Foliar Chlorophyll and Carotenoid Contents of European Aspen Assessed by Airborne Imaging Spectroscopy Across Protected and Nonprotected Boreal Forests

Abstract

Airborne imaging spectroscopy (AIS) captures the spectral reflectance of a forest canopy, which can be used to assess the pigments content. We investigated intraspecific variation in foliar chlorophyll (Chl) and carotenoid (Car) contents in the upper canopy of a keystone tree species, European aspen (Populus tremula L.), and assessed the tree and forest traits that co-occurred with a high or low Chl content. We also evaluated the accuracy of the pigment content estimation by AIS in an 83 km2 study area comprising protected and non-protected areas in Southern Finland. Protected and non-protected forests differed in their aspen profile, so that aspens in protected forests were taller with a shorter crown length than the ones in nonprotected areas. As thick aspens as occurred at protected sites did not occur at nonprotected sites. Chl and Car contents on a leaf area basis showed similar values in protected areas as in non-protected ones, and a high variation in both. Tree density influenced Chl content in non-protected forests and roadsides so that high tree density co-occurred with low Chl content, and vice versa, which implies the influence of light environment. The high intraspecific variation in the pigment contents of aspen allowed pigment estimation based on airborne spectral reflectance. Estimation accuracy for Chl and Car content of aspen leaves using crown mean spectral reflectance was moderate by random forest models with 11.7 % and 13.2 % relative root mean square error (RMSE%), respectively. Chl and Car contents were predicted by partial least squares regression (PLSR) model, with 12.3 % and 13.2 % RMSE%, respectively. We show that 1) aspens with a high foliar Chl content implying high vigor occur in both old-growth and managed forests, 2) remote sensing data revealed a connection between Chl content and tree density in nonprotected areas and roadsides, and 3) green hump wavelengths can be used retrieve to foliar Chl content retrieval from airborne imaging spectroscopy.

Keywords: Populus tremula, Intraspecific variation, Chlorophyll, Carotenoids, Airborne imaging spectroscopy