Incorporating rhizosphere processes into field-scale (agro)ecosystem models

Three different strategies for incorporating rhizosphere processes within field-scale models are compared, taking triple-cropped irrigated rice production as a common system and CH₄ emission as a common focus of interest. The strategies may be characterised as homogeneous (model I; root C deposition is added to the bulk soil compartment), areal (model II; roots contribute via aerenchymatous exchange to an increased soil–atmosphere interfacial surface area), and volumetric (model III; roots create around themselves a specific rhizosphere compartment). Model I is simpler than model II, which is simpler than model III. With identical parameters all models lead to similar seasonally integrated CH₄ emissions, but when the pattern of emission and the simulated CH₄ concentration in the soil is brought into the reckoning, the following order of precedence (greater is better) becomes clear: model IIImodel II>model I. Current field-scale models of soil organic matter (SOM) transformation, especially in rice soils, could be improved by taking explicit account of the rhizosphere and the processes which occur within it.     

干旱区荒漠稀疏植被覆盖度提取及尺度扩展效应

选择线性混合像元分解模型、亚像元模型、最大三波段梯度差法模型以及修正的三波段梯度差法的2个变异模型来提取植被覆盖度,结合地面实测数据,探讨了提取干旱区荒漠稀疏植被覆盖度信息的适宜模型,并以简单平均法模拟了不同尺度的覆盖度影像,通过尺度上推检验了模型在MODIS尺度上的反演效应.结果表明：线性混合像元分解模型反演覆盖度的精度高于其他模型,适于稀疏植被地区,但端元的正确选取较难,从而影响其运用;亚像元分解模型是一个通用模型,植被分类图越精细,通过亚像元分解模型得到的覆盖度精度越高,但这也同时意味着该模型需要测定大量的输入参数;最大三波段梯度差法的算法简单、易于操作,其在农田等中高植被覆盖区及裸土区的预测值与实测值接近,但对干旱区稀疏植被的估计精度偏低;修正后的三波段最大梯度差法模型在稀疏植被覆盖区的预测值与实测值基本一致,在不同尺度上反演的覆盖度信息与实测值的一致性较好.该方法可有效提取干旱区低覆盖度植被信息.     

植物气孔导度的环境响应模拟及其尺度扩展

气孔导度是衡量植物和大气间水分、能量及CO2平衡和循环的重要指标,探讨气孔导度与环境因子的关系及其模拟,以及气孔导度在叶片、冠层及区域尺度间的尺度转换及累积效应,对更好地认识植被与大气间的水热运移过程,合理评价植被在陆面过程中的地位和作用都具有重要意义。从植物气孔导度与环境因子的关系、气孔导度模拟以及尺度扩展三个方面,对前人的研究成果进行了概括总结。从叶片和冠层两个尺度出发,归纳总结了前人对于不同植物气孔导度与环境因子关系的研究成果,发现由于不同植物的遗传特性、测定时的环境、时间尺度的不同,以及未考虑各个环境因子的相互作用对气孔导度的影响,由此得到的气孔导度与环境因子之间的关系也不尽一致。对各单一环境因子与气孔导度的关系,给出了生理学解释,从根本上说明了环境因子变化对气孔导度的影响,而研究环境因子对气孔导度的综合影响时,应对各环境因子进行系统控制与同步观测。模拟计算植物气孔导度的模型主要有Jarvis模型和BWB模型两类,这些模型的模拟能力随着研究对象、试验区域、环境条件的改变而存在一定的差异,在具体使用时应结合实际情况选择最优模型进行模拟。除上述常用模型外,还总结了其他学者分别从不同角度提出的新的模型,对现有气孔导度模型进行了全面的总结。从叶片-冠层、冠层-区域两个方面归纳总结了前人关于气孔导度尺度扩展的研究成果,发现叶片-冠层的尺度扩展研究较成熟而冠层-区域的尺度扩展在模拟精度的验证方面存在困难。针对以下几个方面提出了今后气孔导度的研究重点:(1)结合研究对象所在的区域及环境条件,选择最优模型进行模拟;(2)综合考虑环境因子之间的相互作用及其对气孔导度的累积影响;(3)BWB模型与光合模型的耦合;(4)提高大尺度范围内的气孔导度模拟精度。     

