For a labeled tree $T$ on $\{1,2,...,n\}$ an *inversion* of $T$ is a pair $1 < i < j \leq n$ such that $j$ belongs to the unique path from $1$ to $i$ (we think of $T$ as being rooted at $1$). Let $\mathrm{inv}(T)$ denote the number of inversions of $T$.

Define the generating function $f(q) := \sum_{T} q^{\mathrm{inv}(T)}$ where the sum is over all labeled trees on $\{1,2,...,n\}$.

Then it is known that $f(-1)$ is the number of [alternating permutations][1] in $\mathfrak{S}_n$ (i.e., the so-called "Euler number"). See e.g. Goulden-Jackson 3.3.49(d).

**Question**: Is there a simple proof of this result via a sign-reserving involution?

  [1]: https://en.wikipedia.org/wiki/Alternating_permutation