Lords of the rings: understanding tree ring science

Ask any second grader what you can do with the rings on a tree, and they'll respond, "Learn the age of the tree!" They're not wrong, but dendrochronology — the dating of trees based on patterns in their rings—is more than just counting rings. The hundred year-old discipline has given scientists access to extraordinarily detailed records of climate and environmental conditions hundreds, even thousands of years ago.

The ancient Greeks were the first people known to realize the link between a tree's rings and its age but, for most of history, that was the limit of our knowledge. It wasn’t until 1901 that an astronomer at Arizona's Lowell Observatory was hit with a very terrestrial idea—that climatic variations affected the size of a tree's rings. The idea would change the way scientists study the climate, providing them with over 10,000 years of continuous data that is an important part of modern climate models.

A. E. Douglass, the astronomer in question, is revered as the father of dendrochronology even though one of the field's basic concepts—crossdating, or the matching of ring patterns between trees—was independently discovered on four earlier occasions. (Pioneering computer scientist Charles Babbage was among that group.) Douglass was the first to apply truly scientific rigor to the study of tree rings, using a quantitative approach to tie variations in ring width to available climate records.

For the next dozen years, Douglass scoured Arizona for Ponderosa pine—dead or alive—to construct his first chronology. Completed in 1914, Douglass's chronology stretched back nearly 500 years, a feat accomplished by crossdating. Months later, Douglass teamed with an anthropologist to date timbers in pueblos in the American Southwest. For the rest of his life, Douglass continued to develop the science of dendrochronology. Though he was never able to tie sunspot activity to ring patterns—his original inspiration—his new field found favor with climatologists.

A bit of the basics

Dendrochronology operates under three major principles and a handful of other ground rules. The uniformitarian principle is perhaps the most important. It implies that the climate operates today in much the same way it did in the past. The uniformitarian principle does not imply that the climate today is the same as it was in the past, or even that today's climatic conditions ever occurred in the past. It simply states that the basic processes and limiting factors are consistent through time.

The second principle is that of limiting factors, where an organism's growth rate is constrained by the resource which is most limited. The third principle—crossdating—we've already covered. In addition to these three pillars, dendrochronologists must also pay close attention to where the trees grow. The best specimens are those at the margins of suitable habitat, which are sensitive to minor changes in environmental conditions. Finally, like all good scientists, dendrochronologists must be sure to gather a sufficient number of samples.

Before we get too far, we should cover the basic biology behind tree rings. Rings are formed by changes in a tree's growth rate throughout the year. As trees grow, the thin layer of living tissue just beneath their bark (the cambium) lays down new cells on top of older ones. As the cambium expands outwards, the oldest cells die off, leaving their lignin-hardened cellulosic exoskeletons behind.

Early in the season, trees grow rapidly, and the cells they build at this time are the largest of the year. As the season continues, the growth rate tapers off and cells become progressively smaller until just before winter, when growth slows or ceases. This change in cell size produces the characteristic banding pattern seen in cut wood. Trees in temperate climates produce the most obvious rings, thanks to the large difference in growth rates between the early and late season. Most tropical trees grow at a more or less constant rate throughout the year, making their rings less distinct.

Dendroclimatology, or the study of climatic influences on tree rings, is a large speciality within dendrochronology, and it's easy to see why. Trees not only recorded the last 10,000 years of climate history, they did so on every continent except Antarctica, bequeathing climatologists with an exquisite data set rich in both spatial and temporal detail.

Stressed is best

Trees—both living and dead—are not hard to find, but quality tree ring series are not as easy to come by. Forests with rich soil, gentle topography, and plenty of rainfall will produce gorgeous trees with lousy rings. On the contrary, trees living on steep slopes and in harsh climates are prized for their sensitivity. Scientists seek out trees living on the edge, sometimes literally. One of my undergraduate research projects demanded a rope and safety harness just to sample the trees.

Finding a tree is only half the battle. Once researchers find a stressed tree, they need to estimate where the pith sits within the tree, bore into it with what amounts to an overpriced, hollowed-out drill bit, and remove the core in one piece. Once removed, they take the core back to the lab, mount it, surface it (with a sander or a razor blade), and measure its rings. Scientists used to do that last part by hand, but software coupled with a properly calibrated scanner can now both identify rings and measure their widths.

Raw measurements, though, are of little use. As trees age, their growth slows, and declining trends must be eliminated from the data. These standardized measurements are the meat and potatoes of dendroclimatology. Big fat rings indicate a favorable year, small narrow rings are evidence of the opposite, and the overall pattern gives a snapshot of the climate throughout the tree's life. With these measurements in hand, dendrochronologists can use the principle of crossdating to search for other samples with matching patterns.

Unfortunately, ring data is not always straightforward. Trees are fickle organisms. Sometimes they appear to skip growing for a year. Other times they may grow on the southern side and not the northern. Sometimes spring comes early, the summer is cool and dry, and the fall is warm and wet, causing the tree to grow quickly, then slowly, then quickly again before winding down for the winter, giving the illusion of two years of growth.

To weed out these problematic patterns, dendrochronologists gather dozens of cores from each site and time period. Using statistical analyses, a site chronology can be created that integrates ring data from each individual tree, reducing the influence of tree-to-tree variation and maximizing the climate signal. From numerous site chronologies, master chronologies can be constructed that further reduce the noise caused by variations from different sites.

The "divergence problem"

Recently, dendroclimatologists have discovered that many rings from trees growing in far northern latitudes have not been keeping pace with rising temperatures. This trend seems to have started in the mid-20th century; it's now called the "divergence problem," and is a relatively new discovery. Given the early stage of research into the divergence problem and the uncertainty that remains, climate change deniers have jumped on the issue.

