ALBERT

Notes: Visual observations of smoke dispersion in a wind tunnel and a computational fluid dynamics (CFD) model were used to evaluate methods of improving the performance of Free-Air COT Enrichment (FACE) Systems for field studies of the effects of elevated CO2 on vegetation. A special baffle, named an Enhanced Local Mixing (ELM) system, was observed to increase the turbulence and consequent dilution of fumigant gas in the atmosphere. Modelling results suggest that the ELM design reduces the spatial variation of fumigant gas concentration in small experimental plots. The potential reduction in CO2 use and costs warrants further evaluation under Held conditions.

Notes: We examined whether the exposure of Quercus robur L. to elevated UV-B radiation (280–315 nm) during growth would influence leaf decomposition rate through effects on litter quality. Saplings were exposed for eight months at an outdoor facility in the UK to a 30% elevation above the ambient level of erythemally weighted UV-B radiation under UV-B treatment arrays of fluorescent lamps filtered with cellulose diacetate, which transmitted both UV-B and UV-A (315–400 nm) radiation. Saplings were exposed to elevated UV-A alone under control arrays of lamps filtered with polyester and to ambient radiation under unenergised arrays of lamps. Abscised leaves from saplings were enclosed in 1 mm2 mesh nylon bags, placed in a Quercus–Fraxinus woodland and were sampled at 0.11, 0.53, 1.10 and 1.33 years for dry weight loss, chemical composition and saprotrophic fungal colonization.At abscission, litters from UV-A control arrays had ≈ 7.5% higher lignin/nitrogen ratios than those from UV-B treatment and ambient arrays (P 〈 0.06). Dry weight loss of leaves treated with elevated UV-B radiation during growth was 2.5% and 5% greater than that of leaves from UV-A control arrays at 0.53 and 1.33 years, respectively. Litter samples from UV-B treatment arrays lost more nitrogen and phosphorus than samples from ambient arrays and more carbon than samples from UV-A control arrays. The annual fractional weight loss of litter from UV-B treatment arrays was 8% and 6% greater than that of litter from UV-A control and ambient arrays, respectively. Regression analyses indicated that the increased decomposition rate of UV-B treated litters was associated with enhanced colonization of leaves by basidiomycete fungi, the most active members of the soil fungal community, and that the frequency of these fungi was negatively associated with the initial lignin/nitrogen ratio of leaves.

Notes: The effects of elevated UV-B (280–315 nm) radiation on the long-term decomposition of Quercus robur leaf litter were assessed at an outdoor facility in the UK by exposing saplings to elevated UV-B radiation (corresponding to a 30% increase above the ambient level of erythemally weighted UV-B, equivalent to that resulting from a c. 18% reduction in ozone column) under arrays of cellulose diacetate-filtered fluorescent UV-B lamps that also produced UV-A radiation (315–400 nm). Saplings were also exposed to elevated UV-A radiation alone under arrays of polyester-filtered fluorescent lamps and to ambient solar radiation under arrays of nonenergized lamps. After 8 months of irradiation, abscised leaves were placed into litter bags and allowed to decompose in the litter layer of a mixed deciduous woodland for 4.08 years. The dry weight loss of leaf litter from saplings irradiated with elevated UV-B and UV-A radiation during growth was 17% greater than that of leaf litter irradiated with elevated UV-A radiation alone. Annual fractional weight loss of litter (k), and the estimated time taken for 95% of material to decay (3/k) were respectively increased and decreased by 27% for leaf litter exposed during growth to elevated UV-B and UV-A radiation, relative to that exposed to UV-A alone. The present data corroborate those from a previous study indicating that UV-B radiation applied during growth accelerates the subsequent decomposition of Q. robur leaf litter in soil, but indicate that this effect persists for over four years after abscission.

