Validating and Linking the GIMMS Leaf Area Index (LAI3g) with Environmental Controls in Tropical Africa

Marion Pfeifer,Rob Marchant,Alemu Gonsamo,Dereje Denu,Veronique Lefebvre,Petri Pellikka,Philip Platts

doi:10.3390/rs6031973

Abstract

The recent Global Inventory Modeling and Mapping Studies (GIMMS) LAI3g product provides a 30-year global times-series of remotely sensed leaf area index (LAI), an essential variable in models of ecosystem process and productivity. In this study, we use a new dataset of field-based LAITrue to indirectly validate the GIMMS LAI3g product, LAIavhrr, in East Africa, comparing the distribution properties of LAIavhrr across biomes and environmental gradients with those properties derived for LAITrue. We show that the increase in LAI with vegetation height in natural biomes is captured by both LAIavhrr and LAITrue, but that LAIavhrr overestimates LAI for all biomes except shrubland and cropland. Non-linear responses of LAI to precipitation and moisture indices, whereby leaf area peaks at intermediate values and declines thereafter, are apparent in both LAITrue and LAIavhrr, although LAITrue reaches its maximum at lower values of the respective environmental driver. Socio-economic variables such as governance (protected areas) and population affect both LAI responses, although cause and effect are not always obvious: a positive relationship with human population pressure was detected, but shown to be an artefact of both LAI and human settlement covarying with precipitation. Despite these complexities, targeted field measurements, stratified according to both environmental and socio-economic gradients, could provide crucial data for improving satellite-derived LAI estimates, especially in the human-modified landscapes of tropical Africa.

Highlights

Leaf area index (LAI) is defined as one half of the total leaf surface area per unit ground surface area
Plantation had significantly lower LAITrue compared to forest, but significantly higher LAITrue compared to shrubland and bushland (Figure 2)
We find that very few plantations are identified by the MODIS product, despite large areas of forests being converted to plantations in both Kenya and Ethiopia

Summary

Introduction

Leaf area index (LAI) is defined as one half of the total leaf surface area per unit ground surface area (projected on the local horizontal datum). It is a key biophysical vegetation property describing biome-specific canopy structure [1], and an essential variable in models of ecosystem processes and productivity [2,3], crop productivity [4] and hydrology [5]. Others are produced using empirical and semi-empirical relationships between LAI and spectral vegetation indices [12,13]

Methods

Results

Discussion

Conclusion