Geoscience Reference
In-Depth Information
type of record can capture climate variability over a wide range of timescales, from
interannual to millennial. In this chapter, these critical issues will be examined, with
specific reference to tree rings as natural archives of past climate.
Tree rings are uniquely widespread relative to all comparable natural archives
of climate. Woody plants with reliably annual rings are formed wherever the local
climate imposes a single dormant season and a single growth season each and every
year. Such conditions are widespread in the boreal, temperate, and subtemperate
regions, and in some parts of the tropics. At middle and high latitudes or elevations,
this pattern of one dormant and one growth season per year in the formation of wood
is imposed by annual day length and temperature cycles. In some tropical locations,
it may be imposed by the existence of a single dry season-induced dormant period
each year, as in teak through much of its range in south Asia.
The abundance of potentially useful tree-ring records and the relative ease with
which they may be collected has resulted in unique approaches to their use as natural
archives of past climate. In this chapter some of these specific approaches, which
lead to characteristic features of tree-ring records of past climate, will be described,
so as to help the reader place the chapters that follow in a wider context. Specifically,
consideration will be given to four key issues:
(1) sample design in dendroclimatology and the importance of networks
(2) identifying climate signal in tree rings by empirical-statistical and process-
modeling approaches
(3) stability of the climate signal
(4) the quest for unbiased chronologies
2.2 Sample Design in Dendroclimatology
2.2.1 Natural Archives and Proxy Climate Records
The layer of new xylem or wood laid down each year under the bark of a tree (the
annual ring) is a natural archive of growth that year. The environmental conditions
influencing that growth may leave an imprint on the properties of the ring. Thus
the size, structure, and composition of the ring may contain information on those
conditions; for example, climate. Estimates of those properties in turn may be used
as proxy climate records.
In the case of tree rings, the most commonly used properties are structural;
namely, the total ring width (TRW) and maximum latewood density (MXD). There
are other structural properties of tree rings that can contain climate information; for
example, earlywood and latewood width measured separately, or tracheid (conifer
wood cell) dimensions. Their use will likely increase as a result of technical
of these structural properties and their measurement).
Many measurements of the composition of wood in annual rings could contain
climate information. In recent years considerable progress has been made in using
