Quantifying Woody Plant Encroachment in Grasslands: A Review on Remote Sensing Approaches

Grasslands are an important global ecosystem, providing essential ecological and economic ecosystem services. Over the last couple decades, as a result of climate change and human activities, nearly 50% of global grasslands have degraded. Woody plant encroachment (WPE), one of the outcomes of climate change and human-related activities, negatively affects grasslands’ ecology, as well as their ability to produce food for livestock, habitats for wildlife, and economic returns for rangeland managers. Long-term monitoring of grassland status can facilitate grassland restoration. Additionally, the study of factors that influence grassland dynamics (e.g., grazing, fire, land use, or climate) can help in the restoration of grasslands. Remote sensing (RS) has been used to map the spatiotemporal distribution of WPE by using a wide variety of sensors and methods, necessitating a review on the effectiveness of RS data for WPE monitoring. Based on the importance of RS data and the rate at which grassland ecosystems are changing, this paper provides a literature review on a theoretical basis for quantifying WPE using RS and on existing RS approaches for WPE monitoring. Lastly, it identifies the current challenges associated with quantifying spatio-temporal variability in WPE that future research will need to overcome.