Search

International audience; The objective of this study was to examine the interactive influence of tree diversity and water availability on microbial functioning in surface soil (0-10 cm). The study was conducted at two field experiments (ages 4.2 and 9.0 years, respectively) of the TreeDivNet platform that established plantations with a gradient of tree species richness exposed to high and low water availability treatments. Soil microbial biomass, activity, functional diversity and functional identity were estimated using the MicroResp (TM) method. Soil organic carbon and several other variables potentially associated with soil processes (soil moisture, soil and air temperature, soil pH, soil C:N ratio, tree biomass) were also measured. Our results indicate that tree species richness significantly increased soil microbial biomass and, to a lesser extent, soil microbial activity in the high water availability treatment, at both trial sites. However, tree species richness effects on microbial processes were not expressed in the low water availability treatment, contrary to the stress-gradient hypothesis. Tree species richness and water treatment interaction impacted microbial biomass and activity through their effects on microbial functional diversity and identity at one site and air temperature at the other site. In conclusion, the mechanisms through which tree diversity creates bottom-up effects on biological soil processes depend on environmental conditions and time following the establishment of tree communities. Future research relating diversity and water availability effects on soil microbial processes should consider functional traits of tree species and a larger range of trophic levels including herbivores and soil fauna to better understand biodiversity-soil ecosystem function relationships in forest ecosystems.

Increasing biodiversity has been linked to higher primary productivity in terrestrial ecosystems. However, the underlying ecophysiological mechanisms remain poorly understood. We investigated the effects of surrounding species richness (monoculture, two‐ and four‐species mixtures) on the ecophysiology of Lithocarpus glaber seedlings in experimental plots in subtropical China. A natural rain event isotopically labelled both the water uptaken by the L. glaber seedlings and the carbon in new photoassimilates through changes of photosynthetic discrimination. We followed the labelled carbon (C) and oxygen (O) in the plant–soil–atmosphere continuum. We measured gas‐exchange variables (C assimilation, transpiration and above‐ and belowground respiration) and δ13C in leaf biomass, phloem, soil microbial biomass, leaf‐ and soil‐respired CO 2 as well as δ18O in leaf and xylem water. The 13C signal in phloem and respired CO 2 in L. glaber in monoculture lagged behind those in species mixture, showing a slower transport of new photoassimilates to and through the phloem in monoculture. Furthermore, leaf‐water 18O enrichment above the xylem water in L. glaber increased after the rain in lower diversity plots suggesting a lower ability to compensate for increased transpiration. Lithocarpus glaber in monoculture showed higher C assimilation rate and water‐use efficiency. However, these increased C resources did not translate in higher growth of L. glaber in monoculture suggesting the existence of larger nongrowth‐related C sinks in monoculture. These ecophysiological responses of L. glaber, in agreement with current understanding of phloem transport are consistent with a stronger competition for water resources in monoculture than in species mixtures. Therefore, increasing species diversity in the close vicinity of the studied plants appears to alleviate physiological stress induced by water competition and to counterbalance the negative effects of interspecific competition on assimilation rates for L. glaber by allowing a higher fraction of the C assimilated to be allocated to growth in species mixture than in monoculture.

