Tree- and needle-age-dependent variations in antioxidants and photoprotective pigments in Norway spruce needles at the alpine timberline

To cope with environmental stress, plants are equipped with antioxidative (e.g., ascorbate, glutathione and alpha-tocopherol) and photoprotective (e.g., xanthophyll cycle pigments) defense systems. We investigated the defense capacities of three tree age classes (mature, sapling and seedling) of Norway spruce (Picea abies (L.) Karst.) at a field site near the timberline. Biochemical data were expressed on both a needle dry mass and a surface area basis. Compared with current-year needles, previous-year needles contained higher mass- and area-based concentrations of chlorophylls and alpha-tocopherol, and a larger xanthophyll cycle pool that was in a more epoxidized state. Total glutathione concentration was lower, the glutathione pool was more reduced and the ascorbate pool was more oxidized in previous-year needles than in current-year needles. Needle concentrations of glutathione and alpha-tocopherol increased and chlorophyll concentration decreased with increasing tree age when expressed on a surface area basis. On a dry mass basis, these trends were reversed or nonexistent. The ascorbate pool was more reduced and the glutathione pool was more oxidized in needles of mature trees than in needles of saplings and seedlings. The proportion of protective xanthophyll cycle pigments decreased and the de-epoxidation state increased with increasing tree age. We conclude that tree age and the basis of expression of antioxidant concentration--surface area or dry mass--are important in scaling from seedlings to large trees.     

The effect of seaweed concentrate on the growth of Pinus pinea seedlings

Seedlings of Pinus pinea L. growing in plastic containers were treated with seaweed concentrate (SWC). Different concentrations of SWC were applied, 0 to 3 times, to the roots or shoots of the seedlings. Shoot application increased plant weight mainly by increasing shoot growth. This was manifested as increased shoot length and weight and a decrease in the root/shoot ratio. Root drenches did not change the total plant weight but it accelerated root growth and increased lateral root dry weight. Root growth capacity (RGC) tests for both shoot and root applications indicated an increase in root length and some increases in root number when applied as a root drench. This study indicates that root application of SWC improved seedling quality and increased the ability of seedlings to survive transplanting into pots.Abbreviations GC-MS Gas Chromatography-Mass Spectrometry - RGC root growth capacity - SWC seaweed concentrate     

Nitrogen availability modifies the ozone responses of Scots pine seedlings exposed in an open-field system

Three-year-old Scots pine (Pinus sylvestris L.) seedlings were exposed to either ambient or elevated (1.5-1.6 x ambient) ozone concentration ([O3]) for three growing seasons in an open-field fumigation facility where they were irrigated during the growing season with a nutrient solution providing nitrogen (N) at 70 (LN treatment), 100 (control) or 150% (HN treatment) of the optimum supply rate. Treatment effects were most evident during the third year of exposure, when the ambient [O3] + HN treatment enhanced whole-plant biomass, root/shoot dry weight ratio, needle pigment concentrations and the number of chloroplast plastoglobuli in the mesophyll cells in current-year (C) needles, whereas it reduced starch accumulation in C needles and abscission of 2-year-old (C+2) needles. In the control fertilization, 3 years of exposure to elevated [O3] decreased stem-base diameter and increased K concentration and electron density of chloroplast stroma in C needles. Plants in the HN treatment exposed for 3 years to elevated [O3] had significantly lower heights, current-year main shoot length and root/shoot dry mass ratio than control plants, and increased abscission of C+2 needles. In contrast, O3-induced changes in the ultrastructure of mesophyll cells were most evident in seedlings grown for 3 years in the LN treatment. We conclude that, in Scots pine, a relatively O3-tolerant species, chronic O3 exposure leads to cumulative growth reduction, increased needle abscission and changes in carbon allocation that are strongly influenced by plant N availability.     

