Worldwide mangrove deterioration has been attributed to anthropogenically or climatically induced alterations in environmental conditions. However, due to the lack of long-term data sets, causal relationships are hard to prove. The aim of this study was to investigate the potential of Rhizophora mucronata wood as a proxy for changes in environmental conditions. Using cambial marking, as well as trees of known age (plantation trees), annual growth rings were discovered in this mangrove tree species. The annual growth rings provide information on the age and the growth rates of the trees. However, ring width time-series failed to synchronize, indicating that the inter-annual variability in tree growth could not be related to a common external (environmental) factor and consequently, that ring width cannot be used as a proxy to study changes in environmental conditions on an annual resolution. Intra-annual variability in vessel density and size revealed a trade-off between hydraulic efficiency (large vessels) during the rain season and hydraulic safety (small, more numerous vessels) during the dry season. In addition to this annual earlywood/latewood signal, a semi-annual signal was also discovered after Fourier transformation of the high-resolution vessel density and diameter profiles. The annual and semi-annual signal was also observed in the Fourier spectra of the precipitation and relative humidity and is related to the bimodal distribution of the rainfall in Kenya, resulting in a long and short rain season (semi-annual signal) and one distinct dry season (annual signal). The similarity in the Fourier spectra of the climate data and the vessel features offers strong additional evidence for a climatic driving force controlling the vessel density and diameters, either through changes in the relative humidity or through changes in the pore water salinity caused by precipitation, or both. High-resolution stable isotope profiles revealed a clear annual cyclicity with minima and maxima occurring in the vicinity of the latewood/earlywood and earlywood/latewood boundaries, respectively. The presence of this annual cyclicity offers a high potential for identifying growth rings in tropical trees that lack visible anatomical growth rings. Similarity between the high-resolution isotope profiles of the mangrove trees and temperate tree species suggests that the annual cyclicity is the result of a climatic signal superimposed on the signal caused by post-photosynthetic processes. Intra-annual and inter-annual trends of element distribution in the tree rings of R. mucronata provided evidence for limited radial translocation of Ca, Sr, K, Mg, Na, S, Rb and Mn, while P was only translocated in older trees. In addition, only Ca showed a dependence on the growth rate, resulting in a dilution effect of the Ca concentration at high growth rates. These results indicate that the concentration of elements in the tree rings may provide information on environmental conditions. However, interpretation of the data is complicated by the complex interaction between plants, soil and elements. This study demonstrates the potential of R. mucronata as an environmental proxy, however, also highlights new directions further research should take. In addition, the techniques developed during this study offer potential to the field of tropical dendrochronology as a whole.
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