1,720,994 research outputs found
Testing for tree-ring divergence in the European Alps
No full textEvidence for reduced sensitivity of tree growth to temperature has been reported from
multiple forests along the high northern latitudes. This alleged circumpolar phenomenon
described the apparent inability of temperature-sensitive tree-ring width and density
chronologies to parallel increasing instrumental temperature measurements since the
mid-20th century. In addition to such low-frequency trend offset, the inability of
formerly temperature-sensitive tree growth to reflect high-frequency temperature signals
in a warming world is indicated at some boreal sites, mainly in Alaska, the Yukon and
Siberia. Here, we refer to both of these findings as the ‘divergence problem’ (DP), with
their causes and scale being debated. If DP is widespread and the result of climatic
forcing, the overall reliability of tree-ring-based temperature reconstructions should be
questioned. Testing for DP benefits from well-replicated tree-ring and instrumental data
spanning from the 19th to the 21st century. Here, we present a network of 124 larch and
spruce sites across the European Alpine arc. Tree-ring width chronologies from 40 larch
and 24 spruce sites were selected based on their correlation with early (1864–1933)
instrumental temperatures to assess their ability of tracking recent (1934–2003) temperature
variations. After the tree-ring series of both species were detrended in a manner that
allows low-frequency variations to be preserved and scaled against summer temperatures,
no unusual late 20th century DP is found. Independent tree-ring width and density
evidence for unprecedented late 20th century temperatures with respect to the past
millennium further reinforces our results
Low-frequency noise in delta C-13 and delta O-18 tree ring data: A case study of Pinus uncinata in the Spanish Pyrenees
No full textIsotopic discrimination measurements in tree rings are becoming increasingly important estimators of past environmental change. Potential biases inherent to these parameters, including age trend and level offset are, however, not well understood. We here perform measurements on a new millennium-long data set of decadally resolved δ18O and δ13C discrimination from 25 high-elevation pine trees in the Spanish Pyrenees to investigate whether such low-frequency biases exist and how they alter the long-term behavior of derived time series. Alignment of the tree ring data by biological age reveals age trends over the first one to four centuries after germination. On average, isotope values change by -0.089‰ δ18O and +0.064‰ δ13C per decade over the first 100 years of tree growth. This trend persists into the forth century after germination for δ18O but diminishes to ∼0‰ over the 100-390 year period for δ13C. We also find level offsets up to 7‰ δ18O and 3‰ δ 13C between single trees. Analysis of the shape of age trends reveals negative exponential functions as reasonable choices for detrending of δ18O and (inverted) δ13C time series. The detrended isotope chronologies share low-frequency variance with traditional ring width and density measurements beyond statistical chance, suggesting that the various parameters reflect some of the same environmental forcing. Our results show that δ18O and δ13C from these Spanish pines need to be detrended to assess long-term environmental changes. To evaluate the general applicability of this conclusion, production of (1) well-replicated, (2) nonpooled, and (3) composite chronologies from other species and regions will be required. Increases in measurement speed and technology will make these tests feasible in the coming years. Copyright 2010 by the American Geophysical Union
Five centuries of Central European temperature extremes reconstructed from tree-ring density and documentary evidence
No full textFuture climate change will likely influence the frequency and intensity of weather extremes. As such events are by definition rare, long records are required to understand their characteristics, drivers, and consequences on ecology and society. Herein we provide a unique perspective on regional-scale temperature extremes over the past millennium, using three tree-ring maximum latewood density (MXD) chronologies from higher elevations in the European Alps. We verify the tree-ring-based extremes using documentary evidences from Switzerland, the Czech Republic, and Central Europe that allowed the identification of 44 summer extremes over the 1550-2003 period. These events include cold temperatures in 1579, 1628, 1675, and 1816, as well as warm ones in 1811 and 2003. Prior to 1550, we provide new evidence for cold (e.g., 1068 and 1258) and warm (e.g., 1333) summers derived from the combined MXD records and thus help to characterize high-frequency temperature variability during medieval times. Spatial coherence of the reconstructed extremes is found over Switzerland, with most signatures even extending across Central Europe. We discuss potential limitations of the tree-ring and documentary archives, including the (i) ability of MXD to particularly capture extremely warm temperatures, (ii) methodological identification and relative definition of extremes, and (iii) placement of those events in the millennium-long context of low-frequency climate change. © 2010 Elsevier B.V
Lateglacial environmental variability from Swiss tree rings
No full textEvidence of annually resolved environmental variations during the Allerød interstadial is presented using 81 fossil Scots pine tree-ring series from Gaenziloo and Landikon, near Zurich, Switzerland. The absolute age of the trees ranges between 11,920 and 10,61014C BP, which was determined by wiggle-matching radiocarbon ages to the Cariaco14C data set. From the two sites we created a composite floating Allerød chronology on the basis of their 632 years of overlap (r=0.57), after individual spline detrending. Merging both data sets resulted in a Lateglacial tree-ring chronology covering 1050 years. Regional curve standardization (RCS) was applied to preserve low-frequency information. Growth behavior of the fossil trees was compared with a recent composite pine data set from the central Swiss Alps and reveals distinct differences. The new Allerød RCS chronology reveals major Lateglacial variations, such as the Gerzensee oscillation, the abrupt climate shift towards the Younger Dryas and some short-term events. Radiocarbon ages agree well with those from other sites on the Swiss Plateau. For hemispheric comparisons we used the annual layer thickness record from the NGRIP ice core and the gray-scale varve record from the Cariaco basin. Even though the amplitudes are not yet fully understood, similarities on decadal-to-centennial scales are apparent. © 2007 Elsevier Ltd. All rights reserved
Three centuries of insect outbreaks across the European Alps
No full text• Knowledge of the persistence of regular larch budmoth outbreaks is limited in
space and time. Although dendrochronological methods have been used to reconstruct
insect outbreaks, their presence may be obscured by climatic influences.
