Chlorophyll content estimation in an open-canopy conifer forest with Sentinel-2A and hyperspectral imagery in the context of forest decline

P.J Zarco-Tejada,A Hornero,P.S.A Beck,T Kattenborn,P Kempeneers,R Hernández-Clemente

doi:10.1016/j.rse.2019.01.031

Abstract

With the advent of Sentinel-2, it is now possible to generate large-scale chlorophyll content maps with unprecedented spatial and temporal resolution, suitable for monitoring ecological processes such as vegetative stress and/or decline. However methodological gaps exist for adapting this technology to heterogeneous natural vegetation and for transferring it among vegetation species or plan functional types. In this study, we investigated the use of Sentinel-2A imagery for estimating needle chlorophyll (Ca+b) in a sparse pine forest undergoing significant needle loss and tree mortality. Sentinel-2A scenes were acquired under two extreme viewing geometries (June vs. December 2016) coincident with the acquisition of high-spatial resolution hyperspectral imagery, and field measurements of needle chlorophyll content and crown leaf area index. Using the high-resolution hyperspectral scenes acquired over 61 validation sites we found the CI chlorophyll index R750/R710 and Macc index (which uses spectral bands centered at 680 nm, 710 nm and 780 nm) had the strongest relationship with needle chlorophyll content from individual tree crowns (r2 = 0.61 and r2 = 0.59, respectively; p < 0.001), while TCARI and TCARI/OSAVI, originally designed for uniform agricultural canopies, did not perform as well (r2 = 0.21 and r2 = 0.01, respectively). Using lower-resolution Sentinel-2A data validated against hyperspectral estimates and ground truth needle chlorophyll content, the red-edge index CI and the Sentinel-specific chlorophyll indices CI-Gitelson, NDRE1 and NDRE2 had the highest accuracy (with r2 values >0.7 for June and >0.4 for December; p < 0.001). The retrieval of needle chlorophyll content from the entire Sentinel-2A bandset using the radiative transfer model INFORM yielded r2 = 0.71 (RMSE = 8.1 μg/cm2) for June, r2 = 0.42 (RMSE = 12.2 μg/cm2) for December, and r2 = 0.6 (RMSE = 10.5 μg/cm2) as overall performance using the June and December datasets together. This study demonstrates the retrieval of leaf Ca+b with Sentinel-2A imagery by red-edge indices and by an inversion method based on a hybrid canopy reflectance model that accounts for tree density, background and shadow components common in sparse forest canopies.

Highlights

Slow-acting disturbance processes, including droughts and pathogen outbreaks, appear to be increasing in various forest ecosystems (Hartmann et al, 2018)
Using the highresolution hyperspectral scenes acquired over 61 validation sites we found the CI chlorophyll index R750/R710 and Macc index had the strongest relationship with needle chlorophyll content from individual tree crowns (r2 = 0.61 and r2 = 0.59, respectively; p < 0.001), while TCARI and TCARI/OSAVI, originally designed for uniform agricultural canopies, did not perform as well (r2 = 0.21 and r2 = 0.01, respectively)
We investigated the retrieval of needle chlorophyll content in an open coniferous canopy undergoing decline using Sentinel-2A imagery at two timepoints (June and December) of large sun angle variation

Summary

Introduction

Slow-acting disturbance processes, including droughts and pathogen outbreaks, appear to be increasing in various forest ecosystems (Hartmann et al, 2018) This trend may become clearer and stronger as further climate change is likely to exacerbate droughts (Trenberth et al, 2014), cause new biotic disturbance regimes, while it shifts the suitable habitats of many tree species geographically (Allen et al, 2015; Gauthier et al, 2015; Millar and Stephenson, 2015). Chlorophyll a + b (Ca+b) Carotenoid content (Cx+c) Dry matter (Cm) Equivalent water thickness (Cw) Structural Parameter (N). The high-spatial resolution imagery acquired with the hyperspectral sensor (ground sampling distance 40 cm) enabled the simulation of pixel aggregation at the Sentinel-2A resolution, as well as the identification of individual scene components (i.e., individual tree crowns, crown shadows and understory) (Fig. 2b).

Methods

Results

Conclusion