August 28, 2013 · West Virginia University is well known for its research in the energy fields of coal and natural gas, but for the past several years, trees in another country have taken the spotlight for SOME researchers. One professor is using dendrochronology to come up with new ideas about the rise of Genghis Khan.
Associate chair of the geography department at WVU, Amy Hessl says her interest in Mongolia came after a visit there while she attended some workshops. The country between China and Russia is mostly dominated by wide arid grasslands, but Hessl explains that in the north the landscape is more mountainous with forests.
“If you landed in Mongolia magically,” Hessl says, “you might think you landed in Montana or Idaho—particularly the northern part of the country. It’s very similar in terms of the forest, grasslands, and climate.”
In fact, initially, Hessl went to study climate change and forest fires. While she was there, she and her team came upon trees growing on an 8,000-year-old lava field. It was there where she discovered what are essentially preserved natural records of times gone by.
“We are looking at a cross section of a log called sinus siberica—so Siberian Pine that we collected from a lava flow in central Mongolia. And the inner ring date of this tree is 96BC. So the first year of this tree is 96BC. And this was the first piece of wood that we dated that broke the BC/CE boundary so we were pretty excited about that.”
So how do you date rings of a tree? Hessl says the technique, called crossdating, was developed in the 20s and it’s pretty much the foundation of dendrochronology.
“The idea is that you match periods of contemporaneous growth, times when trees were growing at the same time, and they have unique ring width signatures over multiple decades that overlap with each other. So it’s sort of like finding the same piece of music in a concerto. The trees were listening to that same piece of music, so their rings reproduce that tune. ”
So collecting hundreds of samples of cross sections of logs, and studying the patterns there, Hessl and her team noticed an anomaly. Around the 13th century there was a period of about fifteen years when it must have been exceedingly wet in Mongolia.
“And it just so happens that that really wet period coincides with the initial growth of the Mongol Empire when Genghis Khan amassed his armies and started his conquering of inner Asia.”
Not much is known about how Genghis Khan rose from obscurity to lead an army that claimed more territory in 25 years than the Romans conquered in 400. Hessl explains that for centuries historians have hypothesized that it was variable climate and drought in particular that drove the development of the Mongol empire.
“The Mongol economy is traditionally based in nomadic pastoralism. They were able to capitalize on that wet period, we think, and increase the number of grazing animals, particularly horses.”
Hessl says historians guess one key reason the empire spread so effectively might have been because of their horses. It’s speculated that each warrior had three to five at their disposal.
Warriors could travel long distances and would drink mare’s milk or dry it out to a paste, keep it in a pouch and then simply add water when they needed it, or even make a small cut on one of the horse’s veins and drink the blood or mix it with the milk for emergency nourishment. Ultimately, their horses gave the Mongols military might, creating what one historian called “the intercontinental ballistic missiles of the thirteenth century.”
“In order for that to happen they would have had to develop large herds of horses. And they wouldn’t have been able to do that given variable climate. So we have hypothesized that this period of really wet conditions might have allowed them to develop these really large herd sizes and literally use that horsepower to expand their empire.”
Hessl says now armed with this dendrochronological data, she hopes to collaborate with European and Mongolian archeologist to date various sites throughout Mongolia.