Effects of Season and Successional Stage on Leaf Area Index and Spectral Vegetation Indices in Three Mesoamerican Tropical Dry Forests1

We compared plant area index (PAI) and canopy openness for different successional stages in three tropical dry forest sites: Chamela, Mexico; Santa Rosa, Costa Rica; and Palo Verde, Costa Rica, in the wet and dry seasons. We also compared leaf area index (LAI) for the Costa Rican sites during the wet and dry seasons. In addition, we examined differences in canopy structure to ascertain the most influential factors on PAI/LAI. Subsequently, we explored relationships between spectral vegetation indices derived from Landsat 7 ETM+ satellite imagery and PAI/LAI to create maps of PAI/LAI for the wet season for the three sites. Specific forest structure characteristics with the greatest influence on PAI/LAI varied among the sites and were linked to climatic differences. The differences in PAI/LAI and canopy openness among the sites were explained by both the past land‐use history and forest management practices. For all sites, the best‐fit regression model between the spectral vegetation indices and PAI/LAI was a Lorentzian Cumulative Function. Overall, this study aimed to further research linkages between PAI/LAI and remotely sensed data while exploring unique challenges posed by this ecosystem.     

基于数码相片Gamma校正的水稻叶面积指数估算

随着数码相机的日益普及,利用数码相机进行作物叶面积指数(LAI)测量不断得以应用。由于数码相机成像时会对入射光辐射强度进行Gamma编码变换,输出的相片DN(Digital Number)值与入射光辐射强度呈非线性关系,会造成在确定相片中植被叶片与背景的分割阈值时出现误差,并最终导致LAI估算存在较大不确定性。以水稻为研究对象,获取不同生长期水稻冠层相片并结合同步LAI 2000测量的LAI数据,基于相片Gamma校正原理,对水稻不同生长期冠层相片进行Gamma校正,在此基础上利用冠层孔隙率方法,估算不同生长期水稻LAI。结果表明,经过Gamma校正相片估算的水稻LAI总体精度有显著提高,相片估算的IMAGE LAI与LAI-2000测量值比较的决定系数达到0.71(P0.05)。在整个观测期内,两种方法观测的LAI值在时间变化趋势上表现一致,但在不同生长期内存在差别,在水稻分蘖期和拔节期相片估算的IMAGE LAI要高于LAI-2000测量值,孕穗期到抽穗期期间IMAGE LAI低于LAI-2000测量值,乳熟期到成熟期IMAGE LAI又高于LAI-2000的观测结果。     

筑坝扩容下高原湿地拉市海植物群落分布格局及其变化

基于遥感与地理信息系统技术、结合实地调查与验证,对高原湿地拉市海筑坝扩容13a来湿地植物群落类型、物种组成、空间分布格局进行研究,对比分析筑坝扩容前后植物群落变化特征。结果表明,拉市海当前分布有水葱 (Scirpus tabernaemontani)、两栖蓼 (Polygonum amphibium) 等2个挺水植物群落,鸭子草 (Potamogeton tepperi)、菱 (Trapa bispinosa)等2个浮叶植物群落,穗状狐尾藻 (Myriophyllum spicatum)、篦齿眼子菜 (Potamogeton pectinatus)、菹草 (Potamogeton crispus)、穿叶眼子草 (Potamogeton perfoliatus)、小叶眼子菜 (Potamogeton pusillns)等5个沉水植物群落,草甸植被分布于湖周。湿地植物物种共计61种,隶属于25科、48属,物种丰富度随沉水→浮叶→挺水→草甸逐渐增加。沉水植物群落分布面积最大(615.08 hm²),其次是草甸(214.60 hm²)、浮叶植物群落(140.01 hm²),挺水植物群落分布面积最小 (9.34 hm²),群落垂直层次随沉水→浮叶→挺水呈复杂化的趋势。筑坝13a来,拉市海植物群落类型从单一的沉水型植物群落发展成为由沉水、浮叶、挺水型组成的、水平空间多样化配置的湿地植被系统,其中穗状狐尾藻、篦齿眼子菜、小眼子菜等植物群落在筑坝蓄水13a后没有发生演替得以保留,而扇叶水毛茛(Butrachium bungei)、马来眼子菜(Potamogeton malaianus)和轮藻(Chara spp.)群落发生演替而消失。研究掌握了筑坝扩容下拉市海湿地植物群落分布格局及其变化特征,为科学评估筑坝蓄水对湿地生态系统的影响提供了基础性数据,同时也为水文改变下高原湿地生态系统的保护、管理以及资源可持续利用提供了一定的理论依据。     

