Long-term effects of environmental change and species diversity on tree radial growth in a mixed European forest

Norway spruce (Picea abies), European beech (Fagus sylvatica), silver fir (Abies alba) and Scots pine (Pinus sylvestris) typically co-occur in European forests, but show contrasting response to climate and environmental change. Sustainable forest management therefore depends on species- and regional-specific information. Here, we use tree-ring width measurements of 334 beech, 280 fir, 144 spruce and 63 pine trees from 75 inventory plots in Slovakia to assess the predominant climatic factors that control radial stem growth of Europe’s economically most important forest species. All four species exhibit significant shifts in stem growth over the past 100 years. Ring width patterns were, however, not significantly affected by tree species diversity and site elevation. The resistance, resilience and recovery of all species to the extreme summer droughts between 1950 and 2003 suggest that spruce is the species most unsuitable for the predicted warmer and drier future. Silver fir may benefit from warmer conditions, although we cannot conclude that it will not suffer from predicted increased frequency of climate extremes. Forest management in this locality should aim to avoid significant loss of forest cover by replacing Norway spruce monocultures with mixed stands of silver fir and European beech

Radiocarbon offsets and old world chronology as relevant to Mesopotamia, Egypt, Anatolia and Thera (Santorini)

The new IntCal20 radiocarbon record continues decades of successful practice by employing one calibration curve as an approximation for different regions across the hemisphere. Here we investigate three radiocarbon time-series of archaeological and historical importance from the Mediterranean-Anatolian region, which indicate, or may include, offsets from IntCal20 (~0–22 14C years). While modest, these differences are critical for our precise understanding of historical and environmental events across the Mediterranean Basin and Near East. Offsets towards older radiocarbon ages in Mediterranean-Anatolian wood can be explained by a divergence between high-resolution radiocarbon dates from the recent generation of accelerator mass spectrometry (AMS) versus dates from previous technologies, such as low-level gas proportional counting (LLGPC) and liquid scintillation spectrometry (LSS). However, another reason is likely differing growing season lengths and timings, which would affect the seasonal cycle of atmospheric radiocarbon concentrations recorded in different geographic zones. Understanding and correcting these offsets is key to the well-defined calendar placement of a Middle Bronze Age tree-ring chronology. This in turn resolves long-standing debate over Mesopotamian chronology in the earlier second millennium BCE. Last but not least, accurate dating is needed for any further assessment of the societal and environmental impact of the Thera/Santorini volcanic eruption.</p

Radiocarbon offsets and old world chronology as relevant to Mesopotamia, Egypt, Anatolia and Thera (Santorini)

The new IntCal20 radiocarbon record continues decades of successful practice by employing one calibration curve as an approximation for different regions across the hemisphere. Here we investigate three radiocarbon time-series of archaeological and historical importance from the Mediterranean-Anatolian region, which indicate, or may include, offsets from IntCal20 (~0–22 14C years). While modest, these differences are critical for our precise understanding of historical and environmental events across the Mediterranean Basin and Near East. Offsets towards older radiocarbon ages in Mediterranean-Anatolian wood can be explained by a divergence between high-resolution radiocarbon dates from the recent generation of accelerator mass spectrometry (AMS) versus dates from previous technologies, such as low-level gas proportional counting (LLGPC) and liquid scintillation spectrometry (LSS). However, another reason is likely differing growing season lengths and timings, which would affect the seasonal cycle of atmospheric radiocarbon concentrations recorded in different geographic zones. Understanding and correcting these offsets is key to the well-defined calendar placement of a Middle Bronze Age tree-ring chronology. This in turn resolves long-standing debate over Mesopotamian chronology in the earlier second millennium BCE. Last but not least, accurate dating is needed for any further assessment of the societal and environmental impact of the Thera/Santorini volcanic eruption.ISSN:2045-232

Contrasting effects of environmental change factors on the radial growth of co-occurring European beech and fir trees across Europe

Under predicted climate change, silver fir (Abies alba) and European beech (Fagus sylvatica) are the most likely replacement species for the more heat- and drought-sensitive monocultures of Norway spruce (Picea abies) planted across large parts of continental Europe. Our current understanding of the climate-related adaptation potential of fir and beech mixtures is, however, limited. Here we compile and analyse 2855 tree-ring width (TRW) series from 17 mixed beech-fir forest sites in five European countries. Dendroecological techniques that combine various detrending methods with an application of documented environmental change trends reveal wide variation of radial growth between fir and beech across space and time. Coincidental with peak SO2 emissions, the growth of silver fir declined between 1950 and 1980 at most sites, whereas beech growth increased during this period. Correspondent to a significant warming trend from 1990–2010, average beech growth declined, but silver fir growth increased. Our observations of long-term growth trends support the replacement of spruce monocultures with multi-species mixtures that have demonstrated a higher tolerance of environmental changes

Congruency in fungal phenology patterns across dataset sources and scales

As citizen science and digitization projects bring greater and larger datasets to the scientific realm, we must address the comparability of results across varying sources and spatial scales. Independently assembled fungal fruit body datasets from Switzerland and the UK were available at large, national-scales and more intensively surveyed, local-scales. Phenology responses of fungi between these datasets at different scales (national, intermediate and local) resembled one another. Consistently with time, the fruiting season initiated earlier and extended later. Phenology better correlated across data sources and scales in the UK, which contain less landscape and environmental heterogeneity than Switzerland. Species-specific responses in seasonality varied more than overall responses, but generally fruiting start dates were later for most Swiss species compared with UK species, while end dates were later for both. The coherency of these results, across the data sources, supports the use of presence-only data obtained by multiple recorders, and even across heterogeneous landscapes, for global change phenology research

