The pilot of the turbine powered (jet sustainer) motorglider reported that during descent to the airport, he deployed and operated the jet sustainer for 90 seconds. After turning off the jet sustainer, the pilot forgot to stow it, resulting in increased drag. After entering the traffic pattern, the pilot configured the glider for landing by extending the flaps and landing gear, but the jet sustainer remained extended. The pilot noticed a sink rate in excess of normal, and attempted to resolve the condition by reducing flaps, but inadvertently placed the flaps in the full negative position, which deployed the spoilers and increased the sink rate further. He continued in the airport traffic pattern; however, during the base leg to final leg turn, he realized that impact with a tree line was likely and decided to level the wings to make contact with the trees in a controlled fashion. The glider subsequently impacted the trees resulting in substantial damage to its right wing. The pilot stated that there were no preimpact mechanical malfunctions with the glider.