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stage of the vegetation determines the radiative budget, energy balance partitioning, evapotranspiration and carbon dioxide flux. Here, we synthesize energy balance measurements from across the western boreal zone of North America as a function of stand age following fire. The data are from 22 sites in Alaska, Saskatchewan and Manitoba collected between 1998 and 2004 for a 150-year forest chronosequence. The summertime albedo immediately after a fire is about 0.05, increasing to about 0.12 for a period of about 30 years and then averaging about 0.08 for mature coniferous forests. A mature deciduous (aspen) forest has a higher summer albedo of about 0.16. Wintertime albedo decreases from a high of 0.7 for 5- to 30-year-old forests to about 0.2 for mature forests (deciduous and coniferous). Summer net radiation normalized to incoming solar radiation is lower in successional forests than in more mature forests by about 10%, except for the first 1–3 years after fire. This reduction in net radiative forcing is about 12–24 W m−2 as a daily average in summer (July). The summertime daily Bowen ratio exceeds 2 immediately after the fire, decreasing to about 0.5 for 15-year-old forests, with a wide range of 0.3–2 for mature forests depending on the forest type and soil water status. The magnitude of these changes is relatively large and may affect local, regional and perhaps global climates. Although fire has always determined stand renewal in these forests, increased future area burned could further alter the radiation balance and energy partitioning, causing a cooling feedback to counteract possible warming from carbon dioxide released by boreal fires.

This paper assesses the resilience of Alaska's boreal forest system to rapid climatic change. Recent warming is associated with reduced growth of dominant tree species, plant disease and insect outbreaks, warming and thawing of permafrost, drying of lakes, increased wildfire extent, increased postfire recruitment of deciduous trees, and reduced safety of hunters traveling on river ice. These changes have modified key structural features, feedbacks, and interactions in the boreal forest, including reduced effects of upland permafrost on regional hydrology, expansion of boreal forest into tundra, and amplification of climate warming because of reduced albedo (shorter winter season) and carbon release from wildfires. Other temperature-sensitive processes for which no trends have been detected include composition of plant and microbial communities, long-term landscape-scale change in carbon stocks, stream discharge, mammalian population dynamics, and river access and subsistence opportunities for rural indigenous communities. Projections of continued warming suggest that Alaska's boreal forest will undergo significant functional and structural changes within the next few decades that are unprecedented in the last 6000 years. The impact of these social--ecological changes will depend in part on the extent of landscape reorganization between uplands and lowlands and on policies regulating subsistence opportunities for rural communities. Resume: Cet article evalue la resilience du systeme forestier boreal de l'Alaska face aux changements rapides du climat. Le rechauffement recent est associe a la reduction de croissance des especes d'arbre dominantes, aux maladies des plantes et aux epidemies d'insectes, au rechauffement et a la fonte du pergelisol, a l'assechement des lacs, a l'augmentation de l'etendue des incendies de forets, au recrutement accru d'especes feuillues apres un feu et a la diminution de la securite des chasseurs qui se deplacent sur le lit glace des rivieres. Ces changements ont modifie des caracteristiques structurales, des retroactions et des interactions fondamentales dans la foret boreale, incluant la reduction des effets du pergelisol des hautes terres sur l'hydrologie regionale, l'expansion de la foret boreale vers la toundra et l'amplification du rechauffement climatique a cause de la diminution de l'albedo (saison hivernale plus courte) et des emissions de carbone provenant des incendies de foret. D'autres processus sensibles a la temperature pour lesquels aucune tendance n'a ete detectee incluent la composition des communautes vegetales et microbiennes, les changements a long terme a l'echelle du paysage dans les stocks de carbone, le debit des cours d'eau, la dynamique de population des mammiferes ainsi que l'acces aux rivieres et les perspectives de subsistance des communautes indigenes rurales. Les projections concernant la poursuite du rechauffement indiquent que la foret boreale de l'Alaska va subir au cours des prochaines decennies des changements fonctionnels et structuraux sans preecedents depuis les 6000 dernieeres anneees. L'impact de ces changements socio-ecologiques depend en partie de l'ampleur de la reorganisation du paysage entre les zones seches et humides et des politiques qui regissent les perspectives de subsistance des communautes rurales. [Traduit par la REdaction], Introduction The boreal forest is the northernmost and coldest forested biome. It is underlain by discontinuous permafrost (permanently frozen ground) that governs stand-level biogeochemistry and regional hydrology. The continental climate [...]

