Environmental and biophysical controls of evapotranspiration from Seasonally Dry Tropical Forests (Caatinga) in the Brazilian Semiarid

Highlights

• In-situ observation of evapotranspiration (ET) in a Brazilian STDF (Caatinga) using eddy covariance.

• Maximum surface conductance values occurred at approximately 09:00 (local time).

• During the two studied years the surface did not decouple from the atmosphere.

• During the wet season, ET was mainly controlled by available energy while surface conductance controlled ET during the dry season.

ABSTRACT

Seasonally dry tropical forests are among the most important biomes regarding regional and global hydrological and carbon fluxes. Thus, the objective of this study was to evaluate the seasonal and interannual variability of evapotranspiration (ET) and its biophysical control and characteristics (surface conductance—G_s; decoupling coefficient—Ω; ratio between actual evapotranspiration and equilibrium evapotranspiration—ET/ET_eq) in a preserved Caatinga Biome environment during two dry years in the Northeast Brazil region. A study on this subject with this level of detail in this biome is unprecedent. Measurements were carried out using an eddy covariance system during the period from 1^st January 2014 to 31^st December 2015. The lowest ET values were observed in the dry season of both experiment years (0.3 and 0.2 mm day⁻¹) as a consequence of poor water availability, which favored partial stomatal closure and reduced G_s values (0.22 and 0.13 mm s⁻¹). The opposite occurred in the wet season, when ET (2.6 and 1.7 mm day⁻¹) and G_s (3.74 and 2.13 mm s⁻¹) means reached higher values. Regarding annual values, differences between total annual rainfall in both years is the most probable cause for the differences observed in annual ET values. In 2014, annual ET was of 473.3 mm while in 2015 it was 283.4 mm, which incurred in an overall decrease in G_s, Ω and ET/ET_eq values. Leaf senescence and extremely low G_s values during the dry season suggest that the trees of the Caatinga Biome are more resilient regarding the use of water and are avoiding water stress caused under low water availability.

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 (G_s) and its relationship with environmental and biological factors.

The quantification of G_s 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 G_s was calculated. According to Tan et al. (2019), if there is a viable method to reliably obtain G_s 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 G_s is challenging, since it is regulated by the physical environment but also varies between species, especially in tropical forests (Tan et al., 2019). Usually G_s 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/ET_eq). 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 G_s), 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.