Maple sap uptake, exudation, and pressure changes correlated with freezing exotherms and thawing endotherms

Sap flow rates and sap pressure changes were measured in dormant sugar maple trees (Acer saccharum Marsh.). In the forest, sap flow rates and pressure changes were measured from tap holes drilled into tree trunks in mature trees and sap flow rates were measured from the base of excised branches. Excised branches were also brought into the laboratory where air temperature could be carefully controlled in a refrigerated box and sap flow rates and sap pressures were measured from the cut base of the branches.

Under both forest and laboratory conditions, sap uptake occurred as the wood temperature declined but much more rapid sap uptake correlated with the onset of the freezing exotherm. When sap pressures were measured under conditions of negligible volume displacement, the sap pressure rapidly fell to −60 to −80 kilopascals at the start of the freezing exotherm. The volume of water uptake and the rate of uptake depended on the rate of freezing. A slow freezing rate correlated with a large volume of water uptake, a fast freezing rate induced a smaller volume of water uptake. The volume of water uptake ranged from 0.02 to 0.055 grams water per gram dry weight of sapwood. The volume of water exuded after thawing was usually less than the volume of uptake so that after several freezing and thawing cycles the sapwood water content increased from 0.7 to 0.8 grams water per gram dry weight.

These results are discussed in terms of a physical model of the mechanism of maple sap uptake and exudation first proposed by P. E. R. O'Malley. The proposed mechanism of sap uptake is by vapor distillation in air filled wood fiber lumina during the freezing of minor branches. Gravity and pressurized air bubbles (compressed during freezing) cause sap flow from the canopy down the tree after the thaw.

     

大连4种城市绿化乔木树种夜间液流活动特征

夜间液流有助于树木物质运输及其体内水分的补充(water recharge), 它不仅对植物的生长发育具有重要的生理生态学意义, 而且对大尺度植物蒸腾耗水的估算可能产生重要影响。2008年6月1日至8月31日, 以热扩散探针(thermal dissipation probe, TDP)技术对大连市劳动公园内的雪松(Cedrus deodara)、大叶榉(Zelkova schneideriana)、丝棉木(Euonymus bungeanus)和水杉(Metasequoia glyptostroboides) 4种乔木树种的不同径阶样木树干边材液流进行了测定, 并结合同步土壤水分与小气候观测结果分析了树木夜间(18:00至次日5:00)液流特征。实验结果表明, 树木普遍存在可感夜间液流, 夜间液流总量占观测期液流总量的比例在样木个体间呈现显著差异, 其变化范围为0.44%-75.96%; 观测期雨天夜间液流波动活跃, 显著高于晴天, 其单日夜间液流总量可持平, 甚至高于日间液流。相关分析表明: 水汽压亏缺(vapor pressure deficit, VPD)和风速的变化与夜间蒸腾显著相关, 它们能够较好地解释液流变化(R² > 0.6); 树木夜间液流主要用于夜间蒸腾和自身水分补充, 夜间液流现象主要发生在前半夜, 后半夜液流平稳且极接近0, 夜间液流量与相应的日间流量(R² = 0.356, p = 0.00)及胸径(R²_Spearman > 0.80)显著相关, 说明植物本身的结构和生理特点也是影响树木夜间液流的重要因子。单株样木夜间液流占全天总蒸腾量的比例低于14.4%, 如不考虑夜间液流的影响, 根据日间液流通过尺度扩展推算的森林生态系统年蒸腾量可能偏低。     

Nocturnal sap flow in the C<Subscript>3</Subscript>-CAM species, <Emphasis Type="Italic">Clusia minor</Emphasis>

