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Vegetation composition shifts, and in particular, shrub expansion across the Arctic tundra are some of the most important and widely observed responses of high-latitude ecosystems to rapid climate warming. These changes in vegetation potentially alter ecosystem carbon balances by affecting a complex set of soil-plant-atmosphere interactions. In this review, we synthesize the literature on (a) observed shrub expansion, (b) key climatic and environmental controls and mechanisms that affect shrub expansion, (c) impacts of shrub expansion on ecosystem carbon balance, and (d) research gaps and future directions to improve process representations in land models. A broad range of evidence, including in-situ observations, warming experiments, and remotely sensed vegetation indices have shown increases in growth and abundance of woody plants, particularly tall deciduous shrubs, and advancing shrublines across the circumpolar Arctic. This recent shrub expansion is affected by several interacting factors including climate warming, accelerated nutrient cycling, changing disturbance regimes, and local variation in topography and hydrology. Under warmer conditions, tall deciduous shrubs can be more competitive than other plant functional types in tundra ecosystems because of their taller maximum canopy heights and often dense canopy structure. Competitive abilities of tall deciduous shrubs vs herbaceous plants are also controlled by variation in traits that affect carbon and nutrient investments and retention strategies in leaves, stems, and roots. Overall, shrub expansion may affect tundra carbon balances by enhancing ecosystem carbon uptake and altering ecosystem respiration, and through complex feedback mechanisms that affect snowpack dynamics, permafrost degradation, surface energy balance, and litter inputs. Observed and projected tall deciduous shrub expansion and the subsequent effects on surface energy and carbon balances may alter feedbacks to the climate system. Land models, including those integrated in Earth System Models, need to account for differences in plant traits that control competitive interactions to accurately predict decadal- to centennial-scale tundra vegetation and carbon dynamics.

Predation structures food webs, influences energy flow, and alters rates and pathways of nutrient cycling through ecosystems, effects that are well documented for macroscopic predators. In the microbial world, predatory bacteria are common, yet little is known about their rates of growth and roles in energy flows through microbial food webs, in part because these are difficult to quantify. Here, we show that growth and carbon uptake were higher in predatory bacteria compared to nonpredatory bacteria, a finding across 15 sites, synthesizing 82 experiments and over 100,000 taxon-specific measurements of element flow into newly synthesized bacterial DNA. Obligate predatory bacteria grew 36% faster and assimilated carbon at rates 211% higher than nonpredatory bacteria. These differences were less pronounced for facultative predators (6% higher growth rates, 17% higher carbon assimilation rates), though high growth and carbon assimilation rates were observed for some facultative predators, such as members of the genera Lysobacter and Cytophaga, both capable of gliding motility and wolf-pack hunting behavior. Added carbon substrates disproportionately stimulated growth of obligate predators, with responses 63% higher than those of nonpredators for the Bdellovibrionales and 81% higher for the Vampirovibrionales, whereas responses of facultative predators to substrate addition were no different from those of nonpredators. This finding supports the ecological theory that higher productivity increases predator control of lower trophic levels. These findings also indicate that the functional significance of bacterial predators increases with energy flow and that predatory bacteria influence element flow through microbial food webs.IMPORTANCE The word "predator" may conjure images of leopards killing and eating impala on the African savannah or of great white sharks attacking elephant seals off the coast of California. But microorganisms are also predators, including bacteria that kill and eat other bacteria. While predatory bacteria have been found in many environments, it has been challenging to document their importance in nature. This study quantified the growth of predatory and nonpredatory bacteria in soils (and one stream) by tracking isotopically labeled substrates into newly synthesized DNA. Predatory bacteria were more active than nonpredators, and obligate predators, such as Bdellovibrionales and Vampirovibrionales, increased in growth rate in response to added substrates at the base of the food chain, strong evidence of trophic control. This work provides quantitative measures of predator activity and suggests that predatory bacteria-along with protists, nematodes, and phages-are active and important in microbial food webs.

