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Nutrient additions typically increase terrestrial ecosystem productivity, reduce plant diversity and alter plant community composition; however, the effects of P additions and interactions between N and P are understudied. We added both N (10 g m⁻²) and three levels of P (2.5, 5 and 10 g m⁻²) to a native, ungrazed tallgrass prairie burned biennially in northeastern Kansas, USA, to determine the independent and interactive effects of N and P on plant community composition and above‐ground net primary productivity (ANPP). After a decade of nutrient additions, we found few effects of P alone on plant community composition, N alone had stronger effects, and N and P additions combined resulted in much larger effects than either alone. The changes in the plant community were driven by decreased abundance of C₄ grasses, perhaps in response to altered interactions with mycorrhizal fungi, concurrent with increased abundance of non‐N‐fixing perennial and annual forbs. Surprisingly, this large shift in plant community composition had little effect on plant community richness, evenness and diversity. The shift in plant composition with N and P combined had large but variable effects on ANPP over time. Initially, N and N and P combined increased above‐ground productivity of C₄ grasses, but after 4 years, productivity returned to ambient levels as grasses declined in abundance and the community shifted to dominance by non‐N‐fixing and annual forbs. Once these forbs increased in abundance and became dominant, ANPP was more variable, with pulses in forb production only in years when the site was burned. Synthesis. We found that a decade of N and P additions interacted to drive changes in plant community composition, which had large effects on ecosystem productivity but minimal effects on plant community diversity. The large shift in species composition increased variability in ANPP over time as a consequence of the effects of burning. Thus, increased inputs of N and P to terrestrial ecosystems have the potential to alter stability of ecosystem function over time, particularly within the context of natural disturbance regimes.

1. Forest ecosystems are critical for the global regulation of carbon (C), a substantial portion of which is stored in above-ground biomass (AGB). While it is well understood that taxonomic and functional composition, stand structure and environmental gradients influence spatial variation in AGB, the relative strengths of these drivers at landscape scales have not been investigated in temperate forests. Furthermore, when biodiversity enhances C storage, it is unclear whether it is through mass-ratio effects (i.e. the dominant trait in communities regulates AGB) or through niche complementarity (i.e. increased AGB due to interspecific resource partitioning). 2. To address these mechanisms, we analysed data from a census of 28,262 adult trees sampled across 900 ha of temperate deciduous forest in southwestern Pennsylvania. We used data on four key plant functional traits to determine if (1) there is a positive relationship between species diversity and AGB and (2) whether this is due to mass-ratio effects or niche complementarity. We also sought to (3) identify the physical stand structural attributes and topographic variables that influence AGB across this landscape. 3. We found AGB was positively related to species richness and negatively related to species evenness, albeit weakly, while functional diversity indices had neutral effects. Above-ground biomass was enhanced in communities dominated by traits related to greater maximum tree height, deeper minimum rooting depths and larger seeds. Most importantly, areas with high AGB were dominated by Acer saccharum and Liriodendron tulipifera. Overall, these results support mass-ratio effects, with little evidence for niche complementarity. 4. Synthesis. Stand structure, topography, and species and functional composition, but not taxonomic or functional diversity, were found to be key drivers of above-ground biomass at landscape scales (<900 ha) in this temperate deciduous forest. Our findings suggest that simultaneously managing for both high diversity and for above-ground carbon storage may prove challenging in some forest systems. Our results further indicate that the impact of tree biodiversity loss on above-ground carbon stocks will depend greatly on the identity of the species that are lost.

