Transpiration and canopy conductance in a pristine broad-leaved forest of Nothofagus: an analysis of xylem sap flow and eddy correlation measurements

Summary Tree transpiration was determined by xylem sap flow and eddy correlation measurements in a temperate broad-leaved forest of Nothofagus in New Zealand (tree height: up to 36 m, one-sided leaf area index: 7). Measurements were carried out on a plot which had similar stem circumference and basal area per ground area as the stand. Plot sap flux density agreed with tree canopy transpiration rate determined by the difference between above-canopy eddy correlation and forest floor lysimeter evaporation measurements. Daily sap flux varied by an order of magnitude among trees (2 to 87 kg day^–1 tree^–1). Over 50% of plot sap flux density originated from 3 of 14 trees which emerged 2 to 5 m above the canopy. Maximum tree transpiration rate was significantly correlated with tree height, stem sapwood area, and stem circumference. Use of water stored in the trees was minimal. It is estimated that during growth and crown development, Nothofagus allocates about 0.06 m of circumference of main tree trunk or 0.01 m² of sapwood per kg of water transpired over one hour.Maximum total conductance for water vapour transfer (including canopy and aerodynamic conductance) of emergent trees, calculated from sap flux density and humidity measurements, was 9.5 mm s^–1 that is equivalent to 112 mmol m^–2 s^–1 at the scale of the leaf. Artificially illuminated shoots measured in the stand with gas exchange chambers had maximum stomatal conductances of 280 mmol m^–2 s^–1 at the top and 150 mmol m^–2 s^–1 at the bottom of the canopy. The difference between canopy and leaf-level measurements is discussed with respect to effects of transpiration on humidity within the canopy. Maximum total conductance was significantly correlated with leaf nitrogen content. Mean carbon isotope ratio was –27.76±0.27 (average ±s.e.) indicating a moist environment. The effects of interactions between the canopy and the atmosphere on forest water use dynamics are shown by a fourfold variation in coupling of the tree canopy air saturation deficit to that of the overhead atmosphere on a typical fine day due to changes in stomatal conductance.This paper is dedicated to Prof. Dr. O.L. Lange on the occasion of his 65th birthday     

疏伐对黄土丘陵区刺槐林蒸腾的影响

黄土高原地区植被建设已达到土壤水分承载力的阈值,需要对现有林分进行结构优化并提升其生态功能。不合理的林分密度是导致黄土丘陵区刺槐林土壤干化、生长衰退的主要原因之一。疏伐可以优化林分结构,并能够通过控制蒸腾耗水来调控土壤水分,是促进刺槐林可持续生长的有效手段。疏伐对黄土丘陵区刺槐林蒸腾有何影响,目前并不清楚。研究基于树干液流法估算了4个不同疏伐强度（样地1：52%、样地2：48%、样地3：35%、样地4：未疏伐）下刺槐单株尺度的液流速率与林分尺度的日平均蒸腾量,并分析了不同时间尺度下液流速率与环境因子的关系,以阐明疏伐对黄土丘陵区刺槐林蒸腾的影响。结果表明：（1） 单株尺度刺槐蒸腾速率（即液流速率）随疏伐强度减小（林分密度增大）呈现下降趋势（样地1：0.53 kg cm^-2 d^-1、样地2：0.41 kg cm^-2 d^-1、样地3：0.31 kg cm^-2 d^-1、样地4：0.33 kg cm^-2 d^-1）;（2） 观测期林分尺度日平均蒸腾量随疏伐强度减小呈现上升趋势（样地1：0.90 mm/d、样地2：1.18 mm/d、样地3：1.04 mm/d、样地4：1.44 mm/d）;（3） 在半小时尺度与日尺度上,各样地液流速率与环境因子的关系没有显著差异,半小时尺度单株液流速率均与太阳辐射相关性最高（相关系数0.883-0.908）,液流速率日变化过程与环境因子日变化过程存在时滞现象;日尺度单株液流速率与饱和水汽压亏缺相关性最高（相关系数0.843-0.913）,样地间日尺度单株液流速率的差异性随着饱和水汽压亏缺增大而增大。研究结果初步反映了疏伐导致的林分密度变化对刺槐蒸腾的影响,将为黄土丘陵区刺槐林的结构改造、功能提升和土壤水分调控提供理论支持。     

