Critical Loads Approach Research Articles

Buckling disappearance via merging/divergence in a nonlinear three-d.o.f. system with linear constitutive law

The phenomenon of buckling disappearance, occurring in a parameter-dependent family of systems admitting a nontrivial fundamental path, is studied. Two different forms of disappearance are detected, namely: (i) the divergence, in which the critical load continuously tends to infinity, and (ii) the merging, in which two critical loads approach each other, coalesce, and then disappear at a finite value of the critical load. It is shown that the two phenomena can be exhibited by the same mechanical system, when a suitable elasto-geometric parameter is varied. More importantly, it is proved that merging continuously changes into divergence when a second parameter is changed. A paradigmatic system is chosen to illustrate the two forms of buckling, i.e., a three degree-of-freedom spherical pendulum, elastically constrained at the ground, loaded by a transverse force and/or a conservative couple, made of two longitudinal potential forces. The springs are taken elastically linear, to stress the fact that divergence not necessarily calls for introducing a nonlinear constitutive law, as also mentioned in literature. Only a linear bifurcation analysis is carried out here, aimed to find the bifurcation points along the nonlinear fundamental path. However, due to the presence of non-negligible prestrains, such a bifurcation problem is governed by nonlinear algebraic equations, whose number of roots cannot be predicted in advance.

Assessing critical loads and exceedances for acidification and eutrophication in the forests of East and Southeast Asia: A comparison with EANET monitoring data

Spatial variations in sulfur (S) and nitrogen (N) deposition have changed in East and Southeast Asia in recent decades. Nevertheless, in this region, including the tropics, regional-scale assessments of the long-term risk of acidification and eutrophication (N saturation) for terrestrial ecosystems using a critical load approach have not been updated since 2001. To evaluate future risks, maps of critical loads and exceedances were updated using recently acquired spatial datasets of soil properties, soil minerals, climate, tree plantations, and the annual S and N depositions estimated using the Community Multiscale Air Quality (CMAQ) model. The resulting maps were verified using data on long-term trends in soil pH and nitrate concentration in surface water acquired by the Acid Deposition Monitoring Network in East Asia (EANET). It was found that N deposition exceeded the critical load for eutrophication not only in East Asia but also in some parts of the tropical monsoon and humid regions in Southeast Asia, whereas S deposition partly exceeded the critical load for soil acidification in China and small parts of the tropical monsoon region. The high-risk areas for eutrophication coincided well with the EANET sites, where the increase in nitrate concentration in the surface water was significant over the last 20 years. Hence, the estimated map of the critical load exceedance for eutrophication is more plausible for assessing the risk in East and Southeast Asia than that for acidification, although the critical load exceedance for acidification would be sufficiently significant as an updated risk map based on the latest input values. This update also suggests that increased N deposition around megacities, water discharge, and tree plantations may play an important role in the spatial variability of eutrophication risks in the tropics of Southeast Asia.

A modified ECBL approach for cold-formed steel rack section members

The calibration of appropriate buckling curves for interactive local/distortional-global buckling of thin-walled cold-formed steel members is a task well covered by the Erosion of Critical Bifurcation Load (ECBL) approach. In this context, here a modified formulation of the ECBL approach, based on a new definition of the mode interaction point, is presented. The Generalized Beam Theory is used to find the mode interaction points and the related imperfections. Pallet rack uprights in compression are studied to show the potentialities of the modified ECBL approach and mark the differences with the classical one. The buckling curves coming from the modified ECBL approach are also compared with experimental results and with the buckling curves evaluated following the EN15512:2009.

