The influence of the spatial pattern of foliage removal on regrowth was investigated in the field with a tussock grass, Agropyron desertorum. Tussocks were hand clipped in different spatial patterns that represented extremes of defoliation patterns which could be inflicted by natural herbivores. All defoliated tussocks exhibited increases in specific growth rates following clipping in mid-May. When foliage was removed from the upper portion of the canopy (younger foliage), regrowth rates and season-long aboveground biomass production were less than if the same amount of foliage was removed from low in the canopy (older foliage). The spatial pattern of defoliation also influenced tussock regrowth in a late-May clipping experiment, but differences in the effects of the clipping patterns were associated with the removal of apical meristems rather than with the age or location of foliage removed. Changes in tussock carbon dioxide and water vapor exchange that were associated with changes in growth following mid-May clipping were explored. All clipped tussocks showed increases in integrated daytime carbon dioxide uptake per unit foliage area after defoliation. Differences among treatments in the response of net daytime carbon gain during the first 24 hours after clipping corresponded well with differences in tussock regrowth over a 14-day period following clipping. Increased carbon gain of clipped tussocks was associated with increases in tussock water vapor conductance and intercellular carbon dioxide concentration, and decreases in the ratio of carbon dioxide uptake to water vapor loss. Differences among treatments in daytime carbon gain and regrowth were paralleled by the response of instantaneous rates of light-saturated net photosynthesis for entire tussocks. Defoliation increased the proportion of foliage directly illuminated within the tussock at solar noon. Changes in the fraction of sunlit foliage and the relative amounts of different-aged foliage in tussock canopies were correlated with the responses of light-saturated photosynthesis. Thus, the effects of the spatial pattern of foliage removal on canopy light microclimate and the age of remaining foliage had important implications for carbon gain and regrowth of tussocks following mid-May defoliation.

The incidence of baculovirus disease in susceptible insect hosts may be used to estimate future host population sizes and estimate the relative defoliation of these populations for economically important insects. We developed an Enzyme-linked immunosorbent assay (ELISA) that was specific to two native baculovirus pathogens of the Douglas-fir tussock moth (DFTM), that was more sensitive and accurate (R2=0.99) than microscopic counts of virus polyhedron inclusion bodies, and more accurate than PCR or Southern hybridization for the direct detection of baculovirus disease. We also developed a field-based "dipstick" assay that was sensitive enough to detect virus disease prior to host mortality and did not require virus purification from macerated insect tissues, supporting its use for on-site field surveys. Finally, we used ELISA to establish, for the first time, a method to detect persistent sublethal baculovirus infections. Our work may contribute to more accurate understanding of the incidence and influence of persistent sublethal infections on insect hosts.

The relative competitive abilities of Agropyron desertorum and Agropyron spicatum were compared using Artemisia tridentata transplants as indicator plants. Although these two tussock grasses have similar shoot growth forms and shoot physiological characteristics, they have substantial differences in their competitive abilities. Artemisia had lower survival, growth, reproduction, and water potential when transplanted into neighborhoods of A. desertorum than in neighborhoods of A. spicatum. Plant attributes associated with the differences in competitive ability were explored. Agropyron desertorum and~ spicatum have remarkably similar potential growth rates at warm soil temperatures. In a prolonged cold soil temperature treatment in the greenhouse, A. desertorum had a 66% greater aboveground relative growth rate than A. spicatum. These differences, however, were not apparent for early spring tiller growth rates in the field. Distinct differences in timing of root growth were found between the two tussock grasses. Aqropyron desertorum exhibited greater root growth during winter and early spring and invaded disturbed soil space more rapidly than A. spicatum, especially if the disturbance occurred soon after the snow had melted. Similarly, A. desertorum proliferated its roots in zones of nutrient enrichment created early in the spring sooner than A. spicatum. No differences in root growth were found between species in zones of nutrient enrichment that were created later in the growing season. Despite differences in early spring root growth, water extraction and radiophosphorus acquisition early in the spring were similar for the two grass species. Later in the spring, A. desertorum extracted more water and radiophosphorus than A. spicatum. Differences in resource extraction between the two species in a specific soil layer occurred weeks before A. spicatum, but not A. desertorurn, had obtained maximum root length. Early root growth probably provides A. desertorum an important head start over A. spicatum in soil exploration each growing season. Differences in resource extraction, however, do not become apparent between the two species of Agropyron until plant demand exceeds soil supply rate to the roots.

