Parameterizing a model of Douglas fir water flow using a tracheid-level model

The theory of tree water flow proposed in Aumann & Ford (submitted) is assessed by numerically solving the model developed from this theory under a variety of functional parameterizations. The unknown functions in this nonlinear partial differential equation model are determined using a tracheid-level model of water flow in a block of Douglas fir tracheids. The processes of flow, cavitation, pit aspiration/deaspiration, flow through the cell wall and ray exudation in a block of approximately 79 000 tracheids are modeled. Output from the tracheid model facilitates determination of the hydraulic conductivities in the sapwood as a function of saturation and interfacial area between liquid and gaseous phases of water, the function governing the rate of change in saturation, and the function governing the rate of change in interfacial area. The models show complementary things. The tracheid model shows that capacitance, or the change in saturation per change in pressure, is not constant. When all refilling is stopped, it takes over 180 days for the hydraulic conductivity in the vertical direction to reach 1/4 of its maximal value, showing the robustness of the transpiration stream for conducting water. The shape of the functions determined with the tracheid model change with different tracheid-level assumptions. When these functions are used in the differential equation model, it is shown that cell-wall conductivity plays an important part in the lag in flow observed in many conifers. The flow velocities and rates of change in saturation predicted by the differential equation model agree with those measured in Douglas fir. Both models support the theory of tree water flow presented in Aumann & Ford (submitted) and undermine the theory that water flow in trees is analogous to the flow of current in electric circuits.     

Whole-tree water transport scales with sapwood capacitance in tropical forest canopy trees

The present study examines the manner in which several whole‐tree water transport properties scale with species‐specific variation in sapwood water storage capacity. The hypothesis that constraints on relationships between sapwood capacitance and other water relations characteristics lead to predictable scaling relationships between intrinsic capacitance and whole‐tree behaviour was investigated. Samples of sapwood from four tropical forest canopy tree species selected to represent a range of wood density, tree size and architecture, and taxonomic diversity were used to generate moisture release curves in thermocouple psychrometer chambers, from which species‐specific values of sapwood capacitance were calculated. Sapwood capacitance was then used to scale several whole‐tree water transport properties determined from measurements of upper branch and basal sap flow, branch water potential, and axial and radial movement of deuterated water (D₂O) injected into the base of the trunk as a tracer. Sapwood capacitance ranged from 83 to 416 kg m^?3 MPa^?1 among the four species studied and was strongly correlated with minimum branch water potential, soil‐to‐branch hydraulic conductance, daily utilization of stored water, and axial and radial movement of D₂O. The species‐independent scaling of several whole‐tree water transport properties with sapwood capacitance indicated that substantial convergence in plant function at multiple levels of biological organization was revealed by a simple variable related to a biophysical property of water transport tissue.     

Hydrodynamics of vertical subsurface flow constructed wetlands: Tracer tests with rhodamine WT and numerical modelling

Typical unsteady unsaturated conditions can profoundly affect the hydrodynamics of vertical subsurface flow constructed wetlands. In this study we analyzed the hydrodynamics of a 33 m² vertical flow pilot plant, treating municipal secondary effluents. Three different saturation conditions were analyzed under several constant flux regimes: complete saturation, partial saturation with the free water table 20 cm over the bottom of the bed, and complete drainage. Tracer tests were performed in steady state conditions by dosing rhodamine WT as square input signals. Breakthrough curves were analyzed by means of both a classical residence time distribution analysis and an originally developed numerical plug-flow model with longitudinal dispersion adapted to the unsaturated conditions. We found that the degree of global mixing in the vertical flow constructed wetland increased as the water content increased; this effect was controlled by the hydraulic residence time of the system. Conversely, the degree of local mixing was inversely affected by water content; the dispersivity was 4.5, 10, and 14 cm for fully saturated, partially saturated and draining conditions, respectively. We explain the dependency of dispersivity on water content in physical terms; however, further studies are needed to mathematically include this relationship in numerical models that describe the behaviour of vertical flow constructed wetlands.     

