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Combining hydraulic- and carbon-related measurements helps to understand drought-induced plant mortality. Here, we investigated the role that plant respiration ( R) plays in determining carbon budgets under drought., We measured the hydraulic conductivity of stems and roots, and gas exchange and nonstructural carbohydrate ( NSC) concentrations of leaves, stems and roots of seedlings of two resprouting species exposed to drought or well-watered conditions: Ulmus minor (riparian tree) and Quercus ilex (dryland tree)., With increasing water stress (occurring more rapidly in larger U. minor), declines in leaf, stem and root R were less pronounced than that in leaf net photosynthetic CO2 uptake ( Pn). Daytime whole-plant carbon gain was negative below −4 and −6 MPa midday xylem water potential in U. minor and Q. ilex, respectively. Relative to controls, seedlings exhibiting shoot dieback suffered c. 80% loss of hydraulic conductivity in both species, and reductions in NSC concentrations in U. minor. Higher drought-induced depletion of NSC reserves in U. minor was related to higher plant R, faster stomatal closure, and premature leaf-shedding., Differences in drought resistance relied on the ability to maintain hydraulic conductivity during drought, rather than tolerating conductivity loss. Root hydraulic failure elicited shoot dieback and precluded resprouting without root NSC reserves being apparently limiting for R. [ABSTRACT FROM AUTHOR]

Soil-borne pathogens are a key component of the belowground community because of the significance of their ecological and socio-economic impacts. However, very little is known about the complexity of their distribution patterns in natural systems. Here, we explored the patterns, causes and ecological consequences of spatial variability in pathogen abundance in Mediterranean forests affected by oak decline., We used spatially explicit neighborhood models to predict the abundance of soil-borne pathogen species ( Phytophthora cinnamomi, Pythium spiculum and Pythium spp.) as a function of local abiotic conditions (soil texture) and the characteristics of the tree and shrub neighborhoods (species composition, size and health status). The implications of pathogen abundance for tree seedling performance were explored by conducting a sowing experiment in the same locations in which pathogen abundance was quantified., Pathogen abundance in the forest soil was not randomly distributed, but exhibited spatially predictable patterns influenced by both abiotic and, particularly, biotic factors (tree and shrub species). Pathogen abundance reduced seedling emergence and survival, but not in all sites or tree species., Our findings suggest that heterogeneous spatial patterns of pathogen abundance at fine spatial scale can be important for the dynamics and restoration of declining Mediterranean forests. [ABSTRACT FROM AUTHOR]

Two-year-old seedlings of Sitka spruce were exposed to 70 nI I-1 ozone or to filtered air over three successive summers in outdoor large-scale fumigation chambers (Solardomes). Seven months after the last period of exposure to the pollutant and just prior to budburst, upper-surface chlorisis affecting only the older needles of ozone-exposed trees was observed. In many respects, the symptoms appeared to be similar to those characteristic of type 1 spruce damage occurring in parts of mainland Europe. Chlorophyll pigments were reduced in the ozone-exposed older foliage, but no change in the ratio of chlorophylls to carotene was observed. The content of ascorbic acid was clearly related to the amount of foliar damage observed on the trees exposed to ozone and the largest increases were seen in those trees which were most visibly damaged. Although none of the foliage examined was deficient in any of the nutrient cations which were measured, the concentration of Mg in the older needles was significantly reduced by exposure to ozone, irrespective of damage symptoms. Exposure to ozone also resulted in increases in the ratios of K : Mg and Ca: Mg. In the older needles, leaching of Mg-2 and K- by 0.5 mM H2SO4, pH 3.0, was enhanced by prior exposure to ozone, but the amounts removed were small (< 6%). It is suggested that long-term exposure to ozone has a cumulative effect on plant tissue and that the observed chlorosis was the result of accelerated senescence. [ABSTRACT FROM AUTHOR]

Chlorophyll fluorescence decay profiles have been measured in the wavelength range 680-720 nm for needles from Norway spruce [Picea abies (L.) Karst] trees which have been exposed to ozone. All profiles required three exponential components of lifetime 100-150 ps, 400-600 ps and 3.5-5.0 ns to fit the experimental data. Compared to control samples, the ozone-treated needles exhibited a greater amount of the longest-lived chlorophyll fluorescence and a redistribution in intensity for both the other components from 720+ to 690 nm. These observations are interpreted in terms of disruption of energy transfer and break-up of light-harvesting complexes on exposure to ozone. The potential for use of the technique in monitoring forest decline is discussed. [ABSTRACT FROM AUTHOR]

