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.
Highlights
A better understanding of plant water availability and water use is of great importance for reliable assessments of ecosystem’s resilience to droughts [1]
In this region, our results show that intra-annual ET was inversely related to vapor pressure deficit (VPD) (p < 0.05) (Figure 5b), most likely because, despite the higher VPD, plant species are mostly leafless from September to November
Seasonal signals of vegetation indices green chromatic coordinate (GCC) and normalized difference vegetation index (NDVI) derived from optical remote sensing (RS) data collected for Caatinga vegetation were strongly related to water use, as represented by the late-morning community level sap flow data
Summary
A better understanding of plant water availability and water use is of great importance for reliable assessments of ecosystem’s resilience to droughts [1]. Information on plant water balance allows the development of more realistic soil water assessment tools and land surface models, which is a widely acknowledged requirement for research related to the plant–soil–atmosphere continuum [2]. Plant species have their own specific adaptive mechanisms to cope with droughts [3,4], which are important in water-limited ecosystems such as seasonally dry tropical forests (SDTFs). The deciduity and absence of a grassy layer are important characteristics that distinguish SDTFs from other mild seasonally dry tropical biomes such as the savannas [8]
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