Cajander larch (Larix cajanderi Mayr.) forests of the Siberian Arctic are experiencing increased wildfire activity in conjunction with climate warming. These shifts could affect postfire variation in the density and arrangement of trees and understory plant communities. To better understand how understory plant composition, abundance, and diversity vary with tree density, we surveyed understory plant communities and stand characteristics (e.g., canopy cover, active layer depth, and soil organic layer depth) within 25 stands representing a density gradient of similarly-aged larch trees that established following a 1940 fire near Cherskiy, Russia. Understory plant diversity and mean total plant abundance decreased with increased canopy cover. Canopy cover was also the most important variable affecting individual species' abundances. In general, tall shrubs (e.g., Betula nana subsp. exilis) were more abundant in low-density stands with high light availability, and mosses (e.g., Sanionia spp.) were more abundant in high-density stands with low light availability. These results provide evidence that postfire variation in tree recruitment affects understory plant community composition and diversity as stands mature. Therefore, projected increases in wildfire activity in the Siberian Arctic could have cascading impacts on forest structure and composition in both overstory and understory plant communities. Key words: Cajander larch, Larix cajanderi, Betula nana subsp. exilis, wildfire, disturbance. Les forets de meleze cajander (Larix cajanderi Mayr.) de l'Arctique siberien connaissent une augmentation de l'activite des feux de foret conjointement avec le rechauffement climatique. Ces changements pourraient avoir un impact sur la variation apres feu de la densite et de la composition des arbres ainsi que des communautes vegetales du sous-bois. Dans le but de mieux comprendre de quelle facon la composition, l'abondance et la diversite des plantes de sous-bois varient en fonction de la densite des arbres, nous avons inventorie les communautes vegetales du sous-bois et les caracteristiques du peuplement (par example, couvert forestier, profondeur de la strate active et profondeur de l'horizon organique du sol) dans 25 peuplements representant un gradient de densite de melezes d'age similaire qui se sont etablis a la suite de l'incendie de 1940 pres de Cherskiy, en Russie. La diversite des plantes de sous-bois et l'abondance totale moyenne des plantes diminuaient avec l'augmentation du couvert forestier, lequel etait aussi la plus importante variable ayant un impact sur l'abondance des especes prises individuellement. En general, les grands arbustes (p. ex., Betula nana subsp. exilis) etaient plus abondants dans les peuplements avec une faible densite et une grande disponibilite de lumiere, et les mousses (p. ex., Sanionia spp.) etaient plus abondantes dans les peuplements avec une densite elevee et une faible disponibilite de lumiere. Ces resultats fournissent la preuve que la variation apres feu dans le recrutement des arbres a un impact sur la composition et la diversite de la communaute vegetale du sous-bois a mesure que les peuplements deviennent matures. Par consequent, les augmentations prevues de l'activite des feux de foret dans l'Arctique siberien pourraient avoir des repercussions en cascade sur la structure et la composition de la foret parmi les communautes vegetales du sous-bois et de l'etage dominant. [Traduit par la Redaction] Mots-cles : meleze cajander, Larix cajanderi, Betula nana subsp. exilis, feu de foret, perturbation., Introduction Rapid rates of global change make boreal forests particularly susceptible to forest loss and (or) transition to novel forest conditions (Johnstone et al. 2010; Scheffer et al. 2012). Boreal [...]

In the Arctic tundra, warming is anticipated to stimulate nutrient release and potentially alleviate plant nutrient limitations. Typically simulated by fertilization experiments that saturate plant nutrient demand, future increases in soil fertility are thought to favor ectomycorrhizal (EcM) over ericaceous shrubs and have often been identified as a key driver of Arctic shrub expansion. However, the projected increases in fertility will likely vary in their alleviation of nutrient limitations. The resulting responses of shrubs and their mycorrhizae across the gradient of nutrient limitation may be nonlinear and could contradict the current predictions of tundra vegetation shifts. We compared the functional responses of two dominant shrubs, EcM dwarf birch (Betula nana) and ericaceous Labrador tea (Rhododendron tomentosum), across a long‐term nitrogen and phosphorus fertilization gradient experiment in Arctic Alaska. Using linear mixed‐effects modeling, we tested the responses of shrub cover, height, and root enzyme activities to soil fertility. We found that B. nana cover and height linearly increased with soil fertility. In contrast, R. tomentosum cover initially increased, but decreased after surpassing the intermediate levels of increased soil fertility. Its height did not change. Enzyme activity did not respond to soil fertility on EcM‐colonized B. nana roots, but sharply declined on R. tomentosum roots. Overall, the nonlinear responses of shrubs to our fertility gradient demonstrate the importance of experiments grounded in replicated regression design. Our results indicate that under moderate increases in soil fertility, Arctic shrub expansion may not only include deciduous EcM shrubs but also ericaceous shrubs. Regardless of shifts aboveground, changes in root enzyme activity belowground point to some EcM shrub species playing a more influential role in tundra soils; as EcM roots remained steady in their liberation of soil organic nutrients with heightened soil fertility, degradative root enzyme activity on the dominant ericaceous shrub dropped—in some instances with even the slightest increase in fertility. [ABSTRACT FROM AUTHOR]