Disentangling the relative response sensitivity of soil autotrophic (Ra) and heterotrophic respiration (Rh) to nitrogen (N) enrichment is pivotal for evaluating soil carbon (C) storage and stability in the scenario of intensified N deposition. However, the mechanisms underlying differential sensitivities of Ra and Rh and relative contribution of Rh to soil respiration (Rs) with increasing N deposition remain elusive. A manipulative field experiment with multi‐level N addition rates was conducted over 3 years (2015–2017) in an alpine meadow to explore the relative impact of N enrichment on Ra and Rh and the response of Rh/Rs ratio to the gradient of N addition. Soil respiration components had different sensitivities to N enrichment, with Ra decreasing more than Rh, leading to a higher Rh/Rs ratio as a function of increasing N addition rates. Ra and Rh decreased nonlinearly as N addition rates increased, with a critical load of 8 g N m−2 year−1 above which N enrichment significantly inhibited them. Ra and Rh were controlled by different abiotic and biotic factors, and the regulation of controlling factors on soil respiration components varied over time. N‐induced reduction in the relative abundance of forb significantly affected Ra, and this effect was mainly evident in the second and third years. Nitrogen enrichment significantly changed Rh in the third year, and the decreased Rh under high doses of N addition could be attributed to the changes in microbial biomass C, soil substrate quality and microbial composition. Our study highlights the leading role of Ra in regulating Rs responses to N enrichment and the enhancement of Rh/Rs ratio with increasing N addition. We also emphasize that N‐induced shifts in plant community composition play a vital role in regulating Ra instead of Rh. The changing drivers of Ra and Rh with time suggests that long‐term experiments with multiple levels of N addition are further needed to test the nonlinear responses and underlying mechanisms of soil respiration components in face to aggravating N deposition. A free Plain Language Summary can be found within the Supporting Information of this article. A free Plain Language Summary can be found within the Supporting Information of this article.

For plants, herbivory and interactions with their surrounding soil ecosystem are crucial factors influencing individual performance and plant-community composition. Until now, research has mostly focused on individual effects of herbivory or plant–soil feedbacks (PSFs) on plant growth and community composition, but few studies have explicitly investigated herbivory in the context of PSFs. These few studies, however, were performed under greenhouse conditions even though PSFs and herbivory may differ between greenhouse and field conditions. Therefore, we performed a field experiment in a grassland, testing the growth responses of three grass species that consistently differ in local abundance, on soils previously conditioned by these species. We tested these PSF effects for the three species both in the presence and in the absence of aboveground herbivores. Without herbivores, the two subdominant species suffered from negative PSF effects. However, in the presence of herbivores and on heterospecific soils, the same two species experienced a significant loss of shoot biomass, whereas, in contrast, enhanced root growth was observed on conspecific soils, resulting in overall neutral PSF effects. The dominant species was not damaged by herbivores and showed overall neutral PSF effects in the field with and without herbivores. Our study provides empirical evidence that negative PSF effects that exist under natural field conditions in grasslands can be overwhelmed by aboveground herbivory. Hence, potential PSF effects might not be detected in the field, because other abiotic and biotic interactions such as aboveground herbivory have stronger effects on plant performance and might therefore mask or override these PSF effects.

Background and aims Current knowledge of soil heterogeneity-diversity relationships (HDR) is largely based on studies manipulating single factor, but the advancements in HDR may require a comprehensive experiment incorporating multiple factors. Methods We conducted a three-year field experiment in which a seed mixture of 16 common grassland species was sown in plots with heterogeneous soils consisting of small (10 cm × 10 cm) or large patches (30 cm × 30 cm) of low and high nutrients or low and high pH, and homogeneous soils with an even mixture of low and high nutrient/pH soils. Soil nutrients and pH were manipulated in separate treatments. We determined plant species richness and diversity at two focal scales (40 cm × 40 cm plot-scale and 10 cm × 10 cm patch-scale). Results Plot-scale richness and diversity were not influenced by soil heterogeneity, but patch-scale richness was lower in plots with heterogeneous nutrients than in plots where nutrients were distributed homogeneously. There was no difference between the two heterogeneous nutrient soils with different grain sizes. Patch-scale diversity was higher in heterogeneous pH soils of large patch size than in heterogeneous pH soils of small patch size or the homogeneous pH soil at the final harvest. Species richness and diversity quantified at both plot and patch scales declined in all soils over time. Conclusions The influence of soil heterogeneity on plant species diversity depends on whether the soil varies in nutrients or pH, and on the temporal-spatial scale at which species diversity and soil heterogeneity are measured. These results indicate that soil heterogeneity has the potential to promote plant coexistence and future HDR studies should consider multiple soil factors at various temporal-spatial scales.

