Fundamentals Of Dendroclimatology

A cross section of most temperate forest trees will show an alternation of lighter and darker bands, each of which is usually continuous around the tree circumference. These are seasonal growth increments produced by meristematic tissues in the tree's cambium. When viewed in detail (Fig. 10.1) it is clear that they are made up of sequences of large, thin-walled cells (earlywood) and more densely packed, thick-walled cells (latewood). Collectively, each couplet of earlywood and latewood comprises an annual growth increment, more commonly called a tree ring. The mean width of a ring in any one tree is a function of many variables, including the tree species, tree age, availability of stored food within the tree and of important nutrients in the soil, and a whole complex of climatic factors (sunshine, precipitation,

Resin Duct Wood

Bark

Cambium

False ring

Annual ring

Latewood

Earlywood

Resin duct

Bark

Cambium

False ring

Annual ring

Latewood

Earlywood

Resin duct

FIGURE 10.1 Drawing of cell structure along a cross section of a young stem of a conifer. The earlywood is made up of large and relatively thin-walled cells (tracheids); latewood is made up of small, thick-walled tracheids. Variations in tracheid thickness may produce false rings in either earlywood or latewood (Fritts, 1976).

temperature, wind speed, humidity, and their distribution throughout the year). The problem facing dendroclimatologists is to extract whatever climatic signal is available in the tree ring data and to distinguish this signal from the background noise. Furthermore, the dendroclimatologist must know precisely the age of each tree ring if the climatic signal is to be chronologically useful. From the point of view of pa-leoclimatology, it is perhaps useful to consider the tree as a filter or transducer which, through various physiological processes, converts a given climatic input signal into a certain ring width output that is stored and can be studied in detail, even thousands of years later (Fritts, 1976; Schweingruber, 1988, 1996).

Climatic information has most often been gleaned from interannual variations in ring width, but there has also been a great deal of work carried out on the use of density variations, both inter- and intra-annually (densitometric dendroclimatology). Wood density is an integrated measure of several properties, including cell wall thickness, lumen diameter, size and density of vessels or ducts, proportion of fibers, etc. (Polge, 1970). Tree rings are made up of both earlywood and latewood, which vary markedly in average density and these density variations can be used, like ring-width measurements, to identify annual growth increments and to cross-date samples (Parker, 1971). It has also been shown empirically that density variations contain a strong climatic signal and can be used to estimate long-term climatic variations over wide areas (Schweingruber et al., 1979, 1993). Density variations are measured on x-ray negatives of prepared core sections (Fig. 10.2) and the optical density of the negatives is inversely proportional to wood density (Schweingruber et al., 1978).

Density variations are particularly valuable in dendroclimatology because they have a relatively simple growth function (often close to linear with age). Hence standardization of density data may allow more low-frequency climatic information to be retained than is the case with standardized ring-width data (see Section 10.2.3). Generally, two values are measured in each growth ring: minimum density and maximum density (representing locations within the earlywood and latewood layers, respectively), although maximum density values seem to be a better climatic indicator than minimum density values. For example, Schweingruber et al. (1993) showed that maximum density values were strongly correlated with April-August mean temperature in trees across the entire boreal forest, from Alaska to Labrador, whereas minimum and mean density values and ring widths had a much less consistent relationship with summer temperature at the sites sampled (D'Arrigo et al., 1992). Maximum latewood density values are calibrated in the same way as with the ring-width data using the statistical procedures described in Section 10.2.4. However, optimum climatic reconstructions may be achieved by using both ring widths and densitometric data to maximize the climatic signal in each sample (Briffa et al., 1995).

Isotopic variations in wood have been studied as a possible proxy of temperature variations through time, but the complexities of fractionation both within the hydrological system, and in the trees themselves, make simple interpretations very difficult (see Section 10.4). Ring-width and densitometric and isotopic approaches to paleoclimatic reconstruction are complementary and, in some situations, could be used independently to check paleoclimatic reconstructions based on only one of

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■©■ maximum density •* ** earlywood width Wk eariywood density

A minimum density • ■ latewood width tgj latewood density

■©■ maximum density •* ** earlywood width Wk eariywood density

A minimum density • ■ latewood width tgj latewood density

FIGURE 10.2 Example of a tree-ring density plot based on an x-ray negative of a section of wood (top of figure). Minimum and maximum densities in each annual ring are clearly seen, enabling the annual ring width to be measured as well as the width of both the earlywood and latewood (courtesy of F. Schweingruber).

the methods, or collectively to provide more accurate reconstructions (Briffa etal., 1992a).

