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  • 1. Grogan, P
    et al.
    Jonasson, S
    Controls on annual nitrogen cycling in the understory of a subarctic birch forest2003In: Ecology, ISSN 0012-9658, E-ISSN 1939-9170, Vol. 84, no 1, p. 202-218Article in journal (Refereed)
    Abstract [en]

    Characterization of the controls on annual nitrogen (N) cycling is critical to understanding the functioning of high-latitude ecosystems and to predicting their responses to perturbations. Here, we describe an experimental evaluation of the effects of season and vegetation on annual N cycling in the understory heath vegetation of a subarctic birch forest. Our approach was to follow the partitioning of an isotopically enriched (NH4Cl)-N-15 addition between microbes, plants, and the soil solution at intervals through winter, and in the following summer. To investigate the direct influence of vegetation on ecosystem N cycling, the isotope was added to control plots and to plots from which plant had been removed early in the previous growing season. Our results indicate that the dynamics of both microbial carbon and N were similar in treatment and control plots, suggesting that the presence of intact plants had negligible influence on seasonal patterns of microbial growth or N accumulation in the Soils of this ecosystem. Instead, ecosystem N cycling was dominated by a substantial turnover of recently acquired N from microbes during the winter-summer transition that corresponded to a significant increase in understory plant N-15 uptake. Vascular plant N-15 uptake and accumulation in belowground tissue was Substantial at the onset of winter, but ceased once full winter conditions developed. By contrast, there was a rapid resumption of vascular plant N uptake and strong allocation to aboveground tissue at the onset of summer. Vascular plant types varied strongly in N-15 enhancement. depending on physiological differences in N uptake capacity as well as differences in biomass, In particular, Vaccinium vitis-idaea, Vaccinium myrtillus, and the herbs exhibited strong N-15 acquisition capacities, despite their relatively low biomass. We found no consistent evidence that the evergreen vs. deciduous leaf habit contributed to species differences in N uptake capacity, Species capacity for allocation of N-15 to new shoot tissue was closely correlated with aboveground production per unit total N and tended to be greatest in herbs and V myrtillus, suggesting that inherent physiological capacities for N uptake and tissue allocation are important determinants of species productivity. Our results suggest a hierarchy of controls on annual N cycling. The importance of wintertime influences on annual ecosystem N cycling was indicated by ongoing net mineralization by microbes beneath snow cover and a critical N release from microbes during the winter-summer transition. By contrast, plant N uptake was confined to the snow-free season. Thus, seasonal environmental changes caused a rapid microbial turnover of recently acquired N that then became available for plant uptake in the subsequent growing season. Although vascular plant species appeared to compete strongly with each other to acquire this N, neither their presence nor their total N uptake appreciably affected microbial N-cycling activity. Together, these results suggest that season exerts primary control on annual N cycling in this ecosystem, and that its effect on microbial N turnover is a major process supplying N to plants. By comparison, the effects of understory vegetation on N cycling were small within the time frame of this experiment.

  • 2. Hansson, Lars-Anders
    et al.
    Hylander, Samuel
    Sommaruga, Ruben
    Escape from UV threats in zooplankton: A cocktail of behavior and protective pigmentation2007In: Ecology, ISSN 0012-9658, E-ISSN 1939-9170, Vol. 88, no 8, p. 1932-1939Article in journal (Refereed)
    Abstract [en]

    In order to avoid environmental threats, organisms may respond by altering behavior or phenotype. Using experiments performed in high-latitude Siberia and in temperate Sweden, we show for the first time that, among freshwater crustacean zooplankton, the defense against threats from ultraviolet radiation (UV) is a system where phenotypic plasticity and behavioral escape mechanisms function as complementary traits. Freshwater copepods relied mainly on accumulating protective pigments when exposed to UV radiation, but Daphnia showed strong behavioral responses. Pigment levels for both Daphnia and copepods were generally higher at higher latitudes, mirroring different UV threat levels. When released from the UV threat, Daphnia rapidly reduced (within 10 days) their UV protecting pigmentation-by as much as 40%-suggesting a cost in maintaining UV protective pigmentation. The. evolutionary advantage of protective pigments is, likely, the ability to utilize the whole water column during daytime; conversely, since the amount of algal food is generally higher in surface waters, unpigmented individuals are restricted to a less preferred feeding habitat in deeper waters. Our main conclusion is that different zooplankton taxa, and similar taxa at different latitudes, use different mixes of behavior and pigments to respond to UV radiation.

