Process based model sheds light on climate sensitivity of Mediterranean tree-ring width

We use the process-based VS (Vaganov-Shashkin) model to investigate whether a regional <i>Pinus halepensis</i> tree-ring chronology from Tunisia can be simulated as a function of climate alone by employing a biological model linking day length and daily temperature and precipitation (AD 1959–2004) from a climate station to ring-width variations. We check performance of the model on independent data by a validation exercise in which the model's parameters are tuned using data for 1982–2004 and the model is applied to generate tree-ring indices for 1959–1981. The validation exercise yields a highly significant positive correlation between the residual chronology and estimated growth curve (<i>r</i>=0.76 <i>p</i><0.0001, <i>n</i>=23). The model shows that the average duration of the growing season is 191 days, with considerable variation from year to year. On average, soil moisture limits tree-ring growth for 128 days and temperature for 63 days. Model results depend on chosen values of parameters, in particular a parameter specifying a balance ratio between soil moisture and precipitation. Future work in the Mediterranean region should include multi-year natural experiments to verify patterns of cambial-growth variation suggested by the VS model

Biogeographic, Atmospheric, and Climatic Factors Influencing Tree Growth in Mediterranean Aleppo Pine Forests

There is a lack of knowledge on how tree species respond to climatic constraints like water shortages and related atmospheric patterns across broad spatial and temporal scales. These assessments are needed to project which populations will better tolerate or respond to global warming across the tree species distribution range. Warmer and drier conditions have been forecasted for the Mediterranean Basin, where Aleppo pine (Pinus halepensisMill.) is the most widely distributed conifer in dry sites. This species shows plastic growth responses to climate, being particularly sensitive to drought. We evaluated how 32 Aleppo pine forests responded to climate during the second half of the 20th century by using dendrochronology. Climatic constraints of radial growth were inferred by fitting the Vaganov-Shashkin (VS-Lite) growth model to ring-width data from our Aleppo pine forest network. Our findings reported that Aleppo pine growth decreased and showed the highest common coherence among trees in dry, continental sites located in southeastern and eastern inland Spain and Algeria. In contrast, growth increased in wetter sites located in northeastern Spain. Overall, across the Aleppo pine network tree growth was enhanced by prior wet winters and cool and wet springs, whilst warm summers were associated with less growth. The relationships between site ring-width chronologies were higher in nearby forests. This explains why Aleppo pine growth was distinctly linked to indices of atmospheric circulation patterns depending on the geographical location of the forests. The western forests were more influenced by moisture and temperature conditions driven by the Western Mediterranean Oscillation (WeMO) and the Northern Atlantic Oscillation (NAO), the southern forests by the East Atlantic (EA) and the august NAO, while the Balearic, Tunisian and northeastern sites by the Arctic Oscillation (AO) and the Scandinavian pattern (SCA). The climatic constraints for Aleppo pine tree growth and its biogeographical variability were well captured by the VS-Lite model. The model performed better in dry and continental sites, showing strong growth coherence between trees and climatic limitations of growth. Further research using similar broad-scale approaches to climate-growth relationships in drought-prone regions deserves more attention

Old World megadroughts and pluvials during the Common Era

Climate model projections suggest widespread drying in the Mediterranean Basin and wetting in Fennoscandia in the coming decades largely as a consequence of greenhouse gas forcing of climate. To place these and other “Old World” climate projections into historical perspective based on more complete estimates of natural hydroclimatic variability, we have developed the “Old World Drought Atlas” (OWDA), a set of year-to-year maps of tree-ring reconstructed summer wetness and dryness over Europe and the Mediterranean Basin during the Common Era. The OWDA matches historical accounts of severe drought and wetness with a spatial completeness not previously available. In addition, megadroughts reconstructed over north-central Europe in the 11th and mid-15th centuries reinforce other evidence from North America and Asia that droughts were more severe, extensive, and prolonged over Northern Hemisphere land areas before the 20th century, with an inadequate understanding of their causes. The OWDA provides new data to determine the causes of Old World drought and wetness and attribute past climate variability to forced and/or internal variability

