All underground parts of terrestrial orchids must either accommodate the endophyte or actively reject it. The mycotrophy of the protocorm is obligate, and most roots are also mycotrophic in varying degrees, but in many species the rhizome loses its mycotrophic function while the plant is still young. Storage organs such as root-stem tubers and corms are usually not infected.

An apparently important aspect of the mycotrophic organs is their longevity; although in the tuberous species some roots function for only about 9 months, orchid roots typically continue to function for a number of years.

Protocorm

The germinating embryo develops into the protocorm. Unlike other angiosperm seedlings it has no radicle, since the suspensor end of the embryo becomes specialized to form mycotrophic tissue. Neither root cap nor meristem is formed, and the suspensor end of the embryo remains stationary in the soil (Fig. 5.6, p. 111; Fig. 6.1c, p. 114; Fabre, 1856; Stojanow, 1916). As far as is known all orchid seedlings are modified to form mycotrophic tissue in the basal part, even in Bletilla which has the most highly differentiated embryo yet observed (Bernard, 1904).

The opposite, i.e. chalazal, end with the functional meristem is solely responsible for all increase in width and length in the seedling; from this end a condensed or elongated mycorhizome develops which allows the shoot tip with leaf primordia to progress in the soil.

Strictly speaking the protocorm comprises only that part of the seedling axis that develops below the lowermost leafy appendage and corresponds to the radicle and hypocotyl in seedlings of other plants. The boundary between protocorm and mycorhizome is indistinct, however, since the first scale leaves are often inconspicuous and occasionally vestigial.

Abstract

Investigations have been carried out for the purpose of understanding and predicting different types of storm damage to forest, such as stem, stock or root breaks. The tree sway mechanical model used enables the determination of the modal parameters, natural frequency and shape deflection of the stem, for the two or three first oscillation modes of the structure. Inputs are specific dendrometric data, including mass distribution and conicity of the stem, orientation and biomass of branches. The purpose of this model is to allow the estimation of the fundamental oscillation mode (responsible for stock or root breaks) and the first harmonic (probably responsible for stem breaks) and to simulate the influence on those parameters of silvicultural practices such as pruning and pollarding. For the moment, in our description, the tree stem is considered as a cantilever beam of circular section, rigidly embedded in the soil. Experimentally, the signal delivered by an extensometer plugged under the bark on the tree stem is analysed using a dual-channel signal analyser. The modal parameters of the dynamic behaviour of the tree are identified, including resonance frequencies and associated damping factors. The methodology is first applied to a laboratory tree scale model and a good fit of the experimental results to predicted values is shown. Preliminary experimental results from a young budding red oak indicate the sensitivity of the dynamic spectrum to such a biological change.

Introduction

For about 4 years the research team in wood mechanics of the Laboratory of Wood Rheology of Bordeaux (LRBB) has been conducting an analysis of the mechanical behaviour of tree structure.

Introduction

Insects are susceptible to a wide range of viral pathogens, but viruses of the family Baculoviridae, which occur only in arthropods, have received most attention because their high pathogenicity, narrow host range, and complete safety to vertebrates and plants, make them particularly promising candidates for biological pest control agents (see Huber, Chapter 18). Although natural strains of these viruses can in many cases provide highly satisfactory levels of control, the past 5 years has seen several attempts to produce genetically engineered baculoviruses with improved insecticidal properties. The chief target for these improvements has been reduction in the crop damage that continues to occur after infection of the insects.

A second reason for using genetically engineered baculovirus insecticides is to provide marked strains for ecological studies in a manner somewhat analogous to the way that animals are often marked physically in order to study in a more detailed way their behaviour and survival. Marked viruses may contain simply a short stretch of introduced DNA in their genome. This can be detected by various methods such as dot blotting, Southern blotting, restriction enzyme analysis (especially if the introduced DNA contains additional restriction sites), or, with much greater sensitivity, by use of the polymerase chain reaction. Alternatively, marked viruses may contain a reporter gene that expresses a protein, usually an enzyme, which can be detected easily and specifically. The E. colt lacZ gene, which codes for P-galactosidase, has been found particularly useful.

Introduction

Progress in agronomy over the last 30 years has allowed world food production to almost double. In part, this was possible through the control of diseases caused by air-borne pathogens. However, the damage caused by soil-borne pathogens has increased during the same period; this has resulted in important yield reduction in protected and unprotected crops. To date, satisfactory methods of control have not been developed. Few plant varieties are resistant to soil-borne pathogens. Unfortunately, crop rotation is often not possible for economic reasons. Pesticides are seldom effective in soil and growing concern for the environment restricts their use. Biological control of root diseases can be considered as an alternative for the future. The pseudomonads are currently receiving attention world-wide because some strains, added at a few grams or milligrams per hectare, effectively increase crop yield and improve plant health. These beneficial effects are well documented, however the mechanisms involved are only partially understood. A better understanding of these mechanisms is crucial to safety if pseudomonads are to be released on a large scale. Data on safety are scarce because the investigation of potential risks associated with the release of pseudomonads has just begun. This review will mainly focus on the results obtained with strain CHAO of Pseudomonas fluorescens.

