Summer arrives in the northern hemisphere and with it the fires that devastate the forests in Spain, the United States and other areas of the planet. But fire is not the only enemy of trees. There is another threat that is killing them more slowly. It started at the end of the 18th century, with the so-called Industrial Revolution and its growing greenhouse gas emissions. Its accumulation in the atmosphere is warming the planet and its consequences are now being felt: there are jungles where forest life expectancy has been reduced by half. And in others, the mortality events caused by droughts have multiplied in the last decade.
In Australia, the tree mortality rate has doubled since the mid-1980s. A study with thousands of trees from twenty plots of different latitudes and ecosystems shows that something is not right. One of the strengths of this study, published today in Nature, is the detail of your inventory, with data on forest health (leaf mass per area, wood density, photosynthesis capacity…). The other is that they have information that goes back half a century. Until 1984, the forests grew or remained, with a number of dead trees similar to those that replaced them. But the trend has since reversed. Those who grow up now have reduced their life expectancy and, if the trend continues, many will die before their time.
Oxford University forest ecologist and lead author of the study Nature David Bauman details the extent of the change in trend: “Survival goes, on average, from 99% between 1971 and 1984 to 98% now in the species studied. In other words, we went from 1% to 2% of annual mortality, which, accumulated over decades, leads to profound changes in the dynamics of these forests. Going from 1% to 2% means that life expectancy is divided by two”. A one percentage point reduction doesn’t sound like much, but the math says otherwise. Bauman shows it with a very simplified example: “If you have 1,000 trees with an annual mortality rate of 1%, after 50 years, you will have 665 trees left. But if the rate is 2%, it will only be 364″.
This reduction in life expectancy is happening in most of the 80 species studied and in almost all plots at different latitudes. In the overall sample, the area has decreased by 12.2% due to deforestation. And here the fires that punished the continent in 2020 have had nothing to do with it.
“Death comes from an interaction of factors that affected him in the past. One year it can be a drought and then comes a pathogen or a heat wave”
David Bauman, an ecologist at the University of Oxford and the Smithsonian Institution’s Center for Environmental Research
The ultimate cause of death is not easy to establish. To the long life of the trees, which complicates their monitoring, is added the complication of their slow death. Bauman highlights it: “To be a tree is to be slow, this evolutionary strategy has allowed them to survive changes in the environment, also extremes. Death comes from an interaction of factors that affected you in the past. One year it can be a drought and then comes a pathogen or a heat wave.”
In the humid forests of Australia, the extent of the damage and the areas where it is being most severe gives clues to the culprit. Researchers have observed that the risk of mortality is greater in the drier areas and, in the case of humid forest plots, in their drier margins. In other words, everything points to changes in climatic conditions caused by global warming. “The more the temperature increases, the greater the pressure on the dynamics of evapotranspiration, the leaves of the trees evaporate more water than their roots can capture. But its response time is slow and decades of data are needed to detect this mortality,” says Bauman, a researcher also at the University of Montpellier (France). For this scientist, it is difficult to know if this increase in the mortality rate is happening in other regions of the planet, “but the engine behind it, the vapor pressure deficit, has increased in the other humid forests of the rest of the world. balloon”, he concludes.
The scientist of Ecophyslab from the University of Florida (United States) and one of the promoters of the International Network on Tree Mortality William Hammond agrees with Bauman: “We are still not sure that tree mortality is not increasing in all forests on the planet. For example, it has risen in the Amazon, Europe and North America, but the opposite trend has been observed in the Congo Basin in a recent study in which I have participated”, he says in an email.
What do seem universal are the episodes of great mortality. Unlike annual tree mortality, a ratio that is typically ranging between 1% and 4%, Hammond led research published in April that shows the global nature of what he calls mortality pulses. “These events represent significant deviations from those rates produced in a single year, with at least 15% deaths and sometimes much more,” he says. The work, based on observations in more than 1,800 plots around the planet, from the Mediterranean forest at sea level to the Alpine forests, has discovered a kind of footprint or characteristic mark of the most intense droughts. “This is the set of conditions (dry air, dry soils, high temperature, high vapor pressure deficit…) that we have associated with the mortality events of recent years on the planet,” Hammond details. That this mark appears in forests and jungles of all biomes on Earth “demonstrates the evident role of heat in what has come to be called drought-induced mortality.”
Last Monday, researchers from the Helmholtz Center for Environmental Research (Germany) published a paper confirming that the droughts experienced in Europe between 2018 and 2020 they have been the most pronounced in this part of the world since the 18th century. In addition, unlike previous droughts, the thermal anomaly has been greater, with 2.8º above the average of the last 250 years.
The forestry researcher at the Pablo de Olavide University Raúl Sánchez Salguero has dedicated his scientific career to studying the decline of forests, the slow death of trees. “The problem cannot be reduced to a lack of water or an increase in temperatures. Water stress is suffered by the Mediterranean forest, but in the tropical forests the availability of water has not decreased, but the temperature has risen”, he says from Benamahona (Sierra de Grazalema, Cádiz), where he is studying one of the last pinsapares that remain in Europe. It is the heat that increases atmospheric drought “and this vapor pressure deficit is being seen all over the world,” he notes. This process weakens the forests in the face of the following threats, be it a fire, insects, a cyclone or a new drought.
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