VIRUS DISEASES OF CACAO IN WEST AFRICA IX. STRAIN VARIATION AND INTERFERENCE IN VIRUS 1A

Cacao virus iA, the most important and prevalent of the viruses that attack cacao in the Gold Coast, occurs in strains that differ widely in their virulence towards cacao. Outbreaks usually contain trees infected with different strains and individual trees are often infected simultaneously with more than one strain; this can be demonstrated by coppicing the trees, and by inoculating sets of test plants with grafts from different parts of one tree. Neither mild nor virulent strains seemed to be consistently dominant in roots or in other parts of cacao trees.
Cacao plants infected with mild strains were nearly always protected against the effects of infection by virulent strains; however, virulent strains entered hosts already infected with mild strains, but usually without causing any symptoms unless the plants were coppiced. The severe symptoms that developed on new growth from such coppiced plants were seldom repeated in later growth. Mealybugs transmitted the virulent strains from leaves with symptoms characteristic of infection by the latter, but not from leaves free from such symptoms. These results suggest that the multiplication of a virulent strain is impeded in plants infected with a mild strain.
In the field, infection with a mild strain protected mature trees against the effects of virulent strains spread by mealybugs. During 3 years in which 273 out of 387 previously uninfected trees became severely diseased, only 35 out of 416 infected with themildstrain developed symptoms of infectionwiththevirulent strain. Five years after infection with the mild strain, trees were yielding I pod per tree more than in the year they were infected, whereas the decrease on trees infected with the virulent strain was 16 pods per tree. Some limitations in the practical application of protection by mild strains, and objections to its use as a control measure, are discussed.     

VIRUS DISEASES OF CACAO IN WEST AFRICA

Of the tested plants which are indigenous to West Africa, three species in the Bombacaceae and four in the Steruliaceae are susceptible to one or more of four viruses from cacao, three occurring in the Gold Coast and one in Nigeria. These species are less affected than cacao by the viruses; some show transient leaf symptoms and others are symptomless carriers. The development of spines on the stems of Ceiba pentandra seedlings is suppressed by infection with virus 1 A.
In general, the indigenous species are more difficult to infect than cacao, and mealybugs do not become infective as readily when feeding on them as when feeding on infected cacao. The availability of the viruses to vectors seems to be correlated with severity of symptoms, and transmission from infected plants to cacao becomes less frequent with increasing duration of infection.
Ceiba pentandra trees were found naturally infected in the Gold Coast and Nigeria. In the Western Province of the Gold Coast, Cola chlamydantha trees in cacao farms and forests were naturally infected with viruses apparently identical with those causing swollen shoot of cacao there.
There is little doubt that C. chlamydantha trees are an important source of virus for cacao trees. Whenever possible these and other alternative hosts growing near to cacao should be destroyed.     

COPPICING EXPERIMENTS ON THE SPREAD AND CONTROL OF CACAO SWOLLEN-SHOOT DISEASE IN NIGERIA

A coppicing technique was used to determine the incidence and distribution of latent and unrecognized infection around naturally occurring outbreaks of cacao swollen-shoot virus disease in Western Nigeria. All the apparently healthy trees within 30 yards of eighty outbreaks of various sizes were coppiced and most of the infected stumps which regenerated showed symptoms within a year.
The distribution of the infected stumps around thirty-five outbreaks studied in detail is expressed by the equation log₁₀ I = a + bx. I is the estimated intensity of infection in the coppiced stumps at distance x from the nearest infected tree removed at the time of coppicing. The constant a determines the height of the peak of the infection gradient and increases with outbreak size, whereas the slope of the gradient, determined by the negative constant b , is similar around all outbreaks.
The results are consistent with information on the movement of the mealybugs and spread of cacao swollen-shoot virus in outbreaks. Moreover, they indicate that outbreaks are controlled most economically by removing all obviously infected trees and adjacent apparently healthy ones. Control does not require the destruction of all the trees around outbreaks up to a distance of 30 yards as done previously, but can be achieved by felling fewer trees, the actual number depending on the size of the outbreak.     

