  We compute in small temperature expansion the two-loop renormalization
constants and the three-loop coefficient of the beta-function, that is the
first non-universal term, for the sigma-model with O(N) invariance on the
triangular lattice at N=-1. The partition function of the corresponding
Grassmann theory is, for negative temperature, the generating function of
unrooted forests on such a lattice, where the temperature acts as a chemical
potential for the number of trees in the forest. To evaluate Feynman diagrams
we extend the coordinate space method to the triangular lattice.