Critical levels and loads and the regulation of industrial emissions in northwest British Columbia, Canada

Northwest British Columbia, Canada, a sparsely populated and largely pristine region, is targeted for rapid industrial growth owing to the modernization of an aluminum smelter and multiple proposed liquefied natural gas (LNG) facilities. Consequently, air quality in this region is expected to undergo considerable changes within the next decade. In concert, the increase in LNG capacity driven by gas production from shale resources across North America has prompted environmental concerns and highlighted the need for science-based management decisions regarding the permitting of air emissions. In this study, an effects-based approach widely-used to support transboundary emissions policy negotiations was used to assess industrial air emissions in the Kitimat and Prince Rupert airsheds under permitted and future potential industrial emissions. Critical levels for vegetation of SO2 and NO2 and critical loads of acidity and nutrient nitrogen for terrestrial and aquatic ecosystems were estimated for both regions and compared with modelled concentration and deposition estimates to identify the potential extent and magnitude of ecosystem impacts. The critical level for SO2 was predicted to be exceeded in an area ranging from 81 to 251 km2 in the Kitimat airshed owing to emissions from an existing smelter, compared with <1 km2 in Prince Rupert under the lowest to highest emissions scenarios. In contrast, the NO2 critical level was not exceeded in Kitimat, and ranged from 4.5 to 6 km2 in Prince Rupert owing to proposed LNG related emissions. Predicted areal exceedance of the critical load of acidity for soil ranged from 1 to 28 km2 in Kitimat and 4–10 km2 in Prince Rupert, while the areal exceedance of empirical critical load for nutrient N was predicted to be greater in the Prince Rupert airshed (20–94 km2) than in the Kitimat airshed (1–31 km2). The number of lakes that exceeded the critical load of acidity did not vary greatly across emissions scenarios in the Kitimat (21–23 out of 80 sampled lakes) and Prince Rupert (0 out of 35 sampled lakes) airsheds. While critical loads have been widely used to underpin international emissions reductions of transboundary pollutants, it is clear that they can also play an important role in managing regional air emissions. In the current study, exceedance of critical levels and loads suggests that industrial emissions from the nascent LNG export sector may require careful regulation to avoid environmental impacts. Emissions management from LNG export facilities in other regions should consider critical levels and loads analyses to ensure industrial development is synergistic with ecosystem protection. While recognizing uncertainties in dispersion modelling, critical load estimates, and subsequent effects, the critical levels and loads approach is being used to inform regulatory decisions in British Columbia to prevent impacts that have been well documented in other regions.

Comparative Study on the Determination of Creep Properties by Using Different Types of Small Specimens

The application of small specimen creep test techniques in determination of creep properties of materials in-service has been increasing. There exists a number of small specimen creep testing techniques, such as sub-size uniaxial specimen, small punch specimen, shear punch specimen, impression specimen, three-point bending specimen, cantilever beam specimen, beam specimen with fixed constraint, circular ring specimen and tow-bar specimen. The diversity in specimen types and dimensions will affect the evaluation of creep properties. It is hence necessary to make a comparison among different small specimen creep tests. A comprehensive review has been made on the effect of the specimen dimension, stress state, conversion equation, creep properties, strain sensitivity and other influence factors. The results show that different small specimens result in different creep curves. Influenced by many factors, the small punch specimen has a very complicated data interpretation. With a large equivalent gauge length, the cantilever beam and circular ring specimens can obtain rather accurate creep data. Large deformation effect exists for the three-point bending, cantilever beam and circular ring specimens, which can be controlled by the critical load approach. Because of the low frictional sensitivity, the beam specimen with fixed constraint can obtain creep rupture data of high accuracy. A code of practice is then recommended for selecting a proper small specimen to evaluate the creep properties for in-service components.

Water quality assessment of aquatic ecosystems using ecological criteria – case study in Bulgaria

