Spatial variability of photosynthetically active radiation in European beech and Norway spruce

The vertical and horizontal variability of solar radiation within a mature European beech (Fagus sylvatica L.)-Norway spruce (Picea abies [L.] Karst) mixed stand in Southern Germany is investigated. A large dataset with more than one million spectral measurements of photon fluence rates at six vertical levels within the stand is analyzed with respect to tree species, meteorological sky conditions, and the influence of solar elevation angle on canopy penetration. Irradiance probability density functions of the photosynthetically active waveband are used to describe the three-dimensional radiation field. For a quantification of umbra, penumbra, and sunfleck frequencies, in-canopy fractions of photon fluence rates within the photosynthetically active waveband are investigated. Different phenological stages of beech and their effects on the in-canopy light climate are compared. The results show that during overcast conditions (OVC) fractions of photosynthetically active radiation (PAR) are higher at all canopy levels than during clear sky (CS) conditions due to their exclusively diffuse character. The lowest median PAR level of less than 1% of above-canopy PAR can be observed in the shade crown of beech and at ground level. More PAR can penetrate the canopy at a higher solar elevation under CS conditions. This effect is more pronounced for spruce than for beech due to the conical crown shape of the conifers that allows photons from higher angles to enter the gaps inbetween trees in contrast to the more homogeneously closed beech canopy. Solar elevation is not an important factor at uniformly overcast conditions. Differences in the vertical distribution of umbra and penumbra can be detected when comparing species or different sky conditions. The frequency of sunflecks differs more by species and by the vertical position within the canopy than by sky condition.     

Vertical gradients of potential enzyme activities in soil profiles of European beech,Norway spruce and Scots pine dominated forest sites

Management of forest sites has the potential to modulate soil organic matter decomposition by changing the catalytic properties of soil microorganisms within a soil profile. In this study we examined the impact of forest management intensity and soil physico-chemical properties on the variation of enzyme activities (β-glucosidase, β-xylosidase, α-glucosidase, phenol oxidase, N-acetyl-glucosaminidase, l-leucine aminopeptidase, phosphatase) in the topsoil and two subsoil horizons in three German regions (Schorfheide-Chorin, Hainich-Dün, Schwäbische Alb). The sandy soils in the Schorfheide-Chorin (SCH) showed lower ratios of the activity of carbon (C) acquiring enzymes (β-glucosidase) relative to nitrogen (N) acquiring enzymes (N-acetyl-glucosaminidase + l-leucine aminopeptidase), and activity of C acquiring enzymes relative to phosphorous (P) acquiring enzymes (phosphatase) than the finer textured soils in the Hainich-Dün (HAI) and Schwäbische Alb (ALB), indicating a shift in investment to N and P acquisition in the SCH. All enzyme activities, except phenol oxidase activity, decreased in deeper soil horizons as concentrations of organic C and total N did, while the decrease was much stronger from the topsoil to the first subsoil horizon than from the first subsoil to the second subsoil horizon. In contrast, phenol oxidase activity showed no significant decrease towards deeper soil horizons. Additionally, enzyme activities responsible for the degradation of more recalcitrant C relative to labile C compounds increased in the two subsoil horizons. Subsoil horizons in all regions also indicate a shift to higher N acquisition, while the strength of the shift depended on the soil type. Further, our results clearly showed that soil properties explained most of the total variance of enzyme activities in all soil horizons followed by study region, while forest management intensity had no significant impact on enzyme activities. Among all included soil properties, the clay content was the variable that explained the highest proportion of variance in enzyme activities with higher enzyme activities in clay rich soils. Our results highlight the need for large scale studies including different regions and their environmental conditions in order to derive general conclusions on which factors (anthropogenic or environmental) are most influential on enzyme activities in the whole soil profile in the long term at the regional scale.     

