Historical droughts in Mediterranean regions during the last 500 years: a data/model approach

International audienceWe present here a new method for comparing the output of General Circulation Models (GCMs) with proxy-based reconstructions, using time series of reconstructed and simulated climate parameters. The method uses k-means clustering to allow comparison between different periods that have similar spatial patterns, and a fuzzy logic-based distance measure in order to take reconstruction errors into account. The method has been used to test two coupled ocean-atmosphere GCMs over the Mediterranean region for the last 500 years, using an index of drought stress, the Palmer Drought Severity Index. The results showed that, whilst no model was able to exactly simulate the reconstructed changes, all simulations were an improvement over using the mean climate. Further, a good match was found after 1650 with a model run that took into account changes in volcanic forcing, solar irradiance, and greenhouse gases. A more detailed investigation of the output of this model showed the existence of a set of atmospheric circulation patterns linked to the patterns of drought stress: 1) a blocking pattern over northern Europe linked to dry conditions in the south prior to the Little Ice Age (LIA) and during the 20th century; 2) a NAO-positive like pattern with increased westerlies during the LIA; 3) a NAO-negative like period shown in the model prior to the LIA, but that occurs most frequently in the data during this period. The results of the comparison emphasise the importance of the inclusion of the various forcings in the models and help to understand the atmospheric changes connected to reconstructed climate changes

Influence de la densité des peuplements sur la croissance en hauteur et radiale de Pinus sylvestris L. en région méditerranéenne française.

Un des principaux objectifs de cette étude est de mettre en évidence et de quantifier l'influence de la densité des peuplements sur la croissance en hauteur et radiale du pin sylvestre afin de pour pouvoir ensuite dissocier ce phénomène des autres causes de modification de la croissance (changements climatiques)

Detection of a single synchronous growth change in several tree-ring series of south-east France

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Influence de la densité des peuplements sur la croissance en hauteur et radiale de Pinus silvestris L. en région méditerranéenne française

[Departement_IRSTEA]GT [TR1_IRSTEA]32 - GECOTER / ECOFRICHNational audienceThis study aimed at quantifying the influence of stands density on height and diameter growth of Pinus silvestris L., to dissociate it from the effects on this growth of short and long term variations of the climate. The study area is situated at 1030 metres of elevation, under Mediterranean influence, in south-eastern France. If the inter-tree competition generally accelerated height growth, to the detriment of the diameter growth, two thresholds of density (low and at the opposite very dense) where found, beyond of which the height growth decreased for lack of vs by excess of competition. For the same fertility level, and at the same cambial age, trees grow faster at present than 50 years ago. The annual growths in height and in diameter are well correlated with climate interannual variations, but with one year of time lag: The annual ring width depends mainly on the climate of the year, whereas the length of the terminal shoot of one year depends widely on the climate of the 2nd half of the previous year.Cette étude met en évidence et quantifie l'influence de la densité des peuplements sur la croissance en hauteur et sur la croissance radiale du pin sylvestre, afin de la dissocier des effets liés aux variations à court et long terme climat. Le site d'étude se situe à 1030 mètres d'altitude, sous influence méditerranéenne, dans le sud-est de la France. Si la compétition entre individu favorise en général la croissance, en hauteur au détriment de la croissance en diamètre, deux seuils de densité (un faible et un très dense) sont mis en évidence, au-delà desquels la croissance en hauteur diminue par manque ou par excès de concurrence entre individus. A fertilité égale, et au même âge, les arbres poussent plus vite actuellement qu'il y a 50 ans. Les croissances annuelles en hauteur et en diamètre sont bien corrélées aux variations interannuelles du climat, mais avec un an de décalage: la croissance en diamètre dépend surtout du climat de l'année du cerne, tandis que l'élongation de la pousse terminale d'une année dépend largement du climat du 2ème semestre de l'année précédente

Using a biogeochemistry model in simulating forests productivity responses to climatic change and [CO2] increase: example of Pinus halepensis in Provence (south-east France)

International audienceTree-ring chronologies provide long-term records of growth in natural environmental conditions and may be used to evaluate the impacts of climatic change and [CO2] increase on forest productivity. This study focuses on 21 Pinus halepensis forest stands in calcareous Provence (in the south-east France). Changes in productivity are simulated using the global biogeochemistry model BIOME3, that we have adapted to run with chronological data. Tree-ring data (width and density) were used to estimate, for each stand, an observed series of changes in productivity. Simulated and observed productivity changes are then compared to validate the chronological biogeochemistry model BIOME3C. Variations in productivity were well reconstructed at 15 sites. After this validation, BIOME3C was used to simulate forest productivity changes for a 2 x CO2 scenario. The 2 x CO2 climate used as input was obtained using results from Meteo-France's ARPEGE atmospheric general circulation model (ALCM), downscaled to local meteorological stations. Productivity increases moderately for all stands (from 17 to 24%) when climatic changes alone were taken into account. The main factor responsible for this increase is a reduction in summer drought severity. Productivity increases highly for all stands (from 72 to 86%) when the physiological fertilising effect of the [CO2] increase is considered separately. When both climatic changes and the [CO2] increase were taken into account, productivity increases highly, from 107% (for Moustier) to 141 % (for La Ciotat). The direct fertilising effect of [CO2] increase has a greater influence on the forest stands productivity than the indirect climatic changes effect. These results also exhibit the importance of the synergy between the effects of climate change and [CO2] increase, as the increase in productivity resulting from the combined effects are more than the sum of the individual CO2 and climate effects. Although the detected effects of global change during the 20th century were slight, acceleration of these changes is likely to lead to great changes in the future productivity of P. halepensis forests. (C) 2003 Elsevier B.V. All rights reserved

