Effects of enemy exclusion on biodiversity-productivity relationships in a subtropical forest experiment

Interspecific niche complementarity is a key mechanism posited to explain positive species richness-productivity relationships in plant communities. However, the exact nature of the niche dimensions that plant species partition remains poorly known.Species may partition abiotic resources that limit their growth, but species may also be specialized with respect to their set of biotic interactions with other trophic levels, in particular with enemies including pathogens and consumers. The lower host densities present in more species-diverse plant communities may therefore result in smaller populations of specialized enemies, and in a smaller associated negative feedback these enemies exert on plant productivity.To test whether such host density-dependent effects of enemies drive diversity-productivity relationships in young forest stands, we experimentally manipulated leaf fungal pathogens and insect herbivores in a large subtropical forest biodiversity-ecosystem functioning experiment in China (BEF-China).We found that fungicide spraying of tree canopies removed the positive tree-species richness-productivity relationship present in untreated control plots. The tree species that contributed the most to this effect were the ones with the highest fungicide-induced growth increase in monoculture. Insecticide application did not cause comparable effects. Synthesis. Our findings suggest that tree species diversity may not only promote productivity by interspecific resource-niche partitioning but also by trophic niche partitioning. Most likely, partitioning occurred with respect to enemies such as pathogenic fungi. Alternatively, similar effects on tree growth would have occurred if fungicide had eliminated positive effects of a higher diversity of beneficial fungi (e.g. mycorrhizal symbionts) that may have occurred in mixed tree species communities.

Differential uptake of nitrogen forms by two herbs in the Gurbantunggut desert, Central Asia

Understanding how plants adjust their requirements for different N forms can help elucidate plant coexistence strategies in N-limited desert ecosystems. To understand the mechanisms involved, we investigated whether two desert herbs can directly absorb dissolved organic nitrogen (N) and tested whether the patterns changed over different growth stages. Two dominant herbaceous species, Astragalus arpilobus and Arnebia decumbens, from the southern edge of the Gurbantunggut desert, China, were selected. Short-term (24 h) ¹⁵ N-labelled tracer (¹⁵ N-NO₃ , ¹⁵ N-NH₄ , 2-¹³ C-¹⁵ N-Glycine) treatments were conducted at two soil depths (0-5 cm and 5-15 cm) in the season of rapid growth (June) and in the peak biomass season (July). Enrichment in ¹³ C and ¹⁵ N was assessed in the two species receiving glycine. The ratio ¹³ C:¹⁵ N was 0.21-1.39 at the 24-h harvest, suggesting that approximately 10.5-69.5% of glycine had been absorbed. The amount of absorbed ¹⁵ N was significantly affected by species, month, soil depth and N form. The two species absorbed most ¹⁵ N from the 0-5 cm soil layer, and the absorption rate in July was higher than that in June. The absorption of ¹⁵ N-NO₃ and ¹⁵ N-NH₄ was significantly higher than that of 2-¹³ C-¹⁵ N-Glycine. The results indicate that these herbs could use amino acids in the N-deficient desert ecosystem. The two co-existing species used different forms of inorganic N for their requirements and maintained a specific preference throughout various growth stages.

Using plant traits to understand the contribution of biodiversity effects to annual crop community productivity

Increasing biodiversity generally enhances productivity through selection and complementarity effects not only in natural, but also in agricultural, systems. However, the quest to explain why diverse cropping systems are more productive than monocultures remains a central goal in agricultural science. In a mesocosm experiment, we constructed monocultures, two- and four-species mixtures from eight crop species with or without fertilizer and both in temperate Switzerland and dry, Mediterranean Spain. We measured physical factors and plant traits and related these in structural equation models to selection and complementarity effects to explain seed yield differences between monocultures and mixtures. Increased crop diversity increased seed yield in Switzerland. This positive biodiversity effect was driven to almost the same extent by selection and complementarity effects, which increased with plant height and specific leaf area (SLA), respectively. Also, ecological processes driving seed yield increases from monocultures to mixtures differed from those responsible for seed yield increases through the diversification of mixtures from two to four species. Whereas selection effects were mainly driven by one species, complementarity effects were linked to larger leaf area per unit leaf weight. Seed yield increases due to mixture diversification were driven only by complementarity effects and were not mediated through the measured traits, suggesting that ecological processes beyond those measured in this study were responsible for positive diversity effects on yield beyond two-species mixtures. By understanding the drivers of positive biodiversity-productivity relationships, we can improve our ability to predict species combinations that enhance ecosystem functioning and can promote sustainable agricultural production.

