Does functional soil microbial diversity contribute to explain within-site plant beta-diversity in an alpine grassland and a <i>dehesa</i> meadow in Spain?

Questions: Once that the effects of hydrological and chemical soil properties have been accounted for, does soil microbial diversity contribute to explain change in plant community structure (i.e. within-site beta-diversity)? If so, at which spatial scale does microbial diversity operate? Location: La Mina in Moscosa Farm, Salamanca, western Spain (dehesa community) and Laguna Larga in the Urbión Peaks, Soria, central-northern Spain (alpine grassland). Methods: The abundance of vascular plant species, soil gram-negative microbial functional types and soil chemical properties (pH, available phosphorus, and extractable cations) were sampled at both sites, for which hydrological models were available. Redundancy analysis (RDA) was used to partition variation in plant community structure into hydrological, chemical and microbial components. Spatial filters, arranged in scalograms, were used to test for the spatial scales at which plant community structure change. Results: In the case of the dehesa the diversity of soil gram-negative microbes, weakly driven by soil pH, contributed to a small extent (adj-R2 = 2%) and at a relative medium spatial scale to explain change in plant community structure. The abundance of a few dehesa species, both annual (Trifolium dubium, Vulpia bromoides) and perennial (Poa bulbosa, Festuca ampla), was associated with either increasing or decreasing soil microbial diversity. In the alpine meadow the contribution was negligible. Conclusions: Microbial diversity can drive community structure, though in the hierarchy of environmental factors structuring communities it appears to rank lower than other soil factors. Still, microbial diversity appears to promote or restrain individual plant species. This paper aims to encourage future studies to use more comprehensive and insightful techniques to assess microbial diversity and to combine this with statistical approaches such as the one used here

Pflanzliche Interaktionen in ressourcenarmen Sandökosystemen : die Bedeutung funktioneller und morphologischer Wurzelparameter für die unterirdische Konkurrenzfähigkeit

