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.

中文版

【科學新知】「樹源學」（dendroprovenance）與「年輪年代測定法」（dendrochronology）

安納沙西人留下的波尼托村（Pueblo Bonito）遺址。圖片來源：wiki

中世紀神秘地消失在沙漠中的安納沙西人（Anasazi），留下的明布雷斯文化遺址讓人驚歎不已。矗立在沙漠中的波尼托村，是個五層樓高、有五百間房間的大廈，估計動用了24萬棵樹。

這麼多樹，究竟是從哪裡來的呢？雖然在當時查柯峽谷（Chaco Canyon）還有樹林，但是樹種是不相同的。

過去以鍶同位素進行辨識時認為，這些木頭應該是來自於曲司卡山（Chuska）與聖馬刁山（San Mateo）；但是最近所謂的「樹源學」（dendroprovenance）進行的研究發現，其中有一部份應該是來自於南方75公里的Zuni 山。在公元1020年以後，安納沙西人不再去Zuni山伐木，改去峽谷以西75公里的曲司卡山（Chuska）砍樹。

鍶同位素測定，主要是因為岩石中的銣（Rb）會衰變為鍶（Sr），產生鍶八七與鍶八六；而鍶由於與鈣化學性質相近的關係，常會與鈣一同被植物吸收。由於不同地區的銣含量有不同，因此可以由鍶的含量推出木頭可能是來自哪一座山。但是「樹源學」是什麼呢？

「樹源學」以及「年輪年代測定法」都是透過
仔細觀察樹木的年輪來進行的。
圖片：林芷佑

樹源學的英文dendroprovenance是由兩個部分拼起來的。其中provenance是「來源」的意思，而dendr(o)-則是「樹」的意思（請參考第十九課（一））。

我們都知道樹木有年輪，而不同年年輪的寬窄反映了當年雨量的情形。過去，科學家們發明了「年輪年代測定法」（dendrochronology），用來看過去若干年前的雨量變化。

怎麼看呢？若我們今天砍下一棵樹（A），我們當然知道最外面的一圈就是當年的年輪；往裡算如果有一百圈，就代表這棵樹有一百年那麼老。如果今天有一根木頭（B），我們知道也是砍自於附近的區域，我們將這根木頭取下的年輪與剛剛砍下的樹的年輪比較，因為每個區域每年雨量變化情形不同，長期的、整體的比較，若能找出一個重疊的區域，就知道B究竟有多老、是多少年前砍下來的。接著，我們便可以經由觀察B，了解更久遠以前當地的雨量變化情形。這就是所謂的「年輪年代測定法」！

而「年輪年代測定法」的英文dendrochronology也是由字首與字尾拼起來的唷！其中的dendr(o)-是什麼，就不再贅述了；而chron(o)-是「時間」的意思（請參考第二十四課（二）），-logy這個字尾則是學科、科學的意思（science, study，請參考第十六課（五））。所以，dendr(o)-、chron(o)-、與-logy加起來成了「樹-時間-科學」，就是「年輪年代測定法」啦！

而「樹源學」又是怎麼一回事呢？這是從歐洲發展出來的實驗方法。前面提到，每個區域每年雨量變化情形不同，雖然整個大區域的氣候變化不會太大，但是不同的小區域之間的氣候，還是會有一些細微的不同；這些不同經過仔細比對就可以找到樹木到底是來自哪一座山。這樣經由比較樹木年輪的細微之處，用來鑑定樹木的來源的方法，就稱為「樹源學」。因此，它的英文就用了「樹木」（dendr(o)-）與「來源」（provenance）這兩個字來組合囉！

所以，樹源學是把樹木的年輪當作人的指紋一樣用來做鑑定的依據。未來，如果能建立起各區域的樹木指紋，或許在抓山老鼠的時候，也能像柯南一樣的說：「根據『樹木指紋』，我可以說：兇手就是你！」喔！

參考文獻：

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

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