Climate News Network

Nine vital signs found for forest health

January 3, 2019, by Tim Radford

Forests’ response to climate change needs more study. Image: By Johannes Plenio on Unsplash

Forests help to moderate climate change, which can itself affect forest health. Researchers still puzzle over how the canopy affects the global carbon exchange.

LONDON, 3 January, 2019 – It is a given of climate science that forest health, the consequence of protected and biodiverse forests, will play a vital role in containing global warming. Now a new study for the first time offers foresters, botanists and conservationists the tools to test the health of a vast woodland.

And a second, separate study confirms an ominous discovery: trees can be counted upon to greedily consume ever more atmospheric carbon dioxide – but only while the natural supply of nitrogen holds out.

Trees use photosynthesis to build tissue from atmospheric carbon dioxide, and store the carbon in the form of leaves, fruits and timber while respiring oxygen. In doing so, they reduce levels of global warming.

Humans – by clearing forests, ploughing fields, grazing cattle and burning fossil fuels – tip about 34 billion tonnes of the greenhouse gas carbon dioxide into the atmosphere each year, and the world’s trees take up an estimated 11 bn tonnes of it. But quite how, and how reliably, forests store carbon is still a puzzle.

“The limes, planes, magnolias and poplars that line boulevards and shade city parks could be just as significant to carbon budget calculations as tropical rainforests”

US researchers report in the Proceedings of the National Academy of Sciences that they decided to find out. They analysed data from 421 plots of forest around the world, and took direct samples in 66 of them. They measured temperature, rainfall, vapour pressure, sunlight and wind speed.

Their search spanned 100 degrees of latitude and more than 3,300 metres in altitude. Altogether the scientists gathered information on 55,983 individual trees greater than 2 cms in diameter and divided into 2,701 tree species.

By the time they had finished they had identified nine vital signs that might help with a diagnosis of a forest’s health. These are two different measures of leaf area, as well as wood density, tree height, the counts of leaf carbon, nitrogen and phosphorus and the important ratio of nitrogen to phosphorus.

Armed with these measures, they began to look at precisely how climate might affect a tree population. Two climatic factors in particular had a disproportionate impact.

New pointers

One was temperature variability – that is, the swing from the lowest to the highest mercury levels – and the other was vapour pressure. And they confirmed that, overall, the measured traits are responding to overall global warming.

Such research offers a new set of signposts for understanding how atmosphere, climate and forests interact. The response of the woodlands has become one of the big unresolved questions.

Researchers have found, a little to their surprise, the “urban forests” – the limes, planes, magnolias and poplars that line boulevards and shade city parks – could be just as significant to carbon budget calculations as tropical rainforests.

They have measured unexpected ways in which trees have responded to the rise of 1°C in global average temperatures in the last century, as carbon dioxide levels in the atmosphere have soared from around 280 parts per million to more than 400 ppm.

Concern over nitrogen

But they have also taken serious stock of the planet’s cover of trees, to find that humans are destroying trees at the rate of 15 billion a year and that climate change and human intrusion pose the threat of extinction to many of the world’s 40,000 tropical tree species.

A second team of the US researchers is now sure of one of the mechanisms that might affect the overall health of forests in a warming world. They report in the journal Nature Ecology and Evolution on an intensive examination of the response of 15,000 trees in the wilds of West Virginia to a steady rise in atmospheric carbon dioxide.

Yes, the extra greenhouse gas is fertilising forest growth. But climate change is extending the growing season, as spring arrives earlier and autumn leaf fall happens ever later. A study of the nitrogen isotopes in the leaves suggests that the supply of that other, all-important nutrient, could be on the way down.

If so, the growth of the forests could soon peak, and with that the capacity of forests to moderate climate change could diminish. – Climate News Network

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