The world is rapidly hotting up. Heatwaves are frequenting in an accelerating trend. The toll on trees is severe, killing off the greatest terrestrial carbon sink.
“Not only have we seen more and longer heatwaves worldwide over the past 70 years, but this trend has markedly accelerated,” alarmed ecologist Sarah Perkins Kirkpatrick reported. “Cumulative heat shows a similar acceleration, increasing globally on average by 1°C-4.5°C each decade – but in some places, like the Middle East, and parts of Africa and South America, the trend is up to 10°C a decade.”
Heatwaves cause drought by evaporatively ripping water out of the soil and diminishing replenishing rainfall.
Trees lose a lot of water through their surfaces. If the soil dries out, the tree cannot replace this water. Trees can reduce their water consumption, but if the soil water reservoir is used up, it’s only a matter of time until cell dehydration kills the tree.
Trees in Europe were severely stressed by an intense, long-lived heatwave and concomitant drought in 2018. “Spruce was most heavily affected. But it was a surprise for us that beech, silver fir and pine were also damaged,” stated startled botanist Ansgar Kahmen.
Trees are well aware of the warming world. Like rock stars, trees worldwide have started to live fast and die young. “There are traits within the fastest-growing trees that make them vulnerable, whereas slower-growing trees have traits that allow them to persist,” said American forest biologist Steve Voelker. “Carbon uptake rates of forests are on the wane as slow-growing and persistent trees are supplanted by fast-growing but vulnerable trees.” “Climate change is accelerating tree mortality, increasingly pushing the world’s forests towards being both younger and shorter,” added British biologist Tom Pugh.
8.6 million hectares of forest are lost worldwide each year, mostly for industrial agriculture. Nearly 95% of South America’s Atlantic forest, which extends along the Atlantic coast of Brazil, Paraguay, and Argentina, has been destroyed. What remains are scattered wooded patches averaging 60–70 hectares. Deforestation there continues.
The relationship between soil and vegetation is intimate. Soil fertility is an active process, involving the microbes within, the plants that cover the ground, and the atmospheric environment, including temperature, rainfall, and air quality.
Trees knit their landscapes together. Their heat death will be irredeemable, and soil too fatally wounded. “Soil, once eroded or depleted, takes centuries – millennia even – to recover,” noted American geologist Frank Rhodes.
The same fate awaits lands taken for crops. Already, half of global agricultural land is degraded. Of this, the majority is so damaged that local farmers have been unable to restore it.
With hotting up and less tree cover, forests are losing their ability to absorb carbon. Forests now take up 1/3rd less carbon than they did 25 years ago. What was the greatest carbon sink on land is rapidly turning into a greenhouse gas emitter. “This is decades ahead of even the most pessimistic climate models,” remarked British geographer Simon Lewis.
The fate of trees presages that of humans. For if forests cannot stand, crops will also fail, and so too civilization.
Ishi Nobu, The Fruits of Civilization (2019).
Fiona Harvey, “Shorter lifespan of faster-growing trees will add to climate crisis, study finds,” The Guardian (8 September 2020).
S. E. Perkins-Kirkpatrick & S. C. Lewis, “Increasing trends in regional heatwaves,” Nature Communications (3 July 2020).
“Heatwave trends accelerate worldwide,” ScienceDaily (6 July 2020).
Bernhard Schuldt et al, “A first assessment of the impact of the extreme 2018 summer drought on Central European forests,” Basic and Applied Ecology (45: 86-103 (June 2020).
“Heat stress: The climate is putting European forests under sustained pressure,” ScienceDaily (16 July 2020).
Nate G. McDowell et al, “Pervasive shifts in forest dynamics in a changing world,” Science (29 May 2020).
Damien Carrington, “Climate crisis making world’s forests shorter and younger, study finds,” The Guardian (28 May 2020).
Wannes Hubau et al, “Asynchronous carbon sink saturation in African and Amazonian tropical forests,” Nature (4 March 2020).
Fiona Harvey, “Tropical forests losing their ability to absorb carbon, study finds,” The Guardian (4 March 2020).