The pilot stated that during a flight to the destination airport, the gyrocopter was in a slow climb at about 60 knots due to higher terrain that he knew was approaching along the route. As the gyrocopter approached a ridge, the pilot noticed that more altitude was needed, so he turned left of course along the ridge while continuing the climb, expecting to turn right at an area that he saw had lower terrain. The pilot said that things were still going well, but the climb rate had decreased somewhat. As the gyrocopter approached the area of lower terrain, the gyrocopter started to descend quickly with a best rate of climb speed of 52 knots. The gyrocopter descended lower than the surrounding trees and "brushed" the tree tops, tipping the gyrocopter forward and to the right. The pilot saw a small clearing and applied corrective control input to maintain an upright attitude of the gyrocopter and to reach the clearing. Just before entering the clearing, the gyrocopter contacted oak brush with its rotor blades, which sustained substantial damage. The gyrocopter landed in the clearing and slid with minimal forward speed to a stop. The aircraft fuselage had a fractured nose and collapsed nose gear. The pilot sustained serious injuries and the passenger sustained minor injuries. The pilot stated that if he had turned right at the approach of the ridge where the terrain was lower, he could have gone around the south end of the ridge. He said he should have expected a down draft on the lee side of the ridge and could also have executed an escape route earlier by turning away from the ridge before the area of down flowing air.