冠层高度对毛竹叶片光合生理特性的影响

借助LI-6400便携式光合作用系统,研究了冠层高度对不同林龄毛竹(Phyllostachys pubescens)叶片光合生理特性和水分利用效率(WUE)的季节性影响,为促进毛竹林碳汇能力和生产力提升的林分结构调整等可持续栽培技术提供理论依据。结果表明:(1)出笋期,不同竹龄毛竹叶片净光合速率(Pn)和蒸腾速率(Tr)的日均值呈现出冠层上部小于冠层下部的梯度变化趋势,且2a生毛竹不同冠层Pn日均值大于3a生毛竹;孕笋行鞭期,不同林龄毛竹各时间点Pn值和日均值、以及2年生毛竹各时间点的Tr值均为冠层上部大于冠层下部。各生长季节,不同林龄毛竹个体叶片的气孔导度(Gs)均与Tr的变化趋势一致。(2)2年生毛竹各季节仅冠层上部叶片会出现"光合午休",而3年生毛竹仅于出笋期时各冠层叶片出现"光合午休"现象。(3)出笋期毛竹叶片WUE日均值随着冠层高度增加而增加,这种变化趋势不受竹龄影响;而孕笋行鞭期,仅2年生毛竹叶片WUE日均值随着冠层高度增加而下降。不同冠层高度的孕笋行鞭期毛竹叶片WUE日均值都显著高于出笋期;冠层高度对毛竹叶片气体交换特性和WUE的影响受生长发育关键期的季节因素影响,且毛竹叶片WUE与Gs之间存在负相关关系,其不受毛竹个体年龄和叶片冠层高度影响。(4)不同生长季节各冠层叶绿素a/b值均随着冠层高度下降而降低,不同林龄毛竹叶片叶绿素含量基本随着冠层自上而下呈逐渐增加的趋势。各生长季节,不同林龄个体叶片氮素含量、比叶重随冠层高度垂直变化趋势与叶片Pn日均值的垂直变化趋势一致。研究认为,毛竹不同冠层部位叶片通过改变形态、氮素含量来适应不同生长季节生长环境的变化,以便充分利用光能提高光合能力。     

Recovery of aSasa tsuboiana population after mass flowering and death

The recovery process of aSasa tsuboiana population after a mass flowering and death in 1977 was investigated by 15 years of observation in the Hira Mountains, Kinki district, western Japan. Seed production was high (6600–13 800 seeds m⁻² inSasa plots and 3900 seeds m⁻² in a forest plot) but emergent seedling density was low (14–21 seedlings m⁻²), probably because of seed predation byMicrotus montebelli occurring between seed shedding and the next spring. The seedling density had decreased further by the next year and theS. tsuboiana population recovered from only a limited number of seedlings. In spite of such a low initial density, theS. tsuboiana population was able to regenerate successfully and attained the previous full stand height in 7–16 years.Miscantbus sinensis invaded and delayed the recovery ofS. tsuboiana in one plot, butS. tsuboiana became dominant as it caught up with the height ofM. sinensis. Seedling growth patterns, such as frequent tillering, the onset of rhizome extension in the early stage of seedling growth and frequent culm production from rhizomes, played important roles in the successful regeneration ofS. tsuboiana.     

长白落叶松人工林热值及其能量现存量

采用量热法对5个径级的长白落叶松各器官热值进行测定,结合生物量的数据,对长白落叶松的能量现存量进行研究。结果表明:长白落叶松各器官能量现存量模型W=aD~b最适推算长白落叶松的能量现存量。长白落叶松各器官热值的测定结果显示:枝皮干叶根。各器官的能量现存量呈现:干根皮枝叶的规律,能量现存总量1988.39×10~9J/hm~2,地上部分能量现存量远远高于地下部分,主要集中在0—12 m的树段,地下部分主要集中在粗根和根坨。对比5个径级根系的能量现存量,20径级根系的能量现存量最大(187.73×10~9J/hm~2),8径级根系的能量现存量最小(7.72×10~9J/hm~2)。对比5个径级长白落叶松的能量现存量,16径级的能量现存量最大(723.45×10~9J/hm~2),8径级的能量现存量最小(46.58×10~9J/hm~2)。对比长白落叶松和樟子松的能量现存量可知:长白落叶松小于樟子松。     