Clusia minor L. is a C₃-CAM species in which Crassulacean acid Metabolism (CAM) is induced, among other factors, by water deficit. We propose that CAM induction by natural drought in C. minor shifts the sap flow pattern from daytime to a night-time one, and that the decreased osmotic potential due to increased malate content in droughted plants aids in the increase in nocturnal sap flow. In order to test these hypotheses, we followed for 2 years the seasonal changes in parameters of water relationships and sap flow velocity in one single, freestanding tree growing in Caracas. Leaf water and osmotic potential were measured psychrometrically, nocturnal proton accumulation by titration of aqueous leaf extracts and sap flow density with thermal dissipation probes. Leaf water, osmotic and turgor potential remained relatively high throughout the seasons. Nocturnal proton accumulation was nil under extreme drought or after frequent and heavy rains, and high after moderate rainfall. Estimated malate and citrate concentrations contributed up to 80 and 60%, respectively, of the value of osmotic potential. The shape of the daily courses of sap flow velocity varied seasonally, from mostly diurnal during the dry season to mostly nocturnal after a short dry spell during the rainy season, when nocturnal acid accumulation attained high values. There was a strong positive relationship between the proportion of the integrated sap flow courses corresponding to the night and dawn [H⁺] (r ² = 0.88). Increased nocturnal sap flow in the CAM stage of the tree of C. minor may be explained by a lower osmotic potential due to an increased acid concentration, together with increased stomatal aperture, as suggested by increased nocturnal acid accumulation probably due to nocturnal CO₂ fixation.     

Transient response of sap flow to wind speed

Transient responses of sap flow to step changes in wind speedwere experimentally investigated in a wind tunnel. A Granier-typesap flow sensor was calibrated and tested in a cylindrical tubefor analysis of its transient time response. Then the sensorwas used to measure the transient response of a well-wateredPachira macrocarpa plant to wind speed variations. The transientresponse of sap flow was described using the resistance–capacitancemodel. The steady sap flow rate increased as the wind speedincreased at low wind speeds. Once the wind speed exceeded 8.0m s^–1, the steady sap flow rate did not increase further.The transpiration rate, measured gravimetrically, showed a similartrend. The response of nocturnal sap flow to wind speed variationwas also measured and compared with the results in the daytime.Under the same wind speed, the steady sap flow rate was smallerthan that in the daytime, indicating differences between diurnaland nocturnal hydraulic function, and incomplete stomatal closureat night. In addition, it was found that the temporal responseof the Granier sensor is fast enough to resolve the transientbehaviour of water flux in plant tissue. Key words: Nocturnal transpiration, sap flow, transient response, wind speed Received 31 July 2008; Revised 6 October 2008 Accepted 20 October 2008     

Water and Nutrient Dynamics in Surface Roots and Soils are not Modified by Short-term Flooding of Phreatophytic Plants in a Hyperarid Desert

Little is known of the mechanisms employed by woody plants to acquire key resources such as water and nutrients in hyperarid environments. For phreatophytic plants, deep roots are necessary to access the water table, but given that most nutrients in many desert ecosystems are stored in the upper soil layers, viable shallow roots may be equally necessary for nutrient uptake. We sought to better understand the interaction between water and nutrient uptake from soil horizons differing in the relative abundance of these resources. To this end, we monitored plant water and nutrient status before and after applying flood irrigation to four phreatophytic perennial plant species in the remote hyperarid Taklamakan desert in western China. Sap flow in the roots of five plants of the perennial desert species Alhagi sparsifolia Shap., Karelina caspica (Pall.) Less., Calligonum caput medusea Schrenk, and Eleagnus angustifolia Hill. was monitored using the heat ratio method (HRM). Additionally we measured predawn and midday water potential, foliar nitrate reductase activity (NRA), xylem sap nutrient concentration and the concentration of total solutes in the leaves before, 12 and 96 h after flooding to investigate possible short-term physiological effects on water and nutrient status. Rates of sap flow measured during the day and at night in the absence of transpiration did not change after flooding. Moderately high rates of sap flow (HRM heat pulse velocity, 5–25 cm h⁻¹) detected during the day in soils that had a near zero water content at the surface indicated that all species had contact to groundwater. There was no evidence from sap flow data that plants had utilised flood water to increase maximum rates of transpiration under similar climatic conditions, and there was no evidence of a process to improve the efficiency of water or nutrient uptake, such as hydraulic redistribution (i.e. the passive movement of water from moist soil to very dry soil via roots). Measurements of plant water status, xylem sap nutrient status, foliar NRA and the concentration of osmotically active substances were also unaffected by flood irrigation. Our results clearly show that groundwater acts as the major source of water and nutrients for these plants. The inability of plants to utilise abundant surface soil–water or newly available nutrients following irrigation was attributed to the absence of fine roots in the topsoil layer.     