In the boreal forests of Alaska, recent changes in climate have influenced the exchange of trace gases, water, and energy between these forests and the atmosphere. These changes in the structure and function of boreal forests can then feed back to impact regional and global climates. In this manuscript, we examine the type and magnitude of the climate feedbacks from boreal forests in Alaska. Research generally suggests that the net effect of a warming climate is a positive regional feedback to warming. Currently, the primary positive climate feedbacks are likely related to decreases in surface albedo due to decreases in snow cover. Fewer negative feedbacks have been identified, and they may not be large enough to counterbalance the large positive feedbacks. These positive feedbacks are most pronounced at the regional scale and reduce the resilience of the boreal vegetation--climate system by amplifying the rate of regional warming. Given the recent warming in this region, the large variety of associated mechanisms that can alter terrestrial ecosystems and influence the climate system, and a reduction in the boreal forest resilience, there is a strong need to continue to quantify and evaluate the feedback pathways. Resume: Dans la forets boreale de l'Alaska, les recents changements climatiques ont influence les echanges de gaz a l'etat de trace, d'eau et d'energie entre ces forets et l'atmosphere. Ces changements dans la structure et la fonction des forets boreales peuvent en retour avoir un impact sur le climat regional et mondial. Dans cet article, nous examinons le type et l'ampleur des retroactions climatiques des forets boreales de l'Alaska. De facon generale, les travaux de recherche indiquent que l'effet net d'un rechauffement climatique est une retroaction regionale positive au rechauffement. Actuellement, les principales retroactions climatiques positives sont probablement reliees a la diminution de l'albedo en surface due a la reduction du couvert nival. Moins de retroactions negatives ont ete identifiees et elles ne sont peut-etre pas assez importantes pour contrebalancer les importantes retroactions positives. Ces retroactions positives sont les plus fortes a l'echelle regionale et reduisent la resilience du systeme vegetation--climat boreal en amplifiant le taux de rechauffement regional. Etant donne le recent rechauffement dans cette region, la grande variete de mecanismes associees qui peuvent modifier les ecosystemes terrestres et influencer le systeme climatique, et une reduction de la resilience de la foret boreale, il est imperatif de continuer a quantifier et a evaluer les modes de retroaction. [Traduit par la Redaction], Introduction Recent warming in northern high latitudes by 2-3 °C over the last 50 years has been about five times greater than the global mean (Arctic Climate Impact Assessment 2004). [...]

Abstract: The boreal forest is the second largest terrestrial biome, and the black spruce (Picea mariana (Mill.) BSP) forest type occupies a large extent of boreal North America. Black spruce [...]

A major challenge in predicting Earth's future climate state is to understand feedbacks that alter greenhouse-gas forcing. Here we synthesize field data from arctic Alaska, showing that terrestrial changes in summer albedo contribute substantially to recent high-latitude warming trends. Pronounced terrestrial summer warming in arctic Alaska correlates with a lengthening of the snow-free season that has increased atmospheric heating locally by about 3 watts per square meter per decade (similar in magnitude to the regional heating expected over multiple decades from a doubling of atmospheric C[O.sub.2]). The continuation of current trends in shrub and tree expansion could further amplify this atmospheric heating by two to seven times., The Arctic provides a test bed to understand and evaluate the consequences of threshold changes in regional system dynamics. Over the past several decades, the Arctic has warmed strongly in [...]