Differences in refuge function for prey and tolerance to crayfish among macrophyte species

The invasive crayfish Procambarus clarkii is an omnivore and an ecosystem engineer whose feeding mechanism has reduced the abundance of many native invertebrates and macrophytes. Since macrophytes provide refuges for aquatic insects, macrophyte depletion by crayfish might have indirect negative effects on animal prey in aquatic habitats. We postulated that the prey refuges provided by macrophytes and macrophyte tolerance to crayfish cutting and feeding vary among macrophyte species. We conducted two experiments to (1) investigate differences in macrophyte refuge function for dragonfly larvae against crayfish, and (2) test the tolerance to crayfish cutting and feeding among macrophyte species. Elodea nuttallii (submerged plant), Potamogeton crispus (submerged plant), and Carex idzuroei (emergent plant) had greater refuge effects than Trapa japonica (floating-leaved plant), an effect that might result from the larger total cover of E. nuttallii, P. crispus, and C. idzuroei, and the hardness of C. idzuroei leaves. Tolerance to crayfish cutting and feeding was greater in C. idzuroei than in the other species. As the macro-invertebrate assemblages in submerged vegetation are more abundant and species-rich than those in emergent and floating-leaf vegetation, conservation of E. nuttallii and P. crispus should be prioritized for restoring native aquatic animals in ecosystems invaded by the introduced crayfish.     

Standardizing the Protocol for Hemispherical Photographs: Accuracy Assessment of Binarization Algorithms

Hemispherical photography is a well-established method to optically assess ecological parameters related to plant canopies; e.g. ground-level light regimes and the distribution of foliage within the crown space. Interpreting hemispherical photographs involves classifying pixels as either sky or vegetation. A wide range of automatic thresholding or binarization algorithms exists to classify the photographs. The variety in methodology hampers ability to compare results across studies. To identify an optimal threshold selection method, this study assessed the accuracy of seven binarization methods implemented in software currently available for the processing of hemispherical photographs. Therefore, binarizations obtained by the algorithms were compared to reference data generated through a manual binarization of a stratified random selection of pixels. This approach was adopted from the accuracy assessment of map classifications known from remote sensing studies. Percentage correct () and kappa-statistics () were calculated. The accuracy of the algorithms was assessed for photographs taken with automatic exposure settings (auto-exposure) and photographs taken with settings which avoid overexposure (histogram-exposure). In addition, gap fraction values derived from hemispherical photographs were compared with estimates derived from the manually classified reference pixels. All tested algorithms were shown to be sensitive to overexposure. Three of the algorithms showed an accuracy which was high enough to be recommended for the processing of histogram-exposed hemispherical photographs: “Minimum” ( 98.8%; 0.952), “Edge Detection” ( 98.1%; 0.950), and “Minimum Histogram” ( 98.1%; 0.947). The Minimum algorithm overestimated gap fraction least of all (11%). The overestimation by the algorithms Edge Detection (63%) and Minimum Histogram (67%) were considerably larger. For the remaining four evaluated algorithms (IsoData, Maximum Entropy, MinError, and Otsu) an incompatibility with photographs containing overexposed pixels was detected. When applied to histogram-exposed photographs, these algorithms overestimated the gap fraction by at least 180%.     

Herbivory and growth in terrestrial and aquatic populations of amphibious stream plants

1. Many amphibious plant species grow in the transition between terrestrial and submerged vegetation in small lowland streams. We determined biomass development, leaf turnover rate and invertebrate herbivory during summer in terrestrial and aquatic populations of three amphibious species to evaluate advantages and disadvantages of aerial and submerged life.
2. Terrestrial populations had higher area shoot density, biomass and leaf production than aquatic populations, while leaf turnover rate and longevity were the same. Terrestrial populations experienced lower percentage grazing loss of leaf production (average 1.2–5.1%) than aquatic populations (2.9–17.3%), while the same plant dry mass was consumed per unit ground area.
3. Grazing loss increased linearly with leaf age apart from the youngest leaf stages. Grazing loss during the lifetime of leaves was therefore 2.4–3.1 times higher than mean apparent loss to standing leaves of all ages. The results imply that variation in density of grazers relative to plant production can account for differences in grazing impact between terrestrial and aquatic populations, and that fast leaf turnover keeps apparent grazing damage down.
4. We conclude that the ability of amphibious plants to grow submerged permits them to expand their niche and escape intense competition on land, but the stream does not provide a refugium against grazing and constrains plant production compared with the terrestrial habitat.     

