Fungal community composition shifts along a leaf degradation gradient in a European beech forest

Aims

The fungal communities in living and decomposed leaves of European Beech (Fagus sylvatica) were compared to identify the phyllosphere fungi involved in litter decomposition at a site in Bavaria, Germany.

Methods

New primers were designed to cover a broad range of fungal ribosomal DNA sequence diversity. Following ‘environmental PCR’, clone libraries from each of five samples of living leaves (surface-sterilized and untreated), freshly fallen, initially and highly decomposed leaves, were screened using RFLP fingerprinting.

Results

Statistical analysis (ANOSIM) revealed that the fungal communities colonizing living (a) and initially decomposed leaves (c) significantly differed between each other and from freshly fallen (b) and highly decomposed leaves (d). Fungal assemblages of a and d were statistically indistinguishable from each other and from the endophyllous fungal community in living leaves.

Conclusions

The results showed that endophyllous fungi play a role throughout the whole decomposition process of beech leaf litter. Therefore, clarification of the life cycle of certain endophytic and/or soil fungi may only be achieved by considering both phyllosphere and soil habitats.     

Root and stem xylem embolism,stomatal conductance,and leaf turgor in Acer grandidentatum populations along a soil moisture gradient

The objective of this study was to determine how adjustment in stomatal conductance (g ^s) and turgor loss point (_tlp) between riparian (wet) and neighboring slope (dry) populations of Acer grandidentum Nutt. was associated with the susceptibility of root versus stem xylem to embolism. Over two summers of study (1993–1994), the slope site had substantially lower xylem pressures (_px) and g ^s than the riparian site, particularly during the drought year of 1994. The _tlp was also lower at the slope (-2.9±0.1 MPa; all errors 95% confidence limits) than at riparian sites (-1.9±0.2 MPa); but it did not drop in response to the 1994 drought. Stem xylem did not differ in vulnerability to embolism between sites. Although slope-site stems lost a greater percentage of hydraulic conductance to embolism than riparian stems during the 1994 drought (46±11% versus 27±3%), they still maintained a safety margin of at least 1.7 MPa between midday _px and the critical pressure triggering catastrophic xylem embolism (_pxCT). Root xylem was more susceptible to embolism than stem xylem, and there were significant differences between sites: riparian roots were completely cavitated at -1.75 MPa, compared with -2.75 MPa for slope roots. Vulnerability to embolism was related to pore sizes in intervessel pit membranes and bore no simple relationship to vessel diameter. Safety margins from _pxCT averaged less than 0.6 MPa in roots at both the riparian and slope sites. Minimal safety margins at the slope site during the drought of 1994 may have led to the almost complete closure of stomata (g _s=9±2 versus 79±15 mmol m^-2 s^-1 at riparian site) and made any further osmotic adjustment of _tlp non-adaptive. Embolism in roots was at least partially reversed after fall rains. Although catastrophic embolism in roots may limit the minimum for gas exchange, partial (and reversible) root embolism may be adaptive in limiting water use as soil water is exhausted.     

Comparison of leaf gas exchange and stable isotope signature of water-soluble compounds along canopy gradients of co-occurring Douglas-fir and European beech

Combined δ(13) C and δ(18) O analyses of water-soluble leaf and twig phloem material were used to determine intrinsic water-use efficiency (iWUE) and variability of stomatal conductance at different crown positions in adult European beech (Fagus sylvatica) and Douglas-fir (Pseudotsuga menziesii) trees. Simultaneous gas exchange measurements allowed evaluation of the differences in calculating iWUE from leaf or phloem water-soluble compounds, and comparison with a semi-quantitative dual isotope model to infer variability of net photosynthesis (A(n) ) between the investigated crown positions. Estimates of iWUE from δ(13) C of leaf water-soluble organic matter (WSOM) outperformed the estimates from phloem compounds. In the beech crown, δ(13) C of leaf WSOM coincided clearly with gas exchange measurements. The relationship was not as reliable in the Douglas-fir. The differences in δ(18) O between leaf and phloem material were found to correlate with stomatal conductance. The semi-quantitative model approach was applicable for comparisons of daily average A(n) between different crown positions and trees. Intracanopy gradients were more pronounced in the beech than in the Douglas-fir, which reached higher values of iWUE at the respective positions, particularly under dry air conditions.     