Tree species rather than type of mycorrhizal association drive inorganic and organic nitrogen acquisition in tree-tree interactions

Mycorrhizal fungi play an important role for the nitrogen (N) supply of trees. The influence of different mycorrhizal types on N acquisition in tree-tree interactions is, however, not well understood, particularly with regard to the competition for growth-limiting N. We studied the effect of competition between temperate forest tree species on their inorganic and organic N acquisition in relation to their mycorrhizal type (i.e., arbuscular mycorrhiza or ectomycorrhiza). In a field experiment, we quantified net N uptake capacity from inorganic and organic N sources using 15N/13C stable isotopes for arbuscular mycorrhizal tree species (i.e., Acer pseudoplatanus L., Fraxinus excelsior L., and Prunus avium L.) as well as ectomycorrhizal tree species (i.e., Carpinus betulus L., Fagus sylvatica L., and Tilia platyphyllos Scop.). All species were grown in intra- and interspecific competition (i.e., monoculture or mixture). Our results showed that N sources were not used complementarily depending on a species' mycorrhizal association, but their uptake rather depended on the competitor, indicating species-specific effects. Generally, ammonium was preferred over glutamine and glutamine over nitrate. In conclusion, our findings suggest that the inorganic and organic N acquisition of the studied temperate tree species is less regulated by mycorrhizal association but rather by the availability of specific N sources in the soil as well as the competitive environment of different tree species.

Ecological and evolutionary approaches to improving crop variety mixtures

Variety mixtures can provide a range of benefits for both the crop and the environment. Their utility for the suppression of pathogens, especially in small grain crops, is well established and has seen some remarkable successes. However, despite decades of academic interest in the topic, commercial efforts to develop, release and promote variety mixtures remain peripheral to normal breeding activities. Here we argue that this is because simple but general design principles that allow for the optimization of multiple mixture benefits are currently lacking. We therefore review the practical and conceptual challenges inherent in the development of variety mixtures, and discuss common approaches to overcome these. We further consider three domains in which they might be particularly beneficial: pathogen resistance, yield stability and yield enhancement. We demonstrate that combining evolutionary and ecological concepts with data typically available from breeding and variety testing programmes could make mixture development easier and more economic. Identifying synergies between the breeding for monocultures and mixtures may even be key to the widespread adoption of mixtures-to the profit of breeders, farmers and society as a whole.

Nitrogen Uptake by Two Plants in Response to Plant Competition as Regulated by Neighbor Density

Plant species may acquire different forms of nitrogen (N) to reduce competition for the same resource, but how plants respond to neighbors with different densities in their N uptake is still poorly understood. We investigated the effects of competition regime on the uptake of different N forms by two hygrophytes, Carex thunbergii and Polygonum criopolitanum, by conducting a hydroponic test of excised roots and an in situ experiment in a subtropical wetland ecosystem. The two species were grown either in monocultures or mixtures with various neighbor densities. Root functional traits and N uptake rates of different N forms were measured. Our results showed that N uptake was mainly determined by N form, rather than species identity. Both species were able to use organic N sources, but they took up relatively more N supplied as NO 3 - than as NH 4 + or glycine, irrespective of competition treatments. Both species preferred NO 3 - when grown in monoculture, but in the presence of competitors, the preference of fast-growing C. thunbergii persisted while P. criopolitanum acquired more NH 4 + and glycine, with stronger responses being observed at the highest neighbor density. The hydroponic test suggested that these divergences in N acquisition between two species might be partially explained by different root functional traits. To be specific, N uptake rates were significantly positively correlated with root N concentration and specific root length, but negatively correlated with root dry matter content. Our results implicated that C. thunbergii has a competitive advantage with relatively more stable N acquisition strategy despite a lower N recovery than P. criopolitanum, whereas P. criopolitanum could avoid competition with C. thunbergii via a better access to organic N sources, partly mediated by competition regimes.