Influence of nitrogen and phosphorous availability and ozone stress on Norway spruce seedlings

Four-year-old Norway spruce (Picea abies L. (Karst.)) seedlings were exposed to ambient and elevated (1.5 x ambient in 1997 and 1.6 x ambient in 1998) ozone concentrations [O3] and three nitrogen (N) and two phosphorus (P) availabilities: "optimal" values (control); 70% of the control N and P values (LN and LP); and 150% of the control N value (HN). Treatments were applied in an open-field ozone fumigation facility during the 1997 and 1998 growing seasons. Effects on growth, mineral and pigment concentrations, stomatal conductance and ultrastructure of needles were studied. The HN treatment increased growth significantly, whereas elevated [O3] had a slight or variable impact on growth and biomass allocation in all N treatments. Although there were no significant effects of the LP treatment on plant growth during the second year, there was a reduction in 1-year-old shoot dry mass in the elevated O3 + LP treatment at the end of the experiment. There were no significant treatment effects on mineral concentrations of current-year and 1-year-old needles at the final harvest. In response to the HN treatment, chlorophyll a and b and carotenoid concentrations increased significantly in current-year needles. Chlorophyll a/b ratio decreased in response to elevated [O3] alone, but increased in seedlings in the O(3) + LP treatment. Stomatal conductance of current-year needles decreased with increasing N availability, but increased in response to elevated [O3]. However, the O3-induced increase in stomatal conductance was less in the LN and LP treatments than in the control treatment. In chloroplasts of current-year needles, increased N availability decreased mean starch grain area, but increased the number of plastoglobuli. We conclude that Norway spruce seedlings are relatively tolerant to slightly elevated [O3], and that nitrogen and phosphorus imbalances do not greatly affect the influence of O3 on this species when the exposure lasts for two growing seasons or less.     

Effects of soil moisture manipulations on fine root dynamics in a mature balsam fir (Abies balsamea L. Mill.) forest

We tested the hypothesis that moisture stress affects fine root dynamics during and after the stress. To this end, we investigated the effects of soil moisture on annual and seasonal fine root production and mortality over 4 years in a mature balsam fir (Abies balsamea L. Mill.) stand using a minirhizotron and soil coring. Droughting and irrigating treatments were imposed for 17 weeks during the third year of the study, and post-treatment recovery was measured during the fourth year. Monthly fine root production was often reduced by low soil water content (SWC) during July-September in the pre-treatment years and by imposed drought. Irrigation resulted in higher summer fine root production than in pre-treatment years. In the recovery year, increased fine root production was observed in the previously droughted plots despite low SWC in August and September. Droughting decreased year-end fine root biomass in the treatment year, but biomass returned to pre-treatment levels during the recovery year. Droughting and irrigating did not affect foliage production during the treatment and recovery years. Our results suggest that for balsam fir, establishment and maintenance of a functional balance between foliage and fine root biomass, with respect to moisture supply and demand, can depend on fine root dynamics occurring over more than one growing season. In addition, our findings provided insights into tree growth responses to interannual variation in moisture supply.     

Genotypic variation in growth and physiological responses of Finnish hybrid aspen (Populus tremuloides x P. tremula) to elevated tropospheric ozone concentration

Saplings of six Finnish hybrid aspen (Populus tremuloides Michx. x P. tremula L.) clones were exposed to 0, 50, 100 and 150 ppb ozone (O3) for 32 days in a chamber experiment to determine differences in O3 sensitivity among genotypes. Based on the chamber experiment, three clones with intermediate sensitivity to O3 were selected for a free-air O3 enrichment experiment in which plants were exposed for 2 months to either ambient air (control) or air containing 1.3 x the ambient O3 concentration. We measured stem height and radial growth, number of leaves, dry mass and relative growth rate of leaves, stem and roots, visible leaf injuries, net photosynthesis and stomatal conductance of the clones. There was high clonal variation in susceptibility to O3 in the chamber experiment, indicated by foliar injuries and differential reductions in growth and net photosynthesis. In the free-air O3 enrichment experiment, ozone caused a shift in resource allocation toward stem height growth, thereby altering the shoot to root balance. In both experiments, low O3 concentrations tended to stimulate growth of most clones, whereas 100 and 150 ppb O3 in the chamber experiment impaired growth of most clones. However, growth of the most O3-tolerant clone was not significantly affected by any O3 treatment.     

Field performance of Quercus petraea seedlings grown under competitive conditions: influence of prior undercutting in the seedbed

Quercus petraea was undercut according to the following procedure: undercutting first year once (July, September, November), twice (July and November), or undercutting in two consecutive years (September of year 1 and either July, September or November of year 2). Undercutting decreased height and dry weight of the seedlings compared to uncut control seedlings. Undercutting in September and July in two consecutive years increased the number of first-order lateral roots. Field performance was evaluated by transplanting under two conditions, (a) competition with a mixture of grasses with no irrigation or fertilisation and (b) standard conditions with fertilisation and irrigation first year and no competition with grass. After two growing seasons under competitive conditions, seedlings undercut in September of year 1 had a significantly higher dry weight compared with that of uncut control seedlings. Numbers of lateral roots and field performance under competitive conditions was related when numbers of lateral roots were small, but not when high. Competitive conditions had limited effect on root growth after one growing season, whereas shoot growth was reduced. After two growing seasons both shoot and root growth were reduced in all treatments compared with standard conditions.     

