Is climate change good or bad for trees?


A new study reveals the answer’s anything but certain.


Life in balance: a mature and vigorous stand of aspen.

Martin D Venturas

By Ian Connellan

The natural world, roughly divided into plants and animals, is a fabulous fit.

Animals need oxygen to survive. Plants need carbon dioxide (CO2) for photosynthesis and expel oxygen as a by-product. Almost enough to have you believe in some sort of higher power making things all nice and neat.

So rising levels of atmospheric carbon dioxide across the globe ought to be good news for plants. Indeed, experiments have shown that increased CO2 allows plants to photosynthesise more and use less water.

But the flip side is that warmer temperatures drive plants to use more water and photosynthesise less.

In a new study published in Proceedings of the National Academy of Sciences, University of Utah, US, researchers address the question of which of these forces – more CO2 or more heat stress – wins during climate change.

Somewhat maddeningly, their answer is it depends – on whether forests and trees are able to adapt to their new environment.

The research team used tree physiology as the basis for their study.

“It’s taking the physiology of individual cells and scaling it up in a computer to make projections of a continents’ worth of forests,” says co-author William Anderegg.

To make sense of the study it’s useful to have a quick refresher course on how trees and plants use water.

Water moves from a tree’s roots through the xylem, the tree’s vascular system, and on to the leaves, where photosynthesis happens.

The undersides of leaves have small pores called stomata that open to admit CO2 for photosynthesis. Water vapour can escape through the stomata, so they close to guard against water loss during dry or hot times.

During an intense drought, trees have to work harder to pull water in and move it through the xylem. If the soil is dry enough, the tension on the water can cause an air bubble to form in the xylem, effectively reducing water transport and injuring or killing the tree. It’s the tree equivalent of a heart attack.

Study co-author John Sperry has spent decades studying the physiology of tree water use.

In recent years he and his colleagues have developed a model of how tree physiology, primarily as it relates to stomata opening, influences photosynthesis and water loss in response to changing conditions, including drought.

This model, he says, enables a new way of predicting the outcome between the competing effects of CO2 increase and heat stress.

Anderegg adds that, importantly, the model allows them to simulate the ability of trees to acclimatise to heat and drought – both on a short timescale, by closing or opening stomata, or a long timescale, by extra tree growth or forest dieback.

“We’re assuming the plants are adapted to be somewhat smart about responding to the climate and the environment,” Anderegg says.

Some acclimatisation has been seen in previous experiments where trees were bathed in CO2-enriched air. It’s also seen in forests that are similar but located in slightly different climates.

“Our present-day models don’t do physiology or [acclimatisation],” Anderegg says.

“They matter absolutely enormously to the future of forests. We came up with ways to incorporate those.”

Sperry says results suggest that the winner of the climate-change competition doesn’t depend on the absolute amount of CO2 rise or warming – just the ratio between the two.

“So you can have the same forest moving across big gradients in climate change if that ratio is at the neutral point,” he says. “But anything that pushes that ratio to the warming side is going to have potential for serious negative impact.”

If forests aren’t able to acclimatise, the researchers write, then the ratio must be above 89 parts per million CO2 per degree Celsius of warming to avoid significant stress and tree die-off. Only 55% of climate forecasts show this scenario occurring.

But if forests are able to acclimatise, then they can tolerate a lower ratio: 67 parts per million CO2 per degree of warming, which occurs in 71% of forecasts.

But even with acclimatisation, other factors can tip the balance toward forest catastrophe.

The model only takes into account the physiology of the trees, not forest fires or insect infestation, says co-author Martin Venturas – even though stressed forests are more susceptible to both fires and insects.

“It’s improving one piece of the puzzle, but we still need to learn a lot about the other pieces and how they’re integrated,” he says.

The researchers also write that exceptionally dry years can also tip the balance.

“In those cases, if we drop below a soil moisture threshold, we could have the whole forest die,” says Venturas.

The dieoff can happen relatively suddenly. “You see this in your flower pot at home if you forget to water,” Sperry says.

“It’ll look fine up to a certain point but then you hit that moisture threshold and in a matter of days the plant can die. If you don’t get rain in that period, the system goes into a cycle where the soil’s drying out too fast and sends the trees into vascular failure.”

Sperry adds that the study predicts a precarious tightrope of climate conditions for future forests to navigate.

“The study by no means gives a green light to the status quo,” he says.

  1. https://www.pnas.org/content/early/2019/11/19/1913072116
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