Early-season effects of supplemented solar UV-B radiation on seedling emergence, canopy structure, simulated stand photosynthesis and competition for light

Mixtures and monocultures of wheat (Triticum aestivum) and wild oat (Avena fatua), a common weedy competitor of wheat, were exposed to enhanced solar UV-B radiation simulating a 20% reduction in stratospheric ozone to assess the timing and seasonal development of the UV-B effects on light competition in these species. Results from two years of field study revealed that UV-B enhancement had no detectable effect on the magnitude or timing of seedling emergence in either species. End-of-season measurements showed significant UV-B inhibition of leaf insertion height in wild oat in mixture and monoculture in the second year (irrigated year) but not in the first year (drought year). Leaf insertion height of wheat was not affected by UV-B in either year. The UV-B treatment had no detectable effect on monoculture or total (combined species) mixture LAI but did significantly increase (5–7%) the fractional contribution of wheat to the mixture LAI after four weeks of growth in both years. In addition, the UV-B treatment had subtle effects on LAI height profiles with early season mixtures showing significant reductions in wild oat LAI in lower canopy layers in both years while midseason Year 2 mixtures showed significant reductions in wild oat LAI in upper canopy layers. The changes in canopy structure were found to significantly increase (6–7%) the proportional simulated clear sky canopy photosynthesis and light interception of wheat in mixture. These findings, and others, indicate that the effects of UV-B enhancement on competition are realized very early in canopy development and provide additional support for the hypothesis that UV-B enhancement may shift the balance of competition between these species indirectly by altering competitive interactions for light.     

UV-B and PAR in single and mixed canopies grown under different UV-B exclusions in the field

The purpose of this study was to investigate whether differences in canopy architecture due to the investigated species (planophile versus erectophile, single versus mixed canopies) or to UV-B effects on plant morphology, lead to differences in UV-B and UV-B/PAR doses within canopies.The development of a very small (10 mm diameter) UV-B and PAR sensor on a long 5 mm wide stick allowed us to measure the penetration of UV-B and PAR in single and mixed canopies of the grass Dactylis glomerata and white clover, Trifolium repens. The plants were grown in greenhouses covered with different thicknesses (3 and 5 mm) of UV-transmittant plexi (12 and 18% UV-B exclusion).For clover, a planophile vegetation, radiation penetration was very low for both UV-B and PAR. UV-B penetration was much less than for PAR, resulting in low UV-B/PAR ratio's within the canopy. This is explained by the low UV-B transmittance of the leaves (<0.1 %) in combination with the planophile leaves.In the grass species, both UV-B and PAR penetrated much deeper into the canopy due to the erectophile structure. The difference between UV-B and PAR penetration was generally quite small except in very tall canopies.The mixed species canopies showed results comparable to the clover canopies. Due to the strongly increased grass growth in these plots, light penetration was generally much lower than in the single species cultures. The increased growth of grass in these mixed plots could be linked to the lower UV-B/PAR dose they received.In plots grown under the higher UV-B level there was a relative decrease in UV-B/PAR ratio within the canopy for both species, compared to canopies from the lower UV-B greenhouses. This could not be explained by changes in leaf angle or biomass, but might be linked to the increase in leaf transmittance of PAR.     

Modelling canopy photosynthesis using parameters determined from simple non-destructive measurements

Most models for canopy photosynthesis require a large number of parameters as input which have to be determined by means of direct measurements. Such measurements are usually expensive, time consuming and destructive. The objective of the present study was, therefore, to develop a simple but accurate canopy photosynthesis model based on a minimum number of parameters that can be determined non-destructively. The results from previous studies were used to derive an empirical expression which describes the variation in leaf photosynthetic capacity (P_m) as a function of the light distribution in the canopy. The light distribution itself was calculated with a simple model which assumes only three leaf angle classes (0–30°, 30–60° and 60–90°). The leaf area index was determined indirectly from measurements of direct radiation below the canopy. The result was a model for canopy photosynthesis that requires only a few parameters. These parameters are the leaf photosynthetic capacity at the top of the canopy, the relative frequency of leaves in each of the three leaf angle classes, and the fraction of direct radiation below the canopy. Each of these parameters can be determined by means of simple non-destructive measurements. The model was applied to dense stands of two monocotyledonous species: rice (Oryza sativa L.) and pearl millet (Pennisetum americanum (L.) K. Schum.). The rates of canopy photosynthesis thus calculated were compared to those obtained with a more elaborate reference model. The differences between the values obtained with the two models were small. The present photosynthesis model can, therefore, be considered to be a suitable alternative for the more elaborate model. It was further discussed that, since the model is based on purely non-destructive measurements, it will be particularly useful in cases where it is required to estimate canopy photosynthesis at regular intervals over a length of time or in stands of vegetation that cover large areas of land.     

