Experiments can go beyond studying if plant traits have an effect to testing what these effects are . These experiments could also take a microbial trait-based approach to host filtering and identify the genes required for successful transmission, which are still largely unknown . This could provide valuable insights into the genes required for transmission across the different pathways . Furthermore, metagenomic analyses across plants, populations, species, etc., could determine if these transmission-associated genes are common across meta communities. Such information could show if there is functional conservation across microbial communities, even if they are taxonomically variable. Finally, micro-scale experiments can also test how microbial community assembly is impacted by the interplay between deterministic and stochastic processes. In addition to these tests of microbial and plant trait impacts, experiments testing the role of dispersal in seed microbial community assembly among spatial scales should be conducted. At the micro-scale, nft growing system experiments using synthetic microbial communities on stigmas with varying chemistry and topography can demonstrate how dispersal and selection occur between flowers and seeds, and what the role is of plant genetics and microbial adaptations.
At the macro-scale, pollinator exclusion experiments similar to those in Prado et al. could be conducted across sites in natural landscapes. By using sites at varying distances and connectivity levels from each other, and analyzing both within- and among-site seed microbial community variation, one may obtain new information about how pollinators and patch connectivity impact multi-scale dispersal ability. These proposed studies would elucidate how dispersal contributes to meta community assembly among spatial scales. Along with these single-process studies, we envision studying the interactions between processes through both observational and experimental studies. As JSDMs continue to be refined to model nested and continuous meta communities, they will provide a way to analyze seed microbiome patterns and their associated assembly processes that is more sophisticated than previous modeling approaches. Additionally, priority effect experiments conducted at multiple points in the seed life cycle may reveal how historical contingencies impact seed microbiome assembly throughout the seed life cycle. Such experiments would also test the Primary Symbiont Hypothesis , which argues that seed communities are dominated by a single microbe with significant functional consequences for the plant. Finally, questions will need to be asked about seed microbiome assembly that go beyond just testing for spatial mechanisms.
Primary among these questions is: what fitness benefit does transmission into seeds provide to microbes and their host plants? Such a question gets at the eco-evolutionary dynamics in these microbial meta communities, which can have long-term consequences for both microbes and plants. Because microbial communities behave and evolve at shorter time-scales than macro-organisms , it is feasible to design simple experiments testing how microbes evolve in response to plant defenses, nutrient availability, and micromorphology. Such eco-evolutionary studies may have applications in understanding microbial community shifts with crop domestication . Additionally, both microbes and seeds have dormant stages, which can impact meta community dynamics through tradeoffs with dispersal and delayed responses to environmental conditions . The role of dormancy in seed and plant microbial meta community assembly has yet to be explored, so there is much room to study how dormancy impacts these systems over longer temporal scales. Finally, a hot topic in plant microbiome research is how to modify plant microbial communities for climate resilience and other beneficial traits . However, the impacts of climate change-associated disturbances on plant microbiomes have been limited to pattern-based studies in leaves and roots . As such, more work can be done on how disturbances alter seed microbiome assembly processes and outcomes.
Almond is one of California’s most important crops , with an estimated output of $21.5 billion to California’s economic activity. Despite evidence of the benefits of cover cropping on other woody perennial systems globally, cover cropping has never been widely implemented in California . Recognizing California’s unique agroenvironmental history is central to understanding how cover crop adoption evolved in CA almond systems. CA is the home of environmental pioneers: John Muir , founder of the US National Parks and renowned ecologist, Mary Hunter Austin , acclaimed environmental novelist and Gary Snyder , a distinguished writer and environmental activist. Key written work foundational to American environmental history emerged from the minds and sensitivities of Californian pioneers . As a state, CA demonstrated an early engagement in the politics of conservation and environmental protection and is now at the forefront of climate change policies and the Climate Smart Agriculture discourse in the UnitedStates . It was the first state to certify organic agriculture. CA is also un-paralleled in its manipulation of the natural environment. Through the Central Valley Project, started in 1933, CA has effectively diverted water resources across the state through a complex network of federal, state, and local infrastructures, allowing for vast acreages to be claimed for agriculture. CA’s agrarian history is particularly exceptional in that it was never predominantly constituted of small landholders. Instead, large-scale landholdings emerged as the legacy of Spanish and Mexican land grant systems and were subsequently reinforced by GoldRush generated wealth . These immense landholdings were originally utilized for large-scale wheat production and grazing until the droughts of the late 1800s, which instigated the redistribution of land into smaller tenancies. Then between 1890-1914, facilitated by the emerging technical expertise of immigrants, development of irrigation projects, cooperative marketing and cheap labor, Californian land was gradually converted into intensive specialty crop farms and high value perennial production systems . The first records of almonds D.A. Webb planted in CA date back to 1853 . Even though it was recognized that irrigation could enhance almond productivity, growers did not start using irrigation until the 1930s . Major growth in almond production occurred from 1964 to 1985, vertical hydroponic nft system supported by almond post-harvest product development and strong marketing schemes. Californian agriculture particularly benefited from the early development of unified growers’ organizations, many of which were commodity-based. These unions played an important role in regulating their growing industries. One such organization was the Almond Board of California , founded in 1950 as a non-profit organization that substantially contributed to the expansion of the almond industry. Suchcommodity-based grower organizations participated in shaping irrigation districts, resulting in increased irrigated land, a key factor in facilitating the expansion of high-value specialty crops in the Central Valley . With vertical integration, water rights acquisitions and immense wealth potential in almond production, farm management structures stratified making way for tenant and contract farming, thereby making it possible for key stakeholders to live remotely from the land . Today, the CA almond industry continues its relentless growth: it now encompasses 1,530,000 acres and accounts for 84% of the global production . Central to California’s agricultural growth is the state’s early development of strong science-based extension through the Land Grant and Cooperative Extension systems built by the Morrill Acts of 1962-1890, the Hatch Act of 1887, and the Smith-Lever Act of 1914.
