Satellite Remote Sensing of Savannas: Current Status and Emerging Opportunities

Abstract

Savannas cover a wide climatic gradient across large portions of the Earth’s land surface and are an important component of the terrestrial biosphere. Savannas have been undergoing changes that alter the composition and structure of their vegetation such as the encroachment of woody vegetation and increasing land-use intensity. Monitoring the spatial and temporal dynamics of savanna ecosystem structure (e.g., partitioning woody and herbaceous vegetation) and function (e.g., aboveground biomass) is of high importance. Major challenges include misclassification of savannas as forests at the mesic end of their range, disentangling the contribution of woody and herbaceous vegetation to aboveground biomass, and quantifying and mapping fuel loads. Here, we review current (2010–present) research in the application of satellite remote sensing in savannas at regional and global scales. We identify emerging opportunities in satellite remote sensing that can help overcome existing challenges. We provide recommendations on how these opportunities can be leveraged, specifically (1) the development of a conceptual framework that leads to a consistent definition of savannas in remote sensing; (2) improving mapping of savannas to include ecologically relevant information such as soil properties and fire activity; (3) exploiting high-resolution imagery provided by nanosatellites to better understand the role of landscape structure in ecosystem functioning; and (4) using novel approaches from artificial intelligence and machine learning in combination with multisource satellite observations, e.g., multi-/hyperspectral, synthetic aperture radar (SAR), and light detection and ranging (lidar), and data on plant traits to infer potentially new relationships between biotic and abiotic components of savannas that can be either proven or disproven with targeted field experiments.

Contrasting ecosystem vegetation response in global drylands under drying and wetting conditions

“…suggests that there is an inherent resistance to water deficit in stressed ecosystems. This is supported by experimental research that shows mild stress, for example, low availability of a resource, already activates the plants' stress-response system, which is a decline in leaf growth, photosynthesis, and in the rate of acquisition of all resources (Chapin, 1991;Zhang et al, 2020) (Abdi et al, 2022), postfire vegetation recovery in these ecosystems is rapid due to the increased availability of nutrients (Kellman et al, 1985) and the reproduction strategies of different species (Zirondi et al, 2021). It is thus reasonable to assume that this rapid recovery manifests itself as an increase in LAI, which significantly alleviates the negative effect of fire on the carbon balance (Zhou et al, 2022).…”

“…Burn frequency, for instance, showed high SHAP values for both stressed and non‐stressed systems and an increase in burn frequency is linked to a corresponding increase in LAI trends. As drylands comprise many of the world's fire‐prone ecosystems, such as xeric savannas (Abdi et al, 2022), postfire vegetation recovery in these ecosystems is rapid due to the increased availability of nutrients (Kellman et al, 1985) and the reproduction strategies of different species (Zirondi et al, 2021). It is thus reasonable to assume that this rapid recovery manifests itself as an increase in LAI, which significantly alleviates the negative effect of fire on the carbon balance (Zhou et al, 2022).…”

Precipitation and temperature drive woody dynamics in the grasslands of sub-Saharan Africa

“…Characterising the drivers of grassland dynamics and the interactions over time and space is needed to improve predictions of grassland responses to global environmental change and ultimately limit degradation (Abdi et al 2022). This is a complex task because of the many processes, both direct and indirect, linked to climate, soil, and disturbances (Pausas and Bond 2020) and given the functional differences between grasslands and savannas.…”