Masae Shiyomi

Spatial pattern changes in aboveground plant biomass in a grazing pasture

Using gamma distribution and spatial autocorrelation, it was demonstrated that plant biomass per unit area of a pasture grazed by cattle exhibited two kinds of spatial heterogeneity: small-scale heterogeneity caused by grazing and large-scale heterogeneity caused by topography, land aspect, etc. For each of the 10 measurement times from May to August, 100 quadrats 50 cm × 50 cm were arranged along a straight line 50 m long in a pasture, and the plants within the quadrats were harvested at the height of 3 cm above the ground surface to measure the dry weight. The data were aggregated into frequency distributions, and gamma distribution and the parameter values were estimated. This analysis showed that with the progression of grazing the amount of biomass decreased and the degree of spatial heterogeneity in biomass, measured per 0.25 m2, increased, and due to plant regrowth the trends were reversed. By rearranging the 100 biomass data in order of weight, it was suggested that plots with an extremely large biomass were not grazed by cattle and remained in the pasture. For the same data, variations of biomass along the straight line were divided into two parts based on the moving average: the spatial trend and the residuals which cannot be explained by the trend. In this analysis, 48–75% of the total spatial variation was explained by the trend along the straight line. Analysis using spatial autocorrelation for the actual biomass changes showed that the biomass changes within a range of about 10 m on the straight line gave a positive correlation, which indicates a topographical trend in biomass. Spatial autocorrelation for residuals suggested that the spatial changes in biomass along the straight line followed a wave-like or checker-board pattern. Small-scale spatial heterogeneity in plant biomass may be caused by the uneven deposition of excreta by grazing animals, uneven use of the grassland by grazing animals, and uneven dispersal of plant seeds through faeces over the grassland. The possibility that such unevenness might accelerate energy flow in the grassland ecosystem and contribute to grassland sustainability is discussed.

Model for the spatial pattern formed by a small herd in grazing cattle

Abstract The spatial pattern of a small cattle herd within a fenced grazing pasture was investigated using a mathematical model. Only changes in the distance between the two most distant individuals in the pasture, i.e. troop length, were observed. Troop length is a measure of closeness of individuals within a herd. In the model, we assumed that the troop length was determined by: (1) attractive activities (desire to be in a group) and repulsive activities (maintenance of individual space) operating among individuals in the herd; and (2) random activities unrelated to the attractive and repulsive activities. A relationship between these two elements was incorporated into a diffusion equation, and the theoretical frequency distribution of the troop length was derived. The model was tested using experimental data collected in a strip-wise pasture. Activities of a cattle herd were visually divided into three categories such as resting, feeding and moving states. Parameters representing the attractive and repulsive activities, and the equilibrium of troop length were estimated using the model for the three states. The frequency distribution of troop length was compared between the three states and with the frequency distribution derived under the assumption of a random spatial pattern. It was concluded that: (1) the changes in troop length could be described by the newly derived model; (2) the spatial pattern of a cattle herd in an actual pasture was not random but aggregated; and (3) the closeness of individuals within a herd was loosened in the following order: resting, feeding and moving states.

A measure for spatial heterogeneity of a grassland vegetation based on the beta-binomial distribution

. A method is proposed to estimate the frequency and the spatial heterogeneity of occurrence of individual plant species composing the community of a grassland or a plant community with a short height. The measure is based on the beta-binomial distribution. The weighted average heterogeneity of all the species composing a community provides a measure of community-level heterogeneity determining the spatial intricateness of community composition of existing species. As an example to illustrate the method, a sown grassland with grazing cows was analysed, on 102 quadrats of 50 cm × 50 cm, each of which divided into four small quadrats of 25 cm × 25 cm. The frequency of occurrence for all the species was recorded in each small quadrat. Good fits to the beta-binomial series for most species of the community were obtained. These results indicate that (1) each species is distributed heterogeneously with respective spatial patterns, (2) the degree of heterogeneity is different from species to species, and (3) the beta-binomial distribution can be applied for grassland communities. In most of the observed species spatial heterogeneity is often characterized by species-specific propagating traits: seed-propagating plant species exhibited a low heterogeneity/random pattern while clonal species exhibited a high heterogeneity/aggregated pattern. This measure can be applied to field surveys and to the estimation of community parameters for grassland diagnosis.

