Monitoring Mega-Crown Leaf Turnover from Space

Emma R Bush,Edward T A Mitchard,Edmond Dimoto,Pacôme Dimbonda,Loïc Makaga,Katharine Abernethy,Thiago S F Silva

doi:10.3390/rs12030429

Abstract

Spatial and temporal patterns of tropical leaf renewal are poorly understood and poorly parameterized in modern Earth System Models due to lack of data. Remote sensing has great potential for sampling leaf phenology across tropical landscapes but until now has been impeded by lack of ground-truthing, cloudiness, poor spatial resolution, and the cryptic nature of incremental leaf turnover in many tropical plants. To our knowledge, satellite data have never been used to monitor individual crown leaf phenology in the tropics, an innovation that would be a major breakthrough for individual and species-level ecology and improve climate change predictions for the tropics. In this paper, we assessed whether satellite data can detect leaf turnover for individual trees using ground observations of a candidate tropical tree species, Moabi (Baillonella toxisperma), which has a mega-crown visible from space. We identified and delineated Moabi crowns at Lopé NP, Gabon from satellite imagery using ground coordinates and extracted high spatial and temporal resolution, optical, and synthetic-aperture radar (SAR) timeseries data for each tree. We normalized these data relative to the surrounding forest canopy and combined them with concurrent monthly crown observations of new, mature, and senescent leaves recorded from the ground. We analyzed the relationship between satellite and ground observations using generalized linear mixed models (GLMMs). Ground observations of leaf turnover were significantly correlated with optical indices derived from Sentinel-2 optical data (the normalized difference vegetation index and the green leaf index), but not with SAR data derived from Sentinel-1. We demonstrate, perhaps for the first time, how the leaf phenology of individual large-canopied tropical trees can directly influence the spectral signature of satellite pixels through time. Additionally, while the level of uncertainty in our model predictions is still very high, we believe this study shows that we are near the threshold for orbital monitoring of individual crowns within tropical forests, even in challenging locations, such as cloudy Gabon. Further technical advances in remote sensing instruments into the spatial and temporal scales relevant to organismal biological processes will unlock great potential to improve our understanding of the Earth system.

Highlights

Leaves are the primary mechanism of carbon and water exchange between the Earth’s surface and the atmosphere
Remote sensing has great potential for sampling leaf phenology across tropical landscapes, and remote sensing-based products are used in data assimilation frameworks to inform carbon cycle modelling [5]
We present a rare opportunity to assess the ability of satellite data to detect leaf turnover for individual crowns using a candidate tropical tree species

Summary

Introduction

Leaves are the primary mechanism of carbon and water exchange between the Earth’s surface and the atmosphere. Despite the importance of tropical trees to the global carbon cycle, there are relatively few ground-based observations of tropical leaf phenology [2]. Sensitivity to climate change differs markedly between models [3] and most of the uncertainty over tropical land areas is due to disagreement on the modelled impacts of climate and CO2 on primary productivity, itself a function of the leaf area [4]. Remote sensing has great potential for sampling leaf phenology across tropical landscapes, and remote sensing-based products are used in data assimilation frameworks to inform carbon cycle modelling [5]. There have been controversies over the biological interpretation of canopy reflectance data at the plant level, and the impacts of non-leaf artefacts, such as the solar zenith angle and remnant cloud and haze [6,7,8]. Incremental leaf turnover, and data gaps due to cloudiness are serious limitations in the practical use of remotely sensed leaf area products in the tropics [9]

Methods

Results

Conclusion