Changes in stem water content influence sap flux density measurements with thermal dissipation probes

Key message

Stem WC may decline during the day. Zero-flow dT _m increases when WC decreases. Use of nighttime dT _m in the calculation of sap flux density during the day might introduce errors.

Abstract

There is increasing evidence of diel variation in water content of stems of living trees as a result of changes in internal water reserves. The interplay between dynamic water storage and sap flow is of current interest, but the accuracy of measurement of both variables has come into question. Fluctuations in stem water content may induce inaccuracy in thermal-based measurements of sap flux density because wood thermal properties are dependent on water content. The most widely used thermal method for measuring sap flux density is the thermal dissipation probe (TDP) with continuous heating, which measures the influence of moving sap on the temperature difference between a heated needle and a reference needle vertically separated in the flow stream. The objective of our study was to investigate how diel fluctuations in water content could influence TDP measurements of sap flux density. We analysed the influence of water content on the zero-flow maximum temperature difference, dT _m, which is used as the reference for calculating sap flux density, and present results of a dehydration experiment on cut branch segments of American sycamore (Platanus occidentalis L.). We demonstrate both theoretically and experimentally that dT _m increases when stem water content declines. Because dT _m is measured at night when water content is high, this phenomenon could result in underestimations of sap flux density during the day when water content is lower. We conclude that diel dynamics in water content should be considered when TDP is used to measure sap flow.     

Gas-exchange and sap flow measurements of Salix viminalis trees in short-rotation forest

Simultaneous field measurements of transpiration and sap flow were performed on short-rotation Salix viminalis trees ranging in diameter from 1.5 to 3.5 cm (2-year-old shoots on 8-year-old stumps). Transpiration was measured using an open-top ventilated chamber enclosing the whole foliage of a tree. Sap flow was measured using a tree-trunk heat balance (THB) technique with a constant temperature difference and variable heat input. Both the instantaneous and daily values of water flux measured by the two absolute techniques agreed well with a difference of up to about 5%. In July, the hourly transpiration reached a maximum of about 0.2 kg m^–2 (leaf area) or 0.45 kg tree^–1, whereas maximum daily integrals reached 4 kg tree^–1. The response of sap flow rate to abrupt flux change when inducing emboli by cutting-off the stem was very rapid: the registered signal dropped by 85% within 10 min for a specimen with a projected leaf area of 2 m². For S. viminalis trees, transpiration was linearly correlated with stem cross-sectional area and with leaf area.     

Sapwood to heartwood ratio affects whole-tree water use in dry forest legume and non-legume trees

We investigated vegetation structure, seasonal water use and leaf deciduousness in a seasonally dry forest of Dzibilchaltún, Mexico. Legumes, species which tend to dominate these forests, have an array of water-saving traits. We explored whether legume species had reduced water use under similar growth conditions as other non-legume species of this seasonally dry forest. Sap flux and conductive sapwood area were measured for eight legume and 12 non-legume species. Species abundance, diameter at breast height (DBH), wood density and seasonal leaf cover were characterized in 16, 10 × 10 m² plots. Seasonal stand water use was calculated using the sap flux and ecological data. As predicted, legumes presented lower whole-tree water use compared with sympatric non-legume species. This difference, however, was related to a higher allocation to non-conductive heartwood in legumes and not to differences in sap flux density. Differences in allocation were higher in wider stems (>10 cm DBH); legumes above 25 cm DBH presented nearly half the daily water use of non-legumes of similar size. Wet (July) and dry (March) season stand water use was 629,000 and 156,000 kg ha^?1 month^?1, respectively. During the wet season three non-legume species with high basal area dominated the stand water use, but due to early leaf fall in these species, dry season stand water use was dominated by the legumes.     

