Tree and stand growth of mature Norway spruce and European beech under long-term ozone fumigation

In a 50- to 70-year-old mixed stand of Norway spruce (Picea abies (L.) Karst.) and European beech (Fagus sylvatica L.) in Germany, tree cohorts have been exposed to double ambient ozone (2×O₃) from 2000 through 2007 and can be compared with trees in the same stand under the ambient ozone regime (1×O₃). Annual diameter growth, allocation pattern, stem form, and stem volume were quantified at the individual tree and stand level. Ozone fumigation induced a shift in the resource allocation into height growth at the expense of diameter growth. This change in allometry leads to rather cone-shaped stem forms and reduced stem stability in the case of spruce, and even neiloidal stem shapes in the case of beech. Neglect of such ozone-induced changes in stem shape may lead to a flawed estimation of volume growth. On the stand level, 2×O₃ caused, on average, a decrease of 10.2 m³ ha⁻¹ yr⁻¹ in European beech.     

Free-air fumigation of mature trees. A novel system for controlled ozone enrichment in grown-up beech and spruce canopies

A novel system for continuous and controlled free-air fumigation of mature tree canopies with ozone is described. Ozone generated from oxygen is diluted with air in a pressurized tank and conducted into the canopies by a system of 100 PTFE tubes hanging down from a grid fixed above the crowns. With 45 calibrated outlets per tube providing a constant flow of 0.3 l/min each, a total volume of about 10*10*15 m3 comprising 5 beech and 5 spruce canopies is fumigated. The spatial ozone distribution in the fumigated volume as well as surrounding reference tree canopies is controlled by continuous measuring instruments installed at 4 levels and a dense array of passive samplers. The system will later be used for CO2 fumigation as well. Results of the first year of continuous operation, with 2 * ambient ozone levels having been achieved, are reported.     

Ozone exposure-response relationships for biomass and root/shoot ratio of beech (Fagus sylvatica), ash (Fraxinus excelsior), Norway spruce (Picea abies) and Scots pine (Pinus sylvestris)

Current-year seedlings of beech, ash, Norway spruce and Scots pine were exposed during one growing season to different, but moderate, ozone (O(3)) scenarios representative for Switzerland (50, 85, 100% ambient, 50% ambient+30 nl l(-1)) in open-top chambers (OTCs) and to ambient O(3) concentrations in the field. Biomass significantly decreased with increasing O(3) dose in all species except for spruce. Losses of 25.5% (ash), 17.4% (beech), 9.9% (Scots pine) were found per 10 microl l(-1) h accumulated O(3) exposure over a threshold concentration of 40 nl l(-1) during daylight hours (AOT40). Ratios of root/shoot biomass (RSR) also significantly decreased with increasing AOT40 levels in beech and ash, but not in Norway spruce and Scots pine. The data show that the deciduous species beech and ash were more susceptible to O(3) with respect to RSR and biomass than the coniferous species Norway spruce and Scots pine.     

Effects of ozone exposure in open-top chambers on poplar (Populus nigra) and beech (Fagus sylvatica): a comparison

Rooted cuttings of poplar (Populus nigra) and seedlings of beech (Fagus sylvatica) were exposed to ozone in open-top chambers for one growing season. Three treatments were applied: charcoal-filtered (CF), non-filtered (NF) and non-filtered air plus 30 ppb (nl l(-1)) ozone (NF+). Extra ozone was only added on clear days, from 09:00 until 17:00-20:00. The AOT40s (accumulated exposure over a threshold of 40 ppb), calculated from April to September were 4055 ppb.h for the NF and 8880 ppb.h for the NF+ treatments. For poplar ozone exposure caused highly significant reductions in growth rate, light-saturated net CO(2) assimilation rate, stomatal conductance, F(v)/F(m) and chlorophyll content. The largest effects were observed in August at which time ozone concentrations were elevated. A reduction was noticed in new leaf production, while accelerated ageing and visible damage to leaves caused high leaf losses. For beech the responses were similar but less pronounced: ozone exposure resulted in non-significant growth reductions, slight changes in light-saturated photosynthesis and accelerated leaf abscission. The chlorophyll content of beech leaves was not affected by the ozone treatments. The results confirmed previous observations that fast-growing tree species, such as most poplar species and hybrids, are more sensitive and responsive to tropospheric ozone than slower-growing species, such as beech. The growth reductions observed and reported here for beech were within the range of those reported in relationship to the AOT40 (accumulated exposure over a threshold of 40 ppb) critical level for ozone.     

