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

Abstract:

The mountainous island of Corsica in the western Mediterranean basin hosts Mediterranean as well as alpine ecosystems. It is highly affected by current climate change which manifests in an increasing number of heat waves and drought periods. The interdisciplinary bundle project CorsicArchive integrates dendroecological, climatological, and hydrological studies on the island of Corsica. The superordinate aim of the project is to better understand the forcing factors and fractionation processes responsible for spatial and temporal oxygen isotope variability in tree rings and in the hydrological cycle. Here, we present results from the dendroecological part of the project. We established a 30-year long intra-annual oxygen isotope chronology from tree-ring cellulose of Corsican Black pine (Pinus nigra ssp. laricio) growing at an upper tree line site in order to identify typical intra-annual patterns and the climate parameters responsible for the oxygen isotope variability. The isotope ratios in the early part of the early wood are strongly influenced by the amount of snow during winter, while patterns in the remaining part of the tree ring are mainly determined by summer precipitation conditions. In total, three different intra-annual patterns could be observed which can be attributed to different climate conditions during the vegetation period. The stable isotope pattern reflecting dry-warm conditions during summer occurs with higher frequency after 2007, thus confirming a trend towards pronounced drought stress during summer, which might become critical for Black pine growing at dry sites if becoming more extreme in the future.

Abstract:

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.

Abstract:

The western Mediterranean basin is subject to ongoing climate change, exhibiting more pronounced drought periods during the growing season. Therein, the mountainous island of Corsica, France, was recently exposed to two extreme but strikingly different consecutive years: 2017 was especially dry and yielded a record minimum in annual precipitation whereas 2018 was particularly wet with summer precipitation more than doubling the long-term average. However, both years were equally warm, being the second and third warmest summers since 1900 for entire France. Within this setup, 42 pines were equipped with high-resolution band dendrometers spanning an E-W transect across the central mountain range. Circumference growth data were obtained at five study sites, located in three different elevation belts ranging from sea level up to 1600 m asl. We analyse the different reactions of Maritime pine (Pinus pinaster) in low regions and Corsican Black pine (Pinus nigra ssp. laricio) in high elevations, to the aforementioned climatic conditions. Results show distinctive differences in the length of the growing period in all elevation belts. During the summer drought of 2017, the two coastal sites showed stagnation or even shrinkage of tree circumferences, whereas the western part of the transect was more affected than the east. In contrast, all sites exhibited continuous growth over the summer months in 2018. Additionally, obtained microcore samples will help to pinpoint cambial activity and indicate the differences of earlywood and latewood formation in different elevations under inter-annually varying climatic conditions.

Abstract:

The mountainous island of Corsica in the western Mediterranean basin hosts Mediterranean as well as alpine ecosystems. It is highly affected by current climate change which manifests in an increasing number of heat waves and drought periods. These climate conditions can become crucial for the vitality of pine trees on the island. The interdisciplinary bundle project CorsicArchive integrates dendroecological, climatological, and hydrological studies on Corsica. The superordinate aim of the project is to better understand the forcing factors and fractionation processes responsible for spatial and temporal oxygen isotope variability in tree rings of pine trees and in the hydrological cycle. We analyzed stable oxygen isotope data in intra-annual resolution from tree-ring cellulose, needle water, precipitation and underground water at five different locations along an altitudinal gradient ranging from sea level to 1600 m asl. With this approach we were able to disentangle which water sources are used by the trees growing under different climate conditions. Altitudinal differences in absolute isotope ratios were obvious in all materials except tree-ring cellulose. The isotope values of tree-ring cellulose and their intra-annual distribution pattern depend on the water sources used by the trees. We could clearly distinguish different intra-annual oxygen isotope patterns for trees with access to groundwater from trees which depend on precipitation during the growing season as their main water source.

Abstract:

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 2700m 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 1600m 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.

Abstract:

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 2700m 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 1600m 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.

Abstract:

The island of Corsica, France, poses a unique study area. The fourth largest island of the Mediterranean Sea is dominated by a north-south oriented mountain range that peaks at 2700 metres. In previous studies Pinus nigra subsp. laricio proved to be a reliable source of robust ringwidth chronologies at tree line sites. But the characteristics of tree growth in other parts of the island are yet to be unveiled. Due to the distribution of two pine species (Pinus pinaster in lower areas and Pinus nigra subsp. laricio in higher regions) it is possible to cover the altitudinal range of the island from alpine environments to Mediterranean coasts. By means of tree-ring analysis, the relation between tree age and temporal growth changes was evaluated via growth trajectories for pines in different age classes at various sites across the island, ranging from 1600m asl to sea level. We hypothesize that tree growth in high altitudes is limited by temperature whereas the growth limiting factor in the lowlands is summer drought. First analyses on differences and similarities in growth trajectories depending on altitude, species, and east or west slope of the mountain range will be presented.

Abstract:

The western Mediterranean is one of the most prominent climate change hotspots in Europe and also worldwide. A closely related scientific question to such climatic changes is the contribution of rivers, streams and connected groundwater to the amount of carbon dioxide released to the atmosphere from inland waters. The role of rivers and associated groundwater within the global carbon cycle as a major source of CO2 has long been neglected and even more so headwater streams. So far, only few studies have taken into account carbon stable isotopes as a tracer for sources and sinks of carbon. The high-relief Tavignanu River (Corsica, France) with a total length of 89 km and an altitude difference of 1750 m, from a mountainous source lake to the river mouth to the Mediterranean Sea, was investigated for potential processes that influence the riverine carbon budget. These processes include water mixing (especially with groundwater), weathering, changes in lithology, and seasonal variations. Riverine dissolved inorganic carbon (DIC) concentrations and its stable isotope ratios (?13CDIC) showed seasonal variations. In February, ?13CDIC values of –2 to –7 ‰ (VPDB) indicated influences of atmospheric CO2. During springtime, stable isotope values started to gradually decrease to lower values in July that ranged from –9 to –12 ‰, that indicate an increased contribution from soil- and grondwater-derived CO2 to the riverine DIC pool. The median pCO2 values averaged over the entire Tavignanu River course were 160 µatm for February, 340 µatm for May, and 540 µatm for July thus showing the river as a CO2 sink in winter and only a slight source in summer.

Abstract:

Paleoclimate proxies such as tree rings rely on stable isotope ratios for the reconstruction of past climatic conditions. Such information then allows the calibration of models that evaluate and predict ongoing and future effects of global climate change. A region expected to be widely affected is the North-western Mediterranean. In that region, the island of Corsica was subject to isotope hydrology and dendroisotope studies in the recent past. Based on the findings of these previous investigations the new project “CorsicArchive" was launched in 2017. The project is designed to integrate climate modeling, isotope hydrology, wood anatomy and dendroisotopes for a better understanding of key aspects within the hydrologic cycle and climate reconstruction. The project aims to trace the stable isotope ratios of water along its pathway in the hydrologic cycle from its vapor source over precipitation to the uptake by trees where the information is eventually stored. For these purposes we placed 10 rain collectors in an E-W transect over the islands central main mountain range. With this setup, and two additional automatic rain event samplers, we foresee high-resolution sampling to map spatial and temporal variations. Currently conducted sampling of precipitation and soil water in conjunction with xylem water samples from trees are promising to decipher the sources and fate of water from its source to the climatic archives. We will present preliminary results from this exciting study.