Restored wetlands are less productive than natural wetlands, scientists have discovered.

Regeneration can take hundreds of years and sites may never reach the quality of the original wetlands, a study shows.

Study co-author, David Moreno-Mateos, a postdoctoral fellow from the University of California, Berkeley, explains, "Once you degrade a wetland, it doesn't recover its normal assemblage of plants or its rich stores of organic soil carbon, which both affect natural cycles of water and nutrients, for many years.

"Even after one-hundred years, the restored wetland is still different from what was there before, and it may never recover."

With increased development, many wetland sites in the United States have vanished over the last 100 years. Developers often create new wetland sites to replace those they have destroyed, but the report claims they are not as effective as the originals.

Dr Moreno-Mateos says wetlands help store carbon and so are important in the fight to reduce climate change.

"Wetlands accumulate a lot of carbon, so when you dry up a wetland for agricultural use or to build houses, you are just pouring this carbon into the atmosphere.

"If we keep degrading or destroying wetlands, for example through the use of mitigation banks, it is going to take centuries to recover the carbon we are losing."

The study, published in 24 January's issue of the Public Library of Science Biology, says that wetlands under 250 acres, in cold area and separated from river flows and tides take longer to regenerate.

Co-author Mary Power, professor of integrative biology at UC Berkeley, says, "These context dependencies aren't necessarily surprising, but this paper quantifies them in ways that could guide decisions about restoration, or about whether to damage wetlands in the first place."

Wetlands also help promote fish production, water purification, biodiversity conservation and erosion control.

Restored wetlands contain around a quarter less carbon than original wetlands and even after a century, they still contain a quarter fewer species of native plants.

Although they look similar on the surface, and animal and insect populations are much the same as on restored wetland, plants are slower to regenerate.

He says the answer is "preserve the wetland, don't degrade the wetland."

The researchers examined 124 wetland studies and 621 wetlands across the world. Most were in America and some were restored over a century ago.

Dr Moreno-Mateos says half the wetlands in North America, Europe, China and Australia were destroyed in the 20th century.

Although restored wetlands are on average, 25% less productive than natural sites, those in boreal and cold temperate forests recovered more slowly.

One review of wetland restoration in New York State, showed that after 55 years, only 50 half the organic matter had accumulated compared to the original sites.

"Current thinking holds that many ecosystems just reach an alternative state that is different, and you never will recover the original," he says.

Dr Moreno-Mateos plans in future to investigate if the slower carbon accumulation is caused by a slow recovery of the native plant population or by invasion of non-native rivals.

The report's other co-authors are Francisco A. Comin, from the Department of Conservation of Biodiversity and Ecosystem Restoration at the Pyrenean Institute of Ecology, in Zaragoza, Spain and Roxana Yockteng, from the National Museum of Natural History in Paris, France.

The study was backed by the National Center for Earth Surface Dynamics of the U.S. National Science Foundation Science and Technology Center, the Spanish Ministry for Innovation and Science, the Spanish Foundation for Science and Technology.