Effect of SO2 on the reproduction of pea aphids, Acyrthosiphon pisum, and the impact of SO2 and aphids on the growth and yield of peas

Pea aphids feeding from birth to maturity on pea plants (Pisum sativum) exposed to SO(2) concentrations of 50 nl litre(-1) or 80 nl litre(-1) showed a significant 19% increase in the rate of nymph production during the reproductive period, compared to control aphids feeding on plants in charcoal-filtered air. The higher nymph production resulted in a mean 4.6% increase in the intrinsic rate of population increase (rm). In longer term glasshouse fumigation experiments pea aphid populations were, on average, 1.8 times greater on pea plants in ambient air plus 45 nl litre(-1) SO(2) than in ambient air alone. Aphid infestation in ambient air caused a 42% reduction in pea yield and affected most plant parameters adversely. Ambient air plus SO(2) had no direct effect on yield, but, in combination with aphid infestation, a further 10% reduction in yield was recorded.     

Comparison of ozone uptake and sensitivity between a phytotron study with young beech and a field experiment with adult beech (Fagus sylvatica)

Chamber experiments on juvenile trees have resulted in severe injury and accelerated loss of leaves along with reduced biomass production under chronically enhanced O3 levels. In contrast, the few studies conducted on adult forest trees in the field have reported low O3 sensitivity. In the present study, young beech in phytotrons was more sensitive to O3 than adult beech in the field, although employed O3 regimes were similar. The hypotheses tested were that: (1) differences in O3 uptake were caused by the ontogenetically higher stomatal conductance of young compared to adult trees, (2) the experimental settings in the phytotrons enhanced O3 uptake compared to field conditions, and (3) a low detoxification capacity contributes to the higher O3 sensitivity of the young trees. The higher O3 sensitivity of juvenile beech in the phytotrons is demonstrated to relate to both the experimental conditions and the physiological responsiveness inherent to tree age.     

Reversibility of stemflow-induced soil acidification in Swedish beech forest

Stemwater running down the trunks of beech (Fagus sylvatica L.) has an acidifying effect on soil near the stem. The deposition of acidifying substances may be two to four times higher close to the stem compared to in the stand in general. To study reversibility of soil acidification, 72 stumps of beech trees were chosen from five different year classes of felling (3, 6, 9, 14-15, 25-30), representing the years when stemflow ceased to affect the soil. The H(+) concentration (pH KCl) in the topsoil (0-5 cm) differed between the distances 10-30 cm and 230-250 cm from the stumps, the soil close to the stem being more acid. The longer the time since felling the smaller the differences in H(+) concentration became. This reduction in soil acidity amounted to ca. 50% after 15 years, and only small further changes occurred over the next ten years. This indicates that soil may not recover fully from acidification, or does so at a rather slow rate after the initial 15 years of recovery.     

Seasonal ozone response of mature beech trees (Fagus sylvatica) at high altitude in the Bavarian forest (Germany) in comparison with young beech grown in the field and in phytotrons

