Plant community and tissue chemistry responses to fertilizer and litter nutrient manipulations in a temperate grassland

Human-mediated nutrient amendments have widespread effects on plant communities. One of the major consequences has been the loss of species diversity under increased nutrient inputs. The loss of species can be functional group dependent with certain functional groups being more prone to decline than others. We present results from the sixth year of a long-term fertilization and litter manipulation study in an old-field grassland. We measured plant tissue chemistry (C:N ratio) to understand the role of plant physiological responses in the increase or decline of functional groups under nutrient manipulations. Fertilized plots had significantly more total aboveground biomass and live biomass than unfertilized plots, which was largely due to greater productivity by exotic C₃ grasses. We found that both fertilization and litter treatments affected plant species richness. Species richness was lower on plots that were fertilized or had litter intact; species losses were primarily from forbs and non-Poaceae graminoids. C₃ grasses and forbs had lower C:N ratios under fertilization with forbs having marginally greater %N responses to fertilization than grasses. Tissue chemistry in the C₃ grasses also varied depending on tissue type with reproductive tillers having higher C:N ratios than vegetative tillers. Although forbs had greater tissue chemistry responses to fertilization, they did not have a similar positive response in productivity and the number of forb species is decreasing on our experimental plots. Overall, differential nutrient uptake and use among functional groups influenced biomass production and species interactions, favoring exotic C₃ grasses and leading to their dominance. These data suggest functional groups may differ in their responses to anthropogenic nutrient amendments, ultimately influencing plant community composition.     

Foliar and soil δ15N values reveal increased nitrogen partitioning among species in diverse grassland communities

Plant and soil nitrogen isotope ratios (δ¹⁵N) were studied in experimental grassland plots of varying species richness. We hypothesized that partitioning of different sources of soil nitrogen among four plant functional groups (legumes, grasses, small herbs, tall herbs) should increase with diversity. Four years after sowing, all soils were depleted in ¹⁵N in the top 5 cm whereas in non‐legume plots soils were enriched in ¹⁵N at 5–25 cm depth. Decreasing foliar δ¹⁵N and Δδ¹⁵N (= foliar δ¹⁵N ? soil δ¹⁵N) values in legumes indicated increasing symbiotic N₂ fixation with increasing diversity. In grasses, foliar Δδ¹⁵N also decreased with increasing diversity suggesting enhanced uptake of N depleted in ¹⁵N. Foliar Δδ¹⁵N values of small and tall herbs were unaffected by diversity. Foliar Δδ¹⁵N values of grasses were also reduced in plots containing legumes, indicating direct use of legume‐derived N depleted in ¹⁵N. Increased foliar N concentrations of tall and small herbs in plots containing legumes without reduced foliar δ¹⁵N indicated that these species obtained additional mineral soil N that was not consumed by legumes. These functional group and species specific shifts in the uptake of different N sources with increasing diversity indicate complementary resource use in diverse communities.     

Legumes regulate grassland soil N cycling and its response to variation in species diversity and N supply but not CO2

Legumes are an important component of plant diversity that modulate nitrogen (N) cycling in many terrestrial ecosystems. Limited knowledge of legume effects on soil N cycling and its response to global change factors and plant diversity hinders a general understanding of whether and how legumes broadly regulate the response of soil N availability to those factors. In a 17‐year study of perennial grassland species grown under ambient and elevated (+180 ppm) CO₂ and ambient and enriched (+4 g N m^?2 year^?1) N environments, we compared pure legume plots with plots dominated by or including other herbaceous functional groups (and containing one or four species) to assess the effect of legumes on N cycling (net N mineralization rate and inorganic N pools). We also examined the effects of numbers of legume species (from zero to four) in four‐species mixed plots on soil N cycling. We hypothesized that legumes would increase N mineralization rates most in those treatments with the greatest diversity and the greatest relative limitation by and competition for N. Results partially supported these hypotheses. Plots with greater dominance by legumes had greater soil nitrate concentrations and mineralization rates. Higher species richness significantly increased the impact of legumes on soil N metrics, with 349% and 505% higher mineralization rates and nitrate concentrations in four‐species plots containing legumes compared to legume‐free four‐species plots, in contrast to 185% and 129% greater values, respectively, in pure legume than nonlegume monoculture plots. N‐fertilized plots had greater legume effects on soil nitrate, but lower legume effects on net N mineralization. In contrast, neither elevated CO₂ nor its interaction with legumes affected net N mineralization. These results indicate that legumes markedly influence the response of soil N cycling to some, but not all, global change drivers.     