Relations between water content, potential and permeability in stems of conifers

Abstract. The influence of sapwood water content on the conductivity of sapwood to water was measured on stem sections of Pinus contorta. A reduction in relative water content from 100 to 90% caused permeability to fall to about 10% of the saturated value.
Pressure–volume curves of branchwood and stem sapwood of Pinus contorta and Picea sitchensis have been analysed to definè the tissue capacitance and the time constant and resistance for water movement between stored water and the functional xylem as functions of tissue water potential. Three phases in water loss were discernible. In the initial phase at high water potentials (> –0.5 MPa), the capacitance was large, the time constant long and the resistance to flow large in comparison with intermediate water potentials (−0.5 to −1.5 MPa). At still lower water potentials (−1.5 to −3.0 MPa), the time constant and resistance declined still further but the capacitance had a tendency to increase again, especially in the stemwood of Sitka spruce. Typical values in the second phase were for the time constant 5 s, for the resistance 4 × 10⁻¹³ N s m⁻⁵ and for the capacitance (change in relative water content per unit change in potential) 1×10⁻¹¹ m³ Pa⁻¹. These parameters define the availability of stored water and are being used in a dynamic model of water transport in trees.     

Biophysical properties and functional significance of stem water storage tissues in Neotropical savanna trees

Biophysical characteristics of sapwood and outer parenchyma water storage compartments were studied in stems of eight dominant Brazilian Cerrado tree species to assess the impact of differences in tissue capacitance on whole-plant water relations. The rate of decline in tissue water potential with relative water content (RWC) was greater in the outer parenchyma than in the sapwood for most of the species, resulting in tissue-and species-specific differences in capacitance. Sapwood capacitance on a tissue volume basis ranged from 40 to 160 kg m-3 MPa-1, whereas outer parenchyma capacitance ranged from 25 to only 60 kg m-3 MPa-1. In addition, osmotic potentials at full turgor and at the turgor loss point were more negative for the outer parenchyma compared with the sapwood, and the maximum bulk elastic modulus was higher for the outer parenchyma than for the sapwood. Sapwood capacitance decreased linearly with increasing sapwood density across species, but there was no significant correlation between outer parenchyma capacitance and tissue density. Midday leaf water potential, the total hydraulic conductance of the soil/leaf pathway and stomatal conductance to water vapour (gs) all increased with stem volumetric capacitance, or with the relative contribution of stored water to total daily transpiration. However, the difference between the pre-dawn water potential of non-transpiring leaves and the weighted average soil water potential, a measure of the water potential disequilibrium between the plant and soil, increased asymptotically with total stem capacitance across species, implying that overnight recharge of water storage compartments was incomplete in species with greater capacitance. Overall, stem capacitance contributes to homeostasis in the diurnal and seasonal water balance of Cerrado trees.     

Virus movement in soil during saturated and unsaturated flow

Virus movement in soil during saturated and unsaturated flow was compared by adding poliovirus to sewage water and applying the water at different rates to a 250-cm-long soil column equipped with ceramic samplers at different depths. Movement of viruses during unsaturated flow of sewage through soil columns was much less than during saturated flow. Viruses did not move below the 40-cm level when sewage water was applied at less than the maximum infiltration rate; virus penetration in columns flooded with sewage was at least 160 cm. Therefore, virus movement in soils irrigated with sewage should be less than in flooded groundwater recharge basins or in saturated soil columns. Management of land treatment systems to provide unsaturated flow through the soil should minimize the depth of virus penetration. Differences in virus movement during saturated and unsaturated flow must be considered in the development of any model used to simulate virus movement in soils.     

土壤-植物系统水分运移过程的阻容电模拟

把土壤-植物系统水分运移作为一维水流运动由阻容电路进行模拟,在于将D arcy-R ichards方程从对单点的描述扩展到对一段流路的描述。由此出发,考虑到水流的非稳态性,某一流路的水阻定义为其水势差与平均流量之比,水容为其贮水量对平均水势的导数。与D arcy-R ichards方程相对应,水阻、时间常数分别为导水度、水分扩散度的倒数,相应地单位化的水阻率、比时间常数分别为导水率、水分扩散率的倒数。把SP系统沿水流通道分为若干部分,每一局部的水阻与其水容相并联,各局部间相串联。在此基础上,文章给出了土壤-植物系统水流模拟通式、总水容与分水容间的关系式、总水阻与分水阻间的关系式及特定条件下叶水势随时间变化的关系式。     

