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As trees grow in age a layer of wood cells are produced each year that usually extends around the entire perimeter of the tree. This annual ring of growth consists of thin-walled cells formed early in the growing season (earlywood) and thicker-walled cells produced later in the growing season (latewood).  From the start of earlywood growth to the end of the latewood growth constitutes one annual tree ring.  Because new wood is also added upwards, the number of rings through cross sections of the trunk will decrease with height.  Thus to determine the age of the tree, a tree ring researcher (dendrochronologist) typically counts the number of annual rings near the base or at "belt height".  Ideally, to determine the age of a tree a researcher will saw through the tree to get a cross section.  To preserve living trees, researchers use an increment borer, which is a hollow auger-like device that is twisted into the trunk.  When the borer, about a centimeter in diameter, reaches the center of the tree, the interior plug or core of wood is removed.  The core is then taken to the lab where tree wood and rings are analyzed.  In practice, at least 2 samples are usually taken from the same tree to reduce the amount of variability in the tree ring data.

It's a general rule that tree ring widths decrease as the tree ages.  This fact is well supported by the data, especially in the first 100 years of growth. Overall we see annual fluctuations in tree growth most likely due to seasonal differences in climate variables such as temperature and precipitation.  What's really interesting is the periodic fluctuations beginning around 1650 and continuing through 3-4 cycles into the mid 1800's.  Are these an indication that there were 50-year fluctuations in climate in the Pacific Northwest during this period? If so, what climate variables were limiting factors during periods of slow growth?  An excellent exercise for the student is to determine the best fitting curve through the time series or analyze the differences in variation between the two samples.

The study that uses tree rings to reconstruct the past climate of an area is called dendroclimatology, a subfield of dendrochronology -- the science of using tree ring dating to analyze patterns in physical and social sciences.  Other subfields of dendrochronology include dendroarchaelogy (using tree rings to date wooden artifacts), dendrohydrology (using tree rings to study changes in river flow, surface runoff, and lake levels), and dendropyrochronology (using uses tree rings to study past and present changes in wildfires).  An excellent source on dendrochronology is Henri D. Grissino-Mayer's Ultimate Tree Ring Web Pages.

Source of the data:  Earle, C.J., Brubaker, L.B., Segura, G., International Tree Ring Data Base, NOAA/NGDC Paleoclimatology Program, Boulder, Colorado, USA.  http://www.ngdc.noaa.gov/paleo/ftp-treering.html