Linking xylem diameter variations with sap flow measurements

Measurements of variation in the diameter of tree stems provide a rapid response, high resolution tool for detecting changes in water tension inside the xylem. Water movement inside the xylem is caused by changes in the water tension and theoretically, the sap flow rate should be directly proportional to the water tension gradient and, therefore, also linearly linked to the xylem diameter variations. The coefficient of proportionality describes the water conductivity and elasticity of the conducting tissue. Xylem diameter variation measurements could thus provide an alternative approach for estimating sap flow rates, but currently we lack means for calibration. On the other hand, xylem diameter variation measurements could also be used as a tool for studying xylem structure and function. If we knew both the water tension in the xylem and the sap flow rate, xylem conductivity and/or elasticity could be calculated from the slope of their relationship. In this study we measured diurnal xylem diameter variation simultaneously with sap flow rates (Granier-type thermal method) in six deciduous species (Acer rubrum L., Alnus glutinosa Miller, Betula lenta L., Fagus Sylvatica L. Quercus rubra L., and Tilia vulgaris L.) for 7–91 day periods during summers 2003, 2005 and 2006 and analyzed the relationship between these two measurements. We found that in all species xylem diameter variations and sap flow rate were linearly related in daily scale (daily average R ² = 0.61–0.87) but there was a significant variation in the daily slopes of the linear regressions. The largest variance in the slopes, however, was found between species, which is encouraging for finding a species specific calibration method for measuring sap flow rates using xylem diameter variations. At a daily timescale, xylem diameter variation and sap flow rate were related to each other via a hysteresis loop. The slopes during the morning and afternoon did not differ statistically significantly from each other, indicating no overall change in the conductivity. Because of the variance in the daily slopes, we tested three different data averaging methods to obtain calibration coefficients. The performance of the averaging methods depended on the source of variance in the data set and none of them performed best for all species. The best estimates of instantaneous sap flow rates were also given by different averaging methods than the best estimates of total daily water use. Using the linear relationship of sap flow rate and xylem diameter variations we calculated the conductance and specific conductivity of the soil–xylem–atmosphere water pathway. The conductance were of the order of magnitude 10⁻⁵ kg s⁻¹ MPa⁻¹ for all species, which compares well with measured water fluxes from broadleaved forests. Interestingly, because of the large sap wood area the conductance of Betula was approximately 10 times larger than in other species.     

基于稳定同位素和热比率技术的侧柏水分逆向运移特征与过程

植物在一定环境条件下可通过叶片吸水发生水分逆向运移来维持自身生长,尤其是在季节性干旱地区。但这一过程通常被忽视,使得在量化理解干旱胁迫下的森林植被水分利用过程与机制方面仍存在一定的空白。本研究以北京山区为研究区,以其典型造林树种侧柏为研究对象,利用稳定同位素和热比率技术,通过野外布设对比试验和室内控制盆栽试验,分析树木体内水分逆向运移的发生条件和规律,量化逆向运移量及补给率,研究不同部位水分逆向运移变化特征及过程。结果表明: 在野外对比试验中,控制样方在雨后的树木胸径和根系处监测到逆向液流,且根系逆向液流的发生比胸径处会有所延迟,而对比样方则无逆向液流;在室内控制试验中,不同处理组在雨后2 h树木各部位逆向运移补给率达到最高值,除重度和中度干旱处理外,雨后8 h基本恢复初始状态,水分逆向运移对植物的影响一般不超过24 h;叶片吸水量与其产生的对枝条和根际土壤的逆向补给率和土壤前期含水量呈负相关关系,对叶片、枝条和根际土壤的最大补给率分别为(9.5±0.1)%、(5.9±0.3)%和(5.7%±0.6)%;在水分逆向运移过程中,侧柏不同部位水分运移对时间的响应不同。在复杂多变的水分供给条件下,研究植物水分逆向运移的过程与机制,对准确理解其生存和竞争力具有重要意义。     

