68 research outputs found
Local adaptations to frost in marginal and central populations of the dominant forest tree Fagus sylvatica L. as affected by temperature and extreme drought in common garden experiments
Local adaptations to environmental conditions are of high ecological importance as they determine distribution ranges and likely affect species responses to climate change. Increased environmental stress (warming, extreme drought) due to climate change in combination with decreased genetic mixing due to isolation may lead to stronger local adaptations of geographically marginal than central populations. We experimentally observed local adaptations of three marginal and four central populations of Fagus sylvaticaL., the dominant native forest tree, to frost over winter and in spring (late frost). We determined frost hardiness of buds and roots by the relative electrolyte leakage in two common garden experiments. The experiment at the cold site included a continuous warming treatment; the experiment at the warm site included a preceding summer drought manipulation. In both experiments, we found evidence for local adaptation to frost, with stronger signs of local adaptation in marginal populations. Winter frost killed many of the potted individuals at the cold site, with higher survival in the warming treatment and in those populations originating from colder environments. However, we found no difference in winter frost tolerance of buds among populations, implying that bud survival was not the main cue for mortality. Bud late frost tolerance in April differed between populations at the warm site, mainly because of phenological differences in bud break. Increased spring frost tolerance of plants which had experienced drought stress in the preceding summer could also be explained by shifts in phenology. Stronger local adaptations to climate in geographically marginal than central populations imply the potential for adaptation to climate at range edges. In times of climate change, however, it needs to be tested whether locally adapted populations at range margins can successfully adapt further to changing conditions
Divergence of Arctic shrub growth associated with sea ice decline
Arctic sea ice extent (SIE) is declining at an accelerating rate with a wide range of ecological consequences. However, determining sea ice effects on tundra vegetation remains a challenge. In this study, we examined the universality or lack thereof in tundra shrub growth responses to changes in SIE and summer climate across the Pan-Arctic, taking advantage of 23 tundra shrub-ring chronologies from 19 widely distributed sites (56°N to 83°N). We show a clear divergence in shrub growth responses to SIE that began in the mid-1990s, with 39% of the chronologies showing declines and 57% showing increases in radial growth (decreasers and increasers, respectively). Structural equation models revealed that declining SIE was associated with rising air temperature and precipitation for increasers and with increasingly dry conditions for decreasers. Decreasers tended to be from areas of the Arctic with lower summer precipitation and their growth decline was related to decreases in the standardized precipitation evapotranspiration index. Our findings suggest that moisture limitation, associated with declining SIE, might inhibit the positive effects of warming on shrub growth over a considerable part of the terrestrial Arctic, thereby complicating predictions of vegetation change and future tundra productivity
Climate sensitivity of shrub growth across the tundra biome
The tundra biome is experiencing rapid temperature increases that have been linked to a shift in tundra vegetation composition towards greater shrub dominance. Shrub expansion can amplify warming by altering the surface albedo, energy and water balance, and permafrost temperatures. To account for these feedbacks, global climate models must include realistic projections of vegetation dynamics, and in particular tundra shrub expansion, yet the mechanisms driving shrub expansion remain poorly understood. Dendroecological data consisting of multi-decadal time series of annual growth of shrub species provide a previously untapped resource to explore climate-growth relationships across the tundra biome. We analysed a dataset of approximately 42,000 annual growth records from 1821 individuals, comprising 25 species from eight genera, from 37 arctic and alpine sites. Our analyses demonstrate that the sensitivity of shrub growth to climate was (1) heterogeneous across the tundra biome, (2) greater at sites with higher soil moisture and (3) strongest for taller shrub species growing at the northern or upper elevational edge of their range. Across latitudinal gradients in the Arctic, climate sensitivity of growth was greatest at the boundary between low- and high-arctic vegetation zones, where permafrost conditions are changing and the majority of the global permafrost soil carbon pool is stored. Thus, in order to more accurately estimate feedbacks among shrub change, albedo, permafrost thaw, carbon storage and climate, the observed variation in climate-growth relationships of shrub species across the tundra biome will need to be incorporated into earth system models.JRC.H.3-Forest Resources and Climat