Bartelheimer M. Pflanzliche Interaktionen in ressourcenarmen Sandökosystemen : die Bedeutung funktioneller und morphologischer Wurzelparameter für die unterirdische Konkurrenzfähigkeit. Bielefeld (Germany): Bielefeld University; 2005.As one of the most important biotic interactions between plants, competition is considered to be a major force of influence for plant morphology, distribution and abundance up to the structuring of entire biocoenosises. In unproductive ecosystems, where plant growth is limited by edaphic resources, belowground competition is one of the most decisive processes. Due to laborious methods, though, competition experiments examining root properties in detail are still scarce. Different parameters influencing the zone of influence or the resource-uptake of plants can be determining for belowground competitive ability, while the impact on competitive effect (the ability to affect neighbours) can be independent of the impact on competitive response (the ability to tolerate neighbours). Early successional stages in sand ecosystems are commonly resource-poor in terms of water and nutrient availability and are therefore utilised in this study to examine the impact of morphological and functional root parameters in belowground competition in general as well as in competition for nitrate in special. This thesis thus contributes to investigations to identify the most relevant morphological and functional root parameters concerning belowground competition in general and on competition for nitrate in particular. In a controlled field experiment in a sand-pit allowing the unhampered development of roots, five species from early successional stages of sand ecosystems (Festuca psammophila, Corynephorus canescens, Conyza canadensis, Hieracium pilosella and Hypochoeris radicata) were used to examine the importance of root distribution for the competitive ability (chapter 3). Special emphasis was put on the effect that neighbouring plants have on the root distribution of target plants. As far as the competitive effect is concerned, the results from a small-scale fractional harvest of the rooted soil space by application of a modified monolith-method highlighted the importance of a vast zone of influence with predominance in the uppermost soil layers. Interestingly, the competitive response was by contrast rather connected to the root system's capability for a plastic reaction involving both root segregation and aggregation. While root segregation and its biological reason in diminishing competition are well known, the aggregation of roots towards a neighbouring plant is a phenomenon that has yet been unknown and can be connected to self-/nonself recognition and the biological sense to maintain already occupied soil. In a second sand-pit experiment with the above mentioned species, short-term competition for nitrate was assessed by means of a 15N-nitrate application between competing roots (chapter 4). By inclusion of detailed root sizes, evidence for the so far unproven size symmetry of competition for nitrate was provided, meaning that the nitrate uptake by roots is not impeded overproportionally by bigger neighbouring plants. Nonetheless, root system size was again the decisive feature for the competitive effect, while for the competitive response both size and the already mentioned plastic reactions were most important. Moreover, the role of nitrate transporters for the competitive ability in nitrogen-poor sand was examined in a pot-experiment (chapter 5). Their importance, especially in competition, was unclear as not the relatively fast transport over the plasmalemma but the comparably slow movement of nitrate through the substrate is considered to be the rate-limiting step. Plants of the species Arabidopsis thaliana with a T-DNA-insertion in a nitrate-transporter-gene showed weaker competitive response ability to H. pilosella than the wild type, highlighting the important role of nitrate transporters in nitrate uptake especially in competition. Another important, though contrasting result, was the observation of an (over-)compensation of the defect for the competitive effect which hints at a complex regulation and stresses the role of plastic reactions. Concerning the importance of functional and morphological root parameters for belowground competitive ability, this study on the whole presents a novel and detailed picture that especially gains significance from the differentiation of competitive effect and response: for the competitive effect, especially the size of the belowground zone of influence is important, while the competitive response appears to depend on multiple components like the capability for plastic reactions, the localization and size of the zone of influence