  We consider the model of Brownian motion indexed by the Brownian tree. For
every $r\geq 0$ and every connected component of the set of points where
Brownian motion is greater than $r$, we define the boundary size of this
component, and we then show that the collection of these boundary sizes evolves
when $r$ varies like a well-identified growth-fragmentation process. We then
prove that the same growth-fragmentation process appears when slicing a
Brownian disk at height $r$ and considering the perimeters of the resulting
connected components.