黄土丘陵区辽东栎和刺槐树干液流时滞效应与蒸腾特征的关联性

运用Granier热扩散探针法,于2016年7-9月对半干旱黄土丘陵区天然次生林树种辽东栎和人工林树种刺槐的树干液流进行连续测定,并同步监测气象因子和土壤含水量,用错位相关法分析液流通量密度与空气水汽压亏缺日变化的时滞长度,研究2个树种不同径级个体在不同土壤水分条件下液流通量密度与蒸腾驱动因子之间的时滞效应.结果表明:辽东栎和刺槐液流通量密度的日变化节律与气象因子显著相关,空气水汽压亏缺峰值的出现较辽东栎树干液流通量密度滞后118.2 min,较刺槐树干液流通量密度滞后39.5 min;而光合有效辐射的峰值通常滞后于辽东栎12.4 min,提前于刺槐68.5 min.液流通量密度和空气水汽压亏缺的时滞长度与树种和土壤含水量显著相关,辽东栎、刺槐在土壤含水量较高时段的时滞长度分别大于土壤含水量较低时段32.2和68.2 min.时滞长度与径级的相关性整体上未达到显著水平,但在土壤含水量较低时段小径级刺槐的时滞长度大于大径级21.4 min,差异达到了显著水平.两树种液流通量密度与空气水汽压亏缺之间的时滞效应反映了对蒸腾驱动因子的敏感性,较好的土壤水分条件有利于液流通量密度提早达到峰值,较低土壤水分会导致树干液流对气象环境因子响应的敏感性降低;刺槐树干液流受土壤水分的影响更显著.     

北京绿化树种油松、雪松和刺槐树干液流的空间变异特征

城市绿化树木具有多重生态效应, 其耗水量不容忽视。在不了解树干液流空间变异的前提下, 将点的测定值推广到整树或者林段尺度会产生很大的误差。为准确地确定整树耗水, 采用热消散探针法研究了夏秋季北京成年常绿树种油松(Pinus tabulaeformis)、雪松(Cedrus deodara)和刺槐(Robinia pseudoacacia)树干液流的空间变异特征及产生原因。各树种树干液流存在方位变异, 受树干靠南的方向受光较多、木材解剖特征和枝下高高度的影响, 油松和雪松液流密度与方位之间的关系较为固定, 而刺槐液流密度与方位之间的关系表现出随机性。不同方位每小时液流密度之间高度相关(p < 0.000 1)。因此, 可以基于这种关系准确地计算其他方位的液流(R² > 0.91, p < 0.000 1)。油松和雪松树干液流的径向变异显著, 较深处和较浅处树干液流的日变化格局相似, 但是较深处的液流明显滞后于较浅处的树干液流, 且较浅处树干液流对环境因子的响应远高于深处的液流。不同深度树干液流之间密切相关, 因此可以利用较浅处的液流外推其他深度的液流(R² > 0.89, p < 0.000 1)。然而, 同一棵树不同方位径向剖面特征不同, 雪松南向较深处的液流明显高于其他方位, 且滞后不显著, 这与树冠南向受光较多有关。结合误差分析, 采取北向15 mm和75 mm深处的液流密度均值来估算整树耗水较为准确。     

Epidermal diffusive conductance as related to leaf water potential and photon flux density

Response of epidermal diffusive conductance to simultaneous changes in leaf water potential and photon flux density was studied in primary bean leaves. Values of epidermal conductance corresponding to every photon flux density decreased with decreasing leaf water potential below - 6.9 x 10⁵Pa; slight deorease was followed by a rapid one at water potential ranging from - 8.0 to -10.5 x 10⁵ Pa. In the leaves with water potential lower than -10.5 x 10⁵ Pa neither the saturated photon flux density (1200 [xeinstein m^-2s^-1) induced photoactive stomatal opening. Negative influence of one factor could be partially compensated by positive influence of the other. These results were in good agreement with the considered mechanism of action of leaf water potential and photon flux density on epidermal conductance.     