Seasonal nitrogen uptake strategies in a temperate desert ecosystem depends on N form and plant species

Symbiotic plants might be able to regulate a limited nitrogen (N) pool, thus avoiding and reducing competition for resources, through the uptake of different chemical N forms. Our aim was to see whether coexisting herbs showed preference for different forms of N in a temperate desert. We conducted a situ experiment using the ¹⁵ N labeling method in the Gurbantunggut Desert of Northwestern China dominated by Erodium oxyrrhynchum, Hyalea pulchella, Nonea caspica and Lactuca undulata during their growing period (April and May). Four desert herb species preferentially relied on ¹⁵ N-NO₃ for their N nutrition. Multi-factor analysis of variance (ANOVA) analysis results showed that species, N forms, months, and soil depths strongly affected N uptake rate. The uptake rate by herbs was higher in May than in April, and higher at 0-5 cm than at 5-15 cm soil layers. Erodium oxyrrhynchum, N. caspica and L. undulata showed different preference on N form over months. Erodium oxyrrhynchum and L. undulata changed their uptake preference from more ¹⁵ N-Glycine in April to more ¹⁵ N-NH4 in May. Although the N uptake rate of four desert herbs varied across different soil depths and months, all species absorbed more inorganic N compared with organic N. The higher preference for ¹⁵ N-NO₃ and ¹⁵ N-NH4 over ¹⁵ N-Gly possibly reflects adaptation to different N forms in temperate desert.

Limited evidence for spatial resource partitioning across temperate grassland biodiversity experiments

Locally, plant species richness supports many ecosystem functions. Yet, the mechanisms driving these often-positive biodiversity-ecosystem functioning relationships are not well understood. Spatial resource partitioning across vertical resource gradients is one of the main hypothesized causes for enhanced ecosystem functioning in more biodiverse grasslands. Spatial resource partitioning occurs if species differ in where they acquire resources and can happen both above- and belowground. However, studies investigating spatial resource partitioning in grasslands provide inconsistent evidence. We present the results of a meta-analysis of 21 data sets from experimental species-richness gradients in grasslands. We test the hypothesis that increasing spatial resource partitioning along vertical resource gradients enhances ecosystem functioning in diverse grassland plant communities above- and belowground. To test this hypothesis, we asked three questions. (1) Does species richness enhance biomass production or community resource uptake across sites? (2) Is there evidence of spatial resource partitioning as indicated by resource tracer uptake and biomass allocation above- and belowground? (3) Is evidence of spatial resource partitioning correlated with increased biomass production or community resource uptake? Although plant species richness enhanced community nitrogen and potassium uptake and biomass production above- and belowground, we found that plant communities did not meet our criteria for spatial resource partitioning, though they did invest in significantly more aboveground biomass in higher canopy layers in mixture relative to monoculture. Furthermore, the extent of spatial resource partitioning across studies was not positively correlated with either biomass production or community resource uptake. Our results suggest that spatial resource partitioning across vertical resource gradients alone does not offer a general explanation for enhanced ecosystem functioning in more diverse temperate grasslands.

Seasonal variation in N uptake strategies in the understorey of a beech-dominated N-limited forest ecosystem depends on N source and species

In forest ecosystems, species use different strategies to increase their competitive ability for nitrogen (N) acquisition. The acquisition of N by trees is regulated by tree internal and environmental factors including mycorrhizae. In this study, we investigated the N uptake strategies of three co-occurring tree species [European beech (Fagus sylvatica L.), sycamore maple (Acer pseudoplatanus L.) and Norway maple (Acer platanoides L.)] in the understorey of a beech-dominated, N-limited forest on calcareous soil over two consecutive seasons. For this purpose, we studied (15)N uptake capacity as well as the allocation to N pools in the fine roots. Our results show that European beech had a higher capacity for both inorganic and organic N acquisition throughout the whole growing season compared with sycamore maple and Norway maple. The higher capacity of N acquisition in beech indicates a better adaption of beech to the understorey conditions of beech forests compared with the seedlings of other tree competitors under N-limited conditions. Despite these differences, all three species preferred organic over inorganic N sources throughout the growing season and showed similar seasonal patterns of N acquisition with an increased N uptake capacity in summer. However, this pattern varied with N source and year indicating that other environmental factors not assessed in this study further influenced N acquisition by the seedlings of the three tree species.