Plant competition for light analyzed with a multispecies canopy model

Summary Competition for light among species in a mixed canopy can be assessed quantitatively by a simulation model which evaluates the importance of different morphological and photosynthetic characteristics of each species. A model was developed that simulates how the foliage of all species attenuate radiation in the canopy and how much radiation is received by foliage of each species. The model can account for different kinds of foliage (leaf blades, stems, etc.) for each species. The photosynthesis and transpiration for sunlit and shaded foliage of each species is also computed for different layers in the canopy. The model is an extension of previously described single-species canopy photosynthesis simulation models. Model predictions of the fraction of foliage sunlit and interception of light by sunlit and shaded foliage for monoculture and mixed canopies of wheat (Triticum aestivum) and wild oat (Avena fatua) in the field compared very well with measured values. The model was used to calculate light interception and canopy photosynthesis for both species of wheat/wild oat mixtures grown under normal solar and enhanced ultraviolet-B (290–320 nm) radiation (UV-B) in a glasshouse experiment with no root competition. In these experiments, measurements showed that the mixtures receiving enhanced UV-B radiation had a greater proportion of the total foliage area composed of wheat compared to mixtures in the control treatments. The difference in species foliage area and its position in the canopy resulted in a calculated increase in the portion of total canopy radiation interception and photosynthesis by wheat. This, in turn, is consistent with greater canopy biomass of wheat reported in canopies irradiated with supplemental UV-B.     

基于无人机激光雷达遥感的亚热带常绿阔叶林群落垂直结构分析

森林植被高度与树木分布格局是植物群落重要结构特征,也是计算森林生物量分布的重要参数。传统的森林群落调查方法耗费大量人力物力难以进行较大尺度的群落结构测量,而一般的遥感影像也难以获得精确的地形信息及垂直结构。近年来激光雷达(Light Detection and Ranging,LiDAR)技术快速发展,能够较好的进行植被三维特征的提取并被广泛应用于森林生态系统检测模拟。且随着无人机低空摄影技术的发展催生的无人机激光雷达(UAV-Lidar)更增加了激光雷达的灵活性以及获取较大范围植被冠层信息的能力。而受限于激光的穿透性以及不同植被类型郁闭度的影响,该技术的应用多局限于在针叶林群落的垂直结构研究,而在常绿阔叶林的研究中应用较少。为探究现有无人机激光雷达设备及垂直结构提取分析技术应用于常绿阔叶林的可行性,利用无人机载激光雷达遥感技术对哀牢山中山湿性常绿阔叶林3块面积1hm~2的样地进行基于数字表面模型以及数字地表高程模型做差得到树冠高度模型测量的植被冠层高度、基于局部最大值法进行单木位置提取并使用Clark-Evans最近邻体分析方法进行样地内高大乔木分布格局的计算。分析结果显示,植被高度提取精度平均大于95%,与地表实测的植被高度值拟合度较高,相关系数R~2介于0.833—0.927之间;3个样地冠层高度平均值分别为18.79、19.08、17.03 m,标准差分别为8.10、7.34、7.17 m。单木探测百分比平均86.3%,用户精度以及生产者精度平均分别为75.69%和65.15%。实测得出三个样地全部高大乔木空间分布格局均为聚集分布,而激光雷达测量结果显示为随机分布或均匀分布。实验显示基于无人机激光雷达技术能够很好地提取植被冠层高度信息并能够较好地获取树木位置,但对于树木空间分布格局判定的准确性有待于进一步探索。未来研究应从多角度对激光雷达测量造成的误差原因予以分析(如环境因素),并进一步研究更为精确的单木提取以及植被高度提取方法,为通过无人机激光雷达测算森林生物量及各种生态过程提供更加精准的指标数据。     