Beneficial effect of resident populations of pseudomonads

Resident populations of pseudomonads are part of the natural disease suppressiveness of soils and of suppressiveness induced by monoculture or solarization (Smiley, 1979; Scher and Baker, 1980; Rovira and Wildermuth, 1981; Cook and Weller, 1987; Defago and Haas, 1990; Gamliel and Katan, 1991). It is important to realize that natural suppressiveness occurs only in distinct areas.

Abstract

A survey of the directions of coniferous tree windthrow in a subalpine ecosystem reveals a distinct relationship between the airflow and local terrain features. The survey covered a portion of the USDA Glacier Lakes Ecosystem Experiments Site (GLEES), located at an altitude of about 3300 m above sea level in Southeastern Wyoming, USA. The GLEES site is located in the lee of a glacial cirque basin in the upper treeline ecotone. The direction of the surface wind over the GLEES, as determined by meteorological data and the pattern of wind deformation of trees, is predominantly westerly. These winds result from channelling of larger-scale winds that blow around the south side of the mountain to the west of the GLEES. On the other hand, when the large-scale winds blow around the north side of the mountain, they may descend the ridge to the north of the GLEES at high speed from the northwest. The two modes of wind direction agree with observed bimodal windthrow directions.

Introduction

The Glees Project

The Glacier Lakes Ecosystem Experiments Site (GLEES) in the Snowy Range of Southeastern Wyoming is a 600 ha alpine and subalpine watershed at an elevation of 3200–3500 m above mean sea level (msl). Research at the GLEES is coordinated by scientists from the USDA Forest Service Rocky Mountain Forest and Range Experiment Station, the University of Wyoming, and the Medicine Bow National Forest. Nearly 20 cooperating universities and governmental agencies together with scientists from other countries conduct research on ecosystem and atmospheric processes at the GLEES which relate to climate change.

Abstract

The model (Flowstar) is tested against new airflow data from a region of complex terrain and roughness cover (Kintyre, south-west Scotland). The dataset consists of mean flow data at 10 m above the surface at up to 14 sites supported by airborne measurements of the upstream temperature profile and the flow aloft. The structured, stable stratification is shown to exert a strong influence on the mean flow close to the surface. Far from the surface Flowstar reproduces the main features of the stratification-dependent flow. However, additional Flowstar stratification options may improve the model in cases where the upstream profile is complicated. At 10 m above the surface and in near-neutral conditions the reliability of Flowstar predictions of mean flow are comparable with those from similar models applied to simpler terrain shapes. Errors are observed in wake regions where the model overpredicts windspeeds. This is due to a previously known limitation of linear models. Flowstar modelling of the effects of stable stratification on the mean flow at 10 m is encouraging, although some dependence of accuracy on stratification type is noted. Recommendations for the use of Flowstar in practical applications are made.

Introduction

In many areas of the United Kingdom the wind climate is the limiting factor to forest growth (Cannell & Coutts, 1988). A model of wind flow over complex terrain (i.e. terrain that does not approximate to any simple shape) would help foresters to plan planting patterns to minimise damage. Such a model must be adaptable enough to represent a wide range of realistic conditions and accurate enough to allow confident forest planning.

Introduction

The Industries Assistance Commission of Australia, in their review of the economics of agricultural research using the CSIRO Division of Entomology as an example, concluded that the majority of projects were ‘dry holes’ in that they yielded negligible economic return, but that it only needed one or two successes to pay for the entire operation of a large research agency over a decade or more (Marsden et ah, 1980). Biological control was a major area considered in that review and these two principal conclusions are as true for this field as they are for the general case. Less than half of biological control projects produce substantial success, but those that do are the really big winners.

We would like to review briefly the history of application of economics to biological control in Australia and then consider the benefits and costs in a more general sense. There is nothing unique about biological control from an economic viewpoint, though it does have some special properties that assist the process and increase benefits, and others that require particular consideration.

The examples that we propose to cover concern classical (inoculative) or inundative biological control where the natural enemies have been selected from naturally occurring species or biotypes. We do not cover the release of genetically improved natural enemies, whether modified by conventional methods or by genetic engineering. Many of the concepts we canvas are equally relevant to the ‘new technologies’ in terms of assessing the costs of research, the likelihood of success and the ensuing economic benefits. Even the risks are not dissimilar in nature.

