Solar ultraviolet radiation and ozone depletion-driven climate change: effects on terrestrial ecosystems.

In this assessment we summarise advances in our knowledge of how UV-B radiation (280-315 nm), together with other climate change factors, influence terrestrial organisms and ecosystems. We identify key uncertainties and knowledge gaps that limit our ability to fully evaluate the interactive effects of ozone depletion and climate change on these systems. We also evaluate the biological consequences of the way in which stratospheric ozone depletion has contributed to climate change in the Southern Hemisphere. Since the last assessment, several new findings or insights have emerged or been strengthened. These include: (1) the increasing recognition that UV-B radiation has specific regulatory roles in plant growth and development that in turn can have beneficial consequences for plant productivity via effects on plant hardiness, enhanced plant resistance to herbivores and pathogens, and improved quality of agricultural products with subsequent implications for food security; (2) UV-B radiation together with UV-A (315-400 nm) and visible (400-700 nm) radiation are significant drivers of decomposition of plant litter in globally important arid and semi-arid ecosystems, such as grasslands and deserts. This occurs through the process of photodegradation, which has implications for nutrient cycling and carbon storage, although considerable uncertainty exists in quantifying its regional and global biogeochemical significance; (3) UV radiation can contribute to climate change via its stimulation of volatile organic compounds from plants, plant litter and soils, although the magnitude, rates and spatial patterns of these emissions remain highly uncertain at present. UV-induced release of carbon from plant litter and soils may also contribute to global warming; and (4) depletion of ozone in the Southern Hemisphere modifies climate directly via effects on seasonal weather patterns (precipitation and wind) and these in turn have been linked to changes in the growth of plants across the Southern Hemisphere. Such research has broadened our understanding of the linkages that exist between the effects of ozone depletion, UV-B radiation and climate change on terrestrial ecosystems.

Effects of solar ultraviolet radiation on terrestrial ecosystems. Patterns, mechanisms, and interactions with climate change.

Ultraviolet radiation (UV) is a minor fraction of the solar spectrum reaching the ground surface. In this assessment we summarize the results of previous work on the effects of the UV-B component (280-315 nm) on terrestrial ecosystems, and draw attention to important knowledge gaps in our understanding of the interactive effects of UV radiation and climate change. We highlight the following points: (i) The effects of UV-B on the growth of terrestrial plants are relatively small and, because the Montreal Protocol has been successful in limiting ozone depletion, the reduction in plant growth caused by increased UV-B radiation in areas affected by ozone decline since 1980 is unlikely to have exceeded 6%. (ii) Solar UV-B radiation has large direct and indirect (plant-mediated) effects on canopy arthropods and microorganisms. Therefore, trophic interactions (herbivory, decomposition) in terrestrial ecosystems appear to be sensitive to variations in UV-B irradiance. (iii) Future variations in UV radiation resulting from changes in climate and land-use may have more important consequences on terrestrial ecosystems than the changes in UV caused by ozone depletion. This is because the resulting changes in UV radiation may affect a greater range of ecosystems, and will not be restricted solely to the UV-B component. (iv) Several ecosystem processes that are not particularly sensitive to UV-B radiation can be strongly affected by UV-A (315-400 nm) radiation. One example is the physical degradation of plant litter. Increased photodegradation (in response to reduced cloudiness or canopy cover) will lead to increased carbon release to the atmosphere via direct and indirect mechanisms.

Terrestrial ecosystems, increased solar ultraviolet radiation, and interactions with other climate change factors.

