摘要:

Research on air pollution and acidic deposition during the last 15 yr has greatly increased our knowledge of the rates and the processes of atmospheric deposition. The invigoration of the field has been a direct result of interchange and cooperation among ecosystem ecologists, micrometeorologists, and plant physiological ecologists who each approach the study of atmospheric deposition from different perspectives. This has led to the widespread realization among ecologists of the importance of dry and cloud deposition and the introduction of new methods to estimate these fluxes. In this paper I summarize the current understanding of atmospheric deposition processes, measurement methods, and patterns of deposition in North America. Dry deposition measurements are still highly uncertain in most cases, and methodology is still an active area of research and debate, but it seems clear that ecologists will require a suite of different methods to evaluate dry deposition fluxes of the various elements important to ecosystems. Standard model formulations have been developed for estimating dry and cloud deposition, and these models are finding wide use in flux estimation at sites where direct measurements are unavailable. National monitoring networks for wet and dry deposition have been established and are providing information on continental— and regional—scale patterns. Research has demonstrated that deposition rates are increased substantially at high—elevation sites by enhancement of wet, dry, and especially, cloud deposition. Patterns of O3exposure are also different at high—elevation sites. The deposition of O3, while mechanistically similar to that of other gases, has been treated differently in the ecological and botanical literature because of its direct phytotoxicity at ambient concentrations in industrialized areas. Current efforts in ozone exposure research involve determining appropriate exposure indices for interpreting biological responses and coupling models of atmospheric transport with models of ozone disposition within the plant.

ISSN:1051-0761    

DOI:10.2307/1941997

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Human activity is altering the chemistry of the atmosphere, which, in turn is affecting the physiology and growth of plants. The purpose of this article is to develop four ideas that are currently emerging from the work of a diverse group of plant scientists. (1) Air pollution definitions: The definition of air pollution has been broadened, and research activities are expanding to include analysis of plant responses to a wide range of atmospheric chemicals emitted from anthropogenic sources but not previously considered as air pollutants. Thus experiments with CO2and other trace gases are being pursued with approaches developed in air pollution research. (2) Air pollution uptake: Efforts are increasing to better quantify air pollution absorption rates through stomata in order to calculate actual dose vs. plant responses. The flux rates of gaseous pollutants into leaves, especially O3, are largely dependent upon stomatal conductance. Approaches are being developed to calculate stomatal absorption of gaseous pollutants, based on stomatal conductance values for water vapor and ambient air—pollution concentrations. Calculation of air pollution absorption rates will allow responses of plants to pollutants to be assessed in toxicological frameworks and will help characterize the strength of vegetation as sinks for some gaseous pollutants. (3) Compensatory responses: Plant responses to air pollutants can be interpreted as compensatory, i.e., a physiological adjustment to an environmental stress that maximizes productivity above that which would have occurred in the absence of compensation. Examples of compensatory responses to air pollutants are shifts in root—to—shoot ratio and accelerated rates of leaf maturation. Recognition of compensatory responses to air pollutants allows these responses to be placed in a framework that relates to whole—plant processes and ecosystem functions. (4) Air pollution and multiple stresses: Air pollution stress seldom occurs in isolation, and research approaches are being developed around the concept of multiple interacting stresses. Multiple—stress experiments are important because factors such as plant water status, light, and nutrient availability are known to alter plant responses to air pollutants. Multiple—stress studies will involve experiments with model plant species and high degrees of environmental control and monitoring.

ISSN:1051-0761    

DOI:10.2307/1941998

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Changes in the atmospheric concentrations of a number of air pollutants over the last century are hallmarks of the magnitude and extent of human impact on the environment. Some of these changes are important to ecologists because many pollutants, acting singly or in combination, affect ecological systems in general and forests in particular. The greatest concern lies with chronic levels of tropospheric ozone, cumulative deposition of hydrogen ion, nitrogen, and sulfur via wet and dry processes, a select number of airborne chemicals (e.g., mercury) that tend to bioaccumulate in continental landscapes, and ultraviolet—B radiation through the loss of stratospheric ozone. Because the atmospheric residence time of most pollutants of concern to ecologists is measured on time frames extending from a few weeks to decades, pollutant distribution and effects are regional to global in dimension. We present evidence that ambient levels of some air pollutants in North America are affecting managed and unmanaged forests, and that the two most important pollutants are tropospheric ozone and chronic nitrogen loading. Further evidence indicates that while concentrations of some air pollutants have been declining over the last decade in North America, others are expected to remain unchanged or increase, including tropospheric ozone. We conclude that air pollution is affecting many North American forests and some remote forests around the globe. In the immediate future, the concern for air pollution effects on forests and associated natural resources will broaden to include interactions with changes in climate and pollution effects in the world's developing countries. There has been a rapid evolution in air pollution studies in ecology, shifting away from the agricultural paradigm of single—factor experimentation toward new methodologies that are ecologically and multidisciplinarily based. This shift has been promoted by the recognition that air pollution is one of several factors influencing forest productivity, community dynamics, and biogeochemistry, and that effects arise through long—term exposures. This evolution in methodologies will become even more marked in the future as new ecological approaches are adopted and an understanding is developed of how air pollution interacts with changes in climate. One of the most promising methodologies is process level modeling, which utilizes the large base of data in tree physiology and forest ecology, watershed chemistry, and atmosphere— forest canopy meteorology to develop models of tree physiology and growth and to subsequently scale these investigations to the levels of forest stands and landscapes.

