However, experience has shown that the forest must be dense and contain several hundred trees for every 2.5 acres [1 ha], as well as older and younger trees of different species.
في الازمنة الحديثة بنى المهندسون الحواجز المعدنية المثبتة في الاسمنت للحد من أضرار الانهيارات .‏