Sap flow measurements with some thermodynamic methods,flow integration within trees and scaling up from sample trees to entire forest stands

Sap flow measurement techniques and evaluation of data are reviewed. Particular attention is paid to the trunk segment heat balance (THB) and heat field deformation (HFD) methods based on 30 years experience. Further elaboration of sap flow data is discussed in terms of integrating flow for whole stems from individual measuring points, considering variation of radial patterns in sapwood and variation around stems. Scaling up of data from sets of sample trees to entire forest stands based on widely available biometric data (partially on remote sensing images) is described and evaluated with a discussion of the magnitude of errors, the routine procedure applicable in any forest stand and practical examples.     

油松、栓皮栎树干液流速率比较

应用TDP(ThermalDissipationProbe)技术对油松和栓皮栎树干液流进行了初步研究,经过野外近1a的实地定位观测,研究结果显示:栓皮栎月平均树干液流速率在整个生长期都较油松的月平均树干液流速率要高。前者大约是后者的5~10倍。栓皮栎在土壤干旱时期能够在白天产生明显的树干液流。在土壤干旱时期油松白天不产生树干液流而在晚上产生明显树干液流。在土壤相对湿润时期,油松和栓皮栎树干液流速率的波形与太阳总辐射的波形变化一致,但不同的是油松的树干液流速率波形呈明显的单峰状,而栓皮栎树干液流速率波形呈明显的多峰状。在土壤相对湿润时期太阳总辐射很低时能对油松树干液流速率产生明显的降低作用,而对栓皮栎树干液流则没有明显影响。在土壤干旱时期,油松和栓皮栎树干液流速率的峰值分别大约为0.0001cm/s和0.0006cm/s左右;在土壤水分充足时期,油松和栓皮栎树干液流速率的峰值分别大约相等约为0.0015cm/s左右,分别是油松和栓皮栎在干旱日期的液流速率峰值的10倍和2.5倍。     

Absorptive root area and stem resistivity in whole trees of contrasting structure and size – improvement of methods

Aims

The study was focused on comparing the results of the three instrumental methods applied simultaneously for root studies in several tree species representing contrasting situations: root systems of different structure and stems of a wide range of diameters (especially when considering their resistivity). We want to learn properties of the methods, make some improvements and test their validity, before they will be applied to a large series of trees at the stand level.

Material and methods

Douglas fir (Pseudotsuga menziessii (Mirbel) Franco) with very asymmetric root system and Blue spruce (Picea pungens Engelm.) with homogeneous root system growing in the Mendel University Training Forest Enterprise in K?tiny, were selected as the main sample trees. Three variants of stem impedance measurements needed for absorptive root area estimates were applied to an additional series of over 20 trees. In order to characterize vertical and circumferential (around stem) root distribution we applied (1) the sap flow radial patterns measured by the multi-point sensors based on the heat field deformation (HFD) method, and (2) a modified earth impedance (MEI) method from the group of thermodynamic and electric measuring methods and finally we (3) almost harmlessly excavated the whole root system by supersonic air stream. Three steps of absorptive root area measurements were improved: (a) Impact of stem impedance was almost eliminated, (b) Excessive variation of stem impedance values measured too close to stems (in a place with the most heterogeneous materials) was compensated by extrapolation of several close points, (c) Impact of high curvature of small stems was determined and eliminated by an equation.

Results

All the methods gave similar results when considering differences between individual trees as well as between stem sides. Sap flow density was interesting when expressed per measured absorptive root area and leaf area. Experimental data of main and additional sample trees confirmed validity of relationship, which can be applied to improve stem resistivity especially in small trees.

Conclusions

Results indicated, that all the instrumental methods are field applicable and suitable for quantitative measurements, when specific properties of the methods and stem macrostructure are taken into account. Soil electric parameters characterize the important properties related to presence of cracks, water content, and ion concentration, which are being analyzed now.     

