We prove that it is PPAD-hard to compute a Nash equilibrium in a tree polymatrix game with at most twenty actions per player, and path polymatrix game with at most four hundred actions per player. These are the first PPAD hardness results for a game with a constant number of actions per player where the interaction graph is acyclic. Along the way we show PPAD-hardness for finding an $ε$-fixed point of a 2D LinearFIXP instance, when $ε$ is any constant less than $0.5$. This result is tight, since finding a 0.5-fixed point in a 2D LinearFIXP instance is trivial.

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