Influence of saddle height on lower limb kinematics in well-trained cyclists: static vs. Dynamic evaluation in bike fitting

ABSTRACT: Ferrer-Roca, V, Roig, A, Galilea, P, and García-López, J. Influence of saddle height on lower limb kinematics in well-trained cyclists: Static vs. dynamic evaluation in bike fitting. J Strength Cond Res 26(11): 3025-3029, 2012-In cycling, proper saddle height is important because it contributes to the mechanical work of the lower limb joints, thus altering pedaling efficiency. The appropriate method to select optimal saddle height is still unknown. This study was conducted to compare a static (anthropometric measurements) vs. a dynamic method (2D analysis) to adjust saddle height. Therefore, an examination of the relationship between saddle height, anthropometrics, pedaling angles, and hamstring flexibility was carried out. Saddle height outside of the recommended range (106-109% of inseam length) was observed in 56.5% of the subjects. Inappropriate knee flexion angles using the dynamic method were observed in 26% of subjects. The results of this study support the concept that adjusting saddle height to 106-109% of inseam length may not ensure an optimal knee flexion (30-40°). To solve these discrepancies, we applied a multiple linear regression to study the relationship between anthropometrics, pedaling angles, and saddle height. The results support the contention that saddle height, inseam length, and knee angle are highly related (R = 0.963, p < 0.001). We propose a novel equation that relates these factors to recommend an optimal saddle height (108.6-110.4% of inseam length).     

Light distribution and foliage structure in an oak canopy

 Leaf angle distribution and shoot bifurcation ratio were measured and related to photon flux density (PFD) distribution in an oak canopy. Leaf angle distribution deviated substantially from random and changed markedly throughout the canopy. The observed leaf angle distribution was described by an ellipsoidal function with the single parameter of the distribution, x, changing from 1.6 at the top of the canopy to 3.2 in the lowest branches. In vertically homogeneous canopies, the extinction coefficient for diffuse radiation is expected to decline with increasing leaf area index (LAI). However, in the canopy studied here, the leaf angle distribution changed with height such that the effective extinction coefficient remained constant. Both shoot bifurcation ratio and leaf number per shoot declined with decreasing PFD inside the canopy. Based on these observed relationships, a simple canopy growth model that assumes horizontal homogeneity of the canopy was constructed. Calculations showed that a steady state, when growth in the upper of the canopy is in equilibrium with decline of lower canopy, the total canopy LAI should equal to 4.3. This predicted value of equilibrium LAI is larger than that estimated from measurements of PFD transmission (LAI=3.3), but smaller than that directly determined by litter collection (LAI=6.2 in 1998). Possible reasons for these discrepancies are discussed. Received: 22 June 1998 / Accepted: 7 April 1999     

Effects of ultraviolet-B radiation on cotton (Gossypium hirsutum L.) morphology and anatomy

Cotton (Gossypium hirsutum L.) crop, cultivated between 40 degrees N and 40 degrees S, is currently experiencing 2-11 kJ m-2 d-1 of UV-B radiation. This is predicted to increase in the near future. An experiment was conducted to study the effect of enhanced UV-B radiation on vegetative and reproductive morphology and leaf anatomy of cotton in sunlit, controlled environment chambers. From emergence to harvest, cotton plants were exposed to 0, 8 or 16 kJ m-2 d-1 of UV-B in a square wave approach for 8 h from 0800 to 1600 h. Changes in plant height, internode and branch length, mainstem node number, leaf area, length and area of petals and bracts, and anther number per flower were recorded. Epidermal cell and stomatal density, stomatal index, leaf thickness, and epidermal, palisade and mesophyll tissue thickness were also measured. Initial chlorotic symptoms on leaves turned into necrotic patches on continued exposure to enhanced UV-B. Exposure to high UV-B reduced both vegetative and reproductive parameters and resulted in a smaller canopy indicating sensitivity of cotton to UV-B radiation. Enhanced UV-B radiation increased epicuticular wax content on adaxial leaf surfaces, and stomatal index on both adaxial and abaxial leaf surfaces. Leaf thickness was reduced following exposure to UV-B owing to a decrease in thickness of both the palisade and mesophyll tissue, while the epidermal thickness remained unchanged. The vegetative parameters studied were affected only by high levels of UV-B (16 kJ m-2 d-1), whereas the reproductive parameters were reduced at both ambient (8 kJ m-2 d-1) and high UV-B levels. The study shows that cotton plants are sensitive to UV-B at both the whole plant and anatomical level.     

