Biomedical subjects

A H Legge

Publications and source records attributed to A H Legge.

12 recordsLinked to original sources

PubMed2006

New exposure-based metric approach for evaluating O(3) risk to North American aspen forests.

The United States and Canada currently use exposure-based metrics to protect vegetation from O(3). Using 5 years (1999-2003) of co-measured O(3), meteorology and growth response, we have developed exposure-based regression models that predict Populus tremuloides growth change within the North American ambient air quality context. The models comprised growing season fourth-highest daily maximum 8-h average O(3) concentration, growing degree days, and wind speed. They had high statistical significance, high goodness of fit, include 95% confidence intervals for tree growth change, and are simple to use. Averaged across a wide range of clonal sensitivity, historical 2001-2003 growth change over most of the 26 Mha P. tremuloides distribution was estimated to have ranged from no impact (0%) to strong negative impacts (-31%). With four aspen clones responding negatively (one responded positively) to O(3), the growing season fourth-highest daily maximum 8-h average O(3) concentration performed much better than growing season SUM06, AOT40 or maximum 1h average O(3) concentration metrics as a single indicator of aspen stem cross-sectional area growth.

Air Pollution↗

PubMed2001

Saskatoon serviceberry and ambient sulfur dioxide exposures: study sites re-visited, 1999.

Field surveys for symptoms of foliar injury in a regional airshed that is influenced by a number of point sources of SOx, NOx and hydrocarbons, combined with foliar and soil sulfur analyses, confirmed earlier results that Saskatoon serviceberry (Amelanchier alnifolia Nutt.) cv. Smokey can be used as a biological indicator of chronic sulfur dioxide exposures, in the presence of other phytotoxic air pollutants such as ozone.

Air Pollutants↗

PubMed2000

Passive sampling of ambient, gaseous air pollutants: an assessment from an ecological perspective.

During some past two decades there has been a growing interest among air pollution-vegetation effects-scientists to use passive sampling systems for quantifying ambient, gaseous air pollutant concentrations, particularly in remote and wilderness areas. On the positive side, excluding the laboratory analysis costs, passive samplers are inexpensive, easy to use and do not require electricity to operate. Therefore, they are very attractive for use in regional-scale air quality assessments. Passive samplers allow the quantification of cumulative air pollutant exposures, as total or average pollutant concentrations over a sampling duration. Such systems function either by chemical absorption or by physical adsorption of the gaseous pollutant of interest onto the sampling medium. Selection of a passive sampler must be based on its known or tested characteristics of specificity and linearity of response to the chemical constituent being collected. In addition, the effects of wind velocity, radiation, temperature and relative humidity must be addressed in the context of absorbent/adsorbent performance and sampling rate. Because of all these considerations, passive samplers may provide under- or overestimations of the cumulative exposures, compared to the corresponding data from co-located continuous monitors or active samplers, although such statistical variance can be minimized by taking necessary precautions. On the negative side, cumulative exposures cannot identify short-term (<few hours) pollutant episodes or regulatory non-compliance, where appropriate. Equally important, cumulative exposures (e.g. with ozone, a non-accumulating pollutant in plants) cannot account for the stochasticity and the dynamics of air pollutant exposure and plant (particularly deciduous vegetation) response. Although still being tested, statistical methods appear to be available to reconstruct passive sampler data to mimic the dynamics of the co-located continuous monitoring data. Regional level air quality and ecological risk assessments (multi-point models) as a whole should consider this type of approach in the future, using co-located passive samplers, with continuous monitors at selected locations as calibration points for mapping the air quality distribution on temporal and spatial scales.

Journal Article↗

PubMed2000

Application of a stochastic, Weibull probability generator for replacing missing data on ambient concentrations of gaseous pollutants.

A stochastic, three-parameter, Weibull frequency distribution, probability generator was tested by using theoretical data. Subsequently, it was applied to replace missing values of hourly atmospheric concentrations of trace gases that were continuously monitored at three study sites, for 2 years. The results were highly accurate and realistic. The cumulative means and the medians calculated by the Weibull method were intermediate between corresponding values calculated by uniform substitution of missing values with 'zero' or with half of the minimum detection limit of the appropriate measurement instrument used. Furthermore, the Weibull method allowed the replacement of as many as 100 missing values on either side of a measured data sub-set, without altering the overall characteristics of the true frequency distribution of the entire data set.