Methane oxidation in temperate soils: effects of inorganic N

Additions of inorganic nitrogen (N) to an oak soil with significant potential for methane (CH₄) oxidation resulted in differential reduction in CH₄ oxidation capacity depending on N species added. Nitrate, rather than nitrite or ammonium, proved to be the strongest inhibitor of CH₄ oxidation in oak soil. Both high (CH₄ at 10 μl l⁻¹) and low (CH₄ at 5 ml l⁻¹) affinity CH₄ oxidation in oak soil was completely inhibited at a nitrate concentration similar to that present in an alder soil from the same experimental site. The alder soil showed no capacity for low affinity CH₄ oxidation. A ‘low nitrate’ forest soil (oak) showed high affinity, low capacity CH₄ oxidation upto around 1 ml l⁻¹ CH₄, above which both high and low affinity CH₄ oxidation became apparent following a lag phase, indicating either an induced high affinity uptake mechanism or the existence of distinct low affinity and high affinity methanotroph populations. High affinity CH₄ oxidation became saturated at CH₄ concentrations >500 μl l⁻¹, while low affinity CH₄ oxidation became saturated at ∼30 ml l⁻¹ CH₄. In a ‘high nitrate’ forest soil (alder), CH₄ oxidation appeared to be due to high affinity CH₄ oxidation only and became undetectable at CH₄ concentrations >5 ml l⁻¹.     

Microbial biomass and C and N transformations in forest floors under European beech, sessile oak, Norway spruce and Douglas-fir at four temperate forest sites

The purpose of this research was to compare soil chemistry, microbially mediated carbon (C) and nitrogen (N) transformations and microbial biomass in forest floors under European beech (Fagus sylvatica L.), sessile oak (Quercus petraea (Mattuschka) Lieblein), Norway spruce (Picea abies (L.) Karst) and Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco) at four study sites. We measured soil chemical characteristics, net N mineralization, potential and relative nitrification, basal respiration, microbial and metabolic quotient and microbial biomass C and N under monoculture stands at all sites (one mixed stand). Tree species affected soil chemistry, microbial activities and biomass, but these effects varied between sites. Our results indicated that the effect of tree species on net N mineralization was likely to be mediated through their effect on soil microbial biomass, reflecting their influence on organic matter content and carbon availability. Differences in potential nitrification and relative nitrification might be related to the presence of ground vegetation through its influence on soil NH₄ and labile C availability. Our findings highlight the need to study the effects of tree species on microbial activities at several sites to elucidate complex N cycle interactions between tree species, ground vegetation, soil characteristics and microbial processes.     

Direct estimates of C:N ratios of ectomycorrhizal mycelia collected from Norway spruce forest soils

Direct estimates of C:N ratios of ectomycorrhizal (EM) mycelia growing in situ in forest soils have been obtained for the first time. The mycelial samples were collected from sand-filled mesh bags that were buried in the soil and incubated for 12-18 months in two Norway spruce forests in southern Sweden. At harvest the mesh bags were heavily colonized and the mycelia were extracted from the sand with water. The collected mycelia had earlier been identified as belonging to EM fungi based on their C isotopic composition. The mean value of the C:N ratio for mycelia was 20.2±0.8 (n=25). EM mycelia collected at different soil depths (5, 15 and 30 cm) had similar C:N ratios. C:N ratios of microbial biomass obtained by fumigation-extraction of similar soils have usually been lower (6-13) so possible differences in the extraction efficiency of C and N from bacteria and fungi are discussed.     

Carbon dioxide emissions of soils under pure and mixed stands of beech and spruce, affected by decomposing foliage litter mixtures