Sun-shade adaptability of the red mangrove,Rhizophora mangle (Rhizophoraceae): Changes through ontogeny at several levels of biological organization

Rhizophora mangle L., the predominant neotropical mangrove species, occupies a gradient from low intertidal swamp margins with high insolation, to shaded sites at highest high water. Across a light gradient, R. mangle shows properties of both “light-demanding” and “shade-tolerant” species, and defies designation according to existing successional paradigms for rain forest trees. The mode and magnitude of its adaptability to light also change through ontogeny as it grows into the canopy. We characterized and compared phenotypic flexibility of R. mangle seedlings, saplings, and tree modules across changing light environments, from the level of leaf anatomy and photosynthesis, through stem and whole-plant architecture. We also examined growth and mortality differences among sun and shade populations of seedlings over 3 yr. Sun and shade seedling populations diverged in terms of four of six leaf anatomy traits (relative thickness of tissue layers and stomatal density), as well as leaf size and shape, specific leaf area (SLA), leaf internode distances, disparity in blade–petiole angles, canopy spread: height ratios, standing leaf numbers, summer (July) photosynthetic light curve shapes, and growth rates. Saplings showed significant sun/shade differences in fewer characters: leaf thickness, SLA, leaf overlap, disparity in bladepetiole angles, standing leaf numbers, stem volume and branching angle (first-order branches only), and summer photosynthesis. In trees, leaf anatomy was insensitive to light environment, but leaf length, width, and SLA, disparities in bladepetiole angles, and summer maximal photosynthetic rates varied among sun and shade leaf populations. Seedling and sapling photosynthetic rates were significantly depressed in winter (December), while photosynthetic rates in tree leaves did not differ in winter and summer. Seasonal and ontogenetic changes in response to light environment are apparent at several levels of biological organization in R. mangle, within constraints of its architectural baiiplan. Such variation has implications for models of stand carbon gain, and suggest that response flexibility may change with plant age.     

基于地基激光雷达点云的植被表型特征测量

植物表型是基因型与外界环境共同作用的结果。精确测量植物表型对于植物生理特征与功能性状研究具有重要意义。本研究以加拿大一枝黄花(Solidago canadensis)为对象,对20株植株进行3个月室内培养,各月利用地基激光雷达扫描(terrestrial Li DAR scanning,TLS)系统对实验植株进行多站扫描和点云融合,实现对植株生长过程的连续观测。对于扫描获取的离散点云,利用多端点三维坐标重构法获取植株高度,并基于叶片点云的Delaunay三角网重构叶片表面,获得植株的真实高度、叶面积、叶倾角和方位角等结构参量。对比手动测量结果,发现基于点云重构获得的植株高度与真实植株高度对比,二者间相似性的决定系数(R2)为0.991,叶面积、叶倾角、方位角相似性R2分别为0.989、0.949和0.871;基于TLS点云重构法实现了非破坏性的植物表型测量,能够获得高精度的植物表型特征;多时相扫描能精确监测植物生长过程的表型特征变化。     