紫耳箭竹克隆形态可塑性对典型冠层结构及光环境的响应

在重庆金佛山国家自然保护内,选择了3种典型群落类型(落叶阔叶林、常绿落叶阔叶混交林和常绿阔叶林),使用Hemiview数字植物冠层分析系统量化群落冠层结构和光环境特征,并对林下紫耳箭竹(Fargesia decurvata)的形态可塑性特征进行调查,分析冠层结构和光环境特征改变下紫耳箭竹形态可塑性的差异,并探讨它们之间的相互关系。结果表明:(1)随着落叶阔叶林"常绿落叶阔叶混交林"常绿阔叶林演替的进行,群落的冠层开度降低,叶面积指数增加,平均叶倾角变小,趋于水平化,冠层对光的截获能力提高,林下光照的强度降低(P0.05)。(2)随着光照强度的降低,紫耳箭竹分株矮小化,叶片变窄,生物量积累降低,但通过增大比茎长、叶面积率和比叶面积提高对光的利用效率,并增大分枝角度和比隔长有效适应弱光环境。(3)在光照条件差的常绿阔叶林下,紫耳箭竹降低对地下茎的投资,将较多的生物量用于秆的增高增长和叶片的生长;而在光照条件好的落叶阔叶林环境下,紫耳箭竹降低对枝、叶生物量的分配,则加大对地下茎的投资,可认为是克隆植物对水分资源所表现的一种觅食行为。研究表明,紫耳箭竹种群随着冠层结构的改变发生了明显的可塑性变化,这些可塑性变化是种群对冠层结构和光环境差异的适应性反应的结果,有利于增强种群对异质生境中光资源的获取和利用;群落内部可以通过调控冠层结构的改变协调和控制小径竹种群的发展。     

Misting and nitrogen fertilization of shoots of a saltmarsh grass: effects upon fungal decay of leaf blades

We conducted a 12-week field manipulation experiment in which we raised the nitrogen availability (ammonium sulfate fertilization to roots) and/or water potential (freshwater misting) of decaying leaf blades of a saltmarsh grass (smooth cordgrass, Spartina alterniflora) in triplicate 11-m² plots, and compared the manipulated plots to unmanipulated, control plots. The ascomycetous fungi that dominate cordgrass leaf decomposition processes under natural conditions exhibited a boosting (>2-fold) of living standing crop (ergosterol content) by misting at the 1 st week after tagging of senescent leaves, but afterwards, living-fungal standing crop on misted blades was equivalent to that on control blades, confirming prior evidence that Spartina fungi are well adapted to natural, irregular wetting. Misting also caused 2-fold sharper temporal declines than control in instantaneous rates of fungal production (ergosterol synthesis), 5-fold declines in density of sexual reproductive structures that were not shown by controls, and 2-fold higher rates of loss of plant organic mass. Extra nitrogen gave a long-term boost to living-fungal standing crop (about 2-fold at 12 weeks), which was also reflected in rates of fungal production at 4 weeks, suggesting that saltmarsh fungal production is nitrogen-limited. Although bacterial and green-microalgal crops were boosted by manipulations of nitrogen and/or water, their maximal crops remained 0.3 or 2% (bacteria or green microalgae, respectively) of contemporaneous living-fungal crop. The fungal carbon-productivity values obtained, in conjunction with rates of loss of plant carbon, hinted that fungal yield can be high (>50%), and that it is boosted by high availability of nitrogen. We speculate that one partial cause of high fungal yield could be subsidy of fungal growth by dissolved organic carbon from outside decomposing leaves.     