The fact is that the divergence of tree rings from climate data is a relatively recent occurrence, and there are a number of scientifically plausible (and probable) explanations. Most likely of these is that the "divergent" trees have become too stressed to respond normally to climate changes. The trees that diverge the most from the modern climate record occur in the far northern parts of the globe, exactly where average temperatures have risen the most.

Northern boreal trees are, of course, accustomed to growing in cool temperatures—the rate they use water is specifically tuned to the temperatures they face. As the atmosphere has warmed, though, it is possible (and likely) that the amount of available water has not increased correspondingly. Facing a water deficit, the stressed trees will grow more slowly, leading to the divergence. Furthermore, changes in snowpack and melt dates can also affect water availability.

The divergence problem has led some people to question the validity of tree ring data from these northern latitudes. While it's possible these trees may not have accurately recorded past warming events, keep in mind that all chronologies are calibrated against earlier temperature records (before divergence became a problem) and other temperature proxies. Climate change deniers like to point to the divergence problem as evidence that tree ring data is not suitable for climate reconstruction, which is not at all an accurate assessment. That would be throwing the baby out with the bathwater, as only trees living at extreme northern latitudes are affected.

Isotopic madness

Ring widths are not the only window into past climates tree rings offer; stable carbon and oxygen isotopes are another. Chris Lee touched on this in an excellent Ars piece a few years ago, but I'll dive into a quick summary. Both a ring's cellulose and lignin can be used to measure isotope ratios, but cellulose is more widely used for a few reasons. First, it is easy to extract, making results less prone to experimental error. Second, cellulose is not transported between rings like other compounds — once it is laid down, it remains in place, allowing experts to unambiguously date the isotopic information. Finally, the extraction process for cellulose produces samples that integrate isotopic variation from each entire growing year.

Carbon isotopes are widely used in climate studies to reconstruct past temperatures. Carbon has two naturally occurring stable isotopes, carbon-12 and carbon-13. The carbon dioxide in the atmosphere contains both of these species, with about 8 per mille (‰) less carbon-13 than carbon-12. Carbon-13 ratios in tree tissues, though, are much lower, around 20‰ to 30‰ than carbon-12. Biological processes account for this difference, and they're the reason why carbon isotope values can faithfully record temperature.

When carbon dioxide molecules attempt to enter a leaf, 4.4‰ fewer carbon-13 containing molecules make it through. This is due to simple physics—heavier molecules do not travel as far after bouncing off other molecules. Thus, more carbon-12 molecules zip through a leaf's stomata, the small openings on the leaf's surface through which fresh carbon dioxide is admitted and oxygen and water are expelled. If the tree is stressed by high temperatures, it will close its stomata to limit water loss. At the same time, it also restricts the amount of carbon dioxide entering the leaf.

Once inside, carbon dioxide meets Rubisco, the first enzyme in the carbon fixation process (or Calvin cycle, for those of you who couldn't get enough high school biology). Rubisco is picky and prefers carbon dioxide made with a carbon-12 atom, but its diet is not entirely strict. As long as the photosynthetic machinery is operating, Rubisco must provide carbon, so as the level of carbon-12 carbon dioxide drops (which happens when heat closes the stomata), Rubisco processes more carbon-13 carbon dioxide.

Further along the Calvin cycle, these carbon molecules are turned into sugars, which are later used to build the tree's wood. In a warm year, Rubisco processes more carbon-13 carbon dioxide, which shows up in that year's ring.

Oxygen isotopes are also used in climate reconstructions from tree rings. Trees incorporate the oxygen from water into their sugars, which are then used to build cellulose. Oxygen has three naturally occurring stable isotopes, but scientists use the ratio of oxygen-18 to oxygen-16 (the most common species) to infer climate information.

Trees generally get their water from two different sources: direct precipitation or ground water, depending on the species. The oxygen-18 concentration of precipitation varies depending on temperature—warmer weather gives the heavier isotopes an opportunity to evaporate from the oceans and lakes—and amount of precipitation—heavier isotopes fall first, depleting the oxygen-18 ratio of later rainfall. Rainfall further inland is similarly depleted.

Once water enters a tree, it is drawn up from the roots to the leaves, where some of it evaporates, further altering the oxygen-18/oxygen-16 ratio. Water molecules containing oxygen-16 will escape first because of their lighter weight, making that day's sugars richer in oxygen-18. If it's a cool, moist day, less water will evaporate. If the weather continues to be cool and wet for many months, then that year's rings will also be richer in oxygen-18. The converse is true of hot, dry years.

Though many competing factors can determine oxygen-18/oxygen-16 ratios—temperature, soil type, root depth, and so on—scientists can control certain variables by carefully choosing which trees to sample. For example, trees growing on thin soils that do not hold a lot of water must rely heavily on precipitation. As the isotopic signature of precipitation varies most closely with temperature, rings sampled from these trees contain oxygen-18 signatures that provide a clearer picture of past temperature trends.

Father would be proud

Looking back on the last 110 years of dendrochronology, I think A.E. Douglass would be proud. Dendrochronologists have spent the last century scouring the planet for tree ring data that could unveil mysteries or clarify questions, and the amount of information they have collected is massive. If each ring were one page of a book, dendrochronologists could fill entire libraries with their samples.

The recent and rapid development of stable isotope analysis is astounding, and scientists are really only at the beginning of harnessing this new tool. Meanwhile, the old-fashioned methods Douglass developed are still used in cutting-edge research. That many of the principles he pioneered are still used today is a testament to the resiliency and simplicity of the science he founded.

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Author: Tim De Chant | Source: Ars Technicia [January 06, 2011]

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