Mature Beech Trees Research Articles

Differences between tree stem CO2 efflux and O2 influx rates cannot be explained by internal CO2 transport or storage in large beech trees.

Tree stem respiration (RS ) is a substantial component of the forest carbon balance. The mass balance approach uses stem CO2 efflux and internal xylem fluxes to sum up RS , while the oxygen-based method assumes O2 influx as a proxy of RS . So far, both approaches have yielded inconsistent results regarding the fate of respired CO2 in tree stems, a major challenge for quantifying forest carbon dynamics. We collected a data set of CO2 efflux, O2 influx, xylem CO2 concentration, sap flow, sap pH, stem temperature, nonstructural carbohydrates concentrationand potential phosphoenolpyruvate carboxylase (PEPC)capacity on mature beech trees to identify the sources of differences between approaches. The ratio of CO2 efflux to O2 influx was consistently below unity (0.7) along a 3-m vertical gradient, but internal fluxes did not bridge the gap between influx and efflux, nor did we find evidence for changes in respiratory substrate use. PEPC capacity was comparable with that previously reported in green current-year twigs. Although we could not reconcile differences between approaches, results shed light on the uncertain fate of CO2 respired by parenchyma cells across the sapwood. Unexpected high values of PEPC capacity highlight its potential relevance as a mechanism of local CO2 removal, which merits further research.

Late-season biosynthesis of leaf fatty acids and n-alkanes of a mature beech (Fagus sylvatica) tree traced via 13CO2 pulse-chase labelling and compound-specific isotope analysis.

Leaf cuticular waxes play an important role in reducing evapotranspiration via diffusion. However, the ability of mature trees to regulate the biosynthesis of waxes to changing conditions (e.g., drought, light exposition) remain an open question, especially during the late growing season. This holds also true for one of the most widely distributed trees in Central Europe, the European beech tree (Fagus sylvatica L.). In order to investigate the ongoing formation of wax constituents like alkanes and fatty acids, we conducted a 13CO2 pulse-chase labelling experiment on sun-exposed and shaded branches of a mature beech tree during the late summer 2018. The 13C-label was traced via compound-specific δ13C isotope analysis of n-alkanes and fatty acids to determine the de-novo biosynthesis within these compound classes. We did not observe a significant change in lipid concentrations during the late growing season, but we found higher n-alkane concentrations in sun-exposed compared to shaded leaves in August and September. The n-alkane and fatty acid composition showed ongoing modifications during the late growing season. Together with the uptake and following subsequent decrease of the 13C-label, this suggests ongoing de-novo biosynthesis, especially of fatty acids in European beech leaves. Moreover, there is a high variability in the 13C-label among individual branches and between sun-exposed and shaded leaves. At the same time, sun-exposed leaves invest more of the assimilated C into secondary metabolites such as lipids than shaded leaves. This indicates that the investigated mature beech tree could adjust its lipid production and composition in order to acclimate to changes in microclimates within the tree crown and during the investigated period.

Above- and belowground interplay: Canopy CO2 uptake, carbon and nitrogen allocation and isotope fractionation along the plant-ectomycorrhiza continuum.

In forests, mycorrhizal fungi regulate carbon (C) and nitrogen (N) dynamics. We evaluated the interplay among ectomycorrhizas (ECM), ecosystem C fluxes, tree productivity, C and N exchange and isotopic fractionation along the soil-ECM-plant continuum in a Mediterranean beech forest. From bud break to leaf shedding, we monitored: net ecosystem exchange (NEE, a measure of the net exchange of C between an ecosystem and the atmosphere), leaf area index, stem growth, N concentration, δ13 C and δ15 N in rhizosphere soil, ectomycorrhizal fine root tips (ERT), ECM-free fine root portions (NCR) and leaves. Seasonal changes in ERT relative biomass were strictly related to NEE and mimicked those detected in the radial growth. The analysis of δ13 C in ERT, leaves and NCR highlighted the impact of canopy photosynthesis on ERT development and an asynchronous seasonal C allocation strategy between ERT and NCR at the root tips level. Concerning N, δ15 N of leaves was negatively related to that of ERT and dependent on seasonal 15 N differences between ERT and NCR. Our results unravel a synchronous C allocation towards ERT and tree stem driven by the increasing NEE in spring-early summer. Moreover, they highlighted a phenology-dependent 15 N fractionation during N transfer from ECM to their hosts. This evidence, obtained in mature beech trees under natural conditions, may improve the knowledge of Mediterranean forests functionality.

