Showing papers in "Journal of Ecology in 1975"

A test of a modified line intersect method of estimating root length

Abstract: A test of a modified line intersect method of estimating root length , A test of a modified line intersect method of estimating root length , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

Relative growth-rate: its range and adaptive significance in a local flora.

Abstract: Laboratory experimentation in plant ecology has evolved very largely as an attempt to pursue investigations which began with fieldwork. With the widespread development of plant growth-room facilities an alternative approach is possible. This is to measure the characteristics of plants under a variety of controlled conditions, and to use the results to predict their field ecology (Grime & Hodgson 1969). One advantage of this approach is that predictions can be tested against descriptions of the field ecology obtained by independent field investigation. In the long term, however, the most important advantage of the predictive approach is that many growth-room investigations can be reproduced or extended wherever there are adequate facilities: hence data collected on different species or genotypes and in various laboratories can be compared directly. When comparable data are available for a large number of species drawn from a wide range of habitats it may be possible to estimate the limits of variation of a particular plant attribute, to place an individual measurement in context and to attempt to judge its ecological significance. The investigation described in this paper is an attempt to examine the range and pattern of variation in a local flora of one particular plant attribute-the maximum potential rate of dry matter production. Although most data have been obtained from only one field population per species the number of species is large and includes representatives from all the major dry terrestrial habitats of the area. Uncertainty concerning the extent to which each sample is representative of the species does not, therefore, invalidate the exercise either as an estimate of the range of variation or as an attempt to recognize differences between groups of species of contrasted ecology.

Stability of Grazing Systems: An Application of Predator-Prey Graphs

Abstract: In a now classical paper, Rosenzweig & MacArthur (1963) have shown how the general stability properties of simple predator-prey systems can be studied by graphical techniques, supplemented by mathematical analysis of the behaviour near equilibrium points. A similar, graphical analysis of predation functions was suggested by Holling (1965). The stability analysis of predator and prey isoclines in the predator-prey 'phase diagram' has been discussed further by MacArthur & Connell (1966) and Rosenzweig (1969, 1971). The mathematical and graphical analysis of stability in predator-prey systems, and even three-trophic-levels systems, have been extended by Canale (1970), Rosenzweig (1973), Hubbell (1973), May (1971, 1972), Vandermeer (1973), Strebel & Goel (1973) and Maynard Smith & Slatkin (1973). Thus a considerable body of theory has been developed to deal with systems of populations at two or more trophic levels, or 'exploitation' systems (Rosenzweig 1973). So far only few attempts (Salt 1967; Maly 1969; McAllister, Le Brasseur & Parsons 1972) have been made to apply this theory to real ecological systems, whether by directing experiments or observations to test it, or even by comparing its predictions with existing data. Partly this slowness in application may have been due to the feeling that these theoretical models are still too simplified to be expected to apply directly to the manyspecies, variable-environment and spatially heterogeneous predator-prey systems found in nature. However, as pointed out by Rosenzweig (1973), scientific theory often starts by testing rather simple models, even of complex systems. Grazing systems used and controlled by man, from intensive pastures to extensive range, may be considered as a special case of'predator-prey' systems. Much of the theory was developed with explicit or implicit reference to two animal populations (e.g. Rosenzweig & MacArthur 1963; Holling 1965). But herbivore ('predator')-plant ('prey') interaction is sufficiently similar in its general features to make the same approach useful. Some important modifications are necessary, on the basis of biological considerations, but some results are directly transferable between predator-prey and herbivore-plant systems. Grazing systems have some advantages as relatively simple test cases for general ecological theories: the number of species is limited (usually only one herbivore, in some cases one or a few plant species), environmental heterogeneity within a system is often low and movement of animals is controlled. There is a large and increasing number of observations and experiments on pasture and range systems in many parts of the world. On the other hand, grazing systems are one of the types of ecosystems which are of greatest importance to man; if theoretical ecology could contribute to their understanding and to the solution of their practical management problems, this would be a very useful contribution indeed.

