Cyclone Sidr struck the coast of the impoverished South Asian country on Nov. 15 with winds of 250 kph (155 mph). It killed around 3,500 people, made millions homeless and destroyed a large part of the Sundarbans.

At least 60 percent of the 6,000 sq km (2,320 sq mile) mangrove swamps that are home to more than 400 Royal Bengal tigers was devastated by the cyclone.

Top-dying was already endemic among Sundari trees, but the disease has spread and intensified since the cyclone hit, threatening the existence of the forest, a senior forest official said.

Sundari trees constitute 70 percent of trees grown in the swampy forest. The trees grow up to 70 feet (23 metres) in height and are mostly used in boat building and house construction.

Experts have yet to find a cause of the top-dying disease, but they suspect that increased salinity may have something to do with the intensification of the disease.

"Gradual decreasing of oxygen in the soil of the forest may also be a factor for this (top-dying disease)," A.F. Fazlul Haq, a professor in the forestry department of the Khulna University told Reuters.

Some other experts and forest officials say the salinity has been increasing due to decreasing water flow across the forest, which is criss-crossed by a number of rivers and canals.

Water flow has decreased due to urbanisation and the construction of dams further upstream on the rivers, most of which flow in from neighbouring India before emptying into the Bay of Bengal.

The Sundarbans stretch 4,000 sq km (1,545 sq miles) into India's eastern state of West Bengal.