Biophysical and life‐history determinants of hydraulic lift in Neotropical savanna trees

Abstract

Summary 1.Ecological and physiological characteristics of vascular plants may facilitate or constrain hydraulic lift. Studies of hydraulic lift typically include only one or few species, but in species-rich ecosystems a larger number of representative species needs to be studied. 2. Measurements of sap flow in tap roots, lateral roots and stems, as well as stable isotope labelling techniques were used to determine the occurrence and relative magnitude of hydraulic lift in several individuals of nine co-occurring Brazilian savanna (Cerrado) tree species differing in life-history traits, and to assess physical and biological determinants of this process at the tree and ecosystem level. 3. The occurrence of reverse sap flow observed in deciduous and brevideciduous species during the dry season was consistent with hydraulic lift. The evergreen species did not exhibit reverse flow. Consistent with their ability to carry out hydraulic lift, the brevideciduous and deciduous species had both shallow and tap roots (dimorphic root systems), whereas the evergreen species had mostly deep roots (monomorphic root systems). 4. In the deciduous and brevideciduous species, the contribution of tap roots to transpiration increased substantially as the dry season progressed. Seasonal changes in the contribution of tap roots to transpiration were not observed in the evergreen species. 5. There was an inverse relationship between rates of reverse sap flow and seasonal loss of hydraulic conductivity in lateral roots, suggesting that hydraulic lift in Cerrado woody plants may help maintain the functionality of the lateral roots in exploring dry and nutrient rich superficial soil layers without directly enhancing the amount of water uptake. 6. Reverse sap flow in lateral roots of the deciduous and brevideciduous species increased asymptotically as the driving force for water movement from roots to the soil increased. This nonlinear relationship implies that additional sinks for water such as nocturnal transpiration and refilling of internal water storage tissues may compete for internal water resources during the dry season.7. There appears to be a trade-off between greater year-round access to nutrients in the upper soil layers (deciduous and brevideciduous species) and a greater access to deep and more reliable water sources during the dry season (evergreen species), which has implications for whole-ecosystem water, carbon and nutrient balance in Neotropical savannas.

Soil water potential determines the presence of hydraulic lift of Populus euphratica Olivier across growing seasons in an arid desert region

“…SWP threshold referred to the marginal level at which of plant roots releasing water into its adjacent soils in HL. It was believed that the magnitude of SWP threshold was mainly determined by the relative potential differences between RWP and SWP (Caldwell et al 1998;Horton, Hart 1998;Brooks et al 2006;Scholz et al 2008). Due to the difference in response of varieties on soil depth between RWP and SWP, SWP threshold of HL would change among soil layers.…”

“…Within the HL process, it may be not enough water potential difference to lift water from deep soil layers or groundwater to shallow soil layers due to transpiration shutoff at night (Prieto et al 2012). The fluctuations in fine roots distribution, plant requirements in water and minerals might determine the changes in HL across plant growing season (Scholz et al 2008;Prieto et al 2012;Espeleta et al 2014). It was reported minerals and water mainly stored in shallow and deep soil layers in the arid desert region, respectively (Kramer, Boyer 1995;Horton, Hart 1998;Yang, Lü 2011;Yu et al 2013).…”

“…In the process of HL, water is being lifted from the moist soil layers to the drier layers via plant roots (Richards, Caldwell 1987;Prieto et al 2012Prieto et al , 2016, suggesting that the differences in water potential might exist among soil layers and plant roots (Horton, Hart 1998;Prieto et al 2012). Based on the modeling and theoretical research, many studies speculated that water potentials differed among soil layers, and between plant roots and its adjacent soil layers might determine the presence of HL (Kramer, Boyer 1995;Caldwell et al 1998;Scholz et al 2008). However, due to the environmental and experimental restrictions, many empirical studies did not observed the differences in water potential between soils and plant roots during the presence of HL (Scholz 2002;Hao 2009;Prieto et al 2012).…”

“…For a given soil layer, owing to plant roots releasing water into their adjacent soils at the moment of the present of HL (Horton, Hart 1998;Prieto et al 2012), SWP might be lower than an SWP threshold. It is increasingly being reported that the magnitude of SWP threshold of HL depends on the relative relationship between the roots water potential (RWP) and SWP (Kramer, Boyer 1995;Caldwell et al 1998;Scholz et al 2008). Due to the water transportation from plant roots to stem and leaves, previous studies indicated that RWP decreases from deep roots to shallow roots in the plant (Zimmermann 1984;Kramer, Boyer 1995).…”

Plant- and stand-level variation in biophysical and physiological traits along tree density gradients in the Cerrado

“…Leaves of many deciduous and evergreen tree species senesce and drop during the dry season. Because of low water content in the upper soil layers during this period (Bucci et al, 2008;Scholz et al, 2008b), organic matter decomposition mainly occurs after the first large rainy events at the beginning of the wet season. Litter is rapidly decomposed and mineralizing nutrients are rapidly absorbed by roots or immobilized by microbial biomass (Garofalo, 2001).…”