There have been significant advances in our understanding of the effects of UV-B radiation on terrestrial ecosystems, especially in the description of mechanisms of plant response. A further area of highly interesting research emphasizes the importance of indirect UV radiation effects on plants, pathogens, herbivores, soil microbes and ecosystem processes below the surface. Although photosynthesis of higher plants and mosses is seldom affected by enhanced or reduced UV-B radiation in most field studies, effects on growth and morphology (form) of higher plants and mosses are often manifested. This can lead to small reductions in shoot production and changes in the competitive balance of different species. Fungi and bacteria are generally more sensitive to damage by UV-B radiation than are higher plants. However, the species differ in their UV-B radiation sensitivity to damage, some being affected while others may be very tolerant. This can lead to changes in species composition of microbial communities with subsequent influences on processes such as litter decomposition. Changes in plant chemical composition are commonly reported due to UV-B manipulations (either enhancement or attenuation of UV-B in sunlight) and may lead to substantial reductions in consumption of plant tissues by insects. Although sunlight does not penetrate significantly into soils, the biomass and morphology of plant root systems of plants can be modified to a much greater degree than plant shoots. Root mass can exhibit sizeable declines with more UV-B. Also, UV-B-induced changes in soil microbial communities and biomass, as well as altered populations of small invertebrates have been reported and these changes have important implications for mineral nutrient cycling in the soil. Many new developments in understanding the underlying mechanisms mediating plant response to UV-B radiation have emerged. This new information is helpful in understanding common responses of plants to UV-B radiation, such as diminished growth, acclimation responses of plants to UV-B radiation and interactions of plants with consumer organisms such as insects and plant pathogens. The response to UV-B radiation involves both the initial stimulus by solar radiation and transmission of signals within the plants. Resulting changes in gene expression induced by these signals may have elements in common with those elicited by other environmental factors, and generate overlapping functional (including acclimation) responses. Concurrent responses of terrestrial systems to the combination of enhanced UV-B radiation and other global change factors (increased temperature, CO2, available nitrogen and altered precipitation) are less well understood. Studies of individual plant responses to combinations of factors indicate that plant growth can be augmented by higher CO2 levels, yet many of the effects of UV-B radiation are usually not ameliorated by the elevated CO2. UV-B radiation often increases both plant frost tolerance and survival under extreme high temperature conditions. Conversely, extreme temperatures sometimes influence the UV-B radiation sensitivity of plants directly. Plants that endure water deficit stress effectively are also likely to be tolerant of high UV-B flux. Biologically available nitrogen is exceeding historical levels in many regions due to human activities. Studies show that plants well supplied with nitrogen are generally more sensitive to UV-B radiation. Technical issues concerning the use of biological spectral weighting functions (BSWFs) have been further elucidated. The BSWFs, which are multiplication factors assigned to different wavelengths giving an indication of their relative biological effectiveness, are critical to the proper conduct and interpretation of experiments in which organisms are exposed to UV radiation, both in the field and in controlled environment facilities. The characteristics of BSWFs vary considerably among different plant processes, such as growth, DNA damage, oxidative damage and induction of changes in secondary chemicals. Thus, use of a single BSWF for plant or ecosystem response is not appropriate. This brief review emphasizes progress since the previous report toward the understanding of solar ultraviolet radiation effects on terrestrial systems as it relates to ozone column reduction and the interaction of climate change factors.

Water conservation in Artemisia tridentata through redistribution of precipitation.

Water conservation is important for plants that maintain physiologically active foliage during prolonged periods of drought. A variety of mechanisms for water conservation exist including stomatal regulation, foliage loss, above- and below-ground allocation patterns, size of xylem vessels and leaf pubescence. Using the results of a field and simulation study with Artemisia tridentata in the Great Basin, USA, we propose an additional mechanism of water conservation that can be used by plants in arid and semi-arid environments following pulses of water availability. Precipitation redistributed more uniformly in the soil column by roots (hydraulic redistribution of water downward) slows the rate at which this water can subsequently be taken up by plants, thus prolonging water availability during periods of drought. By spreading out water more uniformly in the soil column at lower water potentials following precipitation events, water use is reduced due to lower soil conductivity. The greater remaining soil water and more uniform distribution result in higher plant predawn water potentials and transpiration rates later in the drought period. Simulation results indicate that plants can benefit during drought periods from water storage following both summer rain events (small summer pulses) and overwinter recharge (large spring pulse). This mechanism of water conservation may aid in sustaining active foliage, maintaining root-soil hydraulic connectivity, and increasing survival probability of plants which remain physiologically active during periods of drought.

Impacts of solar ultraviolet-B radiation on terrestrial ecosystems of Tierra del Fuego (southern Argentina). An overview of recent progress.