Four aquatic ecosystems (two rivers and two dams) situated in the western part of Bulgaria were investigated over a three years’ period. The River Egulya and Petrohan dam are situated in mountainous regions at about 1000 m altitude, and are not influenced by any anthropogenic sources. Petrohan dam is a site for long-term ecosystem research as a part of Bulgarian long-term ecological research network. The other two systems belong to populated industrial areas. The River Martinovska flows through a region with former long-term mining activity, while Ogosta dam is near a battery production factory. Both the geochemical and geographical ecosystems’ conditions are different, and their social usage as well. Ogosta dam water is used for irrigation and Petrohan dam for electric supply. The ecosystem sensitivity to heavy metals was evaluated by a critical load approach. Two criteria were used for risk assessment: critical load exceedance and microbial toxicity test. All studied ecosystems were more sensitive to cadmium than to lead deposition. The potential risk of Cd damage is higher for Petrohan dam and the River Egulya, where critical load exceedance was calculated for two years. Pseudomonas putida growth inhibition test detected a lack of toxicity for all studied ecosystems at the time of investigation with the exception of the low water September sample of the River Martinovska. The fast bacterial test is very suitable for a regular measurement of water toxicity because of its simplicity, lack of sophisticated equipment and clear results.

Experimental Evidence of Erosion of Critical Load in Interactive Buckling

AbstractThe paper starts with a selective review of some basic contributions related to local-overall buckling mode interactions in thin-walled steel members. Then, the results of an intensive experimental study carried out at the Politehnica University of Timisoara with the purpose of evaluating the erosion of a theoretical bifurcation load in the distortional-flexural buckling interaction range in pallet rack uprights are presented. Two different sizes of cross sections, with and without perforations, have been studied. To evaluate and quantify the erosion, the erosion of the critical bifurcation load approach is applied. This approach enables one to use the European buckling curves in the Ayrton-Perry format, expressing the imperfection factor (α) in terms of the erosion coefficient (ψ). It allows estimation of the member’s ultimate capacity, taking into account the interaction of sectional (local or distortional) and member (flexural, torsional, or flexural-torsional) instability modes. Test results a...

Controls on soil solution nitrogen along an altitudinal gradient in the Scottish uplands

Nitrogen (N) deposition continues to threaten upland ecosystems, contributing to acidification, eutrophication and biodiversity loss. We present results from a monitoring study aimed at investigating the fate of this deposited N within a pristine catchment in the Cairngorm Mountains (Scotland). Six sites were established along an elevation gradient (486–908m) spanning the key habitats of temperate maritime uplands. Bulk deposition chemistry, soil carbon content, soil solution chemistry, soil temperature and soil moisture content were monitored over a 5year period. Results were used to assess spatial variability in soil solution N and to investigate the factors and processes driving this variability. Highest soil solution inorganic N concentrations were found in the alpine soils at the top of the hillslope. Soil carbon stock, soil solution dissolved organic carbon (DOC) and factors representing site hydrology were the best predictors of NO3− concentration, with highest concentrations at low productivity sites with low DOC and freely-draining soils. These factors act as proxies for changing net biological uptake and soil/water contact time, and therefore support the hypothesis that spatial variations in soil solution NO3− are controlled by habitat N retention capacity. Soil percent carbon was a better predictor of soil solution inorganic N concentration than mass of soil carbon. NH4+ was less affected by soil hydrology than NO3− and showed the effects of net mineralization inputs, particularly at Racomitrium heath and peaty sites. Soil solution dissolved organic N concentration was strongly related to both DOC and temperature, with a stronger temperature effect at more productive sites. Due to the spatial heterogeneity in N leaching potential, a fine-scale approach to assessing surface water vulnerability to N leaching is recommended over the broad scale, critical loads approach currently in use, particularly for sensitive areas.

