Ecological information of ring width, stable carbon isotope composition and wood anatomical variables in tropical tree rings - A contribution to dendrochronology in the tropics

Abstract

Trees are long living organisms and record ecologically relevant information in their growth rings. Studying tree growth on the base of tree rings can help to fill some of the major research gaps concerning the age and life history of trees as well as their response to changing environmental conditions. Forest ecosystems in tropical regions show high species diversity and a large variability of climatic zones, resulting in a multitude of possible wood anatomical ring structures and resultant relations to ecological and environmental parameters. To ensure the successful application of dendrochronological studies a detailed knowledge of wood-anatomical structures and the variability of tree rings is required. Chapter 2 of the dissertation describes macroscopically apparent growth structures and explains the anatomical background of the visibility of tree ring structures. Traditional methods of age determination like direct radiocarbon measurements of the oldest parts of a tree, calculations from repeated diameter measurements as well as the prediction of longevity from mortality rates are discussed and the advantages of tree ring analysis to study age and growth rates in tropical trees are given. In Chapter 3 the dendro-climatic potential of two dominant species from dry forests in Northern Namibia is examined. Both species show a strong correlation between ring width and climate as well as with El Niño Indices, suggesting a common external influence on tree growth on a regional scale. Both tree species are a source of reliable tree ring data, which reflect sensitivity to climate. In Chapter 4 carbon isotope composition of tree ring series from different hydrological sites are compared to define how rainfall variations are influencing 13C fractionation in tropical trees to enable a better interpretation of stable isotope patterns in the wood. The correlation between precipitation time series and tree-ring isotopic data is significantly negative. The results of this study show that tropical trees, like temperate trees, are recording carbon isotopic signals in the wood, which can be linked to common external factors and can be used to date tropical trees. For a better understanding of the relations between external factors and wood structure, wood anatomical variables of tropical trees across taxa and a wide climatic gradient are analyzed in Chapter 5. Vessel diameter shows the strongest and most significant correlations to climate parameters and tree performance. A remarkable result is the observed influence of climate factors on the tissue compostion of trees as only little is known on the significance of this relation. Overall tree morphology has the strongest impact on wood anatomy, followed by climatic influences. Nevertheless, the inter-relation of these factors might shift depending on phylogenetic constraints, individual tree performance as well as climate variability. In summary time series of ring width, stable carbon isotope composition and wood anatomical variables are valuable contributions to dendrochronology in the tropics and are further steps on the way to close the existent research gaps in tropical forest ecology and climate change. In future tree ring analysis methods will help to gain realistic increment data for the implementation of sustainable management plans, reliable proxy data for tropical climate reconstruction and real tree ages for understanding forest and population dynamics.