The status of tree diagrams
As we have seen, tree diagrams are used in transformational generative approaches such as Principles and Parameters Theory and the Minimalist Program, as well as in other non-transformational generative models such as Head-driven Phrase Structure Grammar (HPSG). Tree diagrams have a special status in generative theories. They are not just a convenient ‘shorthand’ for rep resenting grammatical structure. Tree diagrams represent instantiations of the grammatical rules or principles that generative grammarians posit as the basis of a speaker’s knowledge of language. Consider the example in Figure 22.7.
Tree diagrams like this represent a range of information. They represent the word class of each element within the phrase, and they also represent constituency, showing how the elements are grouped together into increasingly complex constructions. In addition, tree diagrams encode information about the kinds of relationships that hold between the subparts of the phrase. For example, in the X-bar model, complements are shown as sister to the head: both found and George are immediately dominated by the same node in the tree, which is the lowest V-bar level. In contrast, modifiers like under the bed are more remote from the head. Tree diagrams also represent linear order. The triangles in this diagram represent phrases whose internal structure is not ‘unpacked’. Finally, this tree is labelled TP because tense is viewed as the head of the clause in the current transformational generative approach. We return to this point below (section 22.2.5).
Cognitive approaches to grammar reject tree diagrams as part of their theoretical model. For example, in Cognitive Grammar, the Content Requirement prohibits tree diagrams. Of course, nothing prevents the cognitive grammarian from sketching out tree diagrams as a convenient shorthand, but they are not admitted as a model of speaker knowledge. This is because the only kind of abstract representation that the Cognitive Grammar model permits (as a usage based model) is the schema that emerges from entrenched patterns. However, Langacker asserts that his model accounts for the same information that is captured by tree diagrams. As we have seen, the Cognitive Grammar model captures word class by means of class schemas. Furthermore, constituency is viewed as ‘just a matter of the step-by-step assembly [. . .] of progressively more elaborate symbolic structures’ (Langacker 2002: 296). It is important to remember, however, that entrenched constructions are not assembled ‘from scratch’ each time they are used in a speech event. Instead, frequently occur ring constructions are stored ‘whole’. Finally, Cognitive Grammar captures linear order by viewing it as temporal order within phonological space. In other words, when two or more units are combined to make a larger construction, the composition of these units at the phonological pole specifies a linear order.
