They are the new bad actors in Indonesian forestry - and across Borneo as well.

The trees under threat in Sumatra are not of the pristine, triple-canopy kind.

They are in the 1.5 million ha of acacia plantation from where the island's thriving pulp and paper industry now gets most of its wood.

But, as The Straits Times has learnt exclusively, the animals are only part of a much wider problem that forestry experts say threatens the sustainability of the plantations and could ultimately drive the multibillion dollar industry to the wall.

They say that by planting a species that is non-indigenous to the island through what is now three seven-year rotations, companies are facing the same ecological disaster that the Germans experienced with imported spruce in the late 1900s.

"They should have a natural forest with mixed species and a longer rotation with sound socio-economic principles, less corruption and better law enforcement," says one expert.

"It is simply not sustainable if they go on as they are."

Nature, it appears, is now taking revenge on Asia Pulp and Paper (APP) and Asia Pacific Resources International (April), the two biggest companies that for years were reviled for the destruction of the Sumatran rainforest.

Since the turn of the current decade, local mammals have developed a taste for maturing Acacia mangium, the monoculture species that delivers commercial wood crops in as little as five years on degraded or infertile soil.

The animals gnaw off the bark to get at the sap and its sweet inner layers of phloem and cambium, either killing the tree or exposing it to ceratocystis - a fungal disease even more difficult to control than root rot.

The animals are causing so much havoc that companies are now busy substituting their acacia for the less tasty Eucalyptus pelita. It grows more slowly, and there is another significant drawback: It does not grow on Sumatra's peatlands.

The monkeys and squirrels have laid waste to half of the Acacia mangium, while root rot is destroying a similar percentage of Acacia crassicarpa, the species grown exclusively on peatland, which makes up about 60 per cent of the overall plantation area.

Dr Chris Beadle, a plant scientist with Australia's Commonwealth Scientific and Industrial Research Organisation, says root rot kills a tree in a year, rendering it commercially useless. Ceratocystis does the same damage in weeks.

Mill owners are already having to cut trees after only four years, which means a sharp reduction in resources.

As in the more valuable oil palm plantations, chemicals could be used to treat the acacia. But this would cost more than the wood itself.

From 1988 to 2003, an estimated 80 per cent of the 185 million cubic m of wood fibre processed by paper mills came from tropical hardwoods.

Under mounting pressure from environmentalists, most companies were expanding capacity at a faster rate than their plantations were reaching maturity.

Its own hardwood resources close to exhaustion, APP finally signed a landmark agreement with The Forest Trust (TFT) and Greenpeace in February last year to end all natural forest-clearing in its supply chain. TFT says the company is sticking to the deal.

Along with April, which has yet to attain plantation sustainability, the two forestry giants produce 6.2 million tonnes, or three-quarters of total pulp capacity, with acacia making up about 80 per cent of the feedstock.

APP controls 2.3 million ha of concessions in South Sumatra and West and East Kalimantan, and operates two pulp mills in Riau and Jambi provinces and seven Java-based paper and packaging mills, in addition to other production facilities in China.

April and its partners manage concessions covering 700,000 ha in Sumatra and East Kalimantan, with a large pulp and paper mill in Riau and a smaller facility in North Sumatra.

April also expanded into China in the mid-2000s.

With an additional nine million ha allocated for plantation conversion by 2020, the forestry ministry wants to boost pulp capacity to 16 million tonnes a year.

That would mean increasing wood supplies from 29 million cu m to 72 million cu m.

The industry's current problems would seem to make this goal a pipe dream, but it has not stopped APP from going ahead with plans to build a new 2.5-million-tonne-a-year pulp mill near Palembang in South Sumatra.

Acacia mangium and Eucalyptus pelita are both indigenous to parts of eastern Indonesia, but when monoculture plantations were introduced into South Sumatra and Kalimantan, acacia was the preferred choice because it grew faster.

Little was done to understand the biology, and by the end of the first rotation, root rot had begun to appear. Over the next two rotations, it developed into a scourge.

Then, two or three years ago, along came the protected macaque. An accomplished swimmer, it inhabits Sumatra's mangrove swamps and riverine forest, often in groups of up to 48 animals feeding off fruit, coconuts, leaves and crabs.

Indonesian foresters say the monkeys only learnt to feast on the acacia sap after local villagers began felling the green belts that were their habitat, following the wholesale destruction of the original rainforest.

Dr Beadle says the macaques made a "massive difference" in a very short period of time, largely because once a tree has been partly debarked or otherwise "wounded", it allows ceratocystis to flourish and spread at an alarming rate.

So far, the monkeys have not shown any interest in eucalyptus, but who knows?

"The logical thing is for the companies to switch to something else," says Dr Beadle. "But you can never tell what the future may hold." (***)

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