The Soundboard of Guzheng: What is "梧桐 wu-tong"?
Silk string instrument has long been known made by 梧桐 "Wu Tong" or 桐 "tong" in the classic Chinese literature. If you check the dictionary for "Wu Tong" you will get "firmiana simplex" as the answer. Detail on firmiana simplex says it's a tropical and tender Perennial plant. It's also called as 青桐 "qing tong" and 桐麻 "tong ma". It grows on moist soil. The bark is green and the flower is yellow-greenish. The seeds are in pea shape. It is mostly populated in the Guangxi province of China, but also commonly seen in the south provinces.
From the classic literature, Chinese people use "wu tong" and "tong" refer to many different species that cover 白桐 "Bai Tong" (pauwlonia), " 紫花桐 Zi Hua Tong"(paulownia), 岗桐 "Gang Tong", 油桐 "You Tong"(Vernicia fordii), 青桐 "Qing Tong"(firmiana simplex), 法国梧桐 "Fa Guo Wu Tong"(Platanus × hispanica) and etc. That sounds very confusing. Basically, tall straight skinny light colored trees are called "wu tong". A more specific name is assigned based on the color of the
tree skin, color of the flower, other visible character and etc.
There is no doubt that guzheng soundboards are made with paulownia in the modern time. Since Lankao county of Henan developed this big industry growing millions of paulownia tree every year, it's basically been the sole supplier for the guzheng soundboard. With the government support and huge profit involves, all other possibilities have been suppressed. The specific specie grown in Lankao is Paulownia elongata. The Lankao paulownia industry has a relative short history.
The first tree was planted by Jiao Yu Lu, a governor in Henan at the time, in 1965. His intention was to grow trees to fight against the dusty wind, but accidentally created this big industry. (information from Lankao County website http://www.lankao.gov.cn/)
Here is a picture of the paulownia at Jiao's grave:
How about before the Lankao paulownia industry began? What kind of wood was used before in making guzheng?
Literature has always said "wu tong" and "tong". Which specie was used exactly?
The first scientific study book on "wu tong" is written 1051A.D. by Chen Shi of Song dynasty. The name of the book is called 桐谱"Tong Pu" or Guidebook of Tong. In the book, he distinguishes 'wu tong" into 6 different types, such as 白花 桐 white flower tong, 紫花桐 purple flower tong, 油桐 oil tong, 梧桐 wu tong, and etc. He detailed the characteristics of each type and how to grow them. Later on, botanic study proved that all the "tong" in Chen's book are all under paulwonia gentry except the oil tong which is Vernicia fordii. There are other studies say that many of the "wu tong" in the Chinese ancient literature refer to paulownia. An evidence is that literature often praises wu tong's purple flower. (information based on various research papers from Institute of Botany, the Chinese Academy of Sciences.)
An example is a poem prasing "wu tong" written by Yan Shu in the Song dynasty. He described "wu tong" as having purple flowers that covers the sky. Most paulownia have either white or purple flowers, while firmiana simplex only have yellow whitish, yellow or yellow greenish flowers.
Baidu encyclopedia says the people in yellow river region has always been calling "pao tong" as "wu tong". In Shanghai, people called Platanus × hispanica, a tree planted by the French in the 19th century, "wu tong". Some will add "French" in front of "wu tong" to distinguish. In the south, "wu tong" refers to "firmiana simplex".
Botanic study shows firmiana simplex has difficulty grow in the cold weather. Even planting in Tianjing failed. Therefore, it is not some trees that will be found common in the mountains of the northern China. That's also an explanation that the northerners have been calling paulownia "wu tong". Paulownia is a much more common tree in the north. ("Review of taxonomic studies on Sterculiaceae", Guihaia, 2003)
A search on firmiana simplex says it's tropicals and tender Perennials.
