Departures from Tree Structures in Discourse: Shared Arguments in the Penn Discourse Treebank

The term discourse structure is used to denote any structure of a text above that of the sentence. Trees have often been posited as a good abstraction when discourse is taken to have a hierarchical structure (Mann and Thompson 1987; Webber et al. 2003; Marcu 2000; Egg and Redeker 2008). Nevertheless, periodically researchers have commented on the need to depart from the strict singleparent hierarchy of trees to structures which have shared daughters, a move which incorporates multiple inheritance and is therefore an issue for tree representations. This study follows up on the observation in (Lee et al. 2006) about the relative ubiquity of shared structures in the Penn Discourse Treebank or PDTB (Prasad et al. 2008; PDTB-Group 2008)), a recently released corpus which annotates discourse relations and their arguments. We limit our investigation here to cases where the shared discourse structure is a syntactically subordinate clause introduced by a subordinating conjunction (e.g. because, although, when, etc.). We examine annotations in the PDTB where the subordinate clause has been taken to be an argument of both the relation associated with the subordinating conjunction and another relation expressed in the immediately subsequent discourse. We ask what such annotations imply about the link between syntactic subordination and discourse subordination. Our argument is that while syntactic subordination may often correlate with discourse subordination, there are interesting exceptions that might better be captured as discourse coordination. We provide some systematic characterization of these exceptions by appealing to well-motivated discourse factors, and discuss their implications for tree structures.