Interactive effects of tree species mixture and climate on foliar and woody trait variation in a widely distributed deciduous tree

Abstract Despite increasing reports of severe drought and heat impacts on forest ecosystems, community‐level processes, which could potentially modulate tree responses to climatic stress, are rarely accounted for. While numerous studies indicate a positive effect of species diversity on a wide range of ecosystem functions and services, little is known about how species interactions influence tree responses to climatic variability. We quantified the intraspecific variation in 16 leaf and wood physiological, morphological and anatomical traits in mature beech trees (Fagus sylvatica L.) at six sites located along a climatic gradient in the French Alps. At each site, we studied pure beech and mixed stands with silver fir (Abies alba Mill.) or downy oak (Quercus pubescens Willd.). We tested how functional traits differed between the two species mixtures (pure vs. mixed stands) within each site and along the climatic gradient. We found significant changes in many traits along the climatic gradient as conditions progressively got drier and warmer. Independent of the mixture, reduced leaf‐level CO2 assimilation, stomatal size and thicker leaf cuticles, consistent with a more conservative resource use strategy, were found. At the drier sites, higher foliar stable carbon isotopic composition (δ13C), thicker mesophyll tissues and lower specific leaf area (SLA) in pure stands suggest that beech had more acquisitive traits there compared to mixed stands. At the wetter sites, trees in beech‐silver fir mixtures had higher chlorophyll concentration, lower δ13C, larger xylem vessels and higher SLA, suggesting a more acquisitive resource use strategy in mixed stands than in pure stands. Our work revealed that species interactions are significant modulators of functional traits, and that they can be just as important drivers of intraspecific trait variation as climatic conditions. We show that downy oak mixtures lead to an adaptive drought response by common beech in dry environments. In contrast, in milder climates, interactions with silver fir seem to increase beech resource acquisition and productivity. These findings highlight a strong context dependency and imply that incorporating local interspecific interactions in research on climate impacts could improve our understanding and predictions of forest dynamics. A free Plain Language Summary can be found within the Supporting Information of this article.

Mature beech and spruce trees under drought – Higher C investment in reproduction at the expense of whole-tree NSC stores

Under drought, potential plant death from depletion of carbon (C) stores, referred to as carbon starvation, is thought to result from negative carbon balance during (partial) stomatal closure. As evidence for C starvation is rare for mature trees, we investigated the C dynamics in mature beech and spruce under drought, focusing on non-structural carbohydrates (NSC) as an indicator of their C balance. Trees were exposed to complete exclusion of precipitation throughfall over two consecutive growing seasons. We assessed NSC concentrations during the early dormant season in leaves, twigs, stem phloem/xylem, coarse and fine roots. Up-scaling to whole-tree NSC pools was achieved using a process-based single-tree model (“BALANCE”), estimating tree biomass increment. While there were distinct differences in NSC concentration among different tissues in both beech and spruce (root < stem < twig < leaves and xylem < phloem), drought did not affect NSC concentrations. However, compared to controls, the whole-tree NSC pool size significantly decreased under drought in both beech (42 %) and spruce (36 %), in parallel to a significant growth decline of overall 52 % and 57 %, respectively. Nevertheless, drought-stressed beech and spruce invested almost twice as much C in reproductive structures relative to total C investment (i.e. 6.0 ± 3.3 and 52.3 ± 8.71 %) compared to control trees (3.1 ± 1.8 and 29.2 ± 7.8 %). This highlights the high priority of C investment into reproduction relative to growth under drought. Given that NSC concentrations are maintained even under severe drought over two growing seasons, NSC pool sizes appear to be a better proxy to assess whole-tree’s carbon status in mature trees. Overall, trees maintained NSC availability, avoiding carbon starvation, by downregulating a major C sink (i.e. growth) while upholding reproduction.