The southern part of Tierra del Fuego, in the southernmost tip of South America, is covered by dense Nothofagus spp. forests and Sphagnum-dominated peat bogs, which are subjected to the influence of ozone depletion and to increased levels of solar ultraviolet-B radiation (UV-B). Over the last 5 years we have studied some of the biological impacts of solar UV-B on natural ecosystems of this region. We have addressed two general problems: (i) do the fluctuations in UV-B levels under the influence of the Antarctic ozone 'hole' have any measurable biological impact, and (ii) what are the long-term effects of solar (ambient) UV-B on the Tierra del Fuego ecosystems? In this paper, we provide an overview of the progress made during the first 4 years of the project. We highlight and discuss the following results: (1) ambient UV-B has subtle but significant inhibitory effects on the growth of herbaceous and graminoid species of this region (growth reduction < or = 12%), whereas no consistent inhibitory effects could be detected in woody perennials; (2) in the species investigated in greatest detail, Gunnera magellanica, the inhibitory effect of solar UV-B is accompanied by increased levels of DNA damage in leaf tissue, and the DNA damage density in the early spring is clearly correlated with the dose of weighted UV-B measured at ground level; (3) the herbaceous species investigated thus far show little or no acclimation responses to ambient UV-B such as increased sunscreen levels and DNA repair capacity; and (4) ambient UV-B has significant effects on heterotrophic organisms, included marked inhibitory effects on insect herbivory. The results from the experiments summarized in this review clearly indicate that UV-B influences several potentially important processes and ecological interactions in the terrestrial ecosystems of Tierra del Fuego.

Inheritance of UV-B tolerance in seven ecotypes of Arabidopsis thaliana L. Heynh. and their F1 hybrids.

We used a partial diallel mating design to examine morphologic response to supplementary ultraviolet-B (UV-B) radiation of seven ecotypes of Arabidopsis thaliana L. Heynh. from several geographic locations in Europe. We were particularly interested in the inheritance of UV-B tolerance by the F1 generation. Morphologic traits included plant height, rosette diameter, number of shoots (lateral branches from the rosette) and branches (lateral branches above the rosette), and reproductive and vegetative dry mass. To effect a large difference in UV treatments, plants under treatment received 11 kJ/m2/day of biologically effective UV-B radiation while control plants received no UV-B radiation. Genotype effects were observed for all traits (P < .0001), but a significant treatment effect and genotype x treatment interactions were detected only for plant height (P = .0001), rosette diameter (P = .0229), and vegetative (P = .0260) and reproductive dry mass (P = .0900). General combining ability was significant for plant height (P < .0001) and vegetative mass (P = .0563), whereas specific combining ability was significant for rosette diameter (P = .0220) and vegetative mass (P = .0506). These results suggest that both pure lines and hybrids of Arabidopsis can be developed for greater tolerance of UV-B radiation. Similar findings for crop species might lead to the development of UV tolerant varieties.

Ozone depletion and UVB radiation: impact on plant DNA damage in southern South America.

The primary motivation behind the considerable effort in studying stratospheric ozone depletion is the potential for biological consequences of increased solar UVB (280-315 nm) radiation. Yet, direct links between ozone depletion and biological impacts have been established only for organisms of Antarctic waters under the influence of the ozone "hole;" no direct evidence exists that ozone-related variations in UVB affect ecosystems of temperate latitudes. Indeed, calculations based on laboratory studies with plants suggest that the biological impact of ozone depletion (measured by the formation of cyclobutane pyrimidine dimers in DNA) is likely to be less marked than previously thought, because UVA quanta (315-400 nm) may also cause significant damage, and UVA is unaffected by ozone depletion. Herein, we show that the temperate ecosystems of southern South America have been subjected to increasingly high levels of ozone depletion during the last decade. We found that in the spring of 1997, despite frequent cloud cover, the passages of the ozone hole over Tierra del Fuego (55 degrees S) caused concomitant increases in solar UV and that the enhanced ground-level UV led to significant increases in DNA damage in the native plant Gunnera magellanica. The fluctuations in solar UV explained a large proportion of the variation in DNA damage (up to 68%), particularly when the solar UV was weighted for biological effectiveness according to action spectra that assume a sharp decline in quantum efficiency with increasing wavelength from the UVB into the UVA regions of the spectrum.

Changes in biologically active ultraviolet radiation reaching the Earth's surface.

