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Question: Legumes are a key component of rangelands because they play an important role in animal nutrition and the entrance of nitrogen (N) into ecosystems through symbiotic fixation. Legume abundance is commonly low in N‐enriched environments because of competition with grasses and non‐legume forbs. Both haying and livestock grazing remove plant biomass and reduce light limitation to plant growth, with the difference that livestock may selectively consume legumes. This study examines how legumes respond to grazing, haying and fertilization, and what mechanisms explain legume abundance in rangelands. Location: Flooding Pampa, Argentina. Methods: We performed two manipulation field experiments over 3 years. First, a factorial of rangeland management (intact, haying, or grazing) under two nutrient levels (ambient and increased N, phosphorus [P] and potassium [K]); and second, a factorial of rangeland management (intact or haying) and N × P addition. We evaluated legume, grass and non‐legume forb abundance and ground‐level light in three to five replicates of our experiments over 3 years. Results: NPK fertilization increased legume abundance consistently under grazing, and temporarily under haying, but had no effect in the intact grassland. Also, P addition increased legume abundance only under haying when N was not added. Temporal changes in legume abundance were positively associated with changes in ground‐level light, which increased with haying and grazing, but decreased with fertilization in the intact grassland, and negatively with grass abundance. Conclusions: The negative effects of nutrients on legume abundance were offset by the positive effects of livestock. The reduction in grass competition and increase in ground‐level light due to grazing and haying explained the positive responses of legume abundance to nutrients in this temperate grassland. Our results highlight the importance of considering the interactive response of legume abundance to grazing and fertilization, which are becoming common practices in rangelands. [ABSTRACT FROM AUTHOR]

Questions: Restoring diversity in temperate grasslands requires eliminating invaders and recovering different native phenological groups. Clipping and seed addition promote native grass recovery, but these effects could depend on the phenological overlap between invaders and native species. We evaluated the importance of the interaction between two types of clipping, species phenology and sowing to restore temperate grasslands invaded by the cool‐season forage species Festuca arundinacea (tall fescue). Location: Flooding Pampa grassland (Buenos Aires, Argentina). Methods: We applied selective and non‐selective clipping on tall fescue during the peak growing season, combined with the sowing of native cool‐ and warm‐season grasses. Plant species cover was estimated visually for 3 years; species were categorized by functional groups. Moreover, through emergence trials, we evaluated whether the soil seed bank had been impoverished by invasion. Results: Regardless of selectivity, clipping decreased tall fescue cover by 12%/year, which reduced microsite limitation. Consequently, total native richness increased by 5 spp./year and cover increased from 3% to 50% by the last experimental year, independent of clipping type. Moreover, native warm‐season grasses, the group with the least phenological overlap with invaders, increased by nearly 25% in cover and by 5 spp./m2 with clipping and independently of sowing. Contrarily, native cool‐season grasses, with more phenological overlap with invaders, increased 5% with propagule addition. Furthermore, the seed bank was enriched with tall fescue and depleted of cool‐season grasses. Conclusions: Our results reveal that overcoming microsite limitation is a necessary condition to restore plant diversity in grassland invaded by tall fescue. Nevertheless, seed limitation and phenological overlap may be particularly important to restore the diversity of some native functional groups of plants. Considering the phenology of species can help to identify either clipping targets or phenological groups to be sown when restoring native diversity. [ABSTRACT FROM AUTHOR]

Anthropogenic nutrient enrichment is driving global biodiversity decline and modifying ecosystem functions. Theory suggests that plant functional types that fix atmospheric nitrogen have a competitive advantage in nitrogen-poor soils, but lose this advantage with increasing nitrogen supply. By contrast, the addition of phosphorus, potassium, and other nutrients may benefit such species in lownutrient environments by enhancing their nitrogen-fixing capacity. We present a global-scale experiment confirming these predictions for nitrogen-fixing legumes (Fabaceae) across 45 grasslands on six continents. Nitrogen addition reduced legume cover, richness, and biomass, particularly in nitrogen-poor soils, while cover of non-nitrogenfixing plants increased. The addition of phosphorous, potassium, and other nutrients enhanced legume abundance, but did not mitigate the negative effects of nitrogen addition. Increasing nitrogen supply thus has the potential to decrease the diversity and abundance of grassland legumes worldwide regardless of the availability of other nutrients, with consequences for biodiversity, food webs, ecosystem resilience, and genetic improvement of protein-rich agricultural plant species. [ABSTRACT FROM AUTHOR]