Does drought advance the onset of autumn leaf senescence in temperate deciduous forest trees?

Abstract. Severe droughts are expected to become more frequent and persistent. However, their effect on autumn leaf senescence, a key process for deciduous trees and ecosystem functioning, is currently unclear. We hypothesized that (I) severe drought advances the onset of autumn leaf senescence in temperate deciduous trees and (II) tree species show different dynamics of autumn leaf senescence under drought. We tested these hypotheses using a manipulative experiment on beech saplings and 3 years of monitoring mature beech, birch and oak trees in Belgium. The autumn leaf senescence was derived from the seasonal pattern of the chlorophyll content index and the loss of canopy greenness using generalized additive models and piecewise linear regressions. Drought and associated heat stress and increased atmospheric aridity did not affect the onset of autumn leaf senescence in both saplings and mature trees, even if the saplings showed a high mortality and the mature trees an advanced loss of canopy greenness. We did not observe major differences among species. To synthesize, the timing of autumn leaf senescence appears conservative across years and species and even independent of drought, heat and increased atmospheric aridity. Therefore, to study autumn senescence and avoid confusion among studies, seasonal chlorophyll dynamics and loss of canopy greenness should be considered separately.

Do trees have constant branch divergence angles?

Many herbaceous plants feature remarkably regular arrangements of lateral organs along the central axis. These phyllotactic patterns are generated by a constant divergence angle between successive buds (or whorls thereof) that first appears at the shoot apircal meristem and is maintained across later ontogentic stages when it can be observed at the macroscopic scale. Do the branches along a tree trunk exhibit similar patterns? Here we use branch skeleton data derived from terrestrial laser scans to empirically estimate the distributions of the divergence angles between successive branches along the trunks of mature European beech, Norway spruce, and Scots pine trees. We find that rather than clustering around a particular value, species-specific branch divergence angles feature statistical properties characteristic of a uniform distribution. We hypothesise this to be the result of the stochasticity in bud development and branch shedding, and provide a rigorous mathematical proof that even when the divergence angle between successive lateral buds is constant, the observed distribution of branch divergence angles will approximate a uniform distribution if bud mortality and branch shedding rates are high.

Ozone Amplifies Water Loss from Mature Trees in the Short Term But Decreases It in the Long Term

We measured whole-tree transpiration of mature Fagus sylvatica and Picea abies trees exposed to ambient and twice-ambient O3 regimes (1xO3 and 2xO3 free-air fumigation). After eight years, mean daily total transpiration did not vary with the O3 regime over the 31 days of our study, even though individual daily values increased with increasing daily O3 peaks in both species. Although the environmental parameters were similar at 1xO3 and 2xO3, the main factors affecting daily transpiration were vapour pressure deficit in 2xO3 spruce and O3 peaks in beech. For a mechanistic explanation, we measured O3-induced sluggish stomatal responses to variable light (sunflecks) by means of leaf-level gas exchange measurements only in the species where O3 was a significant factor for transpiration, i.e., beech. Stomata were always slower in closing than in opening. The 2xO3 stomata were slower in opening and mostly in closing than 1xO3 stomata, so that O3 uptake and water loss were amplified before a steady state was reached. Such delay in the stomatal reaction suggests caution when assessing stomatal conductance under O3 pollution, because recording gas exchange at the time photosynthesis reached an equilibrium resulted in a significant overestimation of stomatal conductance when stomata were closing (ab. 90% at 1xO3 and 250% at 2xO3). Sun and shade leaves showed similar sluggish responses, thus suggesting that sluggishness may occur within the entire crown. The fact that total transpiration was similar at 1xO3 and 2xO3, however, suggests that the higher water loss due to stomatal sluggishness was offset by lower steady-state stomatal conductance at 2xO3. In conclusion, O3 exposure amplified short-term water loss from mature beech trees by slowing stomatal dynamics, while decreased long-term water loss because of lower steady-state stomatal conductance. Over the short term of this experiment, the two responses offset each other and no effect on total transpiration was observed.