胡杨枝芽生长特征及其展叶物候特征

以5个不同发育阶段的胡杨(Populus euphratica Oliv.)个体为研究对象,观测记录了枝芽展叶物候、枝芽生长特征和叶形变化的空间分布规律。结果表明:不同发育阶段的胡杨个体以及同一个体树冠的不同层次,其枝芽生长及其展叶物候期表现出不同的时空特征。随着树龄的增加和树冠层次的增高(由基向顶),当年新生枝条长度、枝条叶片数和叶形指数逐渐减小,但叶面积和叶片干重逐渐增大。5个不同发育阶段胡杨个体均表现出展叶物候始于树冠顶层,依次向下结束于树冠基部;展叶物候期共性表现在枝芽萌动期均在4月上旬,起始展叶期集中在4月中旬,展叶终期则在5月上旬到下旬;树龄较大的个体其枝芽萌动期、起始展叶期、展叶终期较树龄较小的个体早;其枝芽萌动期到展叶终期的时间进程较树龄较小的个体短;不同发育阶段的个体枝芽萌动期出现的时间较为离散,起始展叶期和展叶终期出现的时间较为集中。相关分析表明,出叶周期与枝条长度、枝条叶片数量和叶形指数呈极显著正相关,与叶面积和叶片干重呈显著负相关。     

Productivity and mineral cycling in an oak coppice forest 2. Annual net production of the forest

Annual net production was estimated in the secondary coppice forest near Tokyo, which was dominated by a deciduous oak,Quercus serrata Thunb. Lateral growth of stems and old branches was directly estimated by examining the annual rings for 35 shoots in a clear-cut quadrat of 10m×10m. Phytomasses of current organs were also weighed in the quadrat. Preharvest losses of current organs were determined by twelve 0.5 m² litter traps for fine litter and twelve 6 m² quadrats for woody litter. Branch production was also assessed indirectly by use of the stem-branch allometry and death of branches. The results of the indirect method were in sufficient agreement with the result of the direct one. Grazing loss of leaves from the canopy was estimated directly from the loss in leaf area and indirectly from the animal faeces caught by the litter traps. Net production of the canopy trees was 149 kg a⁻¹ year⁻¹, in which leaf production was 36.9 kg. Animals grazed about 14% of the leaf area by the end of the growing season. True consumption of leaves by animals was 7.6% of leaf production or 10% of leaf mass. Production of undergrowth, mainly a dwarf bamboo,Pleioblastus chino Makino, was 28 kg a⁻¹ year⁻¹, being 15% of the total stand production. Productivity of this forest was significantly higher than that of cool-temperate deciduous broadleaf forests.     

遮荫处理对梅叶冬青叶片形态、光合特性和生长的影响

为了解遮荫环境对梅叶冬青(Ilex asprella)生长和光合特性的影响,采用遮荫网的方法模拟85%、56%和全光照等3种光照环境,研究了遮荫对其生长、光合参数以及生物量等的影响。结果表明,经一年遮荫处理后,梅叶冬青的叶绿素a、b和叶绿素总量都随着遮荫强度的增加而显著增高,胡萝卜素含量则显著降低。与对照相比,56%遮荫处理显著提高梅叶冬青的最大净光合速率和光饱和点,分别提高了17.6%和25.2%,但是85%遮阴处理则显著降低最大净光合速率和光饱和点,分别降低了18.2%和24.1%,两种遮荫处理均显著降低了光补偿点。叶长、叶宽、比叶面积、单片叶面积和叶片含水量均随着遮荫强度的增加而显著增加,而叶片厚度则显著减小。遮荫处理明显抑制整株生物量增长,减小根冠比,但是株高、冠幅和径向生长随遮荫处理时间不同而有所变异。因此,梅叶冬青有耐荫偏阳的特性,在林下种植时需及时调控乔冠层的透光率,一般应大于44%。     