Species loss due to an increasing the number of added nutrients has been explained by both light competition through biomass increase and by niche dimension reduction as a result of species‐specific limiting soil resources trade‐offs. Disturbances, by reducing community biomass, species dominance and increasing light availability, may counteract above‐ground nutrient effects. However, it is unknown if diversity loss at local or spatial scales generated by increasing the number of added nutrients can be redressed with canopy disturbance.We evaluated if local (alpha) and spatial scale (beta) diversity loss generated by the number of added nutrients can be reverted by disturbances in Flooding Pampa grasslands, Argentina. In a 4‐year replicated field experiment, we added soil resources combining nitrogen, phosphorus and potassium to obtain 0, 1, 2 or 3 nutrients and manipulated the regime of canopy disturbance by seasonal mowing and biomass removal.We found that the increasing the number of added nutrients strongly reduced local and spatial plant diversity, despite biomass and light changes generated by mowing. In mown plots, nutrient‐driven local diversity loss intensified along time, thus increasing species dominance. While mowing did not affect dominant species loss, increasing the number of added nutrients promoted rare species loss and reduced spatial dissimilarity. Furthermore, mowing increased local and spatial diversity regardless of light or biomass effects, suggesting alternative pathway effects for disturbance.Synthesis. Our results demonstrate that even when disturbance generated a positive effect on local and spatial diversity, it did not completely counteract the negative effect of number of added nutrients. Thus, the relative importance of above‐ and below‐ground resource competition may change when chronic disturbances alter community dominance. Under low light availability, above‐ground competition may drive species richness loss but when disturbance reduces light limitation, the increasing the number of added nutrients may reduce niche dimensionality and thus species coexistence. In sum, faced with the need to manage eutrophized grasslands, our study showed that disturbance may not completely mitigate the negative effect of multiple nutrient inputs on local and spatial grassland diversity. [ABSTRACT FROM AUTHOR]

Background and Aims Phenotypic plasticity and local adaption can contribute to the success of invasive species. While the former is an environmentally induced trait, the latter involves a selection process to filter the best genotype for a location. We examined the evidence for phenotypic plasticity and local adaptation for seed and seedling traits of the invasive tree Gleditsia triacanthos , with three origins distributed along an approx. 10° latitude gradient across three biomes. Methods In sub-tropical forests, dry woodlands and secondary temperate grasslands in Argentina, we harvested seeds from clusters of neighbouring trees (i.e. families) distributed within 15–20 km in each origin (biome). We manipulated the environmental conditions relevant to each biome, assuming that propagule availability did not represent an ecological barrier. In growth chambers, we evaluated seed imbibition and seed germination under different light, temperature and water potential. In a 2 year common garden, we evaluated the impact of resident vegetation removal on seedling survival and growth. Key Results Mean time to complete seed imbibition differed among origins; seeds from temperate grasslands reached full imbibition before seeds from dry woodlands and sub-tropical forests. Germination was always >70 %, but was differentially affected by water potential, and light quantity (dark–light) and quality (red–far red) among origins, suggesting local adaptation. In the common garden, vegetation removal rather than origin negatively affected seedling survival and enhanced seedling growth. Vegetation removal increased basal diameter, leaves per plant and spine number, and reduced the height:basal diameter ratio. Conclusions We conclude that local adaptation in seed germination traits and plastic changes in seedling allometry (e.g. height:diameter) may allow this tree to respond over the short and long term to changes in environmental conditions, and to contribute to shape G. triacanthos as a successful woody invader. Overall, our study revealed how local adaptation and plasticity can explain different aspects of tree invasion capacity across biomes. [ABSTRACT FROM AUTHOR]