An improved single probe method for sap flow measurements using finite heating duration

Because of its low cost and simple fabrication, it is easy to advocate for the single probe method as a method of choice in sap flow studies. An improved single probe method with finite heating duration (F-SPHP) is verified both in cut stem segments and in the field using mature beech (Fagus sylvatica L.) trees. The F-SPHP method is based on an analytical solution of the partial differential equation for combined heat conduction and convection, which shows large relative sensitivity to thermal conductivity (K). The F-SPHP method is able to measure sap flux densities (SFD) between 2 and 36 cm3 cm−2 h−1 (heat pulse velocity (Vh): 3–60 cm h−1) in the cut stem segment experiment. This is an improvement compared to the instantaneous single probe heat pulse (I-SPHP) method which cannot accurately measure low (Vh < 20 cm h−1) sap flow. Sapflow+ (a four-needle heat pulse method) is used for field validation, and when compared with gravimetric measurements, slopes of 0.853 and 0.730 are obtained for F-SPHP and Sapflow+, respectively. Patterns measured with F-SPHP and Sapflow+ in mature beech trees in the forest are similar at all measurement positions and temperature correction is needed for both methods when natural temperature gradients are steep. Compared with multi-probe methods, the single probe method with finite heating duration has the advantage of causing less damage to conductive tissue.

Long-Term Abandonment of Forest Management Has a Strong Impact on Tree Morphology and Wood Volume Allocation Pattern of European Beech (Fagus sylvatica L.)

The three-dimensional (3D) morphology of individual trees is critical for light interception, growth, stability and interactions with the local environment. Forest management intensity is a key driver of tree morphology, but how the long-term abandonment of silvicultural measures impacts trunk and crown morphological traits is not fully understood. Here, we take advantage of a long management intensity gradient combined with a high-resolution terrestrial laser scanning (TLS) approach to explore how management history affects the 3D structure of mature beech (Fagus sylvatica L.) trees. The management gradient ranged from long-term (&gt;50 years) and short-term (&gt;20 years) unmanaged to extensively and intensively managed beech stands. We determined 28 morphological traits and quantified the vertical distribution of wood volume along the trunk. We evaluated the differences in tree morphological traits between study stands using Tukey’s HSD test. Our results show that 93% of the investigated morphological traits differed significantly between the study stands. Significant differences, however, emerged most strongly in the stand where forest management had ceased &gt;50 years ago. Furthermore, we found that the vertical distribution of trunk wood volume was highly responsive between stands with different management intensity, leading to a 67% higher taper top height and 30% lower taper of beech trees growing in long-term unmanaged stands compared to those in short-term unmanaged or managed stands. These results have important implications for management intensity decisions. It is suggested that the economic value of individual beech trees from long-term unmanaged forests can be expected to be very high. This might also translate to beech forests that are extensively managed, but we found that a few decades of implementation of such a silvicultural system is not sufficient to cause significant differences when compared to intensively managed stands. Furthermore, TLS-based high-resolution analyses of trunk and crown traits play a crucial role in the ability to better understand or predict tree growth responses to the current drivers of global change.

