Frankel's refinement of Bochner's theorem on forward complete Finsler manifolds

In this paper, we prove the Frankel's refinement of Bochner's theorem on both compact and forward complete Finsler manifolds. That is, on a compact or forward complete Finsler manifold M0 with non-positive flag curvature and negative (mean) Ricci curvature, any isometry ψ:M0→M0 homotopic to the identity must be the identity itself.