Indicator species analysis, a divisive polythetic method of classification, and its application to a survey of native pinewoods in Scotland

Abstract: Indicator species analysis is a divisive polythetic method of numerical classification applicable to large sets of qualitative or quantitative data. It incorporates a key which enables new data to be assigned to the classificatory framework. Each dichotomy is established in several steps. First, a one-dimensional reciprocal averaging ordination is computed. The stands are then divided into two groups according to whether they fall on one side or the other of the centre of gravity of the ordination. Five indicator species are then identified which discriminate as well as possible between the two groups of stands. These are then used to construct a secondary ordination. The balance between the indicator species in the secondary ordination provides an objective and easily applied criterion for identifying the two groups of stands which can be used conveniently in the form of a key. The method has been applied to a survey of the native pinewoods of Scotland. The main floristic variation in these woods is related to two gradients: wetness and pH. The indicator species analysis helped to highlight the gradients, and provided a means of pigeon-holing the stands into acceptably homogeneous groups as a basis for further, more detailed, studies

Competition and stand structure in some even-aged plant monocultures

Abstract: Clements defined competition in the following terms: 'When the immediate supply of a single factor necessary (for growth) falls below the combined demands of the individual plants competition begins' (Donald 1963). It is a difficult process to study directly and proof of the causal link between the growth of some individuals, environmental depletion and the reduction in relative growth rate of others has not been obtained. Work on competition in monocultures has concentrated on studies of population structures or on changes in individual plant characteristics observed under conditions where competition is thought to take place; it is the assumed outcome of competition which is studied. The technique most frequently used is to establish a series of communities with different planting densities and to observe them over a time interval and from such studies three symptoms have been described which Harper (1967) considered as indicative of a community under 'density stress'. Firstly, from a frequency distribution of seed or seedling weight which is normal a skewed distribution of individual plant weights develops, there being a large number of plants slightly smaller than the mean and a small number of plants much greater than the mean, referred to as a 'hierarchy of exploitation' by Harper (1967). This type of distribution has a positive value of the third moment, g1 (Sokal & Rohlf 1969), and is commonly referred to in the plant competition literature as 'log normal', an expression used by Koyama & Kira (1956) although they did not attempt to fit this particular distribution to their skewed frequency distributions. The closer the initial spacing of plants the sooner skewness appeared from which Koyama & Kira inferred that the competitive effect developed sooner. Skewness is commonly found in stem size distributions of even-aged forest monocultures, e.g. Bliss & Reinker (1964), Jack (1971), Ford & Newbould (1970). A second phenomenon taken as indicative of density stress is density dependent mortality. The proportion of plants which die in a set interval of community development increases the closer that initial plantings are made above a threshold value, e.g. Davidson & Donald (1958), Yoda et al. (1963). Thirdly, under closer spacings there are alterations in the morphology of the 'mean' plant, a phenomenon referred to as a 'plastic response' by Harper (1967). The proportions of the plants change; mean height increases, there is an alteration in mean leaf weight/area and a reduction in weight of seed produced by each unit of weight of vegetative tissue, e.g. Donald (1963), Hiroi & Monsi (1966). The phrase 'hierarchy of exploitation' suggests that when competition is taking place

Phenology and Dynamics of Root Growth of Three Cool Semi-Desert Shrubs Under Field Conditions

Abstract: Many cool arid land shrub species exhibit much higher root/shoot biomass ratios than many warm desert shrubs even though mean potential evaporation/precipitation ratios are usually less severe in these cooler areas (Barbour 1973; Rodin & Basilevich 1965; Shalyt & Zhivotenko 1968; Sveshnikova 1968; Bjerregaard 1971; Fernandez 1974). These high root/shoot biomass ratios are particularly apparent in regions where most of the annual precipitation occurs in the winter months (Rodin & Basilevich 1965; L. E. Rodin, personal communication; Bjerregaard 1971). Cool semi-desert communities in northern Utah exemplify this situation. Root/shoot biomass ratios are of the order of 9 for the dominant shrub species. Soil moisture recharge results primarily from winter precipitation in the form of snow. When the upper part of the soil profile thaws in early spring, the soil is recharged to a depth of approximately 80-90 cm. Subsequent summer season precipitation is of significance only for recharge of the uppermost soil layers. Therefore, during the course of the main growing season from March to September, there is a steady decline of soil moisture in most of the profile. Soil moisture potentials during the drier portion of the year of less than -70 atm are not uncommon in these soils (Moore & Caldwell 1972). Three prominent shrub species of this region, Atriplex confertifolia (Torr. and Frem.) S. Wats., Ceratoides lanata Nevski, and Artemisia tridentata Nutt. subsp. wyomingensis Beetle were studied as examples of cool semi-desert perennial species which possess profuse root systems composed almost entirely of fine root elements distributed in the upper metre of soil. The extensive root systems maintained by these species may be necessary in order to extract efficiently the available moisture infused throughout the upper metre of soil by the annual spring recharge. This study was undertaken to assess the seasonal root system dynamics and root morphology of such plants in the field in order to investigate the hypothesis that, even though these plants maintain an extensive and profuse root system throughout the zone of primary soil moisture recharge, only a small fraction of the roots are actively growing at any one time. A phased activity of a large root system should allow a prolonged period of root system growth during much of the year facilitating an efficient utilization of the annual soil moisture allotment.