Research into mixed-forests has increased substantially in the last decades but the extent to which the new knowledge generated meets practitioners' concerns and is adequately transmitted to them is unknown. Here we provide the current state of knowledge and future research directions with regards to 10 questions about mixed-forest functioning and management identified and selected by a range of European forest managers during an extensive participatory process. The set of 10 questions were the highest ranked questions from an online prioritization exercise involving 168 managers from 22 different European countries. In general, the topics of major concern for forest managers coincided with the ones that are at the heart of most research projects. They covered important issues related to the management of mixed forests and the role of mixtures for the stability of forests faced with environmental changes and the provision of ecosystem services to society. Our analysis showed that the current scientific knowledge about these questions was rather variable and particularly low for those related to the management of mixed forests over time and the associated costs. We also found that whereas most research projects have sought to evaluate whether mixed forests are more stable or provide more goods and services than monocultures, there is still little information on the underlying mechanisms and trade-offs behind these effects. Similarly, we identified a lack of knowledge on the spatio-temporal scales at which the effects of mixtures on the resistance and adaptability to environmental changes are operating. Our analysis may help researchers to identify what knowledge needs to be better transferred and to better design future research initiatives meeting practitioner's concerns. This article is based upon work from COST Action FP1206, EuMIXFOR, supported by COST (European Cooperation in Science and Technology). The authors want to specially thank the large number of forest managers that were involved in the participatory process and kindly responded to the surveys. The first author also thanks the Spanish Ministry of Economy and Competitiveness for funding the project AGL2015-70425-R which partly supported his contribution to this study. MM acknowledges support by grant IUT21-4 of the Estonian Ministry of Education and Research and DS support by project III 43007 (2011–2017) from Ministry of Education, Science and Technological Development of Republic of Serbia.

There is increasing interest in tall fescue (Festuca arundinacea Schreb.) in Western Europe and elsewhere, mainly because of its better drought resistance and yield potential compared with perennial ryegrass (Lolium perenne L.). Important drawbacks of tall fescue, compared with perennial ryegrass, are its lower digestibility and voluntary intake. Mixtures of both species might combine the advantages of each, and species interactions may eventually lead to transgressive overyielding. We compared the agronomic performance of tall fescue, perennial ryegrass and tall fescue–perennial ryegrass mixtures, as pure-grass swards or in association with white clover (Trifolium repens L.). Tall fescue–perennial ryegrass mixtures differed in the proportion and ploidy of the perennial ryegrass component. Yield, feed quality and botanical composition were measured in the 3 years after the sowing year. We found significant effects of ploidy of the ryegrass variety and of the proportion of ryegrass in the initial seed mixture on the botanical composition of the swards. Nevertheless, all swards were dominated by tall fescue at the end of the experiment. No overyielding of the mixtures compared with that of single-species swards was found, but feed quality was intermediate between that of the single-species swards. Mixed swards had better drought resistance than L. perenne and higher feeding quality than F. arundinacea swards.

Summary 1. Although a major part of plant biomass is underground, we know little about the contribution of different species to root biomass in multispecies communities. We summarize studies on root dis- tributions and plant responses to species-specific soil biota and formulate three hypotheses to explain how root responses may drive species coexistence and ecosystem productivity. 2. Recent studies suggest that root growth of some species may be stimulated in species mixtures compared with monocultures without hampering the growth of other species, leading to below- ground overyielding. Further studies suggest that these responses are the result of reduced impair- ment of growth by species-specific plant pathogens that accumulate in monocultures. 3. First, we hypothesize that due to pathogen-constrained growth, monocultures are ‘under- rooted’, i.e. they do not have enough roots for optimal acquisition of nutrients. Although elevated root production in mixtures represents a cost to the plant, improved nutrition will eventually result in improved plant performance. 4. Second, due to the plant species specificity of the soil biotic communities, we suggest that plant species in mixtures develop an intransitive competitive network in which none of the species is competitively superior to all other species. Competitive intransitivity is proposed as a mechanismof species coexistence. 5. As a final hypothesis, we suggest that pathogen-mediated root overproduction in species mix- tures determines the patterns of community productivity and overyielding, both directly, by improving plant performance, and indirectly, by releasing more carbon into the soil, resulting in enhanced availability of nutrients. 6. Synthesis. Recent evidence suggests that species coexistence and ecosystem productivity may be the result of an interplay between pathogen-driven plant responses and nutritional consequences. We suggest that responses of the roots are an important yet mostly overlooked intermediary between soil biota and plant community responses to biodiversity. Key-words: biodiversity, ecosystem functioning, intransitive competition, niche differentia- tion, plant–soil (below-ground) interactions, root distributions, root ecology, soil nutrient availability, transgressiveoveryielding

