Mountain Formation — and Ensuing Monsoons — Created Biodiversity Hotspot in Asia

A plant press used by the researchers doing fieldwork in the Hengduan Mountains. (Photo courtesy of Deren Eaton)

(CN) — Mountainous regions are home to a breadth of biodiversity found in few other ecosystems on Earth, but where does it all come from? Scientists have traced back plant life in the mountains of China 30 million years to its formation and the ensuing monsoons to find the answer.

By coupling local fossil records with DNA evidence taken from trees, researchers have begun to unravel the mystery of why these particular ecosystems contain such an outsized proportion of life. 

Rick Ree, a curator at Chicago’s Field Museum, and his team studied the vegetation throughout China’s Hengduan Mountains, the Himalayas and the Qinghai-Tibet Plateau in research released Thursday in the journal Science. They attempted to figure out how these mountainous florae are distributed, and how they got to where they are.

The researchers turned their attention to the alpine zone — those plants that grow above the tree line — which contains one-third of all plant species found in China. Alpine plants are particularly hardy and well adapted for the stressful high-altitude environment, yet remain especially vulnerable to global warming for that reason. 

The Hengduan Mountains are considered among the most vulnerable biodiversity hotspots on the planet and contain approximately 3,030 unique varieties of flora.

“This paper addresses the fundamental question of why there are so many species in some parts of the world and not others,” Ree said in a statement. “The formation of this very species-rich community was fueled by ancient mountain-building and then subsequent effects of the monsoon. The biodiversity that we see today has been profoundly shaped by geology and climate.”

The team compared DNA samples taken from 18 groups of flowering plants to determine how closely related they are and how they evolved. By comparing differences in their DNA and information gleaned from fossils, they were able to make an educated guess as to when these plants’ common ancestor lived and began assembling a family tree. 

The authors found that many of the plants studied originated in the Hengduan Mountains, before migrating outward thanks to tectonic activity.

After the Indian tectonic plate collided with the Asian continent and reshaped the landscape around 50 million years ago, increasingly intense monsoons began to pummel the region. 

Shifting wind patterns prompted by the formation of new mountains were the likely culprit, according to the authors. The vast deluge of rain delivered each season by these monsoons allowed this burgeoning ecosystem to flourish during the Miocene period.

“The combined effect of mountain-building and monsoons was like pouring jet fuel onto this flame of species origination,” Ree said. “The monsoon wasn’t simply giving more water for plants to grow, it had this huge role in creating a more rugged topography. It caused erosion, resulting in deeper valleys and more incised mountain ranges.”

These newly contrasting landscape features forever altered the evolution of local plant life by separating and forcing them to evolve independently. Plants in a valley have very different needs from those perched on a hill. Eventually, natural selection ran its course and chose varying traits to propagate depending on where a species wound up, leading to the biodiversity now on display.

“The theory is, if you increase the ruggedness of a landscape, you’re more likely to have populations restricted in their movement because it’s harder to cross a deeper valley than a shallow valley. So any time you start increasing the patchiness and barriers between populations, you expect evolution to accelerate,” Ree said.

The researchers employed model-based simulations of historical sequences of speciation, extinction and colonization to estimate the rate of species accumulation through time in both alpine and non-alpine environments. These models indicate that alpine flora in the region originated earlier than previously suspected — millions of years before their Rocky Mountain counterparts.

In addition to improving our understanding of how this vast array of flora developed, Ree notes that the geological changes examined in the study may help us to predict and mitigate threats to these species going forward.

“Mountain ecosystems tend to be very sensitive to things like global warming, because the organisms that live there are dependent on a tight range of elevation and temperature,” he said. “Understanding how historical environmental change affected alpine plants 20 million years ago can help us predict how today’s climate change will affect their descendants.”

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