Comparison of optical methods for estimating the opening of the canopy and leaf area in leafy forests

Based on inversion of gap fraction data (Poisson model of foliage distribution), three optical methods using the Demon, the Plant Canopy Analyzer LAI-2000 (PCA) and hemispherical photographs, have been compared to estimate canopy openness (CO) and leaf area index (LAI) in a mature, neutrophil, oak-beech-hornbeam forest on mull in eastern France. Mean CO over the whole hemisphere was similar for PCA (7.9%) and hemispherical photographs (8.0%). The needle method, a vertical point quadrat method, applied to the litter after leaf fall has served as a reference to LAI (4.7). The Demon provided the estimate (4.9) closest to the reference value. The PCA and hemispherical photographs underestimate mean LAI by 30% (3.3) and 19% (3.8), respectively, if used without correction. Based on fish-eye sensors, LAI estimates can be improved if 3 annuli (4.2) or 2 annuli (4.5) are used in place of 5 with the PCA, or by means of logarithmic averaging of gap fractions over azimuth at an appropriate angular resolution (180 degrees: 4.6, or 120 degrees: 5.2) with hemispherical photographs. Not taking into account azimuthal variation in gap fraction distribution generates a more important error than the error induced by light scattering near horizon. In order to improve LAI estimates, an original iterative procedure is presented, which allows the simultaneous calculation of LAI over a broad range of angular azimuthal resolutions.     

Molecular Adaptation of rbcL in the Heterophyllous Aquatic Plant Potamogeton

Background

Heterophyllous aquatic plants show marked phenotypic plasticity. They adapt to environmental changes by producing different leaf types: submerged, floating and terrestrial leaves. By contrast, homophyllous plants produce only submerged leaves and grow entirely underwater. Heterophylly and submerged homophylly evolved under selective pressure modifying the species-specific optima for photosynthesis, but little is known about the evolutionary outcome of habit. Recent evolutionary analyses suggested that rbcL, a chloroplast gene that encodes a catalytic subunit of RuBisCO, evolves under positive selection in most land plant lineages. To examine the adaptive evolutionary process linked to heterophylly or homophylly, we analyzed positive selection in the rbcL sequences of ecologically diverse aquatic plants, Japanese Potamogeton.

Principal Findings

Phylogenetic and maximum likelihood analyses of codon substitution models indicated that Potamogeton rbcL has evolved under positive Darwinian selection. The positive selection has operated specifically in heterophyllous lineages but not in homophyllous ones in the branch-site models. This suggests that the selective pressure on this chloroplast gene was higher for heterophyllous lineages than for homophyllous lineages. The replacement of 12 amino acids occurred at structurally important sites in the quaternary structure of RbcL, two of which (residue 225 and 281) were identified as potentially under positive selection.

Conclusions/Significance

Our analysis did not show an exact relationship between the amino acid replacements and heterophylly or homophylly but revealed that lineage-specific positive selection acted on the Potamogeton rbcL. The contrasting ecological conditions between heterophyllous and homophyllous plants have imposed different selective pressures on the photosynthetic system. The increased amino acid replacement in RbcL may reflect the continuous fine-tuning of RuBisCO under varying ecological conditions.     

