The species richness–productivity relationship in the herb layer of European deciduous forests

Aim In contrast to non‐forest vegetation, the species richness–productivity (SR‐P) relationship in forests still remains insufficiently explored. Several studies have focused on the diversity of the tree layer, but the species richness of temperate deciduous forests is mainly determined by their species‐rich herb layer. The factors controlling herb‐layer productivity may differ from those affecting tree layers or open herbaceous vegetation, and thus the SR‐P relationship and its underlying processes may differ. However, the few relevant studies have reported controversial results. Here we explore the SR‐P relationship in the forest herb layer across different areas from oceanic to continental Europe, and put the effect of habitat productivity on species richness into context with other key factors, namely soil pH and light availability. Location North‐western Germany, Czech Republic, Slovakia and southern Urals (Russia). Methods We measured herb‐layer species richness and biomass, soil pH and tree‐layer cover in 156 vegetation plots of 100 m² in deciduous forests. We analysed the SR‐P relationship and the relative importance of environmental variables using regression models for particular areas and separate forest types. Results We found a consistent monotonic increase in the herb‐layer species richness with productivity across all study areas and all forest types. Soil pH and light availability also affected species richness, but their relative importance differed among areas. Main conclusions We suggest that the monotonically increasing SR‐P relationship in the forest herb layer results from the fact that herb‐layer productivity is limited by canopy shading; competition within the herb layer is therefore not strong enough to exclude many species. This differs fundamentally from open herbaceous vegetation, which is not subject to such productivity limits and consequently exhibits a unimodal SR‐P relationship. We present a conceptual model that might explain the differences in the SR‐P relationship between the forest herb layer and open herbaceous vegetation.     

Selection harvest in temperate deciduous forests: impact on herb layer richness and composition

Herb layer richness and composition of four forest types and three different management treatments were investigated in 16 deciduous forest stands of northern Germany. Specifically, we compared the species richness and composition occurring in mature forest stands that were single-tree and group selection harvested to those in unmanaged reference stands. Mean species richness of all herb layer species increased significantly with increasing harvest severity. When analyzing plant groups separately, it became obvious that this overall pattern was not consistent. While a negative relationship was detected between vernal herb richness and harvest severity, group selection harvest significantly increased species richness of summer herbaceous forest species and generalists. Woody species richness was not related to harvest severity. Community composition of the spring aspect was not significantly affected by selection harvest, whereas herb layer species composition in the summer aspect differed significantly among the three harvest treatments. A dominance of highly competitive shrub and generalist species was confined to some parts of the most intensively harvested stands. Overall, our results indicate that the herb layer community was not severely adversely affected by selection harvest at the intensities used in the studied stands. It is suggested that selection harvest systems may be feasible tools with which to conserve local forest vascular plant diversity and at the same time to meet the demand for timber products. However, information about forest history and the implementation of the selection harvest system are basic requirements when interpreting the results of studies on understorey response to selection harvest.     

Chemical composition of soil solutions extracted from New Zealand beech forests and West German beech and spruce forests

Concentrations of ions were measured in soil solutions from beech (Nothofagus) forests in remote areas of New Zealand and in solutions from beech (Fagus sylvatica) and Norway spruce (Picea abies) forests in North-East Bavaria, West Germany, to compare the chemistry of soil solutions which are unaffected by acid deposition (New Zealand) with those that are affected (West Germany). In New Zealand, soil solution SO₄ ^2– concentrations ranged between <2 and 58 mol L^–1, and NO₃ ^– concentrations ranged between <1 and 3 mol L^–1. In West Germany, SO₄ ^2– concentrations ranged between 80 and 700 mol L^–1, and NO₃ ^– concentrations at three of six sites ranged between 39 and 3750 mol L^–1, but was not detected at the remaining three sites. At all sites in New Zealand, and at sites where the soil base status was moderately high in West Germany, pH levels increased, and total Al (Al_t) and inorganic monomeric Al (Al_i) levels decreased rapidly with increasing soil depth. In contrast, at sites on soils of low base status in West Germany, pH levels increased only slightly, and Al levels did not decline with increasing soil depth.Under a high-elevation Norway spruce stand showing severe Mg deficiency and dieback symptoms in West Germany, soil solution Mg²⁺ levels ranged between 20 and 60 mol L^–, and were only half those under a healthy stand. Al_t and Al_i levels were substantially higher the healthy stand than under the unhealthy stand, indicating that Al toxicity was not the main cause of spruce decline.     

