Biodiversity influences ecosystem functioning in aquatic angiosperm communities

Knowledge of the connection between aquatic plant diversity and ecosystem processes is still limited. To examine how plant species diversity affects primary productivity, plant nutrient use, functional diversity of secondary producers and population/community stability, we manipulated submerged angiosperm species diversity in a field experiment lasting 15 weeks. Plant richness increased the shoot density for three of four species. Polyculture biomass production was enhanced by increasing richness, with positive complementarity and selection effects causing positive biodiversity effects. Species richness enhanced the community stability for biomass production and shoot density. Sediment ammonium availability decreased with plant diversity, suggesting improved nutrient usage with increasing plant richness. Interestingly, positive multitrophic effects of plant species richness on structural and functional diversity of macrobenthic secondary producers were recorded. The results suggest that mixed seagrass meadows play an important role for ecosystem functioning and thus contribute to the provision of goods and services in coastal areas.     

Habitat structure is more important than nutrient supply in modifying mussel bed assemblage in an upwelling area of the Peruvian coast

Upwelling intensity modifies coastal primary production and influences individual traits of habitat-forming species. Along the Peruvian coast, beds of the mytilid Perumytilus purpuratus provide structurally complex habitats that harbour many organisms. We predict that in the nutrient-rich system of Central Peru, the modification of structural complexity would have stronger effects on the Perumytilus community than nutrient addition. We experimentally examined the effects of nutrient addition on the Perumytilus-dominated assemblage and we evaluated the potential effect of varying shell size on the Perumytilus-dominated assemblage. Nutrient addition to the mussel bed with slow-release fertilizers caused no changes in the total macro- and microalgal biomass and did not affect abundances or composition of the assemblage. To explore the effect of structural complexity on the Perumytilus assemblage, we manipulated mussel size with experimental bags containing small and large individuals. Predators, grazers and mobile organisms were more abundant among smaller mussels, with smaller gap volume, whereas the barnacle Jehlius cirratus was more abundant on larger mussels. In conclusion, point-source nutrient addition to the mussel bed did not enhance primary production. However, the modification of structural characteristics related to mussel size induced changes in the faunal assemblage. Thus, it seems that in this nutrient-rich system, nutrient enhancement would not significantly affect Perumytilus and its assemblage, whereas structural habitat seems to play an important role in shaping this community.     

Combined effects between temporal heterogeneity of water supply,nutrient level,and population density on biomass of four broadly distributed herbaceous species

Temporal heterogeneity of water supply affects grassland community productivity and it can interact with nutrient level and intraspecific competition. To understand community responses, the responses of individual species to water heterogeneity must be evaluated while considering the interactions of this heterogeneity with nutrient levels and population density. We compared responses of four herbaceous species grown in monocultures to various combinations of water heterogeneity, nutrient level, and population density: two grasses (Cynodon dactylon and Lolium perenne), a forb (Artemisia princeps), and a legume (Trifolium repens). Treatment effects on shoot and root biomass were analyzed. In all four species, shoot biomass was larger under homogeneous than under heterogeneous water supply. Shoot responses of L. perenne tended to be greater at high nutrient levels. Although root biomass was also larger under homogeneous water supply, effects of water heterogeneity on root biomass were not significant in the grasses. Trifolium repens showed marked root responses, particularly at high population density. Although greater shoot and root growth under homogeneous water supply appears to be a general trend among herbaceous species, our results suggested differences among species could be found in the degree of response to water heterogeneity and its interactions with nutrient level and intraspecific competition.     