Estimation of deciduous forest leaf area index using direct and indirect methods

This study evaluated one semi-direct and three indirect methods for estimating leaf area index (LAI) by comparing these estimates with direct estimates derived from litter collection. The semi-direct method uses a thin metallic needle to count a number of contacts across fresh litter layers. One indirect method is based on the penetration of diffuse global radiation measured over the course of a day. The second indirect method uses the LAI-2000 plant canopy analyser (PCA) which measures diffuse light penetration from five different sky sectors simultaneously. The third indirect method uses the Demon portable light sensor to measure the penetration of direct beam sunlight at different zenith angles over the course of half a day. The Poisson model of gap frequency was applied to estimate plant area index (PAI) from observed transmittances using the second and third methods. Litter collection from 11 temperate decidous forests gave values of LAI ranging from 1.7 to 7.5. Estimates based on the needle method showed a significant linear relationship with LAI values obtained from litter collections but were systematically lower (by 6–37%). PAI estimates using all three indirect techniques (fixed light sensor system, LAI-2000 and Demon) showed a strong linear relationship with LAI derived from litter collection. Differences, averaged over all forest stands, between PAI estimates from each of the three indirect methods and LAI from litter collections were below 2%. If we consider that LAI=PAI–WAI (wood area index) then, all three indirect methods underestimated LAI by an additional factor close to the value of WAI. One reason could be a local clumping of architectural canopy components: in particular, the spatial dispositions of branchlets and leaves are not independent, leading to a non-random relationship between the distributions of these two canopy components.     

激素对水生植物生理生态的影响及其应用

激素代谢是植物传导信号和调节生长发育的重要途径.陆地植物五大类激素在水生植物中也有分布,尽管近年来环境污染导致水生植物衰退的问题日益得到重视,但水生植物激素的研究和应用却远滞后于陆生植物.在总结了近年来激素类物质在水生植物中的研究成果,分别从激素的种类、激素的生理生态作用、激素生物合成的途径及作用的部位和机制、激素之间的相互作用.激素类物质在实验和实践上的应用等进行了全面阐述,指出了水生植物激素生理生态学研究的发展方向,从利用激素类物质诱导水生植物抗性的表达,提高抗逆性,恢复水生植被,以及研究和开发适于水生植物生产和管理的生长调节剂等方面,就水生植物激素的进一步研究和应用进行了探讨.     

Estimation of leaf area index and gap fraction in two broad-leaved forests by using small-footprint airborne LiDAR

In this study, we evaluated methods for reliably estimating leaf area index (LAI) and gap fraction in two different types of broad-leaved forests by the use of airborne light detection and ranging (LiDAR) data. We evaluated 13 estimation variables related to laser height, laser penetration rate, and laser point attributes that were derived from LiDAR analyses. The relationships between LiDAR-derived estimates and field-based measurements taken from the forests were evaluated with simple linear regressions. The data from the two forests were analyzed separately and as an integrated dataset. Among the laser height variables, the coefficient of variation (CV) of all laser point heights had the highest level of accuracy for estimating both LAI and gap fraction. However, we recommend that more evaluations be conducted prior to the use of CV in forests with complex structures. The simplest laser penetration variable, which represents the ratio of the number of ground points to the total number of all points (P _ALL), also had a high level of accuracy for estimating LAI and gap fraction at the study sites regardless of whether the data were analyzed separately or as an integrated data set. Furthermore, P _ALL values showed near 1:1 relationships with the field-based gap fraction values. Hence, the use of P _ALL may be the most practical for estimating LAI and gap fraction in broad-leaved forests, even when the canopies are heavily closed.     

Temporal changes and vertical distribution of macrophytes in Lake Kawaguchi

 The distribution of macrophytes in Lake Kawaguchi, Japan, was surveyed in August 1999 using a sampling anchor from a boat. The survey revealed that the present aquatic vegetation was composed of 17 submerged species, and no floating-leaved plants were present. The diversity of submerged plants and their vertical growth limits decreased progressively from the west to the east end of the lake. At the time of the survey, the dominant species was Elodea nuttallii (Planch.) St. John, which had once grown explosively in the mid-1980s in the lake, and except for Vallisneria asiatica Miki, all of the species commonly observed in the lake seem to have diminished their growth greatly. Among them, Potamogeton compressus L., which was the most dominant species in the lake before the invasion of Elodea nuttallii, had decreased most severely. It is suggested that the present state of aquatic vegetation in this lake was much influenced by the persistent growth of Elodea nuttallii as well as by environmental factors. Received: August 7, 2001 / Accepted: February 9, 2002     

Application of Geostatistics to Characterize Leaf Area Index (LAI) from Flux Tower to Landscape Scales Using a Cyclic Sampling Design

