Using the definition of a Finsler--Laplacian given by the first author, we
show that two bi-Lipschitz Finsler metrics have a controlled spectrum. We
deduce from that several generalizations of Riemannian results. In particular,
we show that the spectrum on Finsler surfaces is controlled above by a constant
depending on the topology of the surface and on the quasireversibility constant
of the metric. In contrast to Riemannian geometry, we then give examples of
highly non-reversible metrics on surfaces with arbitrarily large first
eigenvalue.Comment: 27 pages, 3 figure