Effect of nutrient availability, grazer assemblage and seagrass source population on the interaction between Thalassia testudinum (turtle grass) and its algal epiphytes

Seagrass leaves are often densely covered by epiphytic algae which can suppress seagrass productivity and has been implicated in declines of seagrass meadows worldwide. The net effect of epiphytes on seagrass growth and morphology depends on the independent and interactive effects of a variety of factors, including nutrient availability and the intensity of grazing on epiphytes. Here I report the results of a mesocosm experiment designed to test the effects of nutrient addition and within-functional group variation (grazer species composition and the source population of seagrass) on the strength of the interactions among grazers, epiphytes, and turtle grass (Thalassia testudinum). Turtle grass ramets from two sites in the northern Gulf of Mexico were cleared of epiphytes and transplanted into common-garden mesocosms. Replicate ramets were grown in a split-split plot design with two levels of dissolved nutrients and four different grazer species combinations (Tozeuma carolinense alone, Pagurus maclaughlinae alone, both species together, and no grazers present). As expected, grazers had a significant negative effect on epiphyte biomass/leaf area and a significant positive effect on turtle grass growth in the mesocosms. The two species were more similar in their direct effects on epiphyte biomass than in their indirect effects on turtle grass growth; this may reflect differences in epiphyte community composition under different grazer treatments. The effect of nutrient addition on turtle grass growth depended critically on the intensity of grazing: in the presence of grazers, turtle grass tended to produce a greater biomass of new leaf tissue in the tanks with nutrients added than in the control tanks. However, when grazers were absent, the direction of the effect was reversed, and plants with nutrients added grew less than the control plants. The two source populations of turtle grass differed significantly in epiphyte biomass/leaf area accrued in the mesocosms as well as in the strength of the effect of grazers on turtle grass growth. This suggests that population differentiation in seagrass interactions with epiphytes, as well as spatial and temporal variation in resources and grazer community composition, can greatly effect the role of epiphytes in limiting seagrass productivity.     

Species-specific responses of plant communities to altered carbon and nutrient availability

In a field microcosm experiment, species‐specific responses of aboveground biomass of two California annual grassland communities to elevated CO₂ and nutrient availability were investigated. One community grows on shallow, nutrient‐poor serpentine‐derived soil whereas the other occurs on deeper, modestly fertile sandstone/greenstone‐derived substrate. In most species, CO₂ effects did not appear until late in the growing season, probably because the elevated CO₂ increased water‐use‐efficiency easing, the onset of the summer drought. Responses of aboveground biomass to elevated CO₂ differed depending on nutrient availability. Similarly, biomass responses to nutrient treatments differed depending on the CO₂ status. For the majority of the species, production increased most under elevated CO₂ with added nutrients (N,P,K, and micro nutrients). Some species were losers under conditions that increased overall community production, including Bromus hordeaceus in the serpentine community (negative biomass response under elevated CO₂) and Lotus wrangelianus in both communities (negative biomass response with added nitrogen). Treatment and competitive effects on species‐specific biomass varied in both magnitude and direction, especially in the serpentine community, significantly affecting community structure. Individual resource environments are likely to be affected by neighbouring plants, and these competitive interactions complicate predictions of species' responses to elevated CO₂.     

Testing a ‘genes‐to‐ecosystems’ approach to understanding aquatic–terrestrial linkages

A ‘genes‐to‐ecosystems’ approach has been proposed as a novel avenue for integrating the consequences of intraspecific genetic variation with the underlying genetic architecture of a species to shed light on the relationships among hierarchies of ecological organization (genes → individuals → communities → ecosystems). However, attempts to identify genes with major effect on the structure of communities and/or ecosystem processes have been limited and a comprehensive test of this approach has yet to emerge. Here, we present an interdisciplinary field study that integrated a common garden containing different genotypes of a dominant, riparian tree, Populus trichocarpa, and aquatic mesocosms to determine how intraspecific variation in leaf litter alters both terrestrial and aquatic communities and ecosystem functioning. Moreover, we incorporate data from extensive trait screening and genome‐wide association studies estimating the heritability and genes associated with litter characteristics. We found that tree genotypes varied considerably in the quality and production of leaf litter, which contributed to variation in phytoplankton abundances, as well as nutrient dynamics and light availability in aquatic mesocosms. These ‘after‐life’ effects of litter from different genotypes were comparable to the responses of terrestrial communities associated with the living foliage. We found that multiple litter traits corresponding with aquatic community and ecosystem responses differed in their heritability. Moreover, the underlying genetic architecture of these traits was complex, and many genes contributed only a small proportion to phenotypic variation. Our results provide further evidence that genetic variation is a key component of aquatic–terrestrial linkages, but challenge the ability to predict community or ecosystem responses based on the actions of one or a few genes.     