Allelopathy among some British grassland species.

Abstract: Allelopathy, the influence of one plant on another by means of chemical substances, has been extensively studied, and there is now strong evidence that this type of interaction does occur (Whittaker & Feeny 1971; U.S. National Committee for the International Biological Programme 1971; Grodzinsky 1973). Most often investigation of a particular species for possible allelopathic activity has started because of field observations; the species may tend to be surrounded by a bare zone, to have few other species growing beneath it, or in some other way shows an ability to suppress other species which is not expected from its size, density of foliage and other morphological characters. However, the possibility should also be considered that there are other allelopathic interactions where the effects are less extreme, and would not therefore be detected by preliminary field observations, but are nevertheless large enough to alter the balance between species. There are several published reports of species being found to contain toxic materials, when field observations on the species suggest no special toxicity (Welbank 1963; Grant & Sallans 1964; del Moral & Cates 1971). Del Moral & Cates (1971) tested forty species and found toxic materials in every one. All of these studies, however, concerned substances which were extracted from chopped-up plants or which were exuded by dying plant parts; none of them demonstrated the exudation of toxic substances from intact, healthy plants. In many studies of allelopathy the test plants to which the extracts or exudates are applied have been chosen for their convenience rather than their ecological relevance. For example, in the study by del Moral & Cates (1971), the plant volatiles or extracts were applied to seedlings of barley, Bromus tectorum L. and Pseudotsuga menziesii (Mirb.) Franco. Of these three only P. menziesii is native to Washington State, the area from which the donor species originated. The argument implicit in such a choice of convenient species is that all species will respond to the exudates in the same way. This is a very basic assumption and merits investigation. In the research described here, eight species were chosen which give no particular indication, from field observations, that they are involved in allelopathic interactions. The species, four grasses and four dicotyledons, were chosen because they are common in permanent neutral grassland in Britain, and all eight can be found growing together. In order to test whether the exudates have different effects on different species, exudates from each of the eight species were applied to each of the eight, in a factorially arranged experiment. The experiment aimed to study the effects of root exudates from plants young enough to have few dying roots. Leaching from above-ground parts was minimized. There has been little previous work on allelopathy among British grassland species.

The Pygmy Forest Region of Northern California: Studies on Biomass and Primary Productivity

Abstract: A most striking effect of soil on vegetation occurs on the coastal terraces of Mendocino County, northern California (longitude 123? 50' W, latitude 39? 20' N). Giant forests of the coastal redwood (Sequoia sempervirenst), 60-100 m tall, occur on slopes, in ravines and on alluvial bottoms, while 'pygmy forests' of two endemic conifers (Cupressus pygmaea and Pinus contorta ssp. bolanderi), 1-2 m tall, occur on level, intensely podzolized soils of terrace flats. These extreme communities are connected by forests of intermediate stature; in particular Bishop pines (P. muricata) in stands 15-30 m tall are dominant on terrace foredunes and on the richer soils of the terraces. The vegetation of this coastal strip has been characterized in relation to a gradient of soil nutrients, and other habitat features, in an earlier paper (Westman 1975). This article presents measurements of the biomass, productivity and surface areas in the pygmy conifer forests and compares these results with estimates for forests of Sequoia sempervirens, Pinus muricata, and other species.

Factors controlling germination of some dune annuals.