Aims Atmospheric nitrogen (N) deposition derived from agricultural intensification and fossil fuel burning can significantly impact plant growth, species diversity, and nutrient cycling. Semi-natural grasslands are of particular concern because their generally low intensity agricultural management suggests they may be very sensitive to enhanced atmospheric N deposition inputs, although previous experimental research indicates highly variable, site-specific responses. Mediating factors such as soil texture that influence actual availability of soil water and nutrients to plants have generally not been investigated. Methods We report the impacts of 16 years of experimental N addition (simulating 2050 atmospheric N input rates) and a separate, single growing season, high-level N and phosphorus (P) factorial experiment to a hayfield of varying loam soil texture (clay-loam – sandy-loam) on plant community structure and above-and belowground biomass. Results The chronic low-level N addition treatment had no significant effects on either species or community aboveground growth, species richness, or diversity. These properties were best explained by variation in soil water-filled pore space, and were substantially larger on plots with relatively clay-rich soils. Conclusions The general lack of responses to the low-level N additions and the lack of a growth response to the short factorial high-level N and P addition experiment, indicate that future atmospheric N deposition increases are unlikely to have major impacts on hay production or species composition in mesic semi-natural grasslands. By contrast, the strong interconnected influences of soil clay content and plant water availability in our results suggest that textural variation – even within loamy soils—will be a primary determinant of the impacts of anticipated future summer warming and reduced rainfall on hayfield vegetation.

1. Large herbivores influence plant community structure and ecosystem processes in many ecosystems. In large parts of the Arctic, reindeer (or caribou) are the only large herbivores present. Recent studies show that reindeer have the potential to mitigate recent warming-induced shrub encroachment in the Arctic and the associated greening of high-latitude ecosystems. This will potentially have large scale consequences for ecosystem productivity and carbon cycling. 2. To date, information on variation in the interactions between reindeer and plants across Arctic landscapes has been scarce. We utilized a network of experimental sites across a latitudinal gradient in the Scandinavian mountains where reindeer have been excluded from 59 study plots for at least 15 years. We used this study system to test the effect of long-term exclusion of reindeer on the abundance of major plant functional groups, the greenness indexes Leaf Area Index (LAI) and Normalized Difference Vegetation Index (NDVI), soil mineral nitrogen (N) and phosphorous (P), and species richness, and to determine whether the effect of reindeer exclusion is dependent on reindeer density, productivity, soil fertility or climate. 3. We found that NDVI and LAI, lichen and deciduous shrub abundances were largely reduced while soil mineral N was enhanced by reindeer. The direction and amplitude of other plant functional group responses to reindeer exclusion differed between forest and tundra as well as shrub- and herbaceous-dominated vegetation. Higher reindeer densities were related to decreased plant species richness in low-productive sites and to increased species richness in productive sites. 4. The relative reduction in LAI and associated absolute reductions of deciduous shrubs in response to reindeer were positively related to reindeer density, while the relative reduction in NDVI was not. Further, relative reductions in LAI and NDVI in response to reindeer were unrelated to climate and soil fertility. 5. Synthesis. Our results provide long-term experimental evidence highlighting the role of reindeer density in regulating plant species richness, global climate change induced greenness patterns and shrub encroachment at regional scales in the Arctic. These findings emphasize the need to consider reindeer in models predicting vegetation patterns and changes in high-latitude ecosystems.