Mature beech trees (Fagus sylvatica) grown at two different altitudes in the Bavarian forest were compared with young beech trees grown at nearby field sites or in phytotrons for their macroscopic and physiological responses to different ozone (O(3)) exposures. Cumulative O(3) exposure expressed as the sum of hourly mean concentrations above the canopy ranged between 100 and 150 microl l(-1) h, with the vertical O(3) profiles at the higher altitude site being enhanced by 30%. O(3) profiles at all sites were reduced by up to 20% with increasing depth within and beneath the canopy. The leaf discoloration that developed in the absence of premature leaf loss was similar in the sun foliage of mature and young trees (including plant grown in the phytotron). Injury became apparent at low O(3) exposures, expressed as accumulated hourly means over a threshold of 40 nl l(-1) (AOT40 <3.5 microl l(-1) h) at the lower site in both the mature trees and the young beech at the field site, but only occurred when AOT40 values reached 7 microl l(-1) h at the upper site, and 6 microl l(-1) h in the phytotrons. However, the association between injury and O(3) exposure was improved when cumulative ozone uptake to sun leaves was the ozone index, used with values of about 3 mmol m(-2) resulting in visible injury in both mature and young beech growing in phytotrons. Under high ozone exposure levels of inositol were lowered, whilst concentrations of lignin-like materials were enhanced in mature beech. Similar responses were observed in young beech grown in phytotrons. As the sun foliage was affected by only a small and variable extent each year, the seasonal O(3) impact at high altitude did not appear to pose an acute risk to mature beech trees.     

Experimental research on recolonisation with <Emphasis Type="Italic">Anemone nemorosa</Emphasis> of the beech forests of the Ruhr district (Germany) floristically impoverished by air pollution

BACKGROUND, AIM, AND SCOPE: High SO(2) concentrations as have been observed over decades in the Ruhr district lead to a remarkable reduction of leaf area in the majority of the characteristic broad-leafed herbs of the Central European beech forests even after only a few months of experimental fumigation. Thus, it is no wonder in the time of high SO(2) pollution, e.g., in the town of Herne (centre of the Ruhr district), that there was not a single beech forest hosting, for instance, Viola reichenbachiana or Anemone nemorosa. As air quality has improved very much over some decades in the Ruhr district, one can expect a recolonisation of the beech forests by the species of former time characteristic for the herb layer. However, one has to consider that only the air pollution was reduced, while soil acidification and contamination with heavy metals and PAH are, on the short run, irreversible. That is why experiments were carried out, considering the question as to whether recolonisation of the forests of the Ruhr district by the aforementioned species is possible and why such a recolonisation up to now has not occurred. MATERIALS AND METHODS: The experiments were carried out in a beech forest situated in the centre of the Ruhr district in the City of Herne. The wood anemone (A. nemorosa) was chosen as test plant because of its high frequency in beech forests on loess soils outside the Ruhr district, and its absence in beech forests in the Ruhr district. The experiments with A. nemorosa were carried out in three variants with different soils: (a): soil of the local forests (R); (b): soil of the local forests whose soot layer was removed (r); (c): imported soil from a clean air region far away from the Ruhr district (Odenwald). RESULTS: Survival of rhizomes of A. nemorosa is possible for some years in the soils of the Ruhr district; however, the establishment of a population could not be achieved. The results obtained by the imported soil show that it is no longer air pollution, but the soil which prevents the establishment of a population. Sexual reproduction is rather impossible because of the thick litter layer with which all of the Ruhr district's beech forests are covered. DISCUSSION: With respect to the unfavourable chemistry of the soil of the Ruhr district and in consideration of the unfavourable attributes of the soot layer, the author expected the following order of the development of shoot numbers: O > r > R. However, the result is: O > R > r. In contrast to the expected result, the soot layer has no negative but slightly positive effects on the implanted rhizomes. A possible explanation is that the soot layer, which is situated immediately below the top soil, prevents the top soil from drying up and thus even protects the rhizomes from desiccation. Also, the possibility has to be considered that the soot layer functions as a nutrient storage area. CONCLUSIONS: At present, a survival of the rhizomes of A. nemorosa in the soils of the Ruhr district is temporarily possible but does not lead to the establishment of a permanent population. This only can be achieved by additional sexual reproduction. However, the thick litter layer present in all beech forests of the Ruhr district prevents the establishment of seedlings, i.e., it does not allow sexual reproduction to contribute to the population. The soot layer situated below the litter layer represents a second hindrance for germination. Other than seedlings, rhizomes are not negatively affected by the soot layer but even a slight stabilisation has to be stated. As a reason for this slightly positive effect, a protection of the upper mineral soil from desiccation by the hydrophob soot layer has to be considered. Secondly, the soot layer may serve as a nutrient storage which is of particular importance in acid soils, because acidification generally leads to a leeching of nutrients. To answer these questions, detailed further research is necessary. RECOMMENDATIONS AND PERSPECTIVES: In order to restore the formerly rich herbaceous layer of the forests of the Ruhr district, experiments (removal of the litter layer; liming; ploughing) should be carried out at broad-scale to solve the question of how the strong negative effects of the established thick raw humus layer can be reduced or even be avoided. When the problem of the humus layer is solved, the beech forests of the Ruhr district today highly impoverished in species will become a vivid ecosystem, rich in flowering herbaceous species and thus much more attractive for the people of the Ruhr district than at present.     

