Dating exotic trees – green ebony under
DIFFABS X-rays

The width of annual rings is influenced to varying degrees, depending on the species, by precipitation, temperature, sunlight, soil type and ambient conditions (human action, diseases, parasites, etc.). Simply put: the wider the ring, the more favorable were the living conditions. If trees of the same species and of the same age are studied, having grown under similar environmental conditions, we find, on the cut trunks or on samples (cores) from living trees, sequences of similar rings, which can act as chronological markers.

These markers, combined with analysis of the chemical composition of these tree rings (nitrogen content, nutrient cations, etc.) and their isotopic composition (essentially, carbon and oxygen), can provide information on the effects of environmental changes on the functioning of trees. The environmental changes often studied at present include, e.g. changes in atmospheric CO₂ concentration and in the climate.

The annual creation of rings is related to the seasonal cycles experienced in the mid-latitudes: in autumn and winter the tree grows little or not at all. With resumed growth in spring, a new ring is created, easily distinguishable from the preceding one. But what happens to trees in the tropics?

In areas where seasonal climatic variations are small and the conditions are always conducive to plant growth, this annual phenomenon is not found. Although tropical trees sometimes have dark rings, they do not appear to be regular. Thus, we do not know what corresponds to a ring or how much time elapses between two consecutive rings. In other words, the growth rate and age of tropical trees have remained, until now, largely unknown. This gap in our information may seem quite incredible when, for example, tools exist for dating plant fossils. Without the presence of annual rings, it is not possible to use dendrochronology to determine the age of trees, nor to reconstruct the variations in growth they experienced during their lifetime. It becomes very difficult to manage tropical forests well when dendrochronology cannot be replaced by another reliable method. However, beyond the scientific interest that is represented by such a quantity and variety of trees, everyone knows how important these forests are for the "health" of our planet.

Section of a tree trunk

This problem, surprisingly little studied by scientists so far, has mobilized the INRA Phytoecology group * in Nancy, which is focusing its attention on the invisible rings of tropical trees. Synchrotron techniques could bring new elements on this subject. We met the group on the DIFFABS beamline.

The samples brought by Stéphane Ponton’s group were somewhat atypical: these were cores from the trunks of tropical trees (Tabebuia serratifolia, Pericopsis elata, etc.), from forests in French Guiana and plantations in the Republic of the Congo. Each "lath", forty centimeters long and about 4 cm in diameter, was placed on a translation table, acquired and adapted by DIFFABS and the mechanics workshop at SOLEIL in anticipation of this series of experiments. With the movement of the table, the entire length of the sample is gradually passed, through successive steps of about a hundred microns, in front of the X-ray beam produced by the beamline. The fluorescence emitted is recorded by a detector placed perpendicular to the axis of the incident X-ray beam (see Figures 1 and 2). Fluorescence spectra are measured at energies spanning the range corresponding to the elements K to Zn.

Figure 1: Stéphane Ponton sets up the experiment on the DIFFABS diffractometer. The bar of green ebony to be analyzed is placed on the marble slab at the bottom right of the image.	Figure 2 : Dominique Thiaudière (Head of DIFFABS) and Stéphane Ponton position the wood sample in front of the X-ray beam output (bottom right). The fluorescence detector is located on the left, perpendicular to the sample axis (above S. Ponton’s arm).

The researchers start from the hypothesis that, even though the tree growth is continuous, the environmental variations that it will undoubtedly experience during this growth (changes in rainfall patterns, physico-chemical changes in the soil, etc.) will be "integrated" into the wood. The aim is to be able to find a cyclical pattern to these variations in the wood, such as repeated peaks of calcium or zinc, etc. Knowing the age of the tree from planting, the frequency of the peak’s occurrence can be calculated and correlated to a given "event", e.g., peaks of calcium and zinc associated with wet seasons, etc.

In its laboratory in Nancy, the Phytoecology group has been using this same fluorescence approach, but the brilliance of synchrotron radiation can increase the sensitivity of the measurements: fluorescence signals of low intensity - as emitted by trace elements present in the sample - can be detected. In addition, each measurement is acquired in seconds, so it becomes possible to analyze a larger number of samples. Among all the data collected, scientists hope to find THE marker or markers that will allow these trees to be dated. Long term work!

Stéphane Ponton and his colleagues in Nancy have come for the first time to SOLEIL, and it will take weeks to analyze the quantity of spectra recorded over 4 days of experiments. This first attempt has allowed them to improve on the assembly with Dominique Thiaudière’s group on DIFFABS, optimize sample selection and, in short, to "smooth the rough edges" of the experiment. The next runs, because they hope to come back, promise to be even more intensive!

* Département EFPA (Ecologie des Forêts, Prairies et milieux Aquatiques), Unité Ecologie et Ecophysiologie Forestière
http://www.nancy.inra.fr/l_inra_en_lorraine/structures/eef