The pilot was departing from a private grass airstrip in his tailwheel equipped airplane. He stated that there was a slight breeze out of the north. As the airplane climbed above the tree line he encountered a rotor and downdraft and was unable to continue climbing. He elected to set the airplane in an area of trees, resulting in substantial damage to the wing lift struts. The pilot stated that there were no preaccident mechanical anomalies with the airplane that would have precluded normal operation. In the recommendation section of the NTSB Accident Reporting Form 6120.1, he also stated that he had just started flying from the airstrip, and was not familiar with the local conditions. After the accident, he learned that there is often a wind shift/rotor that occurs near the tree line at the midfield point of the airstrip.