Reverse flow of sap in tree roots and downward siphoning of water by Grevillea robusta

1. Constant-power heat-balance sap flow gauges were used to compare sap flow in vertical and lateral roots of Grevillea robusta trees growing without access to ground water at a semiarid site in Kenya.
2. Reversal of sap flow occurred when root systems crossed gradients in soil water potential. Measurement of changes in the direction of flow was possible because of the symmetrical construction of the sap flow gauges; gradients in temperature across the gauges, and thus computed rates of sap flow, changed sign when reverse flow occurred.
3. Reverse flow in roots descending vertically from the base of the tree occurred, while uptake by lateral roots continued, when the top of the soil profile was wetter than the subsoil. The transfer of water downwards by root systems, from high to low soil water potential, was termed 'downward siphoning'; this is the reverse of hydraulic lift.
4. Downward siphoning was induced by the first rain at the end of the dry season and by irrigation of the soil surface during a dry period.
5. Downward siphoning may be an important component of the soil water balance where there are large gradients in water potential across root systems, from a wet soil surface downwards. By transferring water beyond the reach of shallow-rooted neighbours, downward siphoning may enhance the competitiveness of deep-rooted perennials.     

Sap feeding by the marsupial Petaurus australis: an enigmatic behaviour?

Summary The Yellow-bellied Glider, Petaurus australis, was observed to feed sporadically but extensively on eucalypt sap. Gliders extracted sap by making incisions into the bark with their procumbent lower incisor teeth and licking the resulting exudate. Less than 1% of possible trees were incised for sap. Sap flow and sap-sugar concentration were estimated for eight sap-site trees and eight non sap-site trees at intervals during an 18 month period. Measurements of sap-sugar concentration differed substantially among trees but fluctuated between sample periods independent of glider use. Sap flow measurements, were always higher for those trees currently in use by gliders, although the particular sap-site trees comprising this group varied between sample periods. The variation in sap flow occurred indenpendent of the incidence of rainfall or the type of microhabitat the tree occupied. Experimental incising of trees, designed to mimic the effects of feeding by gliders, failed to show any effect on sap flow. This suggests that the incidence of sap feeding is determined by a tree's pattern of sap flow and that there may be trees with unusual patterns of sap flow which gliders select as the most favourable trees to incise.     

Instrumental methods for studies of structure and function of root systems of large trees

New methods using different physical principles have been successfully applied in studies of root systems of large trees. The ground-penetrating radar technique provides 3D images of coarse roots (starting with a diameter of about 20 mm) from the soil surface down to a depth of several metres. This can even be done under layers of undisturbed materials such as concrete, asphalt and water. Fine roots cannot be visualized by this method, but the total rooted volume of soil can be determined. The differential electric conductance method has been used for fast measurement of conducting (absorbing) root surfaces. However, more testing is needed. Both these methods are non-invasive. The results can be verified by an almost harmless excavation of whole root systems, including fine roots, using the ultrasonic air-stream (air-spade) method. This method is suitable for all studies, as well as practical operations on roots or objects in their vicinity, where a gentle approach is required. Sap flow measurements on their own or in tandem with soil moisture monitoring play a leading role in studying root function and hydraulic redistribution of flow in the soil. The water absorption function of roots can be studied by measuring sap flow on individual root branches directly (as on crown branches) and also indirectly, by measuring the radial pattern of sap flow in different sapwood depths at the base of a stem. Root zone architecture can also be estimated indirectly by studying its functionality. The heat field deformation method with multi-point sensors has been found to be very convenient for this purpose. A combination of several such methods is recommended whenever possible, in order to obtain detailed information about the root systems of trees.     

