  A grove is a spanning forest of a planar graph in which every component tree
contains at least one of a special subset of vertices on the outer face called
nodes. For the natural probability measure on groves, we compute various
connection probabilities for the nodes in a random grove. In particular, for
"tripartite" pairings of the nodes, the probability can be computed as a
Pfaffian in the entries of the Dirichlet-to-Neumann matrix (discrete Hilbert
transform) of the graph. These formulas generalize the determinant formulas
given by Curtis, Ingerman, and Morrow, and by Fomin, for parallel pairings.
These Pfaffian formulas are used to give exact expressions for reconstruction:
reconstructing the conductances of a planar graph from boundary measurements.
We prove similar theorems for the double-dimer model on bipartite planar
graphs.
