Based on the planting date experiment, early cover planting results in a consistently more abundant cover crop. Winter rainfall is increasingly variable in California, and the late planting date subjects the cover crop to additional uncertainty in rain timing and quantity. This issue was evident in 2020, where the late cover crop planting had to be delayed due to wet conditions in January but subsequently received little rainfall after planting and ultimately produced relatively low biomass. The late planting date sometimes was associated with reduced weed biomass, which we attribute to the extra burndown herbicide treatment ahead of late planting. While an extended cover crop growing season may contribute to cover crop abundance and consistency, it also precludes other weed management practices and therefore effectively extends the weed growing season. Likewise, the multi-species cover crop had more consistent biomass compared to the brassica cover crop across year and planting date, but this was not always reflected in consistent reductions in weed biomass.The multi-species cover crop mix emerged more quickly than the brassica mix, and this effect was similar following both the early and late planting date . This effect could be related to certain component species in the multi-species mix that were particularly quick to emerge. In non-treated plots, grow room where cover crops were not planted, weed emergence rates were similar after both early and late planting dates.
However, when cover crops were present, weed emergence was generally slower after the late planting date, especially in plots seeded with the brassica mix. Weed emergence rates after the late planting could have been affected by existing weed cover at time of late planting, due to continuous weed germination and a slow-acting burndown herbicide prior to the late planting date. Variations in weed emergence could additionally contribute to reductions in weed biomass from late planted treatments. Overall, the multi-species cover crop had faster emergence than weedy plants, and the brassica cover crop had similar emergence rates with weedy vegetation. However, quicker emergence did not always lead to enhanced weed suppression, which is consistent with previous studies that suggest that biomass, rather than functional diversity, is the most important factor in weed suppression . While cover crop mixes did not reliably slow weed emergence in this study, their germination uniformity and predictable emergence could make them a useful management tool compared to less predictable weedy vegetation. The sprayed and forage treatments had similarly increased levels of summer weed coverage compared to the three cover crop treatments that left residues in place, which were similar to one another. These results indicate that cover crop residues suppress summer weed emergence compared to treatments without any cover crop or where cover crop residues have been removed through baling.
Cover crop literature in annual cropping systems supports the value of cover crop residue for reducing summer weed emergence . In perennial systems, the spatial separation of the cover crop from the primary crop provides additional options for cover crop termination, including flexibility related to timing, repeated termination actions, and termination equipment. Future research could focus on these under-explored aspects of cover crop management in perennial cropping systems, such as by focusing on high-residue termination methods such as roller crimpers or delayed cover crop termination in the early summer. In this study, we observed that cover crops are not consistently effective as a weed control tool compared to weed management programs with repeated herbicide applications, but they continue to demonstrate value as component of an orchard vegetation management program. Such vegetation management programs allow some plant growth on the orchard floor but result in predictable plant cover and favorable orchard floor conditions for nut harvest. Orchard cover crops flourished under a variety of management programs but were most abundant with timely planting and adequate moisture during establishment. We worked in orchards that had not previously been managed with cover cropping, and any effects of cover crops on weeds could compound over the lifecycle of orchard, possibly mediated through processes like depletion of weed seed banks or weed community filtering. Increased understanding of the broader contributions to ecosystem services, such as soil health and agroecosystem resilience, can enhance the benefit of cover crops and make them an attractive component of integrated orchard management systems.
Portions of this material are based upon work that is supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under agreement number G165-20-W7503 through the Western Region SARE program under project number GW19-194. USDA is an equal opportunity employer and service provider. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the authors and do not necessarily reflect the view of the U.S. Department of Agriculture. No conflicts of interest have been declared. The authors gratefully acknowledge Amélie Gaudin and Vivian Wauters for their critical feedback on these projects. We also acknowledge fieldwork support from Andres Contreras, Matthew Fatino, Guy Kyser, Guelta Laguerre, Katie Martin, and Seth Watkins. This research was conducted on lands that are the home of the Patwin people, who remain committed to the stewardship of this land.Vehicle electrification is currently considered one of the most attractive means of decarbonizing major segments of the transportation sector and can also directly contribute to improvements in urban air quality and public health. In spite of substantial progress and proactive policy support, the environmental impacts of electric vehicles under the wide range of future deployment scenarios are poorly understood. In certain intermediate-term scenarios where EVs reach a much larger share of the fleet and demand a double-digit share of available electric power , marginal CO2 intensity during EV charging times will typically be higher than annual average CO2 rates from the bulk power grid, upon which many current studies base their projections. On a 24-hour basis, this may be true for off-peak periods as well as certain peak periods . In certain conditions, even now, the average emissions assumption breaks down because of the high variability of generation requirements at the hourly or sub-hourly level in peak periods of the day or year. This research explores vehicle-grid interactions with a focus on environmental impacts for future scenarios in which electric vehicles are on a trajectory toward substantial market share . This project has leveraged and expanded a series of unique datasets and high-fidelity sub-system models that have previously stood alone as independent research contributions by the EVALUATE research team, its collaborators, and other researchers in the field. Those models govern vehicle energy consumption, travel demands, vehicle charging, and temporal emission profiles associated with electric power generation dispatch. Along with the expansion of those datasets and sub-system models, one of the most exciting contributions of this effort has been to develop an integrated methodology that enables high fidelity evaluation of emissions, in a systems-of-systems framework. An initial use case has been explored as a means of validating and tuning the methodology, which has generated some valuable insights in its own right. The scope of this work is initially based on a regional case study for a target vehicle classification . This convergence research has revealed important findings relative to the comparative emissions impact of light-duty vehicle charging during various times of day. Such findings may be valuable to an individual vehicle owner. For instance, under certain simulated scenarios, we observe marginal emissions can be as much as 20% lower in the overnight hours compared to marginal CO2 emissions experienced during an identical charging event during the daytime. This finding suggests that it will be essential to adjust and/or coordinate charging schedules to reduce the environmental impacts of EVs. More specifically, drying cannabis to the extent emissions impacts are prioritized among other objectives, individuals and policymakers should be encouraged or incentivized to charge when marginal emissions are lowest whenever possible.