Priority questions in multidisciplinary drought research

Addressing timely and relevant questions across a multitude of spatio-temporal scales, state-of-the-art interdisciplinary drought research will likely increase in importance under projected climate change. Given the complexity of the various direct and indirect causes and consequences of a drier world, scientific tasks need to be coordinated efficiently. Drought-related research endeavors ranging from individual projects to global initiatives therefore require prioritization. Here, we present 60 priority questions for optimizing future drought research. This topical catalogue reflects the experience of 65 scholars from 21 countries and almost 20 fields of research in both natural sciences and the humanities. The set of drought-related questions primarily covers drought monitoring, impacts, forecasting, climatology, adaptation, as well as planning and policy. The questions highlight the increasingly important role of remote sensing techniques in drought monitoring, importance of drought forecasting and understanding the relationships between drought parameters and drought impacts, but also challenges of drought adaptation and preparedness policies

Mediterranean winter snowfall variability over the past millennium

The brevity of the instrumental record limits our understanding of snowfall variability and its directional patterns in the Mediterranean region. Here, we develop a 1,208-year-long (800-2017CE) reconstruction of central Mediterranean snowfall variability based on documentary evidence from Italy. The record suggests that the recent reduction in Italian snowfall intensity is not unprecedented over the past millennium, since comparable patterns of low snowfall intensity also occurred during the Medieval Climate Anomaly. Increased snowfall during the Little Ice Age, however, was most likely associated with a shift of the Atlantic multi-decadal variability towards negative values, and this overall cold phase further coincided with increased volcanic activity. Our findings on natural snowfall variability over the central Mediterranean in the past millennium provide a unique winter proxy for validating output from climate model simulations

Drought limitation on tree growth at the Northern Hemisphere's highest tree line

The alpine tree line is generally assumed to be controlled by low temperatures, and thus to be experiencing an upward shift under global warming. As global temperatures rise, tree growth at the tree line could either increase if temperature is the limiting factor or decrease if a warming-induced loss of moisture limits growth. Here, we use dendrochronological techniques to understand the abiotic drivers of the Northern Hemisphere's highest tree line ecotones on the southern Tibetan Plateau (TP). Ring-width measurements from three juniper sites between 4680 and 4900 m asl were significantly and negatively correlated with May-June-July evapotranspiration (ET0), and positively correlated with relative humidity and other moisture-related meteorological variables. At the same time, ring widths were negatively correlated with temperature means and sunshine rates. Our results highlight the common sensitivity of tree growth to moisture variations despite the differential growth trends occurring since 1850 (end of the Industrial Revolution) at the three tree line ecotones. These findings indicate that low temperatures may not be the sole driving force behind tree growth and the range dynamics of alpine tree lines. Tree lines in the dry parts of the TP and possibly also beyond are likely to retreat rather than to advance in a warmer world due to water limitations

Weakening of annual temperature cycle over the Tibetan Plateau since the 1870s

The annual cycle of extra-tropical surface air temperature is an important component of the Earth's climate system. Over the past decades, a reduced amplitude of this mode has been observed in some regions. Although attributed to anthropogenic forcing, it remains unclear when dampening of the annual cycle started. Here we use a residual series of tree-ring width and maximum latewood density from the Tibetan Plateau >4,000 m asl to reconstruct changes in temperature seasonality over the past three centuries. The new proxy evidence suggests that the onset of a decrease in summer-to-winter temperature difference over the Tibetan Plateau occurred in the 1870s. Our results imply that the influence of anthropogenic forcing on temperature seasonality might have started in the late nineteenth century, and that future human influence may further contribute to a weakening of the annual temperature cycle, with subsequent effects on ecosystem functioning and productivity

Improved recovery of ancient DNA from subfossil wood. Application to the world's oldest Late Glacial pine forest

Ancient DNA from historical and subfossil wood has a great potential to provide new insights into the history of tree populations. However, its extraction and analysis have not become routine, mainly because contamination of the wood with modern plant material can complicate the verification of genetic information.Here, we used sapwood tissue from 22 subfossil pines that were growing c. 13000yr bp in Zurich, Switzerland. We developed and evaluated protocols to eliminate surface contamination, and we tested ancient DNA authenticity based on plastid DNA metabarcoding and the assessment of post-mortem DNA damage.A novel approach using laser irradiation coupled with bleaching and surface removal was most efficient in eliminating contaminating DNA. DNA metabarcoding confirmed which ancient DNA samples repeatedly amplified pine DNA and were free of exogenous plant taxa. Pine DNA sequences of these samples showed a high degree of cytosine to thymine mismatches, typical of post-mortem damage.Stringent decontamination of wood surfaces combined with DNA metabarcoding and assessment of post-mortem DNA damage allowed us to authenticate ancient DNA retrieved from the oldest Late Glacial pine forest. These techniques can be applied to any subfossil wood and are likely to improve the accessibility of relict wood for genome-scale ancient DNA studies