Root-Root Interactions: Towards A Rhizosphere Framework

Plant scientists have made great progress in understanding molecular mechanisms controlling root responses to nutrients of arabidopsis (Arabidopsis thaliana) plants under controlled conditions. Simultaneously, ecologists and agronomists have demonstrated that root-root interactions involve more than competition for nutrients. Here, we highlight the importance of both root exudates and soil microbes for root-root interactions, ubiquitous in natural and agricultural ecosystems. We argue that it is time to bring together the recent insights from both scientific disciplines to fully understand root functioning in the real world.

Realistic diversity loss and variation in soil depth independently affect community-level plant nitrogen use

Numerous experiments have demonstrated that diverse plant communities use nitrogen (N) more completely and efficiently, with implications for how species conservation efforts might influence N cycling and retention in terrestrial ecosystems. However, most such experiments have randomly manipulated species richness and minimized environmental heterogeneity, two design aspects that may reduce applicability to real ecosystems. Here we present results from an experiment directly comparing how realistic and randomized plant species losses affect plant N use across a gradient of soil depth in a native-dominated serpentine grassland in California. We found that the strength of the species richness effect on plant N use did not increase with soil depth in either the realistic or randomized species loss scenarios, indicating that the increased vertical heterogeneity conferred by deeper soils did not lead to greater complementarity among species in this ecosystem. Realistic species losses significantly reduced plant N uptake and altered N-use efficiency, while randomized species losses had no effect on plant N use. Increasing soil depth positively affected plant N uptake in both loss order scenarios but had a weaker effect on plant N use than did realistic species losses. Our results illustrate that realistic species losses can have functional consequences that differ distinctly from randomized losses, and that species diversity effects can be independent of and outweigh those of environmental heterogeneity on ecosystem functioning. Our findings also support the value of conservation efforts aimed at maintaining biodiversity to help buffer ecosystems against increasing anthropogenic N loading.

Early root overproduction not triggered by nutrients decisive for competitive success belowground

Background

Theory predicts that plant species win competition for a shared resource by more quickly preempting the resource in hotspots and by depleting resource levels to lower concentrations than its competitors. Competition in natural grasslands largely occurs belowground, but information regarding root interactions is limited, as molecular methods quantifying species abundance belowground have only recently become available.

Principal Findings

In monoculture, the grass Festuca rubra had higher root densities and a faster rate of soil nitrate depletion than Plantago lanceolata, projecting the first as a better competitor for nutrients. However, Festuca lost in competition with Plantago. Plantago not only replaced the lower root mass of its competitor, but strongly overproduced roots: with only half of the plants in mixture than in monoculture, Plantago root densities in mixture were similar or higher than those in its monocultures. These responses occurred equally in a nutrient-rich and nutrient-poor soil layer, and commenced immediately at the start of the experiment when root densities were still low and soil nutrient concentrations high.

Conclusions/Significance

Our results suggest that species may achieve competitive superiority for nutrients by root growth stimulation prior to nutrient depletion, induced by the presence of a competitor species, rather than by a better ability to compete for nutrients per se. The root overproduction by which interspecific neighbors are suppressed independent of nutrient acquisition is consistent with predictions from game theory. Our results emphasize that root competition may be driven by other mechanisms than is currently assumed. The long-term consequences of these mechanisms for community dynamics are discussed.

Coexistence, niches and biodiversity effects on ecosystem functioning

General principles from coexistence theory are often invoked to explain how and why mixtures of species outperform monocultures. However, the complementarity and selection effects commonly measured in biodiversity experiments do not precisely quantify the niche and relative fitness differences that govern species coexistence. Given this lack of direct correspondence, how can we know whether species-rich mixtures are stable and that the benefits of diversity will therefore persist? We develop a resource-based included-niche model in which plant species have asymmetric access to a nested set of belowground resource pools. We use the model to show that positive complementarity effects arise from stabilising niche differences, but do not necessarily lead to stable coexistence and hence can be transient. In addition, these transient complementarity effects occur in the model when there is no complementary resource use among species. Including a trade-off between uptake rates and the size of the resource pool stabilised interactions and led to persistent complementarity coupled with weak or negative selection effects, consistent with results from the longest-running field biodiversity experiments. We suggest that future progress requires a greater mechanistic understanding of the links between ecosystem functions and their underlying biological processes.