The short-term influence of herbivory near patch reefs varies between seagrass species

The coexistence of multiple species within a trophic level can be regulated by consumer preferences and nutrient supply, but the influence of these factors on the co-occurrence of seagrass species is not well understood. We examined the biomass and density responses of two seagrass species in the Florida Keys Reef Tract to grazing pressure near patch reefs, and evaluated how nutrient enrichment impacted herbivory dynamics. We transplanted Halodule wrightii (shoalgrass) sprigs into caged and uncaged plots in a Thalassia testudinum (turtlegrass) bed near a patch reef. Nutrients (N and P) were added to half of the experimental plots. We recorded changes in seagrass shoot density, and after three months, we measured above- and belowground biomass and tissue nutrient content of both species. Herbivory immediately and strongly impacted H. wrightii. Within six days of transplantation, herbivory reduced the density of uncaged H. wrightii by over 80%, resulting in a decrease in above- and belowground biomass of nearly an order of magnitude. T. testudinum shoot density and belowground biomass were not affected by herbivory, but aboveground biomass and leaf surface area were higher within cages, suggesting that although herbivory influenced both seagrass species, T. testudinum was more resistant to herbivory pressure than H. wrightii. Nutrient addition did not alter herbivory rates or the biomass of either species over the short-term duration of this study. In both species, nutrient addition had little effect on the tissue nutrient content of seagrass leaves, and N:P was near the 30:1 threshold that suggested a balance between N and P. The different impacts of grazing on these two seagrass species suggest that herbivory may be an important regulator of the distribution of multiple seagrass species near herbivore refuges like patch reefs in the Caribbean.     

Somaclonal variation in plant adaptation to acid soil in the tropical forage legume Stylosanthes guianensis

Somaclonal variation offers the possibility to obtain changes in one or a few characters of an otherwise outstanding cultivar without altering the remaining, and often unique, part of the genotype. It has been shown to be heritable for some species. A check line of Stylosanthes guianensis (Aubl.) Sw., CIAT 2243 and 14 somaclones in the R₄ generation, selected after three generations from the original 114 plants regenerated from callus cultures, were used in a glasshouse trial. The main objective of the study was to evaluate the physiological basis of the differences in agronomic performance of certain somaclones over the check genotype when grown in a sandy loam acid soil at low or high fertility level. Measurements at the time of harvest (170 days of plant age) included dry matter distribution between shoot and roots, leaf area production, nutrient levels in soil and plant parts, and uptake of nutrients from soil. Somaclones differed with the check genotype in terms of (i) partitioning of fixed carbon between the shoot and roots; (ii) root biomass production and (iii) uptake of nitrogen and phosphorus. Positive relationships were found between total nitrogen uptake and total biomass, and total phosphorus uptake and total biomass, and total phosphorus uptake and total nitrogen uptake. The results of this study provide an insight into the potential use of somaclonal variation for the improvement of plant adaptation to acid soil conditions.     

Nutrient Enrichment and Food Web Composition Affect Ecosystem Metabolism in an Experimental Seagrass Habitat

Background

Food web composition and resource levels can influence ecosystem properties such as productivity and elemental cycles. In particular, herbivores occupy a central place in food webs as the species richness and composition of this trophic level may simultaneously influence the transmission of resource and predator effects to higher and lower trophic levels, respectively. Yet, these interactions are poorly understood.

