- (Focus Area) Environment
- Author: Kathy Keatley Garvey
So, here you are, a newly eclosed Western tiger swallowtail, Papilio rutulus, eager to sip some nectar from a Mexican sunflower, Tithonia rotundifola, in a Vacaville garden.
It's a warm, windless day, and you're anxious to score, score, score.
You touch down on a Tithonia, but something whizzes by your tails.
Whoa! What was that?
You're startled, alarmed, and irritated. It's a territorial male long-horned bee, probably a Melissodes agilis. He aims to dislodge you from your blossom in his attempt to save the nectar for his would-be girlfriends.
You teeter, then totter, then take off. You touch down on another Tithonia.
Hey! Bee brain! Quit targeting me? Go away!
You head for another blossom, determined to grab a least "a little" nectar.
Stop it! Leave me alone! Go take a vacation!
But the bee isn't about to take a vacation. And he won't allow your "staycation."
Spoiler alert: The butterfly admits defeat and departs the flower garden, exasperated but with tails intact. The bee emerges victorious, its real estate intact.
Score: Bee, 3, Butterfly, 0.
The turf battle is over for today. Tomorrow? That's another day and another battle.
- Author: Michael D Cahn
Introduction
Traditional winter cereal cover crops planted in the Salinas valley have many potential benefits including, scavenging nitrate in the soil profile, increasing organic matter in the soil, and protecting the soil from erosion during storm events. However, when grown for 3 to 4 months during the late fall and winter, cereal rye, triticale, or barley can accumulate 5 to 6 tons of dry matter biomass that must be incorporated into the soil before planting a spring vegetable crop. Tilling in a high amount of cover crop biomass can be disruptive to spring planting schedules. Consequently, only a small fraction of the vegetable ground in the Salinas valley is cover cropped each year.
Previous studies demonstrated alternative strategies can limit the biomass growth of these cereal cover crop species so that they can more easily be tilled into the soil, and therefore less disruptive to spring planting schedules. After fall land preparation, the cereal cover crops are seeded into listed beds and/or in the furrow bottoms. After they become established they can reduce runoff and protect the soil from erosion during early winter storm events. Before the cover crops grow too big, they are terminated with an herbicide to limit the amount of above ground biomass that needs to be incorporated in the spring. For organic systems, planting a mustard cover crop on listed beds or furrows which can be terminated mechanically by mowing is another strategy to limit biomass. A good target for these low biomass cover crops is between 0.5 to 1 ton of dry matter per acre by the date of termination. Once terminated, the biomass begins to decompose. However, the residue on the surface continues to protect the soil from erosion and can significantly increase infiltration from rain events. This helps to leach accumulated salts in the soil as well as recharge groundwater aquifers. The remaining decomposed residue can easily be incorporated into the soil during bed preparation in the spring.
One risk of this low biomass approach is accessing fields during the winter to terminate the cover crop. If soil conditions are too wet or if there is not enough available labor, it may be difficult to fit in a spray application or to run a flail mower. This termination step also increases the cost of managing the cover crop. A possible solution is to use species that grow slowly during the winter when temperatures are cold. Sudangrass and sorghum-sudangrass hybrid are warm season adapted species that could be used in this low biomass approach to managing winter cover crops.
Field trial with warm season adapted cover crop species
A field trial was conducted with sudangrass and sorghum-sudangrass in the 2023-2024 winter to evaluate biomass growth, and the effect on storm water runoff and soil erosion compared to bare-fallow plots. The site was located on an Arroyo Seco gravelly loam soil with a slope of more than 5%. Plots measuring 1050 ft in length by four 40-inch wide peaked beds were planted with either sudangrass, sorghum sudangrass hybrid, or left bare fallow. Treatments were replicated 4 times. The cover crops were seeded at 60 to 80 lbs/acre on October 4th and were subsequently sprinkle-irrigated several times. Total water applied for establishment was 2.6 inches. One application of the herbicide Bromoxnil (Maestro) was applied about 45 days after planting to kill emerged broadleaf weeds. Air temperature and rainfall were monitored at the field site and flumes were installed in late November at the end of each plot for measuring runoff during winter storm events (Fig. 1). The flumes were equipment with automated sampling pumps that could collect runoff during storm events. Runoff samples were evaluated for sediment and nutrient concentration at the UC Davis Analytical Laboratory.
