Saturday, December 10, 2016

Reforestation season

It’s the wet season in Borneo.  When I first moved here in October it was unbearably hot and sunny, with only scattered rains.  Now the skies are gray, mist hovers over nearby hills, winds buffet our house, and it rains nearly every day, sometimes in deafening downpours that drown out conversation.  And it’s still unbearably hot.

For many trees the rainy season signals the onset of fruiting.  Months of stored nutrients are channeled into fruit production, in anticipation of good growing weather for seedlings.  Here in western Borneo one fruit towers above the rest.  Durian, the king of fruits, is so valued that naturally occurring durian trees are often left standing as the forest around them is cleared for farming or timber.  Massive durian trees now stand over farms and homesteads, remnants of the ancient forest canopy that occupied those sites just years ago.

Our city of Sukadana has even erected a monument to the durian in the center of town.  In the countryside people risk their lives climbing homemade ladders to harvest the fruits from the dizzyingly tall treetops.  Or they camp out at the base of the trees, scooping up fruits as soon as they fall and being careful not to get clobbered by the giant spiky missiles.  Last week, as I entered Borneo from Malaysia, the customs official at the border wiped a smear of durian slime from his passport stamp before welcoming me back.

Rambutans (from rambut, Malay for hair) are one of many native plants that fruit at the start of the wet season

For us the rainy season is planting time.  My organization manages a few reforestation sites within Gunung Palung National Park.  We target patches of the landscape that have been cleared by loggers, farmers, or fires, and try to restore them to natural forest.  The onset of wet weather means a lower threat from fires and better conditions for seedlings.

Our pilot reforestation site is a hilly area that had been clearcut by loggers and then burned repeatedly by fires sweeping in from nearby farms and plantations.  The cleared areas were colonized by alang-alang grass (Imperata cylindrica), bracken ferns (Pteridium caudatum), and invasive acacia trees (Acacia mangium).  The long grass encourages further fires and the bracken ferns release toxins into the soil that impede the survival of young trees.  Without positive intervention to undo some of the damage, these fire-prone barrens stand little chance of recovering.

Our reforestation site is covered by burnt logs, relicts from wasteful loggers who cut down every tree, even those that weren’t harvestable, and the forest fires that followed

Some of the cut trees were centuries old and 3-4 meters in diameter, as demonstrated by one of our volunteers

It takes thousands of years to create oldgrowth forest.  There’s no way for humans to mimic the process.  We can jump start it, however, and provide protection for natural processes to go unhindered.  Starting in 2009, our organization has replanted a few hectares with native rainforest seedlings every year.  We collect seeds and seedlings from remaining forests in the park, rear them to a size where they are likely to survive, and then transplant them to the reforestation site.

Relying on an expert staff of local foresters with experience all over Borneo, we plant native seedlings on degraded areas within the national park

For the most exposed and hottest sites, we often plant fast-growing legumes—mostly jengkol (Archidendron pauciflorum) and petai (Parkia speciosa)—as pioneer species.  We also plant native fruits like mangoes (Mangifera sp.), jambu (Syzygium sp.) and breadfruits (Artocarpus sp.), plus any other native species we can get to survive.  Our most prized seedlings, however, are several species of dipterocarps—a family of giant trees that includes the tallest tropical species in the world.

Dipterocarps usually dominate the canopy in oldgrowth Bornean forests.  Collecting seeds is difficult, however, because the trees only fruit during mast events that occur once every few years.  Nevertheless, we’ve managed to rear a few species of meranti (Shorea sp.).  As mature forest specialists, meranti seedlings often need shade to survive.  So we plant them inside groves of the invasive acacias.  Eventually, the natives will tower over and outcompete the acacias, or we’ll remove the acacias ourselves once the natives have formed a canopy of their own.

Unplanted areas (on the left) are still covered in bracken fern, alang-alang grass, and acacias, whereas the newly-planted plot (right) will soon be covered with native rainforest trees

New seedlings are dwarfed by the charred remains of the site’s original trees, but may one day form an oldgrowth forest.  Once the native trees are tall enough to form their own canopy, the acacias in the background will either be outcompeted or removed by hand.

There are occasional setbacks.  In 2013 a fire escaped from nearby farmland and burnt most of the site, undoing years of effort.  We revised our fire control measures and the planting continued.

Despite setbacks from fires, much of our reforestation site is now covered in young secondary forest

We also haven’t had much success cultivating one of our rarest trees—Bornean ironwood (Eusideroxylon zwageri).  Ironwood is one of the world’s most durable woods, making it a prized timber tree and a cultural symbol for the people of Borneo.  But its durability comes from extremely slow growth.  It takes decades for an ironwood tree to grow a single meter.  They can live for over a thousand years, and those centuries or millennia of tiny incremental growth result in extremely high wood density.  Unfortunately, the combination of high timber value and slow growth means that ironwood has been wiped out from most areas.  While I have seen plenty of ironwood stumps, I have yet to see a full grown individual.

Full-grown Bornean ironwood trees are rare today, but young saplings sometimes sprout from cut trunks

The good news is that Bornean ironwood, if left alone, can resprout from the roots of cut trees.  We have two such individuals at our site, spindly saplings growing straight up from the sides of massive stumps.  Centuries from now ironwood forests may once again be a prominent feature of the landscape.

Until then, we’ll keep planting.


  1. Inspiring determination to do something good for the planet, no matter what.

  2. Thanks a lot Ken! I do have pessimistic days. But all we can do is keep trying.