Summary: Root‐associated fungi (RAF) and root traits regulate plant acquisition of nitrogen (N), which is limiting to growth in Arctic ecosystems. With anthropogenic warming, a new N source from thawing permafrost has the potential to change vegetation composition and increase productivity, influencing climate feedbacks. Yet, the impact of warming on tundra plant root traits, RAF, and access to permafrost N is uncertain.We investigated the relationships between RAF, species‐specific root traits, and uptake of N from the permafrost boundary by tundra plants experimentally warmed for nearly three decades at Toolik Lake, Alaska.Warming increased acquisitive root traits of nonmycorrhizal and mycorrhizal plants. RAF community composition of ericoid (ERM) but not ectomycorrhizal (ECM) shrubs was impacted by warming and correlated with root traits. RAF taxa in the dark septate endophyte, ERM, and ECM guilds strongly correlated with permafrost N uptake for ECM and ERM shrubs. Overall, a greater proportion of variation in permafrost N uptake was related to root traits than RAF.Our findings suggest that warming Arctic ecosystems will result in interactions between roots, RAF, and newly thawed permafrost that may strongly impact feedbacks to the climate system through mechanisms of carbon and N cycling. [ABSTRACT FROM AUTHOR]

Project Chameleon is a school-based educational programme relating to crime, UK law and social moral positioning. Qualitative data were collated from three focus groups with child participants and two focus groups with university psychology student volunteers. The data related directly to the individual experiences of child participants and student volunteers supporting the facilitation of the programme. Three overarching themes emerged from the data: Personal Experiences, relating to individual learning, attitude and behavioural changes and how individuals used the information provided by the programme; Programme Delivery, which examined all aspects of how the programme was taught to child participants; and Session Content and Issues, discussing the topics and any relevant problems occurring whilst participating. Recommendations for future school-based interventions are discussed, including delivery style, interactivity, dosage and tailored content.

Climate warming and drying are modifying the fire dynamics of many boreal forests, moving them towards a regime with a higher frequency of extreme fire years characterized by large burns of high severity. Plot-scale studies indicate that increased burn severity favors the recruitment of deciduous trees in the initial years following fire. Consequently, a set of biophysical effects of burn severity on postfire boreal successional trajectories at decadal timescales have been hypothesized. Prominent among these are a greater cover of deciduous tree species in intermediately aged stands after more severe burning, with associated implications for carbon and energy balances. Here we investigate whether the current vegetation composition of interior Alaska supports this hypothesis. A chronosequence of six decades of vegetation regrowth following fire was created using a database of burn scars, an existing forest biomass map, and maps of albedo and the deciduous fraction of vegetation that we derived from Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery. The deciduous fraction map depicted the proportion of aboveground biomass in deciduous vegetation, derived using a RandomForest algorithm trained with field data sets (n=69, 71% variance explained). Analysis of the difference Normalized Burn Ratio, a remotely sensed index commonly used as an indicator of burn severity, indicated that burn size and ignition date can provide a proxy of burn severity for historical fires. LIDAR remote sensing and a bioclimatic model of evergreen forest distribution were used to further refine the stratification of the current landscape by burn severity. Our results show that since the 1950s, more severely burned areas in interior Alaska have produced a vegetation cohort that is characterized by greater deciduous biomass. We discuss the importance of this shift in vegetation composition due to climate-induced changes in fire severity for carbon sequestration in forest biomass and surface reflectance (albedo), among other feedbacks to climate. © 2011 Blackwell Publishing Ltd.

Boreal forests are an important source of wood products, and fertilizers could be used to improve forest yields, especially in nutrient poor regions of the boreal zone. With climate change, fire frequencies may increase, resulting in a larger fraction of the boreal landscape present in early-successional stages. Since most fertilization studies have focused on mature boreal forests, the response of burned boreal ecosystems to increased nutrient availability is unclear. Therefore, we used a nitrogen (N) fertilization experiment to test how C cycling in a recently-burned boreal ecosystem would respond to increased N availability. We hypothesized that fertilization would increase rates of decomposition, soil respiration, and the activity of extracellular enzymes involved in C cycling, thereby reducing soil C stocks. In line with our hypothesis, litter mass loss increased significantly and activities of cellulose- and chitin-degrading enzymes increased by 45–61% with N addition. We also observed a significant decline in C concentrations in the organic soil horizon from 19.5 ± 0.7% to 13.5 ± 0.6%, and there was a trend toward lower total soil C stocks in the fertilized plots. Contrary to our hypothesis, mean soil respiration over three growing seasons declined by 31% from 78.3 ± 6.5 mg CO2–C m−2 h−1 to 54.4 ± 4.1 mg CO2–C m−2 h−1. These changes occurred despite a 2.5-fold increase in aboveground net primary productivity with N, and were accompanied by significant shifts in the structure of the fungal community, which was dominated by Ascomycota. Our results show that the C cycle in early-successional boreal ecosystems is highly responsive to N addition. Fertilization results in an initial loss of soil C followed by depletion of soil C substrates and development of a distinct and active fungal community. Total microbial biomass declines and respiration rates do not keep pace with plant inputs. These patterns suggest that N fertilization could transiently reduce but then increase ecosystem C storage in boreal regions experiencing more frequent fires.

