Ringel's tree-packing conjecture

Let $T$ be a tree with $n+1$ vertices, and let a tree pack into a graph when edge-disjoint copies of it can be placed in that graph.

Ringel's conjecture. Every tree with $n+1$ vertices packs $2n+1$ times into the complete graph $K_{2n+1}$.

Equivalently, $K_{2n+1}$ should decompose into $2n+1$ edge-disjoint copies of every tree on $n+1$ vertices. The source presents this as a longstanding open problem, alongside an asymptotic version.