Hydraulic redistribution in three Amazonian trees

About half of the Amazon rainforest is subject to seasonal droughts of 3 months or more. Despite this drought, several studies have shown that these forests, under a strongly seasonal climate, do not exhibit significant water stress during the dry season. In addition to deep soil water uptake, another contributing explanation for the absence of plant water stress during drought is the process of hydraulic redistribution; the nocturnal transfer of water by roots from moist to dry regions of the soil profile. Here, we present data on patterns of soil moisture and sap flow in roots of three dimorphic-rooted species in the Tapajós Forest, Amazônia, which demonstrate both upward (hydraulic lift) and downward hydraulic redistribution. We measured sap flow in lateral and tap roots of our three study species over a 2-year period using the heat ratio method, a sap-flow technique that allows bi-directional measurement of water flow. On certain nights during the dry season, reverse or acropetal flow (i.e.,in the direction of the soil) in the lateral roots and positive or basipetal sap flow (toward the plant) in the tap roots of Coussarea racemosa (caferana), Manilkara huberi (maçaranduba) and Protium robustum (breu) were observed, a pattern consistent with upward hydraulic redistribution (hydraulic lift). With the onset of heavy rains, this pattern reversed, with continuous night-time acropetal sap flow in the tap root and basipetal sap flow in lateral roots, indicating water movement from wet top soil to dry deeper soils (downward hydraulic redistribution). Both patterns were present in trees within a rainfall exclusion plot (Seca Floresta) and to a more limited extent in the control plot. Although hydraulic redistribution has traditionally been associated with arid or strongly seasonal environments, our findings now suggest that it is important in ameliorating water stress and improving rain infiltration in Amazonian rainforests. This has broad implications for understanding and modeling ecosystem process and forest function in this important biome.     

华北落叶松夜间树干液流特征及生长季补水格局

在宁夏六盘山北侧半干旱区的叠叠沟小流域,采用热扩散探针法在2011年生长季监测了华北落叶松(Larix principisrupprechtii)人工林的树干液流速率,分析了夜间树干液流和补水量的变化特征及与气象、土壤水分等环境因子的关系.结果表明:树干液流速率日变化表现为典型的单峰宽峰曲线,且整个生长季均存在微弱的夜间液流,一般表现为逐渐减小,特别是在晴天,且晴天的变幅显著大于雨天.除生长季中期雨天夜间液流平均速率显著高于晴天,生长季初期及末期雨天与晴天的差异并不显著.生长季内,夜间树干补水总量为11.03 mm,占总蒸腾量的7.22％;5月份的树干补水量最大(4.19mm),其他月份的树干补水量明显减小,在0.9-1.7mm的范围波动.但不同月份间的补水贡献率存在明显差异,表现为生长季末期(9、10月)＞初期(5月)＞中期(6-8月).相关分析表明,日补水量与各气象因子关系不大,仅与降水量显著正相关(P＜0.05),与土壤含水率、日间蒸腾量、日蒸腾总量极显著正相关(P＜0.01).夜间补水的月蒸腾贡献率与月均土壤含水率、月均气温、月均日间蒸腾量、月总蒸腾量等显著相关(P＜0.05);而夜间补水的日蒸腾贡献率与日最高气温、日均气温、日间蒸腾量、日均饱和水汽压差、日总蒸腾量、日均太阳辐射强度、日最低气温、日均空气相对湿度、日降水量、土壤含水率等极显著相关(P＜0.01),经逐步回归分析建立了日补水量蒸腾贡献率与环境因子的多元线性模型.     

Diel patterns of CO2 exchange for epiphytic cacti differing in succulence

Net CO: uptake over 24-h periods was examined for 6 species of epiphytic cacti. which as a group number about 120 species that vary considerably in stem succulence. Nocturnal CO, uptake as a percentage of the total daily CO, uptake increased from 0% for Rhipsalis houlletiana to 14% for Epipliyl/urn pittieri to 21% for E. oxypetnluin to 51% for R. rhornbea to 87% for Schlumbergera truncata to 98% for Cryplocercus anthonyamis. For R. houlletiana and S. truncata, maximum net CO, uptake occurred at a photosynthetically active radiation of only 200 μmol m⁻² s⁻¹, as is typical for shade plants. Short-term drought of R. rhombea caused a shift favoring nocturnal CO, uptake. For the 6 species examined, increased reliance on nocturnal CO, uptake characteristic of Crassulacean acid metabolism (CAM) was closely correlated with increases in stem thickness, fresh weight per unit area, water mass per unit area, and to a somewhat lesser extent with another measure of succulence, water mass per unit mass of chlorophyll.     

