Only about 3 percent of America's native old-growth forest remains, and many of the trees they hold are those that were not big enough to attract a logger's eye. The result is a generation of trees that barely resemble the native forests that once blanketed the country. That makes some scientists suspect that the surviving forests have lost much of their genetic quality, the molecular muscle that made them dominate the landscape. To answer those questions, the mightiest trees of their types, or genetically identical offspring, must be preserved for study, and that is what is being done by a handful of enthusiasts, including Mr. Flanagan and David Milarch, a nurseryman from Copemish, Mich.