Spatial variability of leaf wetness duration in different crop canopies

The spatial variability of leaf wetness duration (LWD) was evaluated in four different height-structure crop canopies: apple, coffee, maize, and grape. LWD measurements were made using painted flat plate, printed-circuit wetness sensors deployed in different positions above and inside the crops, with inclination angles ranging from 30 to 45°. For apple trees, the sensors were installed in 12 east-west positions: 4 at each of the top (3.3 m), middle (2.1 m), and bottom (1.1 m) levels. For young coffee plants (80 cm tall), four sensors were installed close to the leaves at heights of 20, 40, 60, and 80 cm. For the maize and grape crops, LWD sensors were installed in two positions, one just below the canopy top and another inside the canopy. Adjacent to each experiment, LWD was measured above nearby mowed turfgrass with the same kind of flat plate sensor, deployed at 30 cm and between 30 and 45°. We found average LWD varied by canopy position for apple and maize (P<0.05). In these cases, LWD was longer at the top, particularly when dew was the source of wetness. For grapes, cultivated in a hedgerow system and for young coffee plants, average LWD did not differ between the top and inside the canopy. The comparison by geometric mean regression analysis between crop and turfgrass LWD measurements showed that sensors at 30 cm over turfgrass provided quite accurate estimates of LWD at the top of the crops, despite large differences in crop height and structure, but poorer estimates for wetness within leaf canopies.     

Adjustment of foliage structure and function to a canopy light gradient in two co-existing deciduous trees. Variability in leaf inclination angles in relation to petiole morphology

 Foliar inclination angles, petiole morphology and dry matter partitioning between assimilative and support biomass were studied in shade-intolerant Populus tremula L. and shade-tolerant Tilia cordata Mill. along a natural light gradient across the canopy. The leaves of sub-canopy species T. cordata were on average exposed to lower irradiances, and they were also more horizontal with greater blade inclination angles (ϕ_B, defined as the angle between the leaf fall-line and the horizon; ϕ_B was positive for the leaves inclined upwards, and negative for the leaves inclined downwards) than those in P. tremula. Seasonal average daily integrated quantum flux density (Q _int, mol m^–2 day^–1) and ϕ_B were not related in T. cordata, and only a weak negative effect of Q _int on ϕ_B was detected in P. tremula. Nevertheless, when both species were pooled, there was a strong negative relationship between Q _int and ϕ_B, implying that the leaves became progressively vertical with increasing height in the canopy. Interspecific differences in foliage inclination were mainly related to petiole morphology, in particular to petiole length, rather than to contrasting biomass investment patterns between assimilative and support tissues within the leaf. It was suggested that more horizontal leaves, resulting from the species-specific structure of petioles, partly explain the superior performance of shade-tolerant T. cordata in the understory and the sub-canopy. Received: 13 November 1997 / Accepted: 6 March 1998     

The response of five tropical dicotyledon species to solar ultraviolet-B radiation

Tropical regions currently receive the highest levels of global solar ultraviolet-B radiation (UV-B, 280–320 nm) even without ozone depletion. The influence of natural, present-day UV-B irradiance in the tropics was examined for five tropical species including three native rain forest tree species (Cecropia obtusifolia, Tetragastris panamensis, Calophyilum longifolium) and two economically important species (Swietenia macrophylla, Manihot esculenta). Solar UV-B radiation conditions in a small clearing on Barro Colorado Island, Panama (9° N), were obtained using either a UV-B-excluding plastic film or a film that transmits most of the solar UV-B. Significant differences between UV-B-excluded and near-ambient UV-B plants were often exhibited as increased foliar UV-B absorbing compounds and, in several cases, as reduced plant height with exposure to solar UV-B. Increases in leaf mass per area and reductions in leaf blade length under solar UV-B occurred less frequently. Biomass and photosystem II function using chlorophyll a fluorescence were generally unaffected. The results of this study provide evidence that tropical vegetation, including native rain forest species, responds to the present level of natural solar UV-B radiation. This suggests that even minor ozone depletion in the tropics may have biological implications.     

