Project

CorsicArchive - Altitudinal Gradients and Forest Response: Climate, Hydrology and Isotope Variability of a Mediterranean Ecosystem

Goal: CorsicArchive contributes to a better understanding of past and future climate in the western Mediterranean and enables assessment of hydroclimatic variability as well as changes of forest ecosystem productivity along an elevation gradient on Corsica. Thus, the project integrates climatological, hydrological, and dendroecological studies and focuses on the understanding of oxygen isotope variability in the hydrological cycle and in tree rings of native pines. The sub-projects (climatology, isotope hydrology, dendroecology, and dendroisotopes) are closely linked and allow for a synthesis of current and past hydro-climatic conditions on Corsica.

Date: 1 January 2017 - 31 December 2021

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Project log

Martin Häusser
added a research item
Stem radial variations of Corsican Black pine (Pinus nigra Arnold subsp. laricio Maire) and Maritime pine (Pinus pinaster Aiton) were monitored to quantify the impact of two meteorologically contrasting consecutive years. On the French island of Corsica, in the western Mediterranean basin, the year 2017 was extremely dry, while 2018 was exceptionally wet. We attached electric band dendrometers to 36 pines along an east–west transect, spanning the central mountain range, and set up automated weather stations at all five sites, ranging from 10 m asl to 1600 m asl. Stem radial variations (SRV) were separated into irreversible growth (GRO) and tree water deficit (TWD) periods. During the drought of 2017, the most severe tree water deficits occurred in the western part of the island, whereas trees at higher elevations were more affected than at lower elevations. A prolonged decrease of SRV, even close to the tree line, suggests bimodal growth and reveals high plasticity of growth patterns in both Corsican pines. Stem radial variations correlated significantly with precipitation and temperature. The positive correlations of GRO with precipitation and the negative correlations of TWD with temperature imply that high evapotranspiration led to the intense period of TWD in 2017. A novel approach was used to further investigate the growth/climate relationship by including synoptic-scale pressure situations. This revealed that an elevation gradient in GRO per weather pattern was only present in the wet year and that even rarely occurring weather patterns can have a substantial impact on tree growth. This novel approach provides a more comprehensive insight into meteorological drivers of tree growth patterns by incorporating different scales of the climatic system.
Martin Häusser
added a research item
The French Mediterranean island of Corsica is already today confronted with a clear tendency towards water shortage, leading not only to socio-economical, but also to ecological problems. A potential, but not very widespread source of water is the presence of near-ground clouds, mostly fog. In this study, we investigate fog-low stratus (FLS) frequencies in Corsica, derived from a data set of Meteosat Second Generation SEVIRI, whereby a distinction between fog and low stratus is hardly feasible using remote sensing data. The FLS frequency was studied with respect to its interaction with distinct locally-generated wind and its dependence on the planetary boundary layer height (PBLH) obtained by ERA5 reanalysis (the fifth generation of the European Centre for Medium-Range Weather Forecasts, ECMWF). Results show that radiation FLS is formed in coastal areas at sunrise, with low PBLH. On the other hand, in the interior of the island at sunset, a maximum of advection FLS is formed, fostered by locally-generated and related transport of moisture. On the east side of the island, FLS frequency is lower throughout the year due to frequent lee situations. This situation is reinforced by reduced synoptic moisture transport by westerly winds, so that westerly exposed slopes benefit from moisture input by FLS formation.
