We investigated long-term responses (since 1850) of Fagus sylvatica (Luxembourg; central Europe) to shifts in temperature, precipitation, and nitrogen deposition by analyzing diameter at breast ...height (DBH) increment, basal area increment (BAI), and tree-ring stable isotopes ( delta super(13)C, delta super(15)N). We compared stands on soils with contrasting water supply (Regosols and Cambisols with an available water capacity of ca. 40 and 170 mm, respectively) and of two different age classes (ca. 60 vs. 200 years). All stands showed a peak in DBH increment in the decade 1978-1987, but a decline in increment growth in subsequent decades. In addition, BAI declined in mature stands in the last two decades. Decreasing increment rates were attributable to an increasing drought limitation of stands, mainly induced by increasing temperatures in the last two decades. Contrary to our expectations, stands on Cambisols showed a similar susceptibility to shifts in temperature and precipitation as stands on Regosols, suggesting a strong adaptation of the respective ecotypes grown at dryer sites. This result was in line with long-term trends for tree-ring delta super(13)C signatures, which did not differ significantly between stands on Cambisols and Regosols. Climate impacts on tree-ring delta super(15)N signatures were low. High spring precipitation and temperatures caused increasing and decreasing delta super(15)N values, respectively, but only in mature stands on Cambisols. Stands on Regosols tended to have lower delta super(15)N values than stands on Cambisols. Decreasing delta super(15)N values in recent decades suggest an increasing impact of allochthonous N loads with isotopically lighter N.
Despite a large body of studies investigating soil organic carbon (SOC) stocks and potential influencing factors, the impact of contrasting parent material, particularly in the subsoil, has received ...little attention. To reveal potential effects varying parent materials exert on SOC stocks, we investigated chemical (14C content and overall chemical composition via 13C NMR spectroscopy) and plant/microbial related parameters (root mass, amino sugars) of bulk soil and soil organic matter fractions from topsoil, subsoil, and rhizosphere soil at three European beech stands (Fagus sylvatica L.) only differing in parent material (Tertiary sand, Quaternary loess, and Tertiary basalt).
The results suggest that the clay fraction, its amount being largely dependent on the respective parent material, took a central role in shaping differences in SOC stocks among the investigated sites by affecting soil organic matter stabilization via organo-mineral association and aggregation. This fraction was particularly relevant in the subsoil, where it accounted for up to 80% of the bulk soil SOC stocks that decreased with decreasing amounts of the clay fraction (basalt > loess > sand site). Determining the soil's nutrient composition, parent material likely also indirectly affected SOC stocks by changing rhizosphere traits (such as fine root density or mortality) and by attracting root growth (and thus organic matter inputs) to subsoil with higher nutrient contents, where in situ root inputs in the form of rhizodeposits were likely the prime source of plant-derived SOC. However, root inputs also contributed in large part to topsoil SOC stocks and were associated with higher abundance of microbial compounds (amino sugars), whose relative importance increased with increasing soil depth.
Independent of soil depth and site, amino sugars and the amount of the clay fraction, combined with parameters related to the input of organic matter (root mass and amount of the particulate organic matter fraction) explained more than 90% of the variability in SOC stocks, indicating a key role of these measures in impacting SOC stocks. Because parent material directly or indirectly influenced these parameters, we demonstrate the necessity to consider differences in parent material when estimating and predicting SOC stocks.
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•The clay fraction, particularly in the subsoil, shaped differences in SOC stocks.•In the topsoil, the relevance of the clay fraction was attenuated by root inputs.•Microbial residues were constant or slightly increased with soil depth.•Amino sugars, root mass, clay and POM explained ∼90% of variation in SOC stocks.