Stratospheric ozone levels are near their lowest point since measurements began, so current ultraviolet-B (UV-B) radiation levels are thought to be close to their maximum. Total stratospheric content of ozone-depleting substances is expected to reach a maximum before the year 2000. All other things being equal, the current ozone losses and related UV-B increases should be close to their maximum. Increases in surface erythemal (sunburning) UV radiation relative to the values in the 1970s are estimated to be: about 7% at Northern Hemisphere mid-latitudes in winter/spring; about 4% at Northern Hemisphere mid-latitudes in summer/fall; about 6% at Southern Hemisphere mid-latitudes on a year-round basis; about 130% in the Antarctic in spring; and about 22% in the Arctic in spring. Reductions in atmospheric ozone are expected to result in higher amounts of UV-B radiation reaching the Earth's surface. The expected correlation between increases in surface UV-B radiation and decreases in overhead ozone has been further demonstrated and quantified by ground-based instruments under a wide range of conditions. Improved measurements of UV-B radiation are now providing better geographical and temporal coverage. Surface UV-B radiation levels are highly variable because of cloud cover, and also because of local effects including pollutants and surface reflections. These factors usually decrease atmospheric transmission and therefore the surface irradiances at UV-B as well as other wavelengths. Occasional cloud-induced increases have also been reported. With a few exceptions, the direct detection of UV-B trends at low- and mid-latitudes remains problematic due to this high natural variability, the relatively small ozone changes, and the practical difficulties of maintaining long-term stability in networks of UV-measuring instruments. Few reliable UV-B radiation measurements are available from pre-ozone-depletion days. Satellite-based observations of atmospheric ozone and clouds are being used, together with models of atmospheric transmission, to provide global coverage and long-term estimates of surface UV-B radiation. Estimates of long-term (1979-1992) trends in zonally averaged UV irradiances that include cloud effects are nearly identical to those for clear-sky estimates, providing evidence that clouds have not influenced the UV-B trends. However, the limitations of satellite-derived UV estimates should be recognized. To assess uncertainties inherent in this approach, additional validations involving comparisons with ground-based observations are required. Direct comparisons of ground-based UV-B radiation measurements between a few mid-latitude sites in the Northern and Southern Hemispheres have shown larger differences than those estimated using satellite data. Ground-based measurements show that summertime erythemal UV irradiances in the Southern Hemisphere exceed those at comparable latitudes of the Northern Hemisphere by up to 40%, whereas corresponding satellite-based estimates yield only 10-15% differences. Atmospheric pollution may be a factor in this discrepancy between ground-based measurements and satellite-derived estimates. UV-B measurements at more sites are required to determine whether the larger observed differences are globally representative. High levels of UV-B radiation continue to be observed in Antarctica during the recurrent spring-time ozone hole. For example, during ozone-hole episodes, measured biologically damaging radiation at Palmer Station, Antarctica (64 degrees S) has been found to approach and occasionally even exceed maximum summer values at San Diego, CA, USA (32 degrees N). Long-term predictions of future UV-B levels are difficult and uncertain. Nevertheless, current best estimates suggest that a slow recovery to pre-ozone depletion levels may be expected during the next half-century. (ABSTRACT TRUNCATED)

Downward flux of water through roots (i.e. inverse hydraulic lift) in dry Kalahari sands.

Downward transport of water in roots, in the following termed "inverse hydraulic lift," has previously been shown with heat flux techniques. But water flow into deeper soil layers was demonstrated in this study for the first time when investigating several perennial grass species of the Kalahari Desert under field conditions. Deuterium labelling was used to show that water acquired by roots from moist sand in the upper profile was transported through the root system to roots deeper in the profile and released into the dry sand at these depths. Inverse hydraulic lift may serve as an important mechanism to facilitate root growth through the dry soil layers underlaying the upper profile where precipitation penetrates. This may allow roots to reach deep sources of moisture in water-limited ecosystems such as the Kalahari Desert.

Shading reduces exploitation of soil nitrate and phosphate by Agropyron desertorum and Artemisia tridentata from soils with patchy and uniform nutrient distributions.