Circum-Arctic distribution of chemical anti-herbivore compounds suggests biome-wide trade-off in defence strategies in Arctic shrubs
Spatial variation in plant chemical defence towards herbivores can help us understand variation in herbivore top-down control of shrubs in the Arctic and possibly also shrub responses to global warming. Less defended, non-resinous shrubs could be more influenced by herbivores than more defended, resinous shrubs. However, sparse field measurements limit our current understanding of how much of the circum-Arctic variation in defence compounds is explained by taxa or defence functional groups (resinous/non-resinous). We measured circum-Arctic chemical defence and leaf digestibility in resinous (Betula glandulosa, B. nana ssp. exilis) and non-resinous (B. nana ssp. nana, B. pumila) shrub birches to see how they vary among and within taxa and functional groups. Using liquid chromatography-mass spectrometry (LC-MS) metabolomic analyses and in vitro leaf digestibility via incubation in cattle rumen fluid, we analysed defence composition and leaf digestibility in 128 samples from 44 tundra locations. We found biogeographical patterns in anti-herbivore defence where mean leaf triterpene concentrations and twig resin gland density were greater in resinous taxa and mean concentrations of condensing tannins were greater in non-resinous taxa. This indicates a biome-wide trade-off between triterpene- or tannin-dominated defences. However, we also found variations in chemical defence composition and resin gland density both within and among functional groups (resinous/non-resinous) and taxa, suggesting these categorisations only partly predict chemical herbivore defence. Complex tannins were the only defence compounds negatively related to in vitro digestibility, identifying this previously neglected tannin group as having a potential key role in birch anti-herbivore defence. We conclude that circum-Arctic variation in birch anti-herbivore defence can be partly derived from biogeographical distributions of birch taxa, although our detailed mapping of plant defence provides more information on this variation and can be used for better predictions of herbivore effects on Arctic vegetation
Background invertebrate herbivory on dwarf birch (Betula glandulosa-nana 1 complex) increases with temperature and precipitation across the tundra biome
Circum-Arctic distribution of chemical anti-herbivore compounds suggests biome-wide trade-off in defence strategies in Arctic shrubs
Spatial variation in plant chemical defence towards herbivores can help us understand variation in herbivore top-down control of shrubs in the Arctic and possibly also shrub responses to global warming. Less defended, non-resinous shrubs could be more influenced by herbivores than more defended, resinous shrubs. However, sparse field measurements limit our current understanding of how much of the circum-Arctic variation in defence compounds is explained by taxa or defence functional groups (resinous/non-resinous). We measured circum-Arctic chemical defence and leaf digestibility in resinous (Betula glandulosa, B. nana ssp. exilis) and non-resinous (B. nana ssp. nana, B. pumila) shrub birches to see how they vary among and within taxa and functional groups. Using liquid chromatography-mass spectrometry (LC-MS) metabolomic analyses and in vitro leaf digestibility via incubation in cattle rumen fluid, we analysed defence composition and leaf digestibility in 128 samples from 44 tundra locations. We found biogeographical patterns in anti-herbivore defence where mean leaf triterpene concentrations and twig resin gland density were greater in resinous taxa and mean concentrations of condensing tannins were greater in non-resinous taxa. This indicates a biome-wide trade-off between triterpene- or tannin-dominated defences. However, we also found variations in chemical defence composition and resin gland density both within and among functional groups (resinous/non-resinous) and taxa, suggesting these categorisations only partly predict chemical herbivore defence. Complex tannins were the only defence compounds negatively related to in vitro digestibility, identifying this previously neglected tannin group as having a potential key role in birch anti-herbivore defence. We conclude that circum-Arctic variation in birch anti-herbivore defence can be partly derived from biogeographical distributions of birch taxa, although our detailed mapping of plant defence provides more information on this variation and can be used for better predictions of herbivore effects on Arctic vegetation.Peer reviewe