and the uptake capacity for resources.Als einer der wichtigsten biotischen Interaktionen kommt Konkurrenz eine prägende Rolle für Pflanzen zu, da sie ihre Morphologie, Verteilung und Häufigkeit bis hin zur Strukturierung ganzer Lebensgemeinschaften beeinflusst. In Ökosystemen, in denen das Pflanzenwachstum durch Bodenressourcen limitiert ist, kommt dabei der unterirdischen Konkurrenz die entscheidende Bedeutung zu. Gleichwohl sind Konkurrenzversuche, die Wurzeleigenschaften detailliert untersuchen, aufgrund der aufwändigen Methoden rar. Verschiedene Wurzelparameter, die den unterirdischen Einflussbereich oder die Ressourcenaufnahme von Pflanzen bestimmen, können für die unterirdische Konkurrenzkraft prägend sein, wobei die Bedeutung für den Konkurrenz-Effekt (die Fähigkeit Nachbarpflanzen zu beeinträchtigen) unabhängig sein kann von der Bedeutung für die Konkurrenz-Toleranz (der Fähigkeit Nachbarn zu tolerieren). An Hand von frühen Sukzessionsstadien in Sandökosystemen, die durch Wasser- oder Nährstofflimitierungen als ressourcenarm anzusehen sind, wird deshalb die Bedeutung von morphologischen und funktionellen Wurzelparametern bei unterirdischer Konkurrenz im Allgemeinen und bei Konkurrenz um Nitrat im Speziellen untersucht. In einem kontrollierten Freilandexperiment wurde in einem Sandbeet, das ungehinderte Wurzelentwicklung zulässt, an fünf Pflanzenarten früher Sukzessionsstadien aus Sandökosystemen (Corynephorus canescens, Festuca psammophila, Conyza canadensis, Hieracium pilosella und Hypochoeris radicata) die Rolle der Wurzelverteilung für die Konkurrenzkraft untersucht (Kapitel 3). Dabei lag ein Schwerpunkt auf der Wirkung von Nachbarpflanzen auf die Wurzelverteilung der Zielpflanzen. Die kleinräumige Aufteilung des Wurzelraums nach einer modifizierten Monolith-Methode nach Böhm (1979) zeigte auf, dass ein großer unterirdischer Einflussbereich mit Schwerpunkt in den oberen Bodenschichten wichtig für den Konkurrenzeffekt ist. Interessanterweise war für die Konkurrenz-Toleranz dagegen eher die Reaktionsfähigkeit des Wurzelsystems in Form von Wurzelsegregation bzw. -aggregation entscheidend. Während Wurzelsegregation und ihre Bedeutung in der Minderung von Konkurrenz schon länger bekannt sind, ist die Aggregation von Wurzeln in Richtung der Nachbarpflanze ein bisher unbekanntes Phänomen, das mit Selbst-/Fremderkennung in Verbindung gebracht werden kann und dessen biologischer Sinn wahrscheinlich in der Sicherung eines exklusiven Zugangs zu bereits besetzten Bodenbereichen zu sehen ist. In einem zweiten Sandbeetexperiment mit den oben genannten Pflanzenarten wurde die kurzzeitige Konkurrenz um Nitrat mittels einer 15N-Nitrat-Applikation zwischen konkurrierende Wurzelsysteme betrachtet (Kapitel 4). Dabei gelang durch die Erfassung der Wurzelgrößen u.a. der bisher nicht klar erbrachte Nachweis, dass Konkurrenz um Nitrat größensymmetrisch verläuft, d.h. Wurzeln in ihrer Nitrataufnahme durch größere Nachbarwurzeln nicht überproportional eingeschränkt werden. Gleichwohl war die Wurzelgröße auch hier für den Konkurrenzeffekt entscheidend, während sich für die Konkurrenz-Toleranz neben der Größe die schon beschriebene Reaktionsfähigkeit als wichtig erwies. In einem weiteren Experiment wurde in Topfkultur die Rolle von Nitrattransportern für die Konkurrenzfähigkeit in stickstoffarmem Sand untersucht (Kapitel 5). Für die Bedeutung besonders in Konkurrenz bestand ein Untersuchungsbedarf, weil der vergleichsweise langsame Fluss des Nitrats durch das Substrat als der geschwindigkeitsbestimmende Schritt der Nitrataufnahme anzusehen ist, nicht der vergleichsweise schnelle Transport über das Plasmalemma. Pflanzen der Art Arabidopsis thaliana mit einer T-DNA-Insertion in einem Nitrattransportergen hatten gegenüber H. pilosella eine schwächere Toleranz als der Arabidopsis Wildtyp, womit Nitrattransportern eine wichtige Rolle in der Nitrataufnahme besonders unter Konkurrenzbedingungen zuzuordnen ist. Als weiteres wichtiges Ergebnis konnte dagegen im Konkurrenz-Effekt eine (Über-)Kompensation des Defekts festgestellt werden, was auf eine komplexe Regulation hindeutet und die Rolle von plastischen Reaktionen betont. Bezüglich der Bedeutung von funktionellen und morphologischen Wurzelparametern für die unterirdische Konkurrenzkraft zeigt diese Arbeit im Ganzen ein neuartiges und detailliertes Bild auf, das v.a. durch die Differenzierung nach Konkurrenz-Effekt und -Toleranz an Aussagekraft gewinnt: Für die Effekt-Konkurrenzkraft ist danach v.a. die Größe des unterirdischen Einflussbereichs wichtig, während die Toleranz-Konkurrenzkraft von mehreren Einzelkomponenten wie plastischer Reaktionsfähigkeit, Lokalisation und Größe des Einflussbereichs und der Ressourcenaufnahmekapazität abhängig erscheint

