Environmental and biophysical controls of evapotranspiration from Seasonally Dry Tropical Forests (Caatinga) in the Brazilian Semiarid
Introduction
The reminiscent areas of Seasonally Dry Tropical Forests (SDTF) are located mostly in the Neotropics and extend from the Northwest of Mexico to the North of Argentina, usually in isolated patches or land segments (Miles et al., 2006; Pennington et al., 2000, 2006; Linares-Palomino et al., 2011). Among the SDTF, the Caatinga Biome located in the Brazilian Semiarid region in the Northeast Brazil is the only one consisted of a fully functional ecosystem which comprises a continuous area of approximately 800,000 km², representing roughly 10% of the Brazilian territory (Miles et al., 2006; Särkinen et al., 2011; Santos et al., 2012; Koch et al., 2017).
The Caatinga has a variety of endemic species (Sampaio et al., 1995; Leal et al., 2005; Moura et al., 2013) and its vegetation is composed mainly by xerophyte, woody, thorny, deciduous and semi-deciduous physiognomies with a predominance of trees and shrubs morphologically adapted to sustain water stress (Sampaio et al., 1995; Araújo et al., 2007; Mendes et al., 2017). The Brazilian Semiarid region is characterized by low and irregular rainfall, high temperatures and solar radiation, which increase evaporation and soil desiccation, leading to water deficits during most of the year (Dombroski et al., 2011; Mutti et al., 2019), although rainfall extreme events occur throughout the year (Oliveira et al., 2017; Mutti et al., 2019). The Caatinga has been identified as one of the most important wildlife regions of the globe and most biodiverse dry forests (Mittermeier et al., 2003; Pennington et al., 2006; Santos et al., 2014; Koch et al., 2017). Nevertheless, only 1% of this Biome has been converted into protected areas and the Caatinga and other SDTF have received less attention than tropical rainforests regarding research efforts (Koch et al., 2017; Tomasella et al., 2018).
The CO2 transfer rate from the atmosphere to the carboxylation sites during the photosynthesis process is closely related to water lost to the atmosphere through leaf transpiration. These exchanges are controlled by the interaction of various environmental factors (such as solar radiation, air temperature, vapor pressure deficit—VPD and soil water content), besides biological processes inherent to the vegetation such as leaf emergence and development and stomatal conductance (Zha et al., 2013). At the plant level, physiological control of CO2 absorption and transpiration is carried out by stomata and the modulation of such processes is quantified in terms of leaf stomatal conductance (Fanourakis et al., 2013; Lin et al., 2015; Wehr et al., 2017). At the ecosystem level, the control of both evapotranspiration (ET) and CO2 absorption are quantified in terms of surface conductance (Gs) and its relationship with environmental and biological factors.
The quantification of Gs is important not only for understanding the mechanisms that control ET and CO2 exchange, but also for calibrating the Penmann-Monteith equation for future applications under the conditions in which Gs was calculated. According to Tan et al. (2019), if there is a viable method to reliably obtain Gs values and its environmental controls, then ET can be easily calculated at local and global scales using only usually observed meteorological variables. However, the parameterization of Gs is challenging, since it is regulated by the physical environment but also varies between species, especially in tropical forests (Tan et al., 2019). Usually Gs has been described by the Penman-Monteith equation in its big-leaf version (Tan et al., 2019) and its controls have been analyzed through the relationship between it and different biophysical parameters, such as VPD, vegetation indices and the ratio between actual and equilibrium evapotranspiration (ET/ETeq). These analyses have been performed in different biomes around the world (Ryu et al., 2008; Zha et al., 2013; Ma et al., 2015; Tan et al., 2019).
Recently, observational studies in the Caatinga have been developed in order to better understand the soil-plant-atmosphere relationship in this biome. It has been observed that the closure of the energy balance is better during the wet season and under very unstable conditions, and most of the net radiation is converted into sensible heat flux (Teixeira et al., 2008; Campos et al., 2019). Mutti et al. (2018) showed that during a wet year, ET differences between land cover classes were less noticeable due to soil saturation and the urgency of vegetated surfaces to meet their physiological needs. In a dry year, however, the differences were more evident, with bare soil showing lower ET rates and vegetation classes showing higher ET values in a Semiarid Brazil watershed predominantly covered by the Caatinga. Da Silva et al. (2017) measured the water and energy fluxes over the Caatinga and found that the ET in the dry season was controlled by the vegetation and in the wet season it was controlled by the atmospheric conditions. Studies of this nature are important because they allow a better understanding of the effects of projected climate change scenarios for this biome, which indicate increasing trends in air temperature, higher evapotranspiration rates, decreased rainfall and, consequently, aggravation of water deficit (Magrin et al., 2014; Marengo et al., 2017).
