Rainfall-driven variations in 13 C composition and wood anatomy of Breonadia salicina trees from South Africa between AD 1375 and 1995

Introduction According to climatic predictions by the IPCC, rainfall distribution in southern Africa will change during the next few decades; tropical regions are expected to receive more rain and subtropical regions less rain than today. However, there is a high degree of uncertainty associated with climate prediction models, as the inter-model variability is large for both tropical and subtropical regions of southern Africa. Despite indications of the significance of the tropics for global climate systems on various timescales (for example, refs 2–4), the dynamics of past climates of tropical and subtropical regions have not been studied, nor understood, as thoroughly as the palaeoclimate of temperate regions. During the last decade, however, the amount of high-resolution palaeoclimatic data from southern Africa covering the late Holocene has increased. Studies of speleothems, pollen, lake levels, and trees have contributed to valuable knowledge about past climatic change in the region. The spatial distribution of palaeoclimatic data is still sparse compared to that of temperate regions and a denser pattern of high-resolution data is needed in order to place regional climate change in a global context and to produce trustworthy climatic predictions. In dendroclimatology, different properties of annual tree rings (mainly width and density) are measured in order to extract palaeoclimate information with yearly resolution. In tropical and subtropical regions, dendroclimatology is used to a lesser extent than in temperate regions, because of the weaker temperature and day-length seasonality, which results in fewer trees producing distinct annual rings. However, sometimes rainfall seasonality can trigger trees to produce annual growth structures in the wood. In southern Africa this is observed in only a few, mainly gymnosperm species, where ring formations are often difficult to identify and interpret. In angiosperm trees, the rings may sometimes be identified by thin bands of parenchyma cells (thin-walled cells used mainly for storage and carbohydrate transport) or by gradual changes in the size of vessel cells (ring porosity). Attempts have been made to apply dendroclimatology in southern Africa, but only a few have overcome the problem of cross-dating between trees. These studies confirm that the use of dendrochronology and dendroclimatology in the tropics and subtropics is feasible, even though much more complicated, and more restricted in its timeextension, than in temperate regions. A method complementary to ring-width analysis is the study of chemical properties, such as stable carbon, oxygen and hydrogen isotopes in tree rings. Using carbon isotope composition as a proxy for climate in trees is based on fractionations of the stable isotopes C and C during photosynthetic transpiration and carbon fixation. The relation between C and C, expressed as the C value, is fixed within each growth ring, and each year a certain C signal is stored within the wood. As the magnitude of fractionation during photosynthesis is influenced by factors such as temperature, irradiation and soil moisture, the C variations in the tree rings may be used as a proxy for these climate-related parameters. Another way to retrieve climate information from trees is to analyse changes in wood anatomy, as the size and density of the water transporting cells (vessels) in certain angiosperm species change as a result of soil moisture status (for example, refs 24–26). This is an effective strategy for the tree, as only a small change in vessel diameter will affect the water conductivity dramatically, since the relationship between the factors is exponential. Gillespie et al. confirm a relationship between precipitation amounts and vessel properties for the same tree species investigated in this study, Breonadia salicina. This observation is promising for the use of this species as a palaeoclimate indicator. In the study reported here, we analysed variations in carbon isotope composition and wood anatomy in two B. salicina trees from northern South Africa (Fig. 1). At the start of this project, our expectation was that the visible layers in the trees were seasonal growth rings, and that data with annual resolution could be extracted. AMS datings of the wood revealed, however, that the tree was much older than the ring counts indicated. Thus, although the possibility of obtaining data with annual resolution was lost, the potential of obtaining a climatic record spanning a longer time period was gained. 162 South African Journal of Science 101, March/April 2005 Research Articles