Principal indicators of past climates - Climate Change: Biological and Human Aspects - page 39

2.1 Terrestrialbioticclimaticproxies
2.1.1 Tree-ringanalysis(dendrochronology)
Dendrochronology was developed early in the 20th century, not by a biologist but by
an astronomer, Andrew Douglass. When he was 27, and working at the Lowell
Observatory at Flagstaff, Arizona, Douglass discovered a possible relationship
between climate and plant growth. He recorded the annual rings of nearby pines
and Douglas firs ( Pseudotsuga spp.). In 1911 he made corresponding records among
trees felled around 50 miles to the south west of the observatory, which prompted
his study of sequoias. When viewing a cross-section of a tree, it became clear in
certain species that wide rings were produced during wet years, and narrow rings
were produced during dry years (Webb, 1983).
Douglass coined the term dendrochronology, meaning tree time study, and worked
on exploring the link between tree rings and climate. Between 1919 and 1936 he wrote
a three-volume work, Climate Cycles and Tree Growth . Collectively, the methods he
pioneered are now invaluable for archaeologists to date prehistoric remains. They
were first used this way in 1909 when Clark Wissler, of the American Museum of
Natural History, put him in touch with the Archer M. Huntington Survey of the South-
west. In 1918, Wissler (who recognized the value of tree rings for dating purposes)
arranged for Douglass to receive nine beam sections from Aztec Ruin and Pueblo
Bonito so he could begin cross-dating the two ruins. The final missing link to his
studies came from an unstable beam that was extracted from the Whipple Ruin in
Show Low, Arizona. That beam bridged the gap between living-tree-ring chronology
and the archaeological tree-ring chronology that had been established.
The Laboratory of Tree-Ring Research, subsequently initiated by Douglass, has
the largest accumulation in any research centre of tree-ring specimens from both
living trees and age-old timbers.
Trees respond to seasonal and climatic fluctuations through their annual growth
rings. The thickness of each ring reflects the climate of the principal growing season,
primarily the summer, in which it is formed. The rings are made of xylem. Pith is
found at the centre of the tree stem followed by the xylem, which makes up the
majority of the tree's circumference. The outer cambium layer of the tree trunk keeps
the xylem separated from the rough bark. Each spring or summer a new layer of
xylem is formed, and so the rings are produced, which may be counted as an annual
record. More specifically, a tree ring is a layer of wood cells produced by a tree in
1 year, consisting of thin-walled cells formed in the early growing season (called
earlywood), and thicker-walled cells that are produced later in the growing season
(called latewood). The beginning of the earlywood and the end of the latewood
forms one annual ring. In dendrochronology these rings are then counted and their
thickness compared. However, because (as discussed in the previous section) other
local factors come into play - due to the tree's site, which determines the nutrients,
water and sunlight it receives - statistical analysis of a number of tree-ring samples
from the local area is required to discern the local climatic factor. Even so, trees