Journal Article↗

PubMed1999

Foliar injury symptoms of Saskatoon serviceberry (Amelanchier alnifolia Nutt.) as a biological indicator of ambient sulfur dioxide exposures.

Saskatoon serviceberry or Saskatoon (Amelanchier alnifolia Nutt. cv. Smoky) seedlings were planted at five study sites within a 35,000 km(2) airshed, that is influenced by a number of isolated stationary sources of sulfur dioxide (SO(2)), oxides of nitrogen and hydrocarbons, among others. The locations of the five sites were based on the results of a meteorological dry deposition model for the oxides of sulfur and nitrogen. Visible foliar injury responses of Saskatoon were used as a biological indicator of SO(2) exposures, through monthly field surveys. During late July 1998, unifacial, interveinal chlorosis was observed on some 12% of the seedlings at one study site. By September, the chlorosis had become more severe (necrosis) on some 70% of the plants at that site. Site specific ambient SO(2) levels were relatively low (maximum 5-min concentration of 52.8 ppb). Similar data were unavailable for all, but one other site. Therefore, foliar total S and SO(4)(2-)-S concentrations were analyzed in September at four of the five study sites. Previously soil SO(4)(2-)-S at these sites had been analyzed. There were spatial variabilities among these parameters. Based on the overall examination of these data, it is concluded that the observed visible injury symptoms were due to chronic SO(2) exposures, exacerbated by the presence of ozone (O(3)). Independent of this literature based speculation, visible foliar injury responses of Saskatoon can be used as a biological indicator for acute or chronic ambient SO(2) exposures, in the presence of other phytotoxic air pollutants.

Journal Article↗

PubMed1998

A numerical analysis of the combined open-top chamber data from the USA and Europe on ambient ozone and negative crop responses.

Statistical analysis was performed using selected sets of combined data from the US National Crop Loss Assessment Network and the European Open-Top Chambers Programme to examine the relationships between the occurrences of hourly ambient ozone (O3) concentrations and adverse crop yield responses. The results suggest that the frequency of occurrences of relatively low hourly O3 concentrations ( approximately <35 ppb) are not as important as moderate to higher concentrations in eliciting negative crop biomass responses. They also suggest that daily peak (highest) hourly O3 values ( approximately >90 ppb) may not be as critical, most likely because they frequently do not occur during time periods when conditions that promote atmospheric conductivity (O3 deposition) and plant uptake (O3 absorption) are in coherence.

Journal Article↗

PubMed1995

Air quality and its possible impacts on the terrestrial ecosystems of the North American Great Plains: an overview.

Over the past several decades, numerous studies have been conducted on the impacts of air pollutants (air quality) on terrestrial ecosystems (crops and forests). Although ambient air is always composed of pollutant mixtures, in determining the relative air quality and its ecosystem impacts at a given geographic location and time, a predominant number of studies have shown that at the present time surface-level O(3) is the most important phytotoxic air pollutant. Within the North American Great Plains, the precursors for surface-level O(3) are mainly anthropogenic NO(x) and VOCs (volatile organic compounds). Texas and Alberta are the top regions of such emissions in the United States and Canada, respectively. This appears to be due mainly to the prevalence of natural gas and/or oil industry in the two regions and the consequent urbanization. Nevertheless, the total emissions of NO(x) and VOCs within the North American Great Plains represent only about 25-36% of the corresponding total emissions within the contiguous United States and the whole of Canada. Within the Great Plains many major crop and tree species are known to be sensitive to O(3). This sensitivity assessment, however, is based mainly on our knowledge from univariate (O(3) only) exposure-plant response studies. In the context of global climate change, in almost all similar univariate studies, elevated CO(2) concentrations have produced increases in plant biomass (both crop and tree species). The question remains as to whether this stimulation will offset any adverse effects of elevated surface O(3) concentrations. Future research must address this important issue both for the Great Plains and for all other geographic locations, taking into consideration spatial and temporal variabilities in the ambient concentrations of the two trace gases.

