Passage
READING THE CLIMATE IN TREE RINGS
(1)
[1] The record of climate stored in the growth rings of ancient trees 1 was arguably one of the most valuable natural archives ever available to science. [2] A broad consensus among researchers 2 holds that a tree in a temperate forest adds exactly one ring for each year that it lives. [3] Naturalists long speculated 3 so as to why neighboring trees in a single grove could grow at such different rates, even though the seedlings 4 have long started sprouting in the same season, once the spring thaw softened the ground. [4] The oldest bristlecone pines took root more than four thousand years ago, yet for most of that span no one knew how to read the seasons written in their rings. 5
(2)
Evidently, 6 a ring's width is set largely by that season's weather. Long before the 7 raising of systematic tree-ring study, foresters had noticed that the widths of the rings laid down during a drought were only a small fraction of what 8 they are in a rainy decade. In other words, no single ring, however carefully measured, can reveal a region's climate on its own. 9 Since there is no question that soil, sunlight, and disease each leave a mark on a tree, there is also no question that, to reconstruct a lost century of weather, a long and unbroken sequence is required.
(3)
10 The scientists built their master calendar by measuring the ring patterns of oak beams salvaged from old barns, village churches, and sunken ships across northern Europe. Oak resists decay for centuries in the right conditions, and the run of wide and narrow rings, the pattern preserved in every beam, 11 are directly linked to the wet and dry years the tree endured.
(4)
Specifically, beams 12 were cut from trees that grew in dry lowland valleys were matched against timbers from cooler upland forests known to have 13 higher yearly rainfall.
(5)
The question of how far back a single tree can carry a trustworthy climate record has long 14 vexed researchers. 15 Previous estimate that puts the useful range at only a few centuries during the early years of the field were based on the short-lived species then available for study, leaving open the possibility that far older records still lay buried in bogs and riverbeds.