The role of legumes as a component of biodiversity in a cross-European study of grassland biomass nitrogen

To investigate how plant diversity loss affects nitrogen accumulation in above‐ground plant biomass and how consistent patterns are across sites of different climatic and soil conditions, we varied the number of plant species and functional groups (grasses, herbs and legumes) in experimental grassland communities across seven European experimental sites (Switzerland, Germany, Ireland, United Kingdom (Silwood Park), Portugal, Sweden and Greece). Nitrogen pools were significantly affected by both plant diversity and community composition. Two years after sowing, nitrogen pools in Germany and Switzerland strongly increased in the presence of legumes. Legume effects on nitrogen pools were less pronounced at the Swedish, Irish and Portuguese site. In Greece and UK there were no legume effects. Nitrogen concentration in total above‐ground biomass was quite invariable at 1.66±0.03% across all sites and diversity treatments. Thus, the presence of legumes had a positive effect on nitrogen pools by significantly increasing above‐ground biomass, i.e. by increases in vegetation quantity rather than quality. At the German site with the strongest legume effect on nitrogen pools and biomass, nitrogen that was fixed symbiotically by legumes was transferred to the other plant functional groups (grasses and herbs) but varied depending on the particular legume species fixing N and the non‐legume species taking it up. Nitrogen‐fixation by legumes therefore appeared to be one of the major functional traits of species that influenced nitrogen accumulation and biomass production, although effects varied among sites and legume species. This study demonstrates that the consequences of species loss on the nitrogen budget of plant communities may be more severe if legume species are lost. However, our data indicate that legume species differ in their N₂ fixation. Therefore, loss of an efficient N₂‐fixer (Trifolium in our study) may have a greater influence on the ecosystem function than loss of a less efficient species (Lotus in our study). Furthermore, there is indication that P availability in the soil facilitates the legume effect on biomass production and biomass nitrogen accumulation.     

Butterfly response to floral resources during early establishment at a heterogeneous prairie biomass production site in Iowa, USA

In the Midwestern USA, current biofuel production systems rely on high input monoculture crops that do little to support native biodiversity. The University of Northern Iowa??s Tallgrass Prairie Center is investigating the feasibility of cultivating and harvesting diverse mixes of native prairie vegetation for use as a sustainable biofuel in a manner that also conserves biodiversity and protects soil and water resources. In 2009, we established 48 research plots on three soil types at an Iowa site with a uniform history of row crop production. We seeded each plot with one of four treatments of native prairie vegetation: (1) switchgrass monoculture, (2) warm-season grass mix (5 grass species), (3) biomass mix (16 species of grasses, legumes, and forbs), or (4) prairie mix (32 species of grasses, legumes, forbs, and sedges). In 2010, we measured vegetation characteristics and studied butterfly use of the plots to investigate the hypothesis that more diverse plant communities would support a greater abundance and diversity of butterflies. Habitat characteristics varied significantly among the plots by treatment and soil type, and butterflies responded rapidly to variation in floral abundance and richness. Averaged over the entire growing season, butterflies were six times more abundant and twice as species rich in the biomass and prairie mix plots compared to the warm-season grass and switchgrass plots. Our results suggest that implementation of biomass production using diverse mixes of native prairie vegetation on marginal lands could have positive effects on the maintenance of butterfly populations in agricultural landscapes.     

