Experts believe that a depleted ozone layer contributed to the largest mass extinction ever on Earth, which took place 252 million years ago.
Now, a new study has shown that extreme levels of ultraviolet rays, emanating from the sun and passing through the atmosphere, can cause trees to become sterile.
They believe this may have led to a catastrophic shortage of nutrition, which affected every level of the food chain, during the end-Permian crisis.
Since the 1980s, the ozone has been damaged by man-made chemicals and is at risk of again being destroyed, leaving the planet vulnerable to the harmful radiation.
While the main culprit for ozone depletion has since been banned, the finding suggests stricter measures may be needed for other industrial chemicals.
Extreme levels of ultraviolet (UV) radiation can cause trees to become sterile, says a new study. When subjected to high levels of UV radiation, the pine cones of bonsai trees (left) became shrivelled and infertile (right)
Researchers from the University of California Berkeley found that radiation levels similar to those at the time of the end-Permian crisis were enough to stop the plants reproducing.
They recreated the high UV levels in a lab to see how modern-day plants would cope, irradiating 18-inch (45cm) tall, bonsai-like pines with UV-B dosages up to 13 times stronger than on Earth today.
Experts found that, while the radiation doesn’t kill the plants, it deforms pollen and damages pine cones, making reproduction impossible.
The most intense levels of UV radiation on Earth occurred at the start of the Triassic period.
At this time, nearly 70 per cent of known land animals and 95 per cent of marine life went extinct.
The experiment was designed to simulate the effects of ozone depletion, believed to have been caused by immense volcanic eruptions that occurred at the end of the Permian Period.
Jeffrey Benca, the graduate student who conducted the experiment, said: ‘During the end-Permian crisis, the forests may have disappeared in part or fully because of increased UV exposure.
‘With pulses of volcanic eruptions happening, we would expect pulsed ozone shield weakening, which may have led to forest declines previously observed in the fossil record.
Bonsai trees were placed in a special UV chamber at the University of California Berkeley (pictured) for two months to see the effect the ultraviolet waves had on the small pine trees. Researchers found they survived, but were sterile
‘If you disrupt some of the dominant plant lineages globally repeatedly, you could trigger trophic cascades by destabilising the food web base, which doesn’t work out very well for land animals.’
During the two-month experiment, none of the trees died but all seed cones, or pine cones, shrivelled up only days after emerging.
In three separate indoor UV chambers, Mr Benca exposed the bonsai trees to 7.5, 10 and 13 times normal UV-B intensity.
When placed outside, the trees regained the ability to produce healthy seed cones in later years.
By subjecting the bonsai trees to the same levels of radiation as Earth experienced 250 million years ago, researchers saw that although the trees survived, they couldn’t reproduce. None of the trees died but all seed cones, or pine cones, shrivelled up only days after emerging
The research team found that the pines produced malformed pollen.
At the medium and extreme exposure range, 12 to 15 per cent of pollen grains were misshapen, versus three per cent in normal and low-exposure trees.
Their seed cones shrivelled up before they had a chance to be fertilised.
This seemed to be a systemic reaction to the UV-B stress, since even seed cones hidden among foliage died.
Researchers found that although the high levels of UV light severely damaged the trees and halted reproduction, the trees did not die. The lack of growth and reproduction could have been enough to disrupt ecosystems and kill animals that depend on plants as a food source
Scientists have proposed that ozone depletion caused by periodic volcanic eruptions over nearly a million years was one cause of the end-Permian extinction, but how has been unclear.
The lack of ozone at the end of the Permian period is thought to have been caused by volcanic eruptions in Siberia over hundreds of thousands of years.
These eruptions produced what are known today as the Siberian Traps: lava fields covering much of northern Russia.
They originally encompassed nearly 3 million square miles with an average thickness of about 1,000 feet (300 metres).
The high levels of UV-B radiation caused healthy pollen grains (left) to become malformed and distorted (right). This meant the plants could not reproduce effectively and as a result they became sterile
Acid rain would have been a local effect, but the mass extinction of land animals, marine life and many plant lineages was global.
Although plants could have survived through huge levels of radiation, the damaging effect on growth and reproduction may have had a huge impact on the food chain.
Previous paleoclimate modelling studies suggest the volcanic spurts could have wiped out the ozone layer worldwide, though temporarily.
Nevertheless, even if ancient trees regained their fertility, repeated bouts of sterility could have hampered population growth over time, leading to collapse of the biosphere planet-wide.
Co-author professor Cindy Looy added: ‘Paleontologists have come up with various kill scenarios for mass extinctions.
Regular pollen grains have a regular and predictable appearance, the radiation caused them to mutate and deform to the point where the plant was incapable of reproducing at all
‘But plant life may not be affected by dying suddenly as much as through interrupting one part of the life cycle, such as reproduction, over a long period of time, causing the population to dwindle and potentially disappear.’
The current ozone depletion is as a result of an influx of damaging chemicals into the atmosphere since the 1980s.
Carbon fluorocarbons – CFCs – were common in fridges, aerosol cans and many other products.
These created a huge hole in the ozone layer over the poles, which has been associated with increased rates of cancer and other health concerns.
Since a global ban on the damaging chemicals, the ozone layer has slowly started to repair itself.
But the surprise findings tell scientists something about past extinctions and Earth’s future prospects as climate change, habitat destruction and pollution set us up for Earth’s sixth mass extinction.
The full findings of the research were published in the journal Science Advances.