Soil respiration is the largest terrestrial source of CO₂ to the atmosphere. In forests, roughly half of the soil respiration is autotrophic (mainly root respiration) while the remainder is heterotrophic, originating from decomposition of soil organic matter. Decomposition is an important process for cycling of nutrients in forest ecosystems. Hence, tree species induced changes may have a great impact on atmospheric CO₂ concentrations. Since studies on the combined effects of beech-spruce mixtures are very rare, we firstly measured CO₂ emission rates in three adjacent stands of pure spruce (Picea abies), mixed spruce-beech and pure beech (Fagus sylvatica) on three base-rich sites (Flysch) and three base-poor sites (Molasse; yielding a total of 18 stands) during two summer periods using the closed chamber method. CO₂ emissions were higher on the well-aerated sandy soils on Molasse than on the clayey soils on Flysch, characterized by frequent water logging. Mean CO₂ effluxes increased from spruce (41) over the mixed (55) to the beech (59) stands on Molasse, while tree species effects were lower on Flysch (30-35, mixed > beech = spruce; all data in mg CO₂-C m⁻² h⁻¹). Secondly, we studied decomposition after fourfold litter manipulations at the 6 mixed species stands: the O_i - and O_e horizons were removed and replaced by additions of beech -, spruce - and mixed litter of the adjacent pure stands of known chemical quality and one zero addition (blank) in open rings (20 cm inner diameter), which were covered with meshes to exclude fresh litter fall. Mass loss within two years amounted to 61-68% on Flysch and 36-44% on Molasse, indicating non-additive mixed species effects (mixed litter showed highest mass loss). However, base cation release showed a linear response, increasing from the spruce - over the mixed - to the beech litter. The differences in N release (immobilization) resulted in a characteristic converging trend in C/N ratios for all litter compositions on both bedrocks during decomposition. In the summers 2006 and 2007 we measured CO₂ efflux from these manipulated areas (a closed chamber fits exactly over such a ring) as field indicator of the microbial activity. Net fluxes (subtracting the so-called blank values) are considered an indicator of litter induced changes only and increased on both bedrocks from the spruce - over the mixed - to the beech litter. According to these measurements, decomposing litter contributed between 22-32% (Flysch) and 11-28% (Molasse) to total soil respiration, strengthening its role within the global carbon cycle.     

Tree roots in canopy soils of old European beech trees—An ecological reassessment of a forgotten phenomenon

The formation of adventitious roots in humus accumulations in tree canopies is widely acknowledged from tropical and temperate rainforests, while the occurrence of those canopy roots in temperate tree species under mesic climates has been largely disregarded for ca. 100 years. Moreover, almost nothing is yet known of the ecological growth conditions or the structure or morphology of such canopy root systems. This study reports on the occurrence of tree fine roots in crown humus pockets of old European beech (Fagus sylvatica L.) trees. The aim was to compare these canopy roots with the fine roots in the terrestrial organic layer soil in terms of fine root biomass density, root morphological traits, ectomycorrhizal colonisation and chemical composition of the root tissue, and to relate these root traits to the chemical properties of the respective soils. Fine root biomass density in crown humus pockets was ca. 7 times higher than in the terrestrial organic layer, even though soil chemical properties of both rooting media were similar. Fine roots in the canopy differed from terrestrial fine roots by lower specific root tip abundance, specific root length, and specific root surface area, all of which points to a longer lifespan of the fine roots in the canopy. Moreover, canopy roots revealed a lower percentage of root tips colonised by ectomycorrhizal fungi than terrestrial roots (87% vs. 93%). Chemical composition of the root tissue in canopy and terrestrial soils was similar for most elements, but canopy roots showed lower P, Fe, and Al concentrations and a higher N/P ratio than terrestrial roots. Root P concentrations of both canopy and terrestrial fine roots were closely related to soil P concentration, but not to soil C/P or N/P ratios. On the other hand, tissue N of canopy roots, but not of terrestrial roots, revealed a clear dependence on soil N and C/N values, suggesting a more limited N availability in the canopy soil compared to the terrestrial organic layer. However, the overall small differences in soil chemical properties between canopy and terrestrial organic layer soil cannot explain the markedly higher volumetric root density in the crown humus and the differences in ecomorphological traits between canopy and terrestrial soil. Instead, it is speculated that these differences are more likely a result of temporarily high water availability in crown humus pockets due to high water flow along the surface of branches to the central crown parts of the beech trees.     

Correlations between element concentrations in spruce foliage and forest soils

Abstract

The relationships between foliage element concentrations in red spruce and soil chemical properties were studied to determine if standard soil measurements of individual elements in soils were well correlated with the concentrations of these elements in foliage. Significant positive correlations between O horizon and foliage concentrations existed only for K, Mn and P. Significant negative correlations between the concentrations of the major divalent cations (i.e. Ca, Mg, Mn) and K in the foliage were found suggesting a possible antagonism between the mono‐ and divalent cations for uptake from the soil. Trees with the highest foliage concentrations of Ca also were determined to be growing on soils which were producing the best growth rates. Foliage concentrations of P, and to a lesser extent K and Mg, were below values considered to be adequate for optimum growth in red spruce.     