Productivity and resistance to weed invasion in four prairie biomass feedstocks with different diversity

High‐diversity mixtures of native tallgrass prairie vegetation should be effective biomass feedstocks because of their high productivity and low input requirements. These diverse mixtures should also enhance several of the ecosystem services provided by the traditional monoculture feedstocks used for bioenergy. In this study, we compared biomass production, year‐to‐year variation in biomass production, and resistance to weed invasion in four prairie biomass feedstocks with different diversity: one species – a switchgrass monoculture; five species – a mix of C₄ grasses; 16 species – a mix of grasses, forbs, and legumes; and 32 species – a mix of grasses, forbs, legumes, and sedges. Each diversity treatment was replicated four times on three soil types for a total of 48 research plots (0.33–0.56 ha each). We measured biomass production by harvesting all plant material to ground level in ten randomly selected quadrats per plot. Weed biomass was measured as a subset of total biomass. We replicated this design over a five‐year period (2010–2014). Across soil types, the one‐, 16‐, and 32‐species treatments produced the same amount of biomass, but the one‐species treatment produced significantly more biomass than the five‐species treatment. The rank order of our four diversity treatments differed between soil types suggesting that soil type influences treatment productivity. Year‐to‐year variation in biomass production did not differ between diversity treatments. Weed biomass was higher in the one‐species treatment than the five‐, 16‐, and 32‐species treatments. The high productivity and low susceptibility to weed invasion of our 16‐ and 32‐species treatments supports the hypothesis that high‐diversity prairie mixtures would be effective biomass feedstocks in the Midwestern United States. The influence of soil type on relative feedstock performance suggests that seed mixes used for biomass should be specifically tailored to site characteristics for maximum productivity and stand success.     

Positive interactions between nitrogen-fixing legumes and four different neighbouring species in a biodiversity experiment

The importance of facilitative processes due to the presence of nitrogen-fixing legumes in temperate grasslands is a contentious issue in biodiversity experiments. Despite a multitude of studies of fertilization effects of legumes on associated nonfixers in agricultural systems, we know little about the dynamics in more diverse systems. We hypothesised that the identity of target plant species (phytometers) and the diversity of neighbouring plant species would affect the magnitude of such positive species interactions. We therefore sampled aboveground tissues of phytometers planted into all plots of a grassland biodiversity–ecosystem functioning experiment and analysed their N concentrations, δ¹⁵N values and biomasses. The four phytometer species (Festuca pratensis, Plantago lanceolata, Knautia arvensis and Trifolium pratensis) each belonged to one of the four plant functional groups used in the experiment and allowed the effects of diversity on N dynamics in individual species to be assessed. We found significantly lower δ¹⁵N values and higher N concentrations and N contents (amount of N per plant) in phytometer species growing with legumes, indicating a facilitative role for legumes in these grassland ecosystems. Our data suggest that the main driving force behind these facilitative interactions in plots containing legumes was reduced competition for soil nitrate (“nitrate sparing”), with apparent N transfer playing a secondary role. Interestingly, species richness (and to a lesser extent functional group number) significantly decreased δ¹⁵N values, N concentrations and N content irrespective of any legume effect. Possible mechanisms behind this effect, such as increased N mineralisation and nitrate uptake in more diverse plots, now need further investigation. The magnitude of the positive interactions depended on the identity of the phytometer species. Evidence for increased N uptake in communities containing legumes was found in all three nonlegume phytometer species, with a subsequent strong increase in biomass in the grass F. pratensis across all diversity levels, and a lesser biomass gain in P. lanceolata and K. arvensis. In contrast, the legume phytometer species T. pratense was negatively affected when other legumes were present in their host communities across all diversity levels.     

Effect of plant species richness on invasibility of experimental plant communities

Invasion of unsown species to artificially created assemblages of grassland species was investigated in a 3-year field experiment. In the experiment, assemblages varying in species richness (1, 2, 4, 8, and 16 species) and functional group richness (1–4, grasses, legumes, rosette forbs, and creeping forbs) were grown in control and fertilized plots, without any attempt to prevent the invasion of unsown species or to weed them. The relationship between species and functional group diversity and above-ground biomass was positive for sown species in all study years (2003, 2004 and 2005). In the latter 2 years, weed invader biomass decreased significantly with increasing biomass of sown species and their functional group richness, but not with number of species. However, no suppressive effect of species or functional group richness beyond that by increased biomass of residents was found. In fact, slight but significant positive partial effect of species richness was found, suggesting that the negative effect of the same amount of resident biomass on invaders is stronger when the biomass is composed of fewer species. The negative relationship between the number of functional groups of residents and invader biomass suggested that better coverage of functional trait space could be a mechanism promoting the resistance to invasion. In Addition, species composition of invaders were significantly related to initial composition of sown residents.     