Natural populations of the bloom forming cyanobacterium Microcystis are typically composed of several distinct genotypes. Using Microcystis strains that differ in growth rate, microcystin production and colony formation, we conducted a laboratory experiment in the presence and absence of a grazer, the water flea Daphnia, to investigate whether interactions among strains can be predicted from functional traits, and whether the outcome of competition between strains is influenced by a grazer. Two toxic and two non-toxic Microcystis strains, isolated from a single lake, were grown during four weeks as single strains, in all possible combinations of two strains and all together, in the presence and absence of Daphnia magna. The relative abundance of strains in the populations was assessed using denaturing gradient gel electrophoresis, and the growth rate of each strain in mixed populations was compared to its growth rate in monoculture to determine interactions between strains. The observed interactions were strain-specific, and the relative abundances of strains in mixed populations could be partially explained by taking toxicity and colony formation into account. Importantly, some of the interactions were strongly altered by the presence of Daphnia. Daphnia induced colony formation in one strain, which then became a better competitor. Daphnia grazing also caused a higher evenness in the populations, both through a weakening of competitive interactions as well as by facilitation effects. Strong facilitation effects were due to non-toxic strains benefiting from the protection offered by toxic strains in the presence of predation. Overall, our results emphasize the presence of strong competitive interactions between Microcystis strains in the absence of grazing, whereas indirect positive interactions are prevalent in the presence of a generalist grazer. Our results suggest that differences in functional traits and grazer-mediated facilitation effects may enhance coexistence of Microcystis strains, including toxic and non-toxic strains.

The area of forest plantations is increasing worldwide helping to meet timber demand and protect natural forests. However, with global change, monospecific plantations are increasingly vulnerable to abiotic and biotic disturbances. As an adaption measure we need to move to plantations that are more diverse in genotypes, species, and structure, with a design underpinned by science. TreeDivNet, a global network of tree diversity experiments, responds to this need by assessing the advantages and disadvantages of mixed species plantations. The network currently consists of 18 experiments, distributed over 36 sites and five ecoregions. With plantations 1-15 years old, TreeDivNet can already provide relevant data for forest policy and management. In this paper, we highlight some early results on the carbon sequestration and pest resistance potential of more diverse plantations. Finally, suggestions are made for new, innovative experiments in understudied regions to complement the existing network. ispartof: Ambio vol:45 issue:1 pages:29-41 ispartof: location:Sweden status: published

Summary 1. Although a major part of plant biomass is underground, we know little about the contribution of different species to root biomass in multispecies communities. We summarize studies on root dis- tributions and plant responses to species-specific soil biota and formulate three hypotheses to explain how root responses may drive species coexistence and ecosystem productivity. 2. Recent studies suggest that root growth of some species may be stimulated in species mixtures compared with monocultures without hampering the growth of other species, leading to below- ground overyielding. Further studies suggest that these responses are the result of reduced impair- ment of growth by species-specific plant pathogens that accumulate in monocultures. 3. First, we hypothesize that due to pathogen-constrained growth, monocultures are ‘under- rooted’, i.e. they do not have enough roots for optimal acquisition of nutrients. Although elevated root production in mixtures represents a cost to the plant, improved nutrition will eventually result in improved plant performance. 4. Second, due to the plant species specificity of the soil biotic communities, we suggest that plant species in mixtures develop an intransitive competitive network in which none of the species is competitively superior to all other species. Competitive intransitivity is proposed as a mechanismof species coexistence. 5. As a final hypothesis, we suggest that pathogen-mediated root overproduction in species mix- tures determines the patterns of community productivity and overyielding, both directly, by improving plant performance, and indirectly, by releasing more carbon into the soil, resulting in enhanced availability of nutrients. 6. Synthesis. Recent evidence suggests that species coexistence and ecosystem productivity may be the result of an interplay between pathogen-driven plant responses and nutritional consequences. We suggest that responses of the roots are an important yet mostly overlooked intermediary between soil biota and plant community responses to biodiversity. Key-words: biodiversity, ecosystem functioning, intransitive competition, niche differentia- tion, plant–soil (below-ground) interactions, root distributions, root ecology, soil nutrient availability, transgressiveoveryielding