Environmental effects on soil NO concentrations and root N uptake in beech and spruce forests

This study aimed to investigate the shifts in net nitrogen (N) uptake and N compounds of fine roots over the vegetation period (i.e., spring, summer, autumn) and correlate this with NO concentration in the soil. Soil NO concentration was measured using gas lysimeters for collection and a chemiluminescence analyzer for quantification. Net N uptake by the roots was determined using the ¹⁵N enrichment technique. N pools were quantified using spectrophotometric techniques. Soil NO concentrations at beech and spruce forest sites were highest in spring (June), and lowest in winter (December). Total N of the roots was similar during the seasons and between the two years under study despite considerable variation of different N compounds. Net N uptake generally increased with higher N supply. Correlation analysis revealed a positive relationship between soil NO concentration and net N uptake only for spruce trees. This relationship seemed to be modulated by environmental factors and tree species.     

Nitrogen transformations and pools in N-saturated mountain spruce forest soils

Nitrogen leaching persists in mountain forests of Europe even in the presence of decreasing N depositions. We have hypothesized that this leaching is linked to soil N transformations occurring over the whole year, even at 0°C temperatures. The aims were to estimate (1) the effect of temperature on N transformations and (2) N pools and fluxes. The study sites are situated in the Bohemian Forest (Czech Republic). Litter, humus, and 0–10-cm mineral layers were sampled in early spring, and the effect of temperature on net nitrification, net ammonification, and microbial N immobilization were measured in a short-term incubation experiment without substrate addition. Nitrogen pools were calculated from the concentrations of N forms in the soil and soil pool weights, while daily N fluxes were calculated from daily net rates of processes and soil pool weights. Relationships between temperature and net nitrification, net ammonification, and microbial N immobilization did not follow the Arrhenius type equation; all processes were active close to 0°C, indicating that microbial N transformations occur over the whole year. Microbial N immobilization rate was generally greater than N mineralization rate. The microbial N pool was significantly larger than mineral N pools. Organic layers containing tens of grams of available N per square meter contributed more than 70% to the available N in the soil profile. Daily N fluxes were related to N pools. On average, N fluxes represented daily mineral and microbial N pool changes of 1.14 and 1.95%, respectively. The effect of microbial composition on the C/N ratio of microbial biomass and respiration is discussed.     

Acidification of soils in five catchments in Norway

Soil plots were established in catchments within the frame of the Norwegian Monitoring Programme for Long-Range Transported Air Pollutants. Four soil plots of about 10 m by 20 m were established within each catchment in areas with relatively homogeneous soil and vegetation. Multiple soil cores were taken at 50 points in a grid from each plot (repeated 4 times) and the samples were pooled by depths. Each soil core was divided by soil horizon, by predefined depths or by a combination of these, usually giving one soil sample for each 2-cm-depth down to about 15 cm. The soil pH measured in water suspension varied for all soils between 3.5 and 4.9. Base saturation measured at pH 7 varied between 2% and 22% with the highest values in the upper, more humus rich layers. All of the monitored catchments include acid soils poor in base cations. After 8 years the plots were resampled using the same procedure as for the first sampling. In most of the plots a decreasing pH trend from the first to the second sampling was measured. However, the base saturation was generally higher in all plots at the second sampling compared with the first. There are no clear effects of the different pollution regimes in the various catchments. It must be emphasized, however, that 8 years is a short period for measuring long-term soil changes.     

n-Alkanes and free fatty acids in humus and A1 horizons of soils under beech, spruce and grass in the Massif-Central (Mont-Lozère), France