The present study was performed on mature Norway spruce trees (Picea abies L.) with needle losses in the range of 25-35%. The trees were growing at three altitudes (870, 1270, and 1700 m above sea level) in a mountain forest in the Calcareous Alps of Bavaria. The objectives were to assess the role of climate and pollution in addition to nutrient disorders for damage to spruce trees. During the growing season the average temperature was up to 5 °C lower and the average ozone concentration was about twofold higher at the high altitude in comparison to the low altitude (13 and 18 °C, and 60 and 35 nl l-1 ozone, respectively); the average NOx and SO2 concentrations did not exceed 5 nl l-1. Foliar analysis of pollutants did not indicate critical levels of Pb, Cd, Cl, F and S. Biometrical parameters, foliar element composition, pigment and protein content of the needles were determined during four seasons. Average dry: fresh weight ratios, specific needle weights, projected needle areas, soluble protein and carotenoid contents were similar within the 20 % range for needles from all three altitudes. Foliar analysis of nutrients indicated a low nutrient status especially for nitrogen, and showed a phosphorus deficiency in needles from the three sites. The chlorophyll content of the needles decreased with increasing altitude. Since nutrient imbalances were greatest in needles from the lowest site, it was concluded that the greater damage to spruce trees and lower chlorophyll content of the needles at altitudes > 1000 m above sea level were not caused by nutrient disorders. [ABSTRACT FROM AUTHOR]

The death of many mature red spruce (Picea rubens Sarg.) trees in the northeastern United States has sparked controversy over the roles natural factors and air pollutants play. The goal of this work was to investigate the effects of long-term ozone exposures on development of injury and, in particular, the expression of winter injury in red spruce. Red spruce seedlings were exposed to elevated levels of ozone during two growing seasons, starting just after budbreak, and continuing into the period of early frosts. Trees spent both winter periods under ambient conditions of light and temperature. No visible lesions on foliage that could be attributed to ozone developed during the growing seasons. Yellow and brown needles appeared each winter, with much more injury in the second year. However, in the spring after two seasons of ozone exposure, there was no significant effect of ozone on the overall appearance of the entire population of seedlings. Samples were taken for histology and ultrastructure approximately monthly. Ozone treatments increased mesophyll cell disruption after frosts in early winter in the first year, but not in the second. There was no correlation between histological injury in the autumn and visible symptoms in the spring, indicating that there may be different aspects of the winter injury syndrome. In the second summer from 20 to 40°, of all mesophyll cells were damaged. This damage was not related to ozone treatment, and probably resulted from high summer temperatures. [ABSTRACT FROM AUTHOR]

Three-year-old red spruce seedlings were exposed to various concentrations of ozone, from 0.4 to 3 times ambient, for two consecutive growing seasons in open-top chambers in Ithaca, NY, USA. Exposures, which varied with changes in daylength, were from 30 May to 14 December l987 and 1 June to I December 1988. Seedling biomass, foliar pigments and carbohydrate content of roots and shoots were assessed once or twice per month. Tree biomass increased through the summer and declined slightly in late autumn, regardless of ozone treatment. Biomass and contents of sugar and starch of 1988 fixed growth declined linearly with exposure to increasing levels of ozone. Rates of decline in total shoot biomass and old shoot biomass increased linearly with exposure to increasing concentrations of ozone, but tree height was not affected. Seasonal means of starch content of roots decreased linearly with increasing doses of ozone, but in contrast seasonal mean sugar content and rate of sugar accumulation of roots increased linearly. Chlorophyll a and chlorophyll B contents were not affected by ozone. These data are consistent with the concept that ozone stress is cumulative, since effects on biomass and carbohydrate content of roots and shoots were not detected after the first year of exposure, but were detected after the second year of treatments. Furthermore, biomass was reduced by ozone only in tissues that were initiated during the first year of exposures and developed during the second year of exposures (1988 fixed growth). [ABSTRACT FROM AUTHOR]

Loblolly pine (Pinus taeda L.) seedlings from two families differing in ozone sensitivity, were exposed to ozone and acidic precipitation in modified open-top chambers, beginning in May of 1988. Visible injury (bleached needle tips) occurred after a late-season frost in April, 1989 on newly emerging (length &les; 5 cm) needles. After one week, needle tips turned brown and necrotic. Incidence of injury was recorded in April. May and June, 1989. Injury only occurred on trees exposed to ozone at above-ambient concentrations (1.7 and 2.5 × ambient), and was significantly greater in the ozone-sensitive family (GAKR 15-91) suggesting that this response is under some genetic control. Incidence of injury decreased with time and was more noticeable in the ozone-tolerant family (GAKR 15-23). There was no effect of acid rain, or interaction between ozone and acid rain, revealed by visible injury. In addition, experiments were conducted to determine if ozone and/or freezing temperatures enhanced electrolyte leakage. There were no differences observed in diffusate conductivity among treatments. These findings suggest that the observed injury was due to photo-oxidation induced by an ozone-low temperature interaction, and represent the first report of this type of injury in loblolly pine. [ABSTRACT FROM AUTHOR]