Leaf variation in a tree of Pourouma tomentosa (Cecropiaceae) in French Guiana

In the flora of French Guiana we find considerable within-plant variation in leaf form. We observed entire, two-lobed, and three-lobed leaves within five separate levels (tiers) of the canopy of a single individual ofPourouma tomentosa subsp.maroniensis. Five branches from each of the five tiers of the tree were collected around the axis of the trunk. From these branches five secondary branchlets were selected and all leaves excised with information recorded as to nodal position, number of leaf nodes, and fertility status of the main branch. This design produced 1015 leaves representing about 20 m² of foliar area and about 2.4 kg of blade dry weight. Our objectives were to determine if statistically significant patterns exist for leaf variation and to suggest improvements for future, general collections. The four lower tiers had 62% entire, 10% 2-lobed, and 28% 3-lobed leaves, in contrast to the top tier with 38% entire, 11% 2-lobed, and 51% 3-lobed leaves. The top tier had no fertile branches. in the lower tiers, fertile branches produced 68% entire leaves whereas nonfertile branches produced only 46% entire leaves. In the top tier, lobed leaves made up 73% of surface area, while in the lower tiers, lobed leaves made up only 48% of total surface area. We selected a random subset of 75 leaves from the 1015, for morphometric analysis using two-way ANOVA (tier×leaf type). The boundaries of leaf images were digitized and rendered into Fourier coefficients, yielding leaf surface area and two variables that quantify aspects of shape: dissection index and leaf complexity. The Fourier coefficients were averaged by tier and by leaf type to reconstruct synthetic, average leaf images. Logistic regression was used to predict the position of leaves on the tree and to provide visualization of the relationships between leaf position on the tree and leaf morphological variables. Within the tree crown, leaf surface area and leaf specific mass (LSM) increases with height, although leaf shape does not change with height. LSM does not vary with leaf form; and sun leaves are larger than shade leaves on this tree. We conducted computer sampling experiments based on exact randomization to simulate the process of obtaining all leaf shapes present in an individual tree when making field collections of varying numbers of duplicates. This also points out the importance of noting the presence of within-tree variation in leaf form on herbarium labeds. Failure to recognize leaf variation can lead to incorrect delimitation of species as well as cause overestimates of the number of species in diversity studies.     

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 conditions on the growth and morphological characteristics of F. nitida, and the adaptive responses of F. nitida to different 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 cluster 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. Moreover, the changes in plasticity enable the plants to optimize their light usage efficiency to promote growth and increase access to resources available in heterogeneous light environments. __________ Translated from Acta Ecologica Sinica, 2006, 26(12): 4019–4026 [译自: 生态学报]     

Canopy Photosynthesis and Crop Growth Rate of Eight Temperate Forage Grasses

The rates of canopy and individual leaf photosynthesis, ratesof growth of shoots and roots, and the extinction coefficientfor light of eight temperate forage grasses were determinedin the field during early autumn. Canopy gross photosynthesiswas calculated as net photosynthesis plus dark respiration adjustedfor temperature using a Q₁₀ = 2. The relationships between canopygross photosynthesis and light intensity were hyperbolic, andthe initial slopes of these curves indicated that light wasbeing utilized efficiently at low light intensities. The initialslope depended on the distribution of light in the canopy andthe quantum efficiency of the individual leaves. The maximumrate of canopy gross photosynthesis reflected the maximum rateof individual leaf photosynthesis. Although the maximum rateof canopy gross photosynthesis was correlated with crop growthrate, there was no significant relationship between daily grossphotosynthesis and crop growth rate. Indeed, daily gross photosynthesisvaried by only 22 per cent, whereas the daily growth of shootsand roots varied by 120 per cent. This poor correlation is influencedby a negative correlation (P < 0.01) between the maximumrate of canopy gross photosynthesis and the initial slope ofthe curve relating canopy gross photosynthesis and light intensity.Difficulties in the interpretation of measurements of dark respirationappeared to confound attempts to relate daily net photosynthesisto crop growth rate, individual leaf photosynthesis, and theextinction coefficient for light.     

