Rwenzori Mountains’ first major fire in 12,000 years marked new era for climate

UNIVERSITY PARK, Pa. — For the past several years, Penn State geoscientist Sarah Ivory and her students have been among a team of scientists scaling the East African Rwenzori Mountains, collecting sediment core samples from lakes formed at the end of the last ice age as glaciers began receding in the region some 12,000 years ago.

Among those cores was a surprising revelation: A 2012 wildfire that ravaged 16 square miles of the forest and alpine landscapes at more than 13,000 feet above sea level was unprecedented in at least the last 12,000 years. The researchers also found evidence in fossilized pollen that the fire significantly shifted the region’s ecology. Led by Andrea Mason, a doctoral candidate at Brown University, the team recently published these findings in the journal Nature.

The blaze in the alpine moorland surprised forest experts who assumed the climate was too cold and too wet for fires to start and to spread, Ivory said.

Within the cores, researchers looked at the remnants of charcoal to piece together the fire history of the area since the lakes originated. The cores show no fire activity for about 10,000 years. A slight increase in fire activity about 2,000 years ago — coinciding with an increase in human activity in the region — was recorded in a lower elevation lake. In 2012, the amount of charcoal in the cores shot up more than 100 times, aligning with the timing of the blaze at the border of Uganda and the Democratic Republic of Congo, at higher elevation. Researchers also assessed historic pollen records to determine there were dramatic shifts in the region’s ecology over the past 2,000 years as fire increased.

“The fact that this one fire in 2012 is the only fire that’s happened on this mountain for the entire existence of the lake is mind blowing,” Ivory said. “It’s like the image of a plastic bag in the Mariana Trench. We shouldn’t see human influence in an area that’s this remote, but it’s there.”

The team trekked the mountainside over a period of nearly two weeks, collecting sediment cores from two lakes: Lake Mahoma at about 9,000 feet and Lake Kopello in the alpine zone around 13,000 feet. Lake runoff and wind concentrated indicators into the soft underwater beds captured in the cores that tell us about past plants and climates on the mountain like pollen grains, leaf waxes, fossil bacteria and other biomarkers.

One goal of the project, Ivory said, was to help understand rapid changes to the low-lying village more than 10,000 feet below the Ugandan peaks. The Rwenzori Mountains National Park, a UNESCO World Heritage Site in Uganda, is home to the last remaining glacier that numbered in the dozens roughly 100 years ago. In that time, the region has lost more than 90% of its glacial ice.

In the village of Kilembe, deadly floods, landslides and mudslides have ravaged infrastructure, homes, farmland and livestock in recent years. In her research at large in Africa and parts of the Middle East, Ivory is assessing how warming since the last ice age has and continues to affect ecosystems. She and her students have been assisting with reforestation and forest mapping to combat the effects of climate change in the area.

The fire in the alpine region is another sign of change, Ivory said. Kilembe experienced massive flooding the year following the fire — which burned 18% of the catchment above the village. The unpredictability of the river that runs through the village continues to plague the community, Ivory said, with continued flooding that permanently knocked out power years ago.

“Whenever it rains, everything is disrupted,” Ivory said. “Flooding destroyed bridges and forced boulders into the river, making it difficult to rebuild what was there. It’s really transformed a community just downstream from the 2012 fire.”

Another key finding, Ivory said, was told through the pollen records analyzed at Penn State. The pollen revealed massive changes associated with early human fires as well as more recent fires driven by human-induced climate change to the ecology of the Rwenzori Mountains, one of the most unique and biodiverse mountainous regions on the planet. Much like isolated areas such as the Galapagos Islands offer a glimpse of evolutionary changes, this remote area of Uganda is a hot spot for ecological research and discovery, Ivory said.

The Rwenzori Mountains belong to a network of Afroalpine “sky islands,” isolated, high-altitude environments that support unique plant and animal life found nowhere else on Earth. Comparable systems, such as Mount Kilimanjaro and Mount Kenya, have also seen recent wildfire activity, suggesting that rising temperatures may be altering ecosystems that were once naturally safeguarded by extreme alpine conditions.

Pollen records showed extensive changes in plant life, beginning around 2,000 years ago. This period coincides with an increase in human intervention and agricultural practices. At lower elevation, pollen associated with rainforest trees declined after the period when fires were detected, while pollen associated with bamboo and other grasses increased.

Since 2012, Ivory said, the slow growing mountainside trees have remained damaged and the forest is at risk of ecosystem transformation, especially if fires continue to persist.

“The Rwenzori Mountains have one of the most diverse and pristine expressions of this type of vegetation,” Ivory said. “Similar areas have a lot more disturbance and a lot more fire. Until recently, this was a holdout; it was one of the best examples of this special ecosystem and now that’s under threat.”

In addition to Ivory and Mason, co-authors include Meredith Kelly, Dartmouth College; Bob Nakileza, Makerere University; Eleanor Pereboom and James Russell, Brown University; and Richard Vachula, Auburn University.