New Zealand's South Island tussock grasslands have been highly modified by human activities, including burning, grazing and introductions of exotic plants for pastoralism. Studies suggest that tussock grasslands are degraded, in that native species have declined, and exotic species have increased in both diversity and abundance. These trends are primarily thought to be related to the impacts of grazing and subsequent grazing removal. Few studies have assessed long-term changes that have occurred in tussock grasslands, and those that have are generally limited to one particular location. This thesis aimed to investigate temporal changes in community structure in tussock grasslands, and relate these changes to environmental variables and land tenure. Data were used from 90 permanently-marked vegetation transects, which were set up on 19 geographically widespread properties in areas of tussock grassland across Canterbury and Otago in the South Island of New Zealand. The transects were on land in both conservation and pastoral tenure. Each transect was 100 m, and consisted of 50 0.25 m² quadrats. The transects were measured between 1982 and 1986 (first measurement), were re-measured between 1993 and 1999 (second measurement) and again between 2005 and 2006 (third measurement). A total of 347 vascular species were observed over the 90 transects and three measurement times. Species richness declined between the first and second measurements (first time interval), and increased between the second and third measurements (second time interval), at both the small (quadrat) and large (transect) scales. Both native and exotic species declined in mean quadrat species richness during the first time interval, and then increased during the second time interval. Changes in mean quadrat species richness were similar on transects in both conservation and pastoral tenure. Multivariate analysis of species' occurrences in quadrats identified a long gradient in species composition for these 90 transects. Four key plant communities were identifed along this gradient and differed in their mean elevation: (1) Highly-modified pastoral community, (2) Short-tussock grassland community, (3) Tall-tussock grassland community, (4) Alpine mat-forming species community. A detailed investigation into temporal changes that occurred on 53 transects that occurred in short- and tall-tussock grassland communities showed that changes in species composition were not consistent over time. Transects on different properties changed in species composition by different amounts. Specifically, in ordination space, transects on two properties changed in composition significantly more than transects on one other property. The property that a transect was on also affected the way that it changed in composition, i.e. native species were more likely to have increased on transects on some properties. Transects in conservation tenure did not change in species richness or composition differently from those in pastoral tenure. Considering that many native plants in tussock grasslands are relatively slow-growing, and that these areas have been grazed and burned for more than a century, we may expect it to be some time before we can detect differences in vegetation dynamics on conservation land from that on pastoral land. The changes in the community structure of these tussock grasslands were related to a combination of environmental factors, such as soil chemistry, climate, and management factors. This study has allowed greater understanding of vegetation change in tussock grasslands, and demonstrates the importance of long-term ecological monitoring in making reliable and accurate predictions about landscape-scale changes in tussock grassland community structure.

The study undertaken was carried out in the montane tussock grassland region which in the South Island comprises some six million acres. Field trials were established at Mt Somers, Mid Canterbury. This study attempts to elucidate some of the factors connected with the sulphur nutrition of grasses. For the particular trial area it was hoped to determine four key aspects ; 1. Whether the response of the grass to nitrogen is governed by the sulphur level available. 2. A determination of what level of nitrogen was likely to be provided by clover fixation and transferred to the grass component under improved grassland conditions in the area. 3. What the optimum level of sulphur for the association is, allowing adequate sulphur for maximum possible nitrogen fixation as well as for utilization by the grass of the nitrogen transferred. 4. The effects of nitrogen and sulphur on plant chemical composition and any possible relationships between plant and soil as infuenced by these factors. Following a review of literature, a summary of experimental work is presented, with results, discussion and conclusions.