Ambient UV-B radiation reduces PSII performance and net photosynthesis in high Arctic Salix arctica

Ambient ultraviolet-B (UV-B) radiation potentially impacts the photosynthetic performance of high Arctic plants. We conducted an UV-B exclusion experiment in a dwarf shrub heath in NE Greenland (74°N), with open control, filter control, UV-B filtering and UV-AB filtering, all in combination with leaf angle control. Two sites with natural leaf positions had ground angles of 0° (‘level site’) and 45° (‘sloping site’), while at a third site the leaves were fixed in an angle of 45° to homogenize the irradiance dose (‘fixed leaf angle site’). The photosynthetic performance of the leaves was characterized by simultaneous gas exchange and chlorophyll fluorescence measurements and the PSII performance through the growing season was investigated with fluorescence measurements. Leaf harvest towards the end of the growing season was done to determine the specific leaf area and the content of carbon, nitrogen and UV-B absorbing compounds. Compared to a 60% reduced UV-B irradiance, the ambient solar UV-B reduced net photosynthesis in Salix arctica leaves fixed in the 45° position which exposed leaves to maximum natural irradiance. Also a reduced Calvin Cycle capacity was found, i.e. the maximum rate of electron transport (J_max) and the maximum carboxylation rate of Rubisco (V_cmax), and the PSII performance showed a decreased quantum yield and increased energy dissipation. A parallel response pattern and reduced PSII performance at all three sites indicate that these responses take place in all leaves across position in the vegetation. These findings add to the evidence that the ambient solar UV-B currently is a significant stress factor for plants in high Arctic Greenland.     

Assessing the Geometric Structure of a White Clover (Trifolium repens L.) Canopy using3-D Digitising

A method was developed for assessing the three dimensional (3-D)geometric structure of white clover canopies. 3-D co-ordinatesof pre-defined points on leaves, petioles and stolons were measuredusing a Polhemus Fastrak electromagnetic 3-D digitiser. Digitisingprogressed downwards from the top of the canopy and plant partswere removed after they have been digitised. Leaflets were treatedas four quarter-ellipses, and petiole and stolons were treatedas cylinders. Leaf dimensions and areas calculated from 3-Dco-ordinates were within about 5% and 20% of direct measurementsmade with a ruler and a planimeter, respectively. Special softwareand freeware POV-Ray were used to reconstruct a virtual canopyfrom digitiser records and to calculate canopy characteristicssuch as leaf area index (LAI), petiole intersection area, andprofiles of leaflet areas and inclinations with height. It tookbetween 3 and 7 h to digitise 10 x 10 cm stands of clover andthe resulting information was considerably more comprehensiveand accurate than could have been obtained by the alternative‘point quadrat’ or ‘stratified clipping’methods.Copyright 2000 Annals of Botany Company White clover, Trifolium repens, geometric structure, leaf area, leaf angle, 3-D digitising     

Response of clonal plasticity of Fargesia nitida to different canopy conditions of subalpine coniferous forest

The aim of this study is to explore the effects of canopy conditions on clump and culm numbers, and the morphological plasticity and biomass distribution patterns of the dwarf bamboo species Fargesia nitida. Specifically, we investigated the effects of canopy condi-tions on the growth and morphological characteristics of F. nitida, and the adaptive responses of F. nitida to dif-ferent canopy conditions and its ecological senses. The results indicate that forest canopy had a significant effect on the genet density and culm number per clump, while it did not affect the ramet density. Clumps tended to be few and large in gaps and forest edge plots, and small under forest understory plots. The ramets showed an even distribution under the closed canopy, and clus-ter distribution under gaps and forest edge plots. The forest canopy had a significant effect on both the ramets'biomass and biomass allocation. Favourable light conditions promoted ramet growth and biomass accumulation. Greater amounts of biomass in gaps and forest edge plots were shown by the higher number of culms per clump and the diameter of these culms. Under closed canopy, the bamboos increased their branching angle, leaf biomass allocation, specific leaf area and leaf area ratio to exploit more favourable light conditions in these locations. The spacer length, specific spacer length and spacer branching angles all showed significant differences between gaps and closed canopy conditions. The larger specific spacer length and spacer branching angle were beneficial for bamboo growth, scattering the ramets and exploiting more favourable light conditions. In summary, this study shows that to varying degrees, F nitida exhibits both a wide ecological amplitude and high degree of morphological plasticity in response to differing forest canopy conditions. More-over, the changes in plasticity enable the plants to optimize their light usage efficiency to promote growth and increase access to resources available in heteerogeneous light environoments.     

