Ground-based spectroradiometric estimation of live fine fuel moisture of Mediterranean plants

Abstract

We developed a method to estimate live fine fuel moisture (LFFM) by ground-based, high-resolution plant reflectance measurements in the visible and near infrared regions. NDVI and WI ( R 970/ R 900) were the reflectance indices used because they provided the best estimation of LFFM. The performance of the method was evaluated (1) by measuring monthly plant reflectance and LFFM in adult plants growing in the field, and (2) in a desiccation experiment carried out with potted seedlings in a plastic tunnel. The species studied in the field site consisted of two deep-rooted shrubs ( Arbutus unedo and Quercus coccifera), two shallow-rooted shrubs ( Cistus albidus and C. monspeliensis), and one grass ( Brachypodium retusum). The species studied in experimental conditions were two trees ( Pinus halepensis and Q. ilex), and six shrubs ( Arbutus unedo, Q. coccifera, C. albidus, C. monspeliensis, Phillyrea angustifolia, and Pistacia lentiscus). During desiccation, the 970 nm trough tended to disappear (consequently, the WI increased), and the reflectance in the red increased (consequently, the NDVI decreased). The two reflectance indices NDVI and WI were significantly correlated with LFFM when all the species were considered together and within most of the individual species, both in the field and in potted seedlings. We propose a method consisting of a simple radiometer measuring plant reflectance at 680, 900, and 970 nm, that could speed up the measurement of LFFM, and that could be useful in wildfire risk evaluation.