Given the large area of the Caatinga Biome, one can infer that it plays an important role in regional (or even global) processes related to the biosphere-atmosphere interactions (Moura et al., 2016). Previous studies in this biome have clarified some uncertainties about key subjects such as the role of seasonal rainfall on the closure and partitioning of the energy balance, evapotranspiration and CO2 exchange (Teixeira et al., 2008; da Silva et al., 2017; Campos et al., 2019; Mutti et al., 2019; Santos et al., 2020). On the other hand, a detailed analysis of the characteristics of ET control in the Caatinga Biome has not yet been developed. Although Teixeira et al., (2008) briefly commented the characteristics of surface resistance (reciprocal of Gs), the analysis of environmental controls was not carried out, since it was not the objective of said study. In this sense, more effort is needed in order to better comprehend the environmental and biophysical controls of ET and in which way they affect heat and mass transfer in the Caatinga. Thus, long term detailed studies on the energy and water fluxes between vegetation and the atmosphere are needed due to the vulnerability of this environment to anthropic activities and climate change. Furthermore, the accuracy of climate scenarios and their impacts on climate, biodiversity and the population of these region is still debatable, and uncertainties are high (Magrin et al., 2014; Marengo and Bernasconi, 2015). Such a challenge reinforces the critical need for studies on how responds to environmental variables and how it influences energy and mass fluxes, which is particularly important for understanding future biosphere-atmosphere interactions in the Brazilian Semiarid region.
In this context, the objective of this study was to evaluate the seasonal variability of evapotranspiration and its control mechanisms in a preserved Caatinga Biome environment during two dry years. We also analyzed the daytime pattern of evapotranspiration and compared the sensitivity of surface conductance to VPD.
Section snippets
Site description
The study was conducted in a preserved fragment of the Caatinga Biome, in the Rio Grande do Norte State, Brazilian Semiarid. An 11-meters tall flux tower equipped with an eddy covariance system was installed in the Seridó Ecological Station (ESEC-Seridó; 6°34’42”S, 37°15’05”W, 205 m, above sea level), a conservation unit of the Caatinga Biome located between the Serra Negra do Norte and Caicó cities (Fig. 1). The ESEC-Seridó area is managed by the Chico Mendes Institute for Biodiversity
Seasonal analysis
Seasonal analysis was carried out considering the behavior of daily accumulated rainfall during the experiment. Thus, the following seasons were defined: i) wet season, from February to May in 2014 (419.6 mm) and from February to April in 2015 (381.5 mm); ii) dry season, from August to October in 2014 (6.2 mm) and from August to November in 2015 (0.0 mm); iii) wet-dry transition season, from June to July in 2014 (21.6 mm) and from May to July in 2015 (56.0 mm); and iv) dry-wet transition
Meteorological aspects
The annual analysis of the VPD was consistent when comparing the results of 2014 and 2015, that is, the year with less rainfall volume (2015) was warmer and drier, which resulted in higher VPD values. Seasonal variations were also similar to the values reported in Brazilian tropical savannas (Rodrigues et al., 2014) and deciduous tropical forests in northern Thailand (Igarash et al., 2015), which also present vegetation susceptible to extended drought periods. On the other hand, VPD values
Conclusions
A seasonal analysis of the biophysical control and characteristics of ET was carried out in the Caatinga Biome with a detail level that is unprecedent for this biome. The physiological controls at the ecosystem level were based on surface conductance (Gs) and its relationships with vegetation and atmospheric parameters (NDVI, LAI and VPD). Additionally, we analyzed the decoupling coefficient (Ω), and the ratio between actual evapotranspiration and equilibrium evapotranspiration (ET/ETeq). The
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors are thankful to the Brazilian National Institute of Semi-Arid (INSA) and CNPq/Capes/FACEPE (Process no 465764/2014-2 and 420854/2018-5) for funding the project which originated the eddy covariance data used in this study which is partially based on the Ph.D. Thesis by Thiago Valentim Marques carried out in the Climate Sciences Graduate Program of the Federal University of Rio Grande do Norte. We are also thankful to the CNPq for funding the Research Productivity Grant to the eighth
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