An Assessment of Long-term Risks of Metals in Sudbury: A Critical Loads Approach

Over a century of metal processing activity has resulted in widespread metal contamination of soils in Sudbury, ON, Canada. To assess the potential for recovery from the large reductions in metal deposition, critical loads were estimated for metals at 415 sites in Sudbury using an ‘effects based’ approach that is based on exceedance of provincial soil guidelines using multiple independent estimates of metal partitioning (Kd) for each metal. Sudbury soils are heavily contaminated with copper (Cu) and nickel (Ni), with 74 % of samples currently exceeding the provincial soil guideline for Cu and 87 % of samples exceeding the guideline for Ni. Both metals are strongly correlated with other metals (zinc (Zn), cadmium (Cd), lead (Pb)), although they rarely exceed provincial guidelines Copper and Ni are also strongly correlated with organic matter but not soil pH. Based on the most recent Cu and Ni deposition estimates (mid-1990s), it is estimated that between 20 % and 51 % of the sites receive deposition in excess of the ‘effects based’ critical load for Cu and between 5 % and 97 % of sites receive atmospheric deposition in excess of the ‘effects based’ critical load for Ni. These results suggest that Cu and Ni concentrations in soil will generally decrease resulting in slightly fewer sites that exceed the provincial soil guideline, but that the timeframe of this response will be very slow, with relatively little change occurring over the next 100 years. Even assuming a best case deposition scenario whereby Cu and Ni deposition were to immediately fall to background levels, the percentage of sites with Cu and Ni levels in excess of the OMOE guideline would still be between 69 % and 72 %, and 56 % and 86 %, respectively, demonstrating that while recovery of the Sudbury soils is possible, greater reductions in metal deposition are needed and even so, it will be a process that takes several centuries.

Past, Present, and Future Exceedance of Critical Loads of Acidity for Surface Waters in Finland

A critical load is a deposition limit below which harmful effects for a given ecosystem do not occur; the approach has underpinned European sulfur (S) and nitrogen (N) effects-based emission reduction policies during the last two decades. Surface waters are an important resource in Finland, as such the development of models and determination of critical loads has played a central role in supporting their recovery from acidification or preservation of ecosystem health. Critical loads of acidity for Finnish lakes were determined using the steady-state First-order Acidity Balance (FAB) model in conjunction with comprehensive national surveys of surface waters (headwater lakes; n = 1066) and soils. In the 1980s almost 60% of the study lakes were exceeded, impacting brown trout and perch populations. The steep decline in emissions and acidic (S and N) deposition during the last two decades has reduced exceedance to <10%, and by 2020 exceedance is predicted to reach preindustrial (1880) levels. In concert with these reductions, chemical and biological recovery has been observed. The critical load approach has been instrumental in assessing impacts to surface waters in Finland and directing effects-based emission reduction policies.

Effects of nitrogen deposition and empirical nitrogen critical loads for ecoregions of the United States

Human activity in the last century has led to a significant increase in nitrogen (N) emissions and atmospheric deposition. This N deposition has reached a level that has caused or is likely to cause alterations to the structure and function of many ecosystems across the United States. One approach for quantifying the deposition of pollution that would be harmful to ecosystems is the determination of critical loads. A critical load is defined as the input of a pollutant below which no detrimental ecological effects occur over the long-term according to present knowledge. The objectives of this project were to synthesize current research relating atmospheric N deposition to effects on terrestrial and freshwater ecosystems in the United States, and to estimate associated empirical N critical loads. The receptors considered included freshwater diatoms, mycorrhizal fungi, lichens, bryophytes, herbaceous plants, shrubs, and trees. Ecosystem impacts included: (1) biogeochemical responses and (2) individual species, population, and community responses. Biogeochemical responses included increased N mineralization and nitrification (and N availability for plant and microbial uptake), increased gaseous N losses (ammonia volatilization, nitric and nitrous oxide from nitrification and denitrification), and increased N leaching. Individual species, population, and community responses included increased tissue N, physiological and nutrient imbalances, increased growth, altered root : shoot ratios, increased susceptibility to secondary stresses, altered fire regime, shifts in competitive interactions and community composition, changes in species richness and other measures of biodiversity, and increases in invasive species. The range of critical loads for nutrient N reported for U.S. ecoregions, inland surface waters, and freshwater wetlands is 1–39 kg N·ha−1·yr−1, spanning the range of N deposition observed over most of the country. The empirical critical loads for N tend to increase in the following sequence for different life forms: diatoms, lichens and bryophytes, mycorrhizal fungi, herbaceous plants and shrubs, and trees. The critical load approach is an ecosystem assessment tool with great potential to simplify complex scientific information and communicate effectively with the policy community and the public. This synthesis represents the first comprehensive assessment of empirical critical loads of N for major ecoregions across the United States.