Notes: Saplings of pedunculate oak (Quercus robur L.) were exposed at an outdoor facility to modulated levels of elevated UV-B radiation (280–315 nm) under treatment arrays of cellulose diacetate-filtered fluorescent lamps which also produced UV-A radiation (315–400 nm). Saplings were also exposed to UV-A radiation alone under control arrays of polyester-filtered lamps and to ambient levels of solar radiation under arrays of unenergized lamps. The UV-B treatment corresponded to a 30% elevation above the ambient level of erythemally weighted UV-B radiation. Sapling growth and the occurrence of associated organisms were examined over two years. In both years, leaves of saplings exposed to UV-B treatment were thicker and smaller in area relative to leaves exposed to ambient and control levels of radiation. UV-B treatment also retarded bud burst at one sampling in the first year of the study. Some responses were recorded which were common to both treatment and control arrays, implying that UV-A radiation, or some other factor associated with energized lamps, was responsible for the observed effects. Saplings under treatment and control arrays were taller in the first year of the study, suffered greater herbivory from chewing insects, and had lower root dry weights and greater insertion heights of secondary branches than saplings exposed to ambient levels of radiation. Exposure of saplings to elevated UV-A radiation alone under control arrays increased estimated leaf volumes in the second year of the study and reduced the number of secondary branches and the total number of branches per sapling after two years, relative to both treatment and ambient arrays. There were no effects of elevated ultraviolet radiation on shoot or total plant weight, root/shoot ratios, stem diameter, the numbers or insertion heights of primary or tertiary branches, total leaf number, timing of leaf fall or frequency of ectomycorrhizas. Our study suggests that any increases in UV-B radiation as a result of stratospheric ozone depletion will influence the growth of Q. robur primarily through effects on leaf morphology.

Notes: It has been suggested that field experiments which increase UV-B irradiation by a fixed amount irrespective of ambient light conditions (‘square-wave’), may overestimate the response of photosynthesis to UV-B irradiation. In this study, pea (Pisum sativum L.) plants were grown in the field and subjected to a modulated 30% increase in ambient UK summer UV-B radiation (weighted with an erythemal action spectrum) and a mild drought treatment. UV-A and ambient UV control treatments were also studied. There were no significant effects of the UV-B treatment on the in situ CO2 assimilation rate throughout the day or on the light-saturated steady-state photosynthesis. This was confirmed by an absence of UV-B effects on the major components contributing to CO2 assimilation; photosystem II electron transport, ribulose 1,5-bisphosphate regeneration, ribulose 1,5-bisphosphate carboxylase/oxygenase carboxylation, and stomatal conductance. In addition to the absence of an effect on photosynthetic activities, UV-B had no significant impact on plant biomass, leaf area or partitioning. UV-B exposure increased leaf flavonoid content. The UV-A treatment had no observable effect on photosynthesis or productivity. Mild drought resulted in reduced biomass, a change in partitioning away from shoots to roots whilst maintaining leaf area, but had no observable effect on photosynthetic competence. No UV-B and drought treatment interactions were observed on photosynthesis or plant biomass. In conclusion, a 30% increase in UV-B had no effects on photosynthetic performance or productivity in well-watered or droughted pea plants in the field.

Notes: The phyllosphere microbial populations inhabiting the needles of three conifer species, Scots pine (Pinus sylvestris L.), Sitka spruce (Picea sitchensis L.) and Norway spruce (Picea abies (L.) Karst.), exposed to SO2 and O3, in an open-air fumigation experiment were analysed over a 3 year period using serial dilution after washing, direct plating and a fluorescein diacetate (FDA) enzyme assay. Total fungal populations ranged from 102 to 105 colonyforming units (CPU) g−1 fresh weight of needles. The dominant fungi isolated from needles varied with tree species and isolation technique; Aureobasidium pullulans (de Bary) Arnaud was most common on Scots pine and Norway spruce and white yeasts on Sitka spruce using the dilution plating method. However, direct plating of needle segments onto culture media indicated that Sclerophoma pythiophila (Corda) Hohnel was dominant on Scots pine and A. pullulans on Sitka and Norway spruce. Green needles of Sitka spruce were found to be endophytically colonized by Rhizosphaera kalkhoffii Bubak, but seldom by Lophodermium piceae (Fuckel) Hohn during extensive sampling in 1990. Statistical analyses revealed significant differences (P〈0.05) between plots in the 3 year mean of the total fungal populations or the fungal biomass (FDA assay) on all three tree species. Differences between plots were also observed for a number of dominant component species. Data were also analysed for treatment effects. A significant effect of SO2 treatment was observed on the total fungal populations on Sitka spruce (P〈0.05) which were reduced markedly by the low-SO2 treatment, while the O3 treatment caused a significant increase in total fungal numbers on Scots pine (P〈0.05). The FDA activity on needles of both Scots pine and Sitka spruce was noticeably higher in the 03-only treatment plot, but the overall O3 effect was not significant. Treatment effects were also detected on the occurrence of component species. The serial dilution method revealed an SO2 effect (P〈0.05) of a reduction in the occurrence of pink yeasts on Sitka spruce and an O3 effect (P〈0.05) of an increase in the occurrence of S. pythiophila on Sitka spruce (P〈0.01) but a decrease of Epicoccum nigrum Link and Cladosporium spp. on Scots pine. The direct-plating method revealed an SO2 effect of an increase in S. pythiophila on Norway spruce (P〈0.05). Ozone treatment caused a significant increase in the isolation of a black strain of A. pullulans on Norway spruce (P〈0.05). Endophytic colonization of Sitka spruce needles by R. kalkhoffii was found to be increased on two occasions by O3 exposure.