Ellenberg’s water table experiment put to the test: species optima along a hydrological gradient

An important aspect of niche theory is the position of species' optima along ecological gradients. It is widely believed that a species' ecological optimum takes its shape only under competitive pressure. The ecological optimum, therefore, is thought to differ from the physiological optimum in the absence of interspecific competition. Ellenberg's Hohenheim water table experiment has been very influential in this context. However, the water table gradient in Ellenberg's experiment was produced by varying the soil thickness above the water table, which confounded the potentially disparate impacts of water table depth (WTD) and soil depth on species growth. Accordingly, here we have re-evaluated Ellenberg's work. Specifically, we tested the hypothesis that physiological and ecological optima are identical and unaffected by interspecific interaction. We used the same six grasses as in Ellenberg's experiments, but in our mesocosms, WTD was varied but soil depth kept constant. The design included both an additive component (with/without plant interaction) and a substitutive component (monocultures vs. species mixtures). The results show that the physiological optima along the hydrological gradient varied greatly between species, even in the absence of interspecific interaction. Within species, however, physiological and ecological optima appeared identical in most cases, irrespective of the competition treatment. We conclude that the 'physiological capacity' of species largely determines where they are able to persist and that any impact of interspecific interaction is only marginal. These findings are at variance with Ellenberg's rule, where competition is considered to shift the distribution of a species away from its physiological optimum

Root responses to legume plants integrate information on nitrogen availability and neighbour identity

Rhizobial symbiosis is known to increase the nitrogen availability in the rhizosphere of legumes. Therefore, it has been hypothesized that other plants' roots should forage towards legume neighbours, but avoid non-legume neighbours. Yet, root distribution responding to legume plants as opposed to non-legumes has not yet been rigorously tested and might well be subject to integration of multiple environmental cues. In this study, wedevised an outdoor mesocosm experiment to examine root distributions of the two plant species Pilosella officinarum and Arenaria serpyllifolia in a two-factorial design. While one factor was 'neighbour identity', where plants were exposed to different legume or non-legume neighbours, the other factor was 'nitrogen supply'. In the latter the nutrient-poor soil was supplemented with either nitrogen-free or with nitrogen-containing fertilizer. Unexpectedly, of all treatments that included a legume neighbour (eight different species or factor combinations), we found merely one case of root aggregation towards a legume neighbour (P. officinarum towards Medicago minima under nitrogen-fertilized conditions). In this very treatment, also P. officinarum root-shoot allocation was strongly increased, indicating that neighbour recognition is coupled with a contesting strategy. Considering the various response modes of the tested species towards the different legume and non-legume neighbours, we can conclude that roots integrate information on neighbour identity and resource availability in a complex manner. Especially the integration of neighbour identity in root decisions must be a vital aptitude for plants to cope with their complex biotic and abiotic environment in the field. (C) 2018 Gesellschaft fur Okologie. Published by Elsevier GmbH. All rights reserved

Belowground neighbor perception in Arabidopsis thaliana studied by transcriptome analysis: roots of Hieracium pilosella cause biotic stress

Root-root interactions are much more sophisticated than previously thought, yet the mechanisms of belowground neighbor perception remain largely obscure. Genome-wide transcriptome analyses allow detailed insight into plant reactions to environmental cues. A root interaction trial was set up to explore both morphological and whole genome transcriptional responses in roots of Arabidopsis thaliana in the presence or absence of an inferior competitor, Hieracium pilosella. Neighbor perception was indicated by Arabidopsis roots predominantly growing away from the neighbor (segregation), while solitary plants placed more roots toward the middle of the pot. Total biomass remained unaffected. Database comparisons in transcriptome analysis revealed considerable similarity between Arabidopsis root reactions to neighbors and reactions to pathogens. Detailed analyses of the functional category "biotic stress" using MapMan tools found the sub-category "pathogenesis-related proteins" highly significantly induced. A comparison to a study on intraspecific competition brought forward a core of genes consistently involved in reactions to neighbor roots. We conclude that beyond resource depletion roots perceive neighboring roots or their associated microorganisms by a relatively uniform mechanism that involves the strong induction of pathogenesis-related proteins. In an ecological context the findings reveal that belowground neighbor detection may occur independently of resource depletion, allowing for a time advantage for the root to prepare for potential interactions