Environmental and stomatal control of transpiration, canopy conductance and decoupling coefficient in young lemon trees under shading net

Sap flow, as a measure of transpiration, was monitored in 2-year-old lemon trees growing in pots. Eight trees were used in the experiment, four of which were placed under a rectangular shading net, while the other four were maintained in the open air. Daily averages of canopy conductance and photosynthesis were not affected by shading; however, the daily transpiration was reduced in shaded plants, which displayed an increase in water use efficiency compared with exposed trees. The decoupling coefficient was higher in the shaded trees, indicating that the transpiration of lemon trees was efficiently controlled by stomata in exposed plants, while the transpiration rate was mainly influenced by radiation in the plants growing under the net. This influence was more pronounced in the afternoon, when the whole tree transpiration was largely dominated by equilibrium transpiration in the plants under netting, and the relationship between transpiration and radiation showed a steeper slope in shaded trees. The reduction in transpiration and the maintenance of photosynthesis in shaded plants with respect to exposed trees indicated that screen structures in semi-arid and arid environments could be considered as an intermediate solution for reducing plant water stress and increasing water use efficiency.     

Canopy photosynthetic production in a Japanese larch stand. I. Seasonal and vertical changes of leaf characteristics along the light gradient in a canopy

In an 18 year old Japanese larch stand, leaf characteristics such as area, weight, gross photosynthetic rate and respiration rate were studied in order to obtain basic information on estimating canopy photosynthesis and respiration. The leaf growth courses in area and weight from bud opening were approximated by simple logistic curves. The growth coefficient for the area growth curve was 0.155–0.175 day⁻¹, while that for the weight growth was 0.112–0.117 day⁻¹. The larger growth coefficient in area growth caused the seasonal change in specific leaf area (SLA) that increased after bud opening to its peak early in May at almost 300 cm² g⁻¹ and then decreased until it leveled off at about 140 cm²g⁻¹. The change inSLA indicates the possibility that leaf area growth precedes leaf thickness growth. The relationship between the coefficientsa andb of the gross photosynthetic rate (p)-light flux density (1) curve (p=bI/(1+aI)) and the mean relative light flux density (I′/I ₀) at each canopy height were approximated by hyperbolic formulae:a=A/(I′/I ₀)+B andb=C/(I′/I ₀)+D. Leaf respiration rate was also increased with increasingI′/I ₀. Seasonal change of gross photosynthetic rate and leaf respiration rate were related to mean air temperature through linear regression on semilogarithmic co-ordinates.     

Altitudinal variation in stomatal conductance,nitrogen content and leaf anatomy in different plant life forms in New Zealand

Summary This study is part of a series of investigations on the influence of altitude on structure and function of plant leaves. Unlike most other mountain areas, the Southern Alps of New Zealand provide localities where physiologically effective moisture stress occurs neither at high nor at low elevation, but the changes in temperature and radiation with elevation are similar or even steeper than in most other regions. In comparison with results from other mountains, where moisture may impair plant functioning at low elevation, this study allows an estimation of the relative role of water for the expression of various leaf features typically associated with alpine plants. Maximum leaf diffusive conductance (g), leaf nitrogen content (LN), stomatal density (n) and distribution, as well as area (A), thickness (d) and specific area (SLA) of leaves were studied. Three different plant life forms were investigated over their full altitudinal range (m): trees, represented by Nothofagus menziesii (1,200 m), ericaceous dwarf shrubs (1,700 m), and herbaceous plants of the genus Ranunculus (2,500 m). In all three life forms g, LN, and n increased, while SLA and A decreased with elevation. Recent investigations have found similar trends in other mountains from the temperate zone, but the changes are larger in New Zealand than elsewhere. Herbs show the greatest differences, followed by shrubs and then trees.It is concluded that g is dependent upon light climate rather than water supply, whereas SLA and related structural features appear to be controlled by the temperature regime, as they show similar altitudinal changes under different light and moisture gradients. The higher leaf nitrogen content found at high elevations in all three life forms, suggests that metabolic activity of mature leaves is not restricted by low nitrogen supply at high altitude. In general, the leaves of herbaceous plants show more pronounced structural and functional changes with altitude than the leaves of shrubs and trees.     

