  We provide new evidence that spanning forests of graphs satisfy the same
negative correlation properties as spanning trees, derived from Lord Rayleigh's
monotonicity property for electrical networks. The main result of this paper is
that the Rayleigh difference for the spanning forest generating polynomial of a
series parallel graph can be expressed as a certain positive sum of monomials
times squares of polynomials. We also show that every regular matroid is
independent-set-Rayleigh if and only if every basis-Rayleigh binary matroid is
also independent-set-Rayleigh.