Plant community diversity and composition affect individual plant performance

Effects of plant community diversity on ecosystem processes have recently received major attention. In contrast, effects of species richness and functional richness on individual plant performance, and their magnitude relative to effects of community composition, have been largely neglected. Therefore, we examined height, aboveground biomass, and inflorescence production of individual plants of all species present in 82 large plots of the Jena Experiment, a large grassland biodiversity experiment in Germany. These plots differed in species richness (1–60), functional richness (1–4), and community composition. On average, in more species-rich communities, plant individuals grew taller, but weighed less, were less likely to flower, and had fewer inflorescences. In plots containing legumes, non-legumes were higher and weighed more than in plots without legumes. In plots containing grasses, non-grasses were less likely to flower than in plots without grasses. This indicates that legumes positively and grasses negatively affected the performance of other species. Species richness and functional richness effects differed systematically between functional groups. The magnitude of the increase in plant height with increasing species richness was greatest in grasses and was progressively smaller in legumes, small herbs, and tall herbs. Individual aboveground biomass responses to increasing species richness also differed among functional groups and were positive for legumes, less pronouncedly positive for grasses, negative for small herbs, and more pronouncedly negative for tall herbs. Moreover, these effects of species richness differed strongly between species within these functional groups. We conclude that individual plant performance largely depends on the diversity of the surrounding community, and that the direction and magnitude of the effects of species richness and functional richness differs largely between species. Our study suggests that diversity of the surrounding community needs to be taken into account when interpreting drivers of the performance of individual plants.     

Root responses to nitrogen pulse frequency under different nitrogen amounts

Responses of morphology and biomass allocation of roots to frequency of nitrogen (N) pulse potentially influence the fitness of plants, but such responses may be determined by root size. We grew 12 plant species of three functional groups (grasses, forbs, and legumes) under two N pulse frequencies (high vs. low supply frequency) and two N amounts (high vs. low supply amount). Compared to low-amount N supply, high-amount N supply stimulated biomass accumulation and root growth by either increasing the thickness and length of roots or decreasing the root mass fraction. Compared to low-frequency N supply, high-frequency N supply improved biomass accumulation and root growth in forbs or grasses, but not in legumes. Furthermore, the magnitude of the response to N frequency was significantly negatively correlated with root size at the species scale, but this was only true when the N amount was high. We conclude that root responses to N frequency are related to plant functional types, and non-legume species is more sensitive to N frequency than legume species. Our results also suggest that root size is a determinant of root responses to N frequency when N supply amount is high.     

Plant diversity effects on soil heterotrophic activity in experimental grassland ecosystems

The loss of plant species from terrestrial ecosystems may cause changes in soil decomposer communities and in decomposition of organic material with potential further consequences for other ecosystem processes. This was tested in experimental communities of 1, 2, 4, 8, 32 plant species and of 1, 2 or 3 functional groups (grasses, legumes and non-leguminous forbs). As plant species richness was reduced from the highest species richness to monocultures, mean aboveground plant biomass decreased by 150%, but microbial biomass (measured by substrate induced respiration) decreased by only 15% (P = 0.05). Irrespective of plant species richness, the absence of legumes (across diversity levels) caused microbial biomass to decrease by 15% (P = 0.02). No effect of plant species richness or composition was detected on the microbial metabolic quotient (qCO₂) and no plant species richness effect was found on feeding activity of the mesofauna (assessed with a bait-lamina-test). Decomposition of cellulose and birchwood sticks was also not affected by plant species richness, but when legumes were absent, cellulose samples were decomposed more slowly (16% in 1996, 27% in 1997, P = 0.006). A significant decrease in earthworm population density of 63% and in total earthworm biomass by 84% was the single most prominent response to the reduction of plant species richness, largely due to a 50% reduction in biomass of the dominant `anecic' earthworms. Voles (Arvicola terrestris L.) also had a clear preference for high-diversity plots. Soil moisture during the growing season was unaffected by plant species richness or the number of functional groups present. In contrast, soil temperature was 2 K higher in monocultures compared with the most diverse mixtures on a bright day at peak season. We conclude that the lower abundance and activity of decomposers with reduced plant species richness was related to altered substrate quantity, a signal which is not reflected in rates of decomposition of standard test material. The presence of nitrogen fixers seemed to be the most important component of the plant diversity manipulation for soil heterotrophs. The reduction in plant biomass due to the simulated loss of plant species had more pronounced effects on voles and earthworms than on microbes, suggesting that higher trophic levels are more strongly affected than lower trophic levels.     