Using a biogeochemistry model in simulating forests productivity responses to climatic change and CO2 increase: example of Pinus halepensis in Provence (south-east France)

Tree-ring chronologies provide long-term records of growth in natural environmental conditions and may be used to evaluate the impacts of climatic change and [CO2] increase on forest productivity. This study focuses on 21 Pinus halepensis forest stands in calcareous Provence (in the south-east France). Changes in productivity are simulated using the global biogeochemistry model BIOME3, that we have adapted to run with chronological data. Tree-ring data (width and density) were used to estimate, for each stand, an observed series of changes in productivity. Simulated and observed productivity changes are then compared to validate the chronological biogeochemistry model BIOME3C. Variations in productivity were well reconstructed at 15 sites. After this validation, BIOME3C was used to simulate forest productivity changes for a 2 x CO2 scenario. The 2 x CO2 climate used as input was obtained using results from Meteo-France's ARPEGE atmospheric general circulation model (ALCM), downscaled to local meteorological stations. Productivity increases moderately for all stands (from 17 to 24%) when climatic changes alone were taken into account. The main factor responsible for this increase is a reduction in summer drought severity. Productivity increases highly for all stands (from 72 to 86%) when the physiological fertilising effect of the [CO2] increase is considered separately. When both climatic changes and the [CO2] increase were taken into account, productivity increases highly, from 107% (for Moustier) to 141 % (for La Ciotat). The direct fertilising effect of [CO2] increase has a greater influence on the forest stands productivity than the indirect climatic changes effect. These results also exhibit the importance of the synergy between the effects of climate change and [CO2] increase, as the increase in productivity resulting from the combined effects are more than the sum of the individual CO2 and climate effects. Although the detected effects of global change during the 20th century were slight, acceleration of these changes is likely to lead to great changes in the future productivity of P. halepensis forests. (C) 2003 Elsevier B.V. All rights reserved

Bioclimatic model of tree radial growth: application to the French Mediterranean Aleppo pine forests

International audienceThe potential effects of global changes on forests are of increasing concern. Dendrochronology, which deals with long-term records of tree growth under natural environmental conditions, can be used to evaluate the impact of climatic change on forest productivity. However, assessment of climatic change impacts must be supported by accurate and reliable models of the relationships between climate and tree growth. In this study, a bioclimatic model is used to explore the relationships between tree radial growth and bioclimatic variables closely related to the biological functioning of a tree. This model is at an intermediate level of complexity between purely empirical and process-based models. The method is illustrated with data for 21 Aleppo pine ( Pinus halepensis Mill.) stands grown under a Mediterranean climate in south-east France. The results show that Aleppo pine growth is mainly controlled by soil water availability during the growing season. The bioclimatic variable which best expresses the observed inter-annual tree growth variations is the actual evapotranspiration (AET). Four parameters were adjusted to simulate dendrochronological data: the soil water capacity, the wilting point, the minimum temperature for photosynthesis, and the end of the growing season. The bioclimatic model gives better results than the standard response function and provides better insight into the functional processes involved in tree growth. The convincing results obtained by the bioclimatic model as well as the limited numbers of parameters it requires demonstrate the feasibility of using it to explore future climatic change impacts on Aleppo pine forests.

Preserving long-term fluctuations in standardisation of tree-ring series by the adaptative regional growth curve (ARGC)

International audienceThis paper presents a new method for the standardisation of tree-ring series, which attempts to remove the age effect from the low-frequency variations in the series. standardisation techniques based oil the biological growth trend (RCS) only remove the trend linked to the age of the tree. However, in some trees, the trend is substantially different from the regional curve, and when the site fertility is not taken into account the standardisation process may induce significant biases in the RCS standardised curve An artificial neural network is used here to estimate an adaptive regional growth curve (ARGC) model. For each population or group Of Populations, the predictors are, in addition to the age (used by RCS) of each ring, the initial and the maximum growth rates (measured by the ring increments) of each tree. We have compared this method to the RCS method, Using 20 Pinus cembra sites covering the Southern French Alps. The results show that the ARGC standardisation performs better for growth trend analysis and, by inference, for climate reconstruction (C) 2009 Elsevier GmbH. All rights reserved