Time constant for water transport in loblolly pine trees estimated from time series of evaporative demand and stem sapflow

 The use of stem sap flow data to estimate diurnal whole-tree transpiration and canopy stomatal conductance depends critically upon knowledge of the time lag between transpiration and water flux through the stem. In this study, the time constant for water movement in stems of 12-year-old Pinus taeda L. individuals was estimated from analysis of time series data of stem water flux and canopy transpiration computed from mean daytime canopy conductance, and diurnal vapor pressure deficit and solar radiation measurements. Water uptake through stems was measured using a constant-heat sapflow probe. Canopy transpiration was correlated to stem uptake using a resistance-capacitance equation that incorporates a time constant parameter. A least-squares auto-regression determined the parameters of the resistance-capacitance equation. The time constants for ten loblolly pine trees averaged 48.0 (SE = 2.0) min and the time lag for the diurnal frequency averaged 47.0 (SE = 2.0) min. A direct-cross correlation analysis between canopy transpiration and sap flow time series showed maximum correlation at an approximately 30 min lag. Residuals (model-predicted minus actual stem flow data) increased with increasing soil moisture depletion. While the time constants did not vary significantly within the range of tree sizes studied, hydraulic resistance and capacitance terms were individually dependent on stem cross-sectional area: capacitance increased and resistance decreased with stem volume. This result may indicate an inverse adjustment of resistance and capacitance to maintain a similar time constant over the range of tree sizes studied.     

A feedback control system for the precise and continuous regulation of photosynthetic photon flux density

A simple and inexpensive feedback control system that provides continuous and precise control of photosynthetic photon flux density (PPFD) in a whole plant cuvette is described. A ‘Plexiglass’ tank is interposed between a light source and cuvette and PPFD changed by varying the level of dyed liquid in the tank. The amount of liquid pumped into or drained from the tank is a function of the difference (error) between a defined set point value of PPFD and that measured in the cuvette. The set point can be varied as a function of time, can follow the output of a quantum sensor measuring ambient PPFD or can be driven by values of PPFD read from a data file. Within the 0.4 to 0.64 μm waveband, the dye acts as a neutral density filter so that there is no change in spectral distribution with PPFD. Photosynthetic photon flux density in the cuvette was controlled to better than 20 μmol m⁻²s⁻¹ when the set point was varied from 200 to 1100 μmol m⁻²s⁻¹ over 3 min. When the set point was held constant or changed less rapidly, errors did not exceed 5 μmol m⁻²s⁻¹. Net photosynthesis of Western redcedar (Thuja plicata Donn.) seedlings held at 18 °C closely followed rapid changes in PPFD.     

The accuracy of the thermal dissipation technique for estimating sap flow is affected by the radial distribution of conduit diameter and density