Forests are biodiversity hotspots worldwide with 70% of terrestrial biodiversity being included in forested landscapes. However, deforestation, forest degradation and fragmentation lead to an increasing rate of species extinctions. Hence, predicting the consequences of changes in species numbers, in distribution patterns of taxa, and of shifts in dominance, has become a major challenge for community and ecosystem ecology. However, until now the relationships between biodiversity and ecosystem functioning in forests have been largely underexplored Therefore, the overall aims of the BELSPO cluster project FORBIO are: to review, synthesize and disseminate existing knowledge about the benefits and drawbacks of mixed stands vs monocultures (WP1); and to establish a highly innovative, large-scaled forest biodiversity experiment to evaluate the impact of increasing tree diversity on forest ecosystem functioning (WP2). To achieve the first objective, a so-called ‘white paper’ has been compiled by the FORBIO team members which has been published in Dutch as a special issue of the BosRevue and in French as a special issue of Forêt Wallone. Among stakeholders, many different opinions exist about the functioning of mixed forests and therefore the scientific evidence was confronted with stakeholder perceptions on ecosystem services in mixed forests compared to monocultures. The principal outcome was that stakeholders appear to have quite strong opinions on the functioning of mixed vs monoculture stands, whereas the review of the scientific literature highlighted the lack of specific information on forest ecosystem services in mixed forests compared to monocultures, in particular from studies where confounding factors can be eliminated or accounted for. The second objective was met by establishing two large-scaled tree diversity experiments in Zedelgem (Flanders) and Gedinne (Wallonia). Based on a state-of-the-art experimental design, 32 810 and 33 304 trees of five different species were planted in 42 and 44 experimental plots in Zedelgem and Gedinne, respectively. A third experiment with a similar design will be established in Hecthel-Eksel (Flanders) in 2011. FORBIO’s short-term contributions to sustainable development are mainly related to the fact that the project has introduced the state-of-the-art concepts and empirical support on the various relationships between biodiversity and ecosystem functioning to a large audience of forest owners, managers, users and scientists in Belgium. In the long-run, FORBIO will significantly contribute to a better understanding of the importance of tree species diversity for the functioning of forest ecosystems and the ecosystem services that they provide thanks to the establishment of the two (and soon three) large-scaled tree diversity experiments. Furthermore, the experiments, embedded in the worldwide TreeDivNetwork, will most likely continue to act as an attractor for researchers from Belgium and abroad. nrpages: 59 status: published

A common pattern emerging from studies on the relationship between plant diversity and ecosystem functioning is that productivity increases with diversity. Most of these studies have been carried out in perennial grasslands, but many lasted only two growing seasons or reported data from a single year. Especially for perennial plant communities, however, the long-term effects of diversity are important. The question whether interactions between few species or among many species lead to increased productivity remained largely unanswered. So far, the main mechanism addressed is the increased input of nitrogen by nitrogen-fixing legumes. We report that other mechanisms can also generate strong increases of productivity with diversity. Results from 4 consecutive years of a plant diversity experiment without legumes show that a positive relationship between plant species richness and productivity emerged in the second year and strengthened with time. We show that increased nutrient use efficiency at high species richness is an important underlying mechanism. This mechanism had not been discussed in earlier studies. Furthermore, our results suggest that complementary nutrient uptake in space and time is important. Together, these mechanisms sustain consistently high productivity at high diversity. biodiversity | niche complementarity | nitrogen use efficiency | ecosystem functioning

We investigated the effect of species richness on productivity in randomly assembled grassland communities without legumes. Aboveground biomass increased with increasing species richness and different measures of complementarity showed strong increases with plant species richness. Increasing productivity could not be attributed to a relative increase of highly productive species. Instead, the increase appeared to be caused by the increased performance of several low-productive species. Our results provide evidence that niche complementarity can strongly increase productivity in grasslands, although the communities contained only grasses and forbs.