黄土丘陵区辽东栎树干液流特征对边材面积和土壤水分的响应

运用Granier热扩散探针法对半干旱黄土丘陵区不同胸径辽东栎进行树干液流测定,并对太阳辐射、空气温湿度、降水量、土壤水分等环境因子进行同步观测,分析不同土壤水分条件下不同胸径辽东栎的树干液流变化特征及其对环境因子的响应.结果表明:辽东栎液流日变化特征总体上与太阳辐射和空气水汽压亏缺呈相同趋势,但液流峰值出现时间早于两个气象环境因子的峰值时间.同一树木个体在土壤水分条件较高时期的树干液流通量高于土壤水分较低时期.在相同土壤水分条件下,大径级样本液流通量显著高于小径级样本.采用指数饱和曲线函数对液流通量与太阳辐射和空气水汽压亏缺以及两因子的综合指标进行拟合,效果良好,可以反映液流通量对气象环境因子的响应规律.不同胸径辽东栎在不同土壤水分条件下的拟合曲线特征和拟合参数差异表明,在土壤水分较高时段,液流通量可快速上升至饱和值;在土壤水分较低时段,液流通量上升缓慢.小径级样本对土壤水分变化的反应更加剧烈.单位空气水汽压亏缺的日液流通量值(日液流通量与空气水汽压亏缺的比值)在两种土壤水分条件下的比值与边材面积呈线性相关,且小径级样本的斜率高于大径级样本,说明小径级样本对土壤水分的变化较为敏感,在土壤含水量较低时段,大径级样本较厚的导水组织对土壤水分供应不足起到了缓冲作用.     

Virus movement in soil during saturated and unsaturated flow.

Virus movement in soil during saturated and unsaturated flow was compared by adding poliovirus to sewage water and applying the water at different rates to a 250-cm-long soil column equipped with ceramic samplers at different depths. Movement of viruses during unsaturated flow of sewage through soil columns was much less than during saturated flow. Viruses did not move below the 40-cm level when sewage water was applied at less than the maximum infiltration rate; virus penetration in columns flooded with sewage was at least 160 cm. Therefore, virus movement in soils irrigated with sewage should be less than in flooded groundwater recharge basins or in saturated soil columns. Management of land treatment systems to provide unsaturated flow through the soil should minimize the depth of virus penetration. Differences in virus movement during saturated and unsaturated flow must be considered in the development of any model used to simulate virus movement in soils.     

Granier树干液流测定系统在马占相思的水分利用研究中的应用

介绍了Granier热消散探针在树干液流测定中的工作原理,并利用该系统长期监测广东鹤山马占相思林14株样树的液流密度,分析了树木个体内和个体之间液流密度的差异、整树和林段水分利用的量化特征.由于树木边材结构以及周围微环境的差别,树木内和个体间的液流密度差异非常明显,变异系数的平均值分别为15.51%-37.26%、37.46%-50.73%.尽管液流密度的差异较大,但同一株树木不同方位的液流密度之间却呈现明显的线性相关（p＜0.0001）,这是重要的特征值,使得只需测定某一方位的液流密度经尺度外推计算整树和林段蒸腾成为可能.树木液流对环境因子响应的变化规律取决于所参照的时间尺度,日变化主要受光辐射、水汽压差等气候因子的控制,而土壤水份对液流的季节变化影响较大.形态特征明显影响树木的液流,高大树木由于边材较厚、树干粗壮和冠幅较宽而承载较多的辐射能量,因而水分蒸腾较高.对树木液流密度在径向和方位上进行适当的整合,可较准确地计算整树和林段蒸腾.由液流估测的马占相思整树和林段蒸腾的结果显示,该群落的水分利用在时间和空间上均有明显的分化.     