Enhanced photo- and antioxidative protection,and hydrogen peroxide accumulation in drought-stressed Cistus clusii and Cistus albidus plants

Mechanisms of drought stress resistance were studied in Cistus clusii Dunal and Cistus albidus L., two native Mediterranean shrubs that can withstand severe summer drought. While water deficit, solar radiation and temperature increased from winter to summer in the field, C. clusii and C. albidus reduced leaf area, increased root mass per leaf area, and showed diurnal changes in stomatal conductance to minimize water loss. In both species, the consequent reductions in CO2 assimilation were accompanied by reduced efficiency of photosystem II photochemistry, and protection against stress was afforded by enhanced de-epoxidation of violaxanthin in the xanthophyll cycle and increases in alpha-tocopherol and beta-carotene. In addition, hydrogen peroxide (H2O2) accumulation was observed in mesophyll cell walls of both species during the first stages of drought, although no accumulation of H2O2 was observed in chloroplasts or other organelles during the study. Despite these common responses, C. albidus and C. clusii differed in the extent of photo- and antioxidative protection. In response to drought, C. clusii showed a higher de-epoxidation state of the xanthophyll cycle and higher alpha-tocopherol and beta-carotene concentrations than C. albidus. We conclude that several structural and biochemical mechanisms underlie stress resistance in C. clusii and C. albidus, and are indicative of the different degrees of stress resistance of these shrubs.     

Germination and seedling growth of Quercus (section Erythrobalanus) across openings in a mixed-deciduous forest of southern New England, USA

In this study three species of the genus Quercus section Erythrobalanus (Quercus coccinea, Quercus rubra, Quercus velutina) were investigated. All occur together as canopy trees in forests of southern New England. Acorns of each Quercus species were planted in plots located in five zones that represent a range of forest gap/canopy conditions that can occur within a southern New England forest. These five zones were demarcated adjacent to and across large openings of two physiographic sites—valley and ridgetop. Experiments were designed to monitor germination and initial growth of seedlings for the first three growing seasons. During the start of the first growing season germination was monitored. At the end of the first growing season measurements of height and number of flushes were taken and destructive samples of seedlings made for dry mass of root, stem and leaves. At the end of the third growing season height was recorded for surviving seedlings. Comparisons were made of germination and growth of seedlings located in the different gap/canopy conditions.

Results demonstrated clear differences in patterns of germination and early growth among species and among gap/canopy conditions of the sites. All species showed an increased lag in germination with reduced amounts of light. Highest growth and flushing rate were in the center conditions of the openings for all species during the first growing season. Quercus rubra had the greatest height growth the first growing season but a lower number of flushes than Q. velutina and Q. coccinea. In comparison with the other species, Q. rubra had the greatest total dry mass in most gap/canopy conditions after the first growing season. However, Q. velutina had the greatest total dry mass in the center of the ridgetop opening. In almost all gap/canopy conditions Q. velutina had greater proportions of dry mass allocated to roots compared with the other species.

After 3 years, greatest height growth in any of the gap/canopy conditions was recorded for all three species in the center of the valley site. Under this condition Q. rubra had significantly greater growth than Q. velutina and Q. coccinea. Quercus rubra also had significantly greater height growth and survival beneath the canopy conditions of the valley site than the other species. On the ridgetop site regeneration failed to establish beneath canopy conditions that provided low amounts of light. Quercus velutina showed greatest height growth after three years in the center and edge conditions of the ridgetop opening compared with the other species. Environmental influences that determine species germination and growth performance are suggested.     