Roles of plant biomass and vegetational heterogeneity, and energy-matter cycling in grassland sustainability

A long-term survey of energy, carbon and nitrogen budgets in a grassland has been carried out since 1974 at the National Grassland Research Institute, Japan. Based on the survey, a system model describing energy and matter flows was constructed. Sustainability of grassland ecosystems in relation to productivity was evaluated by simulations. The results indicated that at low and high levels of animal productivity the grassland ecosystems were able to accumulate carbon in soil year by year, and there was no appreciable indication of decrease in the nitrogen content in soil provided that a given amount of fertilizer was applied to the grassland. The heterogeneity in plant biomass can be described by a gamma distribution. Grasslands grazed by domestic animals such as cattle and sheep display a strong spatial heterogeneity. This characteristic is caused by the uneven deposition of excreta by grazing animals, uneven use of the grassland by grazing animals, and uneven dispersal of plant seeds by grazing animals over the grassland. It was suggested that such unevenness may accelerate energy and matter cycling, and then contribute to grassland sustainability.

Measures of spatial heterogeneity for species occurrence or disease incidence with finite-counts

We propose two types of indices with finite-count correction to measure the spatial heterogeneity of binary characteristics of organisms, such as occurrence or non-occurrence of organisms and infected or non-infected plants. We consider the following two examples: plant occurrence in a grassland community, and yellow dwarf disease infection in a rice field. For the grassland community, N quadrats comprising n cells of equal area, were set at random sites in a grassland, and the occurrence of a given species A in each of n cells was recorded. For disease infection, N quadrats, each consisting of n rice plants, were set at random sites in a paddy field, and the number of plants infected with yellow dwarf virus in each quadrat were counted. In these examples, since the number of cells in a quadrat is finite, neither occurrence nor incidence increase infinitely, unlike the number of aphids on a maize leaf. The first category of index belongs to the mean : variance ratio type. The estimated index value for occurrence (or incidence) is the same as that for non-occurrence (or non-incidence). The second category belongs to the k-type of a negative binomial distribution. If some random plants die or recover from the disease then the expected value of the second type of index does not change. For n→∞, the current indices approach the mean : variance ratio and the inverse of k in a negative binomial distribution, respectively. This indicates that these indices are suitable for binary data sets.

Spatial heterogeneity in a grassland community: Use of power law

A new regression analysis was proposed to evaluate the degree of spatial heterogeneity for individual species comprising a plant grassland community. The weighted average of the heterogeneity value of all the species comprising the community provides a measure of community-level heterogeneity. A field survey was carried out, as an example, in order to analyze the spatial heterogeneity of a pasture with grazing cows, using 100 quadrats 50 cm × 50 cm, each of which was divided into four smaller quadrats 25 cm × 25 cm, on a 50 m long line-transect. The frequency of occurrence for all the species in each small quadrat was recorded. The regression associated with the ratio of the theoretical and observed variances of occurence counts was used to analyse the frequency distribution of species in a pasture community. A good fit to the regression for the whole community was obtained. These results indicate that (i) each species in the example was distributed more heterogeneously than a random pattern; and (ii) the regression could well describe the spatial heterogeneity of the grassland plant community. In most of the observed species, spatial heterogeneity is often characterized by species-specific propagation traits and the architecture of plant bodies. Thus, the spatial patterns of a grassland community can be evaluated in detail by this power-law approach. This measure is suitable for field surveys and comparative studies of grassland communities, and for other plant communities that are generally short in height.

