Pine Trees’ Sterilization Linked to Ozone Depletion

(CN) – Ultraviolet radiation at the level scientists believe Earth experienced 252 million years ago during the planet’s largest mass extinction event can temporarily sterilize pine trees, according to a new study that supports the theory that ozone depletion contributed to the die-off.

UC Berkeley researchers subjected dwarf pines (Pinus mugo) to enhanced UV-B radiation in growth chambers for two months. The irradiated pines not only produced malformed pollen, but they dropped all their seed cones, becoming temporarily sterile. (Jeffrey Benca, UC Berkeley)

The report, published Wednesday in the journal Science Advances, finds that high UV exposure could also affect other conifers and potentially deciduous trees. These impacts suggest a need to limit the use of chemicals that deplete the planet’s ozone layer, which is still recovering following a global ban in the 1980s on chlorofluorocarbon refrigerants after holes in the ozone developed over Earth’s poles.

However, some industrial chemicals also destroy atmospheric ozone, leaving Earth vulnerable to excessive UV rays that can cause DNA mutations and expose trees to levels of radiation that could limit or ruin their ability to reproduce, the study finds.

University of California, Berkeley, graduate student Jeffrey Benca irradiated 18-inch bonsai-like pines with UV-B radiation levels up to 13 times stronger than experienced on the planet today. Benca and his team simulate the impact of ozone depletion caused by massive volcanic eruptions that occurred at the end of the Permian Period, about 299 million years ago.

None of the trees died during the two-month experiment. But all pine cones shriveled up days after appearing, leaving the trees sterile.

After being placed outside, the trees were able to regain the ability to produce healthy seed cones years later.

Normal seed cones (left) of the dwarf pine Pinus mugo compared to the shriveled seed cones of trees irradiated with more than 10 times the amount of UV-B that hits Earth’s surface today. Once removed from the UV-B growth chambers, the trees regained their fertility. The irradiated trees also produced malformed pollen, which has been observed in fossils from the end of the Permian Period 252 million years ago. (Jeffrey Benca, UC Berkeley)

Researchers have argued ozone depletion caused by periodic volcanic eruptions over almost a million years was one cause of the extinction at the end of the Permian, but how was unclear.

Acid rain would have been an issue for portions of Earth, but the extinction of many plant lineages, nearly 70 percent of known land animals, and 95 percent of marine life occurred on a global scale.

Previous paleoclimate modeling research suggests volcanic eruptions could have destroyed the ozone layer worldwide temporarily. However, even if ancient trees regained their fertility, repeated episodes of sterility could have hindered population growth over time, resulting in the collapse of the biosphere across Earth, according to Benca.

“During the end-Permian crisis, the forests may have disappeared in part or fully because of increased UV exposure,” Benca said. “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.”

“If you disrupt some of the dominant plant lineages globally repeatedly, you could trigger trophic cascades by destabilizing the food web base, which doesn’t work out very well for land animals.”

The team’s findings can inform researchers about past extinction events and Earth’s future prospects as pollution, climate change and habitat destruction leave the planet vulnerable to a sixth mass extinction.

“Paleontologists have come up with various kill scenarios for mass extinctions, 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,” said co-author Cindy Looy, an associate professor of integrative biology at UC Berkeley.

Co-author Ivo Duijnstee adds that mass extinctions ultimately stem from the decay of evolutionary development outpacing the introduction of new species.

“Global biodiversity catastrophes are not about death but about the pruning of evolutionary branches on the tree of life at a rate much higher than the sprouting of new shoots,” said Duijnstee, an adjunct assistant professor of integrative biology at UC Berkeley.

“Jeff, who used his plant growth chambers as a time machine to test the potential of a hypothesis about what may have happened 252 million years ago, provides an excellent example illustrating how the slowly unfolding extinction on land over maybe tens or hundreds of thousands of years may have been caused by reproductive troubles at the base of the food chain.”


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