Minnesota gained slightly more wetlands than it lost from 2006 to 2011, according to a study released by the DNR on Friday.

The preliminary results, based on comparisons of thousands of aerial photos taken three years apart, indicate that the state is in line with a no-net-loss goal established more than 20 years ago.

The numbers are small — a gain of 3,390 acres over a loss of 1,310 acres, for a net gain of 2,080 acres. That's a small fraction of 1 percent of the state's 10.6 million acres of wetlands, but "statistically significant," said Doug Norris, DNR wetlands program coordinator.

"The fact we did not see losses was very interesting," he said. "Some field biologists have told me that doesn't square with what they're seeing out there. But the data shows what it shows."

At the same time, Norris coated the findings with cautions. The types of wetlands that have increased are not those that have the highest nutrient or habitat value for wildlife, he said. And the results are based on only one round of photo comparisons.

"The real value of this will come many, many cycles down the road," he said.

State, federal agencies and wildlife groups have invested millions of dollars in wetland restorations in recent decades, while crop acreage has increased in a booming farm economy in recent years.

Jon Schneider, manager of Minnesota conservation programs for Ducks Unlimited, hadn't seen the report, but said Minnesota needs to emphasize wetland restoration, particularly in prairie regions. The DNR report indicated that the Prairie Parkland region, which spreads across the state's southwest corner and up the Red River Valley, saw the greatest gains and the greatest losses, with a net gain of 1,230 acres.

"Our position is that it's admirable to have a policy of no net loss," he said. "Stability is better than declining.

"But we've lost so much in the prairie portion of the state that we should be seeing an increase in prairie wetlands. And to my knowledge we're not."

Norris said that in general the scope of land type changes in the study are small because changes take more time than the study has covered so far. Also, some changes are incremental, with varying effects. A field that might grow crops in the summer but hold water the rest of the year might be considered a wetland, with some benefits for migrating waterfowl. But it would not be as valuable to wildlife as a wetland with native aquatic vegetation, Norris said.

The study used photos from 5,000 locations in the state.

Bill McAuliffe • 612-673-7646