Three-year old Norway spruce [Picea abies (L.) Karst.] trees from two clones (4076 and 4102) of a medium altitude southern German provenance were transplanted in May 1986 into an acid brown earth soil. During the summers of 1987 and 1988 the trees were exposed in large-scale fumigation chambers (solardomes) to ozone at 100 or 20 nl l-1 (control). Acid (pH 3.6) or control mist (pH 5.5) was applied daily to trees. The wettability of current and 1-year-old needles was assessed in November 1988 by measuring the contact angle of water droplets placed on needles, and the amount of surface wax was determined. The contact angle of water droplets on current and 1-year-old needles was significantly reduced by 100 nl l-1 PPB ozone and/or acid mist, indicating that the wettability had increased, but the combined effect of ozone and acid mist was no more than additive. Exposure to the pollutants resulted in a slight decrease in the quantity of surface wax, but the effect was not significant. Ozone and acid mist treatment also increased the projected area of needles relative to their dry weight. The possible involvement of such effects in forest decline are discussed. [ABSTRACT FROM AUTHOR]

Two-year-old red spruce seedlings [Picea rubens Sarg. syn. P. rubra Du Roi) Link] were subjected to 6 simulated acid mist treatments (pH 2.5, 2.7, 3.0, 3.5, 4.0, 5.0) in a replicated experiment using open-top chambers. Acid mist solutions containing equimolar (NH4)2SO1 and HNO3 were applied twice weekly for 22 weeks, each application being equivalent to 2 mm of precipitation. Visible symptoms of foliar damage were observed on the 3 most acidic mist treatments (pH 2.5, 2.7, 3.0). The inputs of nitrogen, sulphur and acidity in the most acidic treatment were 55. 42, 1.3 kg ha¹ respectively, over a 10 week period. The plants subjected to the pH 2.5 treatment were found to be most severely damaged with approximately 40% foliar necrosis after 10 weeks of treatment. On approximately 80% of seedlings, necrosis was confined to current year needles only. These damaged needles were initially light brown or light orange in colour turning a deeper red 3 to 5 weeks after initial necrosis. Percentage foliar damage was linearly related to concentration (of NH4-, NO3, SO1² and H-) with 62% foliar damage in the pH 2.5 treatment after a 22-week period. Spray application stopped in December 1987. Observations during the following spring showed that the pH 2.5 and pH 3 treatments induced earlier flushing, requiring 60 day °C less thermal time than the pH 5.0 treatment. In 1988, this decrease in thermal requirement was equivalent to flushing 11 days earlier. There was no evidence of acid mist treatments inducing bud mortality. [ABSTRACT FROM AUTHOR]

Seedlings of red spruce [Picea rubens Sarg. syn. P. rubra (Du Roi) Link] were exposed to mists containing equimolar (NH4)2 SO4 and HNO3 at pHs of 2.5, 2.7, 3.0, 3.5, 4.0 or 5.0. The mists were applied twice each week, amounting to 2 mm precipitation equivalent on each occasion, between July and December, to open-top chambers supplied with charcoal-filtered air. Frost hardiness of shoots excised from seedlings was determined on 6 occasions starting on 21 September, and was found to be strongly influenced by acid mist treatments, seedlings subject to the most acidic mists being the least frost hardy. On 21 September when the first sample was taken the lethal temperature for killing 50% of shoots (LT50) was - 11°C for the least acidic (pH 5.0) mist and -7 °C for the most acidic (pH 2.5). By 19 October, the LT50s of pH 5.0 and pH 2.5 mists were - 27 and - 15 °C respectively. All intermediate treatments ranked according to treatment concentration, with the smaller concentrations causing lower LT50 values. The treatment at pH 30 provided S and N inputs to the seedlings similar to those experienced by red spruce at elevations of about 1000 m in the southern Appalachians. At pH 3.0, the frost hardiness LT10 during October was typically 8 °C higher than the pH 5.0 treatment, leading to a significant increase in the probability of frost damage at the LT10 level in an average October. The proximity of minimum night temperatures during September to December to the LT10 temperatures of red spruce shoots receiving large inputs of SO4&sup2-;, NO3-, NH4+ and H+ suggests that decreases in frost hardiness caused by intercepted cloud water containing large concentrations of these ions may play a significant part in the observed decline at mountain-top locations. [ABSTRACT FROM AUTHOR]