Spring Sap of Trees

In the spring, sap in perennial plants becomes available after winter dormancy. As is well known, substances including minerals and organic nutrients are transported from storage compartments in roots and stems to growth regions. For this solute distribution, xylem as well as phloem are used and probably also the cambial region and the ray system. In the present study, a puncture method has been used to record sap availability in six north European tree species during the spring. Attempts were made to withdraw sap from the trees at points 0.5 m, 1.3 m and 3.0 m above ground with “phloem needles” (constructed according to Hammel 1968), and a syringe. Sap could be withdrawn for the longest period for the sycamore (71 days). Birch (47 days) gave an intermediate period, beech (12 days), oak (12 days) and alder (13 days) gave short periods, and sap could not be withdrawn from ash. Effects of two environmental factors, temperature and light intensity on sap availability were examined. Temperature dependence of sap availability, already observed in the genus Acer, was confirmed in sycamore maple (Acer pseudoplatanus). There also seemed to be a correlation between sap availability and light intensity. Analysis of sugar in the tree sap revealed that sucrose was the only sugar in the sap of sycamore but hexoses (glucose and fructose) were preponderant in the sap of the other trees. The pH of sycamore sap (pH 6.9–5.4) fell as the season advanced. However, the pH-values of sap from the other trees did not vary significantly.     

Whole tree xylem sap flow responses to multiple environmental variables in a wet tropical forest

In order to quantify and characterize the variance in rainforest tree physiology, whole tree sap flow responses to local environmental conditions were investigated in 10 species of trees with diverse traits at La Selva Biological Station, Costa Rica. A simple model was developed to predict tree sap flow responses to a synthetic environmental variable generated by a principle components analysis. The best fit was obtained with a sigmoid function which explained between 74 and 93% of the variation in sap flux of individual trees. Sap flow reached an asymptote where higher light and evaporative demand did not cause sap flux to increase further. Soil moisture had little influence on sap flux. The morphological characteristics of the trees significantly affected sap flow; taller trees responded to changes in environmental variables sooner than shorter trees and high liana cover buffered tree sap flow responses to weather. The effect of species‐specific differences on the model was small; the mean effectiveness of the model was reduced by 6% when parameters were estimated from a single pool of measurements taken from all individuals. The results indicate that sap flow response could be effectively estimated using a simple general model and composite environmental index for these 10 diverse tree species.     

Functional Traits and Water Transport Strategies in Lowland Tropical Rainforest Trees

Understanding how tropical rainforest trees may respond to the precipitation extremes predicted in future climate change scenarios is paramount for their conservation and management. Tree species clearly differ in drought susceptibility, suggesting that variable water transport strategies exist. Using a multi-disciplinary approach, we examined the hydraulic variability in trees in a lowland tropical rainforest in north-eastern Australia. We studied eight tree species representing broad plant functional groups (one palm and seven eudicot mature-phase, and early-successional trees). We characterised the species’ hydraulic system through maximum rates of volumetric sap flow and velocities using the heat ratio method, and measured rates of tree growth and several stem, vessel, and leaf traits. Sap flow measures exhibited limited variability across species, although early-successional species and palms had high mean sap velocities relative to most mature-phase species. Stem, vessel, and leaf traits were poor predictors of sap flow measures. However, these traits exhibited different associations in multivariate analysis, revealing gradients in some traits across species and alternative hydraulic strategies in others. Trait differences across and within tree functional groups reflect variation in water transport and drought resistance strategies. These varying strategies will help in our understanding of changing species distributions under predicted drought scenarios.     

Root distribution of poplar at varying densities on pastoral hill country

Spaced poplar (Populus spp.) trees are used widely in New Zealand for soil conservation on erodible pastoral hill country. Their root distribution in this environment, and factors that affect it, are poorly understood. Robust recommendations on effective tree spacing depend on knowledge of root systems. This study determined the effect of tree density, position between trees, and soil depth (0–90 cm) on root number, root diameter distribution, root area ratio (RAR), and cross sectional area per root for young trees on slopes. Data were collected for lateral roots using trenches. Greater than 80% of roots were < 5 mm diameter and root attributes were highest in shallow soil. Trees at 770 stems per hectare (sph) had 3–12 times more roots and 3–9 times greater RAR than those at densities of ≤ 237 sph, representative of most tree-pasture systems. Mean cross sectional area per root was similar across densities. Positions close to trees had twice as many roots (46 vs. 23/m²) and RAR (109 vs. 52 mm²/m²) as positions midway between trees. The study provided quantitative understanding of variation in root distribution with tree density and information useful for supporting and strengthening recommendations on densities for effective erosion control.     

