What is Dendrochronology?
Dendrochronology is compounded from the words dendro meaning wood, chrono meaning time, and logo meaning study. Literally it means the study of the series of events in time by examining tree rings (or the annual growth rings of the wood of trees).
Growth rings of trees vary. They may be seen as alternating bands of light early wood and dark late wood. Note that the term early wood is because it represents wood formed during the year’s early growing season. During autumn-winter in temperate latitudes and rainless dry months in monsoonal tropics most trees shed leaves slowing wood formation hence the term late wood. Through the lens, wood cells in early wood would have thinner walls but larger lumen (or cavity) but thicker walls with smaller lumen in late wood. Among broad-leaved trees, pores (or vessels) vary in size and density distribution thus forming bands, hence the term ring-porous, for example large pores in concentric bands during early growth gradually succeeded by less dense smaller sized pores. Sometimes growth rings are bounded by white concentric lines which when seen through a lens are composed of small light colored wood cells, termed as marginal parenchyma. When marginal parenchyma is composed of two or more concentric layers of cells, the ring boundaries can be seen by the naked eye. Such parenchyma tissues alternatively may form concentric bands in between rings but because they are thick and widely spaced in early wood and are narrow closely spaced in late wood, they also define growth rings.
The micro-details of a surfaced corewood sample are captured by scanning and then examined through the screen of a personal computer with possible outcomes whether or not a tree species has absent, indistinct or distinct growth rings. |
The distinct growth rings in trees can be seen as some sort of record of the past. Trees grow rapidly when their environmental requirements are optimum during a year and hence present wide rings. Narrow rings therefore mean trees succumbed to one or a combination of environmental limiting factors like for example too little or too much water, too cold or too hot temperature, defoliation by insect pest or by strong typhoon, etc. Indeed if one beholds the whole cross-section of a fallen tree with distinct growth rings, one sees beginning from the pith (center of the wood) to the bark series of growth rings. Some rings are generally wider and narrower, but there are also extremely wide or narrow rings. Extreme cases of tree ring widths are interpreted to be times of extreme environmental pulses such as drought years versus high precipitation years. Thus, dendrochronologists can study and reconstruct rainfall history of a site when variation of growth rings correlates well with instrumentally recorded rainfall. If the correlation is statistically acceptable, then the growth rings of trees are good proxy to past climate reconstruction even deep in the past when there were no instrumental records.
Climate reconstruction is just one of the many applications of dendrochronology. Dendrochronology can also be used to study the gap-dynamics and ecological succession phenomena of forest ecosystems. Other applications include reconstructions of events such as forest fire, prolonged floods, severe forest defoliations due to factors such as insect pest outbreak or very strong typhoons, volcanic eruptions, glaciation events, etc. Archeologists even use dendrochronology to link historical events with extreme climate events. Featured in National Geographic Magazine (July 2009 issue), Dr. Brendan Buckley for example established resonance between megadroughts and kingdom turnovers in tropical continental Southeast Asia.