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1. The experimental manipulation of atmospheric drought: Teasing out the role of microclimate in biodiversity experiments

   https://doi.org/10.1111/1365-2745.13595  

   Aguirre, Beatriz A. ; Hsieh, Brian ; Watson, Samantha J. ; Wright, Alexandra J. ; Callaway, ed., Ray ( February 2021 , Journal of Ecology)

   Drought occurrence is increasing due to anthropogenic climate change. Drought can negatively affect plants via reduced water below‐ground and increased evaporative demand or vapour pressure deficit (VPD) above‐ground. Past work has shown that plant diversity can ameliorate the negative effects of drought in plant communities, but these results are inconsistent between experimental and natural drought studies. Furthermore, while studies on the negative effects of reduced soil moisture on plant growth in drought experiments are abundant, the effects of predicted increases in atmospheric VPD have been neglected.

   We directly manipulated atmospheric relative humidity in a biodiversity and drought experiment at the California State University, Los Angeles (CA, USA) under three atmospheric conditions (ambient, dehumidified and humidified), two treatments of native perennial grass diversity (monoculture and eight species polyculture) and two soil drought treatments (control and drought). We assessed both polyculture plant community and individual species (Poa secunda) responses to atmospheric drought and soil drought.

   We found that soil drought only limits above‐ground biomass production when atmospheric conditions are also dry. We also found thatP. secundawas limited by increased competition in polyculture when ambient atmospheric conditions were humid but was facilitated by diversity when atmospheric conditions were dry.

   Synthesis. Higher diversity ecosystems may be capable of protecting individual species from the negative effects of drought (facilitation). Without careful experimental manipulation of atmospheric drought, this important mechanism will be missed.

    

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2. Plant diversity and grasses increase root biomass in a rainfall and grassland diversity manipulation

   https://doi.org/10.3389/fevo.2023.1259809  

   Podzikowski, Laura Y. ; Heffernan, Megan M. ; Bever, James D. ( November 2023 , Frontiers in Ecology and Evolution)

   The loss of plant productivity with declining diversity is well established, exceeding other global change drivers including drought. These patterns are most clearly established for aboveground productivity, it remains poorly understood whether productivity increases associated with diversity are replicated belowground. To address this gap, we established a plant diversity-manipulation experiment in 2018. It is a full factorial manipulation of plant species richness and community composition, and precipitation. Three and five years post-establishment, two bulk soil cores (20cm depth) were collected and composited from each plot and were processed for roots to determine belowground biomass as root standing crop. We observed a strong positive relationship between richness and aboveground production and belowground biomass, generating positive combined above and belowground with diversity. Root standing crop increased 1.4-fold from years three to five. Grass communities produced more root biomass (monoculture mean 463.9 ± 410.3g m⁻²), and the magnitude of the relationship between richness and root standing crop was greatest within those communities. Legume communities produced the fewest roots (monoculture mean 212.2 ± 155.1g m⁻²), and belowground standing crop was not affected by diversity. Root standing crops in year three were 1.8 times higher under low precipitation conditions, while in year five we observed comparable root standing crops between precipitation treatments. Plant family was a strong mediator of increased belowground biomass observed with diversity, with single family grass and aster families generating 1.7 times greater root standing crops in six compared to single species communities, relationships between diversity and aboveground production were consistently observed in both single-family and multiple family communities. Diverse communities with species from multiple families generated only 1.3 times the root standing crop compared to monoculture average root biomass. We surprisingly observe diverse single family communities can generate increases in root standing crops that exceed those generated by diverse multiple family communities, highlighting the importance of plant richness within plant family for a given community. These patterns have potential implications for understanding the interactions of multiple global change drivers as changes in both precipitation and plant community composition do alter whether plant production aboveground is translated belowground biomass.

    

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3. Resources do not limit compensatory response of a tallgrass prairie plant community to the loss of a dominant species

   https://doi.org/10.1111/1365-2745.13741  

   Chaves, Francis A. ; Smith, Melinda D. ( August 2021 , Journal of Ecology)

   The effect of species loss on ecosystem productivity is determined by both the functional contribution of the species lost, and the response of the remaining species in the community. According to the mass ratio hypothesis, the loss of a dominant plant species, which has a larger proportionate contribution to productivity, is expected to exert an overwhelming effect on this important ecosystem function. However, via competitive release, loss of a dominant species can provide the opportunity for other plant species to establish, thrive and become abundant in the community, potentially compensating for the function lost. Furthermore, if resource limitation is removed, then the compensatory response of function to the loss of a dominant species should be greater and more rapid than if resources are more limiting.

   To evaluate how resources may limit compensation of above‐ground productivity to the loss of a dominant plant species, we experimentally removed the C₄perennial tallgrass,Andropogon gerardii, from intact plant communities. We added water for 4 years, as well as nitrogen in the fourth year, to test the effect of resource limitation on the compensatory response.