Redistribution of water via layering branches between connected parent and daughter trees in Norway spruce clonal groups

Key message

We measured sap flow and shoot water potentials in clonally connected parent and daughter trees. We found bidirectional flow patterns in branches mediating the connection between parent and daughter trees.

Abstract

Layering is an important mode of vegetative reproduction at treeline, in which clonal daughter trees are formed by the rooting of lower (“layering”) branches of the parent tree. These branches mediate the connection between parent (PT) and daughter tree (DT). Here, we measured quantity and direction of sap flow in layering branches as well as PT and DT, and measured shoot water potentials in the crowns of a connected PT and DT. We found bidirectional sap flow pattern in layering branches, with the bidirectionality of the flow resulting from water potential dynamics of the parent and daughter trees varying diurnally. We found that 4.3 % of the total water transpired by the DT was supplied by the PT root system, with up to 25 % of the instantaneous daughter tree sap flow coming from the parent tree. In contrast, water provided by the daughter’s root system to the parent tree comprised only a negligible amount, less than 1 % of the parent’s entire sap flow. Additionally, after experimental excavation of part of the DT roots, layering branch flow towards the DT increased, while flows in the opposite direction almost vanished. This study showed that aboveground clonal connections can facilitate a new type of hydraulic redistribution where water is transported bidirectionally through branches. This transfer of water and nutrients is vital especially in the first years of the daughter tree but supplies considerable amounts of water even several years after the establishment of a new clonal tree.

     

不同树高对荷木液流密度、整树蒸腾量和时滞的效应

人工林面积不断增大,这不仅能解决由于森林砍伐引起的一系列社会问题,而且还对解决水土保持、二氧化碳减排等环境问题起到重要作用。了解人工林的生长特性和蒸腾效率,对植被生长、恢复和管理有着重要意义。为此,该研究连续监测了华南地区12棵不同高度荷木人工林的液流密度,对样树以高度划分等级,采取错位相关法分析不同高度等级胸高处液流与冠层蒸腾的时滞效应。结果表明:气候环境相同时,所有样树胸高处液流日格型相似;荷木林蒸腾量优势木中间木劣势木,所有树木湿季月蒸腾量大于干季月蒸腾量;不同高度等级之间时滞差异显著,劣势木时滞50min,优势木和中间木时滞20min;所有样树干湿季时滞差异不显著,同一高度级两季节时滞差少于10min。这些说明:在干季华南地区土壤水分仍然相对较充足,植物输水阻力没有受到土壤水分降低和长距离水分传导的影响;中间木和优势木时滞短,水力阻力小,蒸腾量大并占据着林段的有利资源;劣势木长势低矮,时滞长,导管阻力大,蒸腾量少,光合作用需要的水热资源少,所以回馈根部的营养物质少,不均衡的营养循环使得林段分化愈明显,劣势木将逐渐从林段中被淘汰。该文指出在荷木人工林生长后期,对于长势低矮,生命力极弱的劣势木应定期砍伐,这样能增加优势木和中间木对光照及水分等有利资源的分配,提高林分质量,增加林地生产力。     

马占相思树干液流与光合有效辐射和水汽压亏缺间的时滞效应

应用Granier热消散探针测定华南丘陵马占相思的树干液流,将液流与对应的光合有效辐射和水汽压亏缺数据列分别进行逐行错位分析和时间序列分析,探讨树干液流与蒸腾驱动因子之间的时滞效应,并对结果进行互相验证.结果表明:马占相思树木蒸腾主要驱动因子是光合有效辐射和水汽压亏缺,树干液流的变化更多地依赖光合有效辐射的变化,而且干季的依赖性比湿季更强;无论是干季还是湿季,树干液流都滞后于光合有效辐射,提前于水汽压亏缺;时滞效应季节差异显著;不同径级马占相思的时滞效应差异不显著;树高、胸径、冠幅并不能解释树干液流与光合有效辐射、水汽压亏缺之间的时滞效应;干季树干液流与水汽压亏缺之间的时滞效应与夜间水分补充量显著相关,湿季则相反.     

