Workpackage 3: Intra-annual Isotope Process Studies


Workpackage Leader:

Gerd Helle (g.helle@fz-juelich.de), Forschungszentrum Jülich GmbH, ICG-V, 52425 Jülich, Germany


Objectives:

To trace the fate of C,O and H stable isotopes from the atmosphere into tree-rings

To evaluate seasonality of radial distribution of C, O and H isotopes and ecophysiological behaviour

To assess the relation of seasonal O and H isotope variations between precipitation and tree-rings

To determine seasonal variations of tree-ring stable isotopes in relation to climatic variables

To provide data these data for the generation of transfer functions/process models


Methodology / work description:

Seasonal changes in ambient environmental conditions are recorded within a tree-ring through the continuous accumulation of wood cells during growth. Climatic factors govern isotopic effects at the site of photosynthesis and subsequent fractionations related to plant physiological processes alter further the isotope signals stored in the tree-rings. Whilst these processes are essentially understood, they remain difficult to quantify/model in climatic terms. High resolution intra-annual process studies will provide a quantitative mechanistic understanding of these processes and their response to seasonal environmental variations. Enhanced knowledge of the seasonal course of these processes will increase our capability to interpret both inter- and intra-annual climatic signals in tree-ring isotopes.

Detailed process studies will be carried out on 40 year old oak and pine trees at an established lysimeter facility in W-Germany. Both oak and pine will be studied owing to their role in the European Isotope Network and characteristic seasonality of growth. The lysimeter facility provides over 35 years of treegrowth measurements, meteorological variables and local water cycling. The site is also situated between two IAEA-GNIP-stations (Emmerich & Bad Salzuflen) providing >20 years of isotopic precipitation data.

C-isotope measurements of different metabolites will be used to trace the fractionations involved in the seasonal interplay of major tree physiological processes. In addition, atmospheric CO2 above and below the canopy will be also be monitored. The fate of oxygen and hydrogen isotopes in precipitation will be traced through soil moisture and compartment specific analyses of the arboreal system. Soil water status and isotopic (O&H) composition of xylem water, leaf water and phloem sap will be analysed. In addition to these measurements bud and leaf organic matter, soluble sugars and starch extracted from leaves, phloem sap and tree-ring material (bulk and cellulose) will help to trace the fate of atmospheric CO2.

Cambium pinning, anatomical and phenological observations and leaf-gas exchange measurements will be carried out to refine the resulting models further. Point-Dendrometer measurements will also be used provide exact dating of intra-annual tree-ring growth and will enable the noninvasive dating of thin sectioned material without resulting the ambiguities of chemical heterogeneitiy induced by cambial wounding methods. Sampling will be carried out at a variety of timescales, from daily and hourly during periods of fast growth to bi-monthly during the dormant period over two complete growing seasons. The resulting information will be utilised in WP 4 for the generation of transfer functions and mechanistic models and supplemented by additional ecophysiolocal measurements obtained through an ongoing DEKLIM project (Germany). The intra-annual investigation will then be extended back over 10 consecutive years (at a 10μm resolution) for two trees. More than 100 subdivisions/year promise intra annual resolutions of less then a week with which to evaluate the seasonal changes of climatic quantities and seasonal changes of isotopes in precipitation.