Differential coordination of stomatal conductance,mesophyll conductance,and leaf hydraulic conductance in response to changing light across species

Stomatal conductance (g_s) and mesophyll conductance (g_m) represent major constraints to photosynthetic rate (A), and these traits are expected to coordinate with leaf hydraulic conductance (K_leaf) across species, under both steady‐state and dynamic conditions. However, empirical information about their coordination is scarce. In this study, K_leaf, gas exchange, stomatal kinetics, and leaf anatomy in 10 species including ferns, gymnosperms, and angiosperms were investigated to elucidate the correlation of H₂O and CO₂ diffusion inside leaves under varying light conditions. Gas exchange, K_leaf, and anatomical traits varied widely across species. Under light‐saturated conditions, the A, g_s, g_m, and K_leaf were strongly correlated across species. However, the response patterns of A, g_s, g_m, and K_leaf to varying light intensities were highly species dependent. Moreover, stomatal opening upon light exposure of dark‐adapted leaves in the studied ferns and gymnosperms was generally faster than in the angiosperms; however, stomatal closing in light‐adapted leaves after darkening was faster in angiosperms. The present results show that there is a large variability in the coordination of leaf hydraulic and gas exchange parameters across terrestrial plant species, as well as in their responses to changing light.     

Carbon isotopic composition and oxygen isotopic enrichment in phloem and total leaf organic matter of European beech (Fagus sylvatica L.) along a climate gradient

This study investigated the influence of climate on the carbon isotopic composition (sigma13C) and oxygen isotopic enrichment (delta18O) above the source water of different organic matter pools in European beech. In July and September 2002, sigma13C and delta18O were determined in phloem carbohydrates and in bulk foliage of adult beech trees along a transect from central Germany to southern France, where beech reaches its southernmost distributional limit. The data were related to meteorological and physiological parameters. The climate along the transect stretches from temperate [subcontinental (SC)] to submediterranean (SM). Both sigma13Cleaf and delta18Oleaf were representative of site-specific long-term environmental conditions. sigma13C of leaves collected in September was indicative of stomatal conductance, vapour pressure deficit (VPD) and radiation availability of the current growing season. delta18O was mainly correlated to mean growing season relative humidity (RH) and VPD. In contrast to the leaves, sigma13Cphloem varied considerably between July and September and was well correlated with canopy stomatal conductance (Gs) in a 2 d integral prior to phloem sampling. The relationship between sigma13C and delta18O in both leaves and phloem sap points, however, to a combined influence of stomatal conductance and photosynthetic capacity on the variation of sigma13C along the transect. delta18Ophloem could be described by applying a model that included 18O fractionation associated with water exchange between the leaf and the atmosphere and with the production of organic matter. Hence, isotope signatures can be used as effective tools to assess the water balance of beech, and thus, help predict the effects of climatic change on one of the ecologically and economically most important tree species in Central Europe.     

