Response of Land Surface Phenology to Variation in Tree Cover during Green-Up and Senescence Periods in the Semi-Arid Savanna of Southern Africa

Moses A Cho,Abel Ramoelo,Luthando Dziba

doi:10.3390/rs9070689

Abstract

Understanding the spatio-temporal dynamics of land surface phenology is important to understanding changes in landscape ecological processes of semi-arid savannas in Southern Africa. The aim of the study was to determine the influence of variation in tree cover percentage on land surface phenological response in the semi-arid savanna of Southern Africa. Various land surface phenological metrics for the green-up and senescing periods of the vegetation were retrieved from leaf index area (LAI) seasonal time series (2001 to 2015) maps for a study region in South Africa. Tree cover (%) data for 100 randomly selected polygons grouped into three tree cover classes, low (<20%, n = 44), medium (20–40%, n = 22) and high (>40%, n = 34), were used to determine the influence of varying tree cover (%) on the phenological metrics by means of the t-test. The differences in the means between tree cover classes were statistically significant (t-test p < 0.05) for the senescence period metrics but not for the green-up period metrics. The categorical data results were supported by regression results involving tree cover and the various phenological metrics, where tree cover (%) explained 40% of the variance in day of the year at end of growing season compared to 3% for the start of the growing season. An analysis of the impact of rainfall on the land surface phenological metrics showed that rainfall influences the green-up period metrics but not the senescence period metrics. Quantifying the contribution of tree cover to the day of the year at end of growing season could be important in the assessment of the spatial variability of a savanna ecological process such as the risk of fire spread with time.

Highlights

Spaceborne remote sensing has provided a cost effective means to study, understand and monitor spatio-temporal changes in vegetated land surfaces [1]
Areas with tree cover greater than 20% showed an earlier day of the year at start of growing season (SGS) or MGR in comparison to areas with tree cover of less than 20%, the difference was statistically not significant at p < 0.05 due to the wide variation in day of the year at SGS or MGR among the low tree cover polygons located across the study region
There was a significant positive trend in the phenological metrics computed from the senescing phase, e.g., day of the year at maximum senescing rate (MSR) or end of growing season (EGS) increases with increasing tree cover

Summary

Introduction

Spaceborne remote sensing has provided a cost effective means to study, understand and monitor spatio-temporal changes in vegetated land surfaces [1]. The land surface phenological signature of a remotely sensed pixel in the African savanna is determined by the spectral characteristics of the Remote Sens. Defined land surface phenological metrics include the dates corresponding to the start, end and length of the growing season. These metrics are important parameters in ecosystem simulation models and coupled biosphere/atmosphere models [7]. The timing and the length of the growing season control spatio-temporal variations of carbon and water cycles and influences latent/sensible heat transport [7,8]. The land surface phenology of Southern African semi-arid savannas is still largely understudied

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