The super(13/12)C ratio in plant roots is likely dynamic depending on root function (storage versus uptake), but to date, little is known about the effect of season and root order (an indicator of ...root function) on the isotopic composition of C-rich fractions in roots. To address this, we monitored the stable isotopic composition of one evergreen (Picea abies) and one deciduous (Fagus sylvatica), tree species' roots by measuring delta super(13)C of bulk, respired and labile C, and starch from first/second and third/fourth order roots during spring and fall root production periods. In both species, root order differences in delta super(13)C were observed in bulk organic matter, labile, and respired C fractions. Beech exhibited distinct seasonal trends in delta super(13)C of respired C, while spruce did not. In fall, first/second order beech roots were significantly depleted in super(13)C, whereas spruce roots were enriched compared to higher order roots. Species variation in delta super(13)C of respired C may be partially explained by seasonal shifts from enriched to depleted C substrates in deciduous beech roots. Regardless of species identity, differences in stable C isotopic composition of at least two root order groupings (first/second, third/fourth) were apparent, and should hereafter be separated in belowground C-supply-chain inquiry.
There is a rising interest in the role of species diversity in ecosystem functioning and services, including productivity. Yet, how the diversity–productivity relationship depends on species identity ...and abiotic conditions remains a challenging issue. We analysed mixture effects on species productivity along site productivity gradients, calculated from a set of abiotic factors, in two biogeographic contexts (highlands and lowlands). We compared the productivity of 5 two‐species mixtures (i.e. 10 cases of mixed species) with that of monocultures of the same species. Five main European tree species were considered: sessile oak (Quercus petraea Liebl.), Scots pine (Pinus sylvestris L.), European beech (Fagus sylvatica L.), silver fir (Abies alba Mill.) and Norway spruce (Picea abies (L.) H. Karst). Our data set was compiled from the 2006 to 2010 French National Forest Inventory data base and covers 2361 plots including pure and mixed stands. Overall productivity of mixtures in highlands, that is European beech–Norway spruce, European beech–silver fir and to a lesser extent, silver fir–Norway spruce, was found to be higher than expected from the productivity of corresponding monospecific stands. Overyielding was mainly due to European beech for the first two mixtures and to silver fir for the third one. No effect of mixture was found for sessile oak–Scots pine and sessile oak–European beech stands in lowlands. Overyielding of sessile oak mixed with Scots pine was not strong enough to significantly increase overall stand productivity. Overyielding of European beech was balanced by an underyielding of sessile oak. The mixture effect changed along site productivity gradients for six cases out of the 10 studied, with a stronger and positive effect on sites with low productivity. The magnitude of this change along site productivity gradients varied up to 89% depending on the tree species. Synthesis. The nature of species interaction in mixtures with regard to productivity changes with species assemblage and abiotic conditions. Overyielding is strongest when species grow in highlands on less productive sites. A negative link between mixture effect and site productivity was found, in line with the stress‐gradient hypothesis.
•Extensive beech and spruce dieback observed after hot droughts in 2018 and 2019.•Dead and healthy spruce and beech trees from central Germany were investigated.•Ca. 50% (7%) of all sampled spruce ...(beech) trees already died in 2018.•Drought signal enhanced by edaphic conditions and species-specific responses.•Dying trees showed lower growth and higher drought sensitivity than healthy trees.
Anthropogenic climate change pushes forest ecosystems globally beyond their limits. Widespread events of forest die-off have been attributed to direct and indirect impacts of increasingly frequent and intense droughts. Here, we focus on an extensive mortality event in Norway spruce (Picea abies (L.) H. Karst.) and European beech (Fagus sylvatica (L.)) forests in Germany, following the successive 2018 – 2019 hot droughts. To examine whether this die-off indeed attributed to observed trends in drought occurrence and intensity, we sampled 143 beech and 186 spruce trees at three low-elevation sites (Spessart, Hassberge, Fichtelberg) with different edaphic properties in northern Bavaria. We analysed long-term hydroclimatic sensitivity and growth responses to extreme events of five site- and species-specific tree-ring width chronologies, including a reference site for each species. Growth of beech was sensitive to drought in April to June, whereas spruce growth was strongly related to drought during June to August, except at slightly higher elevations at the Fichtelberg site, where a summer temperature signal was observed. Trees at the Spessart and Hassberge sites showed an increased response to hydroclimatic conditions in April following the extreme drought in 1976 and from the 1990s onwards at the Fichtelberg site. Spruce bark beetle (Ips typographus L.) outbreaks during the 2018 drought accelerated the high mortality rates in around 50% of the trees at the Spessart and Hassberge site. In 2018, around 7% of all beech trees died at the Hassberge site, the site with the highest clay content. Our results suggest that these widespread mortality events can be attributed to an increasing drought sensitivity and were accelerated by the consecutive recent drought years. Sustainable forest management practices for these ecologically and economically important tree species are required to mitigate the effects of global warming in the future.