Atmospheric nitrogen deposition: Revisiting the question of the importance of the organic component

The organic component of atmospheric reactive nitrogen plays a role in biogeochemical cycles, climate and ecosystems. Although its deposition has long been known to be quantitatively significant, it is not routinely assessed in deposition studies and monitoring programmes. Excluding this fraction, typically 25–35%, introduces significant uncertainty in the determination of nitrogen deposition, with implications for the critical loads approach. The last decade of rainwater studies substantially expands the worldwide dataset, giving enough global coverage for specific hypotheses to be considered about the distribution, composition, sources and effects of organic-nitrogen deposition. This data collation and meta-analysis highlights knowledge gaps, suggesting where data-gathering efforts and process studies should be focused. New analytical techniques allow long-standing conjectures about the nature and sources of organic N to be investigated, with tantalising indications of the interplay between natural and anthropogenic sources, and between the nitrogen and carbon cycles.

Comparison of different critical load approaches for assessing streamwater acid-sensitivity to broadleaf woodland expansion

Due to its potential adverse effects on freshwater acidification, risk assessments of the impacts of forest expansion on surface waters are required. The critical load methodology is the standard way of assessing these risks and the two most widely used models are the Steady-State Water Chemistry (SSWC) and First-order Acidity Balance (FAB) models. In the UK the recommended risk assessment procedure for assessing the impact of forest expansion on freshwater acidification uses the SSWC model, whilst the FAB model is used for guiding emission policy. This study compared the two models for assessing the sensitivity of streamwater to acidification in 14 catchments with different proportions of broadleaf woodland cover in acid-sensitive areas in the UK. Both models predicted the exceedance of streamwater critical loads in the same catchments, but the magnitudes of exceedance varied due to the different treatment of nitrogen processes. The FAB model failed to account for high nitrogen leaching to streamwater, attributed to nitrogen deposition and/or fixation of nitrogen by alder trees in some study catchments, while both models underestimated the influence of high seasalt deposition. Critical load exceedance in most catchments was not sensitive to the use of different acid neutralising capacity thresholds or runoff estimates, probably due to the large difference between critical load values and acidic deposition loadings. However, the assessments were more sensitive to differences in calculation procedure in catchments where nitrogen deposition was similar to the availability of base cations from weathering and/or where critical load exceedance values were < 1 keq H + ha − 1 yr − 1 . Critical load exceedance values from both models agreed with assessments of acid-sensitivity based on indicator macroinvertebrates sampled from the study catchments. Thus the methodology currently used in the UK appears to be robust for assessing the risk of broadleaf woodland expansion on surface water acidification and ecological status.

Freshwater acidification research in Atlantic Canada: a review of results and predictions for the future

Atlantic Canada receives the lowest acid deposition amounts in eastern North America, but has some of the most acidic surface waters on the continent, due to the low buffering provided by regional bedrock and wetlands that produce natural organic acids. Southwestern and eastern parts of Nova Scotia combine poor buffering, high organic acidity, and higher acid deposition, to produce extremely low surface water pH and acid neutralization capacity (ANC) values. Although sulfate deposition is decreasing, concurrent reductions in dissolved base cations, as well as the acid-base characteristics of natural organic acids, are not allowing the recovery of ANC or surface water pH. Spring-time acid pulses occur in Atlantic Canada, though these have been reduced in severity with decreases in winter acid deposition, while autumnal low pH pulses caused by organic acids are a regular occurence in Nova Scotia and must be separated from mineral acidity pulses. Geochemical modeling using both critical load and dynamic approaches, nevertheless predict improvements in the water chemistry of Nova Scotia lakes within the next 20 years. However, re-establishment of pre-acidification water chemistry in most of its lakes will require greater reductions in S emissions than are currently planned in Canada and the United States.