植物功能型研究方法在生态系统模型中的应用

植物功能型(PFTs)是研究不同时空尺度上植被与全球变化之间关系的新方法,被广泛应用于植物生态学模型研究领域.本文介绍了植物功能型的概念由来以及与生活型(life form)、功能团(guild)、生物群区(biome)的区别和联系,总结了植物功能型的5种常见的划分标准,并将近半个世纪以来植物功能型研究方法在生态模型中...     

基于空间变异性的树冠非结构性碳含量估算：以胡桃楸和春榆为例

以胡桃楸和春榆为例,采用枝解析法测定不同基径枝条的非结构性碳水化合物（NSC）浓度的变异,进而估算并评价不同取样方法对树冠NSC含量估算的误差.结果表明： 器官对两种树种树冠NSC浓度的影响显著,叶、新枝、老枝和死枝的可溶性糖与淀粉的总和（TNC）的平均浓度分别为17.6%、12.6%、5.7%和2.9%.叶和新枝的NSC浓度随枝基径、枝龄、枝长和枝相对高度的变异多不显著,但老枝的NSC浓度随枝基径、枝龄和枝长增大而降低,而随枝相对高度增加而增加,其中枝基径是老枝NSC浓度的最佳预测变量（R²在0.87~0.95）.两树种叶、新枝和老枝平均TNC含量分别占其树冠TNC含量的28%、2%和70%.分析树冠枝NSC的空间变异性对树冠NSC含量估算误差的影响发现,采用新枝和直径约3 cm老枝枝段的NSC浓度估算冠层NSC含量是较简易而精确的方法.     

Linking flux network measurements to continental scale simulations: ecosystem carbon dioxide exchange capacity under non-water-stressed conditions

This paper examines long‐term eddy covariance data from 18 European and 17 North American and Asian forest, wetland, tundra, grassland, and cropland sites under non‐water‐stressed conditions with an empirical rectangular hyperbolic light response model and a single layer two light‐class carboxylase‐based model. Relationships according to ecosystem functional type are demonstrated between empirical and physiological parameters, suggesting linkages between easily estimated parameters and those with greater potential for process interpretation. Relatively sparse documentation of leaf area index dynamics at flux tower sites is found to be a major difficulty in model inversion and flux interpretation. Therefore, a simplification of the physiological model is carried out for a subset of European network sites with extensive ancillary data. The results from these selected sites are used to derive a new parameter and means for comparing empirical and physiologically based methods across all sites, regardless of ancillary data. The results from the European analysis are then compared with results from the other Northern Hemisphere sites and similar relationships for the simplified process‐based parameter were found to hold for European, North American, and Asian temperate and boreal climate zones. This parameter is useful for bridging between flux network observations and continental scale spatial simulations of vegetation/atmosphere carbon dioxide exchange.     

On the persistence of memory: Soft clocks and terrestrial biosphere-atmosphere interactions

The circadian clock is considered a central “orchestrator” of gene expression and metabolism. Concomitantly, the circadian clock is considered of negligible influence in the field and beyond leaf levels, where direct physiological responses to environmental cues are considered the main drivers of diel fluctuations. I propose to bridge the gap across scales by examining current evidence on whether circadian rhythmicity in gas exchange is relevant for field settings and at the ecosystem scale. Nocturnal stomatal conductance and water fluxes appear to be influenced by a “hard” clock that may override the direct physiological responses to the environment. Tests on potential clock controls over photosynthetic carbon assimilation and daytime transpiration are scant yet, if present, could have a large impact on our current understanding and modeling of the exchanges of carbon dioxide and water between terrestrial ecosystems and the atmosphere.