Determination of drought tolerance using root activities in Robinia pseudoacada ''''Idaho'''' transformed with mtl-D gene

Idaho locust (Robinia pseudoacacia 'Idaho') is an exotic multi-purpose tree used in landscaping, soil and water conservation, fodder sources and others. To improve its drought tolerance for reclaiming arid land, five lines of transformed mtl-D gene, as osmotic regulator in plant cells, have been selected and managed to determine their drought tolerance under experimental conditions. Qualitative and quantitative variables of transformed plants were studied. The critical value of drought tolerance was determined by detecting the 2,3,5-triphenyl tetrazolium chloride (TTC) reductants in roots and soil water content (SWC). The critical value for drought tolerance was SWC 6% while for the control plants the critical SWC was 8%; a moderate level of SWC is 13% and the highest SWC for plant endurance was 18%. The method proved to be reliable and sensitive in the evaluation of drought tolerance for forest trees.     

Determination of drought tolerance using root activities in Robinia pseudoacacia ''Idaho'' transformed with mtl-D gene

Idaho locust (Robinia pseudoacacia 'Idaho') is an exotic multi-purpose tree used in landscaping, soil and water conservation, fodder sources and others. To improve its drought tolerance for reclaiming arid land, five lines of transformed mtl-D gene, as osmotic regulator in plant cells, have been selected and managed to determine their drought tolerance under experimental conditions.Qualitative and quantitative variables of transformed plants were studied. The critical value of drought tolerance was determined by detecting the 2,3,5-triphenyl tetrazolium chloride (TTC) reductants in roots and soil water content (SWC). The critical value for drought tolerance was SWC 6% while for the control plants the critical SWC was 8%; a moderate level of SWC is 13% and the highest SWC for plant endurance was 18%. The method proved to be reliable and sensitive in the evaluation of drought tolerance for forest trees.     

Silver birch and climate change: variable growth and carbon allocation responses to elevated concentrations of carbon dioxide and ozone

We studied the effects of elevated concentrations of carbon dioxide ([CO2]) and ozone ([O3]) on growth, biomass allocation and leaf area of field-grown O3-tolerant (Clone 4) and O3-sensitive clones (Clone 80) of European silver birch (Betula pendula Roth) trees during 1999-2001. Seven-year-old trees of Clones 4 and 80 growing outside in open-top chambers were exposed for 3 years to the following treatments: outside control (OC); chamber control (CC); 2 x ambient [CO2] (EC); 2 x ambient [O3] (EO); and 2 x ambient [CO2] + 2 x ambient [O3] (EC+EO). When the results for the two clones were analyzed together, elevated [CO2] increased tree growth and biomass, but had no effect on biomass allocation. Total leaf area increased and leaf abscission was delayed in response to elevated [CO2]. Elevated [O3] decreased dry mass of roots and branches and mean leaf size and induced earlier leaf abscission in the autumn; otherwise, the effects of elevated [O3] were small across the clones. However, there were significant interactions between elevated [CO2] and elevated [O3]. When results for the clones were analyzed separately, stem diameter, volume growth and total biomass of Clone 80 were increased by elevated [CO2] and the stimulatory effects of elevated [CO2] on stem volume growth and total leaf area increased during the 3-year study. Clone 80 was unaffected by elevated [O3]. In Clone 4, elevated [O3] decreased root and branch biomass by 38 and 29%, respectively, whereas this clone showed few responses to elevated [CO2]. Elevated [CO2] significantly increased total leaf area in Clone 80 only, which may partly explain the smaller growth responses to elevated [CO2] of Clone 4 compared with Clone 80. Although we observed responses to elevated [O3], the responses to the EC+EO and EC treatments were similar, indicating that the trees only responded to elevated [O3] under ambient [CO2] conditions, perhaps reflecting a greater quantity of carbohydrates available for detoxification and repair in elevated [CO2].     

Drought resistance of two hybrid Populus clones grown in a large-scale plantation