Seasonal changes of leaf surface contamination in beech, oak, and ginkgo in relation to leaf micromorphology and wettability

The leaf surfaces of beech, oak and ginkgo have been investigated with respect to contamination with particles during one growing season. Based on the observation that particles are removed from water-repellent leaves by rain (Lotus effect) the three species were selected because they differ in leaf surface micromorphology and wettability. Leaves of beech are smooth, lacked wax crystals and were ±wettable. Those of ginkgo were rough because their cells were convex and were densely covered by wax crystals, resulting in permanent water repellency. Leaves of oak were covered by waxes and were water repellent when young, but, a few weeks after leaf expansion had ceased the waxes were rapidly eroded. These differences in wettability resulted in different amounts of contamination. Ginkgo collected a very small number of particles during the whole vegetation period. In beech the contamination was significantly higher, but fairly constant, whereas oak leaves accumulated particles with age.     

Non-steady-state, non-uniform transpiration rate and leaf anatomy effects on the progressive stable isotope enrichment of leaf water along monocot leaves

This study focuses on the spatial patterns of transpiration-driven water isotope enrichment (Delta(lw)) along monocot leaves. It has been suggested that these spatial patterns are the result of competing effects of advection and (back-)diffusion of water isotopes along leaf veins and in the mesophyll, but also reflect leaf geometry (e.g. leaf length, interveinal distance) and non-uniform gas-exchange parameters. We therefore developed a two-dimensional model of isotopic leaf water enrichment that incorporates new features, compared with previous models, such as radial diffusion in the xylem, longitudinal diffusion in the mesophyll, non-uniform gas-exchange parameters and non-steady-state effects. The model reproduces well all published measurements of Delta(lw) along monocot leaf blades, except at the leaf tip and given the uncertainties on measurements and model parameters. We show that the longitudinal diffusion in the mesophyll cannot explain the observed reduction in the isotope gradient at the leaf tip. Our results also suggest that the observed differences in Delta(lw) between C(3) and C(4) plants reflect more differences in mesophyll tortuosity rather than in leaf length or interveinal distance. Mesophyll tortuosity is by far the most sensitive parameter and different values are required for different experiments on the same plant species. Finally, using new measurements of non-steady-state, spatially varying leaf water enrichment we show that spatial patterns are in steady state around midday only, just as observed for bulk leaf water enrichment, but can be easily upscaled to the whole leaf level, regardless of their degree of heterogeneity along the leaf.     

A high-performance system of multiple gas-exchange chambers with a laser spectrometer to estimate leaf photosynthesis,stomatal conductance,and mesophyll conductance

Journal of Plant Research - Direct measurements of ecophysiological processes such as leaf photosynthesis are often hampered due to the excessive time required for gas-exchange measurements and the...     

太白山栎属树种气孔特征沿海拔梯度的变化规律

气孔是植物与外界环境进行水分和气体交换的主要通道,调节植物碳同化和水分散失的平衡关系,在一定程度上反映植物对外界环境变化的适应。沿太白山北坡1100—2300 m海拔,测定4种栎属树种的气孔性状,分析气孔性状沿海拔的变化规律和其对环境因子的响应。结果表明:(1)气孔密度与气孔长度间的负相关在4个树种间均显著存在(P0.05);除栓皮栎(Quercus variabilis)外,气孔密度与潜在气孔导度指数的正相关关系均达显著水平;而气孔宽度与气孔长度之间只在栓皮栎和锐齿栎(Q. aliena var. acuteserrata)达到显著水平。(2)栓皮栎和槲栎(Q. aliena)的气孔长度和宽度随海拔升高而下降,气孔密度、潜在气孔导度指数增加,辽东栎(Q. wutaishansea)变化形式则相反;锐齿栎气孔宽度减小,其余性状沿海拔呈单峰变化,在约1600 m处气孔长度达到最小值,气孔密度和潜在气孔导度指数达到最大值。(3)与土壤因子相比,气孔性状主要受气候因素的影响。潜在气孔导度指数与大气温度、空气湿度成极显著正相关(P0.01),与降水量显著负相关(P0.05)。其中,空气相对湿度是影响潜在气孔导度指数的主要因素,能够解释气孔变异的22.9%。本研究结果对于深入认识秦岭太白山地区栎属树种对环境变化的响应和适应提供理论证据。     