Higher drought sensitivity of radial growth of European beech in managed than in unmanaged forests

Climate extremes are predicted to become more frequent and intense in future. Thus, understanding how trees respond to adverse climatic conditions is crucial for evaluating possible future changes in forest ecosystem functioning. Although much information about climate effects on the growth of temperate trees has been collected in recent decades, our understanding of the influence of forest management legacies on climate-growth relationships is still limited. We used individual tree-ring chronologies from managed and unmanaged European beech forests, located in the same growth district (i.e. with almost identical climatic and soil conditions), to examine how forest management legacies (recently managed with selection cutting, >20 years unmanaged, >50 years unmanaged) influence the radial growth of Fagus sylvatica during fluctuating climatic conditions. On average, trees in managed stands had higher radial growth rate than trees in unmanaged stands during the last two decades a 50%. However, the beech trees in the unmanaged stands were less sensitive to drought than those in the managed stands. This effect was most pronounced in the forest with longest management abandonment (>50 years), indicating that the drought sensitivity of mature beech trees is in these forests the lower, the longer the period since forest management cessation is. Management-mediated modifications in crown size and thus water demand are one likely cause of the observed higher climate sensitivity of beech in the managed stands. Our results indicate a possible trade-off between radial growth rate and drought tolerance of beech. This suggests that reducing stem density for maximizing the radial growth of target trees, as is common practice in managed forests, can increase the trees' drought sensitivity. In the prospect of climate change, more information on the impact of forest management practices on the climate-growth relationships of trees is urgently needed.

Drought and irrigation affect transpiration rate and morning tree water status of a mature European beech (Fagus sylvatica L.) forest in Central Europe

AbstractSeverity and frequency of recurrent droughts is likely to be amplified by recent climate changes and may adversely affect large areas of temperate forests. This study addresses the ecophysiological responses of a mature beech forest in Central Europe (Slovakia, 450 m a.s.l.) exposed to drought during the growing seasons of 2012–2014. Sap flow and stem circumference changes of European beech (Fagus sylvatica L.) were measured in two contrasting treatments (drought vs. irrigation). Limited water availability reflected in the morning tree water status (morning stem contraction, ΔW) significantly reduced transpiration. Trees stressed by drought in the years 2012 and 2013 showed remarkably lower values of morning ΔW than those in the irrigated group. This suggests (a) lower water potential of the stem‐conducting tissues and (b) increased use of internally stored water to maintain daily transpiration. Additionally, morning ΔW showed a close relationship with soil water potential, particularly in the nonirrigated trees. Potential evapotranspiration (PET) explained less variability in the transpiration of the control stand than that of the irrigated stand. The use of a simple analysis of covariance model comprising morning ΔW and PET interaction significantly increased the explained variability in transpiration in the control stand. Morning ΔW of mature beech trees seems to be a useful and easily obtainable non‐invasive bioindicator of their ability to reduce and regulate transpiration in relation to atmospheric evaporative demands in response to drought.

Cryptogamic stem covers may contribute to nitrous oxide consumption by mature beech trees

Naturally produced by microbial processes in soil, nitrous oxide (N2O) is an important greenhouse gas contributing to climate change. Accordingly, there is a need to accurately quantify the capability of forest ecosystems to exchange N2O with the atmosphere. While N2O emissions from soils have been well studied, trees have so far been overlooked in N2O inventories. Here, we show that stems of mature beech trees (Fagus sylvatica) may act as a substantial sink of N2O from the atmosphere under conditions of soils consuming N2O. Consistent consumption of N2O by all stems investigated (ranging between −2.4 and −3.8 µg m−2 h−1) is a novel finding in contrast to current studies presenting trees as N2O emitters. To understand these fluxes, N2O exchange of photoautotrophic organisms associated with beech bark (lichens, mosses and algae) was quantified under laboratory conditions. All these organisms were net N2O sinks at full rehydration and temperature of 25 °C. The consumption rates were comparable to stem consumption rates measured under field conditions. Cryptogamic stem covers could be a relevant sink of N2O in European beech forests.

