A new fossil study indicates that Southeast Asia’s forests were more resilient to past climate change than previously thought.
In contrast to the theory that a vast savanna corridor expanded across Southeast Asia during the peak of the last glaciation, the researchers found evidence that a mosaic of forests in fact persisted across much of the region.
The findings emphasize the importance of protecting connected networks of a wide range of forest types to afford the region its best chances of adapting to projected climate change.
In addition to ecological implications, the findings provide insight to theories of how humans historically migrated across and shaped the region’s ecosystems.
Throughout history, philosophers have told us that in order to best prepare for the future, we should look to the past. Wisdom, knowledge and new perspectives can often emerge out of a greater understanding of what went before. And it’s no different when it comes to predicting how nature will respond to projected climate change.
A new paleoecological study based on environmental records going as far back as 119,000 years ago suggests that Southeast Asia’s forests could be more resilient to climate shifts than previously thought, provided a diversity of ecosystems is maintained.
In contrast to a widely held theory that tracts of Southeast Asia’s lowland forests morphed abruptly into a vast savanna corridor in response to the cool and seasonal climate during the peak of the last ice age roughly 20,000 years ago, the international research team found evidence that forests largely persisted across the landscape at that time.
Rather than an abrupt shift from forest to grassland, the researchers documented smooth transitions between lowland rainforest, open-canopy seasonally dry forest with a grassy understory, and tropical montane forest. This mosaic of forest types indicates the region’s ecosystems had the capacity to “resist and recover from” climate stress, the study says.
The new insights back up calls from conservationists to preserve a diverse mix of forest types in well-connected networks across Southeast Asia to afford the region the best chances of adapting to climate change impacts.
“Preservation of forest types that facilitate resilience should be a priority,” Rebecca Hamilton, a paleoecologist at the University of Sydney and lead author of the new study, told Mongabay. “Seasonally dry forest, for instance, often gets overlooked. It’s not seen as locally biodiverse in comparison with tropical rainforest, but it has beta diversity on a landscape level.”
Southeast Asia’s forests have seen unprecedented pressure from human activity in recent decades. Clearance of much of the region’s lowland rainforest to make way for urban expansion, agriculture, and oil palm and rubber plantations is now being followed up with alarming rates of encroachment of upland forest.
To investigate Southeast Asia’s ancient landscapes, Hamilton and her colleagues looked at biochemical data from 59 paleoecological sites across the region. They scrutinized fossil pollen grains preserved in lakebeds to tell which species of trees were growing at a particular point in time; conducted isotopic analyses of carbon in sediment cores to reveal the relative ratios of grassy plants to woody plants extant through time; and even looked at evidence from animal remains and bat guano.
Although their analyses revealed a high abundance of grasses similar to that found in prior studies, the additional presence of woody species in the fossil record led the team to conclude that forests on higher ground likely expanded and flourished during the cool and seasonal climate of the last glacial maximum, while expanses once presumed to have been savanna grassland were more likely seasonally dry forest, which “actually have a lot of grasses in the understory,” Hamilton said.
The last glacial period investigated in the study is by no means an exact analog of projected climate change, the authors note. It was, however, a period of extreme seasonality, which is a “really important factor” in determining whether natural landscapes are dominated by forests or grasslands, they say. With scientists expecting climate change to amplify the seasonality of Southeast Asia’s wet seasons and dry seasons, triggering extreme precipitation and drought events, an enhanced understanding of how ecosystems respond to such conditions is vital, Hamilton said.
The results also suggest that Southeast Asia’s forests might respond to large-scale disturbance in different ways than what’s observed in other parts of the world, according to Hamilton. Forest biologists in South America, for instance, have documented concerning “tipping points” triggered by huge disturbances, such as El Niño-driven forest fires and massive land clearance for agriculture, which lead to irreversible shifts from forest to grassland-dominated savanna systems.
But such tipping points aren’t so clear in Southeast Asia, Hamilton said. Instead, the region’s forests tend toward the “stepping stone” composition of seasonally dry forest, which lies somewhere between forest and grassland. “In Southeast Asia, we don’t really see these abrupt shifts to savanna. We only really see gradual shifts toward more open forest types.”
This doesn’t mean, however, that Asia’s forests are immune to devastating regime shifts. Human activity is persistently undermining the resilience of the region’s forests. “[This] could potentially be quite catastrophic to persistence of rainforest across the tropical zone,” Hamilton said. “We’re seeing [forest] fragmentation often with fire, which is used to clear large swaths of landscapes, which is pretty damaging and can cause abrupt human-caused transitions to savanna [similar] to a lot of the Americas.”
However, we can’t ignore the fact that humans have been shaping the world’s forests for thousands of years, Hamilton said. Previous studies show, for example, that swidden forms of slash-and-burn agriculture have maintained structural forest diversity in parts of mainland Southeast Asia. And the new insights into the extent of forest cover across the region could help elucidate theories of how humans and animals moved through Southeast Asia during the last glacial period.
“Some schools of thought suggest that savanna environments facilitated human migration, [viewing] tropical forests as absolutely inhospitable to humans,” Hamilton said. “Now we know that people use and manage tropical forests and have migrated through at least some forest systems to reach Australia, for example … We don’t have enough evidence to suggest [forest persistence] was a driver of this migration, but it shows that we as a species have historically used resources from a range of ecosystems.”
The findings of the new study “substantiate what at least some of us suspected,” Alice Hughes, an associate professor at the University of Hong Kong, who was not involved in the study, told Mongabay by email. “The savannah corridor hypothesis has long been questioned in Southeast Asia, and [has] longstanding implications for our understanding of regional biogeography.”
Sea levels were much lower during the last glacial period, exposing what’s known as the Sunda Shelf — a low-lying landmass connecting what’s now mainland Southeast Asia to the islands of Borneo, Sumatra, Bali, and many others. Hughes said the new evidence that the Sunda Shelf was home to forests underscores the importance of modern-day forests as important reservoirs of genetic diversity.
“With the rising of sea-levels, remaining forests do represent true refugia, and the last strongholds of genetic diversity of species that in some instances would have had much more expansive ranges across the Sunda shelf,” Hughes wrote. “Undoubtedly, [these forests are] key areas to protect to maintain diversity (and genetic diversity) across the region.”
In the face of global warming, conservation practitioners and policymakers must be mindful of the fact that many of the region’s species have narrow geographic ranges, which renders them particularly vulnerable to impacts from climate change, Hughes added.
“Increasing connectivity across this region will be vital to reconnect forest patches and enable species within them to attempt to track future climate ranges.”
Carolyn Cowan is a staff writer for Mongabay. Follow her on 𝕏, @CarolynCowan11.
Banner image: Rising sea levels after the last ice age formed pockets of forest refugia that constitute important reservoirs of genetic diversity. Image by Rhett A. Butler for Mongabay.
Hamilton, R., Amano, N., Bradshaw, C. J., Saltré, F., Patalano, R., Penny, D., … Roberts, P. (2023). Forest mosaics, not Savanna corridors, dominated in Southeast Asia during the last glacial maximum. Proceedings of the National Academy of Sciences, 121(1). doi:10.1073/pnas.2311280120
Hamilton, R., Penny, D., & Hall, T. L. (2020). Forest, fire & monsoon: Investigating the long-term threshold dynamics of south-east Asia’s seasonally dry tropical forests. Quaternary Science Reviews, 238, 106334. doi:10.1016/j.quascirev.2020.106334