A conditional limit theorem for tree-indexed random walk

We consider Galton-Watson trees associated with a critical offspring distribution and conditioned to have exactly it vertices. These trees are embedded in the real line by assigning spatial positions to the vertices, in such a way that the increments of the spatial positions along edges of the tree are independent variables distributed according to a symmetric probability distribution on the real line. We then condition on the event that all spatial positions are nonnegative. Under suitable assumptions on the offspring distribution and the spatial displacements, we prove that these conditioned spatial trees converge as n --> infinity, modulo an appropriate rescaling, towards the conditioned Brownian tree that was studied in previous work. Applications are given to asymptotics for random quadrangulations. (C) 2005 Elsevier B.V. All rights reserved.