In metropolitan France, forests cover 31% of the territory, i.e. nearly 17 million hectares. The importance of vegetation cover as a lung for the animal kingdom is well known. But all over the world, these green treasures must meet the ecological and social challenges of tomorrow. In particular, to address climate change, strategies must be put in place for the sustainable management of forest ecosystems and the maintenance of forest health.
In February 2021, in France, a petition was launched to stop the cutting of trees in the forests of the Sénart region. The movement is gaining momentum, and the NOF (National Office of Forests) must explain the management of vegetated areas and their sustainability
This article aims to provide a better understanding of this problem and the issues at stake. It also shows the role that technological and scientific progress can play in better understanding the causes and consequences of forest health degradation in threatened regions.
What is forest health?
The concept of a healthy forest has universal appeal. However, forest ecologists and managers are struggling to define this concept exactly. Thus, to date, there is no universally accepted definition for forest health.
Most definitions of forest health can be classified according to their ecological or utilitarian perspective. From an ecological perspective, current understanding of ecosystem dynamics suggests that healthy ecosystems are those that are able to maintain their organisation and autonomy over time while remaining resilient to stress ( Edmonds et al 2011,  Raffa et al 2019).
On the other hand, the utilitarian perspective holds that a forest is healthy if management objectives are met. Consequently, a forest is unhealthy if these objectives are not met. This definition may be appropriate when there is a single, unambiguous management objective, such as timber production or the maintenance of wilderness attributes. It is, however, too narrow when multiple management objectives are required.
Finally, other forest ecologists incorporate both ecological and utilitarian perspectives into their two-part definition of forest health. First, healthy forest management should be based on its intrinsic physical properties (size, elevation, climate). This includes matching basic tree mortality to observed mortality. Secondly, a healthy forest should meet the objectives of the owner/government, provided that these objectives do not conflict with sustainability ( Teale and Castello 2011).
Is forest health a management issue?
Interestingly, forest health is often defined from the “natural” perspective of the forest, or from the perspective of forest managers. We are therefore in a position to ask the questions: what are those management actions that aim to improve forest health? And why simply leave the forests untouched, which intuitively seems to be the best solution?
Forest management: the counter-intuitive but overriding principle of forest health
In 2019, the Food and Agriculture Organisation of the United Nations (FAO) is setting the guidelines for what is intended to be sustainable forest management. In view of the numerous forest fires, too devastating for ecosystems, a strategy was needed. This document, “Fire-smart forest management”, describes the methods, regulations and indicators used to perfect the remedy for poor forest health.
Among these standards are explanations of how to cut down old trees. And so a wave of tree cutting begins. A wave that could be mistaken for deforestation. Indeed, the uninformed walker is indignant to see trunks littering the ground of his favourite forest. However, here are the explanations needed to understand this measure.
One of the main causes of poor health of forest areas is an excess of trees and especially an abundance of trees of the same age. Firstly, too many trees per square metre means that there is little light. This results in few flowers and low vegetation and ultimately very little diversity. The soils under these forests are also of poor quality, lacking in bio-organisms.
We now know that the diversity and cyclicality of ecosystems often make them sustainable. The second point related to the abundance of old trees is fires. Forest fires are not bad in themselves. They have been part of the sustainability of ecosystems since long before man. However, in our case, if a fire breaks out, everything is lost. The soil is no good, nothing will grow back for a long time: the cycle is broken, sustainability is restricted.
(Unhealthy forest, What is a Healthy Forest ? , ©Jeffco Open SPace, Youtube)
Forest diversity: a sustainable solution for forest health
Thus, these two arguments together justify cutting down thousands of old trees and planting new growth. Having a difference in the age of the trees leads to a lower risk of forest loss by fire and less fire in general. Fewer trees brings more diversity. It provides more space for animals and other forest dependents (mosses, flowers, low vegetation). All of these cascading implications result in better soil condition, and thus possible renewal, in other words: a sustainable ecosystem.
(Healthy forest, What is a Healthy Forest ? , ©Jeffco Open SPace, Youtube)
However, assessing the need for and strength of management actions in an area of interest is not an easy task. To achieve this, ecologists and forest rangers are now using computer tools. They allow to quickly visualise these areas as well as the sources of the problems.
Ecological indicators as a guide to prevention
More than ever, it is necessary for environmental decision-makers to use new computer technologies. They allow the observation, understanding and prediction of multiple changes in regions of interest. At Murmuration, as at many other environmental and IT consultancies, satellite images are the main source of this data. In Europe, the Copernicus environmental observation programme has enabled the use of such images through the launch of the Sentinel satellites. Sentinel-2, in particular, has been given the task of providing European countries with comprehensive and up-to-date data for environmental monitoring and surveillance. For the issue of forest health, three main indicators are derived from the processing of these images: NDVI, LAI and FAPAR.
Normalized Difference Vegetation Index (NDVI)
The Normalized Difference Vegetation Index (NDVI) is a simple graphical indicator. It can be used to analyse remote sensing measurements. Often based on a spatial platform, it assesses whether or not the observed target contains living green vegetation.
(NDVI of a forest in Bretagne, France in March 2021, 10mx10m resolution)
Leaf Area Index (LAI)
The Leaf Area Index (LAI) is a dimensionless quantity. It expresses the leaf area of a tree or ecosystem per unit area of soil. It is determined by calculating the total leaf area of the plant on the soil surface it covers.
(The LAI of a forest in Bretagne, France, in March 2021 at 10mx10m resolution)
Fraction of Absorbed Photosynthetically Active Radiation (FAPAR)
FAPAR is the fraction of solar radiation entering the spectral region of photosynthetic radiation that is absorbed by an organism. This biophysical variable is directly related to the productivity of photosynthesis. Some models use it to estimate the assimilation of carbon dioxide in vegetation. As seen previously, this carbon assimilation is very important for understanding the participation of the area under consideration in global gas exchanges.
(FAPAR of a forest in Bretagne, France, in March 2021 at 10mx10m resolution)
Thanks to these satellite images, it is possible to visualise the evolution of ecosystems. This makes it possible to conceptualise the actions to be taken. It is by considering these indicators that the management of forest areas can be done effectively.
Forest health: the basis for sustainable biodiversity
Forests are the lungs of the Earth, they are also the terrestrial receptacles of plant and animal biodiversity. Yet there is a lack of information on their management, whether in France or elsewhere. Contrary to what one might think, leaving a forest alone is not always equivalent to helping it.
At Murmuration we use these environmental indicators to go one step further. Indeed, the jointness and correlation between these variables and other indicators, both ecological and social, allow us to highlight the impacts of humans on biodiversity and the evolution of a vegetated region. Once highlighted, these issues are intended to be the arguments for tomorrow’s decisions.
 – Edmonds, R.L.; Agee, J.K.; Gara, R.I. 2011. Protection et santé de la forêt.
 – Raffa, K.F.; Aukema, B.; Bentz, B.J. [et al]. 2009. Une utilisation littérale de la “santé des forêts” permet de se prémunir contre les abus et les mauvaises applications. Journal de pratique forestière.
 – Teale, S.A. ; Castello, J.D. 2011. Le passé comme clé de l’avenir : une nouvelle perspective sur la santé des forêts.
Authors : Mael Plantec, Remi Nassiri