Frederic Huneau
added a research item
For climate models that use paleo-environment data to predict future climate change, tree-ring isotope variations are one important archive for the reconstruction of paleo-hydrological conditions. Due to the rather complicated pathway of water, starting from precipitation until its uptake by trees and the final incorporation of its components into tree-ring cellulose, a closer inspection of seasonal variations of tree water uptake is important. In this study, branch and needle samples of two pine species (Pinus pinaster and Pinus nigra subsp. laricio) and several water compartments (precipitation, creek, soil) were sampled over a two-year period and analyzed for the temporal variations of their oxygen and hydrogen stable isotope ratios (δ¹⁸O and δ²H) at five sites over an elevation gradient from sea level to around 1600 m a.s.l. on the Mediterranean island of Corsica (France). A new model was established to disentangle temporal relationships of source water uptake of trees. It uses a calculation method that incorporates the two processes mostly expected to affect source water composition: mixing of waters and evaporation. The model results showed that the temporal offset from precipitation to water uptake is not constant and varies with elevation and season. Overall, seasonal source water origin was shown to be dominated by precipitation from autumn and spring. While autumn precipitation was a more important water source for trees growing at mid- (~800–1000 m a.s.l) and high-elevation (~1600 m a.s.l.) sites, trees at coastal sites mostly took up water from late winter and spring. These findings show that predicted decreases in precipitation amounts during the wet season in the Mediterranean can have strong impacts on water availability for pine trees, especially at higher elevations.
Frederic Huneau
added a research item
Drought is a major factor limiting tree growth and plant vitality. In the Mediterranean region, the length and intensity of drought stress strongly varies with altitude and site conditions. We used electronic dendrometers to analyze the response of two native pine species to drought and precipitation events. The five study sites were located along an elevation gradient on the Mediterranean island of Corsica (France). Positive stem increment in the raw dendrometer measurements was separated into radial stem growth and stem swelling/shrinkage in order to determine which part of the trees’ response to climate signals can be attributed to growth. Precipitation events of at least 5 mm and dry periods of at least seven consecutive days without precipitation were determined over a period of two years. Seasonal dynamics of stem circumference changes were highly variable among the five study sites. At higher elevations, seasonal tree growth showed patterns characteristic for cold environments, while low-elevation sites showed bimodal growth patterns characteristic of drought prone areas. The response to precipitation events was uniform and occurred within the first six hours after the beginning of a precipitation event. The majority of stem circumference increases were caused by radial growth, not by stem swelling due to water uptake. Growth-induced stem circumference increase occurred at three of the five sites even during dry periods, which could be attributed to stored water reserves within the trees or the soils. Trees at sites with soils of low water-holding capacity were most vulnerable to dry periods.
Frederic Huneau
added a research item
Water availability is the most important factor for the vitality of forest ecosystems, especially in dry environments. The Mediterranean region is one of the hotspots of future climate change; therefore, data on the water cycle are urgently needed. We measured oxygen isotope compositions in creek water, precipitation, stem water, needle water, and tree-ring cellulose over one growing season to establish the relationship between isotope compositions in different compartments along a fractionation pathway. We analyzed plant material from pine trees (Pinus nigra J.F. Arn subsp. laricio (Poiret) Maire var. Corsicana Hyl. and Pinus pinaster Aiton) at five locations along an elevation gradient from sea level to 1600 m asl. We traced back the oxygen isotope composition from source to sink in tree-ring cellulose in order to identify the water sources used by the trees, and to quantify the extent of isotope fractionation processes. Our results showed that the trees used different water sources over the course of the growing season, ranging from winter snow meltwater to summer precipitation at higher sites and deep soil water reservoirs at coastal sites. Needle water enrichment was higher at higher elevation sites than at coastal locations, highlighting the importance of site-specific climate conditions on the isotopic composition values in tree material. Water availability seems to be most restricted at the highest site, making these trees most vulnerable to climate change.
Frederic Huneau
added 4 research items
Forest fires are an important factor shaping Mediterranean ecosystems and determine the distribution of different species. Information about past forest fires can be obtained with pyrodendroecology. Here, we present a fire history for three sites in the mountain forest belt on the island of Corsica in the Mediterranean Basin. The dating of scars from cores, stem discs, and wedges from 101 pine trees (Pinus nigra and Pinus pinaster) allowed the reconstruction of six definite fire events between 1800 and 2017. Additionally, we reconstructed the spatial extent of a large fire event at AD 2000 with remote sensing data. The study sites are affected by different types of fires. The even-aged forest structure at two sites is clear evidence of past lethal fires, whereas the old-growth Pinus nigra forest at the third site represents the final stage of succession under current climate conditions. The current distribution of both pine species can at least partly be regarded as a result of varying fire frequency at different sites. Although Pinus pinaster stands dominate in areas with frequent fires and can even replace Pinus nigra in cases of high fire frequency, Pinus nigra dominates in areas with low fire frequencies.