The symmetry of competitive interactions in mixed Norway spruce,silver fir and European beech forests

Questions

We aim for a better understanding of the different modes of intra‐ and inter‐specific competition in two‐ and three‐species mixed‐forests. How can the effect of different modes of competitive interactions be detected and integrated into individual tree growth models? Are species interactions in spruce–fir–beech forests more associated with size‐symmetric or size‐asymmetric competition? Do competitive interactions between two of these species change from two‐ to three‐species mixtures?

Location

Temperate mixed‐species forests in Central Europe (Switzerland).

Methods

We used data from the Swiss National Forest Inventory to fit basal area increment models at the individual tree level, including the effect of ecological site conditions and indices of size‐symmetric and size‐asymmetric competition. Interaction terms between species‐specific competition indices were used to disentangle significant differences in species interactions from two‐ to three‐species mixtures.

Results

The growth of spruce and fir was positively affected by increasing proportions of the other species in spruce–fir mixtures, but negative effects were detected with increasing presence of beech. We found that competitive interactions for spruce and fir were more related to size‐symmetric competition, indicating that species interactions might be more associated with competition for below‐ground resources. Under constant amounts of stand basal area, the growth of beech clearly benefited from the increasing admixture of spruce and fir. For this species, patterns of size‐symmetric and size‐asymmetric competitive interactions were similar, indicating that beech is a strong self‐competitor for both above‐ground and below‐ground resources. Only for silver fir and beech, we found significant changes in species interactions from two‐ to three‐species mixtures, but these were not as prominent as the effects due to differences between intra‐ and inter‐specific competition.

Conclusions

Species interactions in spruce–fir–beech, or other mixed forests, can be characterized depending on the mode of competition, allowing interpretations of whether they occur mainly above or below ground level. Our outcomes illustrate that species‐specific competition indices can be integrated in individual tree growth functions to express the different modes of competition between species, and highlight the importance of considering the symmetry of competition alongside competitive interactions in models aimed at depicting growth in mixed‐species forests.

     

Site and temporal variation of soil respiration in European beech, Norway spruce, and Scots pine forests

Global warming and changes in rainfall amount and distribution may affect soil respiration as a major carbon flux between the biosphere and the atmosphere. The objectives of this study were to investigate the site to site and interannual variation in soil respiration of six temperate forest sites. Soil respiration was measured using closed chambers over 2 years under mature beech, spruce and pine stands at both Solling and Unterlüß, Germany, which have distinct climates and soils. Cumulative annual CO₂ fluxes varied from 4.9 to 5.4 Mg C ha^?1 yr^?1 at Solling with silty soils and from 4.0 to 5.9 Mg C ha^?1 yr^?1 at Unterlüß with sandy soils. With one exception soil respiration rates were not significantly different among the six forest sites (site to site variation) and between the years within the same forest site (interannual variation). Only the respiration rate in the spruce stand at Unterlüß was significant lower than the beech stand at Unterlüß in both years. Soil respiration rates of the sandy sites at Unterlüß were limited by soil moisture during the rather dry and warm summer 1999 while soil respiration at the silty Solling site tended to increase. We found a threshold of ?80 kPa at 10 cm depth below which soil respiration decreased with increasing drought. Subsequent wetting of sandy soils revealed high CO₂ effluxes in the stands at Unterlüß. However, dry periods were infrequent, and our results suggest that temporal variation in soil moisture generally had little effect on annual soil respiration rates. Soil temperature at 5 cm and 10 cm depth explained 83% of the temporal variation in soil respiration using the Arrhenius function. The correlations were weaker using temperature at 0 cm (r² = 0.63) and 2.5 cm depth (r² = 0.81). Mean Q₁₀ values for the range from 5 to 15 °C increased asymptotically with soil depth from 1.87 at 0 cm to 3.46 at 10 cm depth, indicating a large uncertainty in the prediction of the temperature dependency of soil respiration. Comparing the fitted Arrhenius curves for same tree species from Solling and Unterlüß revealed higher soil respiration rates for the stands at Solling than in the respective stands at Unterlüß at the same temperature. A significant positive correlation across all sites between predicted soil respiration rates at 10 °C and total phosphorus content and C‐to‐N ratio of the upper mineral soil indicate a possible effect of nutrients on soil respiration.     