Effect of vanadate on bean leaf movement, stomatal conductance, barley leaf unrolling, respiration, and phosphatase activity

Vanadate (Na₃ VO₄) inhibits leaf movement and stomatal conductance of Phaseolus vulgaris L. cv. Carlos Favorit in light-dark cycles as well as photomorphogenetic leaf unrolling of Hordeum vulgare L. cv. Rupal. Inhibition was 50% by 10 to 100 micromolar vanadate and 100% by millimolar vanadate. Leaf unrolling was also inhibited by oligomycin and diethylstilbestrol.     

Coordinated changes in photosynthesis,water relations and leaf nutritional traits of canopy trees along a precipitation gradient in lowland tropical forest

We investigated leaf physiological traits of dominant canopy trees in four lowland Panamanian forests with contrasting mean annual precipitation (1,800, 2,300, 3,100 and 3,500 mm). There was near complete turn-over of dominant canopy tree species among sites, resulting in greater dominance of evergreen species with long-lived leaves as precipitation increased. Mean structural and physiological traits changed along this gradient as predicted by cost–benefit theories of leaf life span. Nitrogen content per unit mass (N_mass) and light- and CO₂-saturated photosynthetic rates per unit mass (P_mass) of upper canopy leaves decreased with annual precipitation, and these changes were partially explained by increasing leaf thickness and decreasing specific leaf area (SLA). Comparison of 1,800 mm and 3,100 mm sites, where canopy access was available through the use of construction cranes, revealed an association among extended leaf longevity, greater structural defense, higher midday leaf water potential, and lower P_mass, N_mass, and SLA at wetter sites. Shorter leaf life spans and more enriched foliar ¹⁵N values in drier sites suggest greater resorption and re-metabolism of leaf N in drier forest. Greater dominance of short-lived leaves with relatively high P_mass in drier sites reflects a strategy to maximize photosynthesis when water is available and to minimize water loss and respiration costs during rainless periods. Overall, our study links coordinated change in leaf functional traits that affect productivity and nutrient cycling to seasonality in lowland tropical forests.     

Use of a model of photosynthesis and leaf microenvironment to predict optimal stomatal conductance and leaf nitrogen partitioning

Abstract. A model of photosynthesis (PGEN) is presented. The model assumes that optimal use is made of the leaf nitrogen available for partitioning between the carboxylase and thylakoid components. This results in predictions of Rubisco and chlorophyll concentrations very similar to those measured elsewhere. A function is incorporated which represents the detrimental effects of negative leaf water potentials on the Calvin cycle, producing a quantitative and mechanistic trade-off between CO2 entering, and H₂O leaving, the leaf. Thus, an optimal stomatal conductance and associated internal partial pressure of CO₂ exists for any given set of environmental conditions. The model calculates this optimal state for the leaf, which is its output. The model was subjected to changes in the following parameters: soil water potential, irradiance, ambient CO2 partial pressure, leaf temperature, leaf-to-air vapour pressure deficit, wind speed, atmospheric pressure, leaf nitrogen content, root dry weight and leaf width. These perturbations resulted in changes in predicted optimal conductance which were very similar to what has been observed. In general, as the capacity of the leaf to fix CO₂ increased, so did the predicted optimal conductance, with the internal partial pressure of CO2 being maintained close to 22Pa.     

