大叶黄杨光谱特征及其叶功能性状对叶面降尘的响应

The Response of Spectral Characteristics and Leaf Functional Traits of Euonymus Japonicus to Leaf Dustfall

城市绿化植物叶片具有滞留、固定大气粉尘颗粒物的重要作用。大叶黄杨是北京市种植面积最大的常绿阔叶景观灌木树种,在冬季承担着主要的粉尘颗粒物滞留的作用。由于城市生态系统作为人类活动最频繁剧烈的区域之一,其环境极为复杂。以往的研究主要以随机、分散的城市环境作为采样点。而植物功能性状在长期的生长发育、繁殖和进化过程中对环境变化具有敏感的响应和可塑性,往往在内部或表型结构表现出一定的生态权衡策略。在叶片滞尘量与光谱特征关系的实际研究过程中,往往忽略了植物在不同生境中的水分、土壤、光照及养护模式的重要影响,导致对其光谱的响应解释不清。基于路面粉尘扩散的规律,根据植物种植位置与路面距离划分高、中、低的粉尘浓度环境,以避免因光照、水分、养分、土壤等因素的干扰。结合植物功能性状特征,探讨大叶黄杨在不同粉尘浓度的影响下,其叶面光谱及其叶功能性状的响应机制及其权衡策略,分析了其高光谱参数与叶面降尘量间的相关关系,并建立滞尘量预测模型,旨在为城市环境中利用高光谱检测植被生长提供重要参考。结果表明:(1)粉尘污染的环境中,大叶黄杨普遍呈现出比叶面积低、叶绿素含量低、叶干物质含量高、叶厚度大的性状组合,体现了植物叶片在结构构建投资与回报之间的权衡策略,也充分说明了植物为了适应城市环境污染的生境特征所造成的不良胁迫,而对自身功能性状进行调整。(2)从可见光到近红外波段(350~2500 nm)范围内,共出现了4个明显的反射峰和4个主要的吸收谷。350~1870 nm区间,光谱反射率总体上与叶面降尘量呈负向相关,由此可以看出随着叶面降尘量的增多,其光谱反射率则随之减小。然而,在1870~2500 nm波段对叶面降尘量的变化比较复杂且无明显规律性。(3)700~1410和1470~1830 nm波段是光谱对叶面降尘量的响应最敏感的波段。在680~780 nm区间出现了"红边效应"。在750~1350 nm区间出现了一个较高的反射平台,这可能是由于叶片水分对该波段具有较强的吸收性。(4)红边斜率、蓝边斜率、黄边斜率、黄边位置对叶面降尘的干扰十分敏感,但红边位置、蓝边位置对其响应不明显。结合叶功能性状的权衡策略可知,由于大叶黄杨在长期的粉尘污染环境影响下,形成了特殊的适应机制,叶面降尘对红边位置和蓝边位置影响不敏感,表现出了较强的抗干扰能力。红边斜率和蓝边斜率对叶面降尘的响应呈负向相关,而黄边斜率对叶面降尘量的响应则呈正向相关。同时,随着叶面降尘量的增加,黄边位置发生了明显的"左移"现象。(5)以叶面水含量指数、叶绿素指数、红边指数、归一化指数、简单比值指数、光合反射指数光谱参数作为自变量,以大叶黄杨的叶面降尘量作为因变量进行回归拟合,分别建立线性、二次多项式和对数形式的叶面降尘量预测模型。所有的模型中,以叶面水含量指数建立的二次多项式预测模型对叶面降尘量具有较高的预测精度(y=-1.1123x^2+0.5439x+0.9911,R^2=0.8289,RMSE=0.122)。

Leaves of urban green plants have an important role in retaining and fixing atmospheric dust particles.Euonymus japonicus is the evergreen broad-leaved shrub species with the largest acreage in Beijing,which play a major role in the retention of dust particles in winter.An urban ecosystem is one of the most frequent and intense areas of human activity.Its environment is extremely complex.In the past studies,the random and scattered urban environment was mainly used as sampling points.However,plant functional traits have a certain ecological trade-off strategy in internal or phenotypic structures due to their sensitive response and plasticity to environmental changes during long-term growth,reproduction and evolution.In the actual research process of the relationship between leaf dust retention and spectral characteristics,the important influences of water,soil,light and conservation mode of plants in different habitats were often neglected,which could not clearly explain the problem of the spectral response.Based on the law of dust particle diffusion,this study divides the high,medium and low dust concentration environment according to plant position and road surface distance,which can better avoid interference caused by light,moisture,nutrients and soil,etc.Combining the characteristics of plant functional traits,the trade-off strategy of leaf surface spectrum and leaf functional traits of Euonymus japonicus under different dust concentrations was investigated,and the relationship between its hyperspectral parameters and leaf surface dustfall was analyzed,and then a prediction model of dust retention was established.It provides an important reference for the use of hyperspectral detection of vegetation growth in urban environments.The results showed that:(1)In the environment of dust pollution,Euonymus japonicus generally showed a combination of traits with lower leaf area,lower leaf area,lower chlorophyll content,high dry matter content and high leaf thickness,which reflected the structural construction of plant leaves.The trade-off strategy between investment and return also fully illustrates the poor coercion caused by plants in order to adapt to the habitat characteristics of urban environmental pollution and adjust their own functional traits.(2)From the visible to near-infrared range(350~2500 nm),there were four distinct reflection peaks and four major absorption valleys.In the 350~1870 nm intervals,the spectral reflectance was generally negatively correlated with the amount of dustfall on the foliar surface.It can be seen that the spectral reflectance decrease with the number of dustfall increases.However,the variation of leaf dustfall in the 1870~2500 nm band was more complicated and had no obvious regularity.(3)The spectra of the 700~1410 and 1470~1830 nm bands were sensitive to the response of foliar dustfall and the"red edge effect"appeared in the 680~780 nm range.A higher reflection platform appeared in the 750~1350 nm range,which may be due to the strong absorption of the leaf moisture in this band.(4)Red edge slope,blue edge slope,yellow edge slope,and yellow edge position are very sensitive to the interference of foliar dustfall,but the red edge position and blue edge position are not obvious.Combined with the trade-off strategy of leaf functional traits,it is known that due to the long-term dust pollution environment,a special adaptation mechanism is formed.Foliar dust reduction is not sensitive to the influence of red edge position and blue edge position,showing strong Anti-interference ability.The red edge slope and the blue edge slope are negatively correlated with the foliar dustfall response,while the yellow edge slope is positively correlated with the leaf surface dustfall response.At the same time,with the increase of the amount of dust on the leaf surface,the position of the yellow edge has a significant"left shift"phenomenon.(5)In this study,foliar water content index,chlorophyll index,red edge index,normalized index,simple ratio index,and photosynthetic reflectance spectral parameters were used as independent variables,and the leaf dust retention of Euonymus japonicus was used as the dependent variable.We establish a prediction model of foliar dustfall in linear,quadratic polynomial and logarithmic forms,respectively.In all models,the quadratic polynomial prediction model based on the foliar water content index has a higher prediction accuracy for foliar dustfall(y=-1.1123x2^+0.5439x+0.9911,R^2=0.8289,RMSE=0.122).