Variation in competitive ability due to variation in soil characteristics is one possible mechanism allowing the local coexistence of plant species. We measured soil water, depth, and nitrogen pools and fluxes in distinct patches of three serpentine grassland species to determine whether soil heterogeneity existed and was correlated with plant species abundance. Through experimental manipulation of species' abundances, we also examined the relative importance of inherent site characteristics vs. plant species' effects in generating heterogeneity in the measured soil characteristics; and measured species' competitive abilities in different patch types. The three common grassland annuals, Calycadenia multi-glandulosum, Plantago erecta, and Lasthenia californica, were segregated with respect to the measured soil characteristics. Differences in soil water, soil depth, soil microbial nitrogen, and soil carbon to nitrogen ratio were due to inherent site characteristics, while differences in nitrate availability were strongly affected by the identity of the species currently growing in a soil patch. Furthermore, all species performed significantly better against one other species in the patch type where they are normally most abundant. These results demonstrate that species diversity within this grassland contributes to soil heterogeneity and suggest that soil heterogeneity could contribute to the coexistence of these species. Key words: California grassland; Calycadenia multiglandulosum; Lasthenia californica; plant coexistence and competition; Plantago erecta; soil characteristics; soil heterogeneity; species diversity., INTRODUCTION Field experiments have established that moderate to strong competition between plants exists in a wide variety of natural communities (Connell 1983, Schoener 1983, Fowler 1986, Goldberg and Barton 1992, [...]

A model of nutrient uptake was used to determine the factors that control the uptake of ammonium, glycine, and nitrate for an important arctic sedge, Eriophorunt vaginatum, because the factors that regulate the nitrogen (N) uptake of plants in natural ecosystems are not well understood and a growing number of studies suggest that organic forms of N, including glycine, are an important source of N for E. vaginatum and other plants. E. vaginatum was selected as an exemplary system to explore nitrogen (N) uptake of a native species in situ, because it appears to be strongly N limited in the field, its N nutrition has been extensively studied, and its root growth habits make it well suited to modeling nutrient uptake. The model accounts for N supply from microbial mineralization and other sources, flux of N through the soil to the root surface, and uptake by the plant root. We included glycine in our simulations of N uptake because laboratory experiments have shown that E. vaginatum can use glycine and other low-molecular-mass amino acids as a source of N. However, the extent to which glycine contributes to the N nutrition of E. vaginatum in the field is unknown. Based on these simulations we find that: (1) Ammonium, glycine, and nitrate could all potentially make significant contributions to the N nutrition of E. vaginatum. The relative contribution of glycine is difficult to assess, because its behavior in the soil has not been characterized. However, glycine and ammonium contributed in roughly equal proportions for most model parameterizations. (2) The importance of factors that regulated modeled ammonium and glycine uptake can be ranked as follows: supply rate [is greater than] [soil factors (buffer capacity and diffusion coefficient) = root density] [is greater than] root uptake kinetics. Supply rate was the only factor that regulated nitrate uptake. These simulation results and other considerations suggest that N uptake kinetics of roots and soil-solution concentrations may not be reliable predictors of whole-plant N uptake in tundra or other natural ecosystems., Keywords: amino-acid uptake by plants; arctic sedge, nitrogen economy of; Eagle Creek, Alaska, USA; Eriophorum vaginatum; modeling nutrient uptake; nitrogen availability cf. uptake; nutrient uptake, physiological factors in; nutrient uptake, [...]