There are conflicting reports on the accuracy of the thermal dissipation probe (TDP, the Granier method) measurement using the original formula, which is widely used to estimate the transpiration of individual trees and forest stands. In this article, six woody species of three wood types were used to study a possible association between TDP measurement accuracy and wood anatomical characteristics, including the vessel diameter and density, as well as sapwood depth. We found that TDP technique with Granier’s original equation underestimated the sap flux density in six species to various degrees, dependent on conduit size and sap flux. Our calibration using two conifers with small diameters and a high density of tracheids was relatively consistent with Granier’s calibration; however, because there were larger diameters and lower densities of vessels in the two diffuse-porous species, the original calibration significantly underestimated sap flow. Two ring-porous species had the largest diameters and lowest densities of vessels. In particular, Robinia pseudoacacia possessed the shallowest sap wood depth, less than a probe length. Our calibration for the ring-porous species, especially R. pseudoacacia, deviated far from the original calibration, which mostly underestimated the sap flow. The degree of underestimation was well associated with sap wood depth and the radial diameter and density distribution of conduits. Our results demonstrated that a new calibration must be operated for each species together with the sapwood depth determination and more probes may be applied for one stem in the field to obtain the more accurate sap flux. In addition, we investigated the effects of different environmental temperature and perfusing fluid composition on the TDP-based sap flux measurement. We found that an environmental temperature reduction from 25 to 0 °C did not alter the values of the maximum temperature difference (ΔT_m) between a heated probe and a reference probe when there was no sap flow, verifying that ΔT_m measured at night can be used as a reference in daytime.     

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.     

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。     

Canopy transpiration in a chronosequence of Central Siberian pine forests

Tree transpiration was measured in 28, 67, 204 and 383‐y‐old uniform stands and in a multicohort stand (140–430 y) of Pinus sylvestris ssp. sibirica Lebed. in Central Siberia during August 1995. In addition transpiration of three codominant trees was monitored for two years in a 130‐y‐old stand. All stands established after fire. Leaf area index (LAI) ranged between 0.6 (28‐y‐old stand) and 1.6 for stands older than 67‐y. Stand xylem area at 1.3 m height increased from 4 cm² m^?2 (28‐y) to 11.5 cm² m^?2 (67‐y) and decreased again to 7 cm² m^?2 in old stands. Above‐ground living biomass increased from 1.5 kg dry weight m^?2 (28‐y) to 14 kg dry weight m^?2 (383‐y). Day‐to‐day variation of tree transpiration in summer was dependent on net radiation, vapour pressure deficit, and soil water stress. Tree‐to‐tree variation of xylem flux was small and increased with heterogeneity in canopy structure. Maximum rates of xylem flux density followed the course of net radiation from mid April when a constant level of maximum rates was reached until mid September when low temperatures and light strongly reduced flux density. Maximum sap flux density (60 g m^?2 s^?1) and canopy transpiration (1.5 mm d^?1) were reached in the 67‐y stand. Average canopy transpiration of all age classes was 0.72 ± 0.3 mm d^?1. Canopy transpiration (E) was not correlated with LAI but related to stand sapwood area SA (E = ? 0.02 + 1.15SA R²) which was determined by stand density and tree sapwood area.     

Seasonal variation of methane emissions from a temperate swamp

Methane flux measurements were made at four sites in a freshwater temperate swamp over the 13 month period of April 1985 through May 1986. Emissions were highly variable both between sites and over time at any one site. Ebullition from sediments was an important component of methane release. Although release of methane through bubbling occurred in only 19% of the measurements made between April and June 1985, when instrumentation allowed us to separate diffusive and bubble fluxes, ebullition accounted for 34% of the total flux during this period. Methane release rates showed a strong seasonal variation, with highest emission rates observed in early spring and again in late summer, which was associated with changes in plant growth and physiology. Emission rates were partially correlated with sediment temperature, but the relationship was not straightforward, and resembled a step function. Emissions responded strongly to temperature change through the range of 10–16°C. At winter sediment temperatures between 4–9°C, CH₄ flux continued at low rates (0–28 mg CH₄ m^–2d^–1; average = 7.9 mg CH₄m^–2d^–1) and appeared insensitive to changes in sediment temperature. Annual methane emission from three constantly flooded sites (mean water depth = 35 cm) was 43.7 +/- 7.8 gm^–2 (standard error); annual flux from a bank site was 41.4 +/- 20.5 gm^–2. A comparison of flux measurements from fresh and saline wetlands in the immediate area of Newport News Swamp emphasizes the importance of edaphic factors in controlling flux.     

