Measurement and remote sensing of LAI in Rocky Mountain montane ecosystems

Nous avons estime l'indice de surface foliaire (LAI) pour le Glacier National Park situe dans le Montana, aux Etats-Unis, en utilisant diverses methodes de mesure de LAI sur le terrain et en etablissant une correlation entre ces resultats et les donnees TM du capteur Landsat. Les valeurs de LAI au sol ont ete estimees au moyen d'equations allometriques reliant LAI au bois d'aubier et par des mesures avec des instruments optiques, incluant un LAI-2000 et un ceptometre Decagon. Comme les estimations de LAI par les methodes optiques sont affectees par un auto-estompage non aleatoire, les valeurs de LAI calculees avec les equations allometriques ont ete comparees aux valeurs obtenues par les mesures optiques dans le but de corriger ces dernieres pour des types de vegetation et de structure de couvert similaires. Des modeles de regression par moindres carres ont ete elabores en regroupant les valeurs de LAI mesurees au sol avec les equations allometriques et les instruments optiques apres correction et a partir d'indices de vegetation provenant des donnees TM de Landsat. Pour des classes specifiques de pente, d'exposition et d'elevation, on a utilise une valeur moyenne de LAI et des indices obtenus par satellite etant donne que les estimations ponctuelles etaient generalement imprecises. L'indice normalise differentiel de vegetation (NDVI) et un ratio infra-rouge moyen simple corrige (SR c ) etaient les valeurs qui concordaient le mieux avec les donnees de LAI prises sur le terrain. Nous avons applique ces deux modeles aux indices TM et evalue les estimations de LAI avec des valeurs independantes mesurees sur le terrain. De plus, nous avons etudie l'influence de la resolution spatiale sur les valeurs de LAI obtenues par satellite en faisant la moyenne des donnees TM sur des cellules (pixels) de 250 x 250 m. Les resultats ont demontre que NDVI offre la meilleure estimation de LAI et que la precision decroit avec l'accroissement de la taille des pixels. SR c surestime la valeur de LAI, principalement a cause de la difficulte d'obtenir une echelle de reflectance appropriee pour la correction dans l'infra-rouge moyen a appliquer a l'indice pour un territoire a plus grande echelle tel que celui etudie ici. Cependant, la correction apportee aux indices TM dans l'infra-rouge moyen constitue un bon indicateur du couvert en sous-etage.

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