You will find here artificial treebanks that consist of all the trees of a given number of vertices. These treebanks were generated with the Linear Arrangement Library. Each treebank is available in two formats text formats: edge list and head vector.
In order to download a particular treebank, download the specific file you are interested in.
Unlabelled free trees: Head vector format, Edge list format
Unlabelled rooted trees: Head vector format, Edge list format
More combinatorial data can be found at Professor Brendan McKay’s webpage, more precisely, here.