Development of heartwood in response to water stress for radiata pine in Southern New South Wales, Australia

Heartwood development and other functional changes in stem conductance in response to water stress in radiata pine were investigated using two contrasting climatic areas (high-altitude sub-alpine vs. warm–dry inland) of the Hume region of New South Wales, Australia. The study included mature (34.5–36.5 years old) and young stands (10–11 years old) measured under normal climate and during an extreme drought. The effect of water stress on heartwood development was examined using sapwood percentage, sapwood saturation, development of dry sapwood and evidence of cavitation in sapwood. Trees at the warm–dry site developed heartwood at faster rates than on the high-altitude site. At breast height, the mature stands of the warm–dry site had 8–14 % less sapwood. Extensive cavitation towards the sapwood/heartwood boundary occurred in some of the mature and young stands on the warm–dry site. We postulated that in water-limiting environments, cavitation of the inner sapwood precedes heartwood formation and is an adaptation mechanism that regulates stem conductance capacity and thus water use in the tree. The drought of 2006 led to decreases in moisture associated with cavitation not previously reported for radiata pine and demonstrated the drought hardiness of the species. In the warm–dry site, breast-height sapwood saturation dropped to 58 and 82 % for suppressed and average-sized trees in a mature unthinned stand; and 75–78 % for two young stands. These saturation levels, however, only imply average values as some cells cavitated whilst others were fully saturated. Cavitation occurred in a localized fashion affecting small to large groups of cells.     

Sapflow+: a four-needle heat-pulse sap flow sensor enabling nonempirical sap flux density and water content measurements

? To our knowledge, to date, no nonempirical method exists to measure reverse, low or high sap flux density. Moreover, existing sap flow methods require destructive wood core measurements to determine sapwood water content, necessary to convert heat velocity to sap flux density, not only damaging the tree, but also neglecting seasonal variability in sapwood water content. ? Here, we present a nonempirical heat-pulse-based method and coupled sensor which measure temperature changes around a linear heater in both axial and tangential directions after application of a heat pulse. By fitting the correct heat conduction-convection equation to the measured temperature profiles, the heat velocity and water content of the sapwood can be determined. ? An identifiability analysis and validation tests on artificial and real stem segments of European beech (Fagus sylvatica L.) confirm the applicability of the method, leading to accurate determinations of heat velocity, water content and hence sap flux density. ? The proposed method enables sap flux density measurements to be made across the entire natural occurring sap flux density range of woody plants. Moreover, the water content during low flows can be determined accurately, enabling a correct conversion from heat velocity to sap flux density without destructive core measurements.     

Axial and radial profiles in conductivities, water storage and native embolism in trunks of young and old-growth ponderosa pine trees

Ponderosa pine has very wide sapwood, and yet the spatial and temporal use of that sapwood for water transport is poorly understood. Moreover, there have been few comparisons of function in tips of old-growth trees in comparison with young trees. In the present study, axial and radial specific conductivity (k_s), leaf specific conductivity (LSC), leaf specific conductance (k_l), native embolism and the compartmentalization of sapwood water storage were characterized in trunks of young and old-growth trees. Trunks of young trees had lower k_s, lower LSC and lower native embolism [corresponding to 5% loss of conductivity (PLC)] than trunks of old-growth trees. However, k_l in young trees was 3.5 times higher than in old-growth trees, supporting the hypothesis that tall trees have a reduced ability to transport water to their leaves. Water storage (capacitance) of young trees was not significantly different than at the base of old-growth trees. Although the top of the old-growth trees had similar k_s, LSC and k_l to the young trees for a given cambial age, they had higher native embolism and lower capacitance. There was no trade-off between k_s and native embolism at any height. In the tree crown, outer sapwood had 35–50% higher k_s than the inner sapwood and 17–25 PLC lower native embolism. At the base of the old trees, there was no significant difference in native embolism between the outer, middle and inner sapwood, showing that refilling of embolisms was complete despite the 130-year difference in wood age among these radial positions. Although during the dry season the inner sapwood tended to be more saturated than the outer sapwood, the outer part of the sapwood contributed up to 60% of the overall stored water. Safer xylem, higher capacitance and higher k_l would appear adaptive in the young trees for regulating their water resource, which is likely to be less reliable than the water availability of older trees with their more developed root system.     

Enhancement of seepage and lateral preferential flow by biopores on hillslopes

Natural soils are generally populated with a wide variety of macropores formed from physical processes and/or biological activity. These macropores can have a large influence on the lateral flow of water in hillslope soils even when those macropores are not continuous or connected directly to ponded water. The concept of self-organization of flow paths described by Sidle et al. (2001) is analyzed through numerical simulation of variably-saturated flow in a large cylinder of soil containing a population of disconnected macropores. It is demonstrated that there is a threshold water pressure at which the macropores will become active, and above this threshold the then active network of macropores significantly increases the effective conductance of the soil volume. In the case examined here the increase exceeded 40%. The analysis presented provides a context for the explanation of soil pipe formation by the process of seepage erosion. An analogy is drawn between percolation theory in porous media and the concept of self-organization of flow pathways at the hillslope scale.     