Societal-level versus individual-level predictions of ethical behavior: a 48-society study of collectivism and individualism
Is the societal-level of analysis sufficient today to understand the values of those in the global workforce? Or are individual-level analyses more appropriate for assessing the influence of values on ethical behaviors across country workforces? Using multi-level analyses for a 48-society sample, we test the utility of both the societal-level and individual-level dimensions of collectivism and individualism values for predicting ethical behaviors of business professionals. Our values-based behavioral analysis indicates that values at the individual-level make a more significant contribution to explaining variance in ethical behaviors than do values at the societal-level. Implicitly, our findings question the soundness of using societal-level values measures. Implications for international business research are discussed
Circum-Arctic distribution of chemical anti-herbivore compounds suggests biome-wide trade-off in defence strategies in Arctic shrubs
Spatial variation in plant chemical defence towards herbivores can help us understand variation in herbivore top?down control of shrubs in the Arctic and possibly also shrub responses to global warming. Less defended, non-resinous shrubs could be more influenced by herbivores than more defended, resinous shrubs. However, sparse field measurements limit our current understanding of how much of the circum-Arctic variation in defence compounds is explained by taxa or defence functional groups (resinous/non-resinous). We measured circum-Arctic chemical defence and leaf digestibility in resinous (Betula glandulosa, B. nana ssp. exilis) and non-resinous (B. nana ssp. nana, B. pumila) shrub birches to see how they vary among and within taxa and functional groups. Using liquid chromatography?mass spectrometry (LC?MS) metabolomic analyses and in vitro leaf digestibility via incubation in cattle rumen fluid, we analysed defence composition and leaf digestibility in 128 samples from 44 tundra locations. We found biogeographical patterns in anti-herbivore defence where mean leaf triterpene concentrations and twig resin gland density were greater in resinous taxa and mean concentrations of condensing tannins were greater in non-resinous taxa. This indicates a biome-wide trade-off between triterpene- or tannin-dominated defences. However, we also found variations in chemical defence composition and resin gland density both within and among functional groups (resinous/non-resinous) and taxa, suggesting these categorisations only partly predict chemical herbivore defence. Complex tannins were the only defence compounds negatively related to in vitro digestibility, identifying this previously neglected tannin group as having a potential key role in birch anti-herbivore defence. We conclude that circum-Arctic variation in birch anti-herbivore defence can be partly derived from biogeographical distributions of birch taxa, although our detailed mapping of plant defence provides more information on this variation and can be used for better predictions of herbivore effects on Arctic vegetation
Shrubs in Space and Time – Alpine and Artic shrub dendroecology: Factors influencing shrub growth and population dynamics in tundra ecosystems
Deutsche Zusammenfassung / German Abstract: Die Klimaerwärmung macht sich besonders in der Arktis bemerkt, wo sich die Durchschnittstemperatur in den letzen vier Jahrzenten doppelt so stark erwärmt hat wie in den gemäßigten Breiten. Untersuchungen, die auf Satellitenbildern basieren und auf der erneuten Aufnahme historischer Landschaftsbilder konnten im selben Zeitraum einen deutlichen Anstieg der pflanzlichen Biomasse – insbesondere eine erhöhte Anzahl und Größe der Sträucher – nachweisen. Diese Strauchexpansion beeinflußt die Vegetation, das Tierleben, den Boden, sowie den Energie und Kohlenstoffhaushalt der arktischen Tundra und das regionale und globale Klima merklich. Wie der Vergleich rezenter und historischer Fotos zeigt, vollzieht sich die Strauchexpansion auf verschiedenen zeitlichen und räumlichen Ebenen und schließt Landschaftseinheiten ein, die eine starke Zunahme an Sträuchern zeigen (expandierende Strauchbestände) und solchen, in denen keine Änderung der Vegetationsdecke feststellbar ist (stagnierende Strauchbestände). Ausreichend hochaufgelöste Satellitenbilder sind allerdings eine relativ rezente Erscheinung, zumal deren Auflösung zum Teil nicht genügt um vor allem Individuen kleinerer Straucharten zu erfassen. Brauchbare historische Fotografien sind selten. Experimentelle Studien haben gezeigt, dass erhöhte Temperaturen und eine erhöhte Nähstoffverfügbarkeit oftmals zu einer erhöhten Strauchbiomasse