Solid/liquid partition coefficients to model trace element critical loads for agricultural soils in Canada

Canadian consumers are demanding a sustainable agricultural industry as well as products delivered under Best Management Practices (BMPs). Trace element accumulation in soils may influence crop productivity, food quality and ecosystem and human health. Canada’s feed and foodstuff export industry has already faced cases of penalties for high trace element content [cadmium (Cd) in durum wheat]. Thus, it is imperative to be able to estimate the accumulation and potential short- and long-term impacts of trace elements in soil. A national-level Trace Element Indicator (TEI) based on present loadings of trace elements to agricultural land is in progress. An Expert Panel including Canadian, American and Australian experts guided the assembly of a proposed methodology for this TEI. The proposed TEI, described briefly here, is a critical load approach with a single expression of the risk of impact from single or multiple trace elements from multiple sources (manures, biosolids, effluents and fertilizers and natural processes), invoked in a stochastic manner. Two key data requirements are the current background levels of trace elements in soil, and the leachability of these trace elements. A survey of total and soluble concentrations of 54 elements in up to 112 soils was completed. Although preliminary in scope, these represent key soil series in Canada. From this, a database of the solid/liquid partition coefficient, Kd, was computed. These Kd values will be used to characterize the leachability of the trace elements. Key words: Cadmium, copper, zinc, lead, nickel, uranium, metals, Kd, distribution coefficient

Developing a critical load approach for national risk assessments of atmospheric metal deposition

The critical load approach has been proposed for evaluation of the need to reduce atmospheric emissions of metals that lead to transboundary transport and deposition across Europe. The present study demonstrates and evaluates the application of a critical load approach for national-scale risk assessment of metal deposition in the United Kingdom. Critical load maps, calculated using critical limits based on pH-dependent free metal ion activities, are presented. Current concentrations of lead and cadmium in soils are compared with two sets of critical limit values: First, limits based on the reactive soil concentration, and second, a pH-dependent free ion critical limit function, which takes into account variable soil characteristics across the country. The use of these two models leads to different conclusions about which areas of the United Kingdom are at greatest risk, partly because of differences in the range of values of pH and organic matter in soils used in ecotoxicological experiments and in the national database. Critical loads were calculated based on free ion critical limits; the critical loads were lowest in the south and east of the country and were associated with higher soil pH, lower runoff, and lower soil organic matter.

Prediction on load carrying capacities of multi-storey door-type modular steel scaffolds

Modular steel scaffolds are commonly used as supporting scaffolds in building construction, and traditionally, the load carrying capacities of these scaffolds are obtained from limited full-scale tests with little rational design. Structural failure of these scaffolds occurs from time to time due to inadequate design, poor installation and over-loads on sites. In general, multi-storey modular steel scaffolds are very slender structures which exhibit significant non-linear behaviour. Hence, secondary moments due to both <TEX>$P-{\delta}$</TEX> and <TEX>$P-{\Delta}$</TEX> effects should be properly accounted for in the non-linear analyses. Moreover, while the structural behaviour of these scaffolds is known to be very sensitive to the types and the magnitudes of restraints provided from attached members and supports, yet it is always difficult to quantify these restraints in either test or practical conditions. The problem is further complicated due to the presence of initial geometrical imperfections in the scaffolds, including both member out-of-straightness and storey out-of-plumbness, and hence, initial geometrical imperfections should be carefully incorporated. This paper presents an extensive numerical study on three different approaches in analyzing and designing multi-storey modular steel scaffolds, namely, a) Eigenmode Imperfection Approach, b) Notional Load Approach, and c) Critical Load Approach. It should be noted that the three approaches adopt different ways to allow for the non-linear behaviour of the scaffolds in the presence of initial geometrical imperfections. Moreover, their suitability and accuracy in predicting the structural behaviour of modular steel scaffolds are discussed and compared thoroughly. The study aims to develop a simplified and yet reliable design approach for safe prediction on the load carrying capacities of multi-storey modular steel scaffolds, so that engineers can ensure safe and effective use of these scaffolds in building construction.