A one-dimensional model of water flow in soil-plant systems based on plant architecture

The estimation of root water uptake and water flow in plants is crucial to quantify transpiration and hence the water exchange between land surface and atmosphere. In particular the soil water extraction by plant roots which provides the water supply of plants is a highly dynamic and non-linear process interacting with soil transport processes that are mainly determined by the natural soil variability at different scales. To better consider this root-soil interaction we extended and further developed a finite element tree hydro-dynamics model based on the one-dimensional (1D) porous media equation. This is achieved by including in addition to the explicit three-dimensional (3D) architectural representation of the tree crown a corresponding 3D characterisation of the root system. This 1D xylem water flow model was then coupled to a soil water flow model derived also from the 1D porous media equation. We apply the new model to conduct sensitivity analysis of root water uptake and transpiration dynamics and compare the results to simulation results obtained by using a 3D model of soil water flow and root water uptake. Based on data from lysimeter experiments with young European beech trees (Fagus silvatica L.) is shown, that the model is able to correctly describe transpiration and soil water flow. In conclusion, compared to a fully 3D model the 1D porous media approach provides a computationally efficient alternative, able to reproduce the main mechanisms of plant hydro-dynamics including root water uptake from soil.     

Cavitation and water storage capacity in bole xylem segments of mature and young Douglas-fir trees

Hydraulic specific conductivity, vulnerability to cavitation and water storage capacity of Douglas-fir sapwood was determined for samples from six young (1.0-1.5 m tall) and six mature trees (41-45 m tall). Measurements on samples from young trees showedthere were no effects of two contrasting sample types (entire stem segments vs sectors chiseled out of entire stems) on any of the calculated hydraulic parameters, for vulnerability to cavitation and water storage capacity. Measurements on mature trees were made on wood from four heights on the bole and from two sapwood depths. Outer and inner sapwood at the base of the tree had higher water storage capacities and were more vulnerable to cavitation than was sapwood from the tree top. At every height, old trees were more vulnerable to cavitation than at 1.0 m from the ground in young trees. The water storage capacities showed three distinct phases at the base of the trunk. Young trees had similar water storage capacity (per unit volume of sapwood) to the topof the mature trees, which was lower than the water storage capacity throughout the rest of the bole xylem. The way in which capacitance was calculated (on a volumetric basis vs a relative water content basis) affected the conclusion one would draw at the low water potentials (<-3 MPa). Within a tree, we found an apparent trade-off between having both hydraulic specific conductivity and stem water storage, and vulnerability to cavitation.     

Comparison between gradient-dependent hydraulic conductivities of roots using the root pressure probe: the role of pressure propagations and implications for the relative roles of parallel radial pathways

Hydrostatic pressure relaxations with the root pressure probe are commonly used for measuring the hydraulic conductivity (Lp(r)) of roots. We compared the Lp(r) of roots from species with different root hydraulic properties (Lupinus angustifolius L. 'Merrit', Lupinus luteus L. 'Wodjil', Triticum aestivum L. 'Kulin' and Zea mays L. 'Pacific DK 477') using pressure relaxations, a pressure clamp and osmotic gradients to induce water flow across the root. Only the pressure clamp measures water flow under steady-state conditions. Lp(r) determined by pressure relaxations was two- to threefold greater than Lp(r) from pressure clamps and was independent of the direction of water flow. Lp(r) (pressure clamp) was two- to fourfold higher than for Lp(r) (osmotic) for all species except Triticum aestivum where Lp(r) (pressure clamp) and Lp(r) (osmotic) were not significantly different. A novel technique was developed to measure the propagation of pressure through roots to investigate the cause of the differences in Lp(r). Root segments were connected between two pressure probes so that when root pressure (P(r)) was manipulated by one probe, the other probe recorded changes in P(r). Pressure relaxations did not induce the expected kinetics in pressure in the probe at the other end of the root when axial hydraulic conductance, and probe and root capacitances were accounted for. An electric circuit model of the root was constructed that included an additional capacitance in the root loaded by a series of resistances. This accounted for the double exponential kinetics for intact roots in pressure relaxation experiments as well as the reduced response observed with the double probe experiments. Although there were potential errors with all the techniques, we considered that the measurement of Lp(r) using the pressure clamp was the most unambiguous for small pressure changes, and provided that sufficient time was allowed for pressure propagation through the root. The differences in Lp(r) from different methods of measurement have implications for the models describing water transport through roots and the potential role of aquaporins.     

