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

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 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:

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.