Treering reconstructions

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The use oftree rings to reconstruct hydroclimate is called dendroclima-tology and dendrohydrology. Most temperate forest trees display concentric annual deposits of tree trunk material forming alternating lighter and darker bands of seasonal growth increments around the tree's circumference. The annual couplets of earlywood and latewood comprise an annual growth increment known as a tree ring (Fig. 8.1). The mean ring width in any tree is a function of tree species, tree age, nutrients available in the tree and soil, and climatic factors including precipitation, temperature, and solar radiation (Saltzman, 2002). Climatic information is obtained from interannual variations in ring widths and from variation in wood density, which is used like ring widths to identify annual growth increments. The earlywood and latewood of the tree rings vary markedly in average density, and density variations contain a strong climatic signal (Bradley, 1999). Tree-ring width and density variations are translated via a statistical transfer function into hydroclimatic variables that reveal temporal patterns of the variables.

Tree rings are the basis for pre-instrument climatic reconstructions for a variety of locations worldwide, including some limited tropical sites. Only a few tropical tree species form distinct annual rings, and tree growth in the tropics is less susceptible to interannual climate variability than at other latitudes (Pumijumnong, 1999). However, subtropical montane forests experience moderate temperature seasonality and large precipitation seasonality that induce dormancy and production of annual rings similar to temperate regions (Villalba et al., 1998). The basic strategy of dendroclimatology is to identify regions where trees are most sensitive to climatic stress so climatic differences are evident in the character of the tree rings. Such locations are commonly at the limit of the natural ranges of temperature and precipitation for a specific tree species. The longest continuous tree-ring records extending over several thousand years are achieved by overlapping the records for individual trees. Long records based on bristlecone pine in the White Mountains in California and oaks and Norway spruce in western Europe have been particularly successful (Hughes, 1996; Wilson et al., 2005).

The width and structure of the tree ring provide information on the temperature and precipitation when the tree ring was formed. In dry regions, the width or thickness of the annual ring is often controlled by the availability of moisture. In cool, moist regions, ring width or maximum wood density is determined by summer temperatures. Tree-ring widths in tropical tree species are most often related to precipitation during the transition from the dry season to the rainy season (Pumijumnong, 1999). The key is that the tree ring is an indicator of the factor that is restricting growth or is the most variable for that environment. By correlating tree-ring characteristics with temperature and precipitation data, a transfer function is developed to convert tree-ring characteristics into weather information. An extensive literature addresses the techniques used to extract climate information from tree rings and how to apply rigorous methods for paleoclimate reconstruction (e.g. Fritts, 1976,1991; Hughes et al., 1981; Hughes, 1996; Tessier et al., 1997), and Loaiciga et al. (1993) focus on tree-ring based hydrologic reconstructions.

It is very important to remember that tree rings provide an indication of the energy and moisture condition and not a direct temperature or precipitation recording. The tree ring is the biological response to the energy or moisture condition. Time resolution is a significant issue involving tree rings. Floods related to high precipitation years may be inferred, but a flood produced by a single storm is not evident in the tree-ring record. Tree rings are more effective for identifying drought than for floods because of the longer time resolution of drought. Temperature reconstructions using ring widths of trees growing in cold environments usually show the influence of warm-season temperatures on growth most strongly (Esper et al., 2002).

Hughes and Graumlich (1996) summarize the extensive tree-ring chronologies for the western United States which form a better tree-ring record for the past 1000 years for this region than for anywhere else in the world. These chronologies include an extraordinary wealth of climatically sensitive tree-ring records that can be used to place the instrumental period in a wider perspective. The most frequently used tree-ring variable for drought and other hydroclimatic studies has been the ring-width index, which measures the departures from normal of annual diameter growth of the tree (Hughes et al., 1981). Both the growth increment of a tree and the annual or seasonal flow of a river are closely related to the water balance of the soil integrated over days, weeks, or months. Statistical studies have repeatedly shown that hydrologic variables and annual growth indices from properly selected trees are highly correlated (Woodside, 2001).

Streamflow reconstructions using tree rings are an extension of the research developed for temperature and precipitation reconstructions. Tree rings are correlated with streamflow records, and streamflow is estimated based on the tree rings. Tree-ring analysis has been employed to examine various facets of the time and space characteristics of streamflow in a variety of global settings (e.g. Meko et al., 2001; Pederson et al., 2001; Gedalof et al., 2004; Davi et al., 2006).

The Colorado River Basin in the western United States is an excellent example of the contribution provided by reconstructed streamflow. More water is diverted out of the Colorado River Basin than any other river basin in the United States, and an international treaty controls delivery of water from the Colorado River to Mexico. The Colorado River Compact was signed in 1922 dividing the flow of the river between upper basin and lower basin states and Mexico based on the average flow at Lees Ferry, Arizona. Streamflow data available in 1922 and beginning around 1900 were used in the Compact for dividing the water among the various states and Mexico. Runoff reconstruction for the Colorado River to 1490 is reported by Woodhouse et al. (2006). The streamflow reconstruction (Fig. 8.2) reveals that the Compact was based on a period of persistently high

1721 1761 1801 1841 1881 1921 1961 2001

Year

Fig. 8.2. Reconstructed annual flow of the Colorado River at Kremmling, Colorado, for 1721-2002 derived from tree-ring analysis and a statistical model. (Data courtesy of NOAA's National Climate Data Center from their website at http://www.ncdc.noaa. gov/paleo/streamflow/kremmling.html.)

1721 1761 1801 1841 1881 1921 1961 2001

Year

Fig. 8.2. Reconstructed annual flow of the Colorado River at Kremmling, Colorado, for 1721-2002 derived from tree-ring analysis and a statistical model. (Data courtesy of NOAA's National Climate Data Center from their website at http://www.ncdc.noaa. gov/paleo/streamflow/kremmling.html.)

flows and subsequent actual water allocations have been faced with the problem of dividing a small and more variable water supply than all of the participants expected. Perhaps the most important point revealed by the streamflow reconstructions is that the modern gauged streamflow record may be an unrepresentative sample. The high flows on the Colorado River in the twentieth century are among the highest in the record, and the twentieth century's most severe dry period is exceeded by several longer duration events in the reconstructed streamflow.

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