Shading may both lessen the demand for soil nutrients and also the energy supply for nutrient acquisition. Since root foraging for nutrients in patchy environments can be energy-costly, especially for an immobile nutrient such as phosphate (P), the effects of shading may be most expected in heterogeneous soils. Plant acquisition of nitrate (N) and phosphate from soils with patchy and uniform nutrient distributions was determined in a field study under open sunlight and with shading for two common perennial Great Basin shrub steppe species, Agropyron desertorum and Artemisia tridentata. Partial shading in a pattern which can occur in shrub steppe vegetation significantly decreased plant N and P acquisition from soils both in the patchy and the uniform nutrient treatments. Artemisia was more affected by the shading than was Agropyron. Exploitation of the rather immobile P ion by both species was reduced to a much greater degree by the shading in the patchy distribution treatment than in the uniform nutrient treatment. As expected, plant acquisition of the more mobile N varied little with nutrient distribution treatment for both species and the depression of N acquisition by shading was the same in both nutrient distributions. The effects of shading appeared to have had its primary influence on different components of root foraging in the two species, especially in the nutrient-rich patches. For Agropyron shading primarily affected root proliferation, as indicated by reduced root density in patches. For Artemisia, shading most influenced root physiological uptake capacity and this was most pronounced in the nutrient-rich patches. While aboveground competition for light may generally reduce nutrient acquisition, the effects appear to be most pronounced if root systems of these steppe species are foraging for nutrients such as P in spatially heterogeneous soils.

Light field heterogeneity among tussock grasses: Theoretical considerations of light harvesting and seedling establishment in tussocks and uniform tiller distributions.

Although the tussock growth form of caespitose graminoids is widespread, the effect of this growth form on light interception and carbon gain of tillers has received little attention. Daily incident photosynthetic photon flux density (PFDinc) and carbon gain in monospecific stands of tussock grasses were compared with those of a hypothetical distribution with the equivalent tiller density per total ground area, but evenly distributed rather than clumped in tussocks. This was computed for two tussock grasses Pseudoroegneria spicata (Pursh) A. Löve (bluebunch wheatgrass) and Agropyron desertorum (Fisch, ex Link) Schult. (creasted wheatgrass) at different plant densities. Daily PFDinc and net photosynthesis (A) were greater if tillers were distributed uniformly rather than clumped in tussocks, except when the density of tussocks was so great as to approach a uniform canopy. When tussock density per ground area was low, much of the difference between tussock and uniform tiller densities in PFDinc and A was due to shading within the tussocks; up to 50-60% of the potential carbon gain was lost in A. desertorum due to shading within tussocks. In a matrix of tussocks, the light field for establishing seedlings was very heterogeneous; potential A ranged from 7 to 96% relative to an isolated seedling. The mean of daily PFDinc and A for seedlings in a tussock stand were nearly identical to the values in corresponding stands of uniform tiller distributions. It is hypothesized that the loss of A resulting from clumping tillers into tussocks is offset by benefits of protecting sequestered belowground resources from invasion by seedlings of competitors.

Soil solution phosphate, root uptake kinetics and nutrient acquisition: implications for a patchy soil environment.

The importance of increased root phosphate (P) uptake kinetics, root proliferation and local increases of soil solution P (P1) for P acquisition from fertile soil microsites was explored with a simulation model and calculated uptake was compared with experimental data. Based on the partitioning of added P in microsites to P1 and P adsorbed on soil particles and the results of a dual-isotope-labeling experiment (Caldwell et al. 1991a), acquisition of P from the fertile microsites was some 20 X that of uptake from an equal volume of soil which received only water. Simulations were in general agreement and also showed that elevation of root P uptake kinetics could contribute more to the increased acquisition than did root proliferation under these circumstances. Although increased physiological uptake capacity for P has generally been considered to be of little benefit because of diffusion limitation, in patchy soil environments selective elevation of P uptake kinetics in fertile microsites may be of considerable benefit. These tests were conducted in calcareous soil which releases much less P into the soil solution than do many other soils. In many noncalcareous soils the benefits of selective elevation of root uptake kinetics would likely be greater.

Effectiveness of phosphate acquisition by juvenile cold-desert perennials from different patterns of fertile-soil microsites.