VIRUS DISEASES OF CACAO IN WEST AFRICA VIII. THE SEARCH FOR VIRUS-RESISTANT CACAO

The methods used and the results of 10 years' search for cacao resistant to swollen-shoot disease are described. Selection among the trees surviving in farms devastated by this virus disease led to the discovery of mild virus strains which can protect trees against virulent strains.
When tested by graft inoculation with virulent virus, none of the selections showed any immunity or resistance save that conferred by previous mild-strain infection. A low degree of tolerance was found in some selections.
Local selections and a range of new introductions were tested by mealybug infection, and only types from the Upper Amazon region of Ecuador were consistently resistant to infection. This genetical resistance seems to be strongest in cacao from the Nanay peninsula, near Iquitos.     

Bird diversity in cacao farms and forest fragments of western Panama

Theobroma cacao plantings, when managed under the shade of rainforest trees, provide habitat for many resident and migratory bird species. We compared the bird diversity and community structure in organic cacao farms and nearby forest fragments throughout mainland Bocas del Toro, Panama. We used this dataset to ask the following questions: (1) How do bird communities using cacao habitat compare to communities of nearby forest fragments? (2) To what extent do Northern migratory birds use shaded cacao farms, and do communities of resident birds shift their abundances in cacao farms seasonally? (3) Do small scale changes in shade management of cacao farms affect bird diversity? Using fixed radius point counts and additional observations, we recorded 234 landbird species, with 102 species that were observed in both cacao and forest fragments, 86 species that were only observed in cacao farms, and 46 species that were restricted to forest fragments. Cacao farms were rich in canopy and edge species such as tanagers, flycatchers and migratory warblers, but understory insectivores were nearly absent from cacao farms. We observed 27 migratory species, with 18 species in cacao farms only, two species in forest only, and seven species that occurred in both habitats. In cacao farms, the diversity of birds was significantly greater where there was less intensive management of the canopy shade trees. Shade tree species richness was most important for explaining variance in bird diversity. Our study shows that shaded cacao farms in western Panama provide habitat for a wide variety of resident and migratory bird species. Considering current land use trends in the region, we suggest that action must be taken to prevent conversion away from shaded cacao farms to land uses with lower biodiversity conservation value.     

Tree species richness and density affect parasitoid diversity in cacao agroforestry

In Brazil, cacao is mostly planted beneath shade trees. The diversity of shade trees varies from monospecific to highly diverse canopies, characteristic of pristine Atlantic Forest. This study evaluates the relationships between family richness of Hymenoptera-Parasitica and Chrysidoidea, and tree species richness and density, the species richness of herbaceous understorey, and the area and age of the cacao agroforestry system. We sampled 16 cacao agroforestry systems, with canopy diversity ranging from one to 22 tree species per hectare, in three seasons: summer (March), winter (August) and spring (November). Parasitoids were sampled using eight Malaise-Townes traps per site. Tree species richness and density were enumerated within 1 ha at each site, and herbaceous plant species richness was calculated in eight 1 m² plots, within the hectare. The number of parasitoid families increased with tree species richness and density in spring and summer, but decreased in winter. Neither species richness of herbaceous plants nor area and age of the system affected parasitoid family richness. We suggest that the increase of parasitoid diversity with tree species richness and density in warmer seasons reflects increasing heterogeneity and availability of resources. The decrease in parasitoid family number with tree density in winter may be due to local impoverishment of resources, leading to parasitoid emigration to neighbouring forest remnants. This result implies that a higher diversity of shade trees will help to maintain high parasitoid levels and, in consequence, higher levels of natural enemies of cacao pests, particularly in the warmer seasons. This prediction is borne out in the experience of cacao producers. The proper management of shade tree diversity will play a vital role in maintaining the sustainability of cacao agroforestry production systems in the tropics and, concurrently, will maintain high biodiversity values in these locations.     