  • 3. Jonasson, Sven
    et al.
    Michelsen, Anders
    Schmidt, Inger K.
    Nielsen, Esben V.
    RESPONSES IN MICROBES AND PLANTS TO CHANGED TEMPERATURE, NUTRIENT, AND LIGHT REGIMES IN THE ARCTIC1999In: Ecology, ISSN 0012-9658, E-ISSN 1939-9170, Vol. 80, no 6, p. 1828-1843Article in journal (Refereed)
    Abstract [en]

    Previous research has shown that experimental perturbations of arctic ecosystems simulating direct and indirect effects of predicted environmental changes have led to strong responses in the plant communities, mostly associated with increased plant nutrient availability. Similarly, changes in decomposition and nutrient mineralization are likely to occur if the soil warms and the soil moisture conditions are altered. Plant and microbial responses have usually been investigated separately, and few, if any, studies have addressed simultaneous responses to environmental changes in plants and soil microorganisms, except in models. We measured simultaneous responses in biomass, nitrogen (N), and phosphorus (P) incorporation in plants and microorganisms after five years of factorial fertilizer addition, air warming, and shading. We expected increased N and P uptake by microorganisms after fertilizer addition and also after warming, due to increases in mineralization rates in warmer soils. Plant productivity and N and P uptake were expected to increase after fertilizer addition but less after warming, because microbes were expected to absorb most of the extra released nutrients. Shading was expected to decrease plant production and also microbial biomass, due to the reduced production of labile carbon (C) in plant root exudates associated with reduced photosynthesis. We found that the plants responded strongly to fertilizer addition by increased biomass accumulation and N and P uptake. They responded less to warming, but more than expected, showing a decline in N and P concentrations in many cases. There were few significant responses to shading. The strongest response was found in combined fertilizer addition and warming treatments. All functional vascular plant groups responded similarly. However, mosses declined under those conditions when vascular plant growth was most pronounced. Contrary to our expectation, microbial C, N, and P did not increase after warming, but microbial N and P increased after shading. As expected, fertilizer addition led to increased microbial P content, whereas microbial N either increased or did not change. In general, microbial C did not change in any treatment. The microbes accumulated extra N and P only when soil inorganic N or P levels increased, suggesting that the soil microorganisms absorbed extra nutrients only in cases of declining N and P sink strength in plants.

  • 4. Mendez, M
    et al.
    Karlsson, P S
    Nutrient stoichiometry in Pinguicula vulgaris: Nutrient availability, plant size, and reproductive status2005In: Ecology, ISSN 0012-9658, E-ISSN 1939-9170, Vol. 86, no 4, p. 982-991Article in journal (Refereed)
    Abstract [en]

    Current understanding of the extent, causes for, and consequences of variation in nutrient composition in plants is limited. Important questions to be addressed include to what extent nutrients covary, how flexible nutrient ratios are within a population or species, how reproduction influences nutrient ratios, and how much the ratios of nutrients to mass and nutrients to each other change through ontogeny. This information is needed to assess the physiological and ecological consequences of plant nutrient composition and to what extent plants function as balanced systems in acquisition and allocation of resources. We studied the variation in nutrient stoichiometry (i.e., the ratio between contents of different nutrients within a plant) in relation to three factors: (1) environmental availability of nitrogen, (2) plant size, and (3) reproductive status. We investigated these questions in 11 populations of the carnivorous plant Pinguicula vulgaris in northern Scandinavia. Dry mass and N and P content were measured for reproductive and vegetative portions of flowering individuals and for winter buds corresponding to four reproductive states: control reproductive individuals, experimentally vegetative individuals (from which flower buds were removed), adult vegetative individuals, and individuals below the threshold size for reproduction. [N], [P], and to a lesser extent, N and P content were positively related to soil N, but not to prey capture. Nutrient stoichiometry was also size dependent; in general, small plants were relatively enriched in N and relatively depleted in P compared to larger plants. Reproductive status affected not only size, but also nutrient stoichiometry of the resulting winter bud. Winter buds derived from reproductive individuals had a higher [N] and lower [P] than those of the different types of nonreproductive individuals. Our findings indicate that studies of nutrient stoichiometry in plants must go beyond links between environmental and plant nutrient concentrations to consider internal processes such as growth and reproduction.