Evaluation of the heat pulse velocity method for measuring sap flow in Pinus patula

Information on the water use of Pinus patula plantations isrequired to predict the impact of forest plantations on waterresources in South Africa. The heat pulse velocity (HPV) methodis a promising technique for measuring water use by trees, andhas been shown to measure sap flows accurately in a varietyof hardwood trees. This method has not been sufficiently verifiedfor pine trees where the presence of a strongly-defined ringstructure in the sapwood gives rise to a complex radial patternof sap flow. The purpose of this study was to compare wateruptake by cut trees to simultaneous HPV sap flow measurementsin the same tree. Fourteen trees were used for this comparison.Results showed that HPV sap flow estimates consistently overestimatedcut-tree uptake by an average of 49%. The bias is attributedto heat averaging across non-conducting latewood rings. Wateruptake was found to be highly correlated to the product of under-barkcross-sectional area and wound-corrected mean HPV, and it issuggested that this empirical relation provides a more appropriateway of estimating water use by this species. Key words: Heat pulse velocity, sap flow, Pinus patula, transpiration     

Estimating tree and stand sapwood area in spatially heterogeneous southeastern Australian forests

Aims Natural and anthropogenic changes in forests can have important influences on transpiration and water production. Understanding the effects of increasing disturbances, due for example to climate change and forest harvesting, requires detailed information on how forest density and structural attributes relate to transpiration. Mean annual transpiration of eucalypt forest communities is often strongly correlated with total cross-sectional sapwood area. Our aim was to test an efficient method for estimating sapwood area at 1.3 m height (SA 1.3) in a large number of trees to understand the spatial heterogeneity of tree and stand sapwood area within and between forest communities, and develop allometric relationships that predict SA 1.3 with forest inventory data. We also apply tree competition models to determine the degree to which the relationship between SA 1.3 and tree basal area at 1.3 m height (BA 1.3) is influenced by competition.Methods We visited 25 recently harvested southeastern Australian forest sites consisting of 1379 trees and 5 Eucalyptus species to evaluate a new efficient data collection method for estimating SA 1.3 with tree taper and stump dimensions data using mixed effects models. The locations of 784 stumps within one 5-ha site were accurately mapped using an unmanned aerial vehicle (UAV), and four distance-dependent tree competition models were applied across the site to explain within-stand variation in the ratio of SA 1.3 to BA 1.3. Data from 24 additional sites, consisting of ten 15 m radial plots per site, were used to analyse within-site variation in R Ha (the ratio of stand sapwood area SA Ha to stand basal area BA Ha). The radial plots were merged within each site to evaluate between-site variations in R Ha across the landscape. For predicting SA Ha with forest inventory data, we computed the relationship between SA Ha and a new index of total stem perimeter per hectare, defined as ? B A H a N T, where N T is tree stocking density.Important findings Our 1379 measured stems represent the most comprehensive measure of sapwood area, surpassing the 757 measured stems in native eucalypt forests published in literature. The species-specific R Ha varied considerably across sites and therefore extrapolating SA Ha with spatially distributed BA Ha maps and a generalized R Ha would introduce local uncertainty. We found that the species-specific stem perimeter index was more effective at capturing variability in SA Ha across the landscape using forest composition, structure and density data (R 2 : 0.72–0.77). The strong correlation between tree SA 1.3 and BA 1.3 improved slightly using tree competition models (R 2 increased from 0.86 to 0.88). Relating SA Ha to routinely measured forest inventory attributes within permanent plots and Light Detection and Ranging (LiDAR) data may provide opportunities to map forest water use in time and space across large areas disturbed by wildfire and logging.