To properly understand what this means for carbon budgets, scientists must solve the puzzle of why trees are dying and how they respond to change.
According to new research, an increase in tree mortality has been observed due to changes in climate and land use . This appears to be transforming forest habitats, with trees getting younger and shorter, the authors add.
Estimates suggest that forests have absorbed up to 30% of anthropogenic carbon emissions in recent decades . Although the overall effects of tree loss on the carbon cycle are complex because old trees and the young trees that replace them absorb carbon at different rates, increased mortality appears to be affecting the ability of forests to retain carbon. .
Researchers in the new study believe that higher mortality rates could begin to exceed the ability of the remaining and new trees to maintain that uptake at the same level, and could lead to an overall reduction in canopy cover and biomass. .
“It is quite concerning because, at the moment, two or three out of every 10 carbon dioxide molecules in the atmosphere are returning to the forests, but we do not know how it will continue in the future,” said Dr. Thomas Pugh, environmental scientist. from the University of Birmingham, UK.
Although the trend towards increasing mortality and changing the composition of trees in forests has been evident in more localized studies, the team’s literature review and data analysis on the change in the use of earth have indicated that this is happening on a large scale , he says.
Dr. Pugh points out, however, that it is not possible to say that this is happening everywhere because many places have not yet been studied. “What we are showing is that if all forests are studied, these trends are generalized, although each place moves at a different rate.”
The death of the tree
Further research is needed to understand how changes in forests relate to future prospects for carbon storage. Many studies have examined photosynthesis-related effects on storage, but another critical piece of the puzzle is understanding the rates and causes of large-scale tree death.
“Until now, getting a big picture of this has been surprisingly difficult,” says Dr. Pugh.
This is partly because trees can live for hundreds of years, and even if death can be observed, the cause is often unclear, he says. If you can get that information for a tree, you need to make thousands of observations to understand trends.
“In the same way, we know how long certain tree species can live, but we don’t know how long they normally live,” added Dr. Pugh.
A project he leads, called TreeMort , which was involved in the new research, is trying to improve understanding of life span by combining measurements made over the past four decades from a wide range of sources, including local studies. , forest inventories, plant trait data and satellite observations. Enough information has recently been gathered to do this, he says.
So far, the team has gained insight into the most visible causes of death, finding that around 12% of tree mortality in terms of biomass loss worldwide is due to major disturbances, such as fires, large-scale uprooting of trees by wind, harvesting, and pest outbreaks.
“I was expecting larger-scale events to represent a larger fraction overall,” Dr. Pugh said. “They have such a huge impact on the landscape, but it turns out that in many forests, most of the action is happening on smaller scales.
At the same time, he was surprised to see in an independent study how many trees in Europe seemed to be dying “overwhelmingly” from the harvest.
The next step, he says, is to better understand the causes and timing when the remaining 88% is due to less visible events, such as smaller-scale harvesting and wind uprooting, competition with neighboring trees, disease, drought and long-term climate effects, such as warmer temperatures.
Over the past two years, Dr. Pugh’s team has been collecting and standardizing data from a wide variety of studies, and now he’s ready to take a closer look.
Pugh believes that getting this information on tree death and adding it to research on the effects of photosynthesis and tree numbers can be a turning point in forecasting carbon budgets with greater precision. “We are not going to remove the uncertainty in any way … but we think we can put much stricter limits on this carbon sink.”
Not only that, but understanding tree mortality has much broader consequences for changes in forest ecosystems in terms of the mix and diversity of trees and the animals they contain. “It opens up a lot of possibilities for work because the rate at which trees die has many implications for ecosystems,” said Dr. Pugh.
The smallest scale
But we still have a lot to learn about trees on a much smaller scale to help improve global predictions of carbon storage over time. Trees lock up carbon in their wood and roots as they grow , so analysis of the growth of woody tissue and how it is affected by climatic variations can shed light on storage prospects.
The INTREE project is trying to do this in the temperate forests of the Alps and Canada through a new approach it has developed to analyze the formation of xylem (woody tissue that conducts water and nutrients).
The method, called “qualitative wood anatomy within the rings,” involves new anatomical studies combined with previous findings from two traditional approaches: long-term analysis of annual tree rings over decades and seasonal monitoring. short-term of weekly woodcuts under the microscope.
According to Dr. Daniele Castagneri, INTREE researcher at the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), observing processes at both scales is key to getting a more complete overview.
Some of the team’s studies have already pointed out the importance of their approach. When scientists investigated how the sprout moth affected European larch ( Larix decidua ) through defoliation (which can suppress growth by hindering photosynthesis) they found that previous studies may have underestimated the resulting biomass loss by about a 25%.
They have also found evidence that warmer weather could have an overall long-term impact on reducing tree growth rates in boreal forests in some of the northernmost latitudes they examined in Canada. This contradicted their expectations based on evidence that trees in that region would actually grow faster in the future due to the corresponding lengthening of the growing season. The researchers suggest that this could also lead to tree death, where the trees die starting at the crown.
“In light of climate change, this means that even boreal forests in a relatively humid climate could be stressed by drought,” said Dr. Castagneri.
Although such studies suggest that some climatic effects could be greater than previously thought, Castagneri cautions that it is currently difficult to generalize due to the need for information on many more tree species and habitats.
Dr. Patrick Fonti, another WSL researcher who oversees INTREE, says there is a growing body of research studying the matter. “We are getting closer to that because there are several groups working in the same direction, so more and more data is being collected,” he said.
In the next year or two, INTREE hopes to link its anatomical-scale findings more directly to the carbon cycle, but Dr. Castagneri says it will be some time before this can be done globally.
“The connections to the global carbon cycle are very complex, and you cannot easily scale from the cellular level to the forest,” he said.
Castagneri hopes the team can finally get a good quantitative understanding of carbon fluxes and share it with other groups to find out even more. Maybe then, he said, we can “have more accurate models that predict what will happen to forests .”
Artículo original
This article was originally published in Horizon, the EU Research and Innovation Magazine