Optimising the environmental benefits of emission reductions from UK coal- and oil-fired power stations: a critical loads approach

This paper describes a method for determining reductions of SO2 emissions from coal- and oil-fired power stations, oil refineries and large industrial units in the UK taking into account their pollution potential. The method is based on the use of two gridded data sets: critical loads, which represent the sensitivity of the environment to acid deposition and modelled estimates of total (wet + dry) sulphur deposition for 646 point sources within the UK. An iterative method is used to identify and subsequently reduce emissions from point sources that contribute most to areas of critical loads exceedance. This paper demonstrates how the method may be used to determine an optimal allocation of emissions across the UK which yields the maximum amount of environmental protection per unit of emission. The paper then goes on to consider the changes that will have to take place within the UK power generation industry in order to meet the revised EC Large Combustion Plant Directive which comes into force on 1 January 2008. Particular emphasis is placed upon proposed emissions trading schemes and the environmental implications of allowing trading between stations with high and low pollution potentials. The paper concludes by suggesting that the emissions trading process should take into account the pollution potential of each source, irrespective of whether the proposed emission is within the plant�s agreed emission limit. An approach based entirely on minimizing environmental damage rather than one which takes cost into account, as in current integrated assessment modelling, could provide an interesting approach across the rest of Europe.

The development of an approach to assess critical loads of acidity for woodland habitats in Great Britain

Abstract. Alongside other countries that are signatories to the UNECE Convention Long Range Transboundary on Air Pollution, the UK is committed to reducing the impact of air pollution on the environment. To advise and guide this policy in relation to atmospheric emissions of sulphur and nitrogen, a critical load approach has been developed. To assess the potential impact of these pollutants on woodland habitats a steady state, simple mass balance model has been parameterised. For mineral soils, a Ca:Al ratio in soil solution has been used as the critical load indicator for potential damage. For peat and organic soils critical loads have been set according to a pH criterion. Together these approaches have been used with national datasets to examine the potential scale of acidification in woodland habitats across the UK. The results can be mapped to show the spatial variability in critical loads of the three principal woodland habitat types (managed coniferous, managed broadleaved/ mixed woodland and unmanaged woodland). The results suggest that there is a wide range of critical loads. The most sensitive (lowest) critical loads are associated with managed coniferous followed by unmanaged woodland on peat soils. Calculations indicate that at steady state, acid deposition inputs reported for 1995–1997 result in a large proportion of all the woodland habitats identified receiving deposition loads in excess of their critical load; i.e. critical loads are exceeded. These are discussed in relation to future modelled depositions for 2010. Whilst significant widespread negative impacts of such deposition on UK woodland habitats have not been reported, the work serves to illustrate that if acid deposition inputs were maintained and projected emissions reductions not achieved, the long-term sustainability of large areas of woodland in the UK could be compromised. Keywords: critical loads, acid deposition, acidification, woodland, simple mass balance model, sustainability

Potential impact of forest harvesting on lake chemistry in south-central Ontario at current levels of acid deposition

The potential impact of harvesting on lake chemistry was assessed for ~1300 lakes in south-central Ontario using a critical loads approach based on the steady-state water chemistry (SSWC) model. The critical load of acidity is currently only exceeded by bulk sulphate deposition in 9% of the lakes if harvesting does not occur. However, the percentage increases to 23%, 56%, and 72% under potential harvesting scenarios that assume wood-only (stem without bark), stem-only, or whole-tree harvesting, respectively. This increase in exceedance of critical load is due to the much lower base cation concentrations in lakes resulting from base cation removals during harvest. For example, only 0.3% of lakes will have Ca2+ concentrations &lt;50 μequiv.·L–1 if harvesting does not occur, whereas 52% of lakes will have Ca2+ concentrations &lt;50 μequiv.·L–1 if whole-tree harvesting occurs. Harvesting clearly has an enormous potential impact on lake chemistry, which will become more apparent as exchangeable base cation pools in soil decline and acid inputs can no longer be buffered.