Jean-Pierre Renaud

Multidimensional scaling of first-return airborne laser echoes for prediction and model-assisted estimation of a distribution of tree stem diameters

Abstract• Key messageWe demonstrate how multidimensional scaling can be used to combine forest inventory field data and airborne laser scanner data to obtain both predictions and model-assisted estimation of a tree stem diameter distribution.• Context The size distribution of forest trees is important both for management planning and analysis purposes. Yet field samples are rarely large enough to assuage a desired accuracy of a direct estimation in all areas of interest. Improvements in spatial coverage and accuracy are possible with a census—or a very large sample of one or more cost-effective auxiliary variables that can inform one about the tree size distribution.• Aims The objective of this study is to demonstrate how a relative frequency distribution of canopy heights from airborne laser scanner data can be used to improve direct estimates of a tree size distribution.• Methods Multidimensional scaling is used to link a relative frequency distribution of canopy heights to an observed plot-level distribution of tree size.• Results A multivariate linear model can be used for both predictions and model-assisted estimation of a tree stem diameter distribution.• Conclusion Multidimensional scaling can provide a multivariate linear link between two relative frequency distributions and is therefore ideally suited for both stand-level predictions and design-based inference of tree size distributions.

Stand-level wind damage can be assessed using diachronic photogrammetric canopy height models

Key messageDiachronic photogrammetric canopy height models can be used to quantify at a fine scale changes in dominant height and wood volume following storms. The regular renewal of aerial surveys makes this approach appealing for monitoring forest changes.ContextThe increasing availability of aerial photographs and the development of dense matching algorithms open up new possibilities to assess the effects of storm events on forest canopies.AimsThe objective of this research is to assess the potential of diachronic canopy height models derived from photogrammetric point clouds (PCHM) to quantify changes in dominant height and wood volume of a broadleaved forest following a major storm.MethodsPCHMs derived from aerial photographs acquired before and after a storm event were calibrated using 25 field plots to estimate dominant height and volume using various modeling approaches. The calibrated models were combined with a reference damage maps to estimate both the within-stand damage variability, and the amount of volume impacted.ResultsDominant height was predicted with a root mean squared error (RMSE) of 4%, and volume with RMSEs ranging from 24 to 32% according to the type of model. The volume impacted by storm was in the range of 42–76%. Overall, the maps of dominant height changes provided more details about within-stand damage variability than conventional photointerpretation do.ConclusionThe study suggests a promising potential for exploiting PCHM in pursuit of a rapid localization and quantification of wind-throw damages, given an adapted sampling design to calibrate models.

Pas de vent, pas de bois. L’apport de la biomécanique des arbres pour comprendre la croissance puis la vulnérabilité aux vents forts des peuplements forestiers

La biomecanique etudie les reactions et adaptations des etres vivants a leur environnement mecanique, par exemple aux oscillations et forces exercees par le vent. Au-dela des theories anciennes de la securite mecanique constante, la mecanobiologie a recemment formalise les signaux mecaniques, la perception des cellules vivantes et les reponses de croissance. Ces mecanismes physiologiques font que l’arbre ne forme vraiment du bois, tissu de soutien, que lorsqu’il est mecaniquement stimule. La croissance est controlee par les deformations mecaniques percues, qui deviennent alors pour l’arbre de bons indicateurs de securite. Pour appliquer ces connaissances aux forets, le projet ANR FOR-WIND a l’ambition de developper une mecanobiologie adaptee aux temps longs et aux conditions naturelles complexes. L’enjeu est de concevoir les pratiques d’amenagement avec de nouveaux indicateurs de vulnerabilite aux vents forts, qui raisonnent l’effet du changement climatique, de la structure du paysage, de la sylviculture ou de l’amelioration genetique au travers des processus cles mais negliges d’endurcissement des arbres aux vents usuels.