Methodology/Principal Findings

Using an experimental seagrass mesocosm system, we factorially manipulated water column nutrient concentrations, food chain length, and diversity of crustacean grazers to address two questions: (1) Does food web composition modulate the effects of nutrient enrichment on plant and grazer biomasses and stoichiometry? (2) Do ecosystem fluxes of dissolved oxygen and nutrients more closely reflect above-ground biomass and community structure or sediment processes? Nutrient enrichment and grazer presence generally had strong effects on biomass accumulation, stoichiometry, and ecosystem fluxes, whereas predator effects were weaker or absent. Nutrient enrichment had little effect on producer biomass or net ecosystem production but strongly increased seagrass nutrient content, ecosystem flux rates, and grazer secondary production, suggesting that enhanced production was efficiently transferred from producers to herbivores. Gross ecosystem production (oxygen evolution) correlated positively with above-ground plant biomass, whereas inorganic nutrient fluxes were unrelated to plant or grazer biomasses, suggesting dominance by sediment microbial processes. Finally, grazer richness significantly stabilized ecosystem processes, as predators decreased ecosystem production and respiration only in the zero- and one- species grazer treatments.

Conclusions/Significance

Overall, our results indicate that consumer presence and species composition strongly influence ecosystem responses to nutrient enrichment, and that increasing herbivore diversity can stabilize ecosystem flux rates in the face of perturbations.     

Functional traits determine tree growth and ecosystem productivity of a tropical montane forest: Insights from a long‐term nutrient manipulation experiment

Trait‐response effects are critical to forecast community structure and biomass production in highly diverse tropical forests. Ecological theory and few observation studies indicate that trees with acquisitive functional traits would respond more strongly to higher resource availability than those with conservative traits. We assessed how long‐term tree growth in experimental nutrient addition plots (N, P, and N + P) varied as a function of morphological traits, tree size, and species identity. We also evaluated how trait‐based responses affected stand scale biomass production considering the community structure. We found that tree growth depended on interactions between functional traits and the type or combination of nutrients added. Common species with acquisitive functional traits responded more strongly to nutrient addition, mainly to N + P. Phosphorous enhanced the growth rates of species with acquisitive and conservative traits, had mostly positive effects on common species and neutral or negative effects in rare species. Moreover, trees receiving N + P grew faster irrespective of their initial size relative to trees in control or to trees in other treatment plots. Finally, species responses were highly idiosyncratic suggesting that community processes including competition and niche dimensionality may be altered under increased resource availability. We found no statistically significant effects of nutrient additions on aboveground biomass productivity because acquisitive species had a limited potential to increase their biomass, possibly due to their generally lower wood density. In contrast, P addition increased the growth rates of species characterized by more conservative resource strategies (with higher wood density) that were poorly represented in the plant community. We provide the first long‐term experimental evidence that trait‐based responses, community structure, and community processes modulate the effects of increased nutrient availability on biomass productivity in a tropical forest.     

Dugong habitat use in relation to seagrass nutrients,tides, and diel cycles

The dugong is the only herbivorous mammal that is strictly marine and a seagrass community specialist. The pasture available to the dugong varies with the tides because seagrass occurs in both intertidal and subtidal areas. We GPS-tracked seven dugongs within a 24 km², intensively used seagrass habitat in subtropical Australia in winter. We modeled resource selection within the habitat by comparing the dugongs’ use of space with the distribution of seagrass in an area defined using the combined space-use of the tracked animals. Selection by dugongs for seagrass quantity (biomass) and quality (nutrients) was analyzed within six time/tide combinations to examine the influences of tidal periodicity and the diel cycle on resource selection. Dugong habitat use was consistently centered over seagrass patches with high nitrogen concentrations, except during the day at low tides when the animals had fewer habitat choices and their space use was centered over high seagrass biomass. The association of dugongs with seagrass high in starch was positive during both day and night high tides when the animals could access the intertidal areas where seagrass biomass was generally low. Associations between dugongs and seagrass species were less definite, reflecting the potential for dugongs to exploit several species. Our model of dugong resource selection suggests that nitrogen is the primary limiting nutrient for dugong populations and also confirms the preference of dugongs for high-energy foods.     

