The critical weed-free period in organically-grown winter wheat

Two experiments were conducted in central southern England between September 1994 and August 1996 to identify the critical weed-free period in organically grown winter wheat (Triticum aestivum, cv. Mercia). In competition with a mixed weed infestation of predominately Alopecurus myosuroides and Tripleurospermum inodorum it was found that wheat yield decreased as the duration of the weed-infested period increased and that the crop needed to be kept free of weeds from sowing in order to completely avoid any yield loss. Also, weeds emerging in the wheat crop (predominately T. inodorum) during the growing season had a significant and detrimental effect on yield. The existence of the critical period, therefore, depends on the imposition of an acceptable yield loss. If a 5% yield loss gives a marginal benefit compared with the cost of weed control, the critical period will begin at 506°C days after sowing (November) and end at 1023°C days after sowing (February). This information could be used by farmers to target mechanical weeding operations to control weeds at a time that will have maximum benefit to the crop.     

Geographic variation in growth response of Douglas‐fir to interannual climate variability and projected climate change

We used 179 tree ring chronologies of Douglas‐fir [Pseudotsuga menziesii (Mirb.) Franco] from the International Tree‐Ring Data Bank to study radial growth response to historical climate variability. For the coastal variety of Douglas‐fir, we found positive correlations of ring width with summer precipitation and temperature of the preceding winter, indicating that growth of coastal populations was limited by summer dryness and that photosynthesis in winter contributed to growth. For the interior variety, low precipitation and high growing season temperatures limited growth. Based on these relationships, we chose a simple heat moisture index (growing season temperature divided by precipitation of the preceding winter and current growing season) to predict growth response for the interior variety. For 105 tree ring chronologies or 81% of the interior samples, we found significant linear correlations with this heat moisture index, and moving correlation functions showed that the response was stable over time (1901–1980). We proceeded to use those relationships to predict regional growth response under 18 climate change scenarios for the 2020s, 2050s, and 2080s with unexpected results: for comparable changes in heat moisture index, the most southern and outlying populations of Douglas‐fir in Mexico showed the least reduction in productivity. Moderate growth reductions were found in the southern United States, and strongly negative response in the central Rocky Mountains. Growth reductions were further more pronounced for high than for low elevation populations. Based on regional differences in the slope of the growth–climate relationship, we propose that southern populations are better adapted to drought conditions and could therefore contain valuable genotypes for reforestation under climate change. The results support the view that climate change may impact species not just at the trailing edges but throughout their range due to genetic adaptation of populations to local environments.     

Perspectives on Palaearctic and Nearctic bird migration; comparisons and overview of life-history and ecology of migrant passerines

In this paper we compare ecological attributes of tropical migrant passerines from the Nearctic and western Palaearctic, focusing particularly on habitat association patterns during both breeding and wintering seasons. Three regions were compared: Europe, western and eastern North America. Breeding bird census data from 32 studies (each including at least four stages of forest succession) were used to assess the association patterns of breeding habitats among tropical migrants. For each species we calculated an index of habitat diversity and habitat preference.
Tropical migrants preferred earlier successional stages than other birds in Europe. The opposite was true in eastern North America. In eastern North America, tropical migrants tended to be associated with a smaller range of serai stages than other passerine species. In their winter quarters, Palaearctic migrants live primarily in open habitats, such as savannas, whereas eastern Nearctic migrants make more frequent use of evergreen forests. Migrants from western North America show the greatest match between breeding and wintering habitats.
We relate the results to the taxonomy and probable history of contemporary avifaunas and vegetation formations of the Old and New World. Taxonomically, tropical migrants from different parts of the Holarctic are less closely related to each other than residents and short-distance migrants. Tropical and temperate avifaunas are more closely related to each other in the New World than in the Old World. Conservation implications of the between-continent differences are briefly discussed.     

Studies on the aggregation reactions and basic dye binding of tobacco mosaic virus. I. Variation of pH,particle asymmetry,acid and base titration results,irreversible binding of methylene blue,ultraviolet absorption,and extent of heat denaturation in tobacco mosaic virus solutions with time of standing

1. Aqueous solutions of tobacco mosaic virus were found to undergo a number of spontaneous changes on standing in the cold. The results of pH measurements, acid and base titrations, intrinsic viscosity determinations, studies on the irreversible binding of methylene blue with the virus, ultraviolet absorption, and the extent of nucleic acid splitting by heat denaturation indicated the occurrence of two successive reactions, the first one causing the release of hydrogen ions and a greater lability of the nucleic acid, and the second one, which involved end-to-end dimerization and which took place after 8 days of standing, requiring hydrogen ions. 2. The first over-all reaction was found to be a mixture of various types of reversible disaggregation and aggregation reactions, the nature of which depended on the pretreatment, the TMV concentration, the time of standing, and the phosphate concentration. For longer times of standing at high protein concentration a sudden drop in ultraviolet absorption is noted after dilution; also the drops in viscosity and pH are largest with a steep rise following, indicating the greatest breakup of end-to-end aggregates with formation of the side-to-side type. For concentrated solutions of TMV in water which have not stood long no drop in ultraviolet absorption is noted on dilution; the decrease in the other quantities is less, indicating that only a less extensive breakdown of end-to-end aggregates occurs. Addition of phosphate to concentrated solutions of TMV causes formation of side-to-side aggregates which break up on dilution. 3. Using the results for the pH increase and the viscosity increase in a given time interval for a given TMV preparation and also the slope of the corresponding titration curve at the pH mean, a value for the number of hydrogen ions taken up per TMV monomer in the formation of the end-to-end dimer was finally calculated. The average result obtained for two preparations was 3300. 4. Methylene blue, in the polymeric form, was demonstrated to cause complete irreversible conversion of TMV monomers to end-to-end dimers. At dye concentrations above 10(-4)M, higher TMV polymers are formed, but these are broken down to dimers on removal of free dye by dialysis. The irreversible binding ratios were shown to be decreased in accordance with the extent of the end-to-end aggregation of the preparation at the time of the experiment, which is in agreement with the concept that the irreversibly bound dye polymers go into the junction formed between two interacting TMV monomers. On the basis that only the monomers initially present in solution can react, maximum binding ratios corresponding to complete conversion of monomers to dimers were calculated from the observed irreversible binding ratios and from the fraction of dimers initially present which was obtained from viscosity data. The average result for three preparations in different states of aggregation was calculated to be 6565 for tetrameric binding or 3230 for dimeric binding, which agrees closely with the result obtained for the uptake of hydrogen ions per TMV monomer in the spontaneous dimerization.