Nitrous oxide emissions from stems of alder, beech and spruce in a temperate forest

Although plants are recognized as conduits for soil-produced N2O, little is known about N2O fluxes from mature trees under field conditions as well as their contribution to total (soil + stem) N2O fluxes. Exclusion of tree-mediated N2O may thus underestimate total forest N2O fluxes. In the present study, our aims were to quantify tree-mediated N2O emissions and their seasonal patterns. We investigated in situ stem and soil N2O fluxes from mature alder trees on poorly-drained soil and mature beech and spruce trees on well-drained soils during March–October 2015. Alder, beech and spruce consistently emitted N2O via stems and all displayed clear seasonal patterns. Soil and air temperature, vapor pressure deficit, and soil N2O concentration influenced the temporal variability of stem N2O fluxes. Stem and soil N2O fluxes from the alder stand were higher than beech and spruce stands. Stem N2O fluxes accounted 8.8–9.8% of the total N2O fluxes in the beech and spruce stands but only 1.1% in the alder stand. Our study reveals the importance of field-based measurements across seasons in order to come up with reliable estimates of stem N2O fluxes. N2O estimates from temperate forests that are solely based on soil N2O flux measurements are probably conservative.

Influence of Root Diameter and Soil Depth on the Xylem Anatomy of Fine- to Medium-Sized Roots of Mature Beech Trees in the Top- and Subsoil.

Despite their importance for water uptake and transport, the xylem anatomical and hydraulic properties of tree roots have only rarely been studied in the field. We measured mean vessel diameter (D), vessel density (VD), relative vessel lumen area (lumen area per xylem area) and derived potential hydraulic conductivity (Kp) in the xylem of 197 fine- to medium-diameter roots (1–10 mm) in the topsoil and subsoil (0–200 cm) of a mature European beech forest on sandy soil for examining the influence of root diameter and soil depth on xylem anatomical and derived hydraulic traits. All anatomical and functional traits showed strong dependence on root diameter and thus root age but no significant relation to soil depth. Averaged over topsoil and deep soil and variable flow path lengths in the roots, D increased linearly with root diameter from ∼50 μm in the smallest diameter class (1–2 mm) to ∼70 μm in 6–7 mm roots (corresponding to a mean root age of ∼12 years), but remained invariant in roots >7 mm. D never exceeded ∼82 μm in the 1–10 mm roots, probably in order to control the risk of frost- or drought-induced cavitation. This pattern was overlain by a high variability in xylem anatomy among similar-sized roots with Kp showing a higher variance component within than between root diameter classes. With 8% of the roots exceeding average Kp in their diameter class by 50–700%, we obtained evidence of the existence of ‘high-conductivity roots’ indicating functional differentiation among similar-sized roots. We conclude that the hydraulic properties of small to medium diameter roots of beech are mainly determined by root age, rendering root diameter a suitable predictor of hydraulic functioning, while soil depth – without referring to path length – had a negligible effect.

Lophozonia tree cavities used for nesting by Slender-billed Parakeets (Enicognathus leptorhynchus) in the central valley of southern Chile: a potentially vanishing keystone resource

BackgroundThe Slender-billed Parakeet (Enicognathus leptorhynchus) is a psittacine endemic to southern Chile and an obligate secondary cavity-nester. In the central valley of southern Chile, most (94%) of the known Slender-billed Parakeet nests have occurred in large, mature southern beech (Lophozonia obliqua) trees (locally known as “pellines”). As relicts of the original old-growth forests of southern Chile, most pellines have been lost due to extensive land-clearing throughout the region, potentially threatening long-term persistence of the Slender-billed Parakeet.MethodsWe conducted our study in the central valley of southern Chile, near the city of Osorno during three consecutive nesting seasons (November–January, 2008–2011). Nest trees used by Slender-billed Parakeets were located by direct observation of parakeet activities and through interviews with local residents, some of whom were former parrot nest poachers. Nest cavities were accessed, inspected and measured using single-rope climbing techniques. We report means, standard errors, 95% confidence intervals and ranges for 11 cavity-related variables. We also report clutch sizes encountered in active nests, and age estimates of nest trees based on known growth rates of Lophozonia trees in southern Chile. Linear regressions were used to evaluate potential relationships between cavity-related variables and clutch size.ResultsWe located and measured 38 Lophozonia tree cavities used for nesting by Slender-billed Parakeets. Compared to those used by other psittacines, nest trees were relatively large, averaging 30.4 ± 1.1 m in height with a mean diameter at breast height of 134.5 ± 4.7 cm. Based on estimated annual diameter increment, ages of nest trees ranged from approximately 209–485 years. Nest cavities entrances averaged 12.5 ± 0.9 m in height above ground level. Cavity entrance widths averaged 51.0 ± 13.3 cm (vertical) by 11.5 ± 0.7 cm (horizontal). Cavity entrance orientations were apparently random, with no directional preferences detected. Nest cavities were also relatively large, with a mean internal diameter of 39.6 ± 2.4 cm and mean depth of 90.3 ± 24.2 cm. Clutch sizes (2–9) were unusually large for psittacines of this size (ca. 280–300 g) and broods of up to seven well-developed nestlings were observed.ConclusionsWe found that the deep and spacious cavities provided by pellines facilitate successful rearing of large broods, thereby maximizing productivity and fitness. The existence of pellines has apparently allowed Slender-billed Parakeets to adapt successfully to a wholesale loss of ancestral habitat to anthropogenic modifications. Immediate and strategic conservation measures, such as protection of existing pellines and the regeneration and recruitment of additional ones, are recommended for ensuring the survival of Slender-billed Parakeet populations throughout the central valley of southern Chile.

