The method used in those tree house project are called: “tree shaping” “arborsculpture” “pooktre” the biggest name in the field so far : Richard Reames
Further explanation (from wikipedia 21Feb2012) :
The practice of training living trees and other woody plants into artistic shapes and useful structures is variously known as Pooktre,arborsculpture, tree training, tree shaping and by several alternative names. There are a few different methods of achieving a shaped tree, which share a common heritage with other artistic horticultural and agricultural practices, such as pleaching, bonsai,espalier, and topiary, and employing some similar techniques.
Many different tree species have been used for this art form, but some trees are better suited than others. Grafting is a unique and distinguishing feature evident in many (but not all) examples of the work is the purposeful inosculation of living trunks, branches, and roots to form artistic designs or functional structures.
Tree shaping has been practiced for at least several hundred years, as demonstrated by the living root bridges built and maintained by the Khasi people of India. Early 20th century practitioners and artisans included banker John Krubsack, Axel Erlandson with his famous circus trees, and landscape engineer Arthur Wiechula. Contemporary designers include artists Peter Cook and Becky Northey, who call their work “Pooktre”, Richard Reames, who coined the term “arborsculpture”, and furniture designer Chris Cattle, who uses the phrase “grownup furniture”
This site(above) talks about an interview with Richard Reames. More on: http://www.cabinetmagazine.org/issues/20/foer.php
This is the best website(picture above) so far. It is blogged by Richard Reames. It is all about tree shaping projects, including the tree houses project i posted previously. more on: http://www.arborsculpture.blogspot.com/, http://arborsmith.com/about.html
The species suitable for trees shaping: (from wikipedia)
宏楓 ,日本雞爪槭槭樹,榿木,樺木,白樺,桉樹,山毛櫸,榕圖,紫薇,海棠蘋果,黃松,懸鈴木梧桐,白楊,櫻桃,紅葉李,黑櫻桃, 番石榴, 砂梨, 栓皮櫟, 軟木橡樹,刺槐, 柳, 垂柳, 柚木, 榆樹
Acer Maple[3][16]
Acer negundo Box Elder[17][18]
Acer palmatum Japanese Maple[19]
Alnus Alder[3][20]
Betula Birch[16]
Betula pendula White Birch[19]
Eucalyptus Eucalyptus[19]
Fagus Beech[3][16]
Ficus Fig[5][21]
Fraxinus Ash[16][19]
Lagerstroemia indica Crape myrtle[citation needed]
Ligustrum Privet[1][20]
Malus Apple[19][20]
Pinus ponderosa Ponderosa pine[19]
Platanus Sycamore[3][20]
Populus Poplar[16][19]
Prunus avium Cherry[3][19]
Prunus cerasifera Myrobalan Plum[22]
Prunus serotina Black Cherry[22]
Psidium Guava[15][23]
Pyrus Pear[19][20]
Quercus Oak[19]
Quercus suber Cork Oak[16][19]
Quercus virginiana Live Oak[1]
Robinia pseudoacacia Locust[19]
Salix Willow[16][22]
Salix babylonica Weeping Willow[16][22]
Tectona grandis Teak[15]
Ulmus Elm[16][20]
Maboren Architecture 