The root zone dynamics of water uptake by a mature apple tree

We report the results from a field experiment in which we examined the spatial and temporal patterns of water uptake by a mature apple tree (Malus domestica Borkh., ‘Splendour’) in an orchard. Time Domain Reflectometry (TDR) was used to measure changes in the soil's volumetric water content, and heat-pulse was used to monitor locally the rates of sap flow in the trunk and roots of the tree. We also measured the tree's distribution of root-length density and obtained supporting data to characterize the soil's hydraulic properties. The experimental data were used to examine the output of the WAVE-model (Vanclooster et al, 1995; Ecol. Model. 81, 183–185) in which soil water transport is predicted using Richards' equation, and where root uptake is represented by a distributed macroscopic sink term. When the surface soil layers were uniformly wet, 70% of the trees water uptake occurred in the top 0.4 m of the root zone, in which approximately 70% of the tree's fine roots were located. When a partial irrigation was applied to just one side of the root zone, the apple tree quickly shifted its pattern of water uptake with an almost two-fold increase in uptake from the wetter soil parts and a corresponding reduction in uptake from the drier parts. The response of root-sap flow to irrigation was almost immediate (i.e. root flow increased within hours of the irrigation). Following subsequent irrigations over the whole soil surface, TDR measurements revealed a surface-ward shift in the pattern of water extraction, and root flow measurements revealed a recovery in the uptake function of seemingly inactive roots located in the previously-dry soil. Via our root sap flow measurements, we observed two roots on the same tree locally responding quite differently to similar events of soil wetting. This observation suggests that there may be considerable functional variability across the apple root system. Our measurement-model calculations yielded similar results and stress the prime role played by the plant in modifying the root zone balance of water. Following an irrigation or rainfall event, root uptake by apple appears to be more dependent upon the near-surface availability of water than it is related to the distribution of fine roots.     

An evaluation of hypotheses to explain the pattern of sap feeding by the yellow-bellied glider,Petaurus australis

Yellow-bellied gliders on occasion extensively incise the trunks of eucalypts to feed on sap but in doing so show strong preference for the species and number of trees utilized. Three hypotheses that may account for aspects of this behaviour were examined. They were: (i) sap feeding is related to the incidence of rain; (ii) sap feeding is influenced by the sap flow idiosyncracies of trees; and (iii) sap feeding is influenced by the availability of alternative food resources. Sap feeding at two sites occurred independent of ‘wet’ or ‘dry’ nights. Sap flow (measured as an index) in two species of eucalypt was highly variable among trees and among sample periods. This is the only hypothesis that accounts for the selectivity of the trees incised. Sap feeding at one site coincided with periods of high sap flow. However, at another site, sap feeding did not occur on measured trees despite their often high sap flow. At such times, flowering trees were abundant and gliders fed extensively on nectar. Thus, elevated sap flow may provide the necessary precondition conducive for sap feeding but may be ignored when other food types are available.     