ISSN:1051-0761    

DOI:10.2307/1941999

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Great Smoky Mountains National Park, located in the southern Appalachians on the border of Tennessee and North Carolina, was designated by Congress in 1977 as a Class I area, where resources are to be protected from damage due to air pollution. The National Park Service, Air Quality Division, collected and then used data on levels of pollutants and response of sensitive resources in the park and found that forests, soils, surface waters, and visibility in the park were experiencing adverse impacts from air pollution. We reviewed the data and process that supported this finding as well as the legal tools available to federal land managers confronted with evidence that sources of air pollution outside the boundaries of Class I areas are affecting resources. We prepared this case study to describe how the National Park Service has used scientific information in regulatory arenas to address air pollution problems at Great Smoky Mountains National Park. Based on this case we show that the requirement for Federal Land Manager review of permits for new stationary sources of air pollution is not sufficient to protect the parks from what has become a regional air quality problem.

ISSN:1051-0761    

DOI:10.2307/1942000

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

We measured productivity and diversity of annual plant communities in human—made pits and mounds (20 ° 30 ° 100 cm) in the Northern Negev of Israel. We differentiated the species according to propagule size, dispersal mode, and geographical distribution in Israel. Our results in 28 experimental units containing a pit, a mound, and a portion of the undisturbed surrounding "matrix" showed that species richness, total plant density, and biomass yield were higher in the pits and mounds than in the matrix. Soil moisture at 0—15 cm depth was higher in the pits, but lower in the mounds relative to the matrix. Species with larger propagules (>5 mm) became more abundant, and those with tinyseeds (<1 mm) became less abundant, in the human—made patches relative to the undisturbed matrix. Wind—dispersed species and those with secondary dispersal by runoff were denser in pits and mounds, while short—distance rain dispersers were more abundant in the matrix. "Edge" species, occurring in the study site at the edge of their geographical distribution in Israel, increased more in density, species richness, and biomass than general "core" species, for which the study site is in the center of their distribution. The annual plant community responded to changes in three environmental factors: (1) microtopographic structure from flat surface to mounds or pits, which function as traps for seeds; (2) soil surface texture from densely packed to loose soil in pits and mounds, which provide suitable sites for larger propagules; (3) soil moisture availability, increasing in the pits but decreasing in the mounds relative to the matrix. The relevance of the study was discussed in relation to (1) combating desertification, (2) sustainable arid land use, and (3) species conservation.

ISSN:1051-0761    

DOI:10.2307/1942001

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

A range in productivity and climate exists along an east—west transect in Oregon. Remote sensing and climatic data for several of the Oregon Transect Ecosystem Research Project (OTTER) forested sites and neighboring shrub sites were combined to determined whether percentage intercepted photosynthetically active radiation (%IPAR) can be estimated from remotely sensed observations and to evaluate climatic constraints on the ability of vegetation to utilize intercepted of radition for production. The Thematic Mappers Simulator (TMS) normalized difference vegetation index (NDVI) provided a good linear estimate of %IPAR (R2= 0.97). Vegetation intercepted from 24.8% to 99.9% of incident photosynthetically active radiation (PAR), and aboveground net primary production (ANPP) ranged from 53 to 1310 g·m—2·yr—1. The ANPP was linearly related to annual IPAR across sites (R2= 0.70). Constraints on the ability of each species to utilize intercepted light, as defined by differential responses to freezing temperatures, drought, and vapor pressure deficit, were quantified from hourly meteorological station measurements near the sites and field physiological measurements. Vegetation could utilize from 30% of intercepted radiation at the eastside semiarid juniper woodland and shrub sites to 97% at the maritime coastal sites. Energy—size efficiency (ϵu), calculated from aboveground production and IPAR modified by the environmental limits, averaged 0.5 g/MJ for the shrub sites and 0.9 g/MJ for the forested sites.