  • 5. Parker, Thomas C.
    et al.
    Sanderman, Jonathan
    Holden, Robert D.
    Blume-Werry, Gesche
    Sjögersten, Sofie
    Large, David
    Castro-Díaz, Miguel
    Street, Lorna E.
    Subke, Jens-Arne
    Wookey, Philip A.
    Exploring drivers of litter decomposition in a greening Arctic: results from a transplant experiment across a treeline2018In: Ecology, ISSN 0012-9658, E-ISSN 1939-9170, Vol. 99, no 10, p. 2284-2294Article in journal (Refereed)
    Abstract [en]

    Abstract Decomposition of plant litter is a key control over carbon (C) storage in the soil. The biochemistry of the litter being produced, the environment in which the decomposition is taking place, and the community composition and metabolism of the decomposer organisms exert a combined influence over decomposition rates. As deciduous shrubs and trees are expanding into tundra ecosystems as a result of regional climate warming, this change in vegetation represents a change in litter input to tundra soils and a change in the environment in which litter decomposes. To test the importance of litter biochemistry and environment in determining litter mass loss, we reciprocally transplanted litter between heath (Empetrum nigrum), shrub (Betula nana), and forest (Betula pubescens) at a sub-Arctic treeline in Sweden. As expansion of shrubs and trees promotes deeper snow, we also used a snow fence experiment in a tundra heath environment to understand the importance of snow depth, relative to other factors, in the decomposition of litter. Our results show that B. pubescens and B. nana leaf litter decomposed at faster rates than E. nigrum litter across all environments, while all litter species decomposed at faster rates in the forest and shrub environments than in the tundra heath. The effect of increased snow on decomposition was minimal, leading us to conclude that microbial activity over summer in the productive forest and shrub vegetation is driving increased mass loss compared to the heath. Using B. pubescens and E. nigrum litter, we demonstrate that degradation of carbohydrate-C is a significant driver of mass loss in the forest. This pathway was less prominent in the heath, which is consistent with observations that tundra soils typically have high concentrations of ?labile? C. This experiment suggests that further expansion of shrubs and trees may stimulate the loss of undecomposed carbohydrate C in the tundra.

  • 6. Quested, H M
    et al.
    Cornelissen, J H C
    Press, M C
    Callaghan, T V
    Aerts, R
    Trosien, F
    Riemann, P
    Gwynn-Jones, D
    Kondratchuk, A
    Jonasson, S E
    Decomposition of sub-arctic plants with differing nitrogen economies: A functional role for hemiparasites2003In: Ecology, ISSN 0012-9658, E-ISSN 1939-9170, Vol. 84, no 12, p. 3209-3221Article in journal (Refereed)
    Abstract [en]

    Although hemiparasitic plants have a number of roles in shaping the structure and composition of plant communities, the impact of this group on ecosystem processes, such as decomposition and nutrient cycling, has been poorly studied. In order to better understand the potential role of hemiparasites in these processes, a comparison of leaf and litter tissue quality, nitrogen (N) resorption, and decomposability with those of a wide range of other plant groups (involving a total of 72 species and including other groups with access to alternative nutrient sources, such as nitrogen fixers and carnivorous plants) was undertaken in several sub-arctic habitats. The foliar N concentration of hemiparasites generally exceeded that of co-occurring species. Further, hemiparasites (and N fixers) exhibited lower N resorption efficiencies than their counterparts with no major alternative N source. As a consequence, annual and perennial hemiparasite litter contained, on average, 3.1% and 1.9% N, respectively, compared with 0.77-1.1% for groups without a major alternative N source. Hemiparasite litter lost significantly more mass during decomposition than many, but not all, co-occurring species. These results were combined with those of a litter trapping experiment to assess the potential impact of hemiparasites on nutrient cycling. The common sub-arctic hemiparasite Bartsia alpina was estimated to increase the total annual N input from litter to the soil by similar to42% within 5 cm of its stems, and by similar to53% across a site with a Bartsia alpina stem density of 43 stems/m(2). Our results therefore provide clear evidence in favor of a novel mechanism by which hemiparasites (in parallel with N-fixing species) may influence ecosystems in which they occur. Through the production of nutrient rich, rapidly decomposing litter, they have the potential to greatly enhance the availability of nutrients within patches where they are abundant, with possible consequent effects on small-scale biodiversity.

