A merge tree is a topological summary of a real-valued function on a graph. Merge trees can be used to find stable features in the data, report the number of connected components above any threshold, or compute other topological descriptors. A local–global merge tree provides a way of distributing a merge tree among multiple processors so that queries can be performed with minimal communication. While this makes them efficient in massively parallel setting, the only known algorithm for computing a local–global merge tree involves global reduction.

Motivated by applications in cosmological simulations, we consider a restricted version of the problem: we compute a local–global tree down to a threshold fixed by the user. We describe two algorithms for computing such a tree via only local exchanges between processors. We present a number of experiments that show the advantage of our method on different simulations.