ISSN:1051-0761    

DOI:10.2307/1942002

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Computer simulations were performed to study the dynamics of disease in species mixtures. Input variables for the simulator EPIMUL were manipulated to reflect factors evaluated in 1989—1990 field experiments using a bean—maize intercrop and the bean rust pathogen. The variables were maize interference effects on spore dispersal, maize—bean competition as it affected spore dispersal, and effects of maize on infection efficiency of the rust pathogen after dispersal. When simulations were run at a low rate of disease progress (conditions less than optimal for disease development), intercrops reduced disease to 32% of the monocrop level using 1989 data, but had no effect using 1990 data. Reductions of infection efficiency were largely responsible for lowered disease in 1989—based simulations. Interference and competition effects on dispersal when acting together reduced disease to 81% of the monocrop level using 1989 data, but only affected epidemics in 1990—based simulations when acting independently. Disease alteration due to dispersal effects was caused by changes in spore escape from plots and not changes in the spore dispersal gradient slope. All treatment effects diminished as the rate of disease progress increased. We have thus identified potential sources of the variability seen in field studies: effects of intercropping may depend highly on the rate of disease progress; infection efficiency effects, probably microclimatic, can have a strong impact on disease; and dispersal effects, which may be qualitatively consistent among years, can produce very different epidemics in different years.

ISSN:1051-0761    

DOI:10.2307/1942003

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

In the shortgrass steppe region of North America there is a controversy about the ability of the dominant species to recruit from seedlings. The prevailing view is that Bouteloua gracilis is incapable of recruitment from seedlings in areas receiving<380 mm of annual precipitation. A common explanation for this situation is that environmental conditions permitting seedling establishment are infrequent. To assess the frequency of environmental conditions appropriate for the recruitment of B. gracilis we used a soil water simulation model and long—term climatic data in conjunction with detailed information about the ecophysiological requirements for seed germination and growth of seminal and adventitious roots. We found that recruitment events occur as frequently as every 30—50 yr on silty clay, silty clay loam, and silty loam soils, but less than once in 5000 yr on sandy soils. Simulated frequencies of recruitment were sufficient to account for the observed abundance of B. gracilis in 7 of 11 soil textures evaluated. The differences in silt content and available water holding capacity accounted for the difference among soil textures in the probability of occurrence of recruitment events. Therefore, soil texture variability may explain the spatial pattern of recruitment and of population recovery after disturbance that occur at the soil type and microsite scales. Annual precipitation explained a large fraction of the temporal variability in recruitment. On average, recruitment occurred in years when precipitation was above the mean. The occurrence of recruitment events in some dry years (precipitationmean), emphasizes the importance of the intraseasonal distribution of precipitation. The sensitivity of recruitment to soil water availability suggests that climate change, particularly changes that increase or decrease the amount or the effectiveness of soil water, could have important effects on the future of populations of B. gracilis.

ISSN:1051-0761    

DOI:10.2307/1942004

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Riparian forests are in a unique position in the landscape since they form a transition between uplands and aquatic systems. These ecosystems may be highly susceptible to nitrogen (N) saturation since they may be subject to high inputs of N from upland areas. We measured potential net N mineralization and nitrification, soil inorganic N levels, microbial biomass carbon (C) and N content, and the N content of litter as indicators of N saturation in two riparian zones on the eastern and western sides of a stream. The sites had similar soils, vegetation, and hydrology, but differing upland land use. The eastern or "enriched" site was downgradient of a dense residential housing development (built in the 1950s) that produced high groundwater nitrate (NO3—) concentrations. The western or "control"site had an undeveloped upland. Our objectives were (1) to evaluate if groundwater NO3—loading had induced changes in surface soil N—cycle processes that are symptoms of N saturation in the enriched site and (2) to evaluate these changes in relation to inputs and outputs of N to the site. Soil inorganic—N levels, litter N content, and potential net N mineralization and nitrification were significantly higher on the enriched site relative to the control site, suggesting that the enriched site and N saturated. However, input—output analysis indicated that the enriched site was still a sink for upland derived NO3—. High rates of denitrification and storage of N in soil organic matter appear to moderate N saturation on the enriched site.

ISSN:1051-0761    

DOI:10.2307/1942005

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Constructed marshes in San Diego Bay (California, USA) have short canopies of the cordgrass Spartina foliosa and soils that are low in organic matter and nitrogen, compared to natural salt marshes of this area. Thus, we hypothesized that organic and nitrogen amendments would accelerate plant growth and enrich soil nitrogen pools. A field experiment in a newly excavated mitigation site examined the effects of adding ammonium sulfate (N), nitrogen—rich alfalfa (with or without added N), and nitrogen—poor straw (with or without added N) prior to planting. Aboveground biomass and stem densities of cordgrass were proportional to the amount of N added, whether in the organic or inorganic form.Alfalfa decomposed more rapidly than straw, but in both cases, N was lost quickly (50—66% in 2 wk). Sediment nitrogen and carbon pools were not increased significantly by the amendments. High decomposition rates and sandy soils apparently prevented accumulation of N in the nutrient and/or organic matter pools. Less than 5% of the nitrogen potentially available for uptake was actually recovered in aerial tissue. With N—poor soil and high nutrient—loss rates, the experimental plots developed less than half the aboveground biomass found in reference natural cordgrass marshes. Amending soils prior to transplantation did not achieve the goal of functional equivalency with natural marshes, and after 2 yr there was no indication that nutrient pools or plant growth rates would achieve natural levels.

ISSN:1051-0761    

DOI:10.2307/1942006

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY