Water Storage of Plants Cannot Be Ignored During The Process of Water Movement

Water migration and use are important processes in trees. However, it is possible to overestimate transpiration by equating the water absorbed by plant roots with that diffused back to the atmosphere via the stomata. The δ2H/δ18O technique and heat ratio method were used to explore the patterns of water use of coniferous and broad-leaved tree species to determine the proportions of water used for transpiration and water storage. Our results indicate that both species showed strong plasticity in their use of water sources. The species Platycladus orientalis (Oriental arbor-vitae) and Quercus variabilis (Chinese cork oak) primarily absorbed water from groundwater and the 60–100 cm soil layer, and Q. variabilis also absorbed water from the 0–20 cm and 20–40 cm soil layers during the dry season. Only P. orientalis was sensitive to precipitation and used water from the 0–20 cm layers. Q. variabilis did not change its water source but increased its uptake of groundwater during the rainy season. We observed reverse flow and hydraulic redistribution in P. orientalis, which alleviated the spatial heterogeneity of soil water and provided water for neighboring trees. Nocturnal sap flow in P. orientalis and Q. variabilis facilitated the storage of water in the trunk. The water used for storage in both species comprised 6–7% of the total quantity and therefore, should be considered in water balance models.

New Representation of Plant Hydraulics Improves the Estimates of Transpiration in Land Surface Model

Transpiration represents more than 30% of the global land–atmosphere water exchange but is highly uncertain. Plant hydraulics was ignored in traditional land surface modeling, but recently plant hydraulics has been found to play an essential role in transpiration simulation. A new physical-based representation of plant hydraulic schemes (PHS) was recently developed and implemented in the Common Land Model (CoLM). However, it is unclear to what extent PHS can reduce these uncertainties. Here, we evaluated the PHS against measurements obtained at 81 FLUXNET sites. The transpiration of each site was estimated using an empirical evapotranspiration partitioning approach. The metric scores defined by the International Land Model Benchmarking Project (ILAMB) were used to evaluate the model performance and compare it with that of the CoLM default scheme (soil moisture stress (SMS)). The bias score of transpiration in PHS was higher than SMS for most sites, and more significant improvements were found in semi-arid and arid sites where transpiration was limited by soil moisture. The hydraulic redistribution in PHS optimized the soil water supply and thus improved the transpiration estimates. In humid sites, no significant improvement in seasonal or interannual variability of transpiration was simulated by PHS, which can be explained by the insensitivity of transpiration demand coupled to the photosynthesis response to precipitation. In arid and semi-arid sites, seasonal or interannual variability of transpiration was better captured by PHS than SMS, which was interpreted by the improved drought sensitivity for transpiration. Arid land is widespread and is expected to expand due to climate change, thus there is an urgent need to couple PHS in land surface models.

Prediction model for sap flow in cacao trees under different radiation intensities in the western Colombian Amazon

In this study, we measured diurnal patterns of sap flow (Vs) in cacao trees growing in three types of agroforestry systems (AFs) that differ in the incident solar radiation they receive. We modeled the relationship of Vs with several microclimatic characteristics of the AFs using mixed linear models. We characterized microclimatic variables that may have an effect on diurnal patterns of sap flow: air relative humidity, air temperature, photosynthetically active radiation and vapor pressure deficit. Overall, our model predicted the differences between cacao Vs in the three different AFs, with cacao plants with dense Musaceae plantation and high mean diurnal incident radiation (HPAR) displaying the highest differences compared to the other agroforestry arrangements. The model was also able to predict situations such as nocturnal transpiration in HPAR and inverse nocturnal sap flows indicative of hydraulic redistribution in the other AFs receiving less incident radiation. Overall, the model we present here can be a useful and cost-effective tool for predicting transpiration and water use in cacao trees, as well as for managing cacao agroforestry systems in the Amazon rainforest.

Hydraulic redistribution by hybrid poplars (Populus nigra x Populus maximowiczii) in a greenhouse soil column experiment

Aims Hydraulic redistribution (HR) enhances water resources for neighboring crops in silvopastoral agroforestry (AF). Here, we tested whether and to what extent water stressed shallow-rooted neighboring plants benefit from water redistributed by deep-rooted poplar plants. Methods We conducted trace experiments with deuterated water (2H2O) in greenhouse soil column experiments. We measured hydraulic lift (HL) by poplars grown at two levels of soil drying and estimated the amount of hydraulically lifted water. In a parallel experiment we grew poplars and barley (Hordeum vulgare) in two columns connected via a small cross-rooting segment. Results Soil moisture measurements and stable isotope signatures of soil and xylem water proved the occurrence of HL in poplar. Additionally, stable isotopes proved the transport of water from deep roots of poplars to shallow roots of barley. Conclusions In conclusion, the experiments showed that poplars are capable to redistribute water during drought spells and that this water can facilitate plant growth of shallow-rooted crops. This result implies evidence for an enhanced soil water supply of plants in agroforest systems under drought conditions.

Interspecific Variation in the Timing and Magnitude of Hydraulic Redistribution in a Forest With Distinct Water Sources

Aims Trees regulate water availability among their rooting strata through a nocturnal, passive transference of water known as hydraulic redistribution (HR). This study investigates differences in HR and groundwater use among common canopy species in longleaf pine ( Pinus palustris Mill., Pinaceae) woodlands and explores environmental factors influencing HR. Methods HR was estimated by sap flux of lateral roots and main stems of three mature canopy species ( P. palustris , Quercus laevis Walter., Fagaceae and Quercus margarettae Ashe., Fagaceae). We used δ 18 O and δD of xylem water, soil water, and groundwater to determine water source. Finally, we related HR to environmental factors (Temperature, VWC, VPD) to better understand controls of HR dynamics. Results Pinus palustris had higher water use than either Quercus species, and also redistributed significantly more water as a nocturnal subsidy. HR fluxes were inversely related with mean daily temperature and independent of shallow soil moisture. Stable isotope mixing models, based on δ 18 O and δD, indicated that all species have access to groundwater, but utilized shallow soil water in differing amounts when available. Conclusions In systems with strong water potential gradients among soil strata, any species with access to a groundwater source is likely capable of HR; however, the magnitude of HR varies significantly by species, even among closely related taxa.

An undiscovered facet of hydraulic redistribution driven by evaporation—A study from a Populus tomentosa plantation

Maintenaining the activity and function of the shallow root system of plants is essential for withstanding drought stress, but the associated mechanism is poorly understood. By investigating sap flow in 14 lateral roots (LRs) randomly selected from trees of a Chinese white poplar (Populus tomentosa) plantation receiving three levels of irrigation, an unknown root-water transport mode of simultaneous daytime bi-directional water flow was discovered. This mode existed in five LRs confined to the surface soil without attached sinker roots. In the longer term, the bi-directional water flow was correlated with the soil water content. However, within the day, it was associated with transpiration. Our data demonstrated that bi-directional root sap flow occurred during the day, and was driven by evaporative demand, further suggesting the existence of circumferential water movement in the LR xylem. We named this phenomenon evaporation-driven hydraulic redistribution (EDHR). A soil-root water transport model was proposed to encapsulate this water movement mode. EDHR may be a crucial drought-tolerance mechanism that allows plants to maintain shallow root survival and activity by promoting root water recharge under extremely dry conditions.