Secondary succession after agriculture abandonment (old-fields) is mostly dominated by exotic grass species. Non-native plant invasions may alter soil fauna, potentially inducing plant-soil feedbacks. Despite their importance in nutrient cycling and plant-soil interactions, meso and macrofauna received less attention than bacteria or fungi. Here we compared the composition of the soil arthropod community in native remnants and plant exotic-dominated old-fields grasslands in the Inland Pampa, Argentina. We sampled independent remnants and old-field grassland plots within a 100 km 2 agricultural landscape to test the hypothesis that the abundance of soil arthropod organisms is related to the quality of the plant biomass, whereas the diversity of the soil biota is related to plant species richness, resulting in a different soil biota composition because of differing plant communities. When compared to non-invaded remnant grasslands, soil activity and soil food-web characteristics of the old-fields sites included: 1. Higher total arthropod abundance, particularly of Isopoda, Pseudoescorpionida and Blattaria; 2. Lower abundance of Hymenoptera and Enthomobryomorpha (Collembola); 3. Lower diversity, and evenness, but similar richness of soil organisms orders; 4. Higher soil respiration rates and soil temperature; and 5. Higher total soil N and K + content, but lower soil P content. These results illustrate that soil arthropod composition can vary widely within grasslands patches depending on plant species composition. Also, the more diverse plant community of remnant grasslands supports a more diverse soil biota, although soil activity is slower. Our results support the strong linkage between plant community and soil arthropod composition and suggest that changes in soil biota composition might promote plant-soil feedback interactions inducing the persistence of these alternative grassland states in new agricultural human-modified landscapes. [ABSTRACT FROM AUTHOR]

Secondary succession may lead to novel, exotic-dominated community states differing in structure and function from the original native counterparts. We hypothesized that grassland soil processes associated with C and N cycling decelerate with community turnover from short-lived forbs and grasses to long-lived native grasses, whereas invasion by exotic perennial grasses maintains fast cycling rates., We measured litter C and N turnover during decomposition, soil respiration, and soil N dynamics in synthetic plant communities resembling four successional stages, established on abandoned farmland in the Inland Pampa, Argentina. We also compared litter chemistry and decay rates of dominant species from each community stage in a common garden, and assessed mass loss for a standard litter type incubated in all communities., Litter decomposition and soil respiration decreased, while litter N retention increased from early through mid to late community stages dominated by forbs short-lived grasses and native perennial grasses, respectively. Soil process rates in exotic perennial grass communities were faster than in native grass communities, but similar to annual grass communities. Further, the standard litter decomposed more slowly in the native perennial than in the exotic perennial grass community. In the common garden, short-lived forbs and grasses decomposed faster than native or exotic perennial grasses, with species' decay rates being negatively related to initial litter C : N ratio., Our results show that changes in soil processes across old-field communities arise chiefly through differences in the quality of litter produced by dominant functional groups. A dominance shift from native to exotic perennial grasses prevented the deceleration of C and N cycling expected with plant successional turnover. Thus, invasion by fast-growing exotic grasses may fundamentally alter ecosystem functioning in novel grasslands., A is available for this article. [ABSTRACT FROM AUTHOR]

Woody encroachment in grasslands represents a global phenomenon with strong consequences on ecosystem functioning. While the causes triggering this process can be multiple, there is consensus on the fact that anthropogenic activities play a central role in woody expansion. In particular, the loss of grass cover increases the chances of woody invasion, whereas the role of defoliation is less known. In this study our objective was to assess the simultaneous effect of competition generated by resident vegetation and woody seedling defoliation on the growth and survival of Gleditsia triacanthos seedlings, a woody invader in Argentina. We established a factorial pot experiment with two main factors: Gleditsia defoliation (2 levels: with and without defoliation) and pasture competition (3 levels: without pasture, clipped pasture and intact pasture). Our results showed that pasture competition reduced Gleditsia survival and tree growth, but that the effect of tree defoliation on tree growth depended on the magnitude of pasture competition. More widely, our results stress the existence of a hierarchy order of factors controlling Gleditsia establishment (survival + growth): grass competition was the main control and tree defoliation became important only in the absence of competition. This evidence suggests that maintaining a competitive grass cover along with a frequency of tree defoliation could diminish tree establishment in herbaceous communities. [ABSTRACT FROM AUTHOR]

