The branch-aided chaotic dispersal of Zelkova abelicea

\emph{Zelkova abelicea} and other members of the relict tree genus \emph{Zelkova} (\emph{Ulmaceae}) show a unique dispersal mechanism. Majority of mature fruits fall together with a part of the branch, containing a twig and dry leaves. These leaves act like a drag-enhancing appendage, carrying the fruits away from the parent tree in a chaotic path. Drop tests allowed to measure: terminal velocity, different flight modes steady for individual fruits, chaotic for the dispersal units and the horizontal distance of the dispersal. \emph{Z. abelicea} presents two dispersal modes: slowly falling dispersal units with chaotic motion and fast falling individual fruits in a straight path. The terminal velocity of \emph{Z. abelicea} dispersal units is $1.53$~m s$^{-1}$, quite similar to that of the East-Asiatic \emph{Z. serrata} ($1.51$~m s$^{-1}$). The falling velocity of individual fruits is instead $2.74$~m s$^{-1}$ in \emph{Z. abelicea}, almost half of $5.36$~m s$^{-1}$ in (\emph{Z. serrata}). Members of the genus \emph{Zelkova} are found in Eurasia, yet their dispersal units have remarkably similar terminal velocity and flight behaviour. The dispersal of \emph{Zelkova} is less efficient than that of other wind dispersed trees. It may have evolved for short-distance ecological spread.