Soil profiles under beech, spruce and a grassland have been analysed to study the evolution of natural n-alkanes in pollution-free ecosystems. The soils had all developed on granitic bedrock, at an altitude of 1300–1500 m in the region of Mont-Lozère (southern Massif-Central, France). In contrast to the grassland soil, the two forest soils both possessed a well-developed acidic moder humus-type horizon. This could be subdivided as follows: fresh litter (OL), fragmentation (OF) and humification (OH) layers; two litters, one fresh (OL1) and one old (OL2) could actually be distinguished in the beech forest soil. The n-alkane signature of the parent plants was preserved in the top litter. Immediately underneath, in the OF layer(s) the original n-alkane signatures were progressively but rapidly replaced by a common signature composed of n-C₂₇ and n-C₂₅ with larger proportions of the former than of the latter. These two hydrocarbons were most probably produced in situ by fungi. These results appear to illustrate the action of soil microorganisms which metabolize the inherited n-alkanes and produce new compounds of the same family. Unlike the alkanes and the low molecular weight fatty acids ≤ C₂₀ (which increase greatly in the OL2 layer under beech as a result of intense microbial activity), the heavy fatty acids (> C₂₀) show no significant change in the organic horizon.     

Mycorrhizal colonization and lead distribution in root tissues of Norway spruce seedlings

Non-inoculated spruce seedlings (Picea abies Karst.) and spruce seedlings colonized with Lactarius rufus Fr., Pisolithus tinctorius Coker & Couch or Paxillus involutus Fr. were grown for 35 or 37 weeks in a microcosm system on two types of natural forest humus differing in Pb content. Using X-ray microanalysis, the distribution and content of Pb in the tissues of mycorrhizal and non-mycorrhizal root tips were compared. No significant difference in the Pb contents of root cortex cell walls of non-mycorrhizal and seedlings colonized by Lactarius rufus, Pisolithus tinctorius, or indigenous mycorrhizal fungi (mainly Tylospora sp.) was found. However, in root tips of seedlings colonized by Paxillus involutus, due to a higher binding capacity for cations, the Pb content in cell walls of the root cortex were higher than in non-mycorrhizal roots. Pb contents in cell walls of the cortex of mycorrhizal and non-mycorrhizal roots were 3 times higher in plants growing in humus with a high Pb content than in plants growing in humus with a low Pb content. It is concluded that increasing contents of Pb in the organic matter may lead to an increased loading of the apoplast with Pb. The mycobionts tested in this investigation did not exclude Pb from root tissues.     

Correlations between soil ph and metal contents in needles of Norway spruce

The concentrations of 21 elements were determined in 1 yr old needles of Norway spruce (Picea abies) at 39 different sites: The following soil parameters were measured at 3 different depths: pH, loss on ignition, total and EDTA-extractable concentrations of 28 elements. Needle concentrations were tested for correlations with soil parameters. The following significant correlations were found with pH: Al, Co, Cs, Mn, and Rb in needles increased with decreasing soil pH, whereas Ba and Sr decreased. Needle concentrations of As, Ca, Cu, Fe, K, La, Mg, P, Sb, Sc, V, and Zn had no significant correlations with soil pH. AI and Ba in needles correlated also with A1 and Ba extracted from the soil. Needle concentrations of the pairs Co and Mn, Cs and Rb, and Ba and Sr showed significant positive interelement correlations, whereas concentrations of P and K had negative interelement correlations with Mn.     

Seasons differently impact the structure of mineral weathering bacterial communities in beech and spruce stands

Bacterial communities play an essential role in the sustainability of forest ecosystems by releasing from soil minerals the nutritive cations required not only for their own nutrition but also for that of trees. If it is admitted that the nutritional needs of trees vary during seasons, the seasonal dynamics of the mineral weathering bacterial communities colonizing the tree rhizosphere remain unknown. In this study, we characterized the mineral weathering efficacy of bacterial strains, from the rhizosphere and the adjacent bulk soil at four different seasons under two different tree species, the evergreen spruce and the deciduous beech, using a microplate assay that measures the quantity of iron released from biotite. We showed that the functional and taxonomic structures of the mineral weathering bacterial communities varied significantly with the tree species as well as with the season. Notably, the Burkholderia strains from the beech stand appeared more efficient to weather biotite that the one from the spruce stand. The mineral weathering efficacy of the bulk soil isolates did not vary during seasons under the beech stand whereas it was significantly higher for the spring and summer isolates from the spruce stand. The weathering efficacy of the rhizosphere isolates was significantly higher for the autumn isolates compared to the isolates sampled in the other seasons under the beech stand and in summer compared to the other seasons under spruce. These results suggest that seasonal differences do occur in forest soil bacterial communities and that evergreen and deciduous trees do not follow the same dynamic.     