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 Douglas Fir Ring Widths Silver Creek, Washington State source: Earle, C.J., Brubaker, L.B., Segura, G.  International Tree Ring Data Base Tree: Sil159     Elevation: 900m  Lat/Long: 46N/121W year mean width  (x .01mm) 1565 258 1566 210 1567 265 1568 299 1569 274 1570 319 1571 357 1572 324 1573 263 1574 280 1575 299 1576 195 1577 245 1578 208 1579 240 1580 255 1581 201 1582 216 1583 235 1584 223 1585 169 1586 186 1587 150 1588 183 1589 185 1590 177 1591 152 1592 154 1593 114 1594 115 1595 123 1596 137 1597 146 1598 155 1599 208 1600 151 1601 144 1602 125 1603 99 1604 135 1605 93 1606 94 1607 95 1608 76 1609 96 1610 105 1611 135 1612 128 1613 138 1614 154 1615 137 1616 103 1617 103 1618 80 1619 52 1620 51 1621 71 1622 84 1623 85 1624 100 1625 85 1626 71 1627 65 1628 68 1629 85 1630 83 1631 70 1632 70 1633 75 1634 68 1635 56 1636 45 1637 60 1638 51.5 1639 44.5 1640 53.5 1641 53.5 1642 52.5 1643 50.5 1644 48 1645 39 1646 38 1647 38.5 1648 55.5 1649 29.5 1650 30 1651 36.5 1652 41 1653 26 1654 31.5 1655 34.5 1656 50.5 1657 59 1658 67.5 1659 68 1660 61 1661 61.5 1662 58 1663 54.5 1664 69.5 1665 48.5 1666 55 1667 69 1668 73.5 1669 83.5 1670 86 1671 79 1672 92 1673 77.5 1674 89 1675 92 1676 76 1677 75 1678 102 1679 85 1680 76.5 1681 104.5 1682 121 1683 103.5 1684 84 1685 98 1686 109 1687 108.5 1688 87 1689 87 1690 89.5 1691 96.5 1692 91 1693 95 1694 107.5 1695 96.5 1696 112.5 1697 102.5 1698 87 1699 106 1700 83.5 1701 70.5 1702 80 1703 94 1704 79 1705 58 1706 55 1707 41.5 1708 38.5 1709 49 1710 66.5 1711 74 1712 65.5 1713 59 1714 66.5 1715 76 1716 87.5 1717 69.5 1718 70.5 1719 79.5 1720 81.5 1721 77 1722 78.5 1723 85 1724 86 1725 82.5 1726 72 1727 78 1728 76.5 1729 92.5 1730 98.5 1731 66 1732 91 1733 76.5 1734 66.5 1735 79.5 1736 60 1737 71.5 1738 95.5 1739 71.5 1740 59.5 1741 46 1742 53 1743 44.5 1744 36 1745 38 1746 35.5 1747 49.5 1748 38.5 1749 37.5 1750 49 1751 50 1752 51.5 1753 54.5 1754 37 1755 54 1756 49 1757 57 1758 46 1759 48.5 1760 57 1761 58 1762 61 1763 67 1764 50.5 1765 55.5 1766 50.5 1767 53 1768 42.5 1769 44 1770 64.5 1771 69.5 1772 58 1773 100 1774 93.5 1775 77.5 1776 60 1777 74 1778 77 1779 82.5 1780 79.5 1781 72.5 1782 65 1783 69 1784 79 1785 89.5 1786 89.5 1787 68.5 1788 99 1789 96.5 1790 83 1791 76 1792 77 1793 89 1794 75 1795 58 1796 50 1797 41.5 1798 54.5 1799 42.5 1800 49.5 1801 73 1802 70.5 1803 59.5 1804 74 1805 70.5 1806 64 1807 64 1808 55 1809 45.5 1810 54.5 1811 41 1812 56 1813 56 1814 75 1815 72 1816 82 1817 75.5 1818 71.5 1819 79.5 1820 72 1821 55.5 1822 76.5 1823 51 1824 51 1825 75 1826 66 1827 47.5 1828 46 1829 62.5 1830 44.5 1831 31 1832 49.5 1833 68.5 1834 64.5 1835 57.5 1836 35 1837 39.5 1838 47.5 1839 51.5 1840 38 1841 44 1842 38 1843 47.5 1844 47.5 1845 59 1846 56 1847 36 1848 38 1849 33 1850 33 1851 48 1852 44 1853 41 1854 37.5 1855 52.5 1856 49 1857 45 1858 48.5 1859 46 1860 56.5 1861 58.5 1862 47 1863 56 1864 41.5 1865 45 1866 52.5 1867 55 1868 54.5 1869 41 1870 52 1871 48 1872 49.5 1873 50 1874 55 1875 60 1876 53 1877 69.5 1878 64 1879 57.5 1880 54.5 1881 48.5 1882 58.5 1883 50 1884 47.5 1885 41.5 1886 39.5 1887 39 1888 46 1889 32 1890 27.5 1891 44 1892 52.5 1893 47.5 1894 46.5 1895 34.5 1896 47.5 1897 35 1898 39 1899 40 1900 47.5 1901 46.5 1902 47.5 1903 43.5 1904 55 1905 31 1906 34 1907 35 1908 37.5 1909 40 1910 35 1911 39 1912 41.5 1913 47.5 1914 50.5 1915 36 1916 41 1917 44.5 1918 37.5 1919 45.5 1920 46.5 1921 47.5 1922 32.5 1923 31 1924 32.5 1925 39.5 1926 33 1927 33 1928 35 1929 37.5 1930 38 1931 58 1932 54 1933 31 1934 23 1935 24.5 1936 16.5 1937 19.5 1938 33 1939 21.5 1940 30 1941 29 1942 41 1943 32.5 1944 33 1945 32.5 1946 24.5 1947 31.5 1948 43.5 1949 29.5 1950 33.5 1951 21 1952 29 1953 34 1954 39 1955 49 1956 32.5 1957 32.5 1958 40 1959 27.5 1960 35 1961 47.5 1962 31.5 1963 33 1964 35.5 1965 32 1966 24 1967 34.5 1968 27 1969 33 1970 38 1971 33 1972 29 1973 34.5 1974 33.5 1975 27 1976 43 1977 39.5 1978 40.5 1979 36 1980 37 1981 51 1982 36 1983 49 1984 50 1985 41 1986 45 1987 39.5
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