N-15-nitrate-labelling demonstrates a size symmetric competitive effect on belowground resource uptake

Bartelheimer M, Steinlein T, Beyschlag W. N-15-nitrate-labelling demonstrates a size symmetric competitive effect on belowground resource uptake. PLANT ECOLOGY. 2008;199(2):243-253.Strong hints exist that belowground competition is generally size-symmetric. While this has frequently been shown by use of integrative indicators like growth or biomass, resource-focussed approaches are still lacking, especially those investigating the competitive effect. Here, we present a correlation between neighbour plants' root sizes and their competitive effect on their target plants' nitrate uptake. This was derived from a controlled field experiment where intra- and interspecific combinations of five different herbaceous species from nutrient poor sand ecosystems were examined in an additive design. Short-term pulses of N-15-labelled nitrate were applied between competing pairs of plant individuals. The sizes of neighbour root systems had high explanatory power for the competitive effect on target plants' nitrate uptake. Equally important, a curve fitting approach revealed that the competitive effect based on N-15-uptake matched predictions of a size-symmetric interaction. With 66% of the variation in competitive effect on nitrate uptake explained by root system size, the degree to which root size results in a belowground overlap of zones of influence is crucial. Within this overlap, further attributes like architecture or uptake capacity may be important. Our data represent experimental support for a size symmetric competitive effect for a specific belowground resource. Since this is not consistent with an overproportional size advantage when mobile soil resources are limiting, it suggests that the survival of small individuals or species should be facilitated by the symmetric nature of belowground competitive effects

Clonal diversity and genetic variation of the sedge Carex nigra in an alpine fen depend on soil nutrients

In this study we analysed the impact of water regime and soil nutrients on the clonal diversity and genetic variation of the sedge Carex nigra in a central alpine fen. For our analysis, we established 16 study plots randomly distributed over the fen. We determined the exact elevation of each plot as an indicator for the water regime and measured the content of phosphorous and potassium in the soil of each plot. Clonal diversity and genetic variation of C. nigra were assessed with nuclear microsatellites using leaf material collected in 20 subplots along a diagonal cross within each study plot. The influence of water regime and soil mineral nutrients on clonal diversity and genetic variation was estimated by Bayesian multiple regression. Our study revealed a clear impact of soil nutrient conditions on clonal diversity and genetic variation of C. nigra, which increased with the concentration of phosphorous and decreased with the concentration of potassium. Key background to these findings seems to be the relative offspring success from generative as compared to clonal propagation. Phosphorous acquisition is essential during seedling establishment. Clonal diversity and genetic variation increase, therefore, at sites with higher phosphorous contents due to more successful recruitment. High levels of clonal diversity and genetic variation at sites of low potassium availability may in contrast be mainly caused by increased plant susceptibility to abiotic stress under conditions of potassium deficiency, which brings about more gaps in C. nigra stands and favors the ingrowth from other clones or recruitment from seeds

Role of mycorrhization and nutrient availability in competitive interactions between the grassland species Plantago lanceolata and Hieracium pilosella

Höpfner I, Beyschlag W, Bartelheimer M, Werner C, Unger S. Role of mycorrhization and nutrient availability in competitive interactions between the grassland species Plantago lanceolata and Hieracium pilosella. Plant Ecology. 2015;216(6):887-899.Arbuscular mycorrhizal fungi (AMF) may serve as an effective substitute for root surface. As mycorrhizal benefits are related to nutrient availability, the trade-off between carbon investments into AMF versus roots may drive competitive interactions. We studied competitive interactions between mycorrhizal and non-mycorrhizal individuals of Hieracium pilosella L. and Plantago lanceolata L., species differing in both mycotrophic degree and carbon allocation to roots. Three fertilization treatments were used to simulate nutritional differences over the course of succession. Species-specific differences in mycotrophy were reflected in markedly larger root/shoot allocation in P. lanceolata and higher mycorrhizal growth dependency in H. pilosella. P. lanceolata dominated competition in all fertilizer treatments, enabled by its comparatively larger root biomass allocation. In contrast, under intermediate and high fertilization, H. pilosella exhibited large investments into clonal shoot growth rather than in roots. Unexpectedly, the competitive imbalance between both species was amplified by the presence of AMF. The poor competitive strength of H. pilosella indicates that AMF-dominated foraging can be less effective than root-dominated foraging in competitive interactions, particularly under high nutrient availabilities. However, the competitive imbalance was reduced in favor of H. pilosella under nutrient deficiency. Our results lend support to the idea of differing competitive success of mycorrhizal- versus root-based foraging strategy over a nutritional gradient, which may play a role in the natural distribution of species over the course of succession