The response of soil solution chemistry in European forests to decreasing acid deposition

Acid deposition arising from sulphur (S) and nitrogen (N) emissions from fossil fuel combustion and agriculture has contributed to the acidification of terrestrial ecosystems in many regions globally. However, in Europe and North America, S deposition has greatly decreased in recent decades due to emissions controls. In this study, we assessed the response of soil solution chemistry in mineral horizons of European forests to these changes. Trends in pH, acid neutralizing capacity (ANC), major ions, total aluminium (Al_tot) and dissolved organic carbon were determined for the period 1995–2012. Plots with at least 10 years of observations from the ICP Forests monitoring network were used. Trends were assessed for the upper mineral soil (10–20 cm, 104 plots) and subsoil (40–80 cm, 162 plots). There was a large decrease in the concentration of sulphate () in soil solution; over a 10‐year period (2000–2010), decreased by 52% at 10–20 cm and 40% at 40–80 cm. Nitrate was unchanged at 10–20 cm but decreased at 40–80 cm. The decrease in acid anions was accompanied by a large and significant decrease in the concentration of the nutrient base cations: calcium, magnesium and potassium (Bc = Ca²⁺ + Mg²⁺ + K⁺) and Al_tot over the entire dataset. The response of soil solution acidity was nonuniform. At 10–20 cm, ANC increased in acid‐sensitive soils (base saturation ≤10%) indicating a recovery, but ANC decreased in soils with base saturation >10%. At 40–80 cm, ANC remained unchanged in acid‐sensitive soils (base saturation ≤20%,  ≤ 4.5) and decreased in better‐buffered soils (base saturation >20%,  > 4.5). In addition, the molar ratio of Bc to Al_tot either did not change or decreased. The results suggest a long‐time lag between emission abatement and changes in soil solution acidity and underline the importance of long‐term monitoring in evaluating ecosystem response to decreases in deposition.     

Water shortage affects the water and nitrogen balance in Central European beech forests

Whilst forest policy promotes cultivation and regeneration of beech dominated forest ecosystems, beech itself is a highly drought sensitive tree species likely to suffer from the climatic conditions prognosticated for the current century. Taking advantage of model ecosystems with cool-moist and warm-dry local climate, the latter assumed to be representative for future climatic conditions, the effects of climate and silvicultural treatment (different thinning regimes) on water status, nitrogen balance and growth parameters of adult beech trees and beech regeneration in the understorey were assessed. In addition, validation experiments with beech seedlings were carried out under controlled conditions, mainly in order to assess the effect of drought on the competitive abilities of beech. As measures of water availability xylem flow, shoot water potential, stomatal conductance as well as delta (13)C and delta (18)O in different tissues (leaves, phloem, wood) were analysed. For the assessment of nitrogen balance we determined the uptake of inorganic nitrogen by the roots as well as total N content and soluble N compounds in different tissues of adult and young trees. Retrospective and current analysis of delta (13)C, growth and meteorological parameters revealed that beech growing under warm-dry climatic conditions were impaired in growth and water balance during periods with low rain-fall. Thinning affected water, N balance and growth mostly of young beech, but in a different way under different local climatic conditions. Under cool, moist conditions, representative for the current climatic and edaphic conditions in beech forests of Central Europe, thinning improves nutrient and water status consistent to published literature and long-term experience of forest practitioners. However, beech regeneration was impaired as a result of thinning at higher temperatures and under reduced water availability, as expected in future climate.     