These information structures spurred the early experimentation of cover cropping in almond. Early records of cover crop trials in orchard systems date back to the 1920s, conducted by the University of California Cooperative Extension . Growers’ objectives in the early use of cover crops in the understories of orchards were “soil conservation and to prolong the life of agriculture” . However, due to concerns over potential increases in water usage and the provision of refuge for pests, combined with the advancement of synthetic fertilizers in the 1900s, cover crops were largely abandoned. In 1994, the BIOS project led by a growers’ alliance built upon previous efforts and developed participatory growers’ networks to demonstrate the possibility for pesticide and synthetic fertilizer reduction by cover cropping. In contrast with early records of soil-focused objectives for cover cropping, BIOS explored other associated ecosystem services, particularly the biological control of insect pests as a key motive to adoption . However, despite early efforts to adapt this practice to Californian almond systems, there is still low adoption of cover cropping, due to remaining concerns, and the forceful pull towards industrialized models to optimize yields. A recent survey by the Almond Board of California indicates that in 2016 only about 6% of almond growers include cover crops in their farming systems . Despite scientific evidence of the benefits of cover cropping, lags in cover crop adoption have been reported in many parts of the United States . We consider that the gap between scientific evidence and the actual uptake of sustainable practices hinders progress. Surveys have been conducted to identify farmers’ concerns and the actual barriers of cover cropping . These works are important to identify the needs of farmers and to inform the development of cover crop research. However, as most farmers’ surveys have been focused on annual systems, there remain important knowledge gaps specific to perennial systems and to Californian farmland. We believe that understanding farmers’ concerns and needs with regards to cover cropping is key to address existing lags in adoption. The objective of our study was to determine differences in users and non-users’ perception of benefits and tradeoffs of cover cropping, to identify how grower characteristics influence the perception of benefits and tradeoffs, and thereby adoption, to survey currently-used cover crop management practices in almond orchards, to identify the most important sources of information about cover crops for growers, and to define most important knowledge gaps to inform research on cover crop adoption in almond production in the Central Valley. Respondents were asked to evaluate the importance of a suite of cover crop benefits pertaining to ecosystem regulating services , supporting services, below ground pest control and weed control) and provisioning services . The information drawn from this survey will provide a platform to tailor research agendas, so as to bring relevant and applicable information that addresses growers’ data needs. Finally, this research will provide valuable information for the development of effective policy measures as well as guide future science extension efforts.A questionnaire was designed in 2017 following the Tailored Design Method and using Qualtrics ©, a cloud-based survey software. Participation was anonymous and voluntary. Questions were chosen to meet four main objectives. In a first section, respondents were asked to evaluate potential opportunities provided by cover cropping in almond systems. In a second section, respondents were asked to define potential barriers respective to ecosystem tradeoffs, which could offset short-term agronomic productivity, the practicality of integrating cover cropping operations in almond orchard systems and potential economic setbacks of cover cropping. In a third section, questions provided insight on currently-used cover crop management practices across the Central Valley and their respective popularity among users. In a final section, the survey ranked farmers’ performance among a list of information sources. Growers were asked to assess current gaps in knowledge, which could be considered as information barriers to cover cropadoption. Questions included Likert-scale, ranking, binary, multiple and single choice, and openended questions. A copy of the survey questions is available in Appendix A – Supplementary Information 1.1.The target group of respondents are individuals involved in the decision-making process of almond farms: owners, managers and/or operators. The target group of respondents did not include external farm advisors nor extension specialists. Respondents of the survey were asked to identify themselves as either users or non-users of cover crops. Two separate questionnaires were developed to target each audience. Questions addressed to both users and non-users sought to identify perceived benefits and concerns associated with cover cropping. The questionnaire addressed specifically to users included questions relating to their experience with cover crops and to their management practices . The definition of users specified in the survey was: “growers having cover crops grown on either part or all of their acreage with a minimum of 1 acre, in at least one growing season in the past five years”. Respondents were not incentivized nor compensated for completing the survey. Considering that relative benefits and tradeoffs of cover crops may vary among CA’s ecozones, respondents were asked to identify the county in which they grow almonds.An IRB human subjects’ approval was obtained for this study for the graduate student and PI. The survey questions were communicated to the Almond Board of California and sent for distribution in December 2017. The paper-version of the survey was distributed for two years at the Almond Board of California Annual Conference.