荒漠防护林典型树种液流特征及其对环境因子的响应

利用基于热补偿理论的SF300分体液流仪对干旱荒漠区人工防护林典型树种(俄罗斯杨、胡杨、榆树、沙枣)树干液流全天候监测,自动气象站同步记录相关环境因子变化。研究表明:①4种防护林树种茎干液流日变化除沙枣树外均存在明显昼夜节律,液流速度在同属种间差异较小,在不同属种间差异显著,俄罗斯杨的日平均液流速度可以达到沙枣的13.8倍,耗水量排序为俄罗斯杨胡杨榆树沙枣树。②水分充足条件下,增加实验地灌溉量使4种树木蒸腾受到抑制,液流流速降低,水分利用效率降低。③液流流速因所处树干径向位点不同而存在差异,俄罗斯杨、榆树、沙枣液流速度表现出由形成层到髓心的递减趋势,胡杨树干径向位点液流没有表现一定规律。④树干液流流速与环境因子进行相关分析,通过逐步回归分析建立了4个典型树种茎干液流速度与环境因子关系估算模型,分析认为4种树木的环境敏感性排序为俄罗斯杨榆树胡杨沙枣。     

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     

Using branch and basal trunk sap flow measurements to estimate whole-plant water capacitance: a caution

Thermometric sap flow sensors are widely used to measure water flow in roots, stems and branches of plants. Comparison of the timing of flow in branches and stems has been used to estimate water capacitance of large trees. We review studies of sap flow in branches and present our own data to show that there is wide variation in the patterns and timing of sap flow of branches in different parts of the crown, owing to the course of daily solar illuminance. In contiguous forest, east-facing and upper branches are illuminated earlier than west-facing and lower branches and most capacitance studies do not include adequate information about branch sampling regimes relative to the overall pattern of crown illuminance, raising questions about the accuracy of capacitance estimates. Measuring only upper branches and normalising these results to represent the entire crown is dangerous because flows at the stem base likely peak in response to maximum crown illuminance (and transpiration) and this will differ compared to the timing of peak flows in upper branches. We suggest that the magnitude of flow lags between branches and stems needs further study, with careful attention to branch position and method application before a robust understanding of capacitance, particularly in woody tissues of large trees, can be formed. We did not detect flow lags in the world’s tallest and largest tree species Sequoia sempervirens and Sequoiadendron giganteum, despite measurement along large pathlengths (∼57 and 85 m), which raises questions as to why large flow lags are often recorded for much smaller species. One conspicuous possibility is the different methods used among studies. Constant-heating methods such as the thermal dissipation probe (and also heat balance methods) include heat capacitance behaviour due to warming of wood tissues, which delays the response of the sensors to changing sap flow conditions. We argue that methods with intrinsic heat-capacitance present dangers when trying to measure water-capacitance in trees. In this respect heat pulse methods hold an advantage.     

Depressed hydraulic redistribution of roots more by stem refilling than by nocturnal transpiration for Populus euphratica Oliv. in situ measurement