Poplar hybrids were grown with irrigation in a large-scale plantation to investigate the mechanisms underlying clonal differences in drought resistance. Beginning in spring 1992, Populus trichocarpa x P. deltoides (TD) and P. deltoides x P. nigra (DN) cuttings received 46, 76, or 137 cm year(-1) of irrigation to supplement the 18-20 cm of annual precipitation, and all trees received the same fertilization regime. Stem volume, assessed as the square of stem diameter at breast height times tree height (D(2)H), and water relations of the trees were studied from the end of their second growing season until the end of their fifth growing season. By the end of the second growing season, stem volume of Clone TD was 40-146% larger than that of Clone DN, but stem volume growth was independent of irrigation in excess of 46 cm year(-1) in both clones. During the third growing season, stem volume growth of both clones was limited by both the 46- and 76-cm irrigation treatments, so that by the end of the third growing season trees in the 46-cm irrigation treatment were only half the size of trees in the 137-cm irrigation treatment. These treatment differences were maintained through the fifth growing season. Although stem volumes of Clone TD trees in the 76- and 137-cm irrigation treatments were larger than the corresponding values for Clone DN trees at the end of the third growing season (1994), these clonal differences gradually decreased in subsequent years and were not detectable after 5 years, because stem volume relative growth rate of Clone DN was greater than that of Clone TD in all treatments. Although both clones exhibited similar predawn leaf water potentials, Clone DN typically maintained higher midday leaf water potentials, suggesting better stomatal control of water loss. Clonal and treatment differences in osmotic potential at full turgor were minimal and could not explain the clonal differences in drought resistance. Root density and root density to stem volume ratio increased more in response to moderate drought in Clone DN than in Clone TD, resulting in enhanced drought resistance (high stem volume growth rate under moderate drought conditions) and an increased capacity to withdraw water from the soil. We conclude that the greater drought resistance of Clone DN compared with Clone TD was the result of the maintenance of a more favorable water balance by stomatal regulation and greater carbon allocation to roots during the early stages of drought. However, the low root density to stem volume ratio in Clone DN growing in the 46-cm irrigation treatment suggests that severe water limitation restricted the preferential allocation of carbon to belowground tissues, so that both root and shoot growth were constrained by severe drought.     

Determination of drought tolerance using root activities in Robinia pseudoacacia ‘Idaho’ transformed with mtl-D gene

Idaho locust (Robinia pseudoacacia ‘Idaho’) is an exotic multi-purpose tree used in landscaping, soil and water conservation, fodder sources and others. To improve its drought tolerance for reclaiming arid land, five lines of transformed mtl-D gene, as osmotic regulator in plant cells, have been selected and managed to determine their drought tolerance under experimental conditions. Qualitative and quantitative variables of transformed plants were studied. The critical value of drought tolerance was determined by detecting the 2,3,5-triphenyl tetrazolium chloride (TTC) reductants in roots and soil water content (SWC). The critical value for drought tolerance was SWC 6% while for the control plants the critical SWC was 8%; a moderate level of SWC is 13% and the highest SWC for plant endurance was 18%. The method proved to be reliable and sensitive in the evaluation of drought tolerance for forest trees.     

Physiological responses of birch (Betula pendula) to ozone: a comparison between open-soil-grown trees exposed for six growing seasons and potted seedlings exposed for one season

Physiological responses of 4-year-old potted saplings of an O3-tolerant clone of Betula pendula Roth to short-term ozone (O3) exposure (one growing season) were compared with those of 6-year-old open-soil-grown trees of the same clone fumigated with O3 for six growing seasons. In the 2001 growing season, both groups of plants were exposed to ambient (control) and 1.6x ambient (elevated) O3 concentration under similar microclimatic conditions in a free air O3 exposure facility. Growth, net photosynthesis, stomatal conductance, stomatal density, visible foliar injury, starch and nutrient concentrations, bud formation and differences in O3 responses between lower, middle and upper sections of the canopy were determined. The potted saplings were unaffected by elevated O3 concentration, whereas the open-soil-grown trees showed a 3-38% reduction in shoot growth, a 22% reduction in number of overwintering buds, a 26-65% decrease in autumnal net photosynthesis, 30% and 20-23% reductions in starch and nitrogen concentrations of senescing leaves, respectively, and disturbances in stomatal conductance. The greater O3 sensitivity of open-soil-grown trees compared with potted saplings was a result of senescence-related physiological factors. First, a lower net photosynthesis to stomatal conductance ratio in open-soil-grown trees at the end of the season promoted O3 uptake and decreased photosynthetic gain, leading to the onset of visible foliar injuries. Second, decreased carbohydrate reserves may have resulted in deleterious carry-over effects arising from the reduced formation of over-wintering buds. Finally, the leaf-level O3 load was higher for open-soil-grown trees than for potted saplings because of slower leaf senescence in the trees. Thus, O3 sensitivity in European white birch increases with increasing exposure time and tree size.     