Modelling climate and societal resilience in the Eastern Mediterranean in the last Millennium

This article analyses high-quality hydroclimate proxy records and spatial reconstructions from the Central and Eastern Mediterranean and compares them with two Earth System Model simulations (CCSM4, MPI-ESM-P) for the Crusader period in the Levant (1095–1290 CE), the Mamluk regime in Transjordan (1260–1516 CE) and the Ottoman crisis and Celâlî Rebellion(1580–1610 CE). During the three time intervals, environmental and climatic stress tested the resilience of complex societies.We find that the multidecadal precipitation and drought variations in the Central and Eastern Mediterranean cannot be explained by external forcings (solar variations, tropical volcanism); rather they were driven by internal climate dynamics. Our research emphasises the challenges, opportunities and limitations of linking proxy records, palaeoreconstructions and model simulations to better understand how climate can affect human history

Predicting species dominance shifts across elevation gradients in mountain forests in Greece under a warmer and drier climate

The Mediterranean Basin is expected to face warmer and drier conditions in the future, following projected increases in temperature and declines in precipitation. The aim of this study is to explore how forests dominated by Abies borisii-regis, Abies cephalonica, Fagus sylvatica, Pinus nigra and Quercus frainetto will respond under such conditions. We combined an individual-based model (GREFOS), with a novel tree ring data set in order to constrain tree diameter growth and to account for inter- and intraspecific growth variability. We used wood density data to infer tree longevity, taking into account inter- and intraspecific variability. The model was applied at three 500-m-wide elevation gradients at Taygetos in Peloponnese, at Agrafa on Southern Pindos and at Valia Kalda on Northern Pindos in Greece. Simulations adequately represented species distribution and abundance across the elevation gradients under current climate. We subsequently used the model to estimate species and functional trait shifts under warmer and drier future conditions based on the IPCC A1B scenario. In all three sites, a retreat of less drought-tolerant species and an upward shift of more drought-tolerant species were simulated. These shifts were also associated with changes in two key functional traits, in particular maximum radial growth rate and wood density. Drought-tolerant species presented an increase in their average maximal growth and decrease in their average wood density, in contrast to less drought-tolerant species

The North American tree-ring fire-scar network

Fire regimes in North American forests are diverse and modern fire records are often too short to capture important patterns, trends, feedbacks, and drivers of variability. Tree-ring fire scars provide valuable perspectives on fire regimes, including centuries-long records of fire year, season, frequency, severity, and size. Here, we introduce the newly compiled North American tree-ring fire-scar network (NAFSN), which contains 2562 sites, >37,000 fire-scarred trees, and covers large parts of North America. We investigate the NAFSN in terms of geography, sample depth, vegetation, topography, climate, and human land use. Fire scars are found in most ecoregions, from boreal forests in northern Alaska and Canada to subtropical forests in southern Florida and Mexico. The network includes 91 tree species, but is dominated by gymnosperms in the genus Pinus. Fire scars are found from sea level to >4000-m elevation and across a range of topographic settings that vary by ecoregion. Multiple regions are densely sampled (e.g., >1000 fire-scarred trees), enabling new spatial analyses such as reconstructions of area burned. To demonstrate the potential of the network, we compared the climate space of the NAFSN to those of modern fires and forests; the NAFSN spans a climate space largely representative of the forested areas in North America, with notable gaps in warmer tropical climates. Modern fires are burning in similar climate spaces as historical fires, but disproportionately in warmer regions compared to the historical record, possibly related to under-sampling of warm subtropical forests or supporting observations of changing fire regimes. The historical influence of Indigenous and non-Indigenous human land use on fire regimes varies in space and time. A 20th century fire deficit associated with human activities is evident in many regions, yet fire regimes characterized by frequent surface fires are still active in some areas (e.g., Mexico and the southeastern United States). These analyses provide a foundation and framework for future studies using the hundreds of thousands of annually- to sub-annually-resolved tree-ring records of fire spanning centuries, which will further advance our understanding of the interactions among fire, climate, topography, vegetation, and humans across North America

Runoff variations in Lake Balkhash Basin, Central Asia, 1779-2015, inferred from tree rings