Native vegetation fragments embedded in anthropogenic landscapes are increasingly threatened by land-use intensification. Managing disturbance regimes and nutrient inputs may help maintain species diversity in such remnants. Yet, it is unclear the extent to which changes in resource availability due to reduced capture by resident plants and/or increased supply rates may trigger native community disassembly and exotic invasions., We examined how mowing disturbance and N fertilizer addition affected plant community recovery after a burning event in a remnant corridor of tussock pampa grassland in Argentina. The percentage cover and richness of native and exotic plant functional groups were monitored over 4 years. According to the 'fluctuating resource theory', we expected invasion to be the highest when both light and N availability were increased simultaneously., Mowing delayed recovery by dominant C4 tussock grasses and promoted subordinate, native C3 grasses and exotic legumes, thus enhancing both native and exotic species richness. Fertilization induced a transient increase in native forbs but decreased total plant richness. Moreover, N addition to mowed grassland led to rapid invasion by short-lived exotic forbs, which were then replaced by exotic perennial grasses. Exotic grasses eventually spread across the grassland corridor, although at different rates depending on the treatment, and in parallel to a generalized decline in native species cover., Synthesis and applications. Community disassembly patterns reflected differential responses of native and exotic functional groups to altered resource supply rates. Synergisms between canopy disturbances and N enrichment posed the greatest threat to preserving a pampa grassland remnant prone to invasion. Establishing buffer zones may be required to enhance the viability of corridor-like grassland remnants in agricultural landscapes. [ABSTRACT FROM AUTHOR]

Abstract: Exotic plants are major constituents of species pools in modern landscapes. Managing succession for restoration of degraded ecosystems thus requires an understanding of novel trajectories unfolding in mixed, native/exotic plant assemblages. We examined trends in native and exotic species abundance over 20years of old-field succession on set-aside farmland in the Inland Pampa, Argentina. Changes in plant cover and species richness were annually monitored on adjacent permanent plots established in different years (1978–1989). Both native and exotic species occurred in early, mid and late successional stages, exhibiting similar life-form replacement patterns, from annual forbs, through annual to perennial grasses. Exotic plant richness declined with plot age. Yet, four exotic grasses remained dominant through succession (50–70% cover), with plots initiated in later years showing increased exotic cover. While native perennial grasses occurred from the onset of succession, increasing from 5 to 12spp/plot, they only showed transient peaks below 30% cover. Cluster analysis of 113 plot-year samples identified alternative community states for early, mid and late successional stages, which were connected by a complex network of interweaving dynamic pathways. Depending on the plot, vegetation dynamics comprised directional temporal trajectories as well as nondirectional pathways, and arrested community states dominated by exotic grasses. Our results illustrate the overwhelming role of exotic species in modern old-field succession, and their potential to hinder recovery of native communities on former agricultural land. Community states with novel, native/exotic plant mixtures could be managed to deliver specific ecosystem services (e.g. forage production, carbon sequestration). However, meeting conservation goals may require active restoration measures, including exotic plant removals and native grass seeding. [ABSTRACT FROM AUTHOR]

Woody encroachment represents a global concern which is mostly studied in natural and semi-natural grasslands and savannas. Woody invasion is a growing phenomenon in agricultural landscapes, but it remains elusive how different crop types and their management could shape woody invasion in croplands. Understanding the mechanisms involved in woody establishment in crops may contribute to design strategies to reduce the magnitude and consequences of invasion. We examined to what extent current agricultural practices affect early establishment of the invasive tree Gleditsia triacanthos (honey locust) in maize and soybean crops, during a wet and a dry growing season. In Pampean croplands (Argentina), we selected zero tillage fields of both crops and controlled management (with/without agricultural practices) and crop presence (with/without soybean or maize plants) during two growing seasons. After maize and soybean harvest, we examined survival, growth, and starch reserves of G. triacanthos seedlings. Results showed that G. triacanthos survival decreased from 76% to 34% in the dry year with respect to the wet year. Crop presence enhanced tree survival, likely by protecting seedlings from herbicide and desiccation, but reduced seedling growth. Seedlings were smaller in soybean than in maize, and larger when crop was removed, independent of crop type. Even so, woody survival was on average 61% and 49% after maize and soybean harvest, respectively. Interestingly, seedling starch reserves were modulated by crop type and management, and were positively associated with G. triacanthos survival. In conclusion, we found a hierarchy of controls acting on G. triacanthos establishment in cropping systems, with growing season rainfall variability, crop type and the applied management modifying tree survival and growth. Overall, our study highlights that, under current management, annually cropped fields may represent a refuge for woody plant invaders and a potential front of woody expansion. Annually cropped fields, influenced by growing season rainfall and crop management, may represent a refuge for woody plant invaders and a potential front of woody expansion. [Display omitted] • In maize and soybean crops, a dry season drastically reduced woody survival. • Crops enhanced survival by protecting trees from herbicides and desiccation. • Contrastingly, crop reduced tree growth by competition. • Maize offered a more favourable environment for woody survival and growth. • Crop fields may act as a refuge for woody invaders and a front of expansion. [ABSTRACT FROM AUTHOR]

