Thirsty and Drunken Christmas trees

views and opinions expressed in this article are those of the author and do not necessarily reflect the official policy or position of the University of Birmingham

“It’s a rapidly worsening situation, but not all is lost. Drunken trees may curve upwards and recover as they resume vertical growth through a clever mechanism known as gravitropism.”

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Coniferous trees have long been an integral part of winter festivals. From garlands, wreaths, and decorated branches, to Christmas trees as we know them today, evergreens symbolise life among the darkness of winter. We can all invoke the idyllic images of northern forests shown on Christmas cards – thick stands of pine, spruce and fir, all covered in snow, with Father Christmas’s sleigh riding above… but will what Dancer and Prancer and the other reindeer see, as they speed through northern skies, conform to that idyll?

Not everywhere. In recent decades, climate warming and thawing of permafrost have disturbed many boreal forest ecosystems. As frozen ground thaws, soils may become less stable, and depressions may appear on the surface. This causes the soil to weaken as an anchoring ground, and trees begin to lean. Locals and forest researchers refer colloquially to the ‘drunken’ appearance of the trees, as if they’ve enjoyed too much seasonal cheer. Eventually, many such trees die, falling into the expanding depressions. Often, these depressions are filled with water, and as they expand into bigger bogs, ponds or lakes, more trees die. It’s tempting to see a festive-season cautionary tale in the watery fate of these ‘drunken’ trees.

However, this is not the only risk for northern trees below the sleigh in a warming world. A ‘droughty’ opposite can also occur, to borrow from Robert Burns’ Tam O’Shanter. Thawing of permafrost may enable increased drainage resulting, surprisingly perhaps, in drier soils. In this case, the combined effects of warmer air temperatures and lower soil moisture can lead to trees suffering from drought-stress. Since trees, like all plants, must release water in order to take in carbon dioxide for photosynthesis, drought-stress is as existential a threat to trees as falling into expanding bogs or lakes.

In our research, we are beginning to unpick which environmental factors govern these contrasting scenarios. This will enable us to better predict the future of forests growing on thawing permafrost. Accurate prediction is important because boreal forests constitute about 1/3rd of the total global forest cover and act as a carbon sink, effectively removing carbon from the atmosphere and locking it up in biomass and soils. In the future, the combined effects of increased air temperatures, permafrost thaw, wildfire and pest outbreaks may shift boreal forests from carbon sinks to carbon sources.

It’s a rapidly worsening situation, but not all is lost. Drunken trees may curve upwards and recover as they resume vertical growth through a clever mechanism known as gravitropism. Forests lost to wetlands and lakes may eventually re-grow as water drains and conditions become drier. Finally, drought-tolerant trees may replace those struggling to adjust to warmer temperatures and drier soils.

In order to give boreal forests the best chance, climate warming must be kept to a minimum, in line with the global initiatives of the Intergovernmental Panel on Climate Change to keep global temperatures increases within 1.5oC. Recent assessments have raised optimism that the Paris Agreement’s goals may be fulfilled, although only if all nations fulfil their promises through prompt, creative, and ambitious actions to meet the carbon and greenhouse gas emission targets.

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