Estimation of carbon allocation to the roots from soil respiration measurements of oil palm

CO₂ flux from the soil was measured in situ under oil palms in southern Benin. The experimental design took into account the spatial variability of the root density, the organic matter in the soil-palm agrosystem and the effect of factors such as the soil temperature and moisture.Measurements of CO₂ release in situ, and a comparison with the results obtained in the laboratory from the same soil free of roots, provided an estimation of the roots contribution to the total CO₂ flux. The instantaneous values for total release in situ were between 3.2 and 10.0 mol CO₂ m^-2 s^-1. For frond pile zones rich in organic matter, and around oil palm trunks, root respiration accounted for 30% of the efflux when the soil was at field capacity and 80% when the soil was dry with a pF close to 4.2. This proportion remained constant in interrow zones at around 75%, irrespective of soil moisture.Subsequently carbon allocation to the roots was determined. Total CO₂ release over a year was 57 Mg of CO₂ ha^-1 yr^-1 (around 1610 g of C per m² per year), and carbon allocation to the roots was approximately 53 Mg of CO₂ ha^-1 yr^-1 of which approximately 13 Mg CO₂ ha^-1 yr^-1 (25%) was devoted to turn-over and 40 Mg CO₂ ha^-1 yr^-1 (75%) to respiration.     

Electron allocation to alternative substrates of Azotobacter nitrogenase is controlled by the electron flux through dinitrogenase

The electron flux through dinitrogenase (MoFe protein, protein containing Mo and Fe) from Azotobacter vinelandii controls the relative effectiveness of alternative substrates as electron acceptors in the nitrogenase system. The electron flux through dinitrogenase reductase (Fe protein) or the concentration of MgATP do not directly control electron allocation but rather control it via their influence on the electron flux through dinitrogenase. Kinetic properties of substrate reduction were studied as a function of the electron flux through dinitrogenase. N₂ was most effective at high electron fluxes, whereas H⁺ was the most effective acceptor at very low rates of electron flow through dinitrogenase. The K_m for acetylene was dependent on the electron flux through dinitrogenase, whereas the K_m for N₂ was much less sensitive to this electron flux. The lag period before the onset of acetylene reduction was proportional to the turnover time of dinitrogenase, and was approx. 12 times greater than the dinitrogenase turnover time. pH has effects on the electron allocation to substrates beyond that expected from the effect of pH on the electron flux; thus, pH may alter the relative ability of the nitrogenase enzyme system to reduce alternative substrates.     

RATE AND TIME OF DNA SYNTHESIS OF INDIVIDUAL CHINESE HAMSTER CELLS

The duration of DNA synthesis of a diploid cell line of Chinese hamster fibroblasts was determined in a comparative study by the FLM technique, and also by a new technique for measuring the rate of DNA synthesis of individual cells. These methods produced comparable results when applied during exponential growth of the cells. The rate of DNA synthesis was measured by means of quantitative autoradiography following a short-term incubation of the cells with 5 × 10^-6 M FUdR and 10^-5 M ¹⁴C-TdR. The choice of the medium for this purpose did not seem to be critical. The autoradiographic silver grains over cells and ¹⁴C-standard sources are counted by microphotometry using incident light bright-field. The direct measurements of DNA synthesis rate are ‘compartment’ statistics which have been converted into ‘flux’ parameters for comparison with the FLM method and applicability in cell-kinetic calculations. Frequency distributions of the rate of DNA synthesis of individual cells thus obtained may resemble normal distributions quite closely. They result from several factors: differences in the rate of synthesis in different parts of the S-phase, the density distribution of cells within the S-phase, the variation in the time of DNA synthesis among individual cells, and the experimental error. In the case of a pronounced partial synchronization as probably has been present in one experiment performed in the lag phase, an incorrect time of DNA synthesis may result from the rate values. Due to the variation in DNA synthesis rate in different parts of the S-phase it is not possible to determine the duration of DNA synthesis of an individual cell. However, the mean values of DNA synthesis time are reliable. The new method will be preferentially applied for determining the duration of DNA synthesis of human cells in as far as difficulties are encountered with the classical methods. In addition, it may be used to advantage for studying cells which make up low percentages in mixed populations. It finally permits a safer morphological classification of the cells under study than is possible with the classical methods.     