This idea also has important implications for the location, type, and ownership models for tomorrow’s charging infrastructure. Translating and operationalizing this type of guidance will require some combination of education, access to rigorous and clear resources, signals between stakeholders , risk management analyses, and behavioral change. The study has also shed light on the critical nature of assumptions made for the dispatch of electricity generation to meet incremental new demand to charge vehicles. Several related observations are important to note and may be valuable for vehicle owners, researchers, and policymakers. First, our study is aimed at comparative analyses which provide insights into how a marginal assumption for CO2 emissions compares to other marginal assumptions, as well as to prevailing approaches . To our knowledge, this has not been done at this level of granularity. Second, in nearly all cases, marginal CO2 assumptions yield higher CO2 impacts than identical simulations that assume weighted average emissions. This variance is broad, ranging from 22% less to 97% greater, depending on a host of casesensitive factors. The team believes its ability to initially quantify and bound this variance represents an important contribution, as it helps decision-makers quantify how important various assumptions are. The research and its findings are provocative for additional reasons. Weighted average emissions in the U.S. are on a gradual decline, driven primarily by the retirement of coal and the addition of renewables over the past decade. This trend has favorable environmental impacts, because the retirement of high-intensity generation resources means they are less likely used to meet marginal demands, and similarly, the addition of low to no carbon-emitting resources has a commensurate impact on the weighted CO2 intensity of the overall grid. However, it is highly unusual for renewables to be used as the principal means of meeting marginal demand because they are generally considered non-dispatchable. This means grid operators will use fossil-generating resources as a means of meeting incremental load. Better foresight and energy storage are two areas that may eventually change this. On foresight, better awareness between stakeholders will help utilities predict and plan for EV charging events, which could presumably result in more holistic management of environmental impacts. Energy storage, at the moment, accounts for a very small fraction of total electrical demand, and it is therefore considered out of scope for the present study. Finally, despite encouraging trends in emissions for the bulk grid, the steady decline may plateau in the future for several reasons including: if transportation demands a large share of electricity, if costs to deliver electricity from zero to low carbon resources is substantially higher than conventional resources, or if additional electrification occurs at scale within other sectors . Still, by quantifying technical parameters related to both the magnitude and the range of possible emissions impacts as compared to multiple baselines , the study’s findings can be useful for education and awareness by all EV users. They also have clear implications on policy and public investment as mentioned, including the urgent need for managed and coordinated charging, and greater attention to resource planning, in terms of generation resources, dispatch decision-making, infrastructure funding, and the long-run environmental benefits and impacts for EVs across a range of use cases and time horizons. The report concludes with several suggestions for future work, including the need to leverage this methodology to consider grid characteristics relative to energy, emissions, decision-making, and planning out to 2030, and the capability of the tool to be scaled and more broadly adapted for conducting similar analyses in other regions.Vehicle electrification is currently considered one of the most attractive means of decarbonizing major segments of the transportation sector and can also directly contribute to improvements in urban air quality and public health. A key advantage of Electric Vehicles compared to internal combustion engine vehicles is that their carbon and emissions footprint is not fixed based on the vehicle technology from a given past model year, but instead can progressively improve in lockstep with a grid that is evolving toward a cleaner and lower carbon generating mix. Driven in part by policy, declining prices, and product availability, EV deployments are accelerating, having surpassed 1,600,000 total vehicles in the U.S. fleet by August 2020. Though EVs still account for less than 1% of the domestic vehicle fleet, this growth is notable compared to a near-zero baseline in 2010. Significant challenges have been overcome during this “first decade” of commercial adoption, including range limitations, charging infrastructure, total cost of ownership, and public acceptance.