Academic Reading — Test 148

Reading passage

Every year, a tree growing in a temperate or seasonal climate adds a new layer of wood beneath its bark. In spring and early summer, when growing conditions are favourable, the tree produces large, thin-walled cells that appear pale to the eye. As the season advances and growth slows, the cells become smaller and darker, forming a denser band. Together, one pale band and one dark band make up a single annual ring. By counting these rings inward from the bark, a researcher can determine the age of the tree. The scientific study of this banding, and of the information locked within it, is known as dendrochronology, a term combining the Greek words for tree, time and the study of something. The central insight of the discipline is that the width of each ring varies from year to year according to the conditions the tree experienced. In a warm, wet year with a long growing season, a tree may lay down a broad ring; in a cold or drought-stricken year, the ring may be narrow. Crucially, trees of the same species growing in the same region tend to respond to these shared conditions in similar ways. This means that the sequence of wide and narrow rings forms a distinctive pattern, almost like a barcode, that is common to many trees in an area. Researchers exploit this shared signal through a procedure called cross-dating, in which ring patterns from different trees are matched against one another. By overlapping the patterns of living trees with those of older, dead timber, and then with even more ancient wood preserved in buildings, bogs or riverbeds, scientists can extend a continuous record back thousands of years. Not every tree is equally useful for this work. The most informative specimens are those growing at the limits of their tolerance, where a single environmental factor strongly controls growth. A pine clinging to a high mountain slope, for instance, is chiefly limited by temperature, so its rings record warmth and cold with particular clarity. A tree in a dry lowland, by contrast, may be governed mainly by rainfall. Because of this, dendrochronologists choose their sites carefully, seeking out places where the climatic signal is strong and the influence of other variables is minimal. They also sample many trees rather than relying on one, since an individual specimen may have been affected by purely local accidents such as fire, insect damage or competition from a neighbour that has nothing to do with the wider climate. To turn a collection of ring measurements into a climate record, researchers must first remove the effects of ageing. As a tree grows older and taller, its rings naturally tend to become narrower, regardless of the weather, simply because the same amount of wood is being spread around an ever-larger trunk. Statisticians correct for this long-term trend through a process called standardisation, which strips out the age-related decline and leaves behind the year-to-year variations that reflect climate. The corrected measurements from many trees are then averaged together to produce a single chronology for the region. Where instrumental weather records overlap with the tree-ring data, the two can be compared and a mathematical relationship established. This relationship is then used to estimate temperature or rainfall for the centuries before thermometers and rain gauges existed. Tree rings hold more than width alone. The density of the latewood, the chemical make-up of the cells and the proportions of certain isotopes within them all carry additional information about past conditions. Isotopes of oxygen and carbon, for example, can shed light on humidity and on how efficiently a tree used water during a given season. These measurements are demanding and expensive to obtain, but they allow scientists to refine and check the conclusions drawn from ring widths, and sometimes to reconstruct aspects of climate that width alone cannot reveal. Dendrochronology has also proved valuable beyond climate science: archaeologists use it to date wooden artefacts and structures, and it has helped to calibrate radiocarbon dating, improving the accuracy of age estimates across many fields. The method does have limits. It works best in regions with a clear seasonal rhythm, because trees in the wet tropics often grow continuously and may form no distinct annual rings at all. Occasionally a tree under severe stress will skip a ring entirely, or produce two in a single year, which can mislead an unwary counter; cross-dating exists partly to catch such errors. The further back a reconstruction reaches, the fewer trees survive to support it, so the oldest portions of a chronology rest on less evidence than the recent ones. Even so, by combining samples from many sites and continents, researchers have built records stretching back over ten thousand years in some places. These long, annually precise archives have become one of the most important tools available for understanding how the climate behaved long before written observations began.