August 10, 2022

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Global forest resilience is declining

Forests play an essential role in life on this planet and provide many important ecosystem services. In some areas, including Europe, its prevalence has also increased over the past 30 years or so. But what if in the meantime the forests became more fragile? This is what Element Recently Posted in temper nature: Analyzing satellite data over the past 20 years, researchers (in this case all men) highlight a decline in forest resilience on a global scale, and analyze the factors most contributing to this decline.

We talk about it with Giovanni Forziri, a researcher at the Ispra Joint Research Center today in the Department of Civil and Forestry Engineering at the University of Florence and first author of the study.

Flexibility measurement

About 30% of our planet is covered with forests, some of which are intact, that is, devoid of human intervention, and others are managed. There are about 41 million square kilometers that not only play an important role in the absorption of carbon dioxide2 in the atmosphere but also contribute to the purification and regulation of water flow, to soil fertility and stability, and to the maintenance of the biodiversity it hosts – not to mention the many products it provides for our activities, from timber to food, such as science on the internet recently called up. In some regions of the world, forests have increased in recent years: in Europe, for example, the area of ​​forests generally increased by 10% between 1990 and 2020.

But what do we know about the health of these forests? What would happen if forests were fragile? Elasticity is a concept that has gained great importance today, which refers to the ability of systems to deal with an external disturbance and thus restore their state of equilibrium. “This is not only for dangerous events, such as a fire or windstorm, but also for events of low volume, such as short-term weather fluctuations,” explains Forziri. However, estimating resilience is not a trivial process, as it requires consideration of criteria other than those readily quantifiable such as forest area.

“The critical threshold at which a system—the forest, in this case—can no longer cope with the upheavals and collapses that the English term denotes.” turning point. Several studies have shown that, as this critical threshold approaches, it is possible to begin to identify “signatures”, that is, characteristics of the forest that act as indicators”, continues the researcher. “In particular, the increase in the oscillations of the system with respect to its equilibrium state is an indication that it is approaching turning point. The metric we used to evaluate these fluctuations, and thus indicate the resilience of the forest, is a parameter called autocorrelation, which expresses how closely a system, at a given moment, is related to its state at an earlier time. When the value is high, the system takes a long time to return to the equilibrium state; Hence, the flexibility is less. On the contrary, systems with low autocorrelation, if exposed to external perturbation, tend to immediately return to equilibrium and show more efficient flexibility.”

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flexibility e driver drop

To estimate forest autocorrelation, the study authors used an index obtained from satellite data, demonstrating the photosynthetic capacity and biomass of forests. To make sure they had robust and consistent estimates over time, they used data provided by the MODIS instrument, which is used by two NASA satellites. “Even if satellite platforms have already been around for about forty years, and therefore we have many observations about the land’s forest cover, MODIS is the only instrument that has not been modified over time, and thus ensures that the data produced is not subject to bias and inconsistencies,” he explains. Forsyari. “This is also for good reason why our analyzes are only focused between 2000 and 2020.”

The research team also used a file machine learning and a series of simulations to analyze the factors influencing changes in resilience and to identify those who themselves bear the most responsibility for the changes. These factors have been divided into three main groups: the average climatology of the planet; Climate change (eg changes in temperature or precipitation) and the conditions of the forests themselves.

This complex investigation has highlighted the declining resilience of forests globally; Not for all types of forests. In fact, if tropical, temperate and arid forests show low resilience, it appears to be increased relative to those in boreal regions. Furthermore, healthy forests generally show greater resilience than managed forests; However, the variance in this resilience over time is comparable. In other words: it’s less fragile forests right now, but the loss of resilience is more or less going at the same rate here too. Not only this: 23% of healthy forests are already close to critical resilience values, as well as showing gradual degradation. “These two numbers, together, are an important alarm signal,” Furzeri comments. “In fact, they point out that these systems suffer significant degradation that can lead to irreversible break points, i.e. critical points“.

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The result of the study is that the decrease in resilience appears to be associated above all with increased climate variability and reduced water availability. Climate variability is not limited to tropical, arid and temperate regions but also affects northern regions. Why, then, does the forest seem less fragile here? In these regions of the planet, there are some mechanisms that tend to offset the negative effects of climate variability, Furzeri explains. One is the average temperature increase, which in the short term tends to yield benefits in terms of increased forest growth; The other is the increase in carbon dioxide2which favors the processes of photosynthesis, accelerating the growth of plants, an effect called “carbon monoxide fertilization”2These two mechanisms also occur in other forests, but without the ability to offset the negative effects associated with climatic changes.

How positive is this data? How much can you reassure us about the health of – at least – the boreal forests? Unfortunately, relatively little, because the increase in elasticity is likely to be transient. “Our work is based on the current situation, but climate projections for the coming decades indicate a decrease in water availability also in the northern regions; At the same time, they indicate that the effects of CO . enrichment2 They will tend to saturate,” explains the researcher. “The combination of these two factors may mean that the beneficial effects in terms of increased resilience may wear off, leading to a trend reversal in these areas in the near future.”

Finally, the researchers also investigated forest productivity. One might think that a healthy forest is also a productive forest – and it often is. However, looking at the global data collected by the study, we note an antagonistic relationship between resilience and forest productivity: about half of the forests that experienced an increase in productive capacity in the past 20 years showed an opposite trend in resilience. previously Other jobs We noticed that resilience and productivity don’t always go hand in hand, again reminding us that in fact forests, even as they grow (largely due to carbon dioxide fertilization)2), however, can be more fragile.

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Mitigation and adaptation for more resilient forests

The studies done so far on forest resilience are actually mainly at the local level, so they only provide information for limited areas. This latest work is the first to provide a global overview of how forests have changed over time in terms of fragility and stability. Although the study does not investigate potential management strategies that could counteract the perceived loss of resilience, the authors remember its importance: “It becomes urgent to take this trend into account when designing effective forest-based mitigation strategies, to avoid future adverse events caused by increased vulnerability. for carbon stocks,” they wrote at the end of the article.

“Ultimately, the resulting deterioration in resilience and fragility could result in forests being no longer able to absorb carbon dioxide.2 But, on the contrary, it can contribute to its release: this is what happens not only due to combustion during fires, but also Due to the deterioration of dead plants’” concludes Forziere. This is also one of the reasons why it is really important to integrate climate change mitigation strategies with adaptation strategies, a challenge that forest sector policies have not yet fully integrated. Pursuing reforestation strategies without a clear view of the new risks to which new forests will be subject is likely to lead to this call maladaptationAdaptation is not only ineffective, it can also be harmful. Forests that are not suitable for the environment in which they are grown, or in areas that are unsuitable for climate, or with biophysical characteristics that are not suitable for the climate of tomorrow, in fact expose them to greater risks, such as fires, wind storms, and the reproduction of harmful insects.”