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That about summer and winter is for our "temperate" climates. But how about the Amazon forest ? Would those trees show no rings because there is less difference between seasons, or would they show a double set because around the equator you can speak of two rainy seasons and two dry seasons in a year ?
Detailed information on long-term growth rates and ages of tropical rain forest trees is important to obtain a better understanding of the functioning of tropical rain forests. Nevertheless, little is known about long-term growth or ages of tropical forest trees, due to a supposed lack of annual tree rings in most tropical tree species. Aim of this PhD thesis was to study the occurrence of annual tree rings in six tropical tree species in the Bolivian Amazon, and to use these rings to reconstruct historical growth patterns and determine ages. These data were also used to evaluate the potential of four timber species for obtaining a sustainable timber harvest.All six tree species included in this study formed annual tree rings due to the occurrence of a distinct dry season, which induces cambial dormancy. Rainfall-growth relationships show a positive relation between tree growth and the amount of rainfall during certain periods of the year, indicating that rainfall plays a major role for tree growth. Ages of trees of 60 cm diameter differed three fold between and within species. The maximum observed age in the whole study was 427 years. Differences between trees in their temporal growth pattern towards the canopy ("canopy accession patterns") were studied by analyzing relative growth changes. This revealed that most trees do not reach the canopy by steady growth, but rather by an irregular pattern of growth spurts (‘releases’) and stand-stills (‘suppressions’), probably mostly caused by temporal variation in light. The four non-pioneer species for which these canopy accession patterns were analyzed showed differences in how they attained the canopy and in the length of periods of slow growth, suggesting differences in shade tolerance. Growth rates were positively autocorrelated among different trees, which means that a tree that has a high growth rate in one year relative to other trees is likely to perform next year also better than other trees. This autocorrelated growth was strong and lasted for long time periods. Incorporation of autocorrelated growth in simulations led to higher and more realistic variation in age estimates, emphasizing the importance of autocorrelated growth for population and growth studies.
Tree-ring analysis in the tropics exists since more than one hundred years. In more than 20 tropical countries and numerous tree species the existence of annual tree-rings is doubtless proven. Rhythmic growth is induced by short drought periods or long lasting inundation, the influence of the photoperiod is questionable. Climatological analyses of tree-ring chronologies show the influence of El Niño on tree growth. Tree-ring based age determinations give maximum ages of not more than 600 years for broad leaf trees in tropical lowlands. Increment estimations by ring-width measurements give reliable results for sustainable management systems in tropical forests.
Records of climate from the terrestrial tropics for the period before instrumentation are very limited. Tropical tree ring research, particularly in the Asian tropics, has been limited by difficulties ranging from problematic annual ring formation, poor understanding of phenology and physiology of thousands of tree species, complicated forest dynamics factors, and political turmoil and resultant effects on access. The need for understanding the potential range of variability in the monsoon regions of Asia is critical for making sound planning decisions in the face of potential hydrological changes associated with global climate change. A growing body of work from the SSEA-DENDRO (South and Southeast Asian Dendrochronology) project, one component of an NSF-funded project "Tree-Ring Reconstruction of Asian Monsoon Climate Dynamics", is beginning to allow analyses of local and regional climate from Monsoon Asian tree rings. We now have continuous records of 500-plus years, that enable analyses of important time periods such as the Little Ice Age (LIA), while "floating" time series span portions of the Medieval Climate Anomaly (MCA). From these records, we see clear evidence of decadal-scale reduced monsoon strength from India to Thailand for much of the 18th century, and we suggest warm SST anomalies in the eastern tropical Pacific as one of the primary factors. We compare our tree-ring based results with evidence from Speleothem research from northeast India that corroborates the decadal-scale monsoon weakening in the LIA, while revealing increased rainfall during the MCA. The role of SST anomalies in the eastern tropical Pacific is seen as significant, with El Ni?o and La Ni?a like conditions resulting in rainfall reductions and increases, respectively, in the study region. Persistent state changes in the SST fields can result in the kinds of decadal-scale patterns we are seeing in monsoon Asia, with far-reaching influence into the western hemisphere as well. More recently, the effects of shifting ENSO influence on monsoon rainfall from India eastward to Southeast Asia over the past few decades is explored and its significance examined.
Records of climate from the terrestrial tropics for the period before instrumentation are limited, and indeed the instrumental data are often insufficient over the modern period. Recently-developed tropical tree ring records from South and Southeast Asia, as part of an NSF-funded project Tree-Ring Reconstruction of Asian Monsoon Climate Dynamics, allow for the analysis of environmental and climate conditions over the past several centuries for the region. The need for understanding the potential range of variability in the monsoon regions of Asia is critical for making sound planning decisions in the face of potential hydrological changes associated with global climate change. We now have some continuous records spanning most of the past five centuries, and these enable analyses of key time periods of the Little Ice Age (LIA), while “floating” time series span portions of the Medieval Climate Anomaly (MCA). From these records, we see clear evidence of decadal-scale reduced summer monsoon strength spanning from India to Thailand for much of the 18th century, and we suggest warm SST anomalies in the central and/or eastern tropical Pacific as one of the primary factors. We compare our tree-ring based results with regional speleothem records that corroborate decadal-scale monsoon weakening for the LIA and increased rainfall during the MCA, at least over northeast India, and coral records that show that portions of the central and eastern tropical Pacific had elevated SSTs during the 18th century. The role of SST anomalies in the eastern and central tropical Pacific is seen as a key component to the monsoon variability over the study region, with El Niño and La Niña like conditions resulting in rainfall reductions and increases, respectively. Persistent state changes in the SST fields can result in the kinds of decadal-scale patterns we see for monsoon Asia, with far-reaching influence into the western hemisphere as well. More recently, the effects of shifting ENSO influence on monsoon rainfall from India eastward to Southeast Asia over the past few decades are explored and their significance examined.
So, yes, reading tree rings in the tropics is more complex than for higher latitudes; but not impossible (although probably with greater variation between species of tree).