Elemental composition of tree rings: A new perspective in biogeochemistry

Abstract

Chemical elements that are associated with the ionic composition of the cytoplasm of cells are crucial components of higher plants. They enable various pro� cesses throughout the lifetime of plants associated with cellular energy, operation of a large number of enzymes, etc. (1). Entering the plant from the soil, chemical elements perform their functions in the plant and with its demise close important biogeochemical cycles (2). Longliving woody plants are themselves components of biogeochemical cycles with varying characteristic times: from one year to centuries. Tree rings that form each year in woody plants record the current climatic conditions on one hand, and they accumulate information about longterm changes of their environment (3) on the other. Wood cells mature in one growing season as cell walls lignify, and upon death, they serve as a waterconducting system. Chemical elements that are rigidly linked to the com� ponents of the cell walls of wood and that preserve this link for many decades are thus valuable. They can be used as a potential source of information about the processes and factors that govern their content that took place during the maturing of wood cells. In this paper we present for the first time the changes in biologicallyimportant elements (P, K, Ca, Mg, Sr, Ba, Cl, Si) from 1300 to 2000 AD in the annual rings of larch ( Larix gmelinii Rupr.) from the Taimyr Peninsula (polar boundary of forest vegetation, 72° N). In this study we only consider selected elements that are considered most informative, although the method allowed us to determine the concentration of a total of 40 elements. There are many publications dedicated to the elemental composition of tree rings over short time intervals, however the potential of using the signal of the elements for making long reconstructions of the paleoenvironmental conditions remains unfulfilled. Such a long dendrochemical record for the elements is obtained for the first time. The conditions of tree growth on the permafrost soils of the Taimyr Peninsula are strictly limited by temperature. Wood samples (tree discs) were collected in 2006. Dating of discs was car� ried out using a long master chronology that was pre� viously obtained for this area (4). The actual lifespan of the trees was no more than 350-400 years; for each calendar period 3-6 samples from discs of different ages were measured. Covering the time interval of 700 years was achieved by using samples from both liv� ing and very well preserved dead trees. The total num� ber of samples analyzed from sixteen trees was 351. A single sample contained a 5�year or 10�year period of growth (5 or 10 calendardated tree rings). Chemi� cal analysis was carried out using the method of mass spectrometry with inductively coupled plasma (ICPMS) employing a quadrupole spectrometer Agilent 7500ce. Measurements were made on solu� tions obtained by dissolving wood specimens in con� centrated nitric acid and their subsequent dilution with water. The method of processing wood samples, their dissolution, and the subsequent sample prepara� tion and measurement is described in detail in (5). The longterm average values as well as the range of changes of concentrations of the elements in wood samples are given in the table. The most abundant ele� ments in the wood samples are K + and Ca 2+ , which are important components of the cytoplasm. Ba and Sr, which are the chemical analogs of Ca and Mg, have the lowest concentrations. Based on the temporal vari� ability of all investigated elements, they can be divided into three groups. The first group consists of K and P, which are characterized by the following dynamics: a relatively stable level of concentration during the period from 1300 to 1950 and then a sharp increase in