OBSERVATIONS ON THE EFFECT OF GROWTH-STIMULATING COMPOUNDS ON THE HEALING OF WOUNDS ON APPLE TREES

Wounds were made at various times of the year on the trunks and large branches of apple trees with a 1 in. bit and treated with a number of organic compounds in lanoline paste. The healing of these wounds was followed through two growing seasons by tracing and measuring the area of exposed wood at different times after wounding.
Callus growth was practically confined to the summer months. Development of callus was improved by applications of lanoline, and the lanoline effect could be further enhanced in the early part of the first growing season following treatment by the addition of certain growth-stimulating compounds including 4-chloro-3:5-dimethylphenoxyacetic acid and 2:4-dichlorophenoxyacetic acid. Indolyl-3-butyric acid probably also caused some stimulation. After the period of initial stimulation the rate of callusing was approximately the same on treated as on untreated wounds.     

Tree Spatial Structure,Host Composition and Resource Availability Influence Mirid Density or Black Pod Prevalence in Cacao Agroforests in Cameroon

Combining crop plants with other plant species in agro-ecosystems is one way to enhance ecological pest and disease regulation mechanisms. Resource availability and microclimatic variation mechanisms affect processes related to pest and pathogen life cycles. These mechanisms are supported both by empirical research and by epidemiological models, yet their relative importance in a real complex agro-ecosystem is still not known. Our aim was thus to assess the independent effects and the relative importance of different variables related to resource availability and microclimatic variation that explain pest and disease occurrence at the plot scale in real complex agro-ecosystems. The study was conducted in cacao (Theobroma cacao) agroforests in Cameroon, where cocoa production is mainly impacted by the mirid bug, Sahlbergella singularis, and black pod disease, caused by Phytophthora megakarya. Vegetation composition and spatial structure, resource availability and pest and disease occurrence were characterized in 20 real agroforest plots. Hierarchical partitioning was used to identify the causal variables that explain mirid density and black pod prevalence. The results of this study show that cacao agroforests can be differentiated on the basis of vegetation composition and spatial structure. This original approach revealed that mirid density decreased when a minimum number of randomly distributed forest trees were present compared with the aggregated distribution of forest trees, or when forest tree density was low. Moreover, a decrease in mirid density was also related to decreased availability of sensitive tissue, independently of the effect of forest tree structure. Contrary to expectations, black pod prevalence decreased with increasing cacao tree abundance. By revealing the effects of vegetation composition and spatial structure on mirids and black pod, this study opens new perspectives for the joint agro-ecological management of cacao pests and diseases at the plot scale, through the optimization of the spatial structure and composition of the vegetation.     

Spider web guilds in cacao agroforestry – comparing tree,plot and landscape‐scale management

Aim Owing to their role as insect predators, web‐building spiders can be important biological control agents within agricultural systems. In complex tropical agroecosystems such as agroforests, management determines plant architecture, vegetation composition and associated ant density, but little is known on how these attributes, together with landscape context, determine spider web density. We hypothesized that all three spatial scales and the presence of Philidris ants significantly contribute to the explanation of spider web density with web types being differently affected. Location In 42 differently managed cacao agroforestry systems in Sulawesi, Indonesia. Methods We surveyed the distribution of five spider‐web types on 420 cacao trees to determine how these relate to habitat variables and a numerically dominant ant species at three different spatial scales, comparing tree, plot and landscape features. We fitted linear mixed‐effects model, selected the best model subset using information‐theoretic criteria and calculated the model‐averaged estimates. We used non‐metric multidimensional scaling (NMDS) to determine and visualize guild level responses to the effects of the tree, plot and landscape‐scale variables. Results The five spider guilds preferred different features of cacao tree architecture. Most frequently recorded webs belonged to the line‐ and orb‐web type. At the tree scale, overall web density was positively related to canopy openness. At the plot scale, a higher number of shade trees was related to a higher web density. At the landscape scale, the altitude determined the distribution patterns of web‐building spiders. Presence of Philidris ants was positively associated with density of orb webs, while no pattern was found for other web types. Main conclusions Results suggest spider web density could be increased by pruning of cacao trees while keeping shade trees at high density in cacao plots. The results emphasize the need to consider scale dependency of crop management and web‐guild‐specific responses that may be related to different functional roles of spiders as a high‐density predator group in agroforestry.     