10.2.1 Sample Selection

In conventional dendroclimatological studies, where ring-width variations are the source of climatic information, trees are sampled in sites where they are under stress; commonly, this involves selection of trees that are growing close to their extreme ecological range. In such situations, climatic variations will greatly influence annual growth increments and the trees are said to be sensitive. In more beneficent situations, perhaps nearer the middle of a species range, or in a site where the tree has access to abundant groundwater, tree growth may not be noticeably influenced by climate, and this will be reflected in the low interannual variability of ring widths (Fig. 10.3). Such tree rings are said to be complacent. There is thus a spectrum of possible sampling situations, ranging from those where trees are extremely sensitive to climate to those where trees are virtually unaffected by interannual climatic variations. Clearly, for

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FIGURE 10.3 Trees growing on sites where climate seldom limits growth processes produce rings that are uniformly wide (left). Such rings provide little or no record of variations in climate and are termed complacent (right): Trees growing on sites where climatic factors are frequently limiting produce rings that vary in width from year to year depending on how severely limiting climate has been to growth.These are termed sensitive (Fritts, 1971).

Rings of uniform width provide Rings of varying width provide little or no record of variations in a record of variations in climate, climate.

FIGURE 10.3 Trees growing on sites where climate seldom limits growth processes produce rings that are uniformly wide (left). Such rings provide little or no record of variations in climate and are termed complacent (right): Trees growing on sites where climatic factors are frequently limiting produce rings that vary in width from year to year depending on how severely limiting climate has been to growth.These are termed sensitive (Fritts, 1971).

useful dendroclimatic reconstructions, samples close to the sensitive end of the spectrum are favored as these would contain the strongest climatic signal. Often, therefore, tree-ring studies at the range limit of trees are favored (e.g., alpine or arctic treeline sites). However, climatic information may also be obtained from trees that are not under such obvious climatic stress, providing the climatic signal common to all the samples can be successfully isolated (LaMarche, 1982). For example, ring widths of bald cypress trees from swamps in the southeastern United States have been used to reconstruct the drought and precipitation history of the area over the last 1000 years or more (Stahle et al., 1988; Stahle and Cleaveland, 1992).

Paleoclimatic reconstructions have also been achieved using teak from equatorial forests in Indonesia (D'Arrigo et al., 1994) as well as from mesic forest trees in Tasmania (Cook et al., 1992a). For isotope dendroclimatic studies (Section 10.4), the sensitivity requirement does not seem to be as critical and it may, in fact, be preferable to use complacent tree rings for analysis (Gray and Thompson, 1978). Sensitivity is also less significant in densitometric studies and good relationships between maximum latewood density and temperature have been found in "normal" trees, which are growing on both moist and well-drained sites (Schweingruber et al., 1991, 1993).

In marginal environments, two types of climatic stress are commonly recognized, moisture stress and temperature stress. Trees growing in semiarid areas are frequently limited by the availability of water, and ring-width variations primarily reflect this variable. Trees growing near to the latitudinal or altitudinal treeline are mainly under growth limitations imposed by temperature and hence ring-width variations in such trees contain a strong temperature signal. However, other climatic factors may be indirectly involved. Biological processes within the tree are extremely complex (Fig. 10.4) and similar growth increments may result from quite different combinations of climatic conditions. Furthermore, climatic conditions prior to the growth period may "precondition" physiological processes within the tree and hence strongly influence subsequent growth (Fig. 10.5). For the same reason, tree growth and food production in one year may influence growth in the following year, and lead to a strong serial correlation or autocorrelation in the tree-ring record. Tree growth in marginal environments is thus commonly corre-

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