"The threats to magnolia vary from country to country and species to species, but in general, deforestation is the main problem," Oldfield said.
Operations ranging from banana plantations in China to coffee plantations in Colombia to logging concessions in the Caribbean threaten magnolia habitat.
Pressure also comes from harvesting the trees for timber and exploiting them for medicinal purposes, the authors say.
The new report is based on data collected by Adrian Newton, of the U.K.'s Bournemouth University, and colleagues.
Newton's team mapped the geographical range of each magnolia species using information from herbaria and previous scientific studies.
The team then overlayed this data on a map of global forest cover obtained from satellite imagery to reveal the health of the forests that house each different magnolia species.
"If the density of trees was less than 40 percent, it was considered to be deforested" and the magnolias there at risk, Newton said.
Magnolia Connections
In some cases the researchers followed up their map data with field studies, trekking through the forest to count magnolia trees and monitor their health.
Some species, like Magnolia wolfii, which only grows in one small region of Colombia, were found to be critically endangered.
During a site visit in August 2006 scientists could find only three fully grown M. wolfii trees and two saplings remaining in a five-acre (two-hectare) remnant of forest surrounded by coffee plantations.
And "a huge range of other flowering plants growing in the same areas as the magnolias are also going to be at risk," said Martin Gardner, a plant conservationist at Scotland's Royal Botanic Garden Edinburgh (RGBE).
What's more, many insects, animals, and birds depend on magnolias for their survival.
"The Mexican endemic species Magnolia schiedeana is pollinated by a beetle, Cyclocephala jalapensis," said Newton, of Bournemouth University.
That beetle is also found naturally only in this region "and appears to depend on the magnolia flowers for adult nutrition."
Seed-eating birds and pollinating insects will also lose a valuable food source if magnolias continue to decline.
Back to the Wild
The report authors and others suggest a number of actions to protect wild trees.
"Quite simply we need to put a stop to unsustainable tree felling," RGBE's Gardner said.
Oldfield, of Botanic Gardens Conservation International, also believes continued cultivation can help bring severely threatened species like M. wolfii back from the brink.
"We need to ensure that all species are in cultivation in botanic gardens," Oldfield said, "so that we can propagate the most vulnerable species and get them back into the wild."
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