A major shift is taking place in U.S. timber ownership, and it could have significant consequences not just for the industry but also for ecosystems across the country. Traditionally, the major private owners of forestlands in the U.S. have been forest product companies, but increasingly, such land is being bought by investment groups hoping to make money on their holdings. In the last four years alone, 15 million acres (an area about three times the size of Massachusetts) have changed hands. Most of that land has been snapped up by timber investment management organizations, or TIMOs, which invest money for institutions and individuals looking to diversify their portfolios. Environmentalists are still trying to determine the likely long-term consequences of the trend. Trees held by timber companies went to feed their mills, while TIMOs have sometimes enabled preservationists to buy large tracts of land. But the knife can cut both ways, since parcels can also be sold off for development. One trend is clear, though: TIMOs are good moneymakers. Between 1985 and 2001, the largest of them, Hancock Natural Resource Group, generated an average annual return of 14.6 percent, making it one of the market’s top-performing investments.