板栗扦插生根过程的解剖结构和生理变化研究

以5年生和30年生板栗插穗以及30年生板栗母树不同月份(5、6、7月)插穗为材料,进行不同浓度(0、200和500mg/L)吲哚丁酸(IBA)处理,观测各类处理材料的生根情况,并对扦插前插穗(初始茎)和扦插后插穗愈伤组织的碳氮代谢物质、褐化相关物质和解剖学结构进行分析观察,探讨不同年龄母树及不同月份插穗扦插生根能力,为板栗扦插生根技术体系奠定理论基础。结果表明:(1)5年生板栗的初始茎和愈伤组织中碳氮代谢相关物质淀粉和可溶性糖含量明显高于30年生板栗,而其可溶性蛋白含量30年生高于5年生板栗,但其γ-氨基丁酸(GABA)含量在不同年龄母树间无明显变化;30年生板栗插穗初始茎中可溶性蛋白、GABA和可溶性糖含量均随着月份的增加而逐渐降低,但是在愈伤组织中GABA、可溶性糖和淀粉含量随着月份增加而逐渐升高。(2)板栗插穗初始茎和愈伤组织的褐化相关物质单宁、酚类和类黄酮含量均表现为30年生不同程度低于5年生;除类黄酮含量在7月份的愈伤组织中有显著升高外,不同月份板栗的单宁、酚类、类黄酮含量均没有明显变化。(3)生根状况及解剖学观察结果显示,5年生板栗插穗的愈伤率和生根率高于30年生的插穗,且30年生插穗6月份扦插的愈伤率最高;板栗插穗属于诱导生根类型,皮层中存在环状厚壁组织;5年生板栗插穗的木质素含量低于30年生板栗,但形成层细胞层数高于30年生。研究认为,板栗扦插生根过程中插穗褐化现象严重,生根率较低;不同年龄母树和不同月份的板栗插穗在扦插过程中的碳氮代谢相关化合物含量都有明显的差异,而褐化相关物质含量仅在不同年龄母树间有较大的差异,不同月份间大部分无明显差异。     

NOTES ON A VIRUS DISEASE OF CACAO

A virus disease of cacao, discovered in Trinidad towards the end of 1943, is apparently confined to the north-western corner of the is land and does not occur in the main cacaogrowing areas of eastern, central and southern Trinidad; nor has it been found in Venezuela, Tobago or the Windward Islands of Grenada, St Vincent, St Lucia and Dominica. The symptoms resemble those of West African swollen shoot disease, consisting of vein-clearing or mosaic of the leaves and red-mottle of the leaves and pods. No swellings are formed, but recent observations indicate that the disease may cause both dieback and reduction in yield. There are two strains closely resembling each other. The spread of the disease under field conditions has been demonstrated, and although the rate of spread has varied somewhat, a mean increase of 41 % over the original number of infected trees has been found over a 10-month period. As with swollen shoot disease, existing diseased areas have increased in size and new outbreaks at short distances have occurred. Both strains were readily transmitted by budding, the incubation period varying from 34 to 136 days, with a mean around 90 days, but no insect vectors have been found. There is at present little reason to believe that any varieties of cacao are immune to the disease or are even symptomless carriers. The question of eradicating this virus, while its distribution remains limited, should be seriously considered.     