  • 7. Soudzilovskaia, N. A.
    et al.
    Cornelissen, J. H. C.
    During, H. J.
    van Logtestijn, R. S. P.
    Lang, S. I.
    Aerts, R.
    Similar cation exchange capacities among bryophyte species refute a presumed mechanism of peatland acidification2010In: Ecology, ISSN 0012-9658, E-ISSN 1939-9170, Vol. 91, no 9, p. 2716-2726Article in journal (Refereed)
    Abstract [en]

    Fen-€“bog succession is accompanied by strong increases of carbon accumulation rates. We tested the prevailing hypothesis that living Sphagna have extraordinarily high cation exchange capacity (CEC) and therefore acidify their environment by exchanging tissue-bound protons for basic cations in soil water. As Sphagnum invasion in a peatland usually coincides with succession from a brown moss-dominated alkaline fen to an acidic bog, the CEC of Sphagna is widely believed to play an important role in this acidification process. However, Sphagnum CEC has never been compared explicitly to that of a wide range of other bryophyte taxa. Whether high CEC directly leads to the ability to acidify the environment in situ also remains to be tested. We screened 20 predominant subarctic bryophyte species, including fen brown mosses and bog Sphagna for CEC, in situ soil water acidification capacity (AC), and peat acid neutralizing capacity (ANC). All these bryophyte species possessed substantial CEC, which was remarkably similar for brown mosses and Sphagna. This refutes the commonly accepted idea of living Sphagnum CEC being responsible for peatland acidification, as Sphagnum’s ecological predecessors, brown mosses, can do the same job. Sphagnum AC was several times higher than that of other bryophytes, suggesting that CE (cation exchange) sites of Sphagna in situ are not saturated with basic cations, probably due to the virtual absence of these cations in the bog water. Together, these results suggest that Sphagna can not realize their CEC in bogs, while fen mosses can do so in fens. The fen peat ANC was 65% higher than bog ANC, indicating that acidity released by brown mosses in the CE process was neutralized, maintaining an alkaline environment. We propose two successional pathways indicating boundaries for a fen–bog shift with respect to bryophyte CEC. In neither of them is Sphagnum CE an important factor. We conclude that living Sphagnum CEC does not play any considerable role in the fen–bog shift. Alternatively, we propose that exclusively indirect effects of Sphagnum expansion such as peat accumulation and subsequent blocking of upward alkaline soil water transport are keys to the fen–bog succession and therefore for bog-associated carbon accumulation.

  • 8. Sundqvist, Maja K.
    et al.
    Liu, Zhanfeng
    Giesler, Reiner
    Wardle, David A.
    Plant and microbial responses to nitrogen and phosphorus addition across an elevational gradient in subarctic tundra2014In: Ecology, ISSN 0012-9658, E-ISSN 1939-9170, Vol. 95, no 7, p. 1819-1835Article in journal (Refereed)
    Abstract [en]

    Temperature and nutrients are major limiting factors in subarctic tundra. Experimental manipulation of nutrient availability along elevational gradients (and thus temperature) can improve our understanding of ecological responses to climate change. However, no study to date has explored impacts of nutrient addition along a tundra elevational gradient, or across contrasting vegetation types along any elevational gradient. We set up a full factorial nitrogen (N) and phosphorus (P) fertilization experiment in each of two vegetation types (heath and meadow) at 500 m, 800 m, and 1000 m elevation in northern Swedish tundra. We predicted that plant and microbial communities in heath or at lower elevations would be more responsive to N addition while communities in meadow or at higher elevations would be more responsive to P addition, and that fertilizer effects would vary more with elevation for the heath than for the meadow. Although our results provided little support for these predictions, the relationship between nutrient limitation and elevation differed between vegetation types. Most plant and microbial properties were responsive to N and/or P fertilization, but responses often varied with elevation and/or vegetation type. For instance, vegetation density significantly increased with N + P fertilization relative to the other fertilizer treatments, and this increase was greatest at the lowest elevation for the heath but at the highest elevation for the meadow. Arbuscular mycorrhizae decreased with P fertilization at 500 m for the meadow, but with all fertilizer treatments in both vegetation types at 800 m. Fungal to bacterial ratios were enhanced by N + P fertilization for the two highest elevations in the meadow only. Additionally, microbial responses to fertilization were primarily direct rather than indirect via plant responses, pointing to a decoupled response of plant and microbial communities to nutrient addition and elevation. Because our study shows how two community types differ in their responses to fertilization and elevation, and because the temperature range across this gradient is ∼3°C, our study is informative about how nutrient limitation in tundra may be influenced by temperature shifts that are comparable to those expected under climate change during this century.

