In theory, more trees should mean a lower risk of dangerous climate change. In practice, it may not be so simple.

LONDON, 6 January, 2021 − The belief that more trees and better-protected forests can help contain climate change looks a little less sure − if only because climate change has already begun to affect the world’s trees and forests.

Researchers have in the last few weeks established a panoply of evidence that higher temperatures and more carbon dioxide may not be recipes for green growth in a greenhouse world.

In the tropics, as the thermometer rises, trees grow more vigorously − but overall lifespans are getting shorter. This must ultimately make the forests less efficient as absorbers of atmospheric carbon.

To compound the hazard to the rainforests, the proportion of the canopy that has always been fire-resistant is showing signs of decrease: in parts of Indonesia, only 10% of the forests remain fireproof.

Climate change and more importantly human disturbance continues to put the survival of whole groups of plants at risk: a new study finds that almost one-third of all the world’s 430 oak species are in danger of extinction.

A separate study of 447 North American trees suggests that they might not have what it takes to keep pace with changes in temperature and rainfall expected in a world of global heating.

Limited gains

And there is yet further evidence that more carbon dioxide does not inevitably mean more potential nourishment for plants: a study by the US space agency Nasa suggests that what scientists call the “carbon dioxide fertilisation effect” has been dwindling since 1982.

Finally, even the gains inevitable with rising temperatures in some regions could be limited. Another Nasa study finds that although Siberia, Canada and Alaska are becoming greener as the mercury rises, the increasing drought and tree death in the Amazon rainforest and others has offset this: another blow for those who hope more growth means more carbon absorption.

None of this should be a great surprise: the more researchers look in fine detail at the challenge of restoring natural habitat as part of the planetary arsenal against climate change, the more problems they have identified.

Although researchers have demonstrated that massive forest planting and restoration could in principle reduce the extra atmospheric carbon amassed over the last century, the details are less certain.

With more heat comes more drought which could turn some forests into sources of carbon rather than sinks. The increasing heat could affect the ability of some species to germinate, thus changing the makeup of the forests.

Trees may not only be dying younger, but growing shorter as conditions change.

“Many regions in the tropics are heating up particularly rapidly and substantial areas will become warmer, on average, than approximately 25°C”

And although spring is occurring ever earlier, so is leaf fall: all these things reduce the efficiency of forests as greedy consumers of carbon.

So the latest harvest of research is simply further confirmation that the global heating to which the world is already committed is going to change the nature of those habitats that have − until now − kept the planet at an even temperature.

That means that restoring forests is not just a matter of planting trees: foresters will need to identify the right trees for climate regimes that have yet to be established.

Tropical rainforests cover only 7% of the planet’s land surface, but they shelter and nourish around half of all the planet’s plants and animal species. Around half of the Earth’s stocks of sequestered carbon are locked in the trunks, branches, leaves and roots.

Researchers report in the Proceedings of the National Academy of Sciences that they examined growth data from more than 100,000 trees of 438 different species found at 3,433 places around the world. They found that as temperatures go beyond 25°C, tree lifespans decline.

“Many regions in the tropics are heating up particularly rapidly and substantial areas will become warmer, on average, than approximately 25°C,” said Emanuel Gloor, of the University of Leeds in the UK, one of the authors.

Human interference

“Our findings, which are the first to demonstrate that there is a temperature threshold, suggest that for trees in this region, their longevity is likely to be negatively affected.”

Rainforests maintain their own microclimates: they keep themselves humid, and therefore more or less fireproof, as long as they remain intact, even during a drought. Researchers report in Communications Earth & Environment that they found 90% of the natural forest cover of Sumatra and Kalimantan had been so badly degraded by human clearance and disturbance that it was no longer fire-resistant. What was true for Indonesia could probably be true too for Central Africa or the Amazon.

“Contrary to the widely-held perception that worsening droughts are threatening the remaining rainforests, tropical forests in Indonesia become susceptible to fire only after human disturbance,” said Tadas Nikonovas of Swansea University in Wales, who led the research.

Human disturbance of natural wilderness threatens not just forests as a whole, but individual species of trees, each of which can be a natural ecosystem, supporting other plants and animals. English oaks, for instance, provide food and shelter to more than 2,300 kinds of moss, fungus, lichen, bird, mammal and insect.

Researchers for the Morton Arboretum in Illinois in the US report that of the world’s 430 species of oak, 113 are threatened with extinction: these include 32 species in Mexico, 36 in China, 20 in Vietnam and 16 in the US.

Tropical trees have naturally faster life-cycles. Trees in cooler regions can on average survive for more than 300 years. Climate change however is likely to happen over a few decades. Can trees keep pace with change at that rate?

Plants need water

Researchers from the University of Maine report in the Journal of Biogeography that they think not. They looked at the climatic ranges most suitable for 447 North American trees and shrubs to find that overall, these were at only 48.6% of their full potential. That is, the trees are no longer in equilibrium with present climate, and must increasingly be at a disadvantage as climate change accelerates.

And although the main driver of global heating and thus climate change − ever-higher ratios of carbon dioxide in the atmosphere − confers some advantage on species that live by photosynthesis, this advantage may not be guaranteed. A space-based study in the journal Science found that over the last four decades, as CO2 ratios in the atmosphere rose, 86% of terrestrial ecosystems became progressively less efficient at absorbing the stuff.

That is, the world’s green canopies have slowed climate change, but their ability to go on doing so may be limited. That is because even though more carbon dioxide should mean more growth, unless there is more nitrogen and more soil moisture as well, a plant’s capacity to respond is limited.

And that, says a second study, in the journal AGU Advances, is less of a problem in some places than others. The Arctic is greening rapidly as average temperatures rise, and there is no shortage of moisture from the thawing permafrost, nor of partly decomposed plant material, to serve as nourishment.

A survey of growth from 1982 to 2016 found that carbon absorption increased in Canada, Alaska and Siberia. But global heating has begun to reduce soil moisture in the tropics, and the gains of the Arctic are not enough to offset losses in what had once been rainforest. Nor are the polar regions likely to go on getting ever-greener.

“I don’t expect that we have to wait another 35 years to see water limitations becoming a factor in the Arctic as well,” said one of the authors, Rolf Reichle, of the Goddard Space Flight Centre in Maryland in the US. − Climate News Network