Abstract: Old fields often become dominated by exotic plants establishing persistent community states. Ecosystem functioning may differ widely between such novel communities and the native-dominated counterparts. We evaluated soil ecosystem attributes in native and exotic (synthetic) grass assemblages established on a newly abandoned field, and in remnants of native grassland in the Inland Pampa, Argentina. We asked whether exotic species alter soil functioning through the quality of the litter they shed or by changing the decomposition environment. Litter decomposition of the exotic dominant Festuca arundinacea in exotic assemblages was faster than that of the native dominant Paspalum quadrifarium in native assemblages and remnant grasslands. Decomposition of a standard litter (Triticum aestivum) was also faster in exotic assemblages than in native assemblages and remnant grasslands. In a common garden, F. arundinacea showed higher decay rates than P. quadrifarium, which reflected the higher N content and lower C:N of the exotic grass litter. Soil respiration rates were higher in the exotic than in the native assemblages and remnant grasslands. Yet there were no significant differences in soil N availability or net N mineralization between exotic and native assemblages. Our results suggest that exotic grass dominance affected ecosystem function by producing a more decomposable leaf litter and by increasing soil decomposer activity. These changes might contribute to the extended dominance of fast-growing exotic grasses during old-field succession. Further, increased organic matter turnover under novel, exotic communities could reduce the carbon storage capacity of the system in the long term. [Copyright &y& Elsevier]

Cool-season grasses establish symbioses with vertically transmitted Neotyphodium endophytes widespread in nature. The frequency of endophyte-infected plants in closed populations (i.e., without migrations) depends on both the differential fitness between infected and non-infected plants, and the endophyte-transmission efficiency. Most studies have been focused on the first mechanism ignoring the second. Infection frequency and endophyte transmission from vegetative tissues to seeds were surveyed in two grasses growing in vegetation units that differ in flood and grazing regimes, and soil salinity. Transmission efficiency and infection frequency for tall fescue did not vary significantly and were 0.98 and 1.00, respectively. For Italian ryegrass, transmission efficiency and infection frequency were 0.88 and 0.57 in humid prairies, and 0.96 and 0.96 in the other vegetation units. Only in humid mesophytic meadows, the observed pattern was irrespective of the presence or absence of grazers. Our results suggest that selection forces for endophyte infection are different for both species. Imperfect transmission was only compensated in tall fescue through an increased fitness of infected plants. Interpreting variations of infection frequency only in terms of differential fitness can be misleading, considering that endophyte transmission can be imperfect and variable in nature. Therefore, this study highlights the importance of measuring transmission efficiency. [ABSTRACT FROM AUTHOR]

Variation in functional community composition is expected to influence the extent of exotic species invasions. Yet, whether resident functional groups control invasion through their relative biomass (mass ratio hypothesis) or by traits other than biomass (identity hypothesis) remains poorly understood., We performed a 6-year experiment to determine the effects of removing different functional groups on exotic species biomass in a Flooding Pampa grassland, Argentina. Functional groups were defined by life-form (grasses or forbs), phenology (winter or summer) and origin (native or exotic). Removal of each functional group was compared against the removal of an equivalent amount of random biomass. Exotic group responses were monitored over 4 years of continuous removals, and after 2 years of recovery without manipulations., Removal of dominant native summer grasses caused the greatest impact on exotic species and overall community composition. Native summer-grass removal significantly increased exotic grass (120%) and forb (730%) biomass beyond the level (46% and 180%, respectively) expected from deleting a similar amount of biomass at random. Exotic annual grasses showed only a transient increase, whereas exotic forb invasion persisted even after 2 years without removals., Removing subordinate, native or exotic winter grasses, and rare native forbs significantly promoted exotic forbs, but to the same level (300%) as random biomass removals. Total grass removal increased exotic forbs to half the extent expected from adding the effects of single grass group removals. Dispersal limitation and harsh abiotic conditions may constrain exotic forb spread into such heavily grass-depleted patches., Synthesis. The impact of losing a functional group on the magnitude and persistence of invasion reflected its relative contribution to community biomass. Identity attributes other than biomass (e.g. phenological niche) further enhanced the biotic control that dominant native grasses exerted on established exotic species. Our findings highlight the community legacies of past disturbances to dominant functional groups. [ABSTRACT FROM AUTHOR]