Soil moisture level and substrate type determine long-term seed lifespan in a soil seed bank

Aims Seeds are usually classified as short- or long-term persistent. It is still hardly understood how environmental conditions influence seed persistence. The study aimed to monitor the long-term effects of different moisture and substrate on seed persistence. Methods Seeds of three Rumex species buried in autumn 2009 in combinations of moisture and substrate were exhumed in spring 2015 and 2021 to test their persistence in the soil after 5.5 and 11.5 years, respectively. Long-term persistence data were compared with data from previous short-term experiment for the same species and environmental conditions reported in Abedi et al. (Plant Soil 374:485-495, 2014). Results No seeds of R. acetosa were found viable after 1.5 years. Seeds of R. acetosella retained viability after 11.5 years mostly in dry-loam (~ 60%) and moist-sand (~ 25%) test conditions and moisture levels were identified as the main driver. R. maritimus retained ≥ 80% viability in moist and wet test conditions and > 40% in the dry test conditions. Conclusions For one (R. acetosella) of the three investigated species, the classification of soil seed bank type depended on environmental conditions, emphasizing the need to introduce a more detailed classification scheme for soil seed persistence and to include the information about extrinsic parameters in databases. However, in the other two species with transient (R. acetosa) and long-term persistent (R. maritimus) seed banks, there are rather intrinsic parameters that affect seed viability. Hence, both site-specific environmental factors as well as seed germination traits need full consideration in the classification of future soil seed bank studies

Identifying mechanisms of competition in multi-species communities

Weigelt A, Schumacher J, Walther T, Bartelheimer M, Steinlein T, Beyschlag W. Identifying mechanisms of competition in multi-species communities. JOURNAL OF ECOLOGY. 2007;95(1):53-64.1 We tested the hypothesis that the competitive effect of multi-species mixtures on a single target plant can be described as a simple additive function of the effects of single species. 2 In a controlled field competition experiment on sand, we used four common species of dry acidic grasslands (Hieracium pilosella, Jasione montana, Corynephorus canescens and Festuca ovina) to study the effects of both, single- and multi-species neighbourhoods on the target species H. pilosella. 3 Target plant biomass points at a competitive ranking of neighbour species with F. ovina as the strongest competitor followed by C. canescens and H. pilosella, while J. montana has a lower effect on the target. The analysis of the competitive response gives no indication of competitive release or amplification with increasing number of neighbouring species. 4 Further analysis with yield-density models reveals that the competition intensity of most multi-species mixtures is well predicted by the effect of pairwise interactions. Hence, for the majority of species combinations, our results indicate additive effects of species competition. 5 Specific species combinations, however, significantly deviate from this result. For combinations of C. canescens with either one of the herbs, the models show a significantly better fit if a non-additivity parameter is included. The important missing factor to describe target plant growth is species-specific biomass of neighbouring plants and here root biomass is the most crucial determinant. The addition of each of these factors as non-additive parameters significantly increases the predictive power of the model. 6 Our work adds to the increasing evidence that interactions in diverse plant communities are not simply a sum of their component species interactions. For a better understanding of the effect of competition on the structure and development of diverse plant communities, future studies need to consider both additive and non-additive effects of competing species