Studies on leaching from spruce twigs and beech leaves

Leaching, depending on the acidity of the eluent, was investigated for K, Mg and Mn. The curves for the leachate concentration versus time could be described by a power function. Leaching from old needles exceeded leaching from young needles to a high degree, but the ratios of the leached elements were almost equal. A method for continuous measurements of pH change in solutions contacting plant materials was developed. The results were in agreement with data on acidity changes in water falling through spruce or beech canopies.     

Air pollution and agricultural aphid pests. II chamber filtration experiments

The responses of four major aphid pest species feeding on three major crops were studied in a series of experimental chambers on the roof of Imperial College in South Kensington, London. The experimental chambers were continually circulated with air which had been subjected to a variety of filtration treatments. In the first series of experiments there were three chambers subject to ambient air, charcoal-filtered air, and charcoal plus 'Purafil'-filtered air; in the subsequent experiments there were four chambers, a charcoal plus 'Purafil' plus charcoal treatment being added. These treatments provided a range of concentrations and mixtures of the ambient gases present at the site. The growth rate of aphids was measured both during filtration and post-filtration, the plants being exposed from sowing for either 42 or 84 days. In all cases there were significant effects on aphid performance which seemed to be most strongly linked to absolute and relative NO concentrations. The pattern of responses by the aphids accorded extremely well with those observed in closed-chamber fumigation experiments with stimulation of performance in relatively polluted air in all cases except Acyrthosiphon pisum (Harris) feeding on Vicia faba L. where the opposite effect was recorded.     

The effects of ozone-exposed sugar maple seedlings on the biological performance and the feeding preference of the forest tent caterpillar (Malacosoma disstria Hbn.)

The effects of exposure of sugar maple (Acer saccharum Marsh.) to ozone on the entire larval stage of a native insect have not been previously investigated. This study reports the effects of sugar maple seedlings exposed to different ozone concentrations on the relative performance and the feeding preference of the forest tent caterpillar (Malacosoma disstria Hbn.). Three-year-old seedlings were set in nine open-top field chambers in the spring of 1992 and 1993. Three ozone concentrations were generated: charcoal-filtered ambient air (0x), ambient air (1x) and three times ambient air (3x). In 1992, female and male larval development time did not differ among ozone treatments. In 1993, female larvae reared on 3x developed faster than those on 0x and 1x, while male larvae were not affected. Ozone treatments did not influence pupal weights except for males in 1993 where pupae reared on 0x were heavier than 1x but did not differ from 3x. Larval and pupal survival rates were not affected by ozone in either year. Finally, 4th and 5th instar larvae showed a significant feeding preference for 3x foliage in 1993 but not in 1992. The response of the forest tent caterpillar to ozone exposed seedlings varied between years and could be more sensitive to annual climatic variations than ozone.     

