  To any free group automorphism, we associate a universal (cone of) limit
tree(s) with three defining properties: first, the tree has a minimal isometric
action of the free group with trivial arc stabilizers; second, there is a
unique expanding dilation of the tree that represents the free group
automorphism; and finally, the loxodromic elements are exactly the elements
that weakly limit to dominating attracting laminations under forward iteration
by the automorphism. So the action on the tree detects the automorphism's
dominating exponential dynamics.
  As a corollary, our previously constructed limit pretree that detects the
exponential dynamics is canonical. We also characterize all very small trees
that admit an expanding homothety representing a given automorphism. In the
appendix, we prove a variation of Feighn--Handel's recognition theorem for
atoroidal outer automorphisms.