Two-year-old red spruce seedlings were exposed to various levels of ozone, from 0.4 to 3 times ambient levels, in open-top chambers in Ithaca, NY, USA. Exposures, which varied with changes in day length, commenced on May 30, 1987 and continued until December 14, 1987. Seedling biomass, carbohydrate contents, pigment contents, and rate of electron transport were assessed twice monthly during the fumigation period. Orthogonal quadratic or cubic polynomials were used to model the response through time of each variable measured. A One-way analysis of variance model was fitted to every regression coefficient in each polynomial model to test for ozone effects on seasonal physiological patterns. Ozone did not influence growth, foliar pigment content, foliar starch content, root carbohydrate content, or rate of electron transport. The Seasonal change of needle raffinose content differed between seedlings exposed to low (0.4 × , 1 × ) and high (2× 3 × ) ozone levels. There was also a trend towards reduced total soluble sugar content in foliage during late autumn in higher ozone treatments. [ABSTRACT FROM AUTHOR]

Scanning electron microscopy was used to investigate the effects of O3 fumigation (70 d of 240 μg O3 m-3 for 6 h d-1, during a 12 h photoperiod) on the condition of the epicuticular wax that fills and surrounds the stomatal antechamber of the needles of seven clones of Norway spruce (Picea abies (L.) Karst.). The rate of change of the physical structure of the wax differed between clones, but in all cases O3 accelerated the process on current and previous year's needles. The change consisted of a transition from a crystalline array of fine tubes to an amorphous, 'melted' layer with no visible structure. This resulted in a higher frequency of stomata occluded by an apparently continuous plug. In four clones, O3 fumigation resulted in 70 - 80% of the stomata on previous year's needles being occluded, in contrast to 7- 28% in charcoal-filtered air. Five of the clones showed a greater effect of O3 on the wax of the previous year's needles than on current needles. The physiological consequences of such O3-induced effects are discussed. Premature wax degradation induced by O3 and/or acid mist may have contributed to the decline of spruce in high-altitude forests of continental Europe. [ABSTRACT FROM AUTHOR]

Three-year-old clonal saplings of Picea abies (L.) Karst, were exposed in chambers to 240 μg O3 m³ for 70 d, 6 h d-1 and then hardened for 14 d at 4°C day/night. The winter hardiness of trees was tested by subjecting excised shoots, bearing previous and current year's needles, to night temperatures of -6, -9, - 12, - 15 and -18 °C in a computer-controlled freezer. No visible injury was observed during the fumigation, hardening period or on the current ear's needles of any of the clones. However, during post-freezing recovery in a mist spray unit four of the clones exposed to ozone developed extensive visible injury on the previous year's needles, in the form of severe, uniform necrosis. The affected needles began to fall prematurely. Induced chlorophyll fluorescence was significantly reduced by O3 in two of the clones that showed post-freezing injury. This suggested that O3-induced membrane injury increased the sensitivity of needles to freezing. One clone (no. 4076), which exhibited no effect of ozone on fluorescence kinetics, showed severe ozone-induced injury of the previous year's needles, developing about 40 d after fumigation independent of temperature. Previous year's needles of O3-treated plants of this clone showed significantly greater rates of cuticular transpiration, so the O3 may have induced gradual desiccation Of the needles. The restriction of damage to the older needles of certain clones of Norway spruce is consistent with symptoms of forest decline affecting high-altitude forests on acid soils. Results indicate that O3 predisposes trees to freezing injury, winter desiccation and drought, and it is proposed that such interactions are important factors contributing to the decline of high-altitude forests of Norway spruce in Europe. [ABSTRACT FROM AUTHOR]

The natural decline of a virgin high-altitude Pinus sylvestris forest during the Little Ice Age (approximately AD 1300-1850) was studied in the Swedish Scandes. Methods included 14C and cross-dating of wood remnants, soil analyses, year-ring chronologies and age structures. A closed pine forest started a gradual decline in the 11th century or somewhat later, which proceeded until the mid-19th century. Long-term failure of regeneration and premature death of trees are postulated to have caused the forest dieback, mainly owing to a negative shift of the carbon budget, including weather-induced loss of photosynthetic tissue and root disfunction. Temperature measurements on the study site stressed its current marginality. Especially thermal conditions in and close to the ground were deduced to be decisive. The decline of the forest is considered to indicate a long-term lowering during the Little Ice Age of the summer (June-September) mean temperature by 1 °C or more, compared with the 1931-1960 mean. [ABSTRACT FROM AUTHOR]

Three-year-old clonal saplings of Picea abies L. Karst. were fumigated for 60 d during the summer of 1985 with four different levels of O[sub3]. Visible injury (in the form of severe, uniform brown necrosis and shedding of affected needles) occurred after a frost in November on the older needles of three of the clones which had received over 200 μg m[sup-3] of O[sub3] during the summer, 47 d previously. No visible injury occurred during the fumigation period or on the current (1985) year needles. It is suggested that the results provide preliminary evidence for an enhancement of frost sensitivity by O[sub3] which may be significant in the current wave of forest decline affecting high-altitude forests in central Europe. [ABSTRACT FROM AUTHOR]