UV-B and PAR in a grass (Lolium perenne L.) canopy

The penetration of natural and artificial UV-B_BE (Biologically Effective UV-B, Caldwell 1971) and PAR (400–700 nm) in a grass canopy with increasing LAI was followed during 2 months. Overall, the transmission of UV-B_BE sunlight is significantly higher than of PAR sunlight. This is mainly due to the higher proportion of diffuse light in the UV-B. Under cloudy conditions no difference between UV-B_BE and PAR could be found. Sun angle and intensity of the radiation were less important in determining the penetration of light. Artificial light penetrates much more through the canopy, resulting in higher irradiation levels in the lower part of the canopy, but a lower UV-B_BE/PAR ratio (since UV-B transmittance of the leaves is lower). The UV-B_BE/PAR ratio reaching the leaves was influenced by LAI, sun angle, percent diffuse light and leaf angle. The large differences in UV-B_BE/PAR ratio per unit leaf area under natural and artificial light conditions are important in understanding the influence of UV-B on plants.     

水稻冠层光合有效辐射的时空分布特征

以2个不同株型水稻品种为材料,设置高、中、低3个施氮水平,利用SunScan冠层分析仪于灌浆期系统测定了不同施氮水平下不同株型水稻品种植株形态和冠层内光合有效辐射（PAR）的时空分布状况.结果表明：施氮量对水稻株高、穗弯曲度和茎叶夹角有明显影响;群体叶面积的垂直分布呈中部>上部>下部的分布特征,最大分层叶面积指数（LAI）出现在0.60相对高度处.冠层内平均PAR透光率从顶部向下递减,且在冠层上中部递减迅速,下部递减缓慢;平均PAR透光率随施氮量的增加而递减;平均PAR透光率日变化表现为早晚较低,中午较高;平均PAR透光率随向下累积LAI的增加呈指数递减,群体消光系数K的日变化表现为早晚较高,中午较低,灌浆期的K值介于0.35～0.50.水稻冠层内PAR的三维空间分布表现为冠层上中部水平面上PAR透光率高,光斑面积大;下部水平面上PAR透光率低,光斑少;同一冠层高度水平面上的PAR光强呈不均匀分布.株型紧凑的水稻品种,冠层透光率高,透光率日变化大,群体消光系数小.     

A two-concentric-loop iterative method in estimation of displacement height and roughness length for momentum and sensible heat

A two-concentric-loop iterative (TCLI) method is proposed to estimate the displacement height and roughness length for momentum and sensible heat by using the measurements of wind speed and air temperature at two heights, sensible heat flux above the crop canopy, and the surface temperature of the canopy. This method is deduced theoretically from existing formulae and equations. The main advantage of this method is that data measured not only under near neutral conditions, but also under unstable and slightly stable conditions can be used to calculate the scaling parameters. Based on the data measured above an Acacia Saligna agroforestry system, the displacement height (d ₀) calculated by the TCLI method and by a conventional method are compared. Under strict neutral conditions, the two methods give almost the same results. Under unstable conditions, d ₀ values calculated by the conventional method are systematically lower than those calculated by the TCLI method, with the latter exhibiting only slightly lower values than those seen under strictly neutral conditions. Computation of the average values of the scaling parameters for the agroforestry system showed that the displacement height and roughness length for momentum are 68% and 9.4% of the average height of the tree canopy, respectively, which are similar to percentages found in the literature. The calculated roughness length for sensible heat is 6.4% of the average height of the tree canopy, a little higher than the percentages documented in the literature. When wind direction was aligned within 5 ° of the row direction of the trees, the average displacement height calculated was about 0.6 m lower than when the wind blew across the row direction. This difference was statistically significant at the 0.0005 probability level. This implies that when the wind blows parallel to the row direction, the logarithmic profile of wind speed is shifted lower to the ground, so that, at a given height, the wind speeds are faster than when the wind blows perpendicular to the row direction. The field experiment of this study was carried out during the senior author’s sojourn at the Blaustein Institute for Desert Research, Ben Gurion University, Israel. Mentioning of brand names and companies is for the convenience of readers only, and does not imply any affiliation or endorsement between the authors and that company.