Genetic Structure and Molecular Diversity of Cacao Plants Established as Local Varieties for More than Two Centuries: The Genetic History of Cacao Plantations in Bahia,Brazil

Bahia is the most important cacao-producing state in Brazil, which is currently the sixth-largest country worldwide to produce cacao seeds. In the eighteenth century, the Comum, Pará and Maranhão varieties of cacao were introduced into southern Bahia, and their descendants, which are called ‘Bahian cacao’ or local Bahian varieties, have been cultivated for over 200 years. Comum plants have been used to start plantations in African countries and extended as far as countries in South Asia and Oceania. In Brazil, two sets of clones selected from Bahian varieties and their mutants, the Agronomic Institute of East (SIAL) and Bahian Cacao Institute (SIC) series, represent the diversity of Bahian cacao in germplasm banks. Because the genetic diversity of Bahian varieties, which is essential for breeding programs, remains unknown, the objective of this work was to assess the genetic structure and diversity of local Bahian varieties collected from farms and germplasm banks. To this end, 30 simple sequence repeat (SSR) markers were used to genotype 279 cacao plants from germplasm and local farms. The results facilitated the identification of 219 cacao plants of Bahian origin, and 51 of these were SIAL or SIC clones. Bahian cacao showed low genetic diversity. It could be verified that SIC and SIAL clones do not represent the true diversity of Bahian cacao, with the greatest amount of diversity found in cacao trees on the farms. Thus, a core collection to aid in prioritizing the plants to be sampled for Bahian cacao diversity is suggested. These results provide information that can be used to conserve Bahian cacao plants and applied in breeding programs to obtain more productive Bahian cacao with superior quality and tolerance to major diseases in tropical cacao plantations worldwide.     

How New Guinea rainforest flower resources vary in time and space: Implications for nectarivorous birds

Abstract Variability in spatial and temporal patterning of flowering by populations of rainforest trees fed upon by honeyeaters and flower-visiting parrots was studied for 2 years in lowland tropical hill forest in Papua New Guinea. All 2200 trees in a 3 ha plot were tagged, identified, mapped and monitored monthly. Of 274 tree species present, 86 flowered during the course of the study; during any given month, approximately 20% of the species flowering that month were visited by nectarivorous birds. Results showed that overall flower resources (total number of species, and number of bird-pollinated species, individuals and flowers) fluctuated during the year, decreased during the dry season and increased during the wet season. In addition, there was a wide range of temporal variation within and among tree species in length and timing of flowering period, percentage of each conspecific population flowering from year to year, and degree of synchrony among flowering conspecifics. Spatial dispersion of tree populations also varied, from clumps to scattered single individuals. Resident bird species were correlated with synchronously flowering trees, whereas nomadic bird species were correlated with asynchronously flowering trees. Resident birds were also associated with smaller blooming displays per tree, whereas nomadic birds were associated with trees that bloomed massively. There was no correlation between avian nomadism and spatial dispersion of tree populations. Thus nomadic birds seem to range in search of rich but unpredictable resources; resident birds may rely more on predictable, but smaller resources.     

The influence of emergent trees on rainfall distribution in a cacao agroforest (Sulawesi,Indonesia)

Emergent trees may have an influence on the volume and the spatial distribution of water input into agroforestry stands and may thus affect water availability for the main crops. Our goal was to analyze the influence of such trees on rainfall distribution in a cacao agroforest area in the rainforest margin zone of Central Sulawesi, Indonesia. The emergent trees studied belong to the species Bischofia javanica (Phyllanthaceae) and were 15 m high remnants from the natural forest. A set of 96 throughfall gauges was systematically distributed underneath canopies of cacao only, and underneath canopies of cacao plus emergent trees (cacao plus trees). From an earlier study we knew that stemflow can safely be estimated with less than 1% of gross precipitation (Pg).     

