Tracing physiological processes of long living tree species and their response on climate change using triple isotope analyses

Kimak, Adam (2015). Tracing physiological processes of long living tree species and their response on climate change using triple isotope analyses (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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The global biomass acts as one of the largest sinks in the global carbon cycle for the human induced fossil fuel carbon emissions. It is continuously influenced by climate variables. Considering that the fundamental structure of plant cells is built by hydrogen, oxygen and carbon atoms taken up as water and atmospheric CO2, respectively, the life-cycle of plants is mainly driven by the availability of the aforementioned elements. Considering the impacts of changing global climate over the last century, namely the rapid increase of atmospheric CO2, air temperature and subsequently water availability among others, the investigation how the plants adapt to these influences became an important task of the climate science community. Implying stable isotope compositions, we have the opportunity to trace the impact of different climate conditions on plants under laboratory circumstances and in field studies. In this work, we applied a novel stable isotope approach that has the capability of simultaneous stable isotope recordings of carbon, hydrogen and oxygen. This innovation is both scientifically and economically advantageous, moreover it also decreases the invested time of complete isotope analysis including sample preparation and measurements (Chapter I).
Furthermore, we investigated the seasonal changes of heterotrophic and autotrophic pathways of cellulose synthesis on leaf level (Chapter II). Here, we collected and analyzed leaves and needles of two deciduous trees (Quercus robur and Fagus sylvatica) and one evergreen tree (Taxus baccata). The stable isotopic composition (carbon, hydrogen and oxygen) of leaf tissues revealed the temporal influence of biochemical processes on cellulose synthesis. In particular, deciduous species start to build the first leaf cells through the heterotrophic pathway and simultaneously apply autotrophic metabolism after the leaves become photo-synthetically active. Moreover, due to leaf development and the increasing capability of photosynthesis during growing season, the usage of heterotrophic metabolism decreases while the opposite was documented for autotrophy until the autotrophic metabolism becomes the dominant source of cellulose synthesis. Because evergreen species have needles from previous years, Taxus baccata forms the necessary carbohydrates via autotrophic metabolism from the beginning of growing season until the next winter dormancy.
For the observation of leaf development we also determined the alpha-cellulose yield that has an increasing trend for all species. The stable isotope analysis of deciduous and evergreen foliage is a developing research area that needs to be supported by information of species specific cellulose content of leaf tissues. This information at hand would be beneficial in relation to time and cost management. Therefore we built a syllabus including nine long living tree species and calculated the required weight of collected leaf material to do triple isotope characterization (Chapter III).
After the analysis of carbon isotopic composition of oak samples from Salvenach for two time periods (1780 - 1825 and 1960 - 1990) documenting information about the usage of allocated carbohydrates under different climate conditions (Chapter IV), we applied our technical improvements to the extended time period. In Chapter V, a 260 years long chronology of deciduous species (Quercus robur) from Salvenach, Switzerland was measured. This included the analysis of carbon, hydrogen, oxygen isotopic composition and tree ring width of inter-annual cross sections (early- and latewood), respectively. According to inter-annual changes of biochemical processes, earlywood is partly formed by carbohydrate reserves from previous years, while latewood cellulose is generally built by new assimilates during summer on the other hand. We were able to gain qualitative and quantitative information from carbon isotopes about the usage of allocated carbohydrates for xylem cellulose synthesis. Thus we found that under optimal conditions the plant used reserves from t-1 (previous year) to t-3 years while under non-optimal conditions reserves usage extended further backwards from t-4 to t-13 years.
Additionally, δD and δ18O isotopes covering 260 years were investigated to trace inter- and intra-annual coherences that are triggered by either water availability or biochemical processes. Surprisingly, we found an increasing trend over the investigated time period for both δD and δ18O. Considering the seasonal pattern of heterotrophic and autotrophic processes (described in Chapter II), the increasing trend might be induced by the increasing degree of heterotrophic (for hydrogen) and autotrophic (for oxygen) metabolism. For a better understanding of the relevant mechanisms we used two different pathways to model the isotopic composition of leaf water: (i) calculation based on a Craig - Gordon model for oxygen and hydrogen independently and (ii) calculation of hydrogen based on the Craig - Gordon modeloxygen simulation. Based on the model results, we hypothesize that the fractionation caused by the NADP+-NADPH cycle varied approximately from -250 to -160 h for earlywood and from -270 to -180 h for latewood. In addition, these fractionations reflect a mixed effect of hetero- and autotrophic metabolic pathways with a changing ratio.
Finally, a short study is presented where we measured hydrogen isotopic composition of Mediterranean evergreen species (Pinus nigra) living under typically arid climate condition (Chapter VII). We individually looked into the last century of seven increment cores providing annual time resolution series from Cazorla, a region in South-East Spain. Due to its location, the region is limited by water availability therefore δD isotopes are closely related to the local climate variability. We found that δD time series show positive correlations with air and sea surface temperature besides negative correlations with precipitation amount and drought index. In this regard hydrogen isotopic composition of tree ring cellulose can provide useful information for climate reconstruction especially for the Mediterranean region.
This work contains new information regarding past carbohydrate remobilization that might be a key component for model studies using stable isotope time series as input data or predict them. At present most of the land biosphere models do not have incorporated this process of remobilization which at least partly can explain differences between model results and observations, especially on the local level. Further, this would most probably help when a downscaling of model simulations with moderate spatial resolution to a given site is necessary. Moreover, tracing biochemical processes of cellulose synthesis by isotope fractionations during leaf development allows us to better understand the pathways of hydrogen and oxygen isotopes from the water uptake to the ring formation. Additionally, the interpretation of hydrogen isotopic composition of tree ring cellulose might be improved since δD of cellulose is mainly driven by the NADP+ - NADPH cycle and the equilibration fractionation of evaporation on the leaf level.

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