摘要:

Production of tallgrass prairie vegetation was measured on experimental plots in which defoliation intensity and frequency were manipulated by mowing and using movable exclosures on areas chronically grazed by cattle. Defoliation history largely controlled whether or not defoliated plants overcompensated (exhibited enhanced production compared to undefoliated controls) for tissue removal. Plants on chronically grazed sites only compensated for foliage removed by grazers. Production on plots mowed prior to the year of measurement was similar to that on chronically grazed sites, while previously unmowed plots exhibited substantial aboveground overcompensation. Aboveground production was maximized by the most frequent mowing treatment and by intermediate mowing heights. Nitrogen and phosphorus concentrations and amounts in aboveground tissues were increased by mowing and grazing. Current mowing regime was more important than mowing history in determining nitrogen concentrations except very early in the growing season. Effects of grazing and mowing on belowground biomass were inconsistent, but frequent mowing appeared to limit accumulation of belowground N reserves and biomass. In North American grasslands, overcompensation is a nonequilibrium plant response to grazing. Photosynthate that would be stored as reserves and used for root growth and flower and seed production instead is used to replace lost leaf area, thereby resulting in higher foliage productivity. However, under chronic grazing or mowing, vegetation is prevented from maintaining high nutrient and water uptake capacity (large root biomass) and accumulating reserves that allow overcompensation responses.

ISSN:1051-0761    

DOI:10.2307/1941800

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

Pest management decisions should take into consideration quantitative information on dispersal of insect pests, but such information is often lacking. The goal of this study was to measure intraforest dispersal in the southern pine beetle (SPB). We developed an analytical formula for interpreting data from mark—recapture studies of insect dispersal. The formula is obtained by postulating a simple model of diffusion with disappearance (e.g., as a result of death) for the spread of marked insects from the release point. Parameters of the model (assumed to be invariant in space) are estimated by fitting the curve to the cumulative number of recaptured insects as a function of the distance from release. We also derived an expression for the distribution of dispersal distances in terms of the fitted parameters. The proposed methodology was applied to a mark—recapture study of SPB dispersal Statistical analysis of recaptures—with—distance curves obtained in 11 replicate releases indicated that the proposed formula provided an accurate description of the data. There were no systematic departures from the functional relationship prescribed by the formula, and the model consistently outperformed another commonly used formula for fitting data on dispersal distances, the exponential curve. We explored the effect of spatial heterogeneity in the host distribution on SPB movement by regressing the deviation from the recapture rate predicted by the model in each trap on the pine basal area around the trap. This correlation was significantly greater than zero, indicating that beetles tended to aggregate in localities where pines were dense. This result suggests that a diffusion model with spatially varying parameters may provide a more accurate description of the redistribution process in the SPB. Quantitative results on SPB intraforest dispersal were summarized by calculatingradii of circles enclosing a given proportion of SPB dispersal distances. For example, we estimated that one half of released beetles dispersed>0.69 km. This result has important implications for evaluating the area—wide consequences of current or novel control tactics that rely on disrupting SPB movements, e.g., cut—and—leave and treatments with anti—congregation pheromone.

ISSN:1051-0761    

DOI:10.2307/1941801

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY