The study of tree rings, known as dendrochronology, can yield annual information on climate conditions for the past millennium or so. Every year, many tree species increase the diameter of their trunks by developing concentric rings formed by a layer of wood cells underneath the bark. While each ring corresponds to one year of growth, the spacing between successive rings is usually not uniform. This unevenness is due to differences in annual growth patterns, signifying climatic changes that have either encouraged or suppressed tree growth. Different tree species are sensitive to moisture and/or temperature changes. These environmental conditions can be inferred from the spacing and thickness of the tree rings. Relatively thick rings with a large spacing between successive rings indicate a more optimal growth setting, whereas narrower, tightly-spaced rings point toward climatic stress conditions (such as drought). Dendrochronology is limited to regions that continually support tree growth.
Another form of proxy data, derived from vegetation, comes from pollen and spores produced by plants. Wind can transport pollen grains to nearby water features, where they settle to the bottom and are preserved in sediment layers. Sediment cores extracted from lake beds are analyzed for the amount and type of preserved pollen found in successive sediment layers. Because pollen grains have unique features, plant types can readily be identified and, through techniques such as radiocarbon dating, indicate the time of deposition. Plant growth and distribution is sensitive to environmental conditions, providing an indicator of temperature and moisture trends on time-scales from centuries to millennia.
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