We introduce and study a variant of the Wasserstein distance on the space
of probability measures, specially designed to deal with measures whose
support has a dendritic, or tree-like structure with a particular direction of
orientation. Our motivation is the comparison of and interpolation between
plants’ root systems. We characterize barycenters with respect to this metric,
and establish that the interpolations of root-like measures, using this new
metric, are also root-like, in a certain sense; this property fails for conventional
Wasserstein barycenters. We also establish geodesic convexity with respect to this
metric for a variety of functionals, some of which we expect to have biological
importance.