北京山区栎林的群落结构与物种组成

在北京东灵山、喇叭沟门和大海陀山地区的辽东栎(Quercus liaotungensis)林以及白龙潭地区的槲树(Q.dentata)-麻栎(Q.acutissima)混交林中共调查了43个样方,利用这些样方资料,对群落进行TWINSPAN聚类和CCA排序,以研究北京地区栎林的群落结构、物种组成以及各群落之间的关系。结果表明：本地区栎林群落结构简单,不同栎林群落的结构差异很大。利用TWINSPAN对乔木层物种进行了分类,按不同指示种划分为7类。CCA排序结果显示第一轴与海拔关系显著,主要反映了温度因子的作用;第二轴与坡向关系显著,反映了水分因子的作用。排序结果与分类结果吻合,显示了不同生境条件对群落结构和物种分布的影响。结果还显示,本地区物种丰富度低,这可能与本地区植被受人为干扰强烈有一定关系。     

Continental‐scale determinants of population trends in European amphibians and reptiles

The continuous decline of biodiversity is determined by the complex and joint effects of multiple environmental drivers. Still, a large part of past global change studies reporting and explaining biodiversity trends have focused on a single driver. Therefore, we are often unable to attribute biodiversity changes to different drivers, since a multivariable design is required to disentangle joint effects and interactions. In this work, we used a meta‐regression within a Bayesian framework to analyze 843 time series of population abundance from 17 European amphibian and reptile species over the last 45 years. We investigated the relative effects of climate change, alien species, habitat availability, and habitat change in driving trends of population abundance over time, and evaluated how the importance of these factors differs across species. A large number of populations (54%) declined, but differences between species were strong, with some species showing positive trends. Populations declined more often in areas with a high number of alien species, and in areas where climate change has caused loss of suitability. Habitat features showed small variation over the last 25 years, with an average loss of suitable habitat of 0.1%/year per population. Still, a strong interaction between habitat availability and the richness of alien species indicated that the negative impact of alien species was particularly strong for populations living in landscapes with less suitable habitat. Furthermore, when excluding the two commonest species, habitat loss was the main correlate of negative population trends for the remaining species. By analyzing trends for multiple species across a broad spatial scale, we identify alien species, climate change, and habitat changes as the major drivers of European amphibian and reptile decline.     

Relationships between the vegetation and soil conditions in beech and beech-oak forests of northern Germany

In this study we examine the relationships between the vegetation of beech and beech-oak forest communities (Hordelymo-Fagetum, Galio-Fagetum, Deschampsio-Fagetum, Betulo-Quercetum) and their soil conditions in the lowlands of northern Germany, based on 84 sample plots. In all plots the vegetation was recorded and soil parameters were analysed (thickness of the O- and the A-horizons, pH, S-value, base saturation, C/N, mean Ellenberg moisture indicator value). The vegetation classification according to the traditional Braun-Blanquet approach was compared with the result of a multivariate cluster analysis. Vegetation-site relationships were analysed by means of an indirect gradient analysis (DCA).Both traditional classification methods and the cluster analysis have produced comparable classification results. So far as the species composition is concerned, a similar grouping of sample plots was found in both approaches. Multivariate cluster analysis thus supports the classification found by the Braun-Blanquet method. The result of the DCA shows that the four forest communities mentioned above represent clearly definable ecological units. The main site factor influencing changes in the species composition is a base gradient, which is best expressed by the S-value. In addition, within the series Hordelymo-Fagetum - Galio-Fagetum - Deschampsio-Fagetum the C/N-ratios and the thickness of the organic layers (O-horizon) increase continuously. By contrast, the floristic differences between oligotrophic forest communities (i.e., Deschampsio-Fagetum and Betulo-Quercetum) cannot be explained by a base gradient and increasing C/N-ratios. It is suggested that a different forest management history in some cases (e.g., promotion of Quercus robur by silvicultural treatments) is responsible for differences in the species composition, but on the other hand the result of the DCA indicates that Fagus sylvatica is replaced by Quercus robur with increasing soil moisture (i.e., with the increasing influence of a high groundwater table). Summarizing these results, it can be concluded that the ecological importance of single site factors affecting the species composition changes within the entire site spectrum covered by the beech and beech-oak forests of northern Germany.     