Key message Intra-annual oxygen isotope patterns of tree-ring cellulose from Pinus nigra subsp. laricio and Pinus pinaster along an elevation gradient can be divided into four distinct patterns ((1) high values in the beginning of the tree ring, declining trend afterwards; (2) low values in the beginning, increasing trend afterwards; (3) high values in the middle of the tree ring; (4) declining trend over the whole tree ring). These patterns reflect the dependency on local site conditions and different water sources. Context Intra-annual oxygen isotope chronologies from tree-ring cellulose can serve as a proxy for past climate conditions at inter- or even intra-seasonal resolution. Aims We explore the potential of these chronologies from pine trees (Pinus nigra J.F. Arn subsp. laricio (Poiret) Maire var Corsicana Hyl. and Pinus pinaster Aiton) growing on the Mediterranean island of Corsica, France, along an elevation gradient ranging from sea level to 1600 m asl. Methods We developed high-resolution intra-annual oxygen isotope chronologies from tree-ring cellulose of eight trees from five sites along the elevation gradient. The analysis period covers the decade 2007–2016 for seven trees and the period 1987–2016 for one tree, respectively. Results The chronologies show a high variability between trees, sites, and years. We identified four different intra-annual oxygen isotope patterns which reflect the dependency of tree growth on different water sources (precipitation during the growing season vs. deeper soil water reservoirs) and climate conditions during the growth season. Trees at coastal sites have access to groundwater, while trees growing in high elevations mostly depend on precipitation during the growth season as water source. Conclusion The different patterns can be attributed to the use of different water sources. The intra-annual chronology covering 30 years indicates an increasing frequency of dry-warm summers after 2007.
The high-relief catchment of the Tavignanu River (Corsica Island, France) with an elevation range from sea level to 2622 m above sea level was investigated for its riverine carbon budget and stable carbon isotopes. Major riverine dissolved inorganic carbon (DIC or TCO2) sources depended on seasons and sub-catchment lithology. In winter δ¹³CDIC values of −2 to −7‰ (VPDB) indicated influences of atmospheric CO2. δ¹³CDIC values decreased gradually to values between −9 and −12‰ in July, which indicates elevated soil CO2 contribution. An observed downstream increase in the total amount of carbon species correlated with inputs from carbonate bearing tributaries and evaporation effects in summer. Main channel partial pressure of CO2 (pCO2) was seasonally highly variable in the upper silicate catchment and the lower coastal plain, where summer values exceed up to six times atmospheric levels. During winter, the central section of the Tavignanu River was found to be undersaturated with respect to atmospheric CO2 levels. The median values for main channel pCO2 were below atmospheric levels in winter and spring and above in summer and autumn. The annual carbon flux across the air-water boundary (FCO2) along the Tavignanu River was calculated with (0.77 ± 0.24) Gg C yr⁻¹, which is about seven times higher than the riverine TCO2 transport to the sea of about 0.11 Gg C yr⁻¹. While large sections of the river experienced year-round atmospheric CO2 uptake or equilibrium, the river as a whole was a small but continuous net source of carbon to the atmosphere. This underlines the important, but so far not well-constrained, contributions of smaller streams and rivers to the terrestrial carbon flux and the need of incorporating them into future global carbon cycle models.