Notes: The aim of the Liphook Project was to assess the effects of SO2 and O3, singly and in combination, on coniferous forest ecosystems. More than 4000 trees of Scots pine (Pinus sylvestris), Norway spruce (Picea abies) and Sitka spruce (P. sitchensis) were fumigated for nearly 4 years using an open-air fumigation technique especially developed for the purpose. The technique eliminated artifacts due to chambers and enabled a variety of effects of the pollutant gases on forest ecosystems to be studied. Most symptoms of forest decline did not occur, but each species reacted in a different way to SO2 stress, providing no evidence for universal forest decline symptoms. However, some of the mechanisms hypothesized to underlie forest declines were observed as an effect of SO2 treatment, though others were not, notably any major effect of O3. The results are assessed against proposed regulatory standards (critical loads and levels) for the protection of forest ecosystems against pollution.

Notes: The development, construction and operation of an open-air fumigation system for exposing young forest trees to controlled concentrations of sulphur dioxide and ozone is described. A computer simulation of gas dispersion was used to design an array of pipework sources which minimized spatial variability in exposure concentrations. Five fumigation plots were constructed using the design and were used to fumigate trees during a 7 year study known as the Liphook Forest Fumigation Project. Rates of gas release were controlled by a small computer to follow predetermined patterns of sulphur dioxide concentration and to maintain an elevation above ambient ozone concentration. Effective control of exposure was demonstrated, and examples of experimentally produced concentration frequency distributions are provided. The advantages and shortcomings of the system are discussed with recommendations for future improvements.

Notes: Foliar elements were analysed in Scots pine, Sitka spruce and Norway spruce over a 6 year period before and during continuous exposure to SO2 and O3 in an open-air fumigation experiment. Sulphur dioxide treatment elevated foliar sulphur concentration in all species, and there were increases in foliar nitrogen in the two spruce species but not in pine. The concentrations of cations were frequently increased by SO2 treatment, but there was no correlation between the sulphur concentration of needles and their total cation charge. SO2-related elevations of foliar magnesium were correlated with the concentration of this element in soil solution, but the mechanism by which other cations were enhanced remains unclear. The only consistent effects on nutrient ratios were for SO2 treatments to increase sulphur/cation ratios.

Notes: Levels of the stilbene glucosides astringin and isorhapontin, the main constitutive antifungal compounds in bark tissues of spruce trees, were not altered in young Picea sitchensis (Bong.) Carr. or Picea abies (L.) Karst. trees exposed to sulphur dioxide and ozone in the Liphook Forest Fumigation Project. These trees had received computer-controlled fumigation treatments with two levels of SO2 (long-term means 13 and 22nmol mol−1) or one level of O3 (1–3.times ambient), or a combination of these treatments, from spring until December.Resistance of bark tissues from these trees to colonization by the root- and butt-rot pathogen Heterobasidion annosum (Fr.) Bref., assessed in vitro using excised stem lengths, was not significantly altered in fumigated plants compared with those exposed to ambient pollutant levels only. This study therefore provided no evidence for altered disease resistance in P. abies and P. sitchensis trees exposed to SO2 and O3.