Abstract: Most dune annuals are characterized by autumn germination and spring flowering and consequently are regarded as 'winter annuals' (Salisbury 1952). In a previous paper (Pemadasa & Lovell 1974a) the factors controlling the timing of flowering of a group of annuals in the dune system at Aberffraw, Anglesey, were discussed. This paper is concerned with the factors determining the precise date of germination of these species. The literature suggests that both innate and enforced dormancy mechanisms (Harper 1957) are involved in controlling the timing of germination of winter annuals, and four plausible explanations can be suggested for their germination in autumn but not in spring or summer. (a) The germination capacity of some species is very, or relatively, low owing to innate dormancy. However, by autumn owing to after-ripening (Baskin & Baskin 1971a, b, 1972; Newman 1963; Ratcliffe 1961) and/or softening of the hard seed coat (Williams & Elliott 1960; Quinlivan 1961), which overcome the innate dormancy, rapid germination is possible. (b) Germination of fresh seeds of some species is favoured by temperatures below those prevailing in the field in spring and summer (Hulbert 1955; Newman 1963), but as the seed ages the temperature range for germination is widened thus enabling germination in autumn. (c) The temperatures in spring and summer are above the maximum or optimum for germination; thus seed is subjected to enforced dormancy. The lower temperatures in autumn allow germination (Juhren, Went & Phillips 1956; Went 1948, 1949; Went & Westagaard 1949), although there is no 'change in temperature response' (Newman 1963) with seed ageing. (d) The soil moisture in summer is below the minimum for germination; thus seeds are forced to remain dormant. The increased precipitation in autumn increases the soil moisture regime to a favourable level allowing germination. Although the generalizations made above may be applicable to a wide variety of winter annuals, it must be emphasized that the germination requirements differ widely between species. This paper concentrates mainly on dormancy, temperature regime and soit moisture regime. Other factors, including light and darkness, are given only brief mention. The species examined were Aira caryophylleat, A. praecox, Cerastium atrovirens, Erophila verna, Mibora minima, Saxifraga tridactylites and Vulpia membranacea. A detailed account of the distribution of these species in the dune system at Aberffraw has * Present address: Department of Botany, University of Sri Lanka, Peradeniya, Sri Lanka. t Nomenclature follows Clapham, Tutin & Warburg (1962).

Gas exchange of three cool semi-desert species in relation to temperature and water stress

Abstract: Arid and semi-arid environments present severe constraints to plant life. Species in arid habitats have evolved different adaptive strategies for avoiding or enduring stress conditions and some involve the processes of gas exchange. Some perennial species may exhibit high photosynthetic and growth rates only during seasons when precipitation and temperature conditions are favourable, becoming largely dormant during harsher periods. Other species appear to be capable of enduring stress conditions through physiological and morphological changes and pose some of the most intriguing and challenging questions from the standpoint of adaptations involving gas exchange. The two shrub species investigated in the present study, Artemisia tridentata Nutt. subsp. tridentata and Gutierrezia sarothrae (Pursh) Britt. & Rusby, are both species possessing the C3 pathway of photosynthesis (Welkie & Caldwell 1970; Williams & Markley 1973). They remain photosynthetically active during periods of water and heat stress, and are frequently found flourishing side by side. These two species none the less exhibit striking differences in phenology and morphology. Artemisia tridentata may attain heights of up to 1-5 m, is deep-rooted, possesses rather dense foliage all the year and has predominantly woody stems. Gutierrezia sarothrae is a much smaller suffrutescent shrub (often c. 0f2 m tall), dies back to a low, woody central stem every year and possesses a shallow, fibrous root system, sparse foliage, and a relatively high proportion of green stem material. In the northern part of its range, this species remains green and vigorous throughout the growing season, even during seasonal drought periods. The general goal of this study was to define differences in the adaptive strategies of gas exchange which allow these species to survive in sirnilar habitats in the same community. A third species present in this community, the perennial bunchgrass Agropyron spicatum (Pursh) Scribn. & Smith var. inerme Heller, which becomes photosynthetically dormant during stress periods, was also studied to present gas exchange characteristics of a species which avoids active gas exchange under extreme physical conditions.t Seasonal changes in the morphological characteristics of each species have been correlated with gas exchange responses. Effects of irradiation were noted, but primary emphasis was placed on gas exchange responses to water stress and temperature.