J. B. Whittaker

Metabolic and faunal activity in litters of tree mixtures compared with pure stands

Abstract Foresters are showing increasing interest in the advantages to timber production of certain mixtures of plantings. We have studied decomposition and nutrient release in litters from tree mixtures compared with pure stands. Comparisons have been made of nutrient dynamics in pure and two-species mixtures of oak (Quercus petraea), alder (Alnus glutinosa) and Norway spruce (Picea abies), planted in a replicated design in 1955. Nutrient availability and tree growth were enhanced in the spruce/pine and depressed in the spruce/alder and spruce/oak mixtures compared with single-species stands. Increased mobilization rates in the spruce/pine and decreased rates in the spruce/alder and spruce/oak were correlated with increased and decreased metabolic activity, respectively, and changes in the decomposer community of the forest floor. It is suggested that a species of litter with a relatively high nutrient concentration, when added to a low-quality litter may enhance decomposition and nutrient release of the latter. It is concluded that part of the explanation for modified growth of trees in mixtures compared with pure stands is to be found in the interactions between invertebrates and microorganisms in the litter, which result in changes in nutrient dynamics.

THE EFFECTS OF AGRICULTURAL MANAGEMENT ON THE SOIL BIOTA OF SOME UPLAND GRASSLANDS

Intensification of sheep farming in the uplands has caused concern in terms of over-production and conservation of wildlife and landscape. Changes in UK agricultural policy to combat these problems are likely to result in reduced pasture improvement and sheep grazing in the uplands. This paper examines the effects, on three adjacent uplands grasslands, of different intensities of sheep management on the soil biota. The effects of short-term removal of sheep grazing by fenced exclosure were also studied. After preliminary investigations the soil Collembola, in particular the fungal-feeding species Onychiurus procampatus, were selected for further study. Field sampling revealed simultaneous trends of decreasing Collembola numbers and increasing total hyphal length and biomass of fungi in the surface soil, together with differences in chemical and physical soil properties, along a gradient of reduced sheep management intensity. Similar changes occurred when sheep grazing was removed by fenced exclosures. The abundance of fluorescein diacetate-active hyphae within the surface soil was shown to vary little along the same gradient of reduced sheep management intensity. The relative abundance of the seven most commonly isolated species of litter fungi varied along the same gradient of sheep management. The findings are discussed in relation to other grassland studies.

Direct and indirect effects of climate change on insect herbivores: Auchenorrhyncha (Homoptera)

1. Novel manipulations of local climate were employed to investigate how warmer winters with either wetter or drier summers would affect the Auchenorrhyncha, a major component of the insect fauna of grasslands. Direct and indirect effects of climate manipulation were found.

Interspecific herbivore interactions in a high CO2 environment - root and shoot aphids feeding on Cardamine.

This study investigated the effects of elevated CO2 on populations of root and/or shoot aphids and their effects on partitioning in Cardamine pratensis. Total plant biomass in elevated ( 600 ppm) CO2 of uninfested Cardamine plants was 52% higher than in ambient ( 350 ppm) concentrations but CO2 effects were not statistically significant. In elevated CO2, feeding by shoot aphids (Aphis fabae fabae) alone and in combination with root aphids (Pemphigus populitransversus), and root aphids alone had no significant effect on plant biomass. No significant effects of elevated CO2 were detected on population size of the shoot or root-feeding species. Interspecific effects were detected between the root and shoot species. Root aphid populations were significantly smaller in the presence of shoot aphids on the same plants. In this system plant growth was unaffected by an elevated CO2 environment. Plant species which are more sensitive to elevated CO2 may show a modified response to herbivore pressure in a future atmospheric environment.

The effect of a foliar disease (rust) on the development of Gastrophysa viridula (Coleoptera: Chrysomelidae)

Abstract. 1 Gastrophysa viridula Degeer (Coleoptera: Chrysomelidae) and the pathogenic rust fungus Uromyces rumicis (Schum.) Wint. both occur on leaves of Rumex crispus L. and R.obtusifolius L. Individual stages of beetle development, and egg laying, were compared on healthy and infected leaves of each plant species in the laboratory. Oviposition choice was investigated in the field and laboratory. 2 Beetles reared on infected leaves of each species had greater larval mortality and slower development than those reared on healthy leaves. Although larvae feeding on infected leaves consumed up to 2.5 times more dry weight than those reared on healthy leaves, they had a lower relative growth rate and pupated at a lower weight. These changes were consistent with the reduced nutritive quality of rust‐infected Rumex leaves. 3 Fecundity of beetles reared on infected leaves of both species was considerably reduced. Eggs laid by beetles feeding on infected R.crispus leaves also had a reduced viability. 4 The beetle developed consistently poorer on healthy R.crispus than on healthy R.obtusifolius throughout its life‐cycle. Differences in larval performance were greater between host species than between infected and healthy leaves. 5 Oviposition was similar on infected and healthy R.crispus in both the laboratory and field. However, adults consumed less, and laid fewer eggs on infected than on healthy R.obtusifolius. The pattern of egg laying on different aged leaves was affected by rust infection: a greater proportion of eggs was laid on the older, infected leaves, than on the equivalent aged leaves on the healthy plants. Few larvae survived from eggs laid on rusted leaves in the field.