Photosynthetic induction state of leaves in a soybean canopy in relation to light regulation of ribulose-1-5-bisphosphate carboxylase and stomatal conductance

Photosynthetic induction state, stomatal conductance and light regulation of ribulose-1,5-bisphosphate carboxylase (rubisco) were examined for leaves in a mature, closed soybean (Glycine max) canopy (leaf area index approximately 5) with the objective to determine the extent to which these factors may be limiting the capacity to respond to light transients during sunflecks. When sampled along a vertical gradient, leaves near the bottom of the canopy had lower rubisco contents and chlorophyll a/b ratios as compared with upper leaves. Leaves sampled at midcanopy showed a wide variation in photosynthetic induction state (ratio of the photosynthetic rate achieved after 1 minute exposure to high light to the steady-state assimilation rate achieved after 20 minutes exposure). Both photosynthetic induction state and the initial rubisco activity varied in parallel with stomatal conductance. By contrast there was no correlation between total rubisco activity and stomatal conductance. The results indicate that induction state, as determined by the light regulation of both rubisco activity and stomatal conductance, is an important limitation to the ability of leaves in a soybean canopy to respond to light transients that occur during sunflecks.     

Summer drought exposure,stand structure,and soil properties jointly control the growth of European beech along a steep precipitation gradient in northern Germany

Increasing exposure to climate warming-related drought and heat threatens forest vitality in many regions on earth, with the trees' vulnerability likely depending on local climatic aridity, recent climate trends, edaphic conditions, and the drought acclimatization and adaptation of populations. Studies exploring tree species' vulnerability to climate change often have a local focus or model the species' entire distribution range, which hampers the separation of climatic and edaphic drivers of drought and heat vulnerability. We compared recent radial growth trends and the sensitivity of growth to drought and heat in central populations of a widespread and naturally dominant tree species in Europe, European beech (Fagus sylvatica), at 30 forest sites across a steep precipitation gradient (500–850 mm year⁻¹) of short length to assess the species' adaptive potential. Size-standardized basal area increment remained more constant during the period of accelerated warming since the early 1980s in populations with >360 mm growing season precipitation (April–September), while growth trends were negative at sites with <360 mm. Climatic drought in June appeared as the most influential climatic factor affecting radial growth, with a stronger effect at drier sites. A decadal decrease in the climatic water balance of the summer was identified as the most important factor leading to growth decline, which is amplified by higher stem densities. Inter-annual growth variability has increased since the early 1980s, and variability is generally higher at drier and sandier sites. Similarly, within-population growth synchrony is higher at sandier sites and has increased with a decrease in the June climatic water balance. Our results caution against predicting the drought vulnerability of trees solely from climate projections, as soil properties emerged as an important modulating factor. We conclude that beech is facing recent growth decline at drier sites in the centre of its distribution range, driven by climate change-related climate aridification.     

Carbonyl sulfide (COS) as a tracer for canopy photosynthesis, transpiration and stomatal conductance: potential and limitations

The theoretical basis for the link between the leaf exchange of carbonyl sulfide (COS), carbon dioxide (CO(2)) and water vapour (H(2)O) and the assumptions that need to be made in order to use COS as a tracer for canopy net photosynthesis, transpiration and stomatal conductance, are reviewed. The ratios of COS to CO(2) and H(2)O deposition velocities used to this end are shown to vary with the ratio of the internal to ambient CO(2) and H(2)O mole fractions and the relative limitations by boundary layer, stomatal and internal conductance for COS. It is suggested that these deposition velocity ratios exhibit considerable variability, a finding that challenges current parameterizations, which treat these as vegetation-specific constants. COS is shown to represent a better tracer for CO(2) than H(2)O. Using COS as a tracer for stomatal conductance is hampered by our present poor understanding of the leaf internal conductance to COS. Estimating canopy level CO(2) and H(2)O fluxes requires disentangling leaf COS exchange from other ecosystem sources/sinks of COS. We conclude that future priorities for COS research should be to improve the quantitative understanding of the variability in the ratios of COS to CO(2) and H(2)O deposition velocities and the controlling factors, and to develop operational methods for disentangling ecosystem COS exchange into contributions by leaves and other sources/sinks. To this end, integrated studies, which concurrently quantify the ecosystem-scale CO(2), H(2)O and COS exchange and the corresponding component fluxes, are urgently needed.