Eutrophication is a widespread environmental change that usually reduces the stabilizing effect of plant diversity on productivity in local communities. Whether this effect is scale dependent remains ...to be elucidated. Here, we determine the relationship between plant diversity and temporal stability of productivity for 243 plant communities from 42 grasslands across the globe and quantify the effect of chronic fertilization on these relationships. Unfertilized local communities with more plant species exhibit greater asynchronous dynamics among species in response to natural environmental fluctuations, resulting in greater local stability (alpha stability). Moreover, neighborhood communities that have greater spatial variation in plant species composition within sites (higher beta diversity) have greater spatial asynchrony of productivity among communities, resulting in greater stability at the larger scale (gamma stability). Importantly, fertilization consistently weakens the contribution of plant diversity to both of these stabilizing mechanisms, thus diminishing the positive effect of biodiversity on stability at differing spatial scales. Our findings suggest that preserving grassland functional stability requires conservation of plant diversity within and among ecological communities.
Changes in livestock loads and eutrophication associated with human activities can modify the stability of grassland's aboveground net primary productivity (ANPP), by modifying the mean (μ) and/or ...standard deviation (σ) of ANPP. The changes in attributes of the plant community (i.e., species richness, species asynchrony, dominance) might in turn explain the ecosystem temporal (inter-annual) stability of grassland production. Here, we evaluated the interactive effects of changes in livestock loads and chronic nutrient addition on the temporal stability of ANPP (estimated as μ/σ) in temperate grasslands. We also assessed the role of different attributes of the plant community on ecosystem stability. We carried out a factorial experiment of domestic livestock exclusion and nutrient addition (10 g.m−2.year−1 of nitrogen, phosphorus, and potassium; n = 6 blocks) during five consecutive years in a natural grassland devoted to cattle production (Flooding Pampa, Argentina). Domestic livestock exclusion reduced ANPP stability by 65%, regardless of nutrient load, mainly by the increase of ANPP standard deviation. This reduction in ANPP stability after livestock exclusion was associated mostly with higher plant species dominance and also with reductions in plant effective richness and in the asynchrony of grassland's species. Despite not finding direct negative effects of eutrophication on ANPP stability, chronic nutrient addition decreased effective species richness and asynchrony, which may translate into reductions in ANPP stability in the future. Our findings highlight that the presence of livestock maintains the temporal stability of ANPP mainly by lowering the dominance of the plant community. However, increases in nutrient loads in grasslands devoted to livestock production may threaten grassland's stability.
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•Livestock removal reduced grassland’s stability (1/CV) by rising plant dominance.•Livestock removal decreased the temporal asynchrony of plant species abundances.•Livestock removal also increased the standard deviation of plant productivity.•Nutrients reduced species richness and asynchrony but did not affect stability.
The reality confronting ecosystem managers today is one of heterogeneous, rapidly transforming landscapes, particularly in the areas more affected by urban and agricultural development. A landscape ...management framework that incorporates all systems, across the spectrum of degrees of alteration, provides a fuller set of options for how and when to intervene, uses limited resources more effectively, and increases the chances of achieving management goals. That many ecosystems have departed so substantially from their historical trajectory that they defy conventional restoration is not in dispute. Acknowledging novel ecosystems need not constitute a threat to existing policy and management approaches. Rather, the development of an integrated approach to management interventions can provide options that are in tune with the current reality of rapid ecosystem change.
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.
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.
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.
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.
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.
Plant functional types that fix atmospheric nitrogen like legumes are central in forage provisioning, but their abundance depends on rangeland management. We explored the effect of grazing, haying, and fertilization on legume abundance in Pampa rangelands. Phosphorus addition, more than nitrogen, boosted legume but only in grazed or hayed plots, which was linked to increased ground‐level light and reduced competition with grasses. The picture shows a fertilized experimental plot under grazing dominated by Lotus glaber, the most abundant legume in the experiment (see detail). Photo credit: Sofía Campana.
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.
