Plant community structure,connectance, niche limitation and species guilds within a dune slack grassland

Species phytomass in 40 mm×40 mm quadrats was used to examine community structure in four homogeneous areas of vegetation in sand dune slacks. There were from 9 to 16 common species in the samples, and most parameters of community structure were found to relate consistently to species number. We used covariance between all species at increasing dimensions of aggregations of quadrats to suggest that sociological interaction between species existed at scales of around 200 mm diameter. Generalised variance was of no help in this enquiry. We show that a connectance estimation arrived at from these parameters is, although a real feature of the analysis, apparently unrelated to connectance as understood in the food web literature. There is strong evidence that guild structure, sociological interaction and niche limitation increase with increasing number of species present, so that most diverse assemblage can be said to have a definite community structure.     

Towards a better understanding of agronomic and soil basis for possible charcoal root rot control and production improvement in bean

Abstract

Macrophomina phaseolina causes charcoal root rot (CRR) in numerous crops worldwide. However, structural progression of CRR epidemics in association with bean agro-ecosystems is little understood. Thus, progression of CRR epidemics in interaction with 13 agro-ecological variables was characterised in 48 commercial bean fields. Frequency of pathogen isolated from root was assessed at vegetative (V3), flowering-podding (R6-7) and pod maturity (R9) growth stages. Two principal factors accounting for 89% of total data variance characterised CRR epidemics. Rhizobial nodulation and cultivation–depth, clay–pH, date–preceding crop, urea–bean class and urea–herbicide interactions accounted for 50% CRR epidemic dynamics. Sixty-five percent of production variance was explained by soil clay, bean class, colonisation, planting date and depth, cultivation method, preceding crop, nodulation, herbicide and urea application. The present findings enabled us to explain a noticeable part of variations in productivity among the bean CRR pathosystems with the help of certain agricultural and environmental events for sustainable agriculture purposes.     

A note on the bias due to omitted confounders

In observational studies some confounders may be unknown andtherefore omitted from the analysis while others are adjustedfor. Approximations to the functions defining the relationshipbetween the parameters in the full and reduced models are proposedleading to asymptotic bias estimates. Numerical calculationsfor logistic and Poisson regression models show good agreementbetween asymptotic and simulation bias. A data set containingthe relationship between low birth weight and smoking (Hosmer& Lemeshow, 1989) is used as an illustration.     

A New Class of Matrices with Application in Experimental Designs

A very general class of balanced matrices is defined in this paper. These matrices are generalisations of the balanced orthogonal designs of Rao (1970) and generalised balanced matrices of Dey and Midha (1976). Some methods of construction of these balanced matrices are discussed. An application of these matrices in experimental designs is also given.     

On the exact null distribution of the generalised likelihood ratio test for analysing unreplicated factorial designs

Al-Shiha and Yang (1999) proposed a multistage procedure for analysing unreplicated factorial experiments, which is based on the statistic that is derived from the generalised likelihood ratio test statistic under the assumption of normality. It was shown by their simulation study that the method is quite competitive with Lenth's (1989) method. In their paper, because of the difficulty of determining the null distribution analytically, the quantiles of the null distribution were empirically simulated. In this paper, we give the exact null distribution of their test statistic, which makes it possible to calculate the critical values of the test.     

Variation in plant species richness in and adjacent to 64 ponds in SE Norwegian agricultural landscapes

All vascular plants, classified by life and growth form into six groups, four groups of hydrophytes (lemnids, nymphaeids, elodeids and isoetids), helophytes, and terrestrial species of pond margins, were inventoried in 64 SE Norwegian agricultural landscape ponds and their adjacent margins. The study sites varied considerably with respect to species richness; 0–4 for each hydrophyte group, 0–9 for helophytes, and 13–77 for terrestrial species. A total of 56 explanatory variables were recorded for each pond and adjacent margin to explain the observed richness variability.