Tree ring studies in the tropics and subtropics

According to the Intergovernmental Panel on Climate Change the global surface temperature has continuously risen since 1861. Increasing temperatures combined with changing precipitation patterns are strong indications for a more active and more intense hydrological cycle in the coming decades. In this respect, it is undisputed that the tropical regions are important for the global climate system. The reaction of tropical forests to enhanced atmospheric CO2 concentrations plays a pivotal role for the land carbon and the land water cycle. Currently our understanding of the physiological reactions such as growth response of tropical trees to rising atmospheric CO2 concentration, partly related to water-use-efficiency (WUE) and climate change is still rather poor and controversially discussed (Frank et al. 2015). Modifications in their carbon uptake and transpiration rate have inevitably global consequences. It is still unclear if tropical trees assimilate more CO2 with constant or slightly reduced water losses in a CO2 richer world or if the carbon gain remains almost unchanged with reduced transpiration.Tree-rings are well suited for environmental investigations with much potential for verifications of their well-being. Tree rings are mostly annually resolved, contain environmental information, are easily sampled and as such valuable archives. Growth limiting factors control the development of trees allowing the derivation of transfer functions that relate tree growth to tree physiological quantities as well as climatic parameters (Cook and Kairiukstis 1992; Fritts 1976).Up to now hemispheric investigations using tree rings are mainly based on data sets of mid and high latitude sites or Nordic tree-line sites. Investigations based on trees of tropical or subtropical regions are comparatively rare. This has several causes. One reason is the difficulty to identify tree-rings or growth increments of annual resolution. Consequently, it is often demanding to construct reliable data sets, i.e. chronologies of annual time resolution, for extracting climatic signals and/or tree physiological parameters. Therefore, tropical or subtropical trees have only just recently come into focus of environmentalists and climatologists. Subsequently tree-ring widths chronologies of tropical tree ensembles are anything but numerous and chronologies, e.g. regarding stable isotopes are almost non-existent