Nutrient dynamics in 3 morphological different tropical seagrasses and their sediments

Seasonality of nutrient dynamics in three morphologically different seagrass species and their sediments was examined for 1 year between November 2006 and November 2007 at four sites in the Andaman Sea, Thailand. The smaller species, Cymodocea serrulata and Halophila ovalis, showed major seasonal variation in shoot density, above- and belowground biomass, much more than expected from seasonal changes in water temperature and light conditions. All parameters showed minimum values in the dry season due to desiccation during neap tides. In contrast Enhalus acoroides showed less seasonal variation. Only limited seasonality was found in tissue N content of all species, whereas tissue P content responded to the low P concentration in the water column during the wet season. There were no differences in sediment conditions among species, and nutrient pools were generally low. Furthermore there were no significant spatial differences in seagrass and sediment nutrient dynamics, despite varying anthropogenic activity at the study sites, reflecting the oligotrophic conditions in this region.     

Climate change effects on plant biomass alter dominance patterns and community evenness in an experimental old‐field ecosystem

Atmospheric and climatic change can alter plant biomass production and plant community composition. However, we know little about how climate change‐induced alterations in biomass production affect plant species composition. To better understand how climate change will alter both individual plant species and community biomass, we manipulated atmospheric [CO₂], air temperature, and precipitation in a constructed old‐field ecosystem. Specifically, we compared the responses of dominant and subdominant species to our climatic treatments, and explored how changes in plant dominance patterns alter community evenness over 2 years. Our study resulted in four major findings: (1) all treatments, elevated [CO₂], warming, and increased precipitation increased plant community biomass and the effects were additive rather than interactive, (2) plant species differed in their response to the treatments, resulting in shifts in the proportional biomass of individual species, which altered the plant community composition; however, the plant community response was largely driven by the positive precipitation response of Lespedeza, the most dominant species in the community, (3) precipitation explained most of the variation in plant community composition among treatments, and (4) changes in precipitation caused a shift in the dominant species proportional biomass that resulted in lower community evenness in the wet relative to dry treatments. Interestingly, compositional and evenness responses of the subdominant community to the treatments did not always follow the responses of the whole plant community. Our data suggest that changes in plant dominance patterns and community evenness are an important part of community responses to climatic change, and generally, that such compositional shifts can alter ecosystem biomass production and nutrient inputs.     

Genotype identity has a more important influence than genotype diversity on shoot biomass productivity in willow short‐rotation coppices

Willow (Salix spp.) short‐rotation coppice is commercially grown to produce lignocellulosic biomass to meet renewable bioenergy demands. Most commercial willow coppices are grown in stands of a single genotype, but biomass productivity may be greater in mixed communities, and the productivity in mixed communities may depend on the specific genotypes involved. We assessed the biomass production of four different Salix genotypes (“Björn,” “Jorr,” “Loden,” “Tora”) grown without additional nutrient fertilization during one cutting cycle at three locations in Europe (Uppsala in Sweden, Rostock and Freiburg in Germany) in plots of pure and mixed communities. We evaluated (i) the effect of genotype diversity on shoot biomass productivity, including the evidence for complementarity and selection effects; (ii) the influence of individual genotypes on mixed community productivity; and (iii) the productivity of individual genotypes in response to pure vs. mixed culture. Mean shoot biomass production after the first cutting cycle decreased in the order Rostock (8.7 Mg ha^?1) > Freiburg (6.9 Mg ha^?1) > Uppsala (5.7 Mg ha^?1), with values similar to those for other nonfertilized willow stands after the first growth cycle. Consistently across all three locations, increasing genotype diversity did not significantly affect shoot biomass production. Using Bayesian statistics, the addition of the genotypes “Jorr” and “Loden” was predicted to enhance shoot biomass production, while “Tora” and “Björn” are more likely to reduce shoot biomass production in mixed communities. In addition, we found evidence for a negative selection effect due to the genotype “Tora” performing better in mixed than in pure communities in two of the sites (Freiburg, Uppsala). In conclusion, our results imply that increasing genetic richness has no negative effect on productivity and that there is a potential to design site‐specific genotype mixtures of short‐rotation coppice promoting both high genetic diversity and high biomass production.     