Isabel Knerr
added a research item
The island of Corsica in the western Mediterranean is characterized by a pronounced topography in which local breeze systems develop in the diurnal cycle. In interaction with the large-scale synoptic situation, various precipitation events occur, which are classified in this study with regard to their duration and intensity. For this purpose, the island was grouped into five precipitation regimes using a cluster analysis, namely the western coastal area, the central mountainous region, the southern coastal area, the northeast coastal area, and the eastern coastal area. Based on principal component analysis using mean sea level pressure (mslp) obtained from ERA5 reanalysis (the fifth generation of the European Centre for Medium-Range Weather Forecasts, ECMWF), six spatial patterns were identified which explain 98% of the large-scale synoptic situation, while the diurnal breeze systems within the regimes characterize local drivers. It is shown that on radiation days with weak large-scale pressure gradients, pronounced local circulations in mountainous regions are coupled with sea breezes, leading to a higher number of short and intense precipitation events. Meridional circulation patterns lead to more intensive precipitation events on the eastern part of the island (30% intensive events with meridional patterns on the east side compared to 11% on the west side). On the west side of Corsica, however, coastal precipitation events are seldom and less intense than further inland, which can be attributed to the influence of the topography in frontal passages.
Sonja Szymczak
added 6 research items
Located in the western Mediterranean basin, the island of Corsica (France) is expected to be strongly affected by climate change. With its diverse topography and altitudinal gradients from sea level to 2700 m asl, it hosts two pine species, i.e. Pinus pinaster in lower areas and P. nigra ssp. laricio in higher regions up to the upper tree line. However, the growth performance of these two dominant forest tree species along different elevation belts is still unknown, so it is not clear how climate change will affect their distribution. The relationships between tree age, elevation, and growth performance were evaluated using growth trajectories derived from tree-ring analysis of pines in different age classes at 13 sites across the island, ranging from sea level to 1600 m asl. Results show that the growth performance of both species under the same environmental conditions is very similar. For both species, we found that growth rates change over time, whereby trees younger than 50 years grew faster than older trees. We also found that P. pinaster is becoming common at higher elevation sites, indicating a migration upslope.
In the ongoing scientific effort to improve the knowledge about climate change, paleoclimate proxies can provide crucial insights into past climatic conditions. This is especially important in the Mediterranean which is predicted to be strongly affected by climate change. Stable oxygen isotopes of tree rings can be used as one of those proxies but seem not to be readily interpretable on the island of Corsica. To fill this gap and identify potential influences on isotope content during the waters pathway from source in the oceans to sink in trees, climatology, dendroecology, isotope-hydrology and dendroisotope research have been united in a unique multi-disciplined project.
Sonja Szymczak
added 4 research items
Oxygen isotope ratios can serve as parameters for water-use efficiency and photosynthetic regulations when high-resolution (intra-annual) isotope chronologies are developed. Additionally, a better understanding of the reasons for small scale isotope variations is essential for a robust paleoclimatic interpretation of long-term tree-ring isotope variability. Our centennial-long oxygen isotope chronologies from Pinus nigra tress growing on the island of Corsica revealed that the δ18O signal is very complex and is influenced by several local environmental factors. Hence, we developed intra-annual isotope chronologies along an east-west transect through the Renoso massif in the south of the island. Due to the alignment, the selected sites cover the full range of Corsican climate from coastal sites with Mediterranean climate to alpine conditions with continuous snow cover for several months. The chronologies were developed for two pine species (Pinus nigra and Pinus pinaster) and cover the last 10 years, thus allowing the exploration of the "normal" intra-annual isotope cycle of the two pine species and the effect of climatic extreme years on the intra-annual cycle. Additional analysis of oxygen isotope ratios in precipitation, needle water, soil water and xylem water will help to interpret the oxygen isotope chronologies and enables us to trace back sources of isotope variability, e.g. different water sources, origin of air masses.