  • 9. Templer, P. H.
    et al.
    Mack, M. C.
    III, F. S. Chapin
    Christenson, L. M.
    Compton, J. E.
    Crook, H. D.
    Currie, W. S.
    Curtis, C. J.
    Dail, D. B.
    D'Antonio, C. M.
    Emmett, B. A.
    Epstein, H. E.
    Goodale, C. L.
    Gundersen, P
    Hobbie, S. E.
    Holland, K
    Hooper, D. U.
    Hungate, B. A.
    Lamontagne, S
    Nadelhoffer, K. J.
    Osenberg, C. W.
    Perakis, S. S.
    Schleppi, P
    Schimel, J
    Schmidt, I. K.
    Sommerkorn, M
    Spoelstra, J
    Tietema, A
    Wessel, W. W.
    Zak, D. R.
    Sinks for nitrogen inputs in terrestrial ecosystems: a meta-analysis of 15N tracer field studies2012In: Ecology, ISSN 0012-9658, E-ISSN 1939-9170, Vol. 93, no 8, p. 1816-1829Article in journal (Refereed)
    Abstract [en]

    Effects of anthropogenic nitrogen (N) deposition and the ability of terrestrial ecosystems to store carbon (C) depend in part on the amount of N retained in the system and its partitioning among plant and soil pools. We conducted a meta‐analysis of studies at 48 sites across four continents that used enriched 15N isotope tracers in order to synthesize information about total ecosystem N retention (i.e., total ecosystem 15N recovery in plant and soil pools) across natural systems and N partitioning among ecosystem pools. The greatest recoveries of ecosystem 15N tracer occurred in shrublands (mean, 89.5%) and wetlands (84.8%) followed by forests (74.9%) and grasslands (51.8%). In the short term (<1 week after 15N tracer application), total ecosystem 15N recovery was negatively correlated with fine‐root and soil 15N natural abundance, and organic soil C and N concentration but was positively correlated with mean annual temperature and mineral soil C:N. In the longer term (3–18 months after 15N tracer application), total ecosystem 15N retention was negatively correlated with foliar natural‐abundance 15N but was positively correlated with mineral soil C and N concentration and C : N, showing that plant and soil natural‐abundance 15N and soil C:N are good indicators of total ecosystem N retention. Foliar N concentration was not significantly related to ecosystem 15N tracer recovery, suggesting that plant N status is not a good predictor of total ecosystem N retention. Because the largest ecosystem sinks for 15N tracer were below ground in forests, shrublands, and grasslands, we conclude that growth enhancement and potential for increased C storage in aboveground biomass from atmospheric N deposition is likely to be modest in these ecosystems. Total ecosystem 15N recovery decreased with N fertilization, with an apparent threshold fertilization rate of 46 kg N·ha−1·yr−1 above which most ecosystems showed net losses of applied 15N tracer in response to N fertilizer addition.

  • 10. Träger, Sabrina
    et al.
    Öpik, Maarja
    Vasar, Martti
    Wilson, Scott D.
    Belowground plant parts are crucial for comprehensively estimating total plant richness in herbaceous and woody habitats2018In: Ecology, ISSN 0012-9658, E-ISSN 1939-9170, Vol. 0, no 0Article in journal (Refereed)
    Abstract [en]

    Most studies consider aboveground plant species richness as a representative biodiversity measure. This approach inevitably assumes that the partitioning of total plant species richness into above- and belowground components is constant or at least consistent within and across vegetation types. However, with studies considering belowground plant richness still scarce and completely absent along vegetation gradients, this assumption lacks experimental support. Novel DNA sequencing techniques allow economical, high-throughput species identification of belowground environmental samples, enabling the measurement of the contributions of both above- and belowground plant components to total plant richness. We investigated above- and belowground plant species richness in four vegetation types (birch forest, heath, low alpine tundra, high alpine tundra) at the scale of herbaceous plant neighborhoods (dm) using 454 sequencing of the chloroplast trnL (UAA) intron to determine the plant species richness of environmental root samples and combined it with aboveground data from vegetation surveys to obtain total plant species richness. We correlated the measured plant species richness components with each other and with their respective plant biomass components within and across vegetation types. Total plant species richness exceeded aboveground richness twice on average and by as much as three times in low alpine tundra, indicating that a significant fraction of belowground plant richness cannot be recorded aboveground. More importantly, no consistent relationship among richness components (above- and belowground) was found within or across vegetation types, indicating that aboveground richness alone cannot predict total plant richness in contrasting vegetation types. Finally, no consistent relationship between plant richness and the corresponding biomass component was found. Our results clearly show that aboveground plant richness alone is a poor estimator of total plant species richness within and across different vegetation types. Consequently, it is crucial to account for belowground plant richness in future plant ecological studies in order to validate currently accepted plant richness patterns, as well as to measure potential changes in plant community composition in a changing environment.

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