不同生境栓皮栎天然更新幼苗植冠构型分析

栓皮栎存在于秦岭南坡的多种林分中,生活在不同生境中的个体往往形成不同的树冠形态和构型特征。为了说明不同生境条件下栓皮栎幼苗的植冠构型变化,采用典型抽样法,对秦岭南坡3种生境中(林冠下、林隙、林缘)的栓皮栎天然更新幼苗的侧枝、叶片特征及其空间分布进行了调查分析,结果表明:3种不同生境中栓皮栎幼苗植冠形态发生了明显的可塑性变化,(1)林冠下的幼苗明显为开阔型树冠,林隙和林缘处的幼苗树冠相对紧密;(2)幼苗的1级侧枝密度与分枝角度在3种生境下均差异显著(P0.05);从Ⅰ到Ⅳ层,林冠下幼苗的分枝角度在冠层内变化幅度不到5°,而林缘处幼苗的分枝角度变化高达40°;发生5个以上1级侧枝的概率以林冠下最大,为0.6;(3)从林缘、林隙到林冠下,幼苗的叶长、叶宽、单叶面积、叶面积指数逐渐降低,数量叶密度和比叶面积则逐渐增大,与其它两种生境相比,林冠下幼苗的叶片逐渐向树冠上层集中,且以更高序的侧枝为主要着生枝条;(4)林隙中栓皮栎幼苗的树高、地径明显优于林缘和林冠下,缩短了苗木进入主林层的时间,林隙对栓皮栎种群更新有利。在今后栓皮栎林的经营中,可以通过适当间伐来增加林隙数量,为森林更新和结构的优化的提供有利条件。     

The roles of ascospores and conidia of Pyrenopeziza brassicae in light leaf spot epidemics on winter oilseed rape (Brassica napus) in the UK

Ascospores of Pyrenopeziza brassicae were produced in apothecia (cup‐shaped ascomata) on oilseed rape debris. The conidia, which were morphologically identical to the ascospores, were produced in acervular conidiomata was greater than for lesions caused by ascospores. In June 2000, on the ground under a crop with light on the surface of living oilseed rape tissues. Ascospores were more infective than conidia on oilseed rape leaves. The proportion of lesions caused by conidia located on leaf veins leaf spot, numbers of petioles with apothecia decreased with increasing distance into the crop from the edge of pathways. Air‐borne ascospores of P. brassicae were first collected above debris of oilseed rape affected with light leaf spot on 5 October 1998 and 18 September 1999,12 or 23 days, respectively, after the debris had been exposed outdoors. P. brassicae conidia were first observed on leaves of winter oilseed rape on 6 January 1999 and 15 February 2000, respectively, after plots had been inoculated with debris in November 1998 and October 1999. In 1991/92, numbers of ascospores above a naturally infected crop were small from January to April and increased in June and July. P. brassicae conidia were first observed in February and the percentage plants with leaves, stems or pods with light leaf spot increased greatly in May and June. In 1992/93, in a crop inoculated with debris, numbers of airborne ascospores were small from October to January and increased from April to June. P. brassicae conidia were first observed on leaves in late November and light leaf spot was seen on stems and pods in March and June 1993, respectively.     

Canopy structure and nitrogen distribution in dominant and subordinate plants in a dense stand of Amaranthus dubius L. with a size hierarchy of individuals

The objective was to investigate how nitrogen allocation patterns in plants are affected by their vertical position in the vegetation (i.e. being either dominant or subordinate). A garden experiment was carried out with Amaranthus dubius L., grown from seed, in dense stands in which a size hierarchy of nearly equally aged individuals had developed. A small number of dominant plants had most of their leaf area in the highest layers of the canopy while a larger number of subordinate plants grew in the shade of their dominant neighbours. Canopy structure, vertical patterns of leaf nitrogen distribution and leaf photosynthetic characteristics were determined in both dominant and subordinate plants. The light distribution in the stands was also measured. Average N contents per unit leaf area (total canopy nitrogen divided by the total leaf area) were higher in the dominant than in the subordinate plants and this was explained by the higher average MPA (leaf dry mass per unit area) of the dominant plants. However, when expressed on a weight basis, average N contents (LNC_av; total canopy N divided by the total dry weight of leaves) were higher in the subordinate plants. It is possible that these higher LNC_av values reflect an imbalance between carbon and nitrogen assimilation with N uptake exceeding its metabolic requirement. Leaf N content per unit area decreased more strongly with decreasing relative photon flux density in the dominant than in the subordinate plants showing that this distribution pattern can be different for plants which occupy different positions in the light gradient in the canopy. The amount of N which is reallocated from the oldest to the younger, more illuminated leaves higher up in the vegetation may depend on the sink strength of the younger leaves for nitrogen. In the subordinate plants, constrained photosynthetic activity caused by shading might have reduced the sink intensity of these leaves.     

