Challenges and opportunities in the use of remote sensing for C3 and C4 grass species discrimination and mapping

Changes in the composition of plant functional type (PFT) activities are expected to accompany a changing climate. In tropical montane grasslands, such changes are predicted to follow shifts in the percentage cover and abundance of species following the C-3 and C-4 photosynthetic pathways. Reliable methods of detecting impacts of such changes on biomass and forage nutrient quality will likely provide a synoptic scale link between carbon sequestration, nutrient cycles and environmental change. Remote sensing approaches provide efficient methods to assess changes in vegetation composition quickly and are efficient for assessing vegetation in large areas. Multitemporal data obtained using broadband multispectral instruments have been used inconsistently to discriminate C-3 and C-4 grasslands and to draw some inferences. Advancements in narrowband hyperspectral systems are expected to offer greater potential for accurate mapping of C-3 and C-4 composition in grasslands. This paper presents an overview of the uses of optical remote sensing for C-3 and C-4 discrimination, which is consistent with the PFT concept used in land-surface modeling schemes. This review describes the structural properties and biochemical characteristics that affect C-3 and C-4 grass reflectance. It highlights critical limitations and evaluates the potential of remote sensing approaches used for C-3 and C-4 studies.