Slow decomposition of leaf litter from mature Fagus sylvatica trees promotes offspring nitrogen acquisition by interacting with ectomycorrhizal fungi

Summary Leaf litter chemistry and ectomycorrhizal (ECM) fungi are key drivers of the below‐ground nitrogen (N) cycling within forest ecosystems. Their combined effects on litter decomposition and N competition between microbial decomposers and plants are still uncertain. We conducted a glasshouse microcosm experiment with low or high ECM‐colonized beech (Fagus sylvatica) saplings, growing with litter collected from old or young beech trees growing on the same loamy soil. After 6 months of growth, we investigated litter decomposition rates, microbial respiration and the N pools within leaf litter, soil (different pools), microbial and plant shoot biomass. We found that the mass loss of the litter from young trees was always higher than the litter from the mature trees. The microbial biomass N per unit soil carbon was low when the litter, especially from mature trees, was added, suggesting that the litter provided recalcitrant compounds and limited soil microbial activity. In contrast to the ‘Gadgil effect’ hypothesis, the high ECM‐colonized roots increased the litter decomposition rate and N immobilization in poorly decomposable litter in comparison with the litter incubated with the low ECM‐colonized roots. Finally, the high ECM‐colonized plants that received the poorly decomposable leaf litter exhibited the highest shoot N amount and biomass and were associated with the lowest microbial biomass N. Two‐way anovas revealed that litter and ECM fungi occurrence together impacted final particulate organic N, microbial biomass N, the amount of shoot N and shoot biomass. Four N pools are key drivers of microbial biomass N and shoot N: particulate organic N, total N, soil ammonium concentration and litter N concentration. Synthesis. Our results support the hypothesis that poorly decomposable leaf litter produced by mature beech trees and ECM fungi together decrease microbial N immobilization but increase tree N acquisition. Increasing N retention within the recalcitrant N forms in soil was identified as a key mechanism by which beech alters soil N cycling with potential positive feedbacks on its acquisition by the plant. Our result emphasizes the importance of considering within‐species litter‐trait variability in litter decomposition.

Reduced stem growth, but no reserve depletion or hydraulic impairment in beech suffering from long-term decline