Number of Samples Required for Estimating Herbaceous Biomass

Abstract Although the precision of herbaceous biomass estimation depends on the sample number, the spatial heterogeneity of the biomass, and sampling procedures, the magnitudes of the influences on the precision have not been clarified. We simulated virtual plant communities based on the gamma distribution to clarify the relationships between the precision of estimating herbaceous biomass and the number of samples, sampling density, spatial heterogeneity of the biomass, and sampling procedures. Using only two parameters, the gamma distribution can approximate the frequency distribution of herbage mass with varying heterogeneity. Our simulations demonstrated that the number of samples is a more influential factor than sampling density on the precision of the herbaceous biomass estimation. Moreover, our simulations confirmed that biomass heterogeneity strongly affected the precision and quantified the magnitude of the influence. When we estimated biomass with random sampling and a 50 × 50 cm quadrat and accepted estimation error of ± 10% of the mean for a confidence interval of 95%, the numbers of samples needed were 200, 77, and 9 for very, intermediate, and less heterogeneous grasslands, respectively. Similarly, when we estimated biomass with a ranked set sampling (RSS), then 24, 15, and 4 samples were needed in very, intermediate and less heterogeneous grasslands, respectively. We came to two conclusions: 1) In less heterogeneous grasslands, good precision of estimation can be obtained with a small number of samples, and it is useful to employ RSS. The cutting method, as well as nondestructive methods, will be practical; and 2) estimation for heterogeneous grassland requires a large number of samples, and it is not so useful to employ RSS. For that reason, more research is needed on nondestructive methods.

Spatial heterogeneity and variability of a large-scale vegetation community using a power-law model

Spatial heterogeneity and stability are fundamental indices for describing vegetation communities. The spatial distribution characteristics of the vegetation in Nenjiang region of northeastern China were evaluated using a variance power-law model. The data fits the model well with estimates given for the levels of heterogeneity for not only single species but also the community as a whole. The linear regression indicates that the species in the community exhibit a consistently organized spatial pattern, as is often discovered in field surveys but rarely seen in artificial systems. The species deviations from the regression line, which exhibit a leptokurtic distribution, may reflect the variability of the community. Thus, the model provides a general tool for management and regulation of ecosystems, especially where there is human disturbances.

Modelling of energy flows and conversion efficiencies in a grassland ecosystem

Abstract A grassland ecosystem is usually made up of many environmental and biological factors which consitute the production factors in the ecosystem, and the interactions between them are intricately entangled with each other. In this study, a system model of energy flows in arable lowland grassland in central Japan is considered. The model is composed of the following seven compartments: above-ground plant biomass, below-ground live biomass, standing dead material, surface litter, herbage ingested by grazing cattle, cattle biomass and feces; and the following two energy flows are dealt with: (1) sun → plants → standing dead material → litter → soil organic matter; and (2) plants → cattle. Energy flows between the seven compartments are described by eight simultaneous differential equations, and parameters in the model were determined from field experiments. The results are as follows: an optimal stocking rate with increases in total cattle body weight obtained during the year can attain a maximum when cattle with a total body weight of 1625 kg/ha, at the beginning of the grazing season, were grazed. Energy flows under such conditions are as follows, in kJ m −2 year −1 : global solar radiation (ca. 473 × 10 4 ) → photosynthetically active radiation (ca. 191 × 10 4 ) → energy fixed in above-ground plant portion (17 460) → energy grazed by cattle (17 468) → net seconndary production (761).

Anthropogenic effects on a tropical forest according to the distance from human settlements.

The protection of tropical forests is one of the most urgent issues in conservation biology because of the rapid deforestation that has occurred over the last 50 years. Even in protected forests, the anthropogenic effects from newly expanding villages such as harvesting of medicinal plants, pasturing cattle and forest fires can induce environmental modifications, especially on the forest floor. We evaluated the anthropogenic effects of the daily activities of neighboring residents on natural forests in 12 plots extending from the village boundary into a natural forest in Thailand. The basal area per unit land area did not present a significant trend; however, the species diversity of woody plants decreased linearly towards the village boundary, which caused a loss of individual density because of severe declines in small saplings compared with adult trees and large saplings in proximity to the village. An analysis of tree-size categories indicates a lack of small samplings near the village boundary. The current forest appears to be well protected based on the adult tree canopy, but regeneration of the present-day forests is unlikely because of the loss of seedlings.