Journal Article↗

PubMed1995

Ambient ozone (O3) and adverse crop response: a unified view of cause and effect.

This paper presents a cohesive view of the dynamics of ambient O(3) exposure and adverse crop response relationships, coupling the properties of photochemical O(3) production, flux of O(3) from the atmosphere into crop canopies and the crop response per se. The results from two independent approaches ((a) statistical and (b) micrometeorological) were analyzed for understanding cause-effect relationships of the foliar injury responses of tobacco cv Bel-W3 to the exposure dynamics of ambient O(3) concentrations. Similarly, other results from two independent approaches were analyzed in: (1) establishing a micrometeorological relationship between hourly ambient O(3) concentrations and their vertical flux from the air into a natural grassland canopy; and (2) establishing a statistical relationship between hourly ambient O(3) concentrations in long-term, chronic exposures and crop yield reductions. Independent of the approach used, atmospheric conditions appeared to be most conducive and the crop response appeared to be best explained statistically by the cumulative frequency of hourly ambient O(3) concentrations between 50 ppb and 90 ppb (100 and 180 microg m(-3)). In general, this concentration range represents intermediate or moderately enhanced hourly O(3) values in a polluted environment. Further, the diurnal occurrence of this concentration range (often approximately between 0900 and 1600 h in a polluted, agricultural environment) coincided with the optimal CO(2) flux from the atmosphere into the crop canopy, thus high uptake. The frequency of occurrence of hourly O(3) concentrations > 90 ppb (180 microg m(-3)) appeared to be of little importance and such concentrations in general appeared to occur during atmospheric conditions which did not facilitate optimal vertical flux into the crop canopy, thus low uptake. Alternatively, when > 90 ppb (180 microg m(-3)) O(3) concentrations occurred during the 0900-1600 h window, their frequency of occurrence was low in comparison to the 50-90 ppb (100-180 microg m(-3)) range. Based on the overall results, we conclude that if the cumulative frequency of hourly ambient O(3) concentrations between 50-62 ppb (100-124 microg m(-3)) occurred during 53% of the growing season and the corresponding cumulative frequency of hourly O(3) concentrations between 50-74 ppb (100-148 microg m(-3)) occurred during 71% of the growing season, then yield reductions in sensitive crops could be expected, if other factors supporting growth, such as adequate soil moisture are not limiting.

Journal Article↗

PubMed1994

Ambient ozone and crop loss: establishing a cause-effect relationship.

This paper provides the results of a retrospective mathematical analysis of the US NCLAN (National Crop Loss Assessment Network) open-top chamber data. Some 77% of the 73 crop harvests examined, showed no statistically significant yield differences between NF (non-filtered open-top chamber) and AA (chamberless, ambient air) treatments (no easily discernable chamber effects on yield). However, among these cases only seven acceptable examples showed statistically significant yield reductions in NF compared to the CF (charcoal filtered open-top chamber) treatment. An examination of the combined or cumulative hourly ambient O3 frequency distribution for cases with yield loss in NF compared to a similar match of cases without yield loss showed that the mean, median and the various percentiles were all higher (>/= 3 X) in the former in contrast to the latter scenario. The combined frequency distribution of hourly O3 concentrations for the cases with yield loss in NF were clearly separated from the corresponding distribution with no yield loss, at O3 concentrations > 49 ppb. Univariate linear regressions between various O3 exposure parameters and per cent yield losses in NF showed that the cumulative frequency of occurrence of O3 concentrations between 50 and 87 ppb was the best predictor (adjusted R2 = 0.712 and p = 0.011). This analysis also showed that the frequency distribution of hourly concentrations up to 87 ppb O3 represented a critical point, since the addition of the frequency distributions of > 87 ppb O3 did not improve the R2 values. In fact as the frequency of hourly O3 concentrations included in the regression approached 50-100 ppb, the R2 value decreased substantially and the p value increased inversely. Further, univariate linear regressions between the frequencies of occurrence of various O3 concentrations between 50 and 90 ppb and: (a) cases with no yield difference in NF and (b) cases with yield increase in NF compared to the CF treatment (positive effect) provided no meaningful statistical relationship (adjusted R2 = 0.000) in either category. These results support the basis that additional evaluation of the frequency of occurrence of hourly O3] concentrations between 50 and 87 ppb for cases with the yield reductions could provide a meaningful ambient O3 standard, objective or guideline for vegetation.

