along the Tanana River floodplain, several turning points have been suggested to characterize the changes in ecosystem structure and function that accompany plant community changes through primary succession. In the past, much of this research focused on a presumed chronosequence that uses space for time substitutions. Within this chronosequence, permanent vegetation plots repeatedly measured over time provide an excellent test of the turning points model. We analyzed both canopy and understory vegetation data collected since 1987 in the Bonanza Creek Experimental Forest at the Bonanza Creek Long Term Ecological Research site to address the following questions: (i) Do long-term changes in the densities of seedling, sapling, and mature trees and shrubs of the dominant woody taxa at each successional stage support the turning points model? (ii) How does the entire plant community change with time at each hypothesized turning point? (iii) Do we see evidence of directional and synchronous shifts in species composition across successional stages? We conclude that some aspects of vegetation change during the last 25 years were consistent with the turning points model; however, many changes were not consistent, indicating the potential roles of biological, environmental, landscape, and climate controls in vegetation patterns. Resume: Le long de la plaine inondable de la riviere Tanana, plusieurs moments decisifs ont ete suggeres pour caracteriser les changements de fonction et de structure de l'ecosysteme qui accompagnent les changements dans la communaute vegetale tout au long de la succession primaire. Dans le passe, la plupart des travaux de recherche ont mis l'accent sur une chronosequence presumee qui suit une approche synchronique. A l'interieur de cette chronosequence, des placettes echantillons permanentes de la vegetation, mesurees a plusieurs reprises dans le temps, constituent un excellent test pour le modele des moments decisifs. Nous avons analyse des donnees de vegetation de la canopee et du sous-etage collectees depuis 1987 dans la foret experimentale de Bonanza Creek, a la station de recherche a long terme en ecologie de Bonanza Creek, dans le but de s'attaquer aux questions suivantes: (i) Est-ce que les changements a long terme de densite des semis, des gaules et des arbres matures ainsi que des arbustes des taxons ligneux dominants a chaque stade de succession supportent le modele des moments decisifs? (ii) De quelle facon la communaute vegetale en entier change avec le temps a chaque moment decisif presume? (iii) Voit-on des signes de changements directionnels et synchrones dans la composition en especes a travers les stades de succession? Nous arrivons a la conclusion que certains aspects de la modification de la vegetation au cours des derniers 25 ans ont ete coherents avec le modele des moments decisifs. Cependant, plusieurs changements n'etaient pas coherents, indiquant que des facteurs biologiques, environnementaux, paysagers et climatiques pourraient avoir un effet determinant sur les patrons de vegetation. [Traduit par la Redaction], Introduction The trajectory and rate of primary succession are influenced by interactions among stochastic events, community-level processes, and species life history traits (Billings 1938; Egler 1954; Connell and Slatyer 1977; [...]

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Northern peatlands represent a long-term net sink for atmospheric CO2, but these ecosystems can shift from net carbon (C) sinks to sources based on changing climate and environmental conditions. In particular, changes in water availability associated with climate control peatland vegetation and carbon uptake processes. We examined the influence of changing hydrology on plant species abundance and ecosystem primary production in an Alaskan fen by manipulating the water table in field treatments to mimic either sustained flooding (raised water table) or drought (lowered water table) conditions for 6 years. We found that water table treatments altered plant species abundance by increasing sedge and grass cover in the raised water table treatment and reducing moss cover while increasing vascular green area in the lowered water table treatment. Gross primary productivity was lower in the lowered treatment than in the other plots, although there were no differences in total biomass or vascular net primary productivity among the treatments. Overall, our results indicate that vegetation abundance was more sensitive to variation in water table than total biomass and vascular biomass accrual. Finally, in our experimental peatland, drought had stronger consequences for change in vegetation abundance and ecosystem function than sustained flooding. [ABSTRACT FROM AUTHOR]