führt, wobei die Ergebnisse zum Teil auch widersprüchlich waren; sich aber in jeden Fall auf eine kleine Versuchsfläche und eine meist kurze Versuchsdauer bezogen. Um die Lücke zwischen diesen methodischen Ansätzen zu schließen wurde in der vorliegenden Dissertation ein dendroökologischer Ansatz gewählt um verschiedene Einflußgrößen auf das Strauchwachstum, sowie die Populationsdynamik zu erfassen. Dafür wurden die häufigsten immergrünen und laubabwerfenden Sträucher sowohl in der Gebirgstundra als auch in der Tundra der nördlichen Breiten beprobt. Ziel dieser Dissertation war es die Anwendbarkeit dendrochronologischer Methoden, mit denen bis dato vor allem Bäume untersucht wurden, auf Sträucher zu testen und etwaige morphologische Unterschiede herauszuarbeiten. Ein weiteres Ziel war es Anhaltspunkte für eine mögliche Strauchexpansion in Skandinavien und die maßgeblichen klimatischen Einflußgrößen auf das Strauchwachstum zu bestimmen. Zudem wollten wir mit Hilfe der Untersuchung der drei häufigsten Straucharten in der Tundra Alaskas und der zugehörigen Bodenparameter der räumlichen Heterogenität der Strauchexpansion auf den Grund gehen. Dabei wurden Standardverfahren der Dendrochronologie angewendet und nur dahingehend modifiziert, dass meist die Jahrringe mehrere Stammscheiben eines Strauchs analysiert wurden. Die vorliegende Arbeit führt erst allgemein in das Thema ein: Dabei werden der Klimawandel in der Arktis, die Wissenschaftsdisziplin Dendroökologie von Sträuchern und ihre Entwicklung sowie die grundlegenden Forschungsfragen und der Aufbau der Dissertation einführend erläutert. Dann folgen sieben wissenschaftliche Fachartikel, deren wichtigste Ergebnisse und Schlußfolgerungen abschließend synthetisiert werden. Aus deren Analyse, auch im Vergleich mit anderen Arbeiten, werden dann unmittelbar Empfehlungen für zukünftige Forschungsansätze destilliert und die Zukunft des Forschungsfeldes reflektiert. Die wichtigsten Erkenntnisse meiner Arbeit sind: I) Die Anwendung dendroökologischer Methoden auf Sträucher ist möglich. Unter ungünstigen Umweltbedingungen bilden sowohl Sträucher als auch Bäume unter Umständen als Anpassungsreaktion keine durchgehenden Jahrringe an der Stammbasis aus. II) Es gibt Anzeichen für eine Strauchexpansion in Skandinavien. III) Es ist mir gelungen, eine kausale Beziehung zwischen der Klimaerwärmung und der zirkumpolaren Strauchexpansion bzw. dem verstärkten Höhen- und Dickenwachstums der Sträucher herzustellen, indem hohe Temperaturen in den Sommermonaten als wichtigste Einflußgröße für das Strauchwachstum identifiziert wurden. IV) Die Ergebnisse meiner Forschungen in der Tundra Alaskas zeigen, dass sich eine erhöhte Schneedecke signifikant und stark negativ auf das Strauchwachstum auswirken kann, wodurch eine populäre Hypothese (snow-shrub-microbe hypothesis) in ihrer generellen Gültigkeit relativiert wird. V) Die räumliche Heterogenität der Strauchexpansion spiegelt sich in deutlichen Unterschieden in der Vegetation und im Boden von expandierenden respektive stabilen Strauchbeständen wider. VI) Diese Unterschiede werden durch unterschiedliche Standorte in der Landschaft mit unterschiedlicher Störungsfrequenz prädisponiert. VII) Strauchringchronologien konnten erfolgreich als natürliches Klimaarchiv genutzt und zur Modellierung der sommerlichen Massenbilanz eines Gletschers verwendet werden. IV) Eine Synthese von Untersuchungen, die an mehr als 25 zirkumpolaren Untersuchungsstandorten von den wichtigsten Forschern auf diesem Gebiet durchgeführt wurden, bestätigt das Vorhandensein einer zirkumpolaren Strauchexpansion, zeigt eine Übersicht der Methoden zur Strauchforschung und weißt zukünftigen Forschungsbedarf auf. Diese Ergebnisse meiner Doktorarbeit zeigen, dass die Anwendung dendroökologischer Methoden auf Sträucher interessante neue Einsichten liefert und dass dadurch unser Verständnis in Bezug auf die Analyse wachstumsbeeinflussender Faktoren, auf die Rekonstruktion vergangener Umweltbedingungen und auf die Erfassung von Populationsdynamiken dieser „Bäume der Tundra“ deutlich verbessert werden konnte.Abstract: The Arctic has experienced a pronounced increase in air temperature over the last four decades, with an average increase of 0.4 °C per decade and thus an increase of almost the double rate than that of temperate regions. Remote sensing studies and repeat photography of historical images have shown large-scale increases of plant productivity in tundra ecosystems over the same time period. A pronounced size, abundance and biomass increase of shrubs has been observed. This so called shrub expansion has important repercussions for the vegetation, the animals, the soil, the energy and the carbon balance of the Arctic tundra and on regional and global climate. As the comparison of historical photographs with recent photographs has shown, this shrub expansion occurs on different temporal and