Effects of trait plasticity on aboveground biomass production depend on species identity in experimental grasslands

Experimental studies with grassland species found a positive relationship between species richness and community biomass production, however the response of individual species was highly variable. The mechanisms behind these patterns are poorly understood. Here we studied aboveground biomass production and plasticity of growth characteristics of four legumes with similar morphology (Lotus corniculatus, Medicago lupulina, Onobrychis viciifolia, Trifolium hybridum) in experimental grasslands varying in species richness (1, 2, 4, 8, 16 and 60) and composition. We identified O. viciifolia and T. hybridum as species that reached higher biomass production in mixtures than expected from monoculture yields, while L. corniculatus and M. lupulina mostly had lower yields than expected. Variation of morphological traits across the species-richness gradient was lowest in the highly competitive O. viciifolia, but increased in the smaller species. The tall-growing O. viciifolia achieved higher biomass production by both, a higher number of plant individuals and an increase in mean mass per individual. Mean shoot number per individual remained constant, but individuals produced heavier shoots. The medium-sized T. hybridum also increased the number of plant individuals, but mean mass per individual did not respond to community species richness. The average mass per shoot was increased in mixtures, but the species developed less shoots per individual. Shoot length and stem weight ratio of T. hybridum increased with community species richness. Morphological changes in the less successful L. corniculatus and M. lupulina with a smaller growth stature were similarly directed as those of T. hybridum. The observed morphological changes are known as typical shade-avoidance mechanisms in dense vegetation. Our study shows that stress responses to changes in resource availability may be a mechanism to enforce higher aboveground biomass production of individual species in mixtures, but it depends on species identity whether trait plasticity is large enough to exceed stress-induced growth limitations.     

Epiphyte grazing enhances productivity of remnant seagrass patches

Anthropogenic nutrient enrichment is increasingly modifying community structure and ecosystem functioning in terrestrial and aquatic ecosystems. In marine ecosystems, the paradigm is that nutrient enrichment leads to a decline of seagrasses by stimulating epiphytic algal growth, which shades and overgrows seagrasses. This ignores the potential for herbivores, which graze upon epiphytic algae, to partially or wholly counter such nutrient effects. We conducted a field experiment to assess the role that the trochid gastropod Calthalotia fragum plays in reducing nutrient impacts on the seagrass, Posidonia australis, in an urbanized Australian estuary, Botany Bay, Sydney. In a field experiment, where nutrient loading and grazer density were orthogonally manipulated, nutrient enrichment failed to promote epiphyte biomass or diminish growth and primary productivity of P. australis. To the contrary, nutrient enrichment enhanced photosynthesis of the seagrass in plots where the grazer was present at higher density. Epiphytic growth was negatively affected by increased C. fragum density, while P. australis shoot growth was positively influenced. Thus, in this study system, grazing appears to play a much greater role in determining seagrass primary productivity and above‐ground growth than moderate nutrient loading, suggesting that the interaction between grazers and nutrients depends on the relative levels of each. Our study contributes to a growing body of literature suggesting that effects of nutrient loading on benthic assemblages are not universally negative, but are dependent on the biotic and abiotic setting.     