Delayed inducible resistance (DIR) is triggered by artificial or herbivore-caused foliar damage and is manifested as decreased performance of herbivore generation(s) feeding on the trees subsequent to the generation during which the damage took place. DIR is associated with increase in concentrations of foliage phenolics and decrease in nitrogen. The growth-differentiation balance hypothesis, and the carbon-nutrient balance (CNB) hypothesis contained in it, claim that DIR is caused by nutritional stress after defoliation of trees growing on nutrient-poor soils. In these environments, nutrient uptake limits plant growth more strongly than does photosynthesis; that is, carbon-based secondary metabolites (e.g., phenolics) are prone to accumulate. According to the CNB hypothesis, an excess of limiting nutrient(s) or reduced photosynthetic rate should lead to elimination of DIR. We tested this using same-aged Betula pubescens ssp. tortuosa trees of five open-pollinated families growing in a common arboretum in northernmost Finland. In addition to unmanipulated control trees, we had three nutritional treatments during three successive growth seasons: N-fertilization, PK-fertilization adding all nutrients except N, and shading to decrease carbon assimilation. Half of the trees in each treatment were artificially defoliated (50% leaf area) in the second study year, one year before the growth trial with geometrid (Epirrita autumnata) larvae. Tree growth measurements showed that N is the growth-limiting nutrient in our study area. N-fertilization and shading of the trees affected foliage chemistry generally as proposed by the CNB hypothesis. For example, they reduced foliar concentrations of total phenolics and condensed tannins. The birch families differed significantly in foliage chemistry, suggesting genetic differences, but the differences were not associated with fertilizations, shading, or defoliation of the trees. Contrary to fertilizing-shade treatments, changes in leaf chemistry and E. autumnata performance caused by defoliation were not consistent with the CNB hypothesis. For example, defoliation caused significant DIR irrespective of N-fertilization or shading. There were no significant differences among the birch families in performance of the moth larvae or in effects of fertilization, shade, or defoliation on larval performance. These responses to defoliation contrast with those of some other studies, especially those on the Betula resinifera-Rheumaptera hastata system in Alaska, which provide clear support for the CNB as an explanation of DIR. We find methodological differences to be an unlikely explanation for the different results but are unable to propose any single mechanism that will explain the diverse plant responses. Key words: Betula pubescens ssp. tortuosa; carbon nutrient balance hypothesis; condensed tannins; delayed inducible resistance; Epirrita autumnata; fertilization; larval performance; manual defoliation; nitrogen; shading; total phenolics., INTRODUCTION Plant secondary metabolites could have evolved as an active defense against herbivores, or their contribution to resistance against herbivores could represent a secondary phenomenon that fortuitously benefits the plant [...]

Element Cycling in Taiga Forests: State-Factor Control Of the 136.8 million hectares comprising interior Alaska, approximately 31% is forested. The remainder of the area is made up of a mosiac [...]

The sizes of major sources and sinks of atmospheric methane ([CH.sub.4]), an important greenhouse gas, are poorly known. [CH.sub.4] from north Siberian lakes contributes ~1.5 teragrams [CH.sub.4] [year.sup.-1] to observed winter increases in atmospheric [CH.sub.4] concentration at high northern latitudes. [CH.sub.4] emitted from these lakes in winter had a radiocarbon age of 27,200 years and was derived largely from Pleistocene-aged carbon., The highest concentration and greatest seasonal amplitude of atmospheric [CH.sub.4] occurs at 65 ° to 70 ° N. Concentrations are highest in March to April and lowest in summer (1). [...]

Humans are altering the composition of biological communities through a variety of activities that increase rates of species invasions and species extinctions, at all scales, from local to global. These changes in components of the Earth's biodiversity cause concern for ethical and aesthetic reasons, but they also have a strong potential to alter ecosystem properties and the goods and services they provide to humanity. Ecological experiments, observations, and theoretical developments show that ecosystem properties depend greatly on biodiversity in terms of the functional characteristics of organisms present in the ecosystem and the distribution and abundance of those organisms over space and time. Species effects act in concert with the effects of climate, resource availability, and disturbance regimes in influencing ecosystem properties. Human activities can modify all of the above factors; here we focus on modification of these biotic controls. The scientific community has come to a broad consensus on many aspects of the relationship between biodiversity and ecosystem functioning, including many points relevant to management of ecosystems. Further progress will require integration of knowledge about biotic and abiotic controls on ecosystem properties, how ecological communities are structured, and the forces driving species extinctions and invasions. To strengthen links to policy and management, we also need to integrate our ecological knowledge with understanding of the social and economic constraints of potential management practices. Understanding this complexity, while taking strong steps to minimize current losses of species, is necessary for responsible management of Earth's ecosystems and the diverse biota they contain. [ABSTRACT FROM AUTHOR]