Abstract

The effects of tree improvement on factors likely to influence tree stability were investigated using clonal Sitka spruce trees that had been grown for 11 years on an unploughed nursery site. The distribution of biomass between root and shoot, and within the root systems of trees from five improved clones, was examined and compared with control trees grown from unimproved cuttings and transplants. The direction (azimuth) of growth and dimensions of the main woody roots were also measured. Differences between clones were found in allocation of biomass between root and shoot, and in root system architecture. Large differences were found between clones in proportions of below-ground biomass allocated to stumps and woody roots (which function for anchorage). These results indicate that root: shoot ratio can be a poor indicator of tree stability when the stump is included as part of the root biomass. The distribution of root origins around the stump showed no significant clumping but the allocation of biomass between roots was found to differ between tree types. On average, the improved clones had allocated biomass to fewer roots than the controls. The amount of branching in the proximal 45 cm of the root system also differed between clones. Distribution of root cross-sectional area around the tree was significantly asymmetric in two of the clones. Overall, root biomass was allocated more to the lee side of the prevailing wind direction. The substantial differences found in allocation between root and shoot, and within the root system, may have implications for the wind stability of trees and could present opportunities for improving stability by clonal selection.

Introduction

Damage by windthrow causes important economic losses to forestry in northern Europe.

Abstract

A small load cell was developed to measure in situ stresses generated within a tree-root system as the surrounding soil mass was being subjected to an external force. The load cells appear to represent a viable method of monitoring tree-root stresses and could readily be modified to register root stresses generated in trees subjected to wind loading. The ability of these instruments to measure in situ both tension and compressional forces will give an opportunity to investigate relationships between various above-ground and belowground tree components during storm conditions.

Introduction

Wind is a serious risk factor in many conifer plantations in New Zealand. Since the early 1940s windthrow has accounted for up to 45 000 ha of damaged trees. Wind damage also has important implications for management of indigenous forests.

Exotic forests on the Canterbury Plains, South Island (lat. 42° 45' S, long. 172°45' E), are often underlain by thin soils and compacted gravels, which tend to produce shallow plate-type root structures of usually less than 1 m total depth. Significant windthrow of exotic forest stands was recorded as early as 1914, with subsequent major damage occurring in 1945, 1964 and 1975 (Somerville, 1979). Genetic research over recent years has resulted in trees with improved form and wood-producing qualities. Whether there has been a corresponding improvement in below-ground qualities has yet to be investigated. However, the genetically improved stock has led to management practices tending towards lower stocking rates. This will increase wind turbulence within the forest and possibly increase windthrow, particularly after final thinning.

Abstract

Wind stability is considered the major problem in Norway spruce silviculture. Traditional thinning models are characterised by a constancy of thinning intensity throughout the rotation. These models will generally result in low wind stability in spruce stands because they violate the important processes involved in wind stability: (1) The regulation of stem number is usually delayed until pulp wood or timber products can be extracted. This causes a high density of trees and high root competition in young stands, where the branching pattern and the long-term increment capacity of the structural root system are fixed. The production of finer roots during the second half of the rotation is thus strongly inhibited by high stem density in the pre-commercial phase. (2) Stem biomass extraction during the middle and last part of the rotation causes severe loss of ‘anchorage biomass’, as stumps do not contribute to neutralising the ‘storm energy’ transferred to the stand canopy. (3) The removal of trees destroys canopy closure: thinning reduces wind protection by neighbouring trees, because wind gusts penetrate deeper into the canopy, and physical crown contact is reduced. (4) A tree adapts to the very specific wind climate defined by the surrounding trees; therefore a change in wind flow in the canopy caused by removal of neighbouring trees makes it susceptible to damage. The negative effects of these last three factors increase with increasing age and with increasing thinning intensity. Regarding wind stability, a ‘D- to A-degree’ stem number reduction model is recommended.

Introduction

To develop successful biocontrol of plant diseases by the introduction of naturally occurring (see Défago et al., Chapter 12) or genetically engineered (see Dowling et al. Chapter 14) antagonistic pseudomonads, careful research is needed to assess risks. This includes viability and translocation of the microbes in soil ecosystems, transfer of genetic material to and from the indigenous soil microbial communities, effects of introductions on biochemical functions of the soil microflora and so on. The development of detection methods is important, not only for risk assessment of survival or dispersal of biocontrol microorganisms, such as genetically engineered microorganisms (GEMs) in soil, but also to evaluate the efficacy of biocontrol agents against plant diseases.

