a hydraulic network interconnects a pump with one or more hydraulic jacks , the jacks being arranged at the ends of parallel segments of the hydraulic network , each segment including a safety check valve for isolating the respective jack of such segment from the remaining portions of the hydraulic network including the pump . the jack or jacks in the system are installed in the backcut of a tree so that forceably extending the ram of each such jack will cause the tree to be felled in the desired direction . each safety check valve includes a camming surface which can be manually positioned in the absence of a substantial load on the respective jack to override the normal automatic operation of the valve thereby maintaining the valve open to allow fluid to return from the respective jack through the valve when it is desired to collapse the jack after the tree has been felled .