Changes in plant species richness and productivity in response to decreased nitrogen inputs in grassland in southern England

Biomass production and plant species diversity in grassland in southern England was monitored before and after a change from conventional to organic farming. Our 18-year study, part of the UK's Environmental Change Network long-term monitoring programme, showed that the cessation of artificial fertiliser use on grassland after conversion to organic farming resulted in a decrease in biomass production and an increase in plant species richness. Grassland productivity decreased immediately after fertiliser application ceased, and after two years the annual total biomass production had fallen by over 50%. In the subsequent decade, total annual grassland productivity did not change significantly, and yields reached 31–66% of the levels recorded pre-management change. Plant species richness that had remained stable during the first 5 years of our study under conventional farming, increased by 300% over the following 13 years under organic farm management. We suggest that the change in productivity is due to the altered composition of species within the plots. In the first few years after the change in farming practice, high yielding, nitrogen-loving plants were outcompeted by lower yielding grasses and forbs, and these species remained in the plots in the following years. This study shows that grassland can be converted from an environment lacking in plant species diversity to a relatively species-rich pasture within 10–15 years, simply by stopping or suspending nitrogen additions. We demonstrate that the trade-off for increasing species richness is a decrease in productivity. Grassland in the UK is often not only managed from a conservation perspective, but to also produce a profitable yield. By considering the species composition and encouraging specific beneficial species such as legumes, it may be possible to improve biomass productivity and reduce the trade-off.     

Soil Environmental Conditions and Microbial Build-Up Mediate the Effect of Plant Diversity on Soil Nitrifying and Denitrifying Enzyme Activities in Temperate Grasslands

Random reductions in plant diversity can affect ecosystem functioning, but it is still unclear which components of plant diversity (species number – namely richness, presence of particular plant functional groups, or particular combinations of these) and associated biotic and abiotic drivers explain the observed relationships, particularly for soil processes. We assembled grassland communities including 1 to 16 plant species with a factorial separation of the effects of richness and functional group composition to analyze how plant diversity components influence soil nitrifying and denitrifying enzyme activities (NEA and DEA, respectively), the abundance of nitrifiers (bacterial and archaeal amoA gene number) and denitrifiers (nirK, nirS and nosZ gene number), and key soil environmental conditions. Plant diversity effects were largely due to differences in functional group composition between communities of identical richness (number of sown species), though richness also had an effect per se. NEA was positively related to the percentage of legumes in terms of sown species number, the additional effect of richness at any given legume percentage being negative. DEA was higher in plots with legumes, decreased with increasing percentage of grasses, and increased with richness. No correlation was observed between DEA and denitrifier abundance. NEA increased with the abundance of ammonia oxidizing bacteria. The effect of richness on NEA was entirely due to the build-up of nitrifying organisms, while legume effect was partly linked to modified ammonium availability and nitrifier abundance. Richness effect on DEA was entirely due to changes in soil moisture, while the effects of legumes and grasses were partly due to modified nitrate availability, which influenced the specific activity of denitrifiers. These results suggest that plant diversity-induced changes in microbial specific activity are important for facultative activities such as denitrification, whereas changes in microbial abundance play a major role for non-facultative activities such as nitrification.     

