Air pollution hampers tree growth in the Arctic | Polarjournal
Large-scale pollution has led to a devastating decline in forests east of Norilsk, Russia. Photo: Dr. Alexander Kirdyanov

Industrial pollution in the Arctic is much worse than we thought. The largest study to date on annual rings of trees from Norilsk in Siberia has shown that the direct and indirect effects of industrial pollution in the region and beyond are far worse than previously thought.

An international team of researchers, led by the University of Cambridge, has combined measurements of the annual ring width and wood chemistry on living and dead trees with soil characteristics and computer modelling to show that the damage caused by decades of nickel and copper mining has not only devastated the local environment, but also affected the global carbon cycle.

The extent of the damage caused to the boreal forest, the largest land biome on earth, can be seen in the annual growth rings of trees near Norilsk, where the decline has spread up to 100 kilometers from the city. The findings are reported in the journal Ecology Letters.

Norilsk in northern Siberia is the northernmost city in the world with more than 100,000 inhabitants and one of the most polluted places. Since the 1930s, the intensive extraction of the region’s vast deposits of nickel, copper and palladium, combined with few environmental requirements, has led to severe pollution. A massive oil spill in May 2020 has contributed to the extreme environmental damage in the region.

Not only the high air emissions of the Norilsk industrial complex are responsible for the direct destruction of some 24,000 square kilometers of boreal forest since the 1960s, but also the surviving trees in large parts of the high northern latitudes suffer as a result. High air pollution leads to declining tree growth, which in turn affects the amount of carbon that can be bound in the boreal forest.

Environmental degradation in Siberia. (a) sampling points in larch and spruce stands (points or triangles) overlaid at different levels of vegetation destruction (grey shading). Points with black midpoints refer to the three locations for which wood chemistry was measured. The red frame in the inserted map places the further study area Norilsk in the context of the boreal forest zone of Russia (green area). Black dots (larch) and a triangle (spruce) in the boreal map show unencumbered reference locations used for the large-scale, process-based modeling experiment. (b) The three images represent different levels and aspects of ecosystem disruption. Illustration: Kirdyanov et al. 2020

Although the link between pollution and forest health is well known, he could not explain the “divergence problem” in dendrochronology (the analysis of annual rings): a decoupling of the annual ring width from rising air temperatures since the 1970s.

Using the largest data set of annual rings of living and dead trees to date, which reconstructs the history and intensity of forest decline in Norilsk, the researchers have shown how the amount of pollutants spilled out into the atmosphere by mines and huts is at least partly responsible for the phenomenon of “darkening the Arctic”, which provides new insights to explain the divergence problem.

“Using the information stored in thousands of tree rings, we can see the impact of the uncontrolled environmental disaster in Norilsk over the past nine decades,” says Professor Ulf Büntgen of the Geographical Institute in Cambridge, who led the study. “While the problem of sulphur emissions and forest deaths has been successfully addressed in large parts of Europe, we have not been able to identify the effects for Siberia, mainly due to the lack of long-term monitoring data.”

The extension of the annual resolution and absolutely dated measurements of the annual ring width compiled by the first author of the study, Alexander Kirdyanov, together with new high-resolution measurements of wood and soil chemistry, allowed the researchers to quantify the extent of the devastating damage to the Norilsk ecosystem that peaked in the 1960s.

Dead forest east of Norilsk. Photo: Dr. Alexander Kirdyanov

“We can see that the trees near Norilsk began to die off massively in the 1960s due to increasing pollution,” says Büntgen. “As air pollution in the Arctic accumulates due to large-scale circulation patterns, we have extended our study far beyond the direct effects of the Norilsk industrial sector and found that trees in the high northern latitudes also suffer as a result.”

Researchers used a process-based model of boreal tree growth to show that the darkening of the Arctic since the 1970s has significantly reduced tree growth.

The darkening of the Arctic is a phenomenon caused by the increase in particles in the Earth’s atmosphere, whether through pollution, dust or volcanic eruptions. The phenomenon partially blocks sunlight, slows down the evaporation process and disrupts the water cycle.

Global warming is expected to increase the growth rate of boreal trees, but the researchers noted that the growth rate of trees in northern Siberia slowed as pollution peaked. They found that atmospheric pollution levels reduced the ability of trees to convert sunlight into energy through photosynthesis, so they were unable to grow as fast or as strong as in areas with less pollution.

“The extent of the damage shows how vulnerable and sensitive the boreal forest is,” says Büntgen. “What surprised us is the widespread spread of the effects of industrial pollution. Given the ecological importance of this biome, pollution in the high northern latitudes could have a huge impact on the entire global carbon cycle.”

Source: University of Cambridge

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