Phosphate uptake was measured for Artemisia tridentata, Agropyron desertorum and Pseudoroegneria spicata, three common perennial North American Great Basin species. Four patterns of nutrient-rich microsites were used in the experiments (different distances, densities and nutrient concentrations) All species were more efficient at taking up P from microsites nearest the plants than from more distant microsites. Artemisia and Agropyron acquired P more rapidly from the distant microsites when there was a larger number of microsites and, therefore, a greater probability of encounter. Uptake from the nearest microsites did not increase after 26 days, while uptake from distant microsites increased and was equal to uptake from the nearest microsites by the end of the experiment. Phosphate uptake was four to five times higher for Artemisia than for Agropyron on a shoot mass basis and seven to eight times greater than for Pseudoroegneria, which reflects species relative growth rates. Differences in shoot dry mass were significant among species, but little evidence was found for interspecific competition. Root density, root dry mass and P uptake in the upper part of the soil mixture was higher for Artemisia than the other species. Phosphate acquisition seems to be influenced by the distance of microsites and their density and the ability of plants to encounter and proliferate absorbing organs in the microsites.

Shading and the capture of localized soil nutrients: nutrient contents, carbohydrates, and root uptake kinetics of a perennial tussock grass.

The ability to exploit spatial and temporal heterogeneity in soil resources can be one factor important to the competitive balance of plants. Competition above-ground may limit selective plant responses to below-ground heterogeneity, since mechanisms such as root proliferation and alterations in uptake kinetics are energy-dependent processes. We studied the effect of shading on the ability of the perennial tussock grassAgropyron desertorum to take up nutrients from enriched soil microsites in two consecutive growing seasons. Roots of unshaded plants selectively increased phosphate uptake capacity in enriched soil microsites (mean increases of up to 73%), but shading eliminated this response. There were no changes in ammonium uptake capacity for roots in control and enriched patches for either shaded or unshaded plants. The 9-day shade treatments significantly reduced total nonstructural carbohydrate (TNC) concentrations for roots in 1990, but had no apparent effect on root carbohydrates in 1991 despite dramatic reductions in shoot TNC and fructan concentrations. Enrichment of the soil patches resulted in significantly greater phosphate concentrations in roots of both shaded and unshaded plants, with less dramatic differences for nitrogen and no changes in potassium concentrations. In many respects the shaded plants did surprisingly well, at least in terms of apparent nutrient acquisition. The effects of aboveground competition on nutrient demand, energy requirements, and belowground processes are discussed for plants exploiting soil resource heterogeneity.

Effect of competition on stable carbon isotope ratios of two tussock grass species.

Previous studies have shown that plant carbon isotope composition varies when plants experience differences in water and nutrient availability. However, none have addressed the effect of root interactions, including competition for these soil resources, on carbon isotope ratios. We studied the effect of interspecific root interactions on the productivity and carbon isotope ratios of two Great Basin tussock grass species (Agropyron desertorum and Pseudoroegneria spicata). We compared grasses grown in mixture with sagebrush (Artemisia tridentara) to grasses in similar mixtures but where root interactions with sagebrush were limited by fiberglass partitions. During both years of the study, tussocks growing in competition with sagebrush produced tissue with more negative δ13C values than grasses experiencing limited root interaction with sagebrush. The magnitude of this difference (0.5 to 0.9%) is similar to that found in other studies when soil fertility and moisture availability were altered.

Rapid physiological adjustment of roots to localized soil enrichment.

SOIL microsites rich in available nutrients are an important source of mineral nutrients for plants in many environments(1-5). Patchiness in nutrient availability below ground is analogous to resource availability in canopy gaps above ground(6). Although the physiological changes occurring in leaves exposed to sun and shade in canopy gaps are well known(7-9), we do not know any studies that show similar physiological changes in roots in enriched soil patches. Here we present evidence of large and rapid increases in the uptake kinetics of plant roots after creating nutrient-rich soil patches in the field. The mean rate of phosphate uptake at a given external phosphate concentration increased by as much as 80% for roots from enriched soil patches compared with roots of control patches treated with distilled water. The changes took place within days of patch treatment. This degree of plasticity was particularly notable for plants growing in soils of very low available phosphorus. These results showing rapid physiological plasticity of roots in fertile soil microsites have important implications for the theory and modelling of nutrient uptake in all soils.

Plant competition for light analyzed with a multispecies canopy model : III. Influence of canopy structure in mixtures and monocultures of wheat and wild oat.