Interactive effects between plant functional types and soil factors on tundra species diversity and community composition

Plant communities are coupled with abiotic factors, as species diversity and community composition both respond to and influence climate and soil characteristics. Interactions between vegetation and abiotic factors depend on plant functional types (PFT) as different growth forms will have differential responses to and effects on site characteristics. However, despite the importance of different PFT for community assembly and ecosystem functioning, research has mainly focused on vascular plants. Here, we established a set of observational plots in two contrasting habitats in northeastern Siberia in order to assess the relationship between species diversity and community composition with soil variables, as well as the relationship between vegetation cover and species diversity for two PFT (nonvascular and vascular). We found that nonvascular species diversity decreased with soil acidity and moisture and, to a lesser extent, with soil temperature and active layer thickness. In contrast, no such correlation was found for vascular species diversity. Differences in community composition were found mainly along soil acidity and moisture gradients. However, the proportion of variation in composition explained by the measured soil variables was much lower for nonvascular than for vascular species when considering the PFT separately. We also found different relationships between vegetation cover and species diversity according the PFT and habitat. In support of niche differentiation theory, species diversity and community composition were related to edaphic factors. The distinct relationships found for nonvascular and vascular species suggest the importance of considering multiple PFT when assessing species diversity and composition and their interaction with edaphic factors. Synthesis: Identifying vegetation responses to edaphic factors is a first step toward a better understanding of vegetation–soil feedbacks under climate change. Our results suggest that incorporating differential responses of PFT is important for predicting vegetation shifts, primary productivity, and in turn, ecosystem functioning in a changing climate.     

Variation in floristic and trait composition along environmental gradients in the herb layer of temperate forests in the transition zone between Central and SE Europe

Species- and trait-environment linkages in forest plant communities continue to be a frequent topic in ecological research. We studied the dependence of floristic and functional trait composition on environmental factors, namely local soil properties, overstory characteristics, climatic parameters and other abiotic and biotic variables. The study area comprised 50 monitoring plots across Slovenia, belonging to the EU ICP Forests monitoring network. Vegetation was surveyed in accordance with harmonized protocols, and environmental variables were either measured or estimated during vegetation sampling. Significant predictors of species composition were identified by canonical correspondence analysis. Correlations between plant traits, i.e. plant growth habit, life form, flowering features and CSR signature, were examined with fourth-corner analysis and linear regressions. Our results show that variation in floristic composition was mainly explained by climatic parameters (mean annual temperature, mean annual precipitation), soil properties (pH) and tree layer-dependent light conditions. Trait composition was most closely related with tree layer characteristics, such as shade-casting ability (SCA, a proxy for light availability in the understory layer), tree species richness and tree species composition. Amongst soil properties, total nitrogen content and soil texture (proportion of clay) were most frequently correlated with different species traits or trait states. The CSR signature of herb communities was associated with tree layer SCA, soil pH and mean annual temperature. The floristic composition of the studied herb-layer vegetation depended on temperature and precipitation, which are likely to be influenced by ongoing climate change (warming and drying). Trait composition exhibited significant links to tree layer characteristics and soil conditions, which are in turn directly modified by forest management interventions.

     

Vegetation in Danish beech forests: the importance of soil,microclimate and management factors,evaluated by variation partitioning

The importance of management regime on floristic variation (mosses and vascular plants) in four Danish beech forests was investigated. Sixty-four blocks were sampled, representing beech stands of different age and management types. Nineteen potential explanatory variables were recorded and tested with Monte-Carlo tests and Canonical Correspondence Analysis. In addition results were evaluated by use of Detrended Correspondence Analysis. Explanatory variables were divided into three groups; soil, microclimatic and management parameters. The amount of variation explained by each group of variables was calculated by use of variation partitioning. The group consisting of management variables explained most variation, on local as well as regional scale. Management related variables explained more variation in vegetation than any other variables together. This indicates the importance of management as determining species composition in Danish beech forests. Management related variables explained most variation on local scale. On a regional scale, soil parameters explained the major part of the variation. The results suggest that thirty years without management are sufficient to change species composition significantly, as compared to managed forests.     