During the night, plant water loss can occur either through the roots, as hydraulic redistribution (HR), or through the leaves via the stoma, as nocturnal transpiration (E_n), which was methodologically difficult to separate from stem refilling (R_e). While HR and E_n have been reported across a range of species, ecosystem, and climate zone, there is little understanding on the interactions between E_n and/or R_e and HR. As water movement at night occurs via gradients of water potential, it is expected that during periods of high atmospheric vapor pressure deficit (VPD), water loss via E_n will override water loss via HR. To test this hypothesis, sap flow in stems and roots of Populus euphratica Oliv. trees, growing in a riparian zone in a hyperarid climate, was measured once in a year. Nocturnal stem sap flow was separated into E_n and R_e using the “forecasted refilling” method. Substantial nocturnal sap flow (38% of 24‐hr flux on average) was observed and positively correlated with VPD; however, the strength of the correlation was lower (R² = .55) than diurnal sap flow (E_d) (R² = .72), suggesting that nocturnal stem sap flow was attributed to both water loss through the canopy and replenishment of water in stem tissues. Partitioning of nocturnal sap flow shows that R_e constituted approximately 80%, and E_n ~20%, of nocturnal sap flow. The amount of root sap flow attributed to redistribution was negatively related to E_d (R² = .69) and the amount of acropetally sap flow in stems, R_e (R² = .41) and E_n (R² = .14). It was suggested that the magnitude of HR is more strongly depressed by R_e that was recharge to the water loss via E_d than by E_n. It was consistent with whole‐tree water balance theory, that the nighttime upward sap flow to xylem, stem refilling and transpiration, may depress hydraulic redistribution of roots.     

Evaluation of the heat pulse velocity technique for measurement of sap flow in rainforest and eucalypt forest species of south-eastern Australia

Sap flow in the stems of two cut saplings each of Eucalyptus maculata (a canopy eucalypt forest tree), Doryphora sassafras and Ceratopetalum apetalum (both canopy rainforest trees of south-eastern coastal Australia) was measured by the heat pulse velocity technique and compared with water uptake from a potometer. Scanning electron micrographs of wounding caused by implantation of temperature sensor and heater probes into the sapwood showed that wounding was similar in rainforest and eucalypt species and was elliptical in shape. A circular wound has been implicitly assumed in previous studies. Accurate measurements of sapling water use were obtained using the smaller transverse wound dimension rather than the larger longitudinal dimension because maximum disruption of sap flow through the xylem vessels occurred in the transverse plane. Accurate measurements of sap flux were obtained above a minimum threshold sap velocity. These velocities were 15·7,10·9 and 9·4 cm h^?1 for E. maculata, C. apetalum and D. sassafras, respectively. Below the threshold sap velocity, however, sap flow could not be accurately calculated from measurements of heat pulse velocity. The minimum threshold sap velocity appeared to be determined by probe construction and xylem anatomy. Despite the elliptical wounding and inaccurate measurement of sap flow below the threshold sap velocity, total sap flow over the experimental period for two saplings of each species was within 7% of water use measured by the potometer.     

Tree water dynamics non-destructively assessed through sap flow measurements and potential evapotranspiration

Sap flow and potential evapotranspiration rates were analyzed for two coniferous tree species (Douglas-fir and Scots pine) and one broadleaf species (sessile oak) in a mixed Carpineto-Quercetum forest during the growing season 2005. The relationship between sap flow and potential evapotranspiration rates, effective crown area as a measure of the relative transpiration and daily relative proportion of the storage water used for transpiration were used as indicators of the tree water dynamics. These indicators were determined on four consecutive days and all three showed good reliability concerning tree water dynamics.     

Determination of the relative uptake of ground vs. surface water by Populus deltoides during phytoremediation

The use of plants to remediate polluted groundwater is becoming an attractive alternative to more expensive traditional techniques. In order to adequately assess the effectiveness of the phytoremediation treatment, a clear understanding of water-use habits by the selected plant species is essential. We examined the relative uptake of surface water (i.e., precipitation) vs. groundwater by mature Populus deltoides by applying irrigation water at a rate equivalent to a 5-cm rain event. We used stable isotopes of hydrogen (D) and oxygen (18O) to identify groundwater and surface water (irrigation water) in the xylem sap water. Pretreatment isotopic ratios of both deuterium and 18O, ranked from heaviest to lightest, were irrigation water > groundwater > xylem sap. The discrepancy in preirrigation isotopic signatures between groundwater and xylem sap suggests that in the absence of a surface source of water (i.e., between rain events) there is an unknown amount of water being extracted from sources other than groundwater (i.e., soil surface water). We examined changes in volumetric soil water content (%), total hourly sapflux rates, and trichloroethene (TCE) concentrations. Following the irrigation treatment, volumetric soil water increased by 86% and sapflux increased by as much as 61%. Isotopic signatures of the xylem sap became substantially heavier following irrigation, suggesting that the applied irrigation water was quickly taken up by the plants. TCE concentrations in the xylem sap were diluted by an average of 21% following irrigation; however, dilution was low relative to the increase in sapflux. Our results show that water use by Populus deltoides is variable. Hence, studies addressing phytoremediation effectiveness must account for the relative proportion of surface vs. groundwater uptake.     

