The effects of increasing carbon dioxide (CO2) and climate on net carbon storage in terrestrial ecosystems of the conterminous United States for the period 1895-1993 were modeled with new, detailed historical climate information. For the period 1980-1993, results from an ensemble of three models agree within 25%, simulating a land carbon sink from CO2 and climate effects of 0.08 gigaton of carbon per year. The best estimates of the total sink from inventory data are about three times larger, suggesting that processes such as regrowth on abandoned agricultural land or in forests harvested before 1980 have effects as large as or larger than the direct effects of CO2 and climate. The modeled sink varies by about 100% from year to year as a result of climate variability.

Although there is a great deal of information concerning responses to increases in atmospheric CO at the tissue and plant levels, there are substantially fewer studies that have investigated ecosystem-level responses in the context of integrated carbon, water, and nutrient cycles. Because our understanding of ecosystem responses to elevated CO is incomplete, modeling is a tool that can be used to investigate the role of plant and soil interactions in the response of terrestrial ecosystems to elevated CO. In this study, we analyze the responses of net primary production (NPP) to doubled CO from 355 to 710 ppmv among three biogeochemistry models in the Vegetation/Ecosystem Modeling and Analysis Project (VEMAP): BIOME-BGC (BioGeochemical Cycles), Century, and the Terrestrial Ecosystem Model (TEM). For the conterminous United States, doubled atmospheric CO causes NPP to increase by 5% in Century, 8% in TEM, and 11% in BIOME-BGC. Multiple regression analyses between the NPP response to doubled CO and the mean annual temperature and annual precipitation of biomes or grid cells indicate that there are negative relationships between precipitation and the response of NPP to doubled CO for all three models. In contrast, there are different relationships between temperature and the response of NPP to doubled CO for the three models: there is a negative relationship in the responses of BIOME-BGC, no relationship in the responses of Century, and a positive relationship in the responses of TEM. In BIOME-BGC, the NPP response to doubled CO is controlled by the change in transpiration associated with reduced leaf conductance to water vapor. This change affects soil water, then leaf area development and, finally, NPP. In Century, the response of NPP to doubled CO is controlled by changes in decomposition rates associated with increased soil moisture that results from reduced evapotranspiration. This change affects nitrogen availability for plants, which influences NPP. In TEM, the NPP response to doubled CO is controlled by increased carboxylation which is modified by canopy conductance and the degree to which nitrogen constraints cause down-regulation of photosynthesis. The implementation of these different mechanisms has consequences for the spatial pattern of NPP responses, and represents, in part, conceptual uncertainty about controls over NPP responses. Progress in reducing these uncertainties requires research focused at the ecosystem level to understand how interactions between the carbon, nitrogen, and water cycles influence the response of NPP to elevated atmospheric CO.

Global warming due to increased concentrations of atmospheric CO 2 is expected to be amplified in middle and higher latitudes. Consequently, ecosystems in these latitudes will experience more pronounced climatic variations. This investigation attempts to assess the sensitivity of Canada's ecosystems to climatic change. Potential ecological impacts resulting from global warming are outlined. With this background, the effects of a doubled CO 2 climate are applied to a classification model derived from the current ecological setting. Results reveal not only major shifts in ecological boundaries but also changes in the character of these broadly distributed ecosystems.

Human monocytic ehrlichiosis (HME) is a zoonotic emerging tick-borne disease with clinical signs that range from mild symptoms to multiple organ failure and death. Ehrlichia chaffeensis, the aetiologic agent of HME, is reported to infect a divergent range of mammals. Although cattle are common hosts of the primary vector of this pathogen, the susceptibility of this host to E. chaffeensis has not been reported to date. This study was undertaken to determine if cattle could provide a useful infection model of E. chaffeensis. Dairy calves were injected with DH82 cells infected with the Arkansas, St Vincent or 91HE17 strain of E. chaffeensis, and monitored for signs of clinical ehrlichiosis and for infection of peripheral blood and ticks by PCR assay. Splenectomized and spleen-intact calves were injected with cryopreserved stabilates of E. chaffeensis-infected DH82 cells for the first experiment. Mild clinical signs were occasionally observed among these calves, and only two blood samples were PCR-positive, while several ticks fed on each calf tested PCR-positive. The second experiment involved injection of normal calves with active cultures of the same E. chaffeensis strains. Interestingly, three of six calves inoculated with active cultures became recumbent and died or had to be euthanized. All of the surviving calves in this experiment tested PCR-positive on multiple dates, but fewer ticks fed on these calves were PCR-positive. These results suggest that a bovine disease model could facilitate the understanding of factors that affect the severity of HME. INTRODUCTIONEhrlichia chaffeensis is an obligate intracellular bacterial parasite and is the aetiologic agent of human monocytic ehrlichiosis (HME), an important emerging tick-borne infectious disease. Clinical outcomes associated with this disease range from asymptomatic to severe and even fatal (Eng et al., 1990b;Everett et al., 1994;Fishbein et al., 1994). E. chaffeensis appears to cause unapparent or mild clinical manifestations in non-primate hosts and this, together with challenges in detecting the pathogen, makes the experimental characterization of E. chaffeensis transmission and pathogenesis difficult. A reliable E. chaffeensis-based animal model of HME would facilitate the understanding of the transmission, pathology and immunology of this malady (Dawson & Ewing, 1992;Dawson et al., 1994;Ewing et al., 1995;Unver et al., 2002;Zhang et al., 2003;Dugan et al., 2004;Loftis et al., 2004).A wide range of mammals are reported to be naturally infected with E. chaffeensis, including white-tailed deer Ewing et al., 1995; Davidson et al., 2001; Varela et al., 2003), domestic goats (Dugan et al., 2000), domestic and wild canids (Dawson et al., 1996;Davidson et al., 1999), and lemurs (Williams et al., 2002 Dermacentor variabilis (Bishopp & Tremblet, 1945;Smith et al., 1946;Lancaster, 1973); both A. americanum and D. variabilis are reportedly infected with E. chaffeensis in nature (Eng et al., 1990a;Anderson et al., 1993;Ewing et al., 1995; Roland et al., 1998; Kram...

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