In order to extract a PCFG from a treebank, the trees need to be interpretable as CFG derivations.
most work has excluded non-local dependencies; either (in PTB-like treebanks) by discarding labeling conventions such as the co-indexation of the trace nodes in the PTB, or (in NeGra/TIGER-like treebanks) by applying tree transformations, which resolve the crossing branches (e.g., Kübler 2005;Boyd 2007).