Reverse flow in roots of Sesbania rostrata measured using the constant power heat balance method

This investigation was performed to examine qualitatively and quantitatively the reverse flow in partially dried roots of Sesbania rostrata using the constant power heat balance method. First, a semi-empirical technique for estimating sheath conductance of sap-flow sensors without assuming that sap flow is zero at night was proposed. Sap flow measured with the heat balance method was compared with water uptake as measured by a potometric method. Sap flow was overestimated by 56·1% for a 3·3-mm-diameter root, and by 40·0% for 6·1 mm and 33·3% for 8·8 mm roots. However, high correlation coefficients between the rates of water uptake and sap flow demonstrated that calibration would provide reliable values for root sap flow. To detect reverse flow, a split root experiment was conducted using a S. rostrata plant with its root system divided between dry and wet compartments. Daily sap flow of the drying compartment declined whereas that in ‘wet’ root increased, suggesting that the decrease in water uptake by ‘dry’ roots was offset by the ‘wet’ roots. Reverse flow was observed at night in the root on the dry side of the container when the soil water potential was less than –0·30 MPa. The total amount of water released into the soil during the night period was estimated to be 22·5 g.     

Laurel forests in Tenerife, Canary Islands

The efficiency of the conductive system in about 40-year-old Laurus azorica trees growing in a laurel forest was evaluated by comparing main stems and leaves (petioles) on the basis of theoretical sap flow values (1) calculated from vessel anatomy (taking vessels as ideal capillaries), (2) derived from measured dye velocity and (3) data taken from direct sap flow measurements. It was found that actual sap flow rate per wood area increases in stems from the pith towards the cambium. The outermost part of the stem is the most important part of the tree for conducting water. Maximum actually measured transpiration (sap flow rate) for the stand was practically identical to the theoretical rate calculated based on petiole anatomy, but it was about 45 times lower than that calculated based on stem anatomy. This illustrates the safety features of stem wood, which due to its high vessel density, is capable of transporting all the water required even when only a small area of its vessels is working. In the petioles, xylem is more efficiently used, but almost all vessels must work in order to supply water to leaves and any disturbance may cause leaf loss.     

Contrasting water relations of three coastal tree species with different exposure to salinity

This field study examined the ecophysiological responses of three tree species to salinity in the Austin Bay Nature Reserve, adjacent to the Peel-Harvey Estuary in Western Australia (115°46' E 32°37' S). The area is at increased risk of flooding with saline water during storm surges due to the construction of a channel between the estuary and Indian Ocean in 1994. Banksia attenuata R.Br. occurs on small sandy ridges adjacent to a seasonal wetland, while Melaleuca cuticularis Labill. and Casuarina obesa Miq. occur in a seasonally flooded wetland. Landscape position determined exposure to salinity, with M. cuticularis and C. obesa experiencing high soil and groundwater salinity during summer (electrical conductivity, EC, up to 70 dS m⁻¹) while B. attenuata was not exposed to soil or groundwater with EC greater than 20 dS m⁻¹. B. attenuata had relatively stable leaf water status throughout the year and did not osmotically adjust as root-zone salinity increased. By contrast, M. cuticularis and C. obesa had large variation in stem water potential and exhibited osmotic adjustment during summer. Whereas the sap flow rates of M. cuticularis and C. obesa remained high throughout the year, sap flow of B. attenuata decreased during summer which may have limited uptake of salt. The three species also exhibited differences in traits associated with tissue-level salt tolerance, as M. cuticularis and C. obesa produced compatible organic solutes (methyl proline in M. cuticularis and proline in C. obesa ), whereas B. attenuata did not. The distributions of these species within the Austin Bay Nature Reserve are determined in part by their tolerance to salinity, which will influence their responses to hydrological disturbance.