Under non-extreme drought conditions, reduced stem growth is not associated with reserve or hydraulic impairment in beech suffering from long-term decline. Global change is expected to increase the frequency and the intensity of drought events in temperate ecosystems. In some regions, this might be associated with an increase in tree decline. Of the ecophysiological mechanisms that have been proposed to explain tree decline, an impairment of the vascular transport system and/or carbon function are two of the most discussed. In a context of long-term decline caused by droughts, we investigated the functional differences between the carbon, nitrogen, and hydraulic functions of healthy and declining mature beech (Fagus sylvatica L.) trees. The study was carried out over two consecutive years with contrasting water availabilities. The radial growth of declining trees was clearly less than that of healthy trees, due to a lower growth rate, associated during the wet year with a shorter growth period. Leaf functional characteristics and hydraulic parameters (native embolism and cavitation vulnerability curves) were similar in healthy and declining trees. However, at the end of a growing season characterized by a dry spring, carbon reserves concentrations in young branches of declining trees were lower than those in healthy trees, though they recovered during the following non-constraining growing season. Our results did not indicate carbon starvation, nitrogen deficiency, or hydraulic failure. However, there seems to be some compensation mechanism related to reserve dynamics in the remaining living tissue of the declining trees. This study shows that the climate conditions of successive years are probably crucial for these functional adjustments to be operational.

Stomatal conductance and intrinsic water use efficiency in the drought year 2003: a case study of European beech

Beech trees were able to cope with the drought of 2003. Harmful water shortage has been avoided by an effective stomatal closure while use of carbon storage pools may have prevented carbon starvation and growth reduction. We applied hydrodynamic modeling together with a tree ring stable isotope approach to identify the physiological responses of beech trees to changing environmental conditions. The drought conditions of the extreme hot and dry summer in 2003 were hypothesized to significantly influence the radial growth of European beech mainly triggered by the stomatal response towards water scarcity leading, in turn, to a decline in carbon assimilation. The functional–structural single tree modeling approach applied, revealed in fact a strong limitation of water use and carbon gain during drought. However, tree ring width data did not show a clear drought response and no differentiation in radial growth during six subsequent years examined (2002–2007) has been observed. Using integrated results from mechanistic carbon–water balance simulations, tree ring carbon and oxygen isotope analysis and tree ring width measurements we postulate that the suggested drought-induced growth decline has been prevented by the remobilization of stored carbohydrates, an early onset in growth and the relatively late occurrence of the severe drought in 2003. Furthermore, we demonstrate that the stomatal response played a significant role in avoiding harmful water tension that would have caused xylem dysfunction. As a result of the combined investigation with physiological measurements (stable isotope approach) and hydrodynamic modeling of stomatal aperture, we could give insights into the physiological control of mature beech tree functioning under drought. We conclude that beech trees have been operating at their hydraulic limits and that the longer or repeated drought periods would have affected the growth considerably.

Spontaneous establishment of late successional tree species English oak (Quercus robur) and European beech (Fagus sylvatica) at reclaimed alder plantation and unreclaimed post mining sites

Nitrogen-fixing trees are often used in disturbed areas to promote soil development and to prepare the ground for more demanding tree species. However, their effect on the establishment of climax tree species has seldom been tested. We mapped the occurrence of volunteer seedlings of European beech (Fagus sylvatica) and English oak (Quercus robur) in patches reclaimed by planting alder (Alnus glutinosa and Alnus incana) and in unreclaimed forest patches (dominated by Salix caprea, Betula pendula and Populus tremula). Both communities were about 30 years old and occurred on one large spoil heap from coal mining near the town of Sokolov (Czech Republic). At the same time, we mapped mature beech and oak trees in the surrounding area. We planted oak and beеch seedling in an alder plantation, an unreclaimed forest stand and a grass-covered area without trees, and recorded their growth for two consecutive years. The closest mature beech grew 1km from the heap whereas seed-producing oaks occurred in its close vicinity and on the heap itself in a 30 to 40-year-old oak plantation. In both species, seedling density decreased with increasing distance from mature trees. Unreclaimed sites supported significantly higher seedling densities than reclaimed ones. Seedlings in the unreclaimed forest stand grew significantly better than in the other two habitats. Oak seedlings exhibited significantly higher mycorrhizal colonization in unreclaimed forest stands than in the other habitats. Unreclaimed forest stands are better habitat for the establishment of climax trees, than the reclaimed alder plantation. Using of spontaneous woody vegetation as nursing cover for climax trees can be recommended in restoration practice. Risk and benefits of nitrogen-fixing trees should be carefully considered.