Using Near-Infrared-Enabled Digital Repeat Photography to Track Structural and Physiological Phenology in Mediterranean Tree–Grass Ecosystems

Yunpeng Luo,M Pilar Martín,Arnaud Carrara,Rosario Gonzalez-Cascon,Gianluca Filippa,Tarek S El-Madany,Mirco Migliavacca,Marta Galvagno,Christine Römermann,Gerardo Moreno,Javier Pacheco-Labrador,Tiana W Hammer,Markus Reichstein,Bernhard Ahrens,Andrew D Richardson,Oscar Perez-Priego,Edoardo Cremonese,Xuanlong Ma

doi:10.3390/rs10081293

Abstract

Tree–grass ecosystems are widely distributed. However, their phenology has not yet been fully characterized. The technique of repeated digital photographs for plant phenology monitoring (hereafter referred as PhenoCam) provide opportunities for long-term monitoring of plant phenology, and extracting phenological transition dates (PTDs, e.g., start of the growing season). Here, we aim to evaluate the utility of near-infrared-enabled PhenoCam for monitoring the phenology of structure (i.e., greenness) and physiology (i.e., gross primary productivity—GPP) at four tree–grass Mediterranean sites. We computed four vegetation indexes (VIs) from PhenoCams: (1) green chromatic coordinates (GCC), (2) normalized difference vegetation index (CamNDVI), (3) near-infrared reflectance of vegetation index (CamNIRv), and (4) ratio vegetation index (CamRVI). GPP is derived from eddy covariance flux tower measurement. Then, we extracted PTDs and their uncertainty from different VIs and GPP. The consistency between structural (VIs) and physiological (GPP) phenology was then evaluated. CamNIRv is best at representing the PTDs of GPP during the Green-up period, while CamNDVI is best during the Dry-down period. Moreover, CamNIRv outperforms the other VIs in tracking growing season length of GPP. In summary, the results show it is promising to track structural and physiology phenology of seasonally dry Mediterranean ecosystem using near-infrared-enabled PhenoCam. We suggest using multiple VIs to better represent the variation of GPP.

Highlights

Phenology is the study of recurring life cycle stages, and their timing and relationship with environmental factors [1,2]
We did not focus on the effects of the fertilization on phenological transition dates (PTDs), productivity, and growth; rather, we focused on the development of the framework to characterize phenology in seasonally dry Mediterranean ecosystems using PhenoCams
In Mediterranean tree–grass ecosystem, meteorology plays an important role in governing seasonal variation of vegetation indices and gross primary productivity (GPP), though the importance of water availability and temperature vary across seasons

Summary

Introduction

Phenology is the study of recurring life cycle stages, and their timing and relationship with environmental factors [1,2]. Phenology controls the seasonality of ecosystem functions and plant feedbacks to climate through diverse processes, such as changes in the surface albedo and the exchange of CO2 between atmosphere and biosphere [3,4,5,6]. Phenology is not always well described in Earth system models [7,8,9], in particular, the environmental factors controlling phenology are still uncertain [6,10]. Conventional visual monitoring of phenology dates back to 705 CE [12], and still plays an important role in evaluating the impacts of climate change on ecosystems [13,14,15,16]. Conventional monitoring requires substantial field work, which limits spatial and temporal representativeness [17]

Objectives

Methods

Results

Discussion

Conclusion