Treebanks are, in general, syntactically annotated corpora, either manually or automatically annotated.
For Dutch, we are aware of the following treebanks available for download.
Some treebanks are also available in Universal Dependency format.
There is a conversion script to convert Alpino trees into UD trees, Alud available from Gihub (an older version, no longer maintained, is lassy2ud)
See also Treebank querying