   Overall, above‐ground biomass production increased in the remaining community with both water and nitrogen addition. However, this increase in biomass production was not sufficient to fully compensate for the loss ofA. gerardii, indicating water and nitrogen were not limiting short‐term compensation in this community.

   Following the removal of the dominant species, there was reordering of species abundances in the community, rather than changes in species richness. The C₄grassBouteloua curtipendulawas the most responsive species, increasing by 57.9% in abundance with water addition and 91.0% with both water and nitrogen addition. Despite this dramatic increase in abundance, its short stature and lower per capita biomass production prevented this species from compensating for the loss ofA. gerardii.

   Synthesis. Short‐term compensation after the loss of a dominant plant species can be hastened by increased resource availability, but ultimately full compensation appears to be limited by the presence and abundance of species in the remaining community that possess traits that allow them compensate for the species lost.

    

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4. Effects of genomic and functional diversity on stand-level productivity and performance of non-native Arabidopsis

   https://doi.org/10.1098/rspb.2020.2041  

   Turner, Kathryn G. ; Lorts, Claire M. ; Haile, Asnake T. ; Lasky, Jesse R. ( October 2020 , Proceedings of the Royal Society B: Biological Sciences)

   null (Ed.)

   Biodiversity can affect the properties of groups of organisms, such as ecosystem function and the persistence of colonizing populations. Genomic data offer a newly available window to diversity, complementary to other measures like taxonomic or phenotypic diversity. We tested whether native genetic diversity in field experimental stands of Arabidopsis thaliana affected their aboveground biomass and fecundity in their colonized range. We constructed some stands of genotypes that we a priori predicted would differ in performance or show overyielding. We found no relationship between genetic diversity and stand total biomass. However, increasing stand genetic diversity increased fecundity in high-resource conditions. Polyculture (multiple genotype) stands consistently yielded less biomass than expected based on the yields of component genotypes in monoculture. This under-yielding was strongest in stands with late-flowering and high biomass genotypes, potentially due to interference competition by these genotypes. Using a new implementation of association mapping, we identified genetic loci whose diversity was associated with stand-level yield, revealing a major flowering time locus associated with under-yielding of polycultures. Our field experiment supports community ecology studies that find a range of diversity-function relationships. Nevertheless, our results suggest diversity in colonizing propagule pools can enhance population fitness. Furthermore, interference competition among genotypes differing in flowering time might limit the advantages of polyculture. 

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5. Short‐ and long‐term effects of nutrient enrichment on salt marsh plant production and microbial community structure

   https://doi.org/10.1111/1365-2745.13756  

   Hanley, Torrance C. ; Bowen, Jennifer L. ; Kearns, Patrick J. ; Hughes, A. Randall ( August 2021 , Journal of Ecology)

   Nutrient enrichment impacts ecosystems globally. Population history, especially past resource environments, of numerically dominant plant species may affect their responses to subsequent changes in nutrient availability. Eutrophication can also alter plant–microbe interactions via direct effects on associated microbial communities or indirect effects on dominant species’ biomass production/allocation as a result of modified plant–soil interactions.

   We combined a greenhouse common garden and a field reciprocal transplant of a salt marsh foundation species (Spartina alterniflora) within a long‐term, whole‐ecosystem, nutrient‐enrichment study to determine whether enrichment affects plant production and microbial community structure differently depending on plant population history. For the greenhouse portion, we collected 20S. alternifloragenotypes—10 from an enriched creek that had received elevated nutrient inputs for 10 years and 10 from an unenriched reference creek—and reared them in a common garden for 1 year. For the field portion, we conducted a 2‐year, fully crossed reciprocal transplant experiment with two gardens each at the enriched and unenriched sites; we examined the effects of source site (i.e. population history), garden site and plant genotype.

   After 2 years, plants in enriched gardens had higher above‐ground biomass and altered below‐ground allocation compared to plants in unenriched gardens. However, performance also depended on plant population history: plants from the enriched site had decreased above‐ground and rhizome production compared to plants from the unenriched site, most notably in unenriched gardens. In addition, almost all above‐ and below‐ground traits varied depending on plant genotypic identity.

   Effects of nutrient enrichment on the associated microbial community were also pronounced. Following 1 year in common garden, microbial community structure varied by plant population history andS. alternifloragenotypic identity. However, at the end of the reciprocal transplant, microbial communities differed primarily between enriched and unenriched gardens.

   Synthesis. Nutrient enrichment can impact plant foundation species and associated soil microbes in the short term. Most importantly, nutrient enrichment can also have long‐lasting effects on plant populations and associated microbial communities that potentially compromise their ability to respond to changing resource conditions in the future.

    

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