Karst forests: the labyrinthian wildlands of green and bedrock

This story is part of the Landscape News series Forgotten Forests.

In the beech forests at the summit of Pikikirunga, a mountain at the northern tip of New Zealand’s South Island, one must take care not to stray from the path. One wrong foot could result in tumbling down a sinkhole into an underground cavern or crevasse, which in these parts can reach a deadly 183 meters (600 feet) deep.

Over millennia, rainwater has carved this mountain’s soft limestone bedrock into elaborate honeycomb structures; local Māori tell stories about the taniwha water guardians that resemble dragons, who both create and dwell in these underground mazes.

This kind of landscape is called karst. Karsts are geological regions featuring caves, underground streams, sinkholes and steep cliffs, created by the erosion of soft bedrock – usually limestone, dolomite or gypsum. The unusual topography creates conditions for special kinds of forests to grow, leading these ecosystems to harbor high numbers of unique and endemic species. They’re found on every continent, with some listed as UNESCO World Heritage sites, like the jagged Tsingy de Bemaraha karst landscape in Madagascar and the otherworldly South China Karst in the lower Chinese provinces.

By their very nature, many karst landscapes are hard to reach and difficult to convert to other land uses, which has helped keep many of these ecosystems safe from human activities. The jagged teeth of the Tsingy de Bemaraha, for example, are all but impenetrable by humans, resulting in the exceptionally large number of endemic species that live there having remained relatively unscathed by external pressures, despite the country’s high national rates of deforestation and poaching.

However, many smaller karst forests across the globe are under threat of destruction – or are already destroyed – by miners digging out limestone, which is used as a building material, component in concrete and soil alkalizer, among other things. This means species are likely wiped out before their existence is even known; scientists often struggle to survey karst landscapes using traditional methods, meaning much remains unknown about the plants and animals that live within them.

When Yves Laumonier, a scientist at the Center for International Forestry Research (CIFOR), went to explore the mountainous karst landscapes of Manusela National Park on eastern Indonesia’s Seram Island, his sneakers were the first things to go. “The wear on your shoes is incredible. They don’t last more than two weeks out there,” he recalls.

“They’re full of caves, holes and sharp bits, so it’s definitely harder to move through than other landscapes,” echoes Cam Webb, a biologist and research affiliate at the University of Alaska who also did fieldwork in Manusela.

But the biological insights made the challenges well worth it, says Laumonier. “It’s very special there in terms of its vegetation, and also its soil and water. For instance, you get some plants there that look like they’re adapted to drought, even though there’s very heavy rain, because the water percolates very quickly through the soil and disappears.”

The pH level of the soil is also unusual. “Most of the tropical soils are fairly acidic,” explains Webb, “but limestone tends to create more alkaline soils, so there are a number of species that occur more frequently there than elsewhere.” That includes a wide variety of orchids and a critically-endangered species of dipterocarp tree (Shorea selanica), as well asmore regionally-common vegetation such as pandanus palms, weeping paperbark trees (Melaleuca leucadendra) and mangroves.

Spanning 189,000 hectares and covering 11 percent of the island, Manusela National Park – which encompasses coastal, swamp, lowland and montane forest types – was established in 1997 as a last bastion for the endangered salmon-breasted cockatoo (Cacatua moluccensis). The reserve is also home to 118 bird species and the greatest number of endemic mammals of any island in Southeast Asia, including the threatened Seram bandicoot (Rhynchomeles prattorum) and Moluccan flying fox (Pteropus chrysoproctus).

While most island-dwellers live in the low-lying coastal parts of Seram that lend themselves to larger-scale agriculture, some live in closer proximity to the karst forests. The soils there, despite being relatively fertile, are very shallow and stony, so farmers are limited in what they can grow. Most locals cultivate crops like bananas, cocoa and sago in mixed, small-scale swidden-style gardens, supplemented by wild foods foraged from the forest and fresh fish.

One high-value export that locals forage from Seram’s limestone caves is the edible nest of the white-nest swiftlet (Aerodramus fuciphagus), which the bird creates out of its own saliva. The nests are among the most expensive products consumed by humans, and can reach up to USD 3,000 per pound (USD 6,600 per kilo). Chinese culture prizes them for their rarity, high nutritional value and rich flavor.

However, local traditions and beliefs put limits on which caves can be accessed, and by whom. “There are some caves where nobody is allowed to go in because it’s so sacred,” says Laumonier. At one cave he visited, a colony of bats would fly out every evening around dusk. “And if you go there and they don’t come out, it’s very bad,” he says. “It means something terrible will happen, or that the visitor is not welcome.” The first time he visited, the bats came out. “But when I came back with friends to show them, it didn’t work! Fortunately, a week later they came back.”

Read the rest of the Forgotten Forests series here.