The pilot took off for a local flight in his experimental amateur-built biplane. During climb out, near 250 feet above ground level, the engine lost power. The pilot performed a forced landing to a field that contained small trees. The lower wing sustained substantial damage on impact with a tree. An examination of the engine revealed that a spark plug wire had detached from its spark plug's terminal. The pilot's safety recommendation stated that installing an Adel clamp on one of the cylinder bolts will prevent the spark plug wire from vibrating off the spark plug terminal.