Ecological Indicators (Nov 2023)
A comprehensive survey on quantifying non-photosynthetic vegetation cover and biomass from imaging spectroscopy
Abstract
Non-photosynthetic vegetation (NPV) is considered a key quantifiable variable in the context of new spaceborne imaging spectroscopy missions. Knowledge of NPV is essential for all terrestrial ecosystems, and its mapping is beneficial for agriculture and forestry. In agriculture, crop residues (CR) play an important role in tillage, erosion control and soil health management. In forest management, NPV mapping supports understanding the dynamics of important ecological processes such as fire, erosion, and land use changes. Whereas the fractional cover of NPV has been extensively studied across managed and natural ecosystems, so far little attention has been paid to the quantification of biomass of senescent material. In this comprehensive survey, we summarize past attempts to quantify landscape-scale NPV or CR cover (in %) and biomass (in g/m2) from optical Earth observation data, with a particular focus on hyperspectral data exploitation. Given three decades of spectroscopy studies on NPV or CR cover detection, we identify the following methodological trends: (1) a shift from unmixing approaches towards regression-based models; (2) a shift from two-band indices towards multi-band equations; and (3) a shift from linear regression towards data-driven machine learning models. (4) In addition, gradual progress in radiative transfer modelling (RTM) in describing the interaction of radiation with non-photosynthetic plant material has been achieved. These trends have enabled progress from merely identifying the presence of NPV and CR to the explicit quantification of NPV biomass over croplands and grasslands. We highlight the potential of recent and upcoming next-generation spaceborne missions and their unprecedented hyperspectral and enhanced multispectral data streams and propose to implement efficient workflows for the operational delivery of global NPV products along with associated uncertainties. In summary, this survey emphasizes the significance of imaging spectroscopy as an efficient way to quantify NPV and support the management of croplands, grasslands, forests, and natural vegetation.