Variation of soil and biomass carbon pools in beech forests across a precipitation gradient

Temperate forests have recently been identified as being continuing sinks for carbon even in their mature and senescent stages. However, modeling exercises indicate that a warmer and drier climate as predicted for parts of Central Europe may substantially alter the source/sink function of these economically important ecosystems. In a transect study with 14 mature European beech (Fagus sylvatica L.) forests growing on uniform geological substrate, we analyzed the influence of a large reduction of annual precipitation (970–520 mm yr^?1) on the carbon stocks in fast and slow pools, independent of the well‐known aging effect. We investigated the C storage in the organic L, F, H layers, the mineral soil to 100 cm, and in the biomass (stem, leaves, fine roots), and analyzed the dependence of these pools on precipitation. Soil organic carbon decreased by about 25% from stands with > 900 mm yr^?1 to those with < 600 mm yr^?1; while the carbon storage in beech stems slightly increased. Reduced precipitation affected the biomass C pool in particular in the fine root fraction but much less in the leaf biomass and stem fractions. Fine root turnover increased with a precipitation reduction, even though stand fine root biomass and SOC in the organic L, F, and H layers decreased. According to regression analyses, the C storage in the organic layers was mainly controlled by the size of the fine root C pool suggesting an important role of fine root turnover for the C transfer from tree biomass to the SOC pool. We conclude that the long‐term consequence of a substantial precipitation decrease would be a reduction of the mineral soil and organic layer SOC pools, mainly due to higher decomposition rates. This could turn temperate beech forests into significant carbon sources instead of sinks under global warming.     

Prediction of macrofungal occurrence in Swedish beech forests from soil and litter variable models

The spatial distribution of 49 macrofungal species in Swedish beech forests was related to the statistical variation in 31 edaphic variables. In order to reduce the multicollinearity problem, the variables were transformed into eight principal components, PCs, which are used in two-group discriminant analysis (on absence/presence patterns) and multiple regression analysis (on number of fruit-bodies). The results suggested that base saturation and organic matter content are of outstanding importance. However, significant relationships were also found with other variables, i.e. Cd or Zn in soil and litter, soil nitrogen mineralization rate, and Na or S in litter. One interesting interpretation of the results is that fungi do not only respond to the main variables of a gradient (soil pH, organic mater, base saturation) but also to other variables. Attempts were made to interpret the PCs to characterize fungal occurrence from the models they formed.     

Ammonium,nitrate and dissolved organic nitrogen in seepage water as affected by compost amendment to European beech,Norway spruce,and Scots pine forests

Fertilization of nutrient-depleted and degraded forest soils may be required to sustain utilization of forests. In some European countries, the application of composts may now be an alternative to the application of inorganic fertilizers because commercial compost production has increased and compost quality has been improved. There is, however, concern that compost amendments may cause increased leaching of nitrogen, trace metals and toxic organic compounds to groundwater. The objective of this study was to assess the risk of ammonium (NH₄ ⁺), nitrate (NO₃ ^–) and dissolved organic nitrogen (DON) leaching following a single compost application to silty and sandy soils in mature beech (Fagus sylvatica L.), pine (Pinus silvestris L.) and spruce (Picea abies Karst.) forests at Solling and Unterlüß in Lower Saxony, Germany. Mature compost from separately collected organic household waste was applied to the soil surface at a rate of 6.3 kg m^–2 in the summer of 1997 and changes in NH₄ ⁺, NO₃ ^– and DON concentrations in throughfall and soil water at 10 and 100 cm soil depths were determined for 32 months. The spruce forests had the highest N inputs by throughfall water and the highest N outputs in both the control and compost plots compared with the pine and beech forests. Overall, the differences in total N outputs at 100 cm soil depth between the control and compost plots ranged between 0.3 and 11.2 g N m^–2 for the entire 32-month period. The major leaching of these amounts occurred during the first 17 months after compost amendments, but there was no significant difference in total N outputs (–0.2 to 1.8 g N m^–2) between the control and compost plots during the remaining 15 months. Most of the mineral soils acted as a significant sink for NO₃ ^– and DON as shown by a reduction of their outputs from 10 to 100 cm depth. Based on these results, we conclude that application of mature compost with high inorganic N contents could diminish the groundwater quality in the first months after the amendments. A partial, moderate application of mature compost with low inorganic N content to nutrient depleted forest soils can minimize the risk of NO₃ ^– leaching.     

