棉花冠层高光谱参数与叶片氮含量的定量关系

建立棉花(Gossypium hirsutum)氮素状况的光谱监测技术对于棉花营养诊断和长势估测具有重要意义。该研究利用冠层高光谱反射率及演变的多种高光谱参数,分析了不同施氮水平下不同棉花品种叶片氮含量与冠层反射光谱的定量关系,建立了棉花叶片氮含量的敏感光谱参数及预测方程。结果显示,棉花叶片氮含量和冠层高光谱反射率随不同施氮水平呈显著变化。棉花叶片氮含量的敏感光谱波段为600~700 nm的可见光波段和750~900 nm的近红外波段,且叶片氮含量与比值植被指数RVI [average (760~850), 700]有密切的定量关系,4个品种的平均决定系数在0.70左右。进一步分析表明,可以用统一的回归方程来描述不同品种、不同生育时期和不同氮素水平下棉花叶片氮含量随反射光谱参数的变化模式,从而为棉花氮素营养的监测诊断与精确施肥提供技术依据。     

利用红边面积形状参数估测水稻叶层氮浓度

研究红边面积参数与叶层氮素状况的定量关系, 有助于水稻(Oryza sativa)生长信息的实时无损获取及精确追氮管理。该研究基于多年不同施氮水平和不同水稻品种的冠层高光谱数据, 系统分析了水稻的红边区域光谱、面积形状特征及其与叶层氮浓度的定量关系。结果表明, 水稻冠层红边区域微分光谱随不同氮素水平变化出现“三峰”现象, 峰值分别出现在700、720和730 nm附近, 且3个波段的峰值高低发生交替变化; 同时, 以3个峰值波段为中心与x坐标轴组成的微分光谱面积和形状相应发生变化。发现基于两两峰值波段划分所得红边子面积所构成的比值(双峰对称度)、归一化差值(归一化对称度)参数与叶层氮浓度具有密切的定量关系, 可作为估测水稻叶层氮浓度的红边面积形状参数。经曲线拟合和模型检验的结果显示, 双峰对称度DPS (A_675-700, A_675-755), 即由675~700 nm区域面积与675~755 nm区域面积的比值, 和DPS (A_730-755,A_675-700) (由730~755 nm区域面积和675~700 nm区域面积的比值)对水稻叶层氮浓度的估测效果最好, 可用于不同水稻品种和生长条件下的叶层氮浓度估测。     

暖温带地区3种森林群落叶面积指数和林冠开阔度的季节动态

以辽东栎(Quercus liaotungensis)为主的落叶阔叶林、华北落叶松(Larix principis-rupprechtii)林和油松(Pinus tabulaeformis)林是暖温带林区具有代表性的森林群落类型。该研究应用国内外流行的半球图方法,通过对这3种森林群落叶面积指数和林冠开阔度的测定和综合比较,分析了叶面积指数和林冠开阔度的季节动态,揭示了暖温带地区不同类型森林群落叶面积指数和林冠开阔度的特征。研究结果表明,落叶阔叶林(优势种为辽东栎、棘皮桦(Betula dahurica)和五角枫(Acer mono))和华北落叶松林两种落叶森林群落的叶面积指数值均随生长季的到来而呈现增长的趋势,最大值出现在8月;林冠开阔度值随着生长季的到来而下降,最大值出现在11月。落叶阔叶林的叶面积指数和林冠开阔度的季节动态较之华北落叶松林明显。油松是常绿树种,其群落叶面积指数和林冠开阔度的变化程度均不明显,但林冠开阔度的变化趋势也是与叶面积指数的变化趋势相反。通过计算得出叶面积指数和林冠开阔度相关显著,并且呈现指数回归的关系。此研究结果为以遥感途径获取暖温带地区叶面积指数提供了地面校正依据,为研究该地区植被林冠的异质性及其造成的影响,以及进一步对该地区林分、景观和区域尺度上碳、水分和通量等方面的模拟提供了基础数据。     

祁连山区青海云杉林冠层叶面积指数的反演方法

叶面积指数(Leaf area index, LAI)是陆地生态系统的一个十分重要的结构参数。随着空间精细化模型的发展和基于过程的分布式模拟技术的应用, 对LAI的区域估算显得越来越重要, 但目前尚缺乏有效的估算手段。该项研究以青海云杉(Picea crassifolia)林为研究对象, 利用LAI-2000冠层分析仪、鱼眼镜头法和经验公式法对林冠层LAI进行了测定, 观测值分别为1.03~3.70、0.48~2.26和2.27~8.20, 显然, 仪器测定值偏低。针对针叶的集聚效应导致仪器测定值偏低的现象, 利用跟踪辐射与冠层结构测量仪(TRAC)测定的青海云杉林聚集系数计算调整系数, 对鱼眼镜头法获取的LAI值进行订正。根据高分辨率的遥感数据反演青海云杉林的植被指数与LAI的关系, 最后获得了较合理的该地区林冠层LAI的空间分布图。     

海岸带复合梨园的光能分布与利用规律

采用方格法和分层法相结合的技术对江苏省如东县棉花原种场林网保护区内13年生梨园的树冠结构特征和光能分布与利用规律进行了系统研究。结果表明:梨树叶幕光能分布状况直接受叶面积系数、树冠结构、太阳位置及光强的影响。各层叶幕中的光合有效辐射(PAR)分布随自然光照条件的变化而改变,其日平均透光率大小和PAR日变化范围均从叶幕外围向内膛随累积叶面积系数(LAI)的增加而减弱;不同生长时期及不同天气变化下树冠光能分布存在明显的差异性;树冠结构与累积LAI对PAR的分布及利用率具有决定性的影响。在生产实践中,通过密度调控、整形修剪等农业技术措施,维持成年梨园盛果期高效光合生产的叶幕厚度在2.5m以上,有效LAI为4左右,是增强梨树光合生产能力、提高产量和品质的有效措施。     

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.     