• More than 5000 tree-ring series from 70 larch host and 73 spruce nonhost sites
within the European Alps and Tatra Mountains were compiled. Site-specific assessment
of growth–climate responses and the application of six larch budmoth detection
methods considering host, nonhost and instrumental time-series revealed
spatiotemporal patterns of insect defoliation across the Alpine arc.
• Annual maps of reconstructed defoliation showed historical persistence of cyclic
outbreaks at the site level, recurring c. every 8–9 yr. Larch budmoth outbreaks
occurred independently of rising temperatures from the Little Ice Age until recent
warmth. Although no collapse in outbreak periodicity was recorded at the local scale,
synchronized Alpine-wide defoliation has ceased during recent decades.
• Our study demonstrates the persistence of recurring insect outbreaks during
AD 1700–2000 and emphasizes that a widely distributed tree-ring network and novel
analysis methods can contribute towards an understanding of the changes in outbreak
amplitude, synchrony and climate dependence
Long-Term effects of climate and land-use change on larch budmoth outbreaks in the French Alps
No full textThe intensity of cyclic larch budmoth (Zeiraphera diniana Guenée; LBM) outbreaks across the European Alps has been reported to have weakened since the early 1980s. In addition to a warmer climate, changes in land-use cover over modern and historical times may have affected the LBM system. Here, we present tree-ring-based reconstructions of LBM outbreaks from a mixed subalpine larch?pine forest in the French Alps for the period 1700?2010. Temporal variation in LBM outbreak severity was mainly driven by land-use changes, including varying forest structure and species composition. Human population pressure and associated resource demands for fuel wood and construction timber not only resulted in a reduction of larch and subsequent suppression of pine, but also supported an overall grassland expansion for livestock. Superimposed on modern land abandonment and pine re-colonization is a strong warming trend, which may also contribute to the observed late 20th-century weakening of Alpine-wide cyclic LBM outbreaks. Our results suggest that a complex interplay of different factors triggered less synchronized LBM outbreaks at broader scales, with overall significantly lower intensities at local scales
Tree-ring-based reconstruction of larch budmoth outbreaks in the Central Italian Alps since 1774 CE
Full text linkThe larch budmoth (Zeiraphera diniana Gn. - LBM) offers a unique example of cyclic fluctuations in insect populations. During regular LBM mass outbreaks, defoliation of European larch (Larix decidua Mill.) subalpine trees results in distinct ring-width reductions in the host trees. Although several observations, reconstructions and models suggest that LBM outbreaks travel from the southwest to the northeast along the Alpine arc, gaps in the underlying data still hamper our mechanistic understanding of the spatio-temporal system dynamics. Evidence of historical LBM outbreaks before 1964 is generally associated with uncertainty and is particularly scarce for the Central Italian Alps. Here, we introduce four new larch ring-width chronologies from Val di Sole in the Central Italian Alps and use time-series analyses and comparisons with non-host trees (Picea abies Karst.) to reconstruct LBM mass outbreaks. We identify distinct fingerprints of 15 spatially-synchronized LBM events that occurred between 1774 and 1964 CE. Our results are important for improving qualitative space-time models to simulate travelling wave dynamics of insect populations, and for correcting ring-width-based summer temperature reconstructions from this part of the Alpine arc
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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