亚热带常绿林不同冠层小枝叶面积-叶生物量关系研究

叶面积与叶生物量的关系对于理解植物叶片的碳收益和投资权衡策略具有重要意义。收益递减假说认为植物的叶面积与叶生物量成显著异速生长关系，其异速生长指数<1.0，但该假说是否适用于不同生活型（常绿与落叶）亚热带木本植物不同冠层高度（上下冠层）当年生小枝的叶片仍不清楚。以江西亚热带常绿阔叶林的69种常绿与落叶木本植物当年生小枝上的叶为研究对象，采用标准化主轴回归估计（standardized major axis estimation，SMA）方法检验不同冠层高度和生活型叶面积与叶生物量的异速生长关系。结果显示：（1）当年生小枝叶生物量在不同冠层高度和生活型的植物中无显著差异（P>0.05），叶面积在常绿和落叶植物中有显著差异（P<0.05），常绿和落叶植物的比叶重存在显著差异（P<0.05），而落叶植物的比叶重在不同冠层高度存在显著差异（P<0.05），同一冠层，常绿植物比叶重显著高于落叶植物（P<0.05）；（2）69种植物的叶面积与叶生物量异速生长指数具有物种特异性，60.9%的物种叶面积与叶生物量呈等速生长关系；（3）不同冠层和生活型植物的叶面积与叶生物量呈等速生长关系，但其异速生长常数在不同冠层高度与生活型间存在差异。这些结果表明冠层高度和生活型未改变叶面积-生物量之间的等速生长关系，不支持"收益递减"假说。     

Temporal and spatial variations in leaf herbivory within a canopy of<Emphasis Type="Italic"> Fagus crenata</Emphasis>

This study investigated spatio-temporal variation in the leaf area consumed by insect herbivores within a canopy of Fagus crenata, with reference to the light conditions of leaf clusters. There was no clear relationship between photosynthetic photon flux density (PPFD) and consumed leaf area (CLA) in May, immediately after leaf flush, but CLA decreased with an increase in PPFD after June. Leaf mass per area, carbon concentration, C/N ratio, concentration of total phenolics, and condensed tannin concentration were higher in leaves under high light intensity than those of leaves under low light. On the other hand, the nitrogen concentration of leaves decreased as light availability increased. Consequently, within-tree variation in light availability affects the consumption of leaves by insect herbivores through temporal changes in leaf characteristics.     

Seasonal variation in standing crop,density and leaf growth rate of the seagrass, Heterozostera tasmanica, in western Port and Port Phillip Bay,Victoria, Australia

Standing crop, density and leaf growth rate of Heterozostera tasmanica (Martens ex Aschers.) den Hartog along with light, temperature, nutrient and sediment characteristics were determined monthly for fifteen months at three study sites in Western Port and one site in Port Phillip Bay, Victoria, Australia. Erect vegetative stems of H. tasmanica were frequently branched, were present throughout the year and accounted for 25–60% of the above-sediment biomass, with the stem proportion higher during winter than summer. At three of the four sites there was a unimodal seasonal pattern in which minimum leaf standing crop (27–61 g dry wt. m^?2), density (600–2000 leaf cluster m^?2) and leaf productivity (0.34–0.77 g dry wt. m^?2 day^?1) generally occurred during winter (June–August) and maximum leaf standing crop (105–173 g dry wt. m^?2), density (2700–5000 leaf cluster m^?2) and leaf productivity (2.6–4.2 g dry wt. m^?2 day^?1) occurred during summer (December–February). A bimodal seasonal pattern with minimum standing crop and density during midsummer occurred at one site. This anomalous seasonal pattern may be due to exposure and desiccation stress during spring low tides. At the site receiving the lowest irradiance, standing crop, density and annual leaf production also were lowest, but length and width of leaves, shoot height and leaf growth rate per leaf cluster were the highest of the four study sites. On average, each leaf cluster at any one of the study sites produced 30–31 leaves per year with mean leaf turnover rates of 1.3–1.7% day^?1. Annual leaf production of H. tasmanica ranged from 410 to 640 g dry wt.m^?2 at the four sites.     

