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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. This content downloaded from 132.174.254.159 on Wed, SUMMARY(1) The dates of budhurst of lateral shoots on 2to 10-year old trees of Picea sitchensis were recorded on fourteen occasions at sites near meteorological stations in lowland and upland Britain between 1960 and 1980.(2) The following relationship accounted for 92% of the variation in thermal time from 1 February to the date of budburst among the fourteen observations: thermal time = 67.4 + 4401.8 exp (-0.042 x chill days)where thermal time was day degrees > 5 IC accumulated from 1 February, and chill days were the number of days <5 "C counted from 1 November, both based on mean daily air temperature ((max. + min.)/2). This model may be used to estimate the date of budburst on young P. sitchensis of most provenances growing in upland Britain.(3) The following features or assumptions of the model were examined with reference to the literature and/or by experimentation: the small effect of provenance; linearity in the relationship between bud growth rate and temperature; the large effect of chilling on thermal time to budhurst; the omission of daylength and soil temperature as variables; the choice of starting dates for effective chilling and thermal time; and the use of simple fixed base temperatures.(4) The model was applied to mean daily temperatures at Eskdalemuir for the period 1912-82. The predicted dates of budburst ranged from 23

Inferential models have long been used to determine pollutant dry deposition to ecosystems from measurements of air concentrations and as part of national and regional atmospheric chemistry and transport models, and yet models still suffer very large uncertainties. An inferential network of 55 sites throughout Europe for atmospheric reactive nitrogen (N_r) was established in 2007, providing ambient concentrations of gaseous NH₃, NO₂, HNO₃ and HONO and aerosol NH₄⁺ and NO₃^&minus; as part of the NitroEurope Integrated Project. <br><br> Network results providing modelled inorganic N_r dry deposition to the 55 monitoring sites are presented, using four existing dry deposition routines, revealing inter-model differences and providing ensemble average deposition estimates. Dry deposition is generally largest over forests in regions with large ambient NH₃ concentrations, exceeding 30–40 kg N ha⁻¹ yr⁻¹ over parts of the Netherlands and Belgium, while some remote forests in Scandinavia receive less than 2 kg N ha⁻¹ yr⁻¹. Turbulent N_r deposition to short vegetation ecosystems is generally smaller than to forests due to reduced turbulent exchange, but also because NH₃ inputs to fertilised, agricultural systems are limited by the presence of a substantial NH₃ source in the vegetation, leading to periods of emission as well as deposition. <br><br> Differences between models reach a factor 2–3 and are often greater than differences between monitoring sites. For soluble N_r gases such as NH₃ and HNO₃, the non-stomatal pathways are responsible for most of the annual uptake over many surfaces, especially the non-agricultural land uses, but parameterisations of the sink strength vary considerably among models. For aerosol NH₄⁺ and NO₃^&minus; discrepancies between theoretical models and field flux measurements lead to much uncertainty in dry deposition rates for fine particles (0.1–0.5 μm). The validation of inferential models at the ecosystem scale is best achieved by comparison with direct long-term micrometeorological N_r flux measurements, but too few such datasets are available, especially for HNO₃ and aerosol NH₄⁺ and NO₃^&minus;

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