The effect of collembolan grazing on fungal activity in differently managed upland pastures: A microcosm study

Laboratory microcosms containing litter from three tussock grasslands were used to assess the impact of grazing by a collembolan, Onychiurus procampatus, on the abundance, nutrient release, and respiration of the saprotrophic fungus, Phoma exigua. The fungal biomass and respiration rate were significantly reduced only when Collembola were present in excess of mean field densities but perhaps more typical of spatial aggregations in the soil. A high efficiency of nutrient immobilization by P. exigua was demonstrated but nutrient release was not significantly affected by the fauna. Problems associated with the use of microcosms in the simulation of field conditions are discussed.

Predicting numbers of an insect (Neophilaenus lineatus: Homoptera) in a changing climate

Measurements of the density of an upland population of the spittlebug Neophilaenus lineatus (L.) (Homoptera, Auchenorrhyncha) were made from 1961 to 1997. Population change from year to year is shown to be largely density-independent. Analyses of meteorological records over the 37-year period showed that the weather variable with the highest correlation with population change was mean minimum temperature in September. A simple model was constructed relating numerical changes from 1961 to 1992 to temperature, and this was found to explain 75% of the change in this population. The model was used to predict numerical change from 1993 to 1997 over which period it explained 70% of the change. The model predicts that a projected rise in mean temperature of 1°C would increase the mean population density of N. lineatus by 50%. Experimental cloches at the upland site which raised mean temperature by 1°C, had N. lineatus densities which were 157% higher than in adjacent control quadrats. An experimental raising of field temperature at a lowland site where population change had been shown to be density dependent did not result in density changes, although dates of hatching were earlier.

The diet and food preferences of Onychiurus procampatus (Collembola) from upland grassland soils.

Specimens of Onychiurus procampatus (Collembola) observed grazing on fungal mycelia were collected from the surface soil of three differently manged upland grasslands. A general trend of an increasing proportion of Collembola with a full gut was found along a gradient of reduced sheep management intensity, which was correlated with increased fungal biomass. In the laboratory, this collembolan showed a consistent order of preference for the mycelium of seven common fungal species isolated from the field sites.

Do birch trees (Betula pendula) grow better if foraged by wood ants

1. Populations of insect herbivores in three main guilds were studied on experimental saplings and natural birch (Betula pendula) trees which were either foraged or not foraged by wood ants (Formica rufa) on the Gait Barrows NNR. 2. The effects of the ants on the insects, and hence on the damage by the herbivores to the trees were measured. 3. Growth parameters of foraged and unforaged trees were measured. 4. Of the seven aphid species feeding on Betula pendula, two had mutualistic relationships with the ants and were increased in numbers by them. The remaining five species were decreased in numbers

Interactions between Rumex spp., herbivores and a rust fungus: the effect of Uromyces rumicis infection on leaf nutritional quality

1. The rust fungus Uromyces rumicis infects leaves of Rumex crispus and R. obtusifolius causing premature senescence and leaf death. We investigated the effect of infection on nutritional quality of leaves with respect to the chrysomelid beetle Gastrophysa viridula and measured larval survival and growth on rusted and unrusted plants. 2. A smaller biomass of G. viridula was produced and a greater area of leaf consumed per unit biomass gained, by egg batches reared on rusted compared to unrusted R. obtusifolius, although no difference was observed between larvae reared on rusted or unrusted R. crispus. 3. Concentrations of total non-structural carbohydrates were not changed in the pustule and non-infected leaves but were increased in the inter-pustule area. Infection changed the partitioning to different carbohydrates, increasing the proportion of starch in R. crispus and decreasing it in R. obtusifolius. 4. The concentration of total nitrogen, both in the pustule and the inter-pustule area, decreased in both species with time after rust infection. 5. In both species, considerably higher oxalate concentrations occurred in the interpustule area and significantly lower concentrations in the uninfected leaves of these plants. Calcium concentrations altered similarly and were correlated with oxalate concentrations. 6. We conclude that rust infection does not improve the nutritional quality of the leaves for G. viridula and is unlikely to do so for other invertebrate herbivores