Resumen
La pérdida de especies debido al aumento en el número de nutrientes del suelo ha sido explicada tanto por la competencia por luz, a través de un aumento en la biomasa aérea, como por una reducción en la dimensión de nicho. Por su parte, los disturbios, debido a que pueden reducir la biomasa de la comunidad y a las especies dominantes aumentando así la disponibilidad de luz, podrían contrarrestar el efecto de los nutrientes sobre la biomasa aérea. Sin embargo, se desconoce si la pérdida de diversidad tanto a escala local o espacial, generada por el aumento en el número de nutrientes agregados al suelo, se puede contrarrestar con disturbios al canopeo.
En este estudio evaluamos si la pérdida de diversidad a escala local (alfa) y espacial (beta), generada por el aumento en el número de nutrientes agregados, puede revertirse por disturbios al canopeo en los pastizales inundables de la Pampa, Argentina. En un experimento de campo replicado durante 4 años, agregamos recursos limitantes del suelo combinando nitrógeno, fósforo y potasio para obtener 0, 1, 2 o 3 nutrientes y manipulamos el régimen de disturbio del canopeo mediante el corte estacional y la eliminación de biomasa.
Encontramos que el aumento en el número de nutrientes agregados al suelo redujo considerablemente la diversidad de plantas a nivel local y espacial, a pesar de los cambios de biomasa y luz generados por el corte. En las parcelas cortadas, la pérdida de diversidad local, debido al aumento en los nutrientes, se intensificó a lo largo del tiempo, aumentando la cobertura de las especies dominantes. Si bien el corte no afectó la pérdida de especies dominantes, el aumento de la cantidad de nutrientes añadidos promovió la pérdida de especies raras y redujo la diversidad espacial. Además, el corte aumentó la diversidad local y espacial independientemente de los efectos de la luz y/o de la biomasa, lo que sugiere efectos alternativos del corte.
Síntesis. Nuestros resultados demostraron que incluso cuando los disturbios generaron un efecto positivo sobre la diversidad local y espacial, no contrarrestaron completamente el efecto negativo del aumento de nutrientes en el suelo. Por lo tanto, la importancia relativa de la competencia aérea y subterránea por los recursos del suelo puede cambiar cuando los disturbios crónicos alteran la dominancia en la comunidad. En condiciones de poca disponibilidad de luz, la competencia aérea puede conducir a la pérdida de riqueza de especies, pero cuando el corte reduce la limitación de luz, el número creciente de nutrientes agregados puede reducir la dimensión del nicho y, por lo tanto, la coexistencia de especies. En síntesis, ante la necesidad de manejar los pastizales eutrofizados, nuestro estudio mostró que los disturbios pueden no mitigar por completo el efecto negativo de múltiples nutrientes en la diversidad local y espacial de los pastizales.
Our study showed that disturbance may not completely mitigate the negative effect of multiple nutrient inputs on local and spatial grassland diversity. Despite mowing generated a positive effect on local and spatial diversity, it did not completely counteract the negative effect of the number of added nutrients. Under low light availability, above‐ground competition may drive species richness loss but when disturbance reduces light limitation, the increasing number of added nutrients may reduce niche dimensionality and thus species coexistence.
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 low-nutrient 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-nitrogen-fixing 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
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./m
2
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.
Summary
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 lay summary is available for this article.
Lay Summary
Anthropogenic nutrient enrichment and shifts in herbivory can lead to dramatic changes in the composition and diversity of aboveground plant communities. In turn, this can alter seed banks in the ...soil, which are cryptic reservoirs of plant diversity. Here, we use data from seven Nutrient Network grassland sites on four continents, encompassing a range of climatic and environmental conditions, to test the joint effects of fertilization and aboveground mammalian herbivory on seed banks and on the similarity between aboveground plant communities and seed banks. We find that fertilization decreases plant species richness and diversity in seed banks, and homogenizes composition between aboveground and seed bank communities. Fertilization increases seed bank abundance especially in the presence of herbivores, while this effect is smaller in the absence of herbivores. Our findings highlight that nutrient enrichment can weaken a diversity maintaining mechanism in grasslands, and that herbivory needs to be considered when assessing nutrient enrichment effects on seed bank abundance.
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