Aim Describe the spatial and temporal properties of transitions in the Arctic and develop a conceptual understanding of the nature of these spatial transitions in the face of directional environmental change. Location Arctic tundra ecosystems of the North Slope of Alaska and the tundra-forest region of the Seward Peninsula, Alaska Methods We synthesize information from numerous studies on tundra and treeline ecosystems in an effort to document the spatial changes that occur across four arctic transitions. These transitions are: (i) the transition between High-Arctic and Low-Arctic systems, (ii) the transition between moist non-acidic tundra (MNT) and moist acidic tundra (MAT, also referred to as tussock tundra), (iii) the transition between tussock tundra and shrub tundra, (iv) the transition between tundra and forested systems. By documenting the nature of these spatial transitions, in terms of their environmental controls and vegetation patterns, we develop a conceptual model of temporal dynamics of arctic ecotones in response to environmental change. Results Our observations suggest that each transition is sensitive to a unique combination of controlling factors. The transition between High and Low Arctic is sensitive primarily to climate, whereas the MNT/MAT transition is also controlled by soil parent material, permafrost and hydrology. The tussock/shrub tundra transition appears to be responsive to several factors, including climate, topography and hydrology. Finally, the tundra/forest boundary responds primarily to climate and to climatically associated changes in permafrost. There were also important differences in the demography and distribution of the dominant plant species across the four vegetation transitions. The shrubs that characterize the tussock/shrub transition can achieve dominance potentially within a decade, whereas spruce trees often require several decades to centuries to achieve dominance within tundra, and Sphagnum moss colonization of non-acidic sites at the MNT/MAT boundary may require centuries to millennia of soil development. Main conclusions We suggest that vegetation will respond most rapidly to climatic change when (i) the vegetation transition correlates more strongly with climate than with other environmental variables, (ii) dominant species exhibit gradual changes in abundance across spatial transitions, and/or (iii) the dominant species have demographic properties that allow rapid increases in abundance following climatic shifts. All three of these properties characterize the transition between tussock tundra and low shrub tundra. It is therefore not surprising that of the four transitions studied this is the one that appears to be responding most rapidly to climatic warming. [ABSTRACT FROM AUTHOR]

Examines the role of black spruce ecosystem in the fire regime of interior boreal forest in Alaska. Component of the landscape; Impact of adding black spruce to the model; Implication of vegetation effects and fire regime dynamics for global models of vegetation dynamics and potential biotic feedbacks to regional climate.

Abstract. The responses of high latitude ecosystems to global change involve complex interactions among environmental variables, vegetation distribution, carbon dynamics, and water and energy exchange. These responses may have important consequences for the earth system. In this study, we evaluated how vegetation distribution, carbon stocks and turnover, and water and energy exchange are related to environmental variation spanned by the network of the IGBP high latitude transects. While the most notable feature of the high latitude transects is that they generally span temperature gradients from southern to northern latitudes, there are substantial differences in temperature among the transects. Also, along each transect temperature co-varies with precipitation and photosynthetically active radiation, which are also variable among the transects. Both climate and disturbance interact to influence latitudinal patterns of vegetation and soil carbon storage among the transects, and vegetation distribution appears to interact with climate to determine exchanges of heat and moisture in high latitudes. Despite limitations imposed by the data we assembled, the analyses in this study have taken an important step toward clarifying the complexity of interactions among environmental variables, vegetation distribution, carbon stocks and turnover, and water and energy exchange in high latitude regions. This study reveals the need to conduct coordinated global change studies in high latitudes to further elucidate how interactions among climate, disturbance, and vegetation distribution influence carbon dynamics and water and energy exchange in high latitudes. [ABSTRACT FROM AUTHOR]

Analyzes the patterns on root and leaf traits of various grassland species in Minnesota. Association of the traits with fertility and growth form; Information on high tissue density species existing as a basal rosette for part of the season; Comparison on the relative abundance between grasses and forbs.