Drought responses of mature trees are still poorly understood making it difficult to predict species distributions under a warmer climate. Using mature European beech (Fagus sylvatica L.), a ...widespread and economically important tree species in Europe, we aimed at developing an empirical stress-level scheme to describe its physiological response to drought. We analysed effects of decreasing soil and leaf water potential on soil water uptake, stem radius, native embolism, early defoliation and crown dieback with comprehensive measurements from overall nine hydrologically distinct beech stands across Switzerland, including records from the exceptional 2018 drought and the 2019/2020 post-drought period. Based on the observed responses to decreasing water potential we derived the following five stress levels: I (predawn leaf water potential >−0.4 MPa): no detectable hydraulic limitations; II (−0.4 to −1.3): persistent stem shrinkage begins and growth ceases; III (−1.3 to −2.1): onset of native embolism and defoliation; IV (−2.1 to −2.8): onset of crown dieback; V (<−2.8): transpiration ceases and crown dieback is >20%. Our scheme provides, for the first time, quantitative thresholds regarding the physiological downregulation of mature European beech trees under drought and therefore synthesises relevant and fundamental information for process-based species distribution models. Moreover, our study revealed that European beech is drought vulnerable, because it still transpires considerably at high levels of embolism and because defoliation occurs rather as a result of embolism than preventing embolism. During the 2018 drought, an exposure to the stress levels III-V of only one month was long enough to trigger substantial crown dieback in beech trees on shallow soils. On deep soils with a high water holding capacity, in contrast, water reserves in deep soil layers prevented drought stress in beech trees. This emphasises the importance to include local data on soil water availability when predicting the future distribution of European beech.
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•We studied the hydraulic behaviour of mature Fagus sylvatica trees.•Data from multiple near-natural soil-plant-atmosphere continua (SPAC) were collected.•The observed physiological drought responses were coherent across sites.•The results were synthesised into an empirical stress scheme for beech.•Our study provides relevant and fundamental information for species range predictions.
Aims: Decomposition of leaf litterfall plays a major role for nitrogen (N) dynamics in soils. However, little is known as to which extent beech leaf litter contributes to N turnover and nitrous oxide ...(N sub(2) O) emissions within one decade after litterfall. Methods: In 1997, we exchanged recently fallen leaf litter by super(15) N-labelled litter in a beech stand (Fagus sylvatica ) at the Solling, Germany. Measurements were conducted 2-3 and 10-11 years after litter exchange. Results: Two years after litter exchange, 92 % of added super(15) N was recovered in the surface 10 cm of the soil. The labelled N was primarily found in the upper part of the F layer of the moder type humus. Eleven years after litter exchange, 73 % of the added super(15) N was lost and the remaining 27 % was mainly recovered in the lower part of the F layer indicating N sequestration. The remaining leaf litter N was subject to measurable N mineralisation (2-3 % of litter N) and N sub(2) O production (0.02 %). Between 0.3 % (eleventh year) and 0.6 % (second year) of total annual N sub(2) O emissions were attributed to beech leaf litter of a single year. Conclusions: Most of the annual N sub(2) O emissions (1.33-1.54 kg N ha super(-1) yr super(-1) ) were probably derived from older soil N pools.
Climate warming will increase the drought exposure of many forests world‐wide. It is not well understood how trees adapt their hydraulic architecture to a long‐term decrease in water availability. We ...examined 23 traits characterizing the hydraulic architecture and growth rate of branches and the dependent foliage of mature European beech (Fagus sylvatica) trees along a precipitation gradient (855–594 mm yr⁻¹) on uniform soil. A main goal was to identify traits that are associated with xylem efficiency, safety and growth. Our data demonstrate for the first time a linear increase in embolism resistance with climatic aridity (by 10%) across populations within a species. Simultaneously, vessel diameter declined by 7% and pit membrane thickness (Tₘ) increased by 15%. Although specific conductivity did not change, leaf‐specific conductivity declined by 40% with decreasing precipitation. Of eight plant traits commonly associated with embolism resistance, only vessel density in combination with pathway redundancy and Tₘ were related. We did not confirm the widely assumed trade‐off between xylem safety and efficiency but obtained evidence in support of a positive relationship between hydraulic efficiency and growth. We conclude that the branch hydraulic system of beech has a distinct adaptive potential to respond to a precipitation reduction as a result of the environmental control of embolism resistance.
