Leaf gas exchange and nitrogen dynamics of N2-fixing, field-grown Alnus glutinosa under elevated atmospheric CO2

Few studies have investigated the effects of elevated CO₂ on the physiology of symbiotic N₂-fixing trees. Tree species grown in low N soils at elevated CO₂ generally show a decline in photosynthetic capacity over time relative to ambient CO₂ controls. This negative adjustment may be due to a reallocation of leaf N away from the photosynthetic apparatus, allowing for more efficient use of limiting N. We investigated the effect of twice ambient CO₂ on net CO₂ assimilation (A), photosynthetic capacity, leaf dark respiration, and leaf N content of N2-fixing Alnus glutinosa (black alder) grown in field open top chambers in a low N soil for 160 d. At growth CO₂, A was always greater in elevated compared to ambient CO₂ plants. Late season A vs. internal leaf p(CO₂) response curves indicated no negative adjustment of photosynthesis in elevated CO₂ plants. Rather, elevated CO₂ plants had 16% greater maximum rate of CO₂ fixation by Rubisco. Leaf dark respiration was greater at elevated CO₂ on an area basis, but unaffected by CO₂ on a mass or N basis. In elevated CO₂ plants, leaf N content (μg N cm^?2) increased 50% between Julian Date 208 and 264. Leaf N content showed little seasonal change in ambient CO₂ plants. A single point acetylene reduction assay of detached, nodulated root segments indicated a 46% increase in specific nitrogenase activity in elevated compared to ambient CO₂ plants. Our results suggest that N₂-fixing trees will be able to maintain high A with minimal negative adjustment of photosynthetic capacity following prolonged exposure to elevated CO₂ on N-poor soils.     

Inhibition of Reproduction in <Emphasis Type="Italic">Trypanosoma lewisi</Emphasis> by Ouabain

Trypanosoma lewisi usually produces a benign parasitaemia in rats. The parasites multiply only during the first few days after infection. Reproducing populations contain organisms that vary greatly in shape, size and total length. The coefficient of variation (CV) of the total length of the parasites measures this variability and is a sensitive index of reproductive activity¹. The CV is high, about 25%, early in the infection when reproduction is intense and thereafter decreases during infection until the trypanosomes become nearly uniform in size at about the tenth day of infection after which mostly non-reproducing forms are found².     

Selection for and against insecticide resistance and possible methods of inhibiting the evolution of resistance in mosquitoes

Abstract. 1. From published data on the rate of build-up of insecticide resistance in wild populations of Anopheles culicifacies and An.stephensi , calculations have been made of the coefficients of selection which would be required to produce these rates of evolution. It is assumed in turn that the resistances might be effectively dominant, recessive or intermediate.
2. The results in the case of DDT resistance in An.culicifacies indicated that the fitness of resistant individuals was 1.28–1.51 × that of susceptibles.
3. Published data have also been examined with reference to the reversions towards susceptibility which occurred when spraying has been withdrawn. The coefficients of selection against resistance, which would be required to produce the observed rates of reversion, have been calculated. In the case of An.culicifacies and DDT these coefficients were found to be inversely related to the initial frequency of the susceptibility gene.
4. The coefficients of selection have been used to assess the feasibility of inhibiting the evolution of resistance by spraying different insecticides in the alternate sectors of a grid pattern and/or periodically releasing susceptible males. If the proportion of migrants from sector to sector were 15–30%, a "grid" system would considerably reduce the rate of evolution of resistance, but to prevent resistance from ultimately building up to high levels periodic releases would also be required. These releases need not be frequent (approximately during one month each year) if resistance is effectively recessive in the field.     

Are there effective resistance management strategies for vectors of human disease?*

The rotation of insecticides used by the Onchocerciasis Control Programme in West Africa is reviewed and the motives for this rotation are shown to be not only management of temephos resistance in the Simulium vectors but also constraints on what compounds are usable at particular seasons. A computer model indicates that without these seasonal constraints there is unlikely to be an advantage in a pre-planned rotation of insecticides, as compared with the prompt switching of compounds as dictated by detection of build up of resistance and switching back to the original compound if the regression of resistance is found to give the opportunity to do so. The latter sequence of events can hardly be called a ‘strategy’ for resistance management, but is what any well-managed pest control programme would be expected to do. The use of an insecticide mixture is different in principle from the use of rotation and depends on the idea that if the mixture is used from the outset, when resistance to both components of the mixture is likely to be rare, the double resistance combination would be so rare as to be dwarfed in numbers by those insects which avoid exposure altogether. The prospects for successful use of a mixture depend on each component killing a very high percentage of the exposed insects which are genetically susceptible to it. Whether this condition is met could be tested, for example, in the case of exposure of mosquitoes to insecticide-treated bednets.     

Post‐Soviet farmland abandonment,forest recovery,and carbon sequestration in western Ukraine

Land use is a critical factor in the global carbon cycle, but land‐use effects on carbon fluxes are poorly understood in many regions. One such region is Eastern Europe and the former Soviet Union, where land‐use intensity decreased substantially after the collapse of socialism, and farmland abandonment and forest expansion have been widespread. Our goal was to examine how land‐use trends affected net carbon fluxes in western Ukraine (57 000 km²) and to assess the region's future carbon sequestration potential. Using satellite‐based forest disturbance and farmland abandonment rates from 1988 to 2007, historic forest resource statistics, and a carbon bookkeeping model, we reconstructed carbon fluxes from land use in the 20th century and assessed potential future carbon fluxes until 2100 for a range of forest expansion and logging scenarios. Our results suggested that the low‐point in forest cover occurred in the 1920s. Forest expansion between 1930 and 1970 turned the region from a carbon source to a sink, despite intensive logging during socialism. The collapse of the Soviet Union created a vast, but currently largely untapped carbon sequestration potential (up to～150 Tg C in our study region). Future forest expansion will likely maintain or even increase the region's current sink strength of 1.48 Tg C yr^?1. This may offer substantial opportunities for offsetting industrial carbon emissions and for rural development in regions with otherwise diminishing income opportunities. Throughout Eastern Europe and the former Soviet Union, millions of hectares of farmland were abandoned after the collapse of socialism; thus similar reforestation opportunities may exist in other parts of this region.     

Effects of Habitat on Competition Between Kit Foxes and Coyotes

Abstract: San Joaquin kit foxes (Vulpes macrotis mutica) are an endangered species with a narrow geographic range whose natural populations are limited by predation by coyotes (Canis latrans). In the warm, arid grassland and shrubland habitats where kit foxes occur, coyotes are more cover dependent than kit foxes, creating the possibility of habitat segregation. Effects of habitat variation on coyote and kit fox competition are unknown. We assessed exploitation and interference competition between coyotes and kit foxes in grassland and shrubland habitats to determine if such competition varies among habitats. With respect to exploitation competition, we evaluated habitat and spatial partitioning, diet, prey abundance, and survival for kit foxes and coyotes at the Lokern Natural Area in central California, USA, from January 2003 through June 2004. Kit foxes partitioned habitat, space, and diet with coyotes. Coyotes primarily used shrubland habitats whereas kit foxes selectively used burned grasslands. Kit foxes and coyotes had high dietary overlap with regards to items used, but proportional use of items differed between the 2 species. Kit foxes selected for Heermann's kangaroo rats (Dipodomys heermanni), which were closely tied to shrub habitats. With respect to interference competition, predation was the primary source of mortality for kit foxes, and survival of individual kit foxes was inversely related to proportion of shrub habitat within their home ranges. Our results suggest that a heterogeneous landscape may benefit kit foxes by providing habitat patches where predation risk may be lower.