The canopy is host to a large percentage of the flora and fauna in tropical wet forests and is distinct from the forest floor in plant richness, soil type and microclimate. In this study, we examined the influence of tree species and season on soil nutrient cycling processes in canopy soils of four tree species common to Costa Rican wet forests. We also compared the canopy soils to the associated forest floor mineral soils. Both tree species and season had strong effects on canopy soil nutrients and processes. Canopy soils from trees with high litter lignin concentrations had higher net N-mineralization rates and higher dissolved inorganic N concentrations than those with low lignin concentrations. During the dry season, net N-immobilization occurred and dissolved organic and inorganic N and available P concentrations were significantly higher than during the wet season. Overall, canopy soils had higher N levels and higher fungi + bacteria richness than forest floor mineral soils. The differences in canopy soil properties observed among tree species indicates that these species have distinct N cycles that reflect differences in both soil origin and biological controls. © 2008 Springer Science+Business Media B.V.

Plant biomass accumulation and productivity are important determinants of ecosystem carbon (C) balance during post-fire succession. In boreal black spruce (Picea mariana) forests near Delta Junction, Alaska, we quantified aboveground plant biomass and net primary productivity (ANPP) for 4 years after a 1999 wildfire in a well-drained (dry) site, and also across a dry and a moderately well-drained (mesic) chronosequence of sites that varied in time since fire (2 to ~116 years). Four years after fire, total biomass at the 1999 burn site had increased exponentially to 160 ± 21 g m⁻² (mean ± 1SE) and vascular ANPP had recovered to 138 ± 32 g m⁻² y⁻¹, which was not different than that of a nearby unburned stand (160 ± 48 g m⁻² y⁻¹) that had similar pre-fire stand structure and understory composition. Production in the young site was dominated by re-sprouting graminoids, whereas production in the unburned site was dominated by black spruce. On the dry and mesic chronosequences, total biomass pools, including overstory and understory vascular and non-vascular plants, and lichens, increased logarithmically (dry) or linearly (mesic) with increasing site age, reaching a maximum of 2469 ± 180 (dry) and 4008 ± 233 g m⁻² (mesic) in mature stands. Biomass differences were primarily due to higher tree density in the mesic sites because mass per tree was similar between sites. ANPP of vascular and non-vascular plants increased linearly over time in the mesic chronosequence to 335 ± 68 g m⁻² y⁻¹ in the mature site, but in the dry chronosequence it peaked at 410 ± 43 g m⁻² y⁻¹ in a 15-year-old stand dominated by deciduous trees and shrubs. Key factors regulating biomass accumulation and production in these ecosystems appear to be the abundance and composition of re-sprouting species early in succession, the abundance of deciduous trees and shrubs in intermediate aged stands, and the density of black spruce across all stand ages. A better understanding of the controls over these factors will help predict how changes in climate and fire regime will affect the carbon balance of Interior Alaska.

Nutrient availability in the arctic is expected to increase in the next century due to accelerated decomposition associated with warming and, to a lesser extent, increased nitrogen deposition. To explore how changes in nutrient availability affect ecosystem carbon (C) cycling, we used radiocarbon to quantify changes in belowground C dynamics associated with long-term fertilization of graminoid-dominated tussock tundra at Toolik Lake, Alaska. Since 1981, yearly fertilization with nitrogen (N) and phosphorus (P) has resulted in a shift to shrub-dominated vegetation. These combined changes have altered the quantity and quality of litter inputs, the vertical distribution and dynamics of fine roots, and the decomposition rate of soil organic C. The loss of C from the deep organic and mineral soil has more than offset the C accumulation in the litter and upper organic soil horizons. In the litter and upper organic horizons, radiocarbon measurements show that increased inputs resulted in overall C accumulation, despite being offset by increased decomposition in some soil pools. To reconcile radiocarbon observations in the deeper organic and mineral soil layers, where most of the ecosystem C loss occurred, both a decrease in input of new root material and a dramatic increase of decomposition rates in centuries-old soil C pools were required. Therefore, with future increases in nutrient availability, we may expect substantial losses of C which took centuries to accumulate. © 2007 Springer Science+Business Media, LLC.