spatial scales with areas of strong increase in shrub cover (expanding patches) and areas without noticeable changes in shrub vegetation (stable patches). While remote sensing approaches for the detection of changes in vegetation are limited in their temporal coverage and so far also in their resolution, historical photographs with high resolution are often not available. Experimental studies have shown that an increase in nutrients or temperature often resulted in increased shrub biomass, but findings were partly contradictory, referred to short term observations and usually confined to small areas. To bridge the gap between spatially limited plot-scale experiments and global large-scale assessment of plant productivity by satellite derived pictures, dendrochronology was used in this thesis to analyze the drivers for and the rate of shrub growth of different widespread evergreen and deciduous shrub species in alpine and arctic tundra and to reconstruct historic environmental conditions. In detail, this doctoral thesis was conducted to study shrub growth and to assess the applicability of traditional dendrochronological methods on shrubs that had been so far mainly applied to trees and to test whether shrubs differed morphologically from trees. Further, I was determined to look for evidence for a possible Scandinavian shrub range expansion and to assess which climatic factors – temperature, precipitation or snow – influenced shrub growth significantly. Moreover, we aimed to find the reason for the observed heterogeneity of the shrub expansion on the landscape and its relevance for the three most common shrubs on the Alaskan tundra. The methods applied followed the routines usually applied for dendrochronological analyses of treerings, with the exception that usually several stem discs of the main stem were analyzed and frequently had to be prepared with help of a microtome as thin-sections, that were stained and sealed on a coverglass before annual shrubrings were measured. The averaged shrubring widths were then compared with environmental factors through correlation and regression methods. This thesis gives first a general introduction to climate change in the Arctic, shrub expansion on the tundra, the scientific discipline of dendrochronology or -ecology on shrubs and its development, the main research questions and the thesis outline. Then seven research papers are presented and the main results and conclusions are synthesized and discussed and finally possible venues of future research are outlined. The most important insights gained from this thesis are the following: I) Dendroecological methods can be applied to shrubs. Insights into shrub morphology have been gained by detecting an interesting mechanism for coping with adverse environmental conditions of both, trees and shrubs that can save resources by confining the production of wood to the upper parts of the stem. II) Further, I found evidence for a shrub expansion in Scandinavia. III) I could establish the causal link between the current climate warming and increased radial and vertical shrub growth by identifying summer temperature as main driver for shrub growth. IV) Results from the Alaskan tundra indicate a strongly adverse role of snow for shrub growth in stable patches, refuting the popular snow-shrub-microbe hypothesis for this extensive area across species. The differing influence of snow is likely linked to the presence of permafrost and shallow active layers and the snow’s contribution to moist or even anoxic conditions in Alaska. V) Furthermore, we found that the different rates and the spatial heterogeneity of shrub expansion are accompanied by strong differences in the surrounding vegetation composition and the soil parameters of expanding (accustomed to more favorable conditions) and stable shrub patches. VI) These differences are predisposed by shrub patch position within the landscape, comprising different levels and rates of disturbance. VII) Additionally, shrub ring records were successfully used as natural archives to model past temperature dynamics respectively summer glacier mass balance with high accuracy. VIII) Finally, a synthesis of the climate-growth relationships of shrubs of more than 25 sites around the Arctic as joined effort together with other leading shrub researchers supports the presence of a circumpolar shrub expansion, gives recommendations for methods used in shrub dendroecology and lays out future research directions. The findings of my dissertation research show that the analysis of shrubs by dendroecological methods yields highly interesting results, and they greatly improved our understanding of factors that influence individual shrub growth, the reconstruction of earlier environmental conditions as well as the reconstruction and assessment of plant population dynamics
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