The physical structure and dynamics of a deep,meromictic crater lake (Lac Pavin,France)

The characteristic feature of the physical structure of Lac Pavin is a distinct and permanent chemically induced density increase between about 60 and 70 m depth. This chemocline separates the seasonally mixed mixolimnion from the monimolimnion, which is characterized by elevated temperature and salinity as well as complete anoxia. Previously published box-models of the lake postulated substantial groundwater input at the lake bottom, and consequently a short water residence time in the monimolimnion and high fluxes of dissolved constituents across the chemocline. We present a new view of the physical structure and dynamics of Lac Pavin, which is based on the results of high-resolution CTD profiles, transient and geochemical tracers (tritium, CFCs, helium), and numerical modeling. The CTD profiles indicate the existence of a sublacustrine spring above rather than below the chemocline. A stability analysis of the water column suggests that vertical turbulent mixing in the chemocline is very weak. A numerical one-dimensional lake model is used to reconstruct the evolution of transient tracer distributions over the past 50 years. Model parameters such as vertical diffusivity and size of sublacustrine springs are constrained by comparison with observed tracer data. Whereas the presence of a significant water input to the monimolimnion can clearly be excluded, the input to the mixolimnion – both at the surface and from the indicated sublacustrine spring – cannot be accurately determined. The vertical turbulent diffusivity in the chemocline is well constrained to K 5×10^-8 m² s^-1, about a factor of three below the molecular diffusivity for heat. Assuming thus purely molecular heat transport, the heat flow through the chemocline can be estimated to between 30 and 40 mW m^-2. With respect to dissolved constituents, the very weak turbulent diffusive exchange is equivalent to a stagnant monimolimnion with a residence time of nearly 100 years. Based on these results and vertical concentration profiles of dissolved species, diffusive fluxes between monimolimnion and mixolimnion can be calculated. A large excess of helium with a ³He/⁴He ratio of (9.09 ± 0.01)×10^-6 (6.57 R _a) is present in the monimolimnion, clearly indicating a flux of magmatic gases into the monimolimnion. We calculate a flux of 1.0×10^-12 mol m^-2 s^-1 for mantle helium and infer a flux of 1.2×10^-7 mol m^-2 s^-1 (72 t year^-1) for magmatic CO₂. The monimolimnion appears to be in steady state with respect to these fluxes.     

祁连山青海云杉树干液流密度的优势度差异

以祁连山排露沟小流域青海云杉林为研究对象,选取有代表性的优势木、亚优势木、中等木和被压木各3—5株,2015年6月16日至10月14日应用热扩散技术对不同优势度青海云杉树干液流密度进行测定,并同步测定相关的林外气象因子。结果表明:(1)青海云杉液流密度呈昼高夜低趋势,晴天液流密度变化幅度较大,而阴雨天变化幅度较小。(2)晴天树木优势度越大,其液流在日内的启动越早,结束越晚,峰值也越大;优势木的平均液流密度为(0.0758±0.0475)m L cm~(-2)min~(-1),是亚优势木的1.5倍,是中等木和被压木的1.68倍。(3)青海云杉平均液流密度基本呈现6月份最大,其次是8月份,9、10月份明显减小,且优势木亚优势木中等木被压木。(4)相关性分析和逐步回归表明,青海云杉日均液流密度与太阳辐射强度、饱和水气压差和空气温度呈正相关关系,与空气相对湿度和降雨量呈负相关关系。影响优势木、亚优势木和中等木液流密度的主要气象因子是太阳辐射强度,被压木液流密度主要受空气相对湿度的影响。