Pseudomonas cepacia Palleroni & Holmes, was initially described as a phytopathogen (Burkholder, 1950) as well as a saprophyte in soil (Sinsabaugh and Howard, 1975). It has been reported to provide biological control of certain soil-borne diseases, such as Fusarium-wilt of onion (Kawamoto and Lorbeer, 1976), damping-off of radish, Fusarium-wilt of tomato and Verticilliumwilt of eggplant (Homma et al., 1985). This bacterium is also known to produce various antibiotics, such as pyrrolnitrin (Elander et al., 1968; Janisiewicz and Roitman, 1988; Homma et al., 1989) and pseudane derivatives (Homma et al 1989), which are supposed to contribute to disease suppression. On the other hand, P. cepacia is extremely resistant to antimicrobial agents and is able to survive and multiply even in purified waters (Carson et at., 1973). In addition, it is known to be an opportunistic pathogen and has been described as a synonym for P. kingii and P. multivorans, and as aetiological agents of clinical infection (Ederer and Matsen, 1972; Randall, 1980).

Introduction

Currently the only one commercialized biological control product against foliar fungal diseases exists, namely, ‘Trichodex’ (Makhteshim Chemical Works) for the control of grey mould of cucumber, tomato and grapevine (Y. Elad, personal communication). A few other products based on fungi are in the final stage of development or registration. Therefore, this review will primarily deal with the state of the art, which ranges from interesting hypotheses to promising research achievements under controlled and natural conditions. Compared with research efforts on biological control of soil-borne diseases, biological control of foliar diseases has up till now received little attention. The main reason for this appears to be the relatively efficient control of many above-ground pathogens by aerial applications of fungicides, while soil-borne pathogens are more difficult targets.

Nowadays, however, the world-wide awareness of the potential danger to the environment from many currently available chemical control agents and the increasing occurrence of fungicide resistance in a number of foliar pathogens has boosted research aimed at alternative means of disease control. For instance, in the Netherlands, a ‘Multi-Year Crop Protection Plan’ has been launched in 1990. This aims at a reduction in total amount of pesticide used of at least 35% by 1995 and 50% by the year 2000 (Anonymous, 1991). More importantly, the average emissions (the average pesticide emissions (to air, soil/ground water, surface water) to the environment are to be reduced to 50% in 1995 and to 30% in the year 2000.

Since the young seedling of terrestrial orchids lives underground, the early life history is largely unknown. Some investigators have painstakingly dup up seedlings and underground structures around the year and endeavoured to deduce the developmental process from this material (Fuchs & Ziegenspeck 1924a, b, 1926a, b, 1927a–c). This is usually the only source of information that we have as regards the life history since many species has never been grown from seed to the adult stage in culture or their life history has not been described when cultured plants were available.

Both excavation of plants in natural populations and observation of cultured specimens have their limitations as methods of investigating the life history. Unless the same individuals are repeatedly dug up and carefully replanted this method gives information only on the sequence of organogenesis and a rough seasonal timetable. Construction of an absolute timetable requires that there is a close correspondence between size and age, but this assumption is unrealistic, since cultivated seedlings of many species show considerable individual variation under uniform culture conditions. Estimates of the duration of seedling stages that have been made on the basis of excavations, particularly by Fuchs & Ziegenspeck (1922–7), have been justly criticized for being exaggerated, but it is mainly the most extreme estimates such as for the underground phase of Orchis ustulata that have been singled out (Summerhayes, 1951) and been met with scepticism. Most of the estimates made by Fuchs & Ziegenspeck do not fall wide of the mark if they are compared with those now emerging from population studies, as can be judged from Table 10.1.

Introduction

While integrated pest management research has sought to reduce the use of pesticides in agriculture over the past 30 years, current public and political pressure has increased dramatically the urgency of the search for alternatives. Few are immediately available, and this places particular reliance on some key biological control agents. One such agent is Bacillus thuringiensis Berliner (Bt), which is the most widely used biopesticide. Sales have doubled in the past 4 years and are conservatively forecast to rise by 20% per year in the future, leading to a market of US$ 300-500 million by 1995 or 5–10% of global insecticide sales by the year 2000 (Anonymous, 1991–6). In addition to the expanding use of Bt as a microbial pesticide, the Bt genes responsible for its efficacy as a pesticide have become a primary source of insect toxins for those who seek to produce transgenic crops resistant to insects. It was reported by Watkinson (1992) that at least 50 plant species have been transformed with Bt genes and the list is being constantly added to. This brings closer the prospect of Bt toxins becoming widely and persistently distributed in the environment.

There is a growing criticism and concern about this genetic approach to pest control (Harris, 1991; van Emden, 1991; Anderson, 1992). Insects have exceeded most projections of their ability to develop resistance to pest control methods applied against them, and this now includes Bt.