SUMMARY
Foliar water uptake (FWU), the direct uptake of water into leaves, is a global phenomenon, having been observed in an increasing number of plant species. Despite the growing recognition of ...its functional relevance, our understanding of how FWU occurs and which foliar surface structures are implicated, is limited. In the present study, fluorescent and ionic tracers, as well as microcomputed tomography, were used to assess potential pathways for water entry in leaves of beech, a widely distributed tree species from European temperate regions. Although none of the tracers entered the leaf through the stomatal pores, small amounts of silver precipitation were observed in some epidermal cells, indicating moderate cuticular uptake. Trichomes, however, were shown to absorb and redistribute considerable amounts of ionic and fluorescent tracers. Moreover, microcomputed tomography indicated that 72% of empty trichomes refilled during leaf surface wetting and microscopic investigations revealed that trichomes do not have a cuticle but are covered with a pectin‐rich cell wall layer. Taken together, our findings demonstrate that foliar trichomes, which exhibit strong hygroscopic properties as a result of their structural and chemical design, constitute a major FWU pathway in beech.
Significance Statement
Foliar water uptake has been shown to alleviate drought effects in many woody plants, yet the water entry pathways remain largely unknown. Understanding this mechanism is, however, crucial to an integrated insight into plant–water relationships under different hydrological conditions. By means of tracer experiments and microscopic analyses, we provide strong evidence that hygroscopic trichomes are the main entry points for water in beech leaves.
Positive relationships between biodiversity and ecosystem functioning are reported in many studies. The underlying mechanisms are, however, only little understood, likely due to the focus on the ...aboveground system and neglecting species-specific traits. We used different clusters of tree species composition to investigate how tree species diversity and tree species identity affect belowground nematode communities. Since soil nematodes comprise different trophic groups and are strongly linked to the microbial community, results can provide insight on how soil food webs are structured. In addition, data on leaf litter and fine root biomass were included to account for different effects of aboveground and belowground resources. Clusters of three trees of one, two and three species were identified within a temperate deciduous forest. Target tree species were European beech (Fagus sylvatica), common ash (Fraxinus excelsior) and lime (Tilia cordata, T. platyphyllos) differing in physiology, leaf litter quality and type of mycorrhiza. Tree species identity strongly affected nematode trophic structure, whereas tree species diversity had no impact. Ash beneficially affected bacterial-feeding nematodes, whereas fungal feeders were suppressed, likely caused by ash litter increasing soil pH. Fostering of the bacterial food chain by ash additionally could be related to rhizodeposition gaining importance after disappearance of high quality ash litter in spring, highlighting seasonal shifts in root and leaf litter-derived resources. The negative effect of ash on fungal-feeding nematodes is suggested to be due to the lack of ectomycorrhizal fungi as ash roots only form arbuscular mycorrhiza. In contrast, beech benefited fungal feeders and detrimentally affected bacterial feeders due to more acidic soil conditions that increase the competitive strength of fungi. Lime tended to negatively impact total nematode density but positively influenced plant-feeding nematodes. Generally, beech and ash strongly but opposingly influenced the trophic structure of nematode communities suggesting that changes in tree species identity result in major shifts in the channeling of energy through decomposer food webs. The results indicate that the structure of soil food webs varies markedly with tree species and point to the importance of basal resources, i.e., leaf litter and rhizodeposits. This suggests bottom-up forces mediated by individual tree species to control major decomposition pathways rather than tree diversity.
► Tree species identity is more important than diversity in structuring soil food webs. ► Beech fostered the fungal energy channel and ash the bacterial energy channel. ► Root and litter-derived resources differently affect soil food webs.
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