The Effect of Genetic Diversity on Ecosystem Functioning in Vegetated Coastal Ecosystems

In species-poor communities, genetic diversity potentially plays an important role for ecosystem functioning, though this is still largely unexplored in marine and estuarine ecosystems. We studied how genetic diversity (sensu genotypic diversity and/or allelic richness) affects ecosystem functioning in marine habitat-forming plant communities. First, we conducted a 15-month field experiment in the highly seasonal Baltic Sea and established mono- and polycultures of different genotypes and genotype combinations of Zostera marina. Second, we reviewed existing literature and performed a meta-analysis of 12 studies including this study. We found no evidence of positive genetic diversity effects on shoot production in the field experiment, but diversity enhanced community stability over time. The literature review revealed that a majority of the included studies observed positive effects of genetic diversity on ecosystem functions such as primary production and nutrient uptake. The results from the meta-analysis support the hypothesis that genetic diversity effects on productivity are stronger during or after periods of stress. These diversity effects were also more positive in the field compared to mesocosm studies. Our results indicate that genetic diversity has positive effects on ecosystem functioning, particularly during increased environmental stress. Thus, local genetic diversity should be preserved especially in species-poor ecosystems, where it potentially provides insurance against environmental change.     

Long-term grazing affects relationships between nitrogen form uptake and biomass of alpine meadow plants

Trade-off between nutrient uptake rate and product accumulation has been found among species characterized as acquisitive and conservative strategies in resource utilization. However, long-term grazing causes changes in soil nutrient availability and plant species abundance by selective foraging and resource allocation between above- and belowground organs, which may cover up such trade-off. However, little is known whether the trade-off can be observed among species in community without grazing disturbance, and how grazing influences the trade-off. We conducted a ¹⁵N labelling experiment in winter grazing and grazing release alpine meadow communities on the Tibet Plateau. We examined changes in N form uptake of 11 common species and relationship of N chemical uptake rate with aboveground biomass. Grazing release increased soil \({\text{NH}}_{4}^{ + }\)–N and \({\text{NO}}_{3}^{ - }\)–N, and increased \({\text{NO}}_{3}^{ - }\)–N uptake rate in two species and \({\text{NH}}_{4}^{ + }\)–N uptake rate in three species. Meanwhile, grazing release decreased aboveground biomass of three species and two of them belong to those species’ increased N uptake rate. Contrarily, grazing release increased aboveground biomass of four species and none belongs to the changed N uptake rate. Thus, grazing release caused changes of plant nutrient uptake rate and aboveground production in different directions, which explains the negative relationship of N uptake rate with aboveground biomass in ungrazed community. Our results indicate that the increase in nutrient uptake is probably one of the mechanisms for acquisitive species to cope with the raising nutrient availability and/or competition from the conservative dominant grasses after grazing release.     

Positive diversity effects on productivity in mixtures of arable weed species as related to density-size relationships

Aims Diversity–productivity relationships among herbaceous species have mostly been studied in grasslands, while less is known about diversity effects among weedy species with a short life cycle.Methods We studied diversity–productivity relationships, shoot density, size and allometry in experimental communities of different species richness (one, three, six, and nine species), functional group number (one to three functional groups: grasses, small herbs and tall herbs) and functional group evenness (even and uneven number of species per functional group) based on a pool of nine arable weed species with a short life cycle in a 2-year experiment.Important findings Higher species richness increased above- and belowground biomass production in both years of the experiment. Additive partitioning showed that positive selection effects increased with increasing species richness and functional group number, while positive complementarity effects were greater when tall herbs were present. Relative yield totals were larger than 1 across all species richness levels but did not increase with species richness, which is consistent with constant positive complementarity effects. Community biomass production and diversity effects increased in the second year of the experiment, when communities achieved greater shoot densities and average shoot sizes. At the community level, varying productivity was mainly attributable to variation in mean shoot sizes. Tall herbs reached greater observed/expected relative yields (=overyielding) due to increased shoot sizes, underyielding of small herbs was mainly attributable to decreased shoot sizes, while grasses partly compensated for reduced shoot sizes by increasing densities. Shifts in community-level density–size relationships and changes in shoot allometry in favour of greater height growth indicated that a greater biomass at a given density was due to increased dimensions of occupied canopy space. We conclude that diversity effects are also possible among short-lived arable weed species, but selection effects through sizes differences among species are key for positive species richness–productivity relationships.