Stand fine root biomass and fine root morphology in old-growth beech forests as a function of precipitation and soil fertility

Only very limited information exists on the plasticity in size and structure of fine root systems, and fine root morphology of mature trees as a function of environmental variation. Six northwest German old-growth beech forests (Fagus sylvatica L.) differing in precipitation (520 – 1030 mm year^–1) and soil acidity/fertility (acidic infertile to basic fertile) were studied by soil coring for stand totals of fine root biomass (0–40 cm plus organic horizons), vertical and horizontal root distribution patterns, the fine root necromass/biomass ratio, and fine root morphology (root specific surface area, root tip frequency, and degree of mycorrhizal infection). Stand total of fine root biomass, and vertical and horizontal fine root distribution patterns were similar in beech stands on acidic infertile and basic fertile soils. In five of six stands, stand fine root biomass ranged between 320 and 470 g m^–2; fine root density showed an exponential decrease with soil depth in all profiles irrespective of soil type. An exceptionally small stand fine root biomass (<150 g m^–2) was found in the driest stand with 520 mm year^–1 of rainfall. In all stands, fine root morphological parameters changed markedly from the topsoil to the lower profile; differences in fine root morphology among the six stands, however, were remarkably small. Two parameters, the necromass/biomass ratio and fine root tip density (tips per soil volume), however, were both much higher in acidic than basic soils. We conclude that variation in soil acidity and fertility only weakly influences fine root system size and morphology of F. sylvatica, but affects root system structure and, probably, fine root mortality. It is hypothesized that high root tip densities in acidic infertile soils compensate for low nutrient supply rates, and large necromasses are a consequence of adverse soil chemical conditions. Data from a literature survey support the view that rainfall is another major environmental factor that influences the stand fine root biomass of F. sylvatica.     

Heterogeneity of soil and soil solution chemistry under Norway spruce (Picea abies Karst.) and European beech (Fagus silvatica L.) as influenced by distance from the stem basis

The present study aims at characterizing plant water status under field conditions on a daily basis, in order to improve operational predictions of plant water stress. Ohm's law analog serves as a basis for establishing daily soil-plant relationships, using experimental data from a water-limited soybean crop: 227-1. The daily transpiration flux, T, is estimated from experimental evapotranspiration data and simulated soil evaporation values. The difference, 227-2, named the effective potential gradient, is derived from i) the midday leaf potential of the uppermost expanded leaves and ii) an effective soil potential accounting for soil potential profile and an effectiveness factor of roots competing for water uptake. This factor is experimentally estimated from field observation of roots. G is an apparent hydraulic conductance of water flow from the soil to the leaves. The value of the lower potential limit for water extraction, required to assess the effective soil potential, is calculated with respect to the plant using the predawn leaf potential. It is found to be equal to –1.2 MPa. It appears that over the range of soil and climatic conditions experienced, the daily effective potential gradient remains constant (1.2 MPa), implying that, on a daily basis, transpiration only depends on the hydraulic conductance. The authors explain this behaviour by diurnal variation of osmotic potential, relying on Morgan's theory (1984). Possible generalization of the results to other crop species is suggested, providing a framework for reasoning plant water behaviour at a daily time step.