Multivariate patterns of biochemical responses of Pinus ponderosa trees at field plots in the San Bernardino Mountains, southern California

Most environmental stress conditions promote the production of potentially toxic active oxygen species in plant cells. Plants respond with changes in their antioxidant and photoprotective systems. Antioxidants and pigments have been widely used to measure these responses. Because trees are exposed to multiple man-made and natural stresses, their responses are not reflected by changes in single stress markers, but by complex biochemical changes. To evaluate such response patterns, explorative multivariate statistics have been used. In the present study, 12 biochemical variables (chloroplast pigments, state of the xanthophyll cycle, alpha-tocopherol, ascorbate and dehydroascorbate, glutathione and oxidized glutathione) were measured in previous-year needles of field-grown Pinus ponderosa Dougl. ex Laws. The trees were sampled in two consecutive years in the San Bernardino Mountains in southern California, where a pollution gradient is overlaid by gradients in natural stresses (drought, altitude). To explore irradiance effects, needle samples were taken directly in the field (sun exposed) and from detached, dark-adapted branches. A principal component analysis on this data set (n = 80) resulted in four components (Components 1-4) that explained 67% of the variance in the original data. Component 1 was positively loaded by concentrations of alpha-tocopherol, total ascorbate and xanthophyll cycle pools, as well as by the proportion of de-epoxides in the xanthophyll cycle. It was negatively loaded by the proportion of dehydroascorbate in the ascorbate pool. Component 2 was negatively loaded by chlorophyll concentrations, and positively loaded by the ratios of lutein and beta-carotene to chlorophyll and by the de-epoxidation state of the xanthophyll cycle. Component 3 was negatively loaded by GSH concentrations and positively loaded by the proportions of GSSG and tocopherol concentrations. Component 4 was positively loaded by neoxanthin and negatively loaded by beta-carotene. The four components could be assigned to the concerted action of the biochemical protection system: high scores on Component 1 represent highly activated antioxidative defense, changes in pigment composition are represented in Components 2 and 4, and the glutathione system, which is important for antioxidant regeneration, is represented in Component 2. Although Component 1 scores were generally higher (indicating activation of antioxidant defense) in light-adapted needles relative to dark-adapted needles, they were also site dependent with increased scores at sites with less pollution, but higher natural stress impacts. High scores of Components 2 and 3 at the highest elevation site, which was only moderately polluted, indicated an increase in photoprotection by pigments and activation of the glutathione system. Significant differences between light- and dark-adapted needles in Components 2 and 3 were only found at the site with the highest pollution. Use of accumulated variables (components) instead of single biochemical variables enabled recognition of response patterns at particular sites and a better comparison with results of other studies is expected. Typical response patterns could be assigned to particular environmental stress combinations, providing a means of assessing potential biological risks within individual forest stands.     

利用Minirhizotron技术监测火炬松新根生长动态

利用Minirhizotron技术 ,研究了位于美国路易斯安那州的 19年生的火炬松新根生长动态。结果表明 ,火炬松新根的根量密度随时间呈增长趋势 ,在生长季末的 1、2月份达到最大。经过林地土壤干旱处理 (通过建立穿透雨隔离系统 ,阻止雨水到达地面 )的树木的根量密度自 9.3cm处 ,随土壤深度增加呈下降趋势 ,未经干旱处理的树木在离土表 80　 10 0cm深处仍有较高的根量密度 ,达 3.4 8　5 .12条·m- 1,是经干旱处理树木对应土壤层次中根量密度的 10多倍。火炬松从 3月份开始发生新根 ,随后迅速增加 ,至 6月份发生最多 ,然后缓慢减少 ,5　 7月的新根发生数占总量的 4 1.8% ,大部分新根发生在离土表 30cm以内的土壤中。 11个月的干旱处理显著地降低火炬松的根量密度 (1 1条·m- 1,未经干旱处理的为 3 2 7条·m- 1)和新根发生数 (5 7条·株- 1,未经干旱处理的为 15 6条·株 - 1) ,干旱处理还可导致新根推迟 1 0　 1.5个月发生。 5a前在土表撒施化肥似乎对新根的发生和生长没有明显影响     

Testing the unifying theory of ozone sensitivity with mature trees of Fagus sylvatica and Picea abies

