Some 25,000 elephants have been slaughtered in Minkebe National Park, an area that had been considered a sanctuary, said the report in the journal Current Biology.
"Because Gabon is thought to hold the largest remaining population of forest elephants, the implication is that forest elephants are in even more trouble than previously believed," said researcher John Poulsen of Duke University and the Agence Nationale des Parcs Nationaux in Gabon.
"With less than 100,000 elephants across all of Central Africa, the subspecies is in danger of extinction if governments and conservation agencies do not act fast."
The poachers are primarily coming into Gabon from the bordering country of Cameroon, the report said.
"We can no longer assume that apparently large and remote protected areas will conserve species -- poachers will go anywhere that a profit can be made," said Poulsen.
To estimate the number of elephants in the forest in 2014, researchers surveyed dung in the forest.
They then compared population size estimates for 2014 to estimates calculated in the same way in 2004.
A key driver of the poaching is demand for ivory, which must be curtailed, researchers said.
"China's recently announced ban of domestic ivory trade will help enormously, if it is effectively implemented," said Poulson.
"The international community needs to put pressure on all remaining nations that allow the trade so that all legal trade is stopped."
Another strategy is to recognize forest elephants as a distinct species from African savanna elephants, to draw attention to their often forgotten plight.
Gabon has taken steps to protect elephants since 2011, elevating forest elephants' conservation status to "fully protected," creating a National Park Police force, doubling the national park agency's budget, and becoming the first African nation to burn all confiscated ivory, the report said.
However, Poulsen said more action is needed, such as coordinated international law enforcement to prosecute wildlife criminals and new multinational protected areas.
"The clock is ticking," he said.