There was not much confusion before since the northerners have always called paulownia as "wu tong". One of the reason that there is a confusion now is probably the Lankao paulownia industry is properly named as "Pao Tong" industry. Since they supply the wood for the instrument industry, so people are wondering why 泡桐 "pao tong" not 梧桐 "wu tong".
Some studies even say 梧桐"wu tong" means 梧"wu" firmiana simplex and 桐" tong" paulownia trees, because in some literature firmiana simplex is called "wu".
Let's take a look at the classical Chinese literature on "tong" that are related to instrument making.
三國陸璣《毛詩草木鳥獸蟲魚疏》分桐為青桐、赤桐、白桐、梧桐四種，稱白桐宜琴瑟,梓實桐皮 為椅，即梧桐。 Lu Ji, Three Kingdoms period.
Lu Ji separates "tong" into 4 kinds. "qing tong" (green tong), "chi tong"(red tong), "bai tong"(white tong), and "wu tong". "Bai tong" is good for making instruments and "wu tong" is good for making chairs.
晉陶弘景《本草集注》分桐為青桐、梧桐、白桐、岡桐四種，青桐葉皮青似梧而 無子；梧桐色白， 葉似青桐而有子；白桐與岡桐無異，唯有花、子；岡桐無子，材中琴瑟。Tao Hongjing, Jing dynasty.
Tao Hongjing separates "tong" in to "qing tong", "wu tong", "bai tong" and "gang tong", and he says "gang tong" is good for musical instruments.
南北朝賈思勰《齊民要術》據《爾雅》及郭璞《爾雅》注之說，以榮桐、襯、梧皆梧桐也，分桐為 青桐、白桐、岡桐三類，謂白桐無子，冬結似子者乃明年之花房，材質可製樂器；梧桐又名青桐，以其 皮青，故名之，其子可食，味似菱芡，而材不中琴瑟...
Jia sixie, Southern and Northern Dynasties.
Jia separates "tong" into "qing tong", "bai tong" and "gang tong". He specificallysays "qing tong"(firmiana simplex), is not good for making instruments, and "baitong" (paulownia) is good for making instruments.
All these literature says 白桐 "bai tong" or white tong or 白花桐 "bai hua tong" isbest for making instruments. Some specifically says 白桐 "bai tong" is a type of 泡桐 paulownia.
I searched for 白桐 "bai tong" and 白花桐 "bai hua tong" and found that it'sPaulownia fortunei (Seem.) Hemsl. It's also commonly called as 白花梧桐 "BaiHua Wu tong".
I also checked the guqin industry and found that there are many makers claimingthey use 白花梧桐 "Bai Hua Wu Tong" as different from 兰考泡桐 "Lankao PaoTong" and selling the guqins in a much higher price. In fact, both wood arepaulownia.
Here is a picture of a Ming dynasty Se now collected in the Shanghai Minxing FolkMusical Instrument Museum. The description says it's made of a whole piece 沙桐 "sha tong", a name used to refer to paulownia growing in Henan's dusty region.
There is no such confusion in Korea and Japan. In Korea, the wood that's used to make kayagum is called "O-Dong" written as 梧桐. The same characters as the Chinese "wu tong". The Japanese use kiri to make koto that is written as 桐 or 梧桐. The characters 梧桐 appeared in Japanese literature wtih instrumentalcontext is as early as in 万葉集 "Manyooshuu" 756A.D. which contains "梧桐製の琴".(A koto made of Wu tong) There is no doubt that the wood used is paulownia. One reason might be that firmiana simplex is rare in Japan and Korea because of the cold weather. So, the Japanese and Korean have been calling paulownia "wu tong" just like the northern Chinese does.
A side note is that the 16-steel string guzhengs from the Chaozhou area were made with 杉木 “shan mu" Cunninghamia Lanceolata. The steel string guzhengs found at the Chaozhou old masters' such as Lin Maogen and Guo Yin are all made of "shan mu". That's also a common tree in the south region. The masters said the "shan mu" gives longer string sustension and is desired for the Chaozhou music.