Environmental Drivers of Water Use for Caatinga Woody Plant Species: Combining Remote Sensing Phenology and Sap Flow Measurements

Abstract

We investigated the water use of Caatinga vegetation, the largest seasonally dry forest in South America. We identified and analysed the environmental phenological drivers in woody species and their relationship with transpiration. To monitor the phenological evolution, we used remote sensing indices at different spatial and temporal scales: normalized difference vegetation index (NDVI), soil adjusted vegetation index (SAVI), and green chromatic coordinate (GCC). To represent the phenology, we used the GCC extracted from in-situ automated digital camera images; indices calculated based on sensors included NDVI, SAVI and GCC from Sentinel-2A and B satellites images, and NDVI products MYD13Q1 and MOD13Q1 from a moderate-resolution imaging spectroradiometer (MODIS). Environmental drivers included continuously monitored rainfall, air temperature, soil moisture, net radiation, and vapour pressure deficit. To monitor soil water status and vegetation water use, we installed soil moisture sensors along three soil profiles and sap flow sensors for five plant species. Our study demonstrated that the near-surface GCC data played an important role in permitting individual monitoring of species, whereas the species’ sap flow data correlated better with NDVI, SAVI, and GCC than with species’ near-surface GCC. The wood density appeared to affect the transpiration cessation times in the dry season, given that species with the lowest wood density reach negligible values of transpiration earlier in the season than those with high woody density. Our results show that soil water availability was the main limiting factor for transpiration during more than 80% of the year, and that both the phenological response and water use are directly related to water availability when relative saturation of the soil profile fell below 0.25.

Satellite Remote Sensing of Savannas: Current Status and Emerging Opportunities

“…Although the distribution of these towers in savanna-dominated areas is low relative to other biomes [94], existing sites provide opportunities for coupling satellite and flux tower data [93] to further enable the partitioning and mapping of plant functional types into their constituent trees and grasses [74]. Some flux towers are equipped with phenological cameras (PhenoCams) that provide site-level repeat photography [95,96]. The so-called "regions of interest" delineated within the field of view of PhenoCam images comprise areas of relatively pure grass and tree canopies that enable the separation of their respective phenology [97] and gross primary production [98].…”

Remote Sensing of Savannas and Woodlands: Editorial

“…The articles then follow global geography starting in the southern hemisphere dealing with TLS in the Australian tropical savanna [21,22]. Next come four articles on various aspects of savanna and woodland vegetation in Brazil [23][24][25][26]. These are followed by two articles on African environments that include a mix of savannas and woodlands and other vegetation [27,28].…”

“…The group of four papers on South America span the ecotone from the heavily altered Mato Grosso forests in Amazonia through the savanna of the Cerrado to the dry forests of the Caatinga. These papers explore the mapping of vegetation types [23], the measurement of woody above-ground biomass (AGB) [24], the long-term change in savanna vegetation [25] and water use [26]. The emphasis on this continuum from tropical rainforest through savanna to dry forest is timely and important as this environment is subject to massive anthropogenic change and complex political forces within Brazil that influence the balance between conservation and exploitation of these ecosystems.…”

“…The paper by Bispo et al [24] combines airborne LiDAR with Landsat 8 and ALOS PALSAR to map AGB for a catchment-scale study region of the Cerrado. The paper by Paloschi et al [26] reports on the strong dependence of water use on soil moisture, and the variation in phenology, and physiology of individual species in this dry, fragile Caatinga environment. The paper by Alencar et al [25] is a new and significant documenting of long-term change in natural native vegetation in the Cerrado using 33 years of Landsat imagery in the Google Earth Engine.…”

“…The featured emphasis on TLS [20][21][22] and the Cerrado [23][24][25][26] points to the technology currently at the cutting edge for remote measurement, and to the focus of concern for the fate of carbon stocks and biodiversity in one of the largest savannas in the world. The adoption of Sentinel 2 [23,30], the application of big-data processing [25] and the combination of SAR, LiDAR and optical data [23,24] signal the future direction of remote sensing of savannas from space.…”