A multispecies canopy photosynthesis simulation model was used to examine the importance of canopy structure in influencing light interception and carbon gain in mixed and pure stands of wheat (Triticum aestivum L.) and wild oat (Avena fatua L.), a common weedy competitor of wheat. In the mixtures, the fraction of the simulated canopy photosynthesis contributed by wheat was found to decline during the growing season and this decline was closely related to reductions in the amount of leaf area in upper canopy layers. For both species in mixture and in monoculture, simulated photosynthesis was greatest in the middle or upper-middle canopy layers and sensitivity analyses revealed that canopy photosynthesis was most sensitive to changes in leaf area and leaf inclination in these layers. Changes in LAI and leaf inclination affected canopy carbon gain differently for mixtures and monocultures, but the responses were not the same for the two species. Results from simulations where the structural characteristics of the two species were substituted indicated that species differences in leaf inclination, sheath area and the fraction of leaf area alive were of minor consequence compared with the differences in total leaf area in influencing relative canopy carbon gain in mixtures. Competition for light in these species mixtures appears to be influenced most by differences in the positioning of leaf area in upper canopy layers which determines, to a great extent, the amount of light intercepted.

Plant competition for light analyzed with a multispecies canopy model : I. Model development and influence of enhanced UV-B conditions on photosynthesis in mixed wheat and wild oat canopies.

Competition for light among species in a mixed canopy can be assessed quantitatively by a simulation model which evaluates the importance of different morphological and photosynthetic characteristics of each species. A model was developed that simulates how the foliage of all species attenuate radiation in the canopy and how much radiation is received by foliage of each species. The model can account for different kinds of foliage (leaf blades, stems, etc.) for each species. The photosynthesis and transpiration for sunlit and shaded foliage of each species is also computed for different layers in the canopy. The model is an extension of previously described single-species canopy photosynthesis simulation models. Model predictions of the fraction of foliage sunlit and interception of light by sunlit and shaded foliage for monoculture and mixed canopies of wheat (Triticum aestivum) and wild oat (Avena fatua) in the field compared very well with measured values. The model was used to calculate light interception and canopy photosynthesis for both species of wheat/wild oat mixtures grown under normal solar and enhanced ultraviolet-B (290-320 nm) radiation (UV-B) in a glasshouse experiment with no root competition. In these experiments, measurements showed that the mixtures receiving enhanced UV-B radiation had a greater proportion of the total foliage area composed of wheat compared to mixtures in the control treatments. The difference in species foliage area and its position in the canopy resulted in a calculated increase in the portion of total canopy radiation interception and photosynthesis by wheat. This, in turn, is consistent with greater canopy biomass of wheat reported in canopies irradiated with supplemental UV-B.

Plant competition for light analyzed with a multispecies canopy model : II. Influence of photosynthetic characteristics on mixtures of wheat and wild oat.

The importance of photosynthetic characteristics such as quantum efficiency or carboxylation efficiency for carbon gain of plants competing for light in dense stands is dependent on several environmental factors and structural features of the canopy. A quantitative analysis of photosynthesis of competing plants in mixed stands of wheat and wild oat (Avena fatua L.), a common weed of wheat, involved measuring photosynthetic parameters of individual leaves at different heights in the canopy throughout the growing season. This information combined with detailed assessments of canopy structure was used with a multispecies canopy model to evaluate the importance of different photosynthetic characteristics for carbon gain in this canopy environment. Independent photosynthesis data sets were used to validate predictions of the model. Carboxylation efficiency (CE) and CO2-and light-saturated photosynthetic capacity (AML) were highly correlated and decreased with depth in the canopy for both species. Quantum efficiency (α) did not tend to decrease with depth in the canopy. Sensitivity analyses with the model for whole-plant carbon gain of each species over entire day periods were conducted. These showed that changes in CE and AML had an influence similar to that of changes in α on carbon gain for both species. This was not necessarily expected from single-leaf photosynthetic behavior in response to changes in CE, AML and α. The influence of α is more pronounced in the lower, more shaded portions of the canopy than are changes in CE and AML. Appreciable differences between the species were apparent for carbon gain under different weather conditions. The differences between the species in carbon gain when in competition for light were associated more with structural features rather than with photosynthetic characteristics.