Long highly-resolved proxies for runoff are in high demand for hydrological forecasts and water management in arid Central Asia. An accurate (R2 = 0.53) reconstruction of October-September discharge of the Ili River in Kazakhstan, 1779–2015, is developed from moisture-sensitive tree rings of spruce sampled in the Tian Shan Mountains. The fivefold extension of the gauged discharge record represents the variability of runoff in the Lake Balkhash Basin for the last 235 years. The reconstruction shows a 40 year long interval of low discharge preceded a recent high peak in the first decade of the 2000s followed by a decline to more recent levels of discharge not seen since the start of the gauged record. Most reconstructed flow extremes (± 2σ) occur outside the instrumental record (1936–2015) and predate the start of large dam construction (1969). Decadal variability of the Ili discharge corresponds well with hydrological records of other Eurasian internal drainages modeled with tree rings. Spectral analysis identifies variance peaks (highest near 42 year) consistent with main hemispheric oscillations of the Eurasian climatic system. Seasonal comparison of the Ili discharge with sea-level-pressure and geopotential height data suggests periods of high flow likely result from the increased contribution of snow to runoff associated with the interaction of Arctic air circulation with the Siberian High-Pressure System and North Atlantic Oscillation

Dendrochronology course in Valsaín forest, Segovia, Spain

Touchan, R.... et al.This report describes an international summer course, >Tree Rings, Climate, Natural Resources, and Human Interaction>, held in Valsaín, Spain, in summer of 2012. The course, with 14 participants from three countries (Spain, Algeria, and Russia), included basic training in dendrochronology skills as well as applied projects in dendroclimatology, dendroecology and dendrogeomorphology. Copyright © 2013 by The Tree-Ring Society.This International Summer School was carried out in collaboration with the Laboratory of Tree-Ring Research, University of Arizona, from the United States, and IGME (Instituto Geológico y Minero de España), IPE-CSIC (Instituto Pirenaico de Ecología - Consejo Superior de Investigaciones Científicas), ARAID foundation, INIA-CIFOR (Instituto Nacional de Investigación y Tecnología Agraria y Alimentaria-Centro de Investigación Forestal) and University of Córdoba, from Spain. The organizers sincerely thank the CENEAM (National Center for Environmental Education), Montes de Valsaín and OAPN (Organismo Autónomo de Parques Nacionales) from the Ministerio de Agricultura, Alimentación, y Medio Ambiente in Spain for the support and facilities to organize this course. We thank Drs. Rafael Herrera, Mar Génova and Guillermo Gea-Izquierdo for their contributions in the course. We thank AEET (Asociación Española de Ecología Terrestre) from Spain for generous grants provided to students. We thank the course sponsors: Tree-Ring Society, National Science Foundation-USA (Grant AGS-Paleo Perspectives on Climate Change Program 1103314), Regents Instruments, Beta Analytic, and Cox Analytical Systems.Peer Reviewe

Dendroclimatology in the Eastern Mediterranean

Dendroclimatology in the Eastern Mediterranean (EM) region has made important contributions to the understanding of climate variability on timescales of decades to centuries. These contributions, beginning in the mid-20th century, have value for resource management, archaeology, and climatology. A gradually expanding tree-ring network developed by the first author over the past 15 years has been the framework for some of the most important recent advances in EM dendroclimatology. The network, now consisting of 79 sites, has been widely applied in large-scale climatic reconstruction and in helping to identify drivers of climatic variation on regional to global spatial scales. This article reviews EM dendroclimatology and highlights contributions on the national and international scale.Center for Mediterranean Archaeology and the Environment (CMATE) Special Issue, Joint publication of Radiocarbon and Tree-Ring Research, also cited as Touchan, R., Meko, D., & Anchukaitis, K. (2014). Dendroclimatology in the Eastern Mediterranean. Radiocarbon, 56(4), S61-S68.This item is part of the Tree-Ring Research (formerly Tree-Ring Bulletin) archive. For more information about this peer-reviewed scholarly journal, please email the Editor of Tree-Ring Research at [email protected]