Declining forest resilience worldwide

Forests play a fundamental role in life on the planet and provide many important ecosystem services. In some areas, including Europe, their extent has also increased over the past thirty years or so. But what if, in the meantime, forests had also become more fragile? This is a recently published article Nature: analyzing the last twenty years of satellite data, the researchers (in this case all men) highlight the decline in forest resilience on a global scale and analyze the factors that contribute most to this decline.

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

Measure durability

About 30% of our planet is covered by forests, some intact, i.e. free from human interference, others managed. There are about 41 million square kilometers that not only play an important role in absorbing CO2 atmospheric but also contribute to the purification and regulation of water flows, the fertility and stability of the soil, the preservation of the biodiversity they host – not to mention the many products they provide for our activities, from timber to food, such as h. Science on the Internet he recently recalled. In some areas of the world, forests have increased in recent years: in Europe, for example, forest area increased by a total of 10% between 1990 and 2020.

But what do we know about the health of these forests? What if they were fragile forests? Resilience is a concept that has gained great importance today, which refers to the ability of systems to withstand an external disturbance and then recover their equilibrium state. “This applies not only to severe events, such as a fire or a windstorm, but also to smaller-scale events, such as short-term climate fluctuations,” explains Forzieri. Estimating resilience, however, is not a trivial process because it requires consideration of parameters other than those that can be easily measured, such as forest cover.

“The critical threshold at which a system—forestry, in this case—is no longer able to cope with a disturbance and collapses is denoted by the English term Crucial point. “Several studies have shown that, as this critical threshold is approached, it is possible to begin to detect ‘signatures’, i.e. the characteristics of the forest that act as indicators,” the researcher continues. “In particular, the increase in the oscillations of the system with respect to its equilibrium state represents an indicator of its approach Crucial point. The metric we used to assess these fluctuations, and therefore indicative of forest resilience, is a parameter called autocorrelation, which expresses how related a system is, at a given point in time, to its state at an earlier time. When the value is high, the system takes a long time to return to its equilibrium state. consequently, durability is lower. Conversely, systems with low autocorrelation, if subjected to an external perturbation, tend to immediately return to equilibrium and exhibit more effective resilience.”

Durability e guide fall

To estimate forest autocorrelation, the study authors used an index derived from satellite data indicative of the photosynthetic capacity of forests and their biomass. To ensure they had robust and consistent estimates over time, they used data provided by the MODIS instrument, which is carried by two NASA satellites. “Even if the satellite platforms already exist for about forty years and therefore we have many observations of the Earth’s forest cover, MODIS is the only instrument that has not been modified over time and therefore guarantees us that the data produced are not subject to bias and inconsistencies,” Forzieri explains. “This is also justified because our analyzes only focus on the period between 2000 and 2020.”

The research team also used a machine learning and a series of simulations to analyze the factors influencing the changes in resilience and identify those most responsible for the changes themselves. These factors have been divided into three main groups: the average climatology of the planet; climatic variability (for example temperature or rainfall variations) and the forest conditions themselves.

This complex research showed a decline in forest resilience worldwide. not for all forest types, though. If, in fact, tropical, temperate and arid forests show a reduced resilience, for those of the northern regions it seems on the contrary increased. In addition, intact forests are overall more resilient than managed forests. However, the variation in this resilience over time is comparable. In other words: it’s less fragile forests at the moment, but the loss of resilience is proceeding more or less at the same rate here. Not only that: 23% of intact forests are already close to critical resilience values, while showing progressive degradation. “These two figures, together, represent a major alarm signal,” comments Forzieri. “In fact, they indicate that these systems are experiencing a significant degradation that could lead to irreversible breaking points, i.e. tipping points“.

The result of the study is that the decline in resilience appears to be mainly linked to increased climate variability and reduced water availability. Climate variability is not limited to tropical, arid and temperate regions, but also affects northern regions. So why do forests seem to have become less fragile here? In these regions of the planet there are some mechanisms that tend to offset the negative effects due to climate variability, explains Forzieri. One is the average temperature rise, which in the short term tends to bring benefits in terms of increased forest growth. the other is an increase in CO2which favors the processes of photosynthesis, accelerating the growth of plants, a phenomenon called “CO2 fertilization”2“. Both of these mechanisms also occur in other forests, but without being able to compensate for the negative effects associated with climate fluctuations.

How positive is this data? How much can you reassure us about the health – at least – of the northern forests? Unfortunately, relatively little, because the increase in durability is likely to be temporary. “Our work is based on the current situation, but climate projections for the coming decades show a decrease in water availability in the northern regions as well. simultaneously indicate that the effects of CO fertilization2 they will tend to saturate,” explains the researcher. “The combination of these two factors could mean that the beneficial effects in terms of increased resilience could wear off, leading to a reversal of the trend 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 indeed it often is. Looking at the global data collected by the study, however, we observe a competitive relationship between resilience and forest productivity: about half of the forests that in the last twenty years showed an increase in productive capacity showed an opposite trend in resilience. Other work has already observed that resilience and productivity do not always go hand in hand, reminding us again that in reality forests, even when growing (mainly due to CO2 fertilization2), however, may be more fragile.

Mitigation and adaptation for more resilient forests

The studies that have been done so far on forest resilience are actually mainly at the local level, so they only provide information on limited areas. This latest work is the first to provide a comprehensive overview of how forests change over time in terms of fragility and stability. And, although the study does not explore potential management strategies that could address the observed loss of resilience, the authors note their importance: “It is urgent to consider this trend when designing effective forest-based mitigation strategies, to avoid future negative events triggered by the increasing vulnerability of carbon stocks”, they write at the end of the article.

“Ultimately, reduced resilience and consequent fragility may lead to forests no longer able to absorb CO2 but which, on the contrary, can contribute to its release: this happens not only due to burning during fires but also due to the degradation of dead plants”, concludes Forzieri. “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 picture of the new risks to which new forests will be subject is likely to lead to this appeal poor fit, an adaptation that is not only ineffective but can also be harmful. “Forests unsuitable for the environment in which they are planted, in climatically unsuitable areas or with biophysical characteristics unsuitable for tomorrow’s climate, actually expose them to higher risks, such as fires, windstorms and proliferation by harmful insects.”

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