Performance of some growth variables

European beech (Fagus sylvatica L.), Norway spruce (Picea abies L. Karst.) and Silver fir (Abies alba Mill.) were exposed to low concentrations of ozone (O(3)) and sulfur dioxide (SO(2)), alone and combined, and simulated acid rain (pH 4.0) in sheltered open-top chambers in Hohenheim (Southwest Germany) for almost five years. The concentrations of O(3) and SO(2) used were related to annual ambient average found in southern West Germany. Two control chambers were ventilated with charcoal filtered air and rainfall was simulated at pH 4.0 and 5.0. Because of large dense plant growth in the chambers it was only possible to measure uncompleted growth of shoots in the upper canopy. Therefore, growth analysis was restricted to this area. The treatment with acidic precipitation decreased the annual shoot growth of beech and reduced leaf surface area of those trees. Exposure to SO(2), O(3) alone and in combination resulted in further reduction of shoot length and leaf surface area. Fumigation with SO(2) and O(3) + SO(2) caused insignificant decreases of shoot length, total dry weight and needle surface area of spruce. The lateral leader shoot growth of spruce exposed to O(3) was significantly reduced only in the last year of the experiment. Growth rates of the spruce exposed to charcoal filtered air and non-acidic precipitation were reduced more than those of beech and fir. Growth variables determined for fir reflected different rates of incremental change. Exposure to O(3) resulted in the largest dry matter production of all fir groups but those exposed to charcoal filtered air and non-acidic precipitation responded with the best lateral leader shoot growth, lowest specific leaf area (SLA) and leaf area ratio (LAR) respectively indicating best metabolic efficiency. At the conclusion of this study a classification of sensitivity was developed for the tree species.     

Photosynthetic and stomatal conductance responses of Norway spruce and beech to ozone, acid mist and frost--a conceptual model

Two-year-old beech and Norway spruce seedlings were exposed to a combination of ozone and acid mist treatments in open-top chambers in Scotland during the months of July through to September 1988. Replicate pairs of chambers received charcoal-filtered air (control), ozone-enriched air (140 nl ozone litre(-1)) or 140 nl ozone litre(-1) plus a synthetic acid mist (pH 2.5) composed of ammonium nitrate and sulphuric acid. Field measurements of assimilation and stomatal conductance were made during August. In addition, measurements of assimilation and conductance were made during September in the laboratory. Light response curves of assimilation and conductance were determined using a GENSTAT nonrectangular hyperbolic model. During February 1988/9 the Norway spruce were subject to a four day warming period at 12 degrees C and the light response of assimilation determined. The same plants were then subject to a 3-h night-time frost of -10 degrees C. The following day the time-course of the recovery of assimilation was determined. It was found that ozone fumigation did not influence the light response of assimilation of beech trees in the field, although stomatal conductance was reduced in the ozone-fumigated trees. The rate of light-saturated assimilation of Norway spruce was increased by ozone fumigation when measured in the field. Measurements of assimilation of Norway spruce made during the winter showed that prior to rewarming there was no difference in the rate of light-saturated assimilation for control and ozone-fumigated trees. However, the ozone plus acid mist-treated trees exhibited a significantly higher rate. The 4-day period of warming to 12 degrees C increased the rate of light-saturated assimilation in all treatments but only the ozone plus acid mist-treated trees showed a significant increase. Following a 3-h frost to -10 degrees C the control trees exhibited a reduction in the rate of light-saturated assimilation (Amax) to 80% of the pre-frost value. In comparison, following the frost, the ozone-fumigated trees showed an Amax of 74% of the pre-frost value. The ozone plus acid mist-treated trees showed an Amax of 64% of the pre-frost trees. The time taken for Amax to attain 50% of the pre-frost value increased from 30 min (control) to 85 min for ozone-fumigated trees to 190 min (ozone plus acid mist). These results are discussed in relation to the impact of mild, short-term frosts, which are known to occur with greater frequency than extreme, more catastrophic frost events. A simple conceptual framework is proposed to explain the variable results obtained in the literature with respect to the impact of ozone upon tree physiology.     

Belowground effects of enhanced tropospheric ozone and drought in a beech/spruce forest (Fagus sylvatica L./Picea abies [L.] Karst)

The effects of experimentally elevated O₃ on soil respiration rates, standing fine-root biomass, fine-root production and δ¹³C signature of newly produced fine roots were investigated in an adult European beech/Norway spruce forest in Germany during two subsequent years with contrasting rainfall patterns. During humid 2002, soil respiration rate was enhanced under elevated O₃ under beech and spruce, and was related to O₃-stimulated fine-root production only in beech. During dry 2003, the stimulating effect of O₃ on soil respiration rate vanished under spruce, which was correlated with decreased fine-root production in spruce under drought, irrespective of the O₃ regime. δ¹³C signature of newly formed fine-roots was consistent with the differing g_s of beech and spruce, and indicated stomatal limitation by O₃ in beech and by drought in spruce. Our study showed that drought can override the stimulating O₃ effects on fine-root dynamics and soil respiration in mature beech and spruce forests.     