ContextAnthropogenic land use can significantly alter insect communities and may threaten services provided by beneficial flower-visiting insects. However, the plant community composition may interact with surrounding land use to affect insects in a way that is not well understood.ObjectivesOur goal was to disentangle the effect of the background plant community on the flowering visiting insect community composition from the independent effect of surrounding land use.MethodsWe planted four fixed community garden plots, three that each contained six species of one plant family (Asteraceae, Fabaceae, Lamiaceae) and one that was a mixed community plot, controlling the number of individuals and species identity of the plants. We then replicated these four fixed plots across five different landscapes in eastern Tennessee and surveyed the insects that visited the flowers for 2 years.ResultsBoth the identity and abundance of floral resources were strong drivers of flower-visiting insect abundance, with floral display being the single largest driver. Independent of the plant community, specific pollinating insects responded to different types of land use at different radii around each site. Total flower visitor and soldier beetle abundance increased with agricultural land use at 500 and 2000 m, respectively. On the other hand, sweat bee abundance increased with semi-natural land use at 2000 m and honey bee abundance increased with developed land use at 1000 m.ConclusionIndependent of plant community composition, surrounding land use affected the abundance, diversity, and composition of flower-visiting insects. However, there was not one consistent land use effect across all flower-visiting insects.

Cliffs are unique ecosystems with an outstanding but relatively unknown plant diversity, harboring rare, endemic and threatened species, but also rock-specialist or generalist species that can become locally common and dominant on cliffs. The rising popularity of climbing represents an increasing threat to cliff biota, affecting community composition and potentially diminishing diversity and species associations. We used a novel sampling design of closely-paired climbed versus unclimbed points along the cliff-face. We sampled along climbing routes of different climbing intensities in El Potrero Chico (Nuevo León, Mexico), identifying plant species and analyzing species associations and community composition in climbed and unclimbed plots. Diversity on the sampled cliffs was high, even greater than in other regional ecosystems. We found reduced abundance, cover, and diversity in climbed plots, irrespective of climbing intensity. Dominant species on the sampled cliffs were the most negatively affected by rock climbing in terms of abundance, and some locally rare species, including endemics and endangered species, were entirely absent from climbed plots. Co-occurrence analysis showed that the number of associations between pairs of dominant and common species were greatly reduced in climbed plots, and that positive associations between locally rare species existed in unclimbed plots but not in climbed plots, which may contribute to the disappearance of endemic and threatened species. Finally, NMDS analysis revealed that the community composition changed significantly due to climbing. Our results indicate that conservation science should convince stakeholders of the need for a holistic conservation of cliff ecosystems and not focus solely on emblematic or rare species, since plant community dynamics and preservation depend on interactions between plant species.

Increasing atmospheric nitrogen (N) deposition has the potential to alter plant diversity and thus the function and stability of terrestrial ecosystems. N-limited alpine ecosystems are expected to be particularly susceptible to increasing N deposition. However, little is known about the critical loads and saturation thresholds of ecosystem responses to increasing N deposition on the Tibetan Plateau, despite its importance to ecosystem management. To evaluate the N critical loads and N saturation thresholds in an alpine ecosystem, in 2010, we treated an alpine meadow with five levels of N addition (0, 10, 20, 40, and 80 kg N ha⁻¹ year⁻¹) and characterized plant and soil responses. The results showed that plant species richness and diversity index did not statistically vary with N addition treatments, but they both changed with years. N addition affected plant cover and aboveground productivity, especially for grasses, and soil chemical features. The N critical loads and saturation thresholds, in terms of plant cover and biomass change at the community level, were 8.8–12.7 and 50 kg N ha⁻¹ year⁻¹ (including the ambient N deposition rate), respectively. However, pronounced changes in soil inorganic N and net N mineralization occurred under the 20 and 40 kg N ha⁻¹ year⁻¹ treatments. Our results indicate that plant community cover and biomass are more sensitive than soil to increasing N inputs. The plant community composition in alpine ecosystems on the Qinghai-Tibetan Plateau may change under increasing N deposition in the future.