Accurate characterization of leaf area index (LAI) is required to quantify the exchange of energy, water, and carbon between terrestrial ecosystems and the atmosphere. The objective of this study was to use a cyclic sampling design to compare the spatial patterns of LAI of the dominant terrestrial ecosystems that comprised the area around the 447-m WLEF television tower, equipped with an eddy flux system, near Park Falls, Wisconsin, USA. A second objective was to compare the efficiency of cyclic, random, and uniform sampling designs in terms of the precision of spatial information derived per unit sampling effort. The vegetation surrounding the tower was comprised (more than 80%) of four major forest cover types: forested wetlands, upland aspen forests, upland northern hardwood forests, and upland pine forests, and a fifth, nonforested cover type, grass (open meadow). LAI differed significantly among the five cover types and averaged 3.45, 3.57, 3.82, 3.99, and 1.14 for northern hardwoods, aspen, forested wetlands, upland conifers, and grass, respectively. The cyclic sampling design maximized information about the variance of vegetation characteristics of the heterogeneous landscape and decreased by 60% the number of plots needed to obtain the same confidence interval width using a random sampling design. The range of spatial autocorrelation for LAI was 147 m, but it was decreased to 117 m when vegetation cover was included as a covariate. The cyclic sampling design has several important advantages over other sampling designs. The cyclic sampling design increased the sampling efficiency by optimizing the placement of plots so they were distributed more efficiently for geostatistical analyses such as semi-variograms, correlograms, and spatial regression and can incorporate covariates (for example, vegetation cover, soil properties, and so on) that may explain the sources of spatial patterns. The cyclic sampling design was used to derive a spatial map of LAI and the average LAI for the 3 × 2 km area centered on the flux tower was 3.51 ± 0.89 (with a minimum of 0 and a maximum of 6.35). Airborne and satellite reflectance data have also been used to characterize LAI, but in this region, and many other forests of the world, remotely sensed vegetation indexes saturate in forests with an LAI greater than 3-5. The cyclic sampling design also provides a general ecological sampling approach that can be used at multiple scales.     

Calibration of LAI-2000 canopy analyser with leaf area index in a young eucalypt stand

Plant area index (PAI) measured with a LI-COR LAI-2000 plant canopy analyser (PCA) was calibrated with leaf area index (LAI) in a young stand of Eucalyptus grandis in the KwaZulu-Natal Midlands, South Africa. Destructive sampling and allometric equations were used to estimate LAI at 2 and 3 years after planting. Significant correlations (P<0.001) were found between LAI and PAI for each age with different equations being generated for the two ages (LAI=1.0594(PAI)−0.892 at 2 years of age, and LAI=1.0393(PAI) at 3 years of age). The equations differed from those reported in other eucalypt studies, as the PCA in this study over-predicted LAI at 2 years, and slightly under-predicted at 3 years, of age. It is argued that the stage of growth influenced this calibration, as the canopy and foliar structure may have been different in the young stands, affecting the basic assumptions for the PCA. A broad conversion from PCA derived PAI to LAI may not necessarily be valid for young, short rotation eucalypt plantations.     

Submerged vegetation of the Rotorua and Waikato lakes 3. Lake Rerewhakaiitu

The submerged and emergent aquatic macrophyte vegetation of the eutrophic Lake Rerewhakaiitu was surveyed in February–March 1973, and is described qualitatively. At the time the lowered lake level had resulted in producing a sward of normally submerged aquatics as land forms. The principal dominant in the sward is Myriophyllum propinquum, the plants being greatly reduced in length to short stems with single entire leaves. At the uppermost levels Tillaea sinclairii is a significant component of the sward, and at lower levels Lilaeopsis lacustris behaves similarly. Below the uppermost submerged belt of macrophytes there is a characean meadow of Chara fibrosa and C. australis which in deeper waters gives way to Potamogeton ochreatus with either C. australis or Nitella hookeri. The high degree of eutrophication and consequent low light penetration has resulted in a vegetation limit at 4.5–5 m, which is lower than that found in other Rotorua lakes.     