华北山区核桃液流变化特征及对不同时间尺度参考作物蒸散量的响应

核桃是华北山区重要的经济树种,但该地区水资源紧缺,因此研究核桃液流耗水变化及其对气象因子的响应对于加强核桃水分管理、确定水分承载力具有指导意义。2008—2010年连续3个生长季节观测分析了华北山区核桃单株液流变化特征,研究了不同时间尺度(日尺度、月尺度)核桃液流与气象因子、参考作物蒸散量(ET0)的关系,并利用2009—2010年数据建立了拟合方程,在此基础上用拟合方程的估算值与2008年液流实测值进行了对比验证。结果表明:核桃液流具有明显的时间变化特征,最大液流量出现在5、6月份,2008—2010年生长季节(4—9月份)核桃液流量分别为893.23 L、854.88 L和841.77 L,日平均液流量分别为4.96 L、4.75 L和4.68 L。液流年际差异主要是由年降雨量引起。核桃液流与不同时间尺度下气象因子、ET0均有较好的相关性,且随着时间尺度的增加,二者间的相关性变大。利用2009—2010年液流实测值与气象因子、ET0建立的拟合方程,对2008年液流值进行了拟合,并与实测值进行了对比验证,发现不同时间尺度下液流拟合值与实测值均具有较好的一致性。在不能实现对液流进行连续观测的情况下,可以结合当地的气象资料、ET0等因子,利用建立的拟合方程对树木液流进行估算,弥补缺失的数据,具有较高的精确性,可用于指导当地核桃水分管理。     

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.     

Resistance to Water Uptake in a Mature Citrus Tree

Resistance to water uptake in an irrigated and a water-stressed22-year-old citrus tree was examined during 7 weeks. Frequentirrigation maintained the soil water potential of one citrustree between 0.0 and –0.016 MPa in the upper 0.6 m, whilethe other tree received no water for 44 d. Sap flow in the trunkswas measured once a week from dawn to dusk at hourly intervalswith a calibrated heat pulse technique. At the same time leafwater potential was measured on sunlit and shaded leaves. Sap flow in the stressed tree decreased with time at a higherrate than the water potential difference between soil and leaf,which implies an increase in the resistance of the transportsystem. Diurnal sap flow and diurnal leaf water potential ofthe irrigated and stressed trees were linearly related throughoutthe experimental period. The slope of the linear relationshipbetween flow and potential for both trees remained nearly constant,indicating that the resistance within the tree remained constant.The effect of drying appeared to be a progressive decline ofthe leaf water potential at zero flow (computed by linear regression).This was ascribed to a reduction of the transport of water inthe soil toward the roots. Key words: Transpiration, Leaf water potential, Plant water relations, Soil water stress     

Function of spiral grain in trees

Summary Through spiral grain, conduits for sap lead from each root to all branches. This uniform distribution of sap is indicated by the paths of vessels and tracheids, and has been proven experimentally by means of dyed sap injected into the base of stems or taken up by roots. Trees receiving water only from roots at one side of the root collar nevertheless stay green and continue growing. Spiral grain in bark distributes food from each branch to other flanks of the stem and to most roots. Experimental interruptions of the sap and food conduits caused the cambial zone to reorient new conduit cells in new directions, bypassing the interruption. In particular, spiral grooves cut into the stem surface caused spiral grain. The new cells reorient through division and growth. Although spiral grain is largely under genetic control, trees appear to have a spiral grain especially where needed for distribution of water when root spheres are dry at one side. Compared with straight-grained trees, spiral-grained stems and branches bend and twist more when exposed to strong wind, in this way offering less wind resistance and being less likely to break. Through the bending and twisting, snow slides down from branches rather than breaking them, but the main function of spiral grain is the uniform distribution of supplies from each root to all branches, and from each branch to many roots.     

Conservation of element concentration in xylem sap of red spruce

We investigated the chemistry of xylem sap as a marker of red spruce metabolism and soil chemistry at three locations in northern New England. A Scholander pressure chamber was used to extract xylem sap from roots and branches cut from mature trees in early June and September. Root sap contained significantly greater concentrations of K, Ca, Mg, Mn, and Al than branch sap. Sap collected in June contained a signficantly greater concentration of Mn than sap collected in September. Sap concentration was related to forest location for N and Mn. Variations in concentrations of N and K were significantly related to the interaction of tree organ and month of collection. Variations in concentrations of P, Cu, Zn, and Fe were not attributable to tree organ, month of collection, or forest location. Patterns of element concentration in xylem sap compared to previously published data on soil solution chemistry indicated a high degree of homeostatic control of xylem sap chemistry. This control likely represents a significant allocation of resources within the tree energy budget.