PHYSIOLOGICAL STUDIES ON THE VERTICILLIUM WILT DISEASE OF TOMATO

The water loss per unit leaf area of tomato plants was decreased after inoculation with Verticillium albo-atrum. When diseased plants began to wilt water loss temporarily increased, but then rapidly decreased to become less than that of healthy plants grown under conditions of adequate or restricted water supply.
The transpiration of excised leaves from plants grown with a restricted water supply was reduced, but not so severely as that of comparable leaves from infected plants. Water loss from leaves on infected plants was reduced irrespective of any blocking of the petiolar xylem.
The rate of water loss from turgid leaf disks on mannitol solutions, and the rate of water uptake of leaf disks on water was similar for disks cut from wilting or turgid leaves of diseased plants or healthy plants grown with an adequate or restricted water supply.
Disease or poor water supply reduced leaf growth but had no effect on the rate of leaf initiation. Although the density of stomata was higher on leaves of diseased plants the stomatal area was less than on healthy plants.
The resistance to water flow in diseased stems was high and was correlated with vessel blockage. About half the blocked vessels contained hyphae. The severity and localization of symptoms in inoculated plants growing on susceptible or resistant rootstocks was directly related to the extent of invasion by the pathogen and to vessel blockage.
Experiments on the wilting activity of cell-free filtrates from cultures of the pathogen in vitro indicated that it produced a stable substance, not an enzyme, that caused wilting in cut shoots by blocking the end of the stem. It is suggested that an increasing internal water shortage causes major symptoms of disease.     

Date of onset of regrowth dieback and its relation to summer drought in eucalypt forest of southern Tasmania

'Regrowth dieback', a disease of Eucalyptus obliqua and E. regnans is causing serious degradation of high yield, even-aged, regrowth forest in south-eastern Tasmania. Dieback symptoms in crowns of diseased trees were first observed in the region in 1964 and since then the disease has continued to develop. Visual symptoms of the disease can be discerned only in crowns of dominant and co-dominant trees since sub-dominant and suppressed trees show similar symptoms due to normal stand competition. This work shows that for increment periods which finished in 1958 or earlier, basal area increments of dominant and co-dominant trees from a number of growth plots in the region were slightly lower in trees which were affected by the disease in 1971 than in those which were not. There were much larger reductions in increments of affected trees measured in increment periods which finished in 1960 or later. It was concluded that an agent very damaging to forest health appeared in the region in 1959 or early 1960. This coincides with the start of a period of several years of unusually dry summers in the region. These findings conform with an earlier suggestion that the disease may have been initiated by drought with secondary damage by the root-rotting fungus Armillaria sp. and/or the defoliating beetle Chrysophtharta bimaculata . The fungus is ubiquitous in the region and has frequently been associated with decayed roots of affected trees. The beetle is believed to have caused episodic tree defoliation in the region. It is suggested that the secondary agents normally cause death or ill-health of a few trees in otherwise healthy forest. Physiological weakening of trees by drought may have allowed the effects of these agents to increase substantially after 1959.     

达维台风对海南尖峰岭热带山地雨林群落的影响

 海南岛地处我国台风的频发地带, 台风是影响海南岛热带雨林的一个重要的自然干扰因子。该文对2005年达维台风前后海南尖峰岭热带山地雨林天然次生林2 600 m²固定样地的物种组成、胸高断面积、植株数目、风倒木的影响因子、物种多样性、生物量变化及碳储量归还进行比较分析。结果显示, 台风后森林内产生大量的风倒木、断枝和落叶, 导致群落郁闭度减少、透光性增强, 并产生大量的林窗。台风后群落的组成结构发生显著变化, 受损株数达514株(占总株数26.1%); 其中风倒木比例较大, 达206株(占总株数10.5%)。方差分析结果显示, 受台风影响产生的风倒木与未明显受损树木的胸径面积、树高和木材密度均无显著差异; 但与树种组成有较紧密的关联, 对乔木层、幼树层和下木层造成的损害存在种类上的明显差异。台风产生的损害可以分为直接性损害和间接性损害两种: 1)直接性损害主要作用于乔木层, 造成大径级植株严重受损; 部分树种重要值降低, 变成伴生种, 或者次优势种和原有优势种成为共同优势种; 2)间接性损害主要通过风倒木作用于幼树层和下木层, 产生的倒木、断枝和落叶压倒其它植株, 表现为小面积范围内的个体死亡, 甚至有种类消失; 部分优势种的个体数减少较多, 但占该种群所有个体数目的比例较小。乔木层的Shannon-Wiener和Simpson多样性指数均略有下降, 而幼树层和下木层均略有上升, 但3个林层均表现为不同程度的种类消失和一些种类的个体数目减少。此次台风还造成该样地至少占台风前总生物量10.42%的生物量归还给林地, 并对森林生态系统碳循环产生重大影响。     