Understory response to varying fire frequencies after 20 years of prescribed burning in an upland oak forest

Ecosystems in the eastern United States that were shaped by fire over thousands of years of anthropogenic burning recently have been subjected to fire suppression resulting in significant changes in vegetation composition and structure and encroachment by invasive species. Renewed interest in use of fire to manage such ecosystems will require knowledge of effects of fire regime on vegetation. We studied the effects of one aspect of the fire regime, fire frequency, on biomass, cover and diversity of understory vegetation in upland oak forests prescribe-burned for 20 years at different frequencies ranging from zero to five fires per decade. Overstory canopy closure ranged from 88 to 96% and was not affected by fire frequency indicating high tolerance of large trees for even the most frequent burning. Understory species richness and cover was dominated by woody reproduction followed in descending order by forbs, C3 graminoids, C4 grasses, and legumes. Woody plant understory cover did not change with fire frequency and increased 30% from one to three years after a burn. Both forbs and C3 graminoids showed a linear increase in species richness and cover as fire frequency increased. In contrast, C4 grasses and legumes did not show a response to fire frequency. The reduction of litter by fire may have encouraged regeneration of herbaceous plants and helped explain the positive response of forbs and C3 graminoids to increasing fire frequency. Our results showed that herbaceous biomass, cover, and diversity can be managed with long-term prescribed fire under the closed canopy of upland oak forests.     

Tallgrass prairie restoration: implications of increased atmospheric nitrogen deposition when site preparation minimizes adventive grasses

Site preparation designed to exhaust the soil seedbank of adventive species can improve the success of tallgrass prairie restoration. Despite these efforts, increased rates of atmospheric nitrogen (N) deposition over the next century could potentially promote the growth of nitrophilic, adventive species in tallgrass restoration projects. We used a field experiment to examine how N addition affected species composition and plant productivity over the first 3 years of a tallgrass prairie restoration that was preceded by the planting of glyphosate‐resistant crops and multiple applications of glyphosate to exhaust the pre‐existing seedbank. We predicted that N addition would increase the percent cover of adventive plant species not included in the original seeding. Contrary to our prediction, only the cover of native species increased with N addition; native non‐leguminous forbs increased substantially, with Conyza canadensis (a weedy native species not part of the restoration seed mix) exploiting the combination of high N and bare ground in the first year, and non‐leguminous forbs (in particular Monarda fistulosa) and native C₃ grasses, all of which were seeded, increasing with N addition by the third year. Native legumes was the only functional group that exhibited lower cover in N addition plots than in control plots. There was no significant response by native C₄ grasses to N addition, and adventive grasses remained mostly absent from the plots. Overall, our results suggest that site pre‐treatment with herbicide may continue to be effective in minimizing adventive grasses in restored tallgrass prairie, despite future increases in atmospheric N deposition.     

N<Subscript>2</Subscript> fixation in three perennial <Emphasis Type="Italic">Trifolium</Emphasis> species in experimental grasslands of varied plant species richness and composition

This study is the first to investigate quantitative effects of plant community composition and diversity on N₂ fixation in legumes. N₂ fixation in three perennial Trifolium species grown in field plots with varied number of neighbouring species was evaluated with the ¹⁵N natural abundance method (two field sites, several growing seasons, no N addition) and the isotope dilution method (one site, one growing season, 5 g N m⁻²). The proportion of plant N derived from N₂ fixation, pNdfa, was generally high, but the N addition decreased pNdfa, especially in species-poor communities. Also following N addition, the presence of grasses in species-rich communities increased pNdfa in T. hybridum and T. repens L., while legume abundance had the opposite effect. In T. repens, competition for light from grasses appeared to limit growth and thereby the amount of N₂ fixed at the plant level, expressed as mg N₂ fixed per sown seed. We conclude that the occurrence of diversity effects seems to be largely context dependent, with soil N availability being a major determinant, and that species composition and functional traits are more important than species richness regarding how neighbouring plant species influence N₂ fixation in legumes.     

