A recent stint of trail-maintenance by the South Okanagan Trail Alliance included hiring a professional faller to clear a number of danger-trees along a popular trail on Campbell Mt. This found me counting tree-rings on the stump of one particular mammoth which was standing-dead and leaning-hard over the trail. Not only were the number of rings impressive for a Ponderosa Pine, but the spacing of same told a wealth of stories.
Dendrochronology; the dating of tree-rings, not only determines the age of a tree, but reveals among other things, the climatic conditions in the plant’s lifetime and even assists in archaeological dating projects. Factors which affect a tree’s growth, be it precipitation, temperature, soil nutrients, fire and physical injuries often appear as a story laid out in rings.
Most of us learned early that tree-trunks add growth-rings each year, building new layers in order to create strong trunks that support countless branches and leaves, no matter the weather. The massive stump that I was examining displayed these growth rings fanning out in irregular shapes in horizontal spacing, telling its story of life.
Pines like that which I sat upon can occasionally skip production of their annual rings, or double-up for two yearly rings, while oaks reliably create rings which showcase their age. Likewise, some trees that grow near a stream can present either enhanced or stunted growth rings. A singular ring’s width, whether thick or thin, offers clues about the growing conditions a tree encountered that year; generally speaking, during good growing years, trees add a thick ring, while challenging years are reflected as narrow and sometime darker rings.
Researchers specializing in dendrochronology may use tree-rings to determine the era a tree was harvested, and even the geographical area that it was felled, as all of the same species of trees in a particular area will usually add rings at the same pace (thickness) if all other growth factors remain constant in that timeframe.
Dendrochronology also works on live trees, where an increment borer is used to drill a half-inch core-sample from the tree. From this sample, ring patterns are plotted by year, providing a history of growth conditions. Chronologies can present a fibrous map of time stretching back thousands of years through employing samples from ancient wood.
If one were seeking to discern when a fallen tree toppled in the forest, they simply need to cross-date (match) its ring patterns to the master chronology for that area. If its rings line up for years 1790 through 1902, you know that’s exactly when it lived and died.
Trees might be considered as massive organic recording devices, containing information about past climate, civilizations, ecosystems and even galactic events; much of it many thousands of years old. Techniques for extracting information from tree rings has been honed and expanded, and new technologies and techniques are able to extract a wider range of information out of trees.
Edward Cook is the director of the Tree Ring Lab at Lamont-Doherty Earth Observatory at the University of Arizona. The facility was founded in the 1930s by A.E. Douglass, an astronomer who used trees and their rings to better understand the connection between sunspots and climate.
The Tree Ring Lab has helped to establish others like it worldwide, which has in turn greatly increased the number of trees studied. A dozen laboratories now exist planet-wide, holding data from more than four thousands study sites on every continent except Antarctica.
Their work – consisting of more than a half-million samples, cumulates in The Tree Ring Data Bank; a Dendrochronology library open to all tree and tree-ring researchers. Each addition to this database forms a deeper understanding of past climate, ecosystems and human civilization.
Technically speaking, research involving tree rings is actually divided into three categories;
1) dendroclimatology: analysis of tree rings for past climate data;
2) dendroarchaeology: the study of tree rings to understand how past climate affected human societies; and
3) dendroecology: reconstruction of past forest ecosystems.
The most common tree rings studied are bristlecone pine, fir and spruce.
An example of how dendroclimatology has presented environmental data lies in a recent two-decade-long drought along a section of the Colorado River – the longest experienced since medieval times when a drought there lasted 62 years.Both the recent and historic lack of moisture was recorded in the rings of trees throughout the area.
An example of how dendroarchaeology has shown societal wisdom (or folly) is exemplified in understanding that tree rings show water in the Colorado River was apportioned to regional states based on flow from 1905 to 1922, some of the wettest years in the last 12 centuries, an era known as a pluvial period.
Tree-ring data presents environmental information in surprising ways; oxygen isotope analysis has unlocked the source of the water that trees drew up through their roots centuries ago, allowing determination of whether an abundance of moisture reflected in the rings was from a hurricane or a severe thunderstorm.
Rings analyzed from ancient trees in multiple locations worldwide demonstrate that after the massive 1568 volcanic eruption of Mount Kelud in Indonesia, the global climate cooled considerably for several years thereafter, as evidenced in narrow tree rings. This was cross-referenced with numerous other sources, including lake sediments, ice core samples, coral and the shells of long-dead geoducks.
Even the universe’s countless stars offer their secrets through trees. Our sun and all other stars emit a form of radiation called cosmic rays; high-energy protons and atomic nuclei which move through space at nearly the speed of light. These energy particles react in the atmosphere with nitrogen and change the levels of carbon 14, which is taken up by every living thing and becomes a tracer for cosmic ray levels.
An extreme cosmic-ray event – likely a large solar-flare, in the year 775, first found in Japanese cedar trees and since, globally, is the strongest cosmic ray event in the tree ring record, with a magnitude larger than the Carrington event, a well-documented solar storm in 1859.
So the next time that you find yourself wandering the forest, and happen upon a stump with clearly delineated tree-rings (which isn’t too difficult in BC these days) ponder not only the number of rings, but also the bigger picture that is laid out before you.