Spectral reflectance from a broccoli crop with vegetation or soil as background: influence on immigration byBrevicoryne brassicae andMyzus persicae

Light reflectance in five wavebands of the spectrum was measured from broccoli (Brassicae oleracea var.botrytis [L].) interplanted with leguminous cover crops (cover crop background) or broccoli grown as monoculture (bare soil background), and fertilized with compost or synthetic fertilizer. AlateBrevicoryne brassicae (L.) andMyzus persicae (Sulzer) (Homoptera: Aphididae) were monitored in yellow pan water traps and on broccoli leaves. Reflectance intensity was higher with a background of bare soil at all wavebands except blue (400–455 nm) in the early-season. Intensity decreased as broccoli canopy cover increased at all wavebands except blue and green (515–550 nm), declining-most dramatically in the yellow (550–590 nm). Highest late-season intensities were in plots with bare soil background and fertilized with compost (those stressed for nitrogen). Few differences in spectral composition, expressed for each waveband as a percentage of total intensity, were recorded. Numbers of alatae were lowest in cover crop background plots in the early season, reached equivalency with bare soil background by mid-season, and showed highest positive correlations with intensity in the yellow (550–590 nm). Results correspond to laboratory findings that aphids are attracted to higher intensity light, especially in the yellow waveband, and support a phototactic explanation for aphid orientation in the field.     

Importance of the gradient in photosynthetically active radiation in a vegetation stand for leaf nitrogen allocation in two monocotyledons

Carex acutiformis and Brachypodium pinnatum were grown with a uniform distribution of photosynthetic photon flux density (PFD) with height, and in a vertical PFD gradient similar to the PFD gradient in a leaf canopy. Distribution of organic leaf N and light-saturated rates of photosynthesis were determined. These parameters were also determined on plants growing in a natural vegetation stand. The effect of a PFD gradient was compared with the effect of a leaf canopy. In Brachypodium, plants growing in a vegetation stand had increasing leaf N with plant height. However, distribution of leaf N was not influenced by the PFD gradient treatment. The gradient of leaf N in plants growing in a leaf canopy was not due to differences within the long, mostly erect, leaves but to differences between leaves. In Carex, however, the PFD gradient caused a clear increase of leaf N with height in individual leaves and thus also in plants. The leaf N gradient was similar to that of plants growing in a leaf canopy. Leaf N distribution was not affected by nutrient availability in Carex. In most cases, photosynthesis was positively related to leaf N. Hence, lightsaturated rates of photosynthesis increased towards the top of the plants growing in leaf canopies in both species and, in Carex, also in the PFD gradient, thus contributing to increased N use efficiency for photosynthesis of the whole plant. It is concluded that in Carex the PFD gradient is the main environmental signal for leaf N allocation in response to shading in a leaf canopy, but one or more other signals must be involved in Brachypodium.     

Leaf position,light levels,and nitrogen allocation in five species of rain forest pioneer trees

We used path analysis to ask whether leaf position or leaf light level was a better predictor of within-plant variation in leaf nitrogen concentration in five species of rain forest pioneer trees (Cecropia obtusifolia, Ficus insipida, Heliocarpus appendiculatus, Piper auritum, and Urera caracasana) from the Los Tuxtlas Biological Station, Veracruz, Mexico. Three hundred seventy-five leaves on 28 plants of the five species were analyzed for leaf nitrogen concentration, leaf mass per area, and leaf light interception at different positions (= nodes) along a shoot. Mean values of leaf nitrogen concentration ranged from 0.697 to 0.993 g/m² in the five species, and varied by as much as 2.24 g/m² among leaves on individual plants. Leaf position on the shoot explained significantly more of the within-plant variation in leaf nitrogen concentration than did leaf light level in four of the five species: Cecropia obtusifolia, Heliocarpus appendiculatus, Piper auritum (branch leaves only), and Urera caracasana. However, individual species differed considerably in the patterns of nitrogen allocation and leaf mass per area among leaves on a shoot. These results suggest that leaf nitrogen deployment in these plants is, in part, developmentally constrained and related to the predictability of canopy light distribution associated with plant growth form.