Leaf area index, canopy structure and photosynthesis of red clover (Trifolium pratense L.)

Abstract. The influence of leaf age, total leaf area and its dispersion in space on canopy photosynthesis were studied using microswards of red clover ( Trifolium pratense L.) which were established in the greenhouse. Two varieties, Renova (flowering) and Molstad (non-flowering), were sown in separate plastic boxes at densities of 225, 400 and 625 plants per m².
Vertical distribution of photosynthetically active radiation (PAR), leaf area, leaf age and ¹⁴CO₂-fixation were determined periodically. Net photosynthesis and dark respiration of canopies were measured. Maximum photosynthetic capacity of individual leaves was measured on plants taken from the intact canopy or from plants where shading of the growing leaves had been prevented.
Net photosynthetic rate of canopies increased linearly with leaf area index (LAI) up to an LAI of 3.5 and then declined at higher LAI, independent of variety and sowing density. Below the optimum LAI, net photosynthesis depended mainly on interception of PAR. Decrease in canopy photosynthesis above the optimum LAI was due to a higher proportion of old leaves with decreased photosynthetic capacity, and not to an increase in respiring plant parts. It is concluded that LAI and position of leaf age categories in the canopy are more important than vertical distribution of leaf area in determining canopy photosynthesis of red clover.     

Carbon gain in a multispecies canopy: the role of specific leaf area and photosynthetic nitrogen-use efficiency in the tragedy of the commons

Models have been formulated for monospecific stands in which canopy photosynthesis is determined by the vertical distribution of leaf area, nitrogen and light. In such stands, resident plants can maximize canopy photosynthesis by distributing their nitrogen parallel to the light gradient, with high contents per unit leaf area at the top of the vegetation and low contents at the bottom. Using principles from game theory, we expanded these models by introducing a second species into the vegetation, with the same vertical distribution of biomass and nitrogen as the resident plants but with the ability to adjust its specific leaf area (SLA, leaf area:leaf mass). The rule of the game is that invaders replace the resident plants if they have a higher plant carbon gain than those of the resident plants. We showed that such invaders induce major changes in the vegetation. By increasing their SLA, invading plants could increase their light interception as well as their photosynthetic nitrogen-use efficiency (PNUE, the rate of photosynthesis per unit organic nitrogen). By comparison with stands in which canopy photosynthesis is maximized, those invaded by species of high SLA have the following characteristics: (1) the leaf area index is higher; (2) the vertical distribution of nitrogen is skewed less; (3) as a result of the supra-optimal leaf area index and the more uniform distribution of nitrogen, total canopy photosynthesis is lower. Thus, in dense canopies we face a classical tragedy of the commons: plants that have a strategy to maximize canopy carbon gain cannot compete with those that maximize their own carbon gain. However, because of this strategy, individual as well as total canopy carbon gain are eventually lower. We showed that it is an evolutionarily stable strategy to increase SLA up to the point where the PNUE of each leaf is maximized.     

白云山国家森林公园不同人为干扰强度的群落冠层结构和光照特征

为探究森林冠层结构与林下光照的变化规律及其相关性,在河南省白云山国家森林公园选取人工林、择伐林、皆伐林和老龄林建立四块面积为1 hm²（100 m×100 m）的固定监测样地,利用半球面影像技术获取冠层结构及林下光照数据。研究发现：1）随着人为干扰强度降低,冠层覆盖度与叶面积指数呈增加趋势,林下散射辐射与直接辐射呈减少趋势;2）择伐林冠层覆盖度与叶面积指数最大,平均叶倾角与透光比最小,林下光照（直接辐射、散射辐射）与冠层覆盖度和叶面积指数都呈显著负相关,林下散射辐射与冠层覆盖度和叶面积指数的负相关关系最强;3）处于不同干扰强度的群落由于冠层结构的差异,形成了不同群落内特定的光照环境。研究结果丰富了暖温带-亚热带生态过渡区森林冠层结构与林下光照动态变化研究资料,同时也为该区域森林的恢复与重建等提供科学的依据。     

Simple scaling of photosynthesis from leaves to canopies without the errors of big-leaf models

In big-leaf models of canopy photosynthesis, the Rubisco activity per unit ground area is taken as the sum of activities per unit leaf area within the canopy, and electron transport capacity is similarly summed. Such models overestimate rates of photosynthesis and require empirical curvature factors in the response to irradiance. We show that, with any distribution of leaf nitrogen within the canopy (including optimal), the required curvature factors are not constant but vary with canopy leaf area index and leaf nitrogen content. We further show that the underlying reason is the difference between the time-averaged and instantaneous distributions of absorbed irradiance, caused by penetration of sunflecks and the range of leaf angles in canopies. These errors are avoided in models that treat the canopy in terms of a number of layers – the multi-layer models. We present an alternative to the multi-layer model: by separately integrating the sunlit and shaded leaf fractions of the canopy, a single layered sun/shade model is obtained, which is as accurate and simpler. The model is a scaled version of a leaf model as distinct from an integrative approach.