Compte-rendu de la journée d’étude « Qualité de vie et cohésion au travail »

Les élèves conservatrices et conservateurs promus DCB30 proposent un retour sur la journée d’étude consacrée à la qualité de vie et cohésion au travail qu’ils ont organisée le 21 mai 2021. Cette journée s’est déclinée autour de deux conférences introductives sur le thème de la qualité de vie au travail, des ateliers pour permettre à de futurs cadres de réfléchir et de mettre en place des actions relatives à ces thèmes et enfin des retours d’expériences

A statistical method for detecting logging-related canopy gaps using high-resolution optical remote sensing

In tropical rainforests, the sustainability of selective logging is closely linked to the extent of collateral stand damage. The capacity to measure the extent of such damage is essential for calculating carbon emissions due to forest degradation under the Reducing Emissions from Deforestation and Forest Degradation (REDD+) process. The use of remote sensing to detect canopy gaps in tropical rainforests is an attractive alternative to ground surveys, which are laborious and imprecise. In French Guiana, the detection of logging-related gaps using very high spatial resolution optical satellite images produced by the Système Pour l'Observation de la Terre (SPOT) 5 sensor is carried out by Office National des Forêts (ONF) (French National Forestry Agency). Gaps are detected using a segmentation method based on computer-assisted photointerpretation. Detection has been automated to improve and accelerate the process. We developed an automatic method, which involves estimating segmentation thresholds using a statistical approach. The principle of the method presented in this article is to model the forest's spectral signature by using a Gaussian distribution and calculate a divergence between that theoretical signature and the image histogram in order to detect gaps that constitute a reduction of forest cover. The segmentation threshold between gap and forest is thus no longer defined in the original radiometric area but as a discrepancy between theoretical distribution and histogram. Computing the divergence to define the threshold made it possible to efficiently automate the detection of all gaps and skid trails with a surface area greater than 100 m2. The proportion of misclassified points measured during field surveys is 12%, which is a high level of precision. The proportion of misclassified points obtained is 12%. This tool could be used to assess the quality of logging operations or biomass loss in other areas where the forest is undergoing deterioration while still remaining predominant in the landscap

Impacts of logging on the canopy and the consequences for forest management in French Guiana

International audienceThis study aimed to correlate logging intensity with canopy opening using medium-resolution satellite images (SPOT-type) on a sample of 15 blocks totaling more than 3300 ha in two French Guianan forests with different reliefs. The maps obtained show the cumulative impacts on the canopy over the entire logging period (2008-2010). The percentages of canopy openness and the unit areas per logged tree were examined with logging statistics and GPS mappings of felled stems for 4 sub-samples. At the block scale, a 20% canopy opening after logging was observed for a logging intensity of 3.5 trees ha−1. The average canopy opening size per harvested tree was 601 m2, which included felling gaps and openings resulting from the construction of roads and trails needed to remove the timber. The coefficients of variation of these variables were low. General linear models (glm) were created to test the relationship between canopy damage, logging intensity and relief at local scales (1 and 4 ha) and at the management unit scale (i.e. 200-300 ha). The model created at management-unit level (block level) did not detect any significant effect of the variables on the percentage of canopy openness. However, a significant and concordant relationship was demonstrated at local levels between relief, logging intensity, their interaction on the one hand and the percentage of canopy openness on the other. A maximum local harvesting intensity of 8 trees per hectare on plateaus makes it possible to obtain an opening of less than 33% at an 85% probability threshold. In contrast, the same canopy openness threshold is obtained at only 5 trees per hectare on hillier terrain, which suggests that it would be advantageous to adjust silviculture recommendations according to forest landscape. The average openness of 20% observed in this sample, considered in relation to the cutting cycle in effect in French Guiana (65 years), implies a complete renewal of forest stands after 325 years, assuming multifunctional management objectives