Der Mittelmeerraum gehört zu den Regionen, die mit am stärksten auf den Klimawandel reagieren und dessen Ökosysteme durch zunehmende Hitze- und Dürreperioden gefährdet sind. Um die klimatische Sensitivität der Vegetation auf vergangene und zukünftige Klimaänderungen analysieren zu können, integriert das Projekt „CorsicArchive - Alitudinal Gradients and Forest Response: Climate, Hydrology and Isotope Variability of a Mediterranean Ecosystem“ dendroökologische, klimatologische und hydrologische Untersuchungen auf der Insel Korsika im westlichen Mittelmeer. Korsika stellt mit seinen steilen Höhengradienten zwischen mediterranen und alpinen Ökosystemen ein einzigartiges Untersuchungsgebiet dar. Während das Klima in den niedrigen Bereichen durch typisches Mittelmeerklima mit Sommertrockenheit geprägt ist, gewinnen mit zunehmender Höhe alpine klimatische Charakteristika (Schneebedeckung, Frosttage) an Bedeutung. Entlang eines Höhengradienten, der das gesamte Klimaspektrum der Mittelmeerinsel abdeckt, werden verschiedene dendroökologische Parameter (Jahrringbreiten, holzanatomische Variablen, Holzdichte, Variationen von Sauerstoffisotopen) an zwei Kiefernarten erfasst: an der im Tiefland verbreiteten Seestrandkiefer (Pinus pinaster) und der im Hochland vertretenen Korsischen Schwarzkiefer (Pinus nigra ssp. laricio). Auf diese Weise lassen sich Einflüsse von Trockenstress und Temperaturmangel auf das Baumwachstum besonders gut anhand der ökologischen Reaktionen der Bäume vergleichen. An den Standorten in den mittleren Höhenlagen, an denen sich die Habitate beider Arten überschneiden, werden die unterschiedlichen Anpassungsstrategien anhand holzanatomischer und wachstumsphänologischer Merkmale untersucht.
Oxygen isotope ratios in tree rings have been established as an important archive for palaeoclimatic research questions. Additionally, they can help to better understand tree physiological processes because they are influenced, beside the isotopic signature of source water, by fractionation processes in leaf water during transpiration, biochemical fractionation during biosynthesis and 18O/16O exchange reaction between xylem water and phloem concentrate. Here we present four annually resolved long-term oxygen isotope chronologies from different upper tree line locations on Corsica. The chronologies are characterized by a high variability between different locations. Trees from the southern sites show lower mean δ18O values which is attributed to precipitation originating from subtropical air masses. Correlations of the δ18O values with climate parameters are very weak; however, significant correlations with winter precipitation may indicate a dependency on the isotopic signature of winter precipitation. This hypothesis is further substantiated by strong correlations of tree-ring δ18O with δ18O values of precipitation and stream water. For a thorough interpretation of the oxygen isotope chronologies, the new established project CorsicArchive (Altitudinal Gradients and Forest Response: Climate, Hydrology and Isotope Variability of a Mediterranean Ecosystem) foresees to measure oxygen isotope ratios in tree rings, needle water, xylem water, soil water and precipitation in intra-annual intervals on the island of Corsica. The superordinate aim of the project is to better understand the processes responsible for spatial and temporal oxygen isotope variability in tree rings and in the hydrological cycle on Corsica. The project contributes to a better understanding and quantification of palaeoclimate in the western Mediterranean and enables assessment of past and future hydroclimate variability and of changes of forest ecosystem productivity along an elevation gradient on a Mediterranean island.
Martin Häusser
added a project goal
CorsicArchive contributes to a better understanding of past and future climate in the western Mediterranean and enables assessment of hydroclimatic variability as well as changes of forest ecosystem productivity along an elevation gradient on Corsica. Thus, the project integrates climatological, hydrological, and dendroecological studies and focuses on the understanding of oxygen isotope variability in the hydrological cycle and in tree rings of native pines. The sub-projects (climatology, isotope hydrology, dendroecology, and dendroisotopes) are closely linked and allow for a synthesis of current and past hydro-climatic conditions on Corsica.