Journal Article↗

PubMed1992

The effects of sulphur gas and elemental sulphur dust deposition on Pinus contorta x Pinus banksiana: cell walls and water relations.

The bulk modulus of elasticity (E) for Pinus contorta (lodgepole pine) x Pinus banksiana (jack pine) hybrids was compared between a site (AI) close to a sour gas processing plant and a control site (AV). The mean bulk modulus of elasticity for branches from AI was 47.5 MPa vs 18.5 MPa for the control site (AV). Site AI had been exposed to S-gas emissions and large amounts of elemental S deposition and had an acidic soil (pH 4.0 at 10 cm depth). During 1981 the needles at AI had more aluminum and iron compared to those at AV (900 ppm vs 390 ppm AI in the 3-year-old needles). Mean leader growth was measured over a 3-year period and was observed to be greater at AI than AV (46+/-7 cm vs 29+/-9 cm for 1988). Histochemically, the needles at AI had higher phenol and lignin content than AV. These results suggest that the S-gas fumigation, S-dust deposition, plus increased concentrations of soluble aluminum and iron had altered the cell wall elastic properties resulting in altered water relations. The implications of this on leaf diffusive resistance and photosynthesis are discussed.

Journal Article↗

PubMed1991

Ozone and the clean troposphere: ecological implications.

In recent years numerous investigations have examined the relationships between chronic ozone (O(3)) exposure and plant responses using statistical or empirical cause-effect models. One of the critically important decisions underlying these studies has been the choice of the control or reference O(3) exposure to which all other treatments are compared. The issue of the control or reference O(3) exposure is a complex one. Does a single ambient control or reference O(3) exposure exist? The approach taken in this paper is to examine the O(3) concentrations in the 'clean troposphere.' One must ask what a clean troposphere is and whether there are any geographic locations in the world today that are totally free of human influences. Data on photochemical oxidants from Fortress Mountain, Alberta, Canada, are examined. A comparison of these data with air quality data from other locations in North America is used to examine the design of vegetation response studies. The need for integrated research design, execution, and evaluation is stressed.

Journal Article↗

PubMed1988

Foliar sulphur species in pine: a new indicator of a forest ecosystem under air pollution stress.

An eleven-year foliar sulphur (S) monitoring program was carried out from 1976 to 1986 near a sulphur recovery-gas plant in west-central Alberta, Canada, as part of a case study designed to determine the effects of chronic, low concentration sulphur gas emissions on the forest ecosystem surrounding the gas plant. Measurements of both foliar total sulphur (ST) and foliar inorganic sulphur (SO4-S) concentration in lodgepole x jack pine trees at the end of each of the 11 growing seasons were taken to provide an indication of S loading of the forest from industrial sulphur emissions. To measure the state of the forest ecosystem, foliar ST was separated into foliar accumulated sulphur (inorganic sulphur or SO4-S) and foliar assimilated sulphur (organic sulphur or S0) and the ratio of SO4-S/S0 taken. Foliar S0 was calculated as the difference between foliar ST and foliar SO4-S. The median SO4-S/S0 ratio, with all three years of needles considered, varied from 0.29 at a reference location (AV) to 0.88 at the location with the highest stress (AI). The corresponding mean values ranged from 0.3 at the reference location to 2.2 at the location of highest stress. The mean seasonal photosynthetic rate of current year's foliage of the pine trees and soil pH were reduced at a stressed location (AI) compared to the reference location (AV), between 1976 and 1981. Over this same time period the mean foliar SO4-S/S0 ratio increased from 0.4 +/- 0.1 to 1.0 +/- 0.3 at the stressed location (AI) and remained nearly the same at the reference location (AV) at 0.3 +/- 0.1. This research suggests that the foliar SO4-S/S0 ratio is a useful indicator of the state of forest ecosystems under S air pollution stress. It is concluded that foliar S separated into various fractions has potential as an early warning environmental management tool.

Journal Article↗