Penjing is nominally split into three types ‘Tree Penjing’, ‘Landscape Penjing’ and ‘Water and Land Penjing’. All these categories overlap in practice.
Chinese Penjing does not have clearly defined styles (Formal Upright, Informal Upright, Slanting, etc.) like Japanese Bonsai, although they do use these categories as points of reference.
Historically, style in Penjing was more to do with regional style, with different areas of China specialising in certain species of tree, certain techniques (clip and grow, bending with rope, bending with wire, etc.) or certain visual ideas (such as trees shaped to represent or suggest dragons).
There are other differences too. In Japanese Bonsai, crossed or tangles roots are usually seen as a real faux pas, whereas in Penjing heavily knotted roots are something which suggests character and age in a tree. Also, in Penjing there is less emphasis on technical perfection (often pruning scars are not hidden). In Penjing, little pagodas and men with fishing rods adorning the scene are seen as a good thing (adding to the beauty), whereas in Bonsai they are just thought of as tacky (distracting you from the tree). The same could be said about pot decoration.
Perhaps the biggest difference is the intention of what is being created. In Bonsai, often and image of a tree in nature in the Platonic sense (or an image of an idealised tree) is being created, whereas in Penjing, a realistic representation of a tree (in the Platonic sense) is not always the aim, although it may be the aim to create a realistic representation on an individual tree, real or imagined. Also, the intended scope of Bonsai and Penjing differ. Bonsai compositions consist of a single tree or a group of trees, whereas Penjing often incorporate other aspects (such as rocks and water) to create a miniature landscape, sometimes these landscapes do not even contain a tree.
Traditionally, Penjing, landscape painting and poetry are and were intimately linked. The Chinese intelligentsia believed that by inderstanding and experiencing nature, they would advance their spiritual development, and bringing nature closer to home in the form of paintings, poetry and Penjing would also aid this development.
Like Japanese haiku poetry, Chinese poetry relied on a very limited amount of content to portray a vast scene in the mind of the listener. Likewise, Chinese landscape painting used a minimal amount of actual brush strokes to portray an impression of a scene. As well as mimicking the visual style of Chinese landscape painting, Penjing also uses a similar method to portray the whole of a scene or idea using a limited number of elements. This is most striking in the ‘literati’ style, which later found its way to Japan. Significantly, it is common for Penjing compositions to be given titles, and in China, they are often regarded as a form of three dimensional landscape painting.
To me, the purpose of Penjing seems more concerned with creating a subtle and metaphorical tale, using trees and other landscape elements as its medium. So the tree or trees become a means to that end, rather than an end in themselves as is more the case with Japanese bonsai.
Some Penjing masters refuse to use wire to shape trees, preferring clip and grow techniques because they think it requires more skill and patience. They see shaping with wire as too easy. Historically, copper wire was very scarce and expensive in China and alternatives were used, such as bending with iron wire or string. But also, ‘clip and grow’ techniques were relied upon for basic shaping and development far more in China than in Japan.