Foroozan Z (2021)
Publication Language: English
Publication Type: Thesis
Publication year: 2021
Publisher: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
City/Town: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Naturwissenschaftliche Fakultät
URI: https://opus4.kobv.de/opus4-fau/frontdoor/index/index/docId/15616
Precise knowledge about the long-term hydroclimatic variability is essential to analyze the impact of natural and anthropogenically induced climate change, especially in the world's arid to semi-arid regions. Iran is located in one of the world's most water-scarce and dry areas in Southwestern Asia. Hence, the country is highly subjected to water scarcity, water-related natural hazards such as drought and flash flood, and thus prone to their consequences on the ecosystem and human society. Besides, the growing population and the thirst for socio-economic development, in turn, increase the region's vulnerability to climate change. However, the current solutions to the aggravating water crisis are inadequate and inefficient without a clear picture of regional water availability in the future. In this context, a major defect is the lack of hydroclimate (precipitation) data of the past and present. A neglected area in this regard is the high potential of natural climate archives for extending our knowledge about past and present variations in hydroclimate.
This research was thus initiated to explore the potential of a natural climate proxy for paleoclimate reconstructions to improve our understanding of long-term regional climate variability in Iran. Accordingly, we evaluated the potential of stable isotopes in tree-ring cellulose to serve as paleoclimate indicators. We determined the climate signal recorded in δ13C and δ18O variations in tree-ring cellulose. Besides, this study tested the ecological responses of two tree species belonging to different plant functional types, an evergreen conifer (Juniperus polycarpos) and a broadleaved deciduous oak (Quercus macranthera), growing under distinct climate conditions to their environments, to increased CO2, and recent climate change by analyzing trends of intrinsic water-use efficiency (iWUE). In this thesis, we evaluated species-specific responses to drought of those tree species to provide baseline data for possible forest management efforts alleviating the impacts of expected climate change.
The observed increasing trends in iWUE indicated reduced stomatal conductance in both study species in response to the changing climate and elevated atmospheric CO2. The mean correlations among individual δ13C series in each site and between the site δ13C chronologies provide strong evidence of a robust common climate signal in isotope series.
The proxy-climate analysis revealed that the interplay between the combined influence of climate and local site conditions and species behavior determines δ13C variations in both species. The results showed that drought stress, which is controlled by precipitation, is the dominant climatic control on the δ13C fluctuations in both oak and juniper trees. In general, juniper responded more sensitive to moisture stress and is thus a more suitable proxy for paleohydroclimatic studies in northern Iran.
Furthermore, δ18O-climate analysis indicated a strong effect of moisture availability on δ18O of tree-ring cellulose in juniper. Accordingly, the first set of analyses and data comparison indicated a high potential of δ18O variations in juniper tree-ring cellulose for paleohydroclimatic investigations in northern Iran.
In the next step, this thesis assessed various pooling techniques for combining cross-dated tree-rings from different tree individuals before isotopic analysis. The evaluations indicated that an alternative approach to individual-tree isotope measurements, combined with high time and cost efficiency, is inter-tree pooling. Accordingly, the results confirmed that a pooled δ18O chronology can be used for paleoclimate reconstructions in northern Iran without compromising high-frequency climate signals.
Subsequently, we were able to develop a well-replicated 501-year spanning tree-ring δ18O chronology, representing the first stable isotope archive with annual resolution for paleohydroclimatic investigations in Iran. The statistical characteristics of the tree-ring δ18O chronology, such as the expressed population signal (EPS), confirmed that the isotope chronology contained a robust climate signal. The regional δ18O chronology showed a significant negative correlation with precipitation and vapor pressure deficit, and a positive correlation with temperature during the growing season. Therefore, it was concluded that the proxy record reflects site moisture conditions controlled mainly by precipitation and is thus useful for the reconstruction of past hydroclimate conditions, like long-term precipitation variations. Consequently, we succeeded in reconstructing hydroclimatic variability for the northern semi-arid region of Iran over the period 1515-2015 using the juniper δ18O chronology. The reconstructed precipitation series captured the magnitude of inter-annual and long-term variability in precipitation and the intensity, duration, and frequency of dry/wet events. During the past 500 years, pronounced phases with wetter (18th century) and drier (16th century) conditions than today are apparent. The reconstructed spring precipitation record gradually declined from the beginning of the 19th century and culminated in a drier period in the early 20th century. It is also evident that since the 18th century, the overall frequency of years with dry events has increased significantly. The study suggests the strong influence of regional atmospheric factors such as the nearby Caspian Sea, the local topography, the high elevation, and the influence of the westerlies on precipitation variations at the study site.
This thesis managed
to obtain conclusive evidence for the high potential of stable isotope
dendroclimatology to investigate long-term hydroclimate variability in a
semi-arid region of Iran. The findings of this study could support
decision-makers in sustainable water resource management and climate change
adaptation plans. An additional important implication of this research is the
contribution to complement the sparse network of available instrumental climate
stations and available proxy records, and the projection of future changes in
the region's climate.
APA:
Foroozan, Z. (2021). Stable isotope variations in tree rings as indicators of environmental change and paleohydroclimate variability in north Iran (Dissertation).
MLA:
Foroozan, Zeynab. Stable isotope variations in tree rings as indicators of environmental change and paleohydroclimate variability in north Iran. Dissertation, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Naturwissenschaftliche Fakultät: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), 2021.
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