The broad range in plant responses to chronic O(3) exposure compels a search for integrative, underlying principles. One such approach is the unifying theory proposed by Reich (1987), which combines the O(3) response of contrasting physiognomic classes of plants on the basis of their intrinsic leaf diffusive conductance and, hence, capacity for O(3) uptake. Physiognomic classes differ in the proportional decline in photosynthesis and growth when compared on the basis of cumulative O(3) exposure per unit time, but converge when compared on the basis of O(3) uptake per unit time or cumulative O(3) uptake over the entire lifetime of the leaf. The theory is based on observations on a large number of contrasting plant species, relying primarily on studies of juvenile trees subjected to short-term O(3) exposure. To test the applicability of the unifying theory to mature trees, broadleaf deciduous European beech (Fagus sylvatica L.) and the evergreen conifer Norway spruce (Picea abies (L.) Karst.) in a mature mixed stand were exposed to either ambient air (control) or air with twice the ambient O(3) concentration delivered into the canopy by means of a free-air fumigation system. We accounted for differences in growing season length, leaf longevity and O(3)-related effects on leaf diffusive conductance in determining total O(3) uptake over the lifetime of the leaf. On this basis, Norway spruce needles required 5 years to take up as much O(3) as did beech leaves in one growing season. The core of the unifying theory on O(3) sensitivity was substantiated in relation to O(3) exposure and uptake. However, contrary to the unifying theory, which was formulated on the basis of results with juvenile trees, the O(3) response of mature trees in a natural stand was more complex. The increased complexity was attributed to additional environmental stressors, stress compensation at the whole-tree level, and differential O(3) sensitivities of leaves according to age class and position within the canopy. Contrary to the theory, photosynthesis was no less sensitive to O(3) in Norway spruce than that of beech, and was reduced in the twice-ambient O(3) regime in the first year of exposure.     

Spatial and temporal patterns of fine root abundance in a mixed oak-beech forest

Spatial distribution and seasonal fluctuation of fine root density (mass per unit soil volume) and abundance (mass or surface area per unit ground surface area) were investigated by the sequential coring technique in a 100–220 year old mixed Fagus sylvatica-Quercus petraea stand on acidic sandy soil in northwest Germany. The fine root systems of the two co-existing species overlapped completely with beech roots being twice as abundant as oak roots. Since Fagus and Quercus occupied equivalent parts of the canopy volume, oak appeared to be under-represented in the below-ground space. There was evidence for some degree of below-ground niche partitioning between the species in both the vertical and the horizontal direction. Oak fine roots were found to be more superficially distributed than beech roots in the organic layers, indicating a vertical stratification of the root systems of the two species. In the forest floor, fine roots were more abundant in the vicinity of aok stems where thicker organic layers occurred. However, this distribution pattern was not a consequence of a greater abundance of oak roots close to their parent stem, but was due to a higher frequency of beech roots here.     

Nutrient availability alters belowground respiration of ozone-exposed ponderosa pine

Exposure to ozone (O(3)) and changes in soil fertility influence both the metabolism of plant roots and their interaction with rhizosphere organisms. Because one indication of altered root metabolism is a change in belowground respiratory activity, we used specially designed measurement chambers to assess the effects of O(3) and nutrient availability on belowground respiratory activity of potted three-year-old ponderosa pine (Pinus ponderosa Dougl. ex Laws.). Seedlings were exposed to a factorial combination of three O(3) treatments and three fertilization treatments in open-top O(3) exposure chambers. Ozone exposure decreased and high nutrient supply increased total plant dry weight, but root/shoot ratios were not affected. In general, exposure to O(3) increased rates of belowground O(2) uptake and CO(2) release and the respiratory quotient (RQ, CO(2)/O(2)), although seasonal differences were detected. In October, following the second season of O(3) exposure, rates of belowground O(2) uptake and CO(2) release and RQ were increased in trees in the high-O(3) exposure treatment by 22, 73 and 32%, respectively, over values in control trees in charcoal-filtered air. Increasing nutrient supply resulted in decreasing rates of belowground O(2) uptake and CO(2) release but it had little effect on RQ. In the high-nutrient supply treatment, rates of belowground O(2) uptake and CO(2) release were decreased by 38 and 39%, respectively, compared with rates in the low-nutrient supply treatment. At the end of the second growing season, the high-nutrient supply treatment had decreased lateral root total nonstructural carbohydrates by 22% compared with the low-nutrient supply treatment. Nutrient availability altered the belowground respiratory response to O(3), such that the response to O(3) was greatest in the low-nutrient supply treatment. Significant O(3) effects on belowground respiratory activity were apparent before any reduction in total plant growth was found, suggesting that roots and rhizosphere organisms may be early indicators of physiological dysfunction in stressed seedlings.