Mycorrhizal fungi can contribute to soil carbon sequestration by immobilizing carbon in living fungal tissues and by producing recalcitrant compounds that remain in the soil following fungal senescence. We hypothesized that nitrogen (N) fertilization would decrease these carbon stocks, because plants should reduce investment of carbon in mycorrhizal fungi when N availability is high. We measured the abundance of two major groups of mycorrhizal fungi, arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) fungi, in the top 10 cm of soil in control and N-fertilized plots within three Alaskan boreal ecosystems that represented different recovery stages following severe fire. Pools of mycorrhizal carbon included root-associated AM and ECM structures; soil-associated AM hyphae; and glomalin, a glycoprotein produced by AM fungi. Total mycorrhizal carbon pools decreased by approximately 50 g C m⁻² in the youngest site under N fertilization, and this reduction was driven mostly by glomalin. Total mycorrhizal carbon did not change significantly in the other sites. Root-associated AM structures were more abundant under N fertilization across all sites, and root-associated ECM structures increased marginally significantly. We found no significant N effects on AM hyphae. Carbon sequestered within living mycorrhizal structures (0.051-0.21 g m⁻²) was modest compared with that of glomalin (33-203 g m⁻²). We conclude that our hypothesis was only supported in relation to glomalin stocks within one of the three study sites. As N effects on glomalin were inconsistent among sites, an understanding of the mechanisms underlying this variation would improve our ability to predict ecosystem feedbacks to global change.

Climate change is driving substantial changes in North American boreal forests, including changes in productivity, mortality, recruitment, and biomass. Despite the importance for carbon budgets and informing management decisions, there is a lack of near‐term (5–30 year) forecasts of expected changes in aboveground biomass (AGB). In this study, we forecast AGB changes across the North American boreal forest using machine learning, repeat measurements from 25,000 forest inventory sites, and gridded geospatial datasets. We find that AGB change can be predicted up to 30 years into the future, and that training on sites across the entire domain allows accurate predictions even in regions with only a small amount of existing field data. While predicting AGB loss is less skillful than gains, using a multi‐model ensemble can improve the accuracy in detecting change direction to >90% for observed increases, and up to 70% for observed losses. Higher stem density, winter temperatures, and the presence of temperate tree species in forest plots were positively associated with AGB change, whereas greater initial biomass, continentality (difference between mean summer and winter temperatures), prevalence of black spruce (Picea mariana), summer precipitation, and early warning metrics from long‐term remote sensing time series were negatively associated with AGB change. Across the domain, we predict nondisturbance‐induced declines in AGB at 23% of sites by 2030. The approach developed here can be used to estimate near‐future forest biomass in boreal North America and inform relevant management decisions. Our study also highlights the power of machine learning multi‐model ensembles when trained on a large volume of forest inventory plots, which could be applied to other regions with adequate plot density and spatial coverage. [ABSTRACT FROM AUTHOR]

Fire is an important pathway for carbon (C) loss from boreal forest ecosystems and has a strong effect on ecosystem C balance. Fires can range widely in severity, defined as the amount of vegetation and forest floor consumed by fire, depending on local fuel and climatic conditions. Here we explore a novel method for estimating fire severity and loss of C from fire using the atmosphere to integrate ecosystem heterogeneity at the watershed scale. We measured the δ13C and Δ14C isotopic values of CO2 emitted from an experimental forest fire at the Caribou-Poker Creek Research Watershed (CPCRW), near Fairbanks, Alaska. We used inverse modeling combined with dual isotope measurements of C contained in aboveground black spruce biomass and soil organic horizons to estimate the amount of C released by this fire. The experimental burn was a medium to severe intensity fire that released, on average, about 2.5 kg Cm−2, more than half of the C contained in vegetation and soil organic horizon pools. For vegetation, the model predicted that approximately 70–75% of pools such as needles, fine branches, and bark were consumed by fire, whereas only 20–30% of pools such as coarse branches and cones were consumed. The fire was predicted to have almost completely consumed surface soil organic horizons and burned about half of the deepest humic horizon. The ability to estimate the amount of biomass combusted and C emission from fires at the watershed scale provides an extensive approach that can complement more limited intensive ground-based measurements.