自然群落中柏达木种群光合产量与光强气温关系的初步研究

关于植物种群的光能利用,植物生产力和净同化率等近年来有不少研究,但其中除了研究植物个体的净同化率外,多为农作物等草本植物群落或者用林业采收法的研究。对于自然木本植物种群及其不同层的植物与光、温度分配关系中用测定光合作用方面的研究工作还不多。 光合作用强度的测定,目前多用离体叶片或盆栽植物进行或者用收集落叶量与砍伐林木     

太原地区某些植物净化二氧化硫潜力的研究

本文以研究某些树木和农作物(以下简称植物)对SO2的净化潜力来评价太原地区大气环境容量中的生物净化参数。内容包括三部分:植物年生物量的调查;用人工模拟熏气试验测定植物吸收SO2的强度和吸收转运周期;以及该地区植物净化SO2潜力的估算。太原地区树木叶片的年生物量约18万吨．树叶平均吸收SO2的强度为1.9%。SO2在树叶中的吸收转运周期约30天。树叶旺盛生活期约90天,相当于三个周期．由此计算可得本地区树木叶片每年吸收SO2的总量约一万吨。郊区农作物年生物量总计为25万吨．农作物吸收SO2的强度为0.45％．SO2吸收转运周期为20天。作物旺盛生活期为40天,相当于两个周期。计算得出本地区农作物吸收 SO2的总量约两千余吨。把树木和农作物年吸收SO2量之和,作为全年植物净化SO2的总量,约1.2万吨。在所研究的525平方公里范围内,平均每平方公里内的植物每年净化SO2的最大潜力约为24吨。在加强污染源治理和控制排放的前提下,利用植物净化大气S02是一条经济、简便和有效的补充措施。     

Diversity and spatial clustering of shade trees affect cacao yield and pathogen pressure in Costa Rican agroforests

The importance of the spatial organisation of individuals in explaining species coexistence within a community is widely recognised. However, few analyses of spatial structure have been performed on tropical agroforests.The main objective of this study was to highlight the links between spatial organisation of shade trees on the one hand, and shade tree species richness and cacao yield on the other, using data from 29 cacao agroforests in Costa Rica.A method of spatial statistics, Ripley's K-function, was used to analyse the spatial organisation of shade and cacao trees in the study plots. For each stand, the X and Y coordinates of ≥2.5-m-tall trees were recorded. In each plot we also assessed shade tree species richness and cacao yield (with total number of pods = number of pods damaged by frosty pod rot + number of healthy pods).Three types of stands were identified: the first was characterised by significant clustering of shade trees, the highest shade tree species richness (S = 6), and the highest number of damaged pods (139 pods ha^?1 year^?1). The second type was characterised by random spatial organisation of shade trees. The third type showed a trend towards regular organisation. Species richness of shade trees did not differ significantly between the last two types (S = 4 for both), nor did the number of damaged pods (56 pods ha^?1 year^?1 and 67 pods ha^?1 year^?1 respectively).Although the trends were not statistically significant for all the variables in our data set, the clustered spatial structure appears to favour a synergy between environmental (tree species richness), and provisioning (cacao production) services.