Food selection in <Emphasis Type="Italic">Microtus arvalis</Emphasis>: the role of plant functional traits

We offered captive common voles (Microtus arvalis) a choice of 11 plant species (representing four ecological groups) growing in vivaria. Selection was evaluated by measuring (1) the biomass of each plant species consumed and (2) functional and life-history plant traits. The legume Trifolium pratense, known for its high nutrient level, and well accessible rosette forbs creating the highest biomass at the soil ground level, were mostly preferred. Voles avoided mainly grasses and the creeping forb Thymus pulegioides. The experiment showed that foraging was strongly plant species-specific. We assessed whether plant functional traits explain selective foraging in common voles. To explore this, we reanalyzed Holišová’s (1959) data about common vole stomach contents and plant trait databases. Regression tree analysis indicated that plant guild and life span were the best predictors of dietary selection, with a probability exceeding 0.5 that voles would eat more grasses and/or legumes than forbs. These results do not correspond with the feeding trial. We suggest that the voles usually consume grasses in the field because grasses are abundant and readily available, but prefer protein-rich forbs when possible.     

Grazing effects on plant functional group diversity in Mediterranean shrublands

Grazing is one of the prevalent human activities that even today are taking place inside protected areas with direct or indirect effects on ecosystems. In this study we analyzed the effects of grazing on plant species diversity, plant functional group (PFG) diversity and community composition of shrublands. We analyzed plant diversity data from 582 sampling plots located in 66 protected areas of the Greek Natura 2000 network, containing in total 1102 plant species and subspecies. We also classified a priori all plant species in seven PFGs: annual forbs, annual grasses/sedges, legumes, perennial forbs, perennial grasses/sedges, small shrubs and tall shrubs. For each site, grazing intensity was estimated in four classes (no grazing, low, medium and high grazing intensity). We found that, at the spatial and temporal scale of this study, as grazing intensity increased, so did total species richness. However, each PFG displayed a different response to grazing. Short-lived species (annual grasses or forbs and legumes) benefited from grazing and their species richness and proportion in the community increased with grazing. Perennial grasses and forbs species richness increased with grazing intensity, but their dominance decreased, since their proportion in the community declined. Short shrub species richness remained unaffected by grazing, while tall shrub diversity decreased. Finally, in sites without grazing the spatial pattern of species richness of the different PFGs was not congruent with each other, while in grazed sites they were significantly positively correlated (with the exception of tall shrubs). This finding may imply that grazing is a selective pressure organizing the community structure, and imposing a certain contribution of each PFG. So, in Mediterranean shrublands in protected areas with a long historical record of grazing, it seems that grazing promotes species diversity and its continuation on a portion of the landscape may be a necessary part of an effective management plan.     

Positive effects of plant species diversity on productivity in the absence of legumes

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.     

Differences in rhizosphere carbon-partitioning among plant species of different families

Interspecific variations in carbon (C) allocation and partitioning in the rhizosphere were investigated on 12 Mediterranean species belonging to different family groups (grasses, legumes, non-legume forbs) and having different life cycles. Plants grown individually in artificial soil, in a greenhouse and inoculated with rhizosphere microflora were labelled with ¹⁴CO₂ for 3 h at the vegetative stage. Rhizosphere respiration was measured during 6 days after which labelled C partitioning between shoots, roots, soil, root washing solution and respiration was estimated. The percentage of assimilated ¹⁴C allocated below ground differed significantly between species (41 – 76%) but no significant difference was found between grasses, legumes and non-legume forbs. When expressed as percentage of below-ground ¹⁴C, rhizosphere respiration was significantly smaller for non-legume forbs (42%) than for grasses (46%) and legumes (51%). Consequently more ¹⁴C was incorporated into root biomass in the former. Half-life of ¹⁴CO₂ evolution through respiration ranged from 23 h in legumes to 27 h for non-legume forbs and 37 h for grasses. This suggested differences in microbial activities due to quantities and quality of root exuded C. Rhizosphere respiration was positively correlated with the amount of ¹⁴C in the solution used to wash the roots on one hand, and root N concentration on the other hand. This led to a functional hierarchy between plant family groups of the overall rhizosphere activity. It went from non-legume forbs being the less active (except Crepis sancta)in terms of respiration and exudation, to grasses and then legumes, the most active but also the richest in nitrogen.