Selection effects of air pollution on gene pools of Norway spruce, European silver fir and European beech.

The effects of industrial pollution on allelic and genotypic structures of Norway spruce. European silver fir and European beech were investigated by means of isozyme analysis. In a mixed Norway spruce-silver fir forest stand in an area heavily polluted by sulphur dioxide and heavy metals in the region of Spis (eastern Slovakia), pairs of neighbouring damaged and apparently healthy trees were selected in two replicates (44 and 69 pairs in a heavily and moderately damaged stand, respectively). Pairwise sampling of trees with contrasting vitality was applied to reduce potential effects of site heterogeneity on the vitality of sampled trees. No significant differences in allelic and genotypic frequencies were found between sets of healthy and declining trees. There were differences in the single-locus heterozygosities, but these were not consistent between the replicates. However, the set of damaged trees exhibited higher levels of genetic multiplicity and diversity, possibly due to the deleterious effect of rare alleles under the conditions of air pollution. Consequently. following the decline of pollutant-sensitive trees, the remaining stand will be depleted of a part of alleles with unknown adaptive value to future selection pressures.     

Use of sap flow measurements to validate stomatal functions for mature beech (Fagus sylvatica) in view of ozone uptake calculations

For a quantitative estimate of the ozone effect on vegetation reliable models for ozone uptake through the stomata are needed. Because of the analogy of ozone uptake and transpiration it is possible to utilize measurements of water loss such as sap flow for quantification of ozone uptake. This technique was applied in three beech (Fagus sylvatica) stands in Switzerland. A canopy conductance was calculated from sap flow velocity and normalized to values between 0 and 1. It represents mainly stomatal conductance as the boundary layer resistance in forests is usually small. Based on this relative conductance, stomatal functions to describe the dependence on light, temperature, vapour pressure deficit and soil moisture were derived using multivariate nonlinear regression. These functions were validated by comparison with conductance values directly estimated from sap flow. The results corroborate the current flux parameterization for beech used in the DO₃SE model.     

Quantifying ozone uptake and its effects on the stand level of common beech (Fagus sylvatica L.) in Southern Germany

Stand level O(3) fluxes were calculated using water balance calculations for 21 Common beech (Fagus sylvatica L.) stands and O(3) data from 20 monitoring stations in Southern Germany. For this intention, the daily loss of water by evapotranspiration per stand area was set against the daily O(3) uptake. During the last 30 years, O(3) uptake ranges between 0 and 187 mmol ha(-1) d(-1) per stand area. Cumulative O(3) uptake (CUO(3)), ranging between 0.1 and 0.7 mmol m(-2) yr(-1) per stand area, shows increasing trends since 1971 with considerably greater values at high altitudes. Effects in radial growth were used to derive an initial approximate critical threshold value for O(3) impacts on the vitality and growth of mature beech stands in Southern Germany. It is concluded that this concept of O(3) flux estimation in combination with dendroecological analyses offers both a site specific and regional applicable approach to derive new critical levels for O(3).     

Spatial variability and temporal stability of throughfall deposition under beech (Fagus sylvatica L.) in relationship to canopy structure

Although the spatial variability of throughfall (TF) in forest ecosystems can have important ecological implications, little is known about the driving factors of within-stand TF variability, particularly in deciduous forests. While the spatial variability of TF water amount and H+ deposition under a dominant beech (Fagus sylvatica L.) tree was significantly higher in the leafed period than in the leafless period, the spatial TF deposition patterns of most major ions were similar in both periods. The semiannual TF depositions of all ions other than H+ were significantly positively correlated (r=0.68-0.90, p<0.05) with canopy structure above sample locations throughout the entire year. The amounts of TF water and H+ deposition during the leafed period were negatively correlated with branch cover. We conclude that the spatial heterogeneity of ion deposition under beech was significantly affected by leaves in the growing period and by branches in non-foliated conditions.