Principal component analysis applied to nutrient balances in organic layers of beech and spruce forests

Yearly nutrient balances were established between August 1979 and October 1981 for the organic layers of beech and spruce forests. Principal component analysis has been applied to the monthly nutrient balances in order to understand the important differences which appear between both types of organic layers. Three main processes affect the redistribution of elements from the organic layers: exchange and solubilization, organic matter mineralization and nitrate immobilization and release. The relative importance of these three processes within the beech and the spruce layers is different. Moreover, it is shown that variations in the rainfall regime is an important factor which govern nitrogen redistribution and that rainfall acidity promotes phosphate leachings from the organic layers of both forests.     

Changes in humus properties and collembolan communities following the replanting of beech forests with spruce

Differences of collembolan communities within the organo-mineral A layer were studied in relation to physico-chemical changes in humus at nine sites of beech forests (Fagus sylvatica L.) and first generation spruce stands (Picea abies (L.) Karst.), planted on former beech forest 30 years ago (Central Pyrenees, France). Changes in humus form were caused by the spruce plantation and occurred mainly within the fermentation horizon where acidifying litter accumulation increased the horizon depth. The recent replacement of beech by spruce induced a shift from mull towards moder humus forms, which is explained by the decreasing organic matter turnover rate. A significant decrease in the three exchangeable cations and pH under spruce was also observed. Collembolan species diversity within the A horizon was significantly lower under spruce at this early stage of the silvogenetic cycle. Differences between species composition of Collembola between the two forest stands is related to changes in environmental conditions (e.g. nutrient availability, soil porosity, soil moisture). This study shows how forest management practices are susceptible to modify biological activity within the A horizon under exotic conifer plantations.     

Bioproductivity of spruce stands in northern European Russia

The bioproductivity of forest ecosystems in terms of forest succession and biodiversity has been studied in northern European Russia near Arkhangelsk since the 1930s. The relationship between forest productivity and forest treatments (thinning and other silvicultural practices) is still not well understood. In this paper, results are reported for spruce-birch stands of natural origin that have undergone thinnings of various intensities over the past 30 years. Changes in stand aboveground and root biomass were recorded in a series of re-measurements and for a series of different thinning intensities and are reported here. A number of biotic and abiotic factors play important roles in determining the productivity and the distribution in biomass components. These factors are changed by thinning treatments. Thus the productivity and structure of foliage as well as other aboveground and belowground biomass components were found to be dependent on the density and spatial distribution of the trees in the stand; production of merchantable stem-wood was also influenced by thinning treatments. The optimum density needed for maximum stand production, however, need not always be best from the economic perspective.     

Correction of magnesium deficiency in a young stand of Norway spruce

A trial testing of the effect of several types of Mg fertilizer was established in a 9 yr old plantation of Norway spruce at the Col du Donon in the Vosgian mountains (France). The plantation, which had exhibited symptoms of Mg deficiency (yellowing), had been fertilized 5 yr prior to this study with 40 kg ha⁻¹ of Mg applied as Epsom-salts. This alleviated the deficiency symptoms, but the yellowing reappeared 3 yr after the fertilization and increased until the year in which the second fertilization trial (described in this paper) was established. Five Mg fertilizers (2 forms of slag, 2 forms of limestone, and Epsom-salts) at two rates of Mg (40 and 80 kg ha⁻¹) were compared to a control. Just after this second fertilization, the trees became green again and small positive effects on circumference and basal area increments has been observed. However, 12 yr after the treatment, there were no differences between the treatments regarding needle coloration, circumference, basal area or volume.