Candidate genes for adaptation to an aquatic habitat recovered from Ranunculus bungei and Ranunculus sceleratus

The molecular mechanisms responsible for plant adaptations to aquatic habitats remain poorly known. In this study, we aimed to detect selection on orthologous genes among the submerged aquatic plant Ranunculus bungei, semi-aquatic R. sceleratus, and terrestrial R. brotherusii and R. cantoniensis, in order to identify candidate genes involved in the adaptive transition from terrestrial habitats to aquatic habitats and if they differ between R. bungei and R. sceleratus. We used 884 previously reported 1:1:1:1 orthologous genes among the four species (one ortholog is represented in each species). We identified 177 genes of R. bungei and R. sceleratus that are potentially under positive selection. Among them, a set of candidate genes for adaptation to an aquatic habitat were recognized. These include several genes that are associated with photosynthesis, vacuoles, ethylene and oxygen receptors, and aerenchyma formation. Gene Ontology analyses suggest that the submerged R. bungei has a higher percentage of positively selected genes associated with the chloroplast and other plastids, while the semi-aquatic R. sceleratus has a higher percentage associated with metabolic processes. This study represents a step forward towards understanding the molecular mechanism of plant adaptations to aquatic habitats.     

干旱对湖北省长湖水生植物多样性的影响

将断面法和地理信息系统（GIS）与全球定位系统（GPS）技术相结合,通过比较湖北省长湖2000年（干旱年）与1999年（非干旱年）的水生植物多样性,探讨了干旱对淡水湖泊水生植物多样性的影响,主要结论如下：（1）干旱对长湖淡水湖泊水生植物物种多样性无影响,但可增加优势种数目（从12个至14个）。（2）干旱使长湖水生植物群丛数目从14个增加至18个,且能显著提高各群丛的Simpson与Shannon-Wiener物种多样性指数。（3）干旱显著提高了长湖水生植被覆盖率与各群丛的盖度,并使全湖水生植被平均单位面积生物量与各群从单位面积生物量显著升高。（4）在干旱年（2000年）,长湖挺水植被消失,但浮水植被面积与沉水植被面积显著扩大：分别从1999年的3.71％与41.32%上升至2000年的12.63%与53.84%（占湖泊总面积的百分比）。（5）干旱条件下淡水湖泊水生植物的生长发育明显加快：2000年7月菹草石芽萌发率、菹草幼株长度和野菱黄叶率显著高于1999年同期值。     

三峡工程中的生物多样性保护

三峡工程导致长江水文情势改变,水库淹没和移民搬迁,成为影响生物多样性的直接的,主要的因素,本文根据最新调查和研究成果,评述了三峡工程对生物多样性的影响,介绍了正在采取的措施及其效果。水库淹没和移民搬迁是影响陆生生物的主要因素。三峡库区有144个植物群系,6388种高等植物,3418种昆虫,500种陆生脊椎动物,其中36个植物群系受到部分或全部淹没影响,4种地方特有植物的野生种群遭到淹没（1种全淹,3种部分淹没）。水文情势改变是影响水生生物的主要因素,长江水系共有鱼类350种,浮游动植物,底栖动物,水生植物等1085种。其中三峡工程对部分珍稀大型水生动物,40余种特有鱼类和“四大家鱼”的影响值得重视,我们将通过建立长期的生物监测站,设立保护区和保护点,采用人工繁殖放流等措施保护生物多样性,减少三峡工程对生物多样性的不利影响,避免因三峡工程而造成物种丧失。     

华南地区亚热带树木叶面积指数的高光谱反演研究

为构建树种叶面积指数的估算模型,以NDVI、RVI、FREP、CIGreen、CIRed-edge、MSAVI2为高光谱特征变量,通过统计分析,确定反演树种叶面积指数的最佳光谱特征变量,构建华南农业大学校园内50种亚热带树木的叶片反射率和叶面积指数(LAI)模型。结果表明,6种高光谱特征变量与树种叶面积指数间都具有极显著相关性,其中红边位置反射率(FREP)和比值植被指数(RVI)与LAI的拟合方程的R2都大于0.8,决定系数分别为0.820和0.811。经过精度验证,FREP估算的均方根误差(RMSE)只有0.13,该回归模型为估测亚热带典型树种的叶片LAI最佳模型。从高光谱遥感的角度结合亚热带植被的群落结构特点来看,建立的红边位置光谱反射率与叶面积指数的回归模型普遍具有较高的拟合度,所以利用高光谱特征变量反演亚热带树木叶片的叶面积指数等植被参数的应用前景较好。