Reconstructing past climates. Tree rings.

Tree rings are an important source of proxy-climate indicators. A year-by-year record or ring pattern is formed that reflects the climatic conditions in which the tree grew. A wide ring means plenty of warm days and sufficient water, whereas a sort one means either short growing seasons because summer was late in coming or due to severe water storage.
Dendrochronology is a science based on the exact calendar dating of annual growth rings in wood (developped by the astronomer A.E. Douglas) and Dendroclimatology is the application of tree ring science, or dendrochronology, to the study of climate (Fritts, 1976) that is it the analysis of tree rings, including the dating of annual rings and study of patterns of ring characteristics, such as widths, density, and isotopic composition. These Properties are a function of the environmental conditions under which the ring formed. Geological and climate event affect the limits on conditions of ring growth and this effect can be recognized in the ring record, then a calendar date or range of dates can be assigned to the particular geomorphic event.
Dendroclimatic records are commonly derived from areas where wood growth is related to climate (in mid- to upper latitudes, or areas where there is seasonality in temperature and/or precipitation, many species of trees form annual growth rings). The tree-ring record now goes back many thousands of years (up to 10,000 kyrs BP), both in North America and in Western Europe, and records are being established on all continents (excepting Antarctica).
.

.

How does a tree produce annual rings?

.

This diagram (Figure 1) shows part of a cross section of a young conifer. The center of the tree is the pith and the outside of the tree is marked by the bark. Just inside the bark is the vascular cambium, where cells that form rings are produced”. The inner portion of a growth ring is formed early in the growing season, when growth is comparatively rapid (hence the wood is less dense) and is known as "early wood" or "spring wood" or "late-spring wood". The outer portion is the "late wood" (and has sometimes been termed "summer wood", often being produced in the summer, though sometimes in the autumn) and is denser. "Early wood" is used in preference to "spring wood", as the latter term may not correspond to that time of year in climates where early wood is formed in the early summer (e.g Canada) or in autumn, as in some Mediterranean species. In parts of the world where a marked seasonal climate does not occur, tree rings tend to be poorly developed.
Figure 1 Diagram of rings in a young conifer. (source: NOAA, Satellite and information services).
.
.

Sampling and dating

The width of the rings: mainly depends on moisture availability, temperature. Complications arise due to competition with other trees for root space, light, nutrients etc. Because of this, trees growing close to their margins make the best for establishing a climatic signal. Tree species have different responses to these three factors – hence the factors can be separated by looking at different species. Usually, we choose trees that are in areas under stress where yearly growth will be sensitive to climate changes (as if climate doesn’t limit growth, one cannot extract any climate signal).

Trees close to their altitudinal limit or close to their moisture limit will typically show the best climatic signal, the ring widths will vary greatly and the tree site is termed “sensitive” (a tree on a steep slope will more likely show moisture limitation).When sampling we must be careful to separate the signal you are looking for from other factors (elevation, slope, and exposure, topographic convergence and potential relative radiation and flooding patterns). Sampled trees should not show signs of disturbance factors such as insect infestation, fire damage, human utilization, etc. (G.C. Wiles et al, 1995).
.

A number of samples must be taking and crosses check them (i.e.: one tree may not grow in a year, throwing the record off). At least 10-20 trees per species are sampled at a site and each tree is cored following specific guidelines to ensure that site selection is appropriately selected and then is transported to the laboratory for cross dating.

Cross-dating

Simple ring counts from living trees are not sufficient in many cases for an accurate age determination to be made. Jacoby et al,(1988). Only rigorous cross-dating could detect such a hiatus in growth and lead to the correct calendar dates on tree-rings.

Figure 2. Diagram showing the fundamental’s of cross-dating (source: NOAA, Satellite and information services).

.
Because the same set of environmental factors influence tree growth throughout a region, the patterns of ring characteristics, such as ring widths, are often common from tree to tree and contain the clues to climate change. At extreme growth sites, where growth is severely limited by temperature or moisture, the ring widths will vary greatly and the tree site is termed “sensitive”. Matching patterns in ring widths or other ring characteristics (such as ring density patterns) among several tree-ring series allow the identification of the exact year in which each tree ring was formed. The variations in rings, particularly from sensitive sites, will correlate over large regions that are dominated by similar climate variations; therefore, tree samples within these climate regimes can be cross-dated. Following these tree-ring patterns from living trees back through time, chronologies can be built up, both for entire regions, and for sub-regions of the world. Thus wood from ancient structures can be matched to known chronologies (a technique called cross-dating) and the age of the wood determined precisely. (See figure 2).
.

An important result of providing a precise tree-ring chronology was the opportunity to calibrate the radiocarbon (14C) time scale by comparing the measured 14C age an individual tree-ring with its “real” age, determined from just counting the annual rings. After cross dating, tree rings parameters other than width such as density (density of early wood, density and width of wood grown late in the season), stable isotopic composition, cell size and wall thickness, resin duct density, and trace metal concentrations, ca be measured. Cross-dating was originally done by visual inspection, until computers were harnessed to do the statistical matching.
Cross dating can also be done by a scanner; they have the advantage over optical microscopes of having great depth of field or 'focus'. Therefore treated wood samples need not be perfectly flat.
.

.
Temperature reconstructions
All palaeoclimatic reconstructions rely on the uniformity principle, although is possible that the role of different factors at a single location or over an entire region could change over time. This possibility has been raised to explain the divergence between temperature and rings parameters (width and maximum latewood density) during the late 20th century. An especially suitable strategy to minimize confounding effects is to sample sites along ecological gradients, such as elevation or latitude.
.

Dendroclimatic studies of the past surface temperature (see diagram as example) are mostly based on ring width or maximum latewood density; the latter usually has a higher correlation with temperature, especially during latewood density and also correlated with ring anatomy as measured by cell number, cell diameter, and cell wall thickness.
.

For air temperature preferred locations are close to the three lines, which represent the latitudinal or altitudinal limit to tree growth (Kullman 1998, Kroner 1999).

.
It is very difficult to distinguish the amount of temporal autocorrelation in tree ring records that is linked to biological processes instead of climatic ones. Dendroclimatic reconstructions often rely on networks of site chronologies. There is a possibility that increasing tree ring widths in modern times might de driven by increasing atmospheric carbon dioxide concentrations, rather than increasing temperatures (Gregory C. Wiles, 1996).
.
.

.
References
Gregory C. Wiles a, Parker E. Calkin b, Gordon C. Jacoby a, Tree-ring analysis and Quaternary geology: Principles and recent applications, Geomorphology 16 (1996) 259-272.
Lamb,H.H,.Climate. History, and the modern world 1995
http://web.utk.edu/~grissino/ by Henri D. Grissino-Mayer, 1994-2008
Surface temperature Reconstruction for the last 2,000 years, National Research council, 2006..
http://earthguide.ucsd.edu/
http://www.ncdc.noaa.gov/paleo/treering.html
http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artjan02/treering.html

Silvia Caloca Casado, 7 de noviembre 2008