Reports on a study which found suppression of microbial decomposition in an annual grassland after continuous exposure to increased carbon dioxide (CO[sub2]). Effects of the increased carbon dioxide, including enhanced plant nitrogen uptake and available carbon for microbes; Results which indicated that increased carbon dioxide can alter the interaction between plants and microbes so that microbial decomposition is slowed and ecosystems increase carbon accumulation.

Focuses on the synthesis of research programs conducted in the Arctic and boreal regions which indicate a strong role of high-altitude ecosystems in the climatic system. Increase in surface air temperature, precipitation, and disturbance rates in boreal forests; Need of a better understanding of vegetation dynamics of biomes; Facts related to the size and direction of atmospheric carbon dioxide flux.

Focuses on the importance of understanding the distribution and function of Arctic and boreal ecosystems to the assessment of global environmental changes. Review of three classes of ecological models; Disadvantages of the three models; Description of climatic, geological and topographic influences.

The progress made in the Land--Atmosphere--Ice Interactions Flux Study over the past 4 yr to fully characterize the biophysical fluxes in the snow-free tundra ecosystem and their relationship to climate and climate change is described. This paper is the result of a synthesis effort to bring together the measurements of surface fluxes at various sites on the North Slope of Alaska in the snow-free period of 1995 with the results of modeling efforts for this region. It is found that methodological and site dissimilarities contribute to measurement dif-ferences at least as much as instrument and sampling error, even for closely collocated and similarly vegetated sites. The regional climate model employed in this study generally simulates fluxes that are within the range of measured fluxes, but tends to overestimate both net radiation and latent heat fluxes. The regional model also captures site to site variations quite well, which appear to be more sensitive to mesoscale meteorological conditions than on specifics of site characteristics. The active layer model employed in this study performs well in estimating ground heat flux but rather more poorly in simulating turbulent fluxes. The global climate model is unable to capture the broad-scale response of the land surface sensible heat flux and net radiation, although it performs rather better in the simulation of latent and ground heat fluxes. Finally, the intended purposes and applications of both data and model simulations have a strong impact on their applicability to other studies. [ABSTRACT FROM AUTHOR]

Presents results of an experimental field study on nutritional controls over nitrogen and phosphorus resorption from Alaskan birch leaves. Methods; Results; Discussion.

Discusses the role of carbon (C) and nitrogen (N) interactions in terrestrial ecosystems. Demonstration that differences in N-use efficiency cause convergence of plant growth; Mechanism of plant-induced negative feedback on production; Effects of addition of inorganic N to ecosystems.

Describes element cycling in taiga forests. Framework for experimental studies of ecosystem processes; Ecosystem controls; State factors in the Alaskan taiga; Control of element cycling in floodplain ecosystems; Control of element cycling in south aspect uplands.

Investigates the ecotypic differentiation in Carex aquatilis on ice-wedge polygons in the Alaskan coastal tundra. Growth and nutrient relations; Microhabitats; Weight per tiller; Leaf-production rate; Root leaf weight ratio; Tissue concentrations of nitrogen and phosphorous.

Presents research which studied recent increases in the seasonal amplitude of atmospheric carbon dioxide (CO2) at high latitudes. Suggestion of a widespread biospheric response to high-latitude warming; Net ecosystem carbon exchange by northern Siberian ecosystems; Effects of warm temperatures on summer carbon influx.