dendroprovenance and dendrochronology

Pueblo Bonito. Source: Wikipedia

Anasazi Indians, who mysteriously "vanish" from the present-day Four Corners region of the United States, comprising southeastern Utah, northeastern Arizona, northwestern New Mexico, and southwestern Colorado in 12 and 13th centuries, left many huge architectural structures in the desert. The biggest one, Pueblo Bonito, is a five-floored building with 500 rooms.

All of those buildings were made by wood. How did Anasazi Indians find so many logs in that region?

Actually, they gathered logs from mountains that are far away. In order to determine where they obtained their building materials, research team checked the strontium (Sr) content in those logs.

Strontium is a decayed product from rubidium (Rb). After rubidium is decayed and strontium is produced, because it is similar to calcium (Ca), it will be absorbed into plant just like calcium. The Rb content in the soil is different in different areas, so scientists can deduce the origin of plant from the Sr content.

From the Sr data, research team determined these logs were from Chuska and San Mateo Mountains. However, dendroprovenance results said otherwise.

Dendroprovenence is a new method developed from dendrochronology. The prefix dendr(o)-, meaning tree, is taught in Lesson 19-1. The middle part of the word, chron(o)-, meaning time, is taught in Lesson 24-2. The last part, the suffix -logy, meas study, science.

Putting them together, dendrochronology means to study the time of the tree.

Source: Ms. Lin

Dendrochronology is used by scientists to study the amount of precipitation in the past. When we chop down a tree (tree A), the outmost ring of tree A represent the amount of precipitation of this very year. Then we can count the rings of the tree. The number of rings tell us how old this tree is. If it has 100 rings, then this tree is 100 years old.

Say we find a log (tree B) also from the same area as tree A. Because precipitation varies year by year, the patterns of thick-and-thin rings in trees that grow in the same area around the same time will be similar because the climate is the same. So if we find one section of tree B is almost identical to one section of tree A, then we can take a good guess about how old tree B is. We can even figure out how long ago tree B was chopped down! This method is called dendrochronology.

As of dendroprovenance, it looks for much smaller details. We knew that precipitation varies year by year and the patterns of tree rings growing in the mountain ranges that surround the same area are similar because the climate is the same. However, each mountain range has its own microclimate. So, if we look more closely to these growth rings, we will find that growth patterns of trees from one mountain range are not identical to those of trees in nearby ranges.

The word provenance means origin or source. Because this method can help scientists find the where are those trees from, it is named dendroprovenance.

In this report, scientists found that some of the trees Anasazi Indians used were obtained from Zuni mountains. However, they switched to Chuska Mountains after 1020 A.D.

Dendroprovenance can be very useful for illegal logging. If we can establish the "fingerprints" of trees in every area, then we can catch those illegal loggers with indisputable evidences!

Reference:

Jared Diamond. 2006. Collapse.(大崩壞)。p.186-7（廖月娟譯）

2015/12/7. Unexpected wood source for Chaco Canyon great houses. Science Daily.

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