It cannot: Θ(n) nodes always means Θ(n) space.

The terminator is copied onto every leaf edge; store it once at the root instead.

The tree can have Θ(n²) edges; store only the branching nodes and drop the leaves.

Each node stores one child pointer per alphabet character; use a hash table at each node instead.

The edge labels together can run to Θ(n²) characters; store each label as a pair of indices into the original string instead of copying it.