Mapping Tree Canopy Cover and Aboveground Biomass in Sudano-Sahelian Woodlands Using Landsat 8 and Random Forest

Martin Karlson,Madelene Ostwald,Eskil Mattsson,Boalidioa Tankoano,Josias Sanou,Heather Reese

doi:10.3390/rs70810017

Abstract

Accurate and timely maps of tree cover attributes are important tools for environmental research and natural resource management. We evaluate the utility of Landsat 8 for mapping tree canopy cover (TCC) and aboveground biomass (AGB) in a woodland landscape in Burkina Faso. Field data and WorldView-2 imagery were used to assemble the reference dataset. Spectral, texture, and phenology predictor variables were extracted from Landsat 8 imagery and used as input to Random Forest (RF) models. RF models based on multi-temporal and single date imagery were compared to determine the influence of phenology predictor variables. The effect of reducing the number of predictor variables on the RF predictions was also investigated. The model error was assessed using 10-fold cross validation. The most accurate models were created using multi-temporal imagery and variable selection, for both TCC (five predictor variables) and AGB (four predictor variables). The coefficient of determination of predicted versus observed values was 0.77 for TCC (RMSE = 8.9%) and 0.57 for AGB (RMSE = 17.6 tons∙ha−1). This mapping approach is based on freely available Landsat 8 data and relatively simple analytical methods, and is therefore applicable in woodland areas where sufficient reference data are available.

Highlights

The Sudano-Sahelian woodlands occupy vast areas between the Saharan desert and the moist forests of the Guinean zone [1,2]
We evaluate the potential of Landsat 8 imagery to map two attributes commonly used to characterize tree cover structure and conditions, namely tree canopy cover (TCC) and aboveground biomass (AGB)
The error rate estimated from the Random Forest (RF) out of bag (OOB) data was used to rank all of the predictor variables by their capacity to predict TCC and AGB

Summary

Introduction

The Sudano-Sahelian woodlands occupy vast areas between the Saharan desert and the moist forests of the Guinean zone [1,2]. Other local case studies show that tree cover conditions have improved substantially since the severe droughts that hit the area in the 1970s and 1980s [12] Such improvements are generally attributed to increased rainfall or farmer managed natural regeneration, with notable cases found in northern Burkina Faso [13] and southern Niger [14,15]. Given these divergent research findings and the importance of trees for local livelihoods, timely information on the extent and conditions of woodlands, including agroforestry landscapes, is of great interest to a number of local actors, such as researchers, natural resource managers and forestry industries [2]

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