Results
Above ground biomass, N uptake, and carbon accumulation
Both cover crops had limited biomass growth, accumulating only 0.35 to 0.5 tons/acre of dry matter by early January and less than 1 ton/acre by mid March (Table 1). Growth was set back by cold conditions that occurred from mid November through early January, occasionally reaching freezing temperatures which caused damage to leaves (Fig. 1). However, the freezing temperatures lasted only a few hours and were not severe enough to kill the cover crops (Fig. 2). By March 13th the cover crops had taken up 45 to 55 lbs N/acre and had a carbon to nitrogen ratio of 15. The C:N ratio of 15 would suggest that after soil incorporation the residue would slowly break down and release N for the following vegetable crop.
Runoff, rainfall infiltration, and control of soil erosion
Total rainfall measured at the trial site was 10.2 inches for the winter season. The most intense period of rainfall occurred in late January and early February which resulted in several runoff events (Fig. 3). During this period about 50% of the rainfall in the bare fallow plots was lost as runoff compared to 15% lost as runoff in the cover crop plots (Fig.4). Over the entire winter season, runoff was reduced by an average of 70% under the cover cropped plots compared to the bare fallow plots, and significantly more rainfall was infiltrated into the ground in the cover cropped plots. In addition, suspended sediment concentration was 90% and 77% less in the sudangrass and sorghum-sudangrass cover crop plots, respectively, compared to the bare plots. Turbidity, total P, and total N concentration in the runoff were also reduced under the cover crop plots compared to the bare fallow plots (Table 2).
Seasonal soil erosion losses could be calculated based on the volume of the runoff and sediment concentration in the runoff. The total loss of sediment averaged more than 3500 lbs per acre in the bare fallow plots during the winter, while erosion losses were reduced by 96% to 98% in the sorghum-sudangrass and sudangrass plots (Fig. 5). Total N losses were reduced by 83% to 86% in the cover crop plots compared to the fallow plots, and total P losses were reduce by 81% to 85% in the cover cropped plots compared to the bare fallow plots.
Conclusions
The use of warm season species such as sudangrass and sorghum-sudangrass hybrids as winter cover crops provides several advantages compared to planting cereal cover crops. The biomass growth through the winter is self-limiting due to the cold conditions that typically occur in the Salinas Valley. Because the final biomass would likely be less than 1 ton per acre, these species can be planted on listed beds in the fall rather than on flat ground. This means that in the spring, the remaining cover crop can be lillistoned into the peaked beds a few weeks before final bedshaping and planting. Cover crops planted on flat ground and have high amounts of biomass usually require many tillage passes to prepare ground for planting. Despite, having less biomass than traditional winter cereal species, sudangrass and sorghum-sudangrass hybrid cover crops provided excellent erosion control compared to leaving the ground bare, and increased infiltration of rainfall during storm events. Also these species may be able to scavenge significant amounts of nitrogen from the soil which can limit nitrate leaching during the winter months. We plan to conduct a second year of field trials with these warm season species to continue evaluating this approach to managing winter cover crops in vegetable systems.
Acknowledgments
This project was funded by the California Leafy Greens Research Board.
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- Author: Kathy Keatley Garvey
The seminars begin Monday afternoon, Sept. 30 and continue every Monday through Dec. 2.
Nematologist Amanda Hodson, assistant professor of soil ecology and pest management, is coordinating the seminars. All, except one, will be held in Briggs Hall. All, but one, will be on Zoom.
The Zoom link:
https://ucdavis.zoom.us/j/95882849672.
Michael Hoffmann, professor emeritus, Cornell University, will deliver the Thomas and Nina Leigh Distinguished Alumni Award Seminar in the Putah Creek Lodge at 4 p.m. on Oct. 14. (See below)
The list of seminars:
Monday, Sept. 30, 4:10 to 5 p.m., 122 Briggs
Kyle Wickings
Department of Entomology, Cornell University
Title: “Composition and Function of Soil Invertebrate Communities in Residential Greenspaces”
Monday, Oct. 7, 4:10 to 5 p.m., 122 Briggs
Juliana Rangel Posada
Professor of Apiculture, Department of Entomology, Texas A&M University
Title: “Don't Compromise: Food Lipid Content Shapes Protein-Lipid Regulation in Honey Bee (Apis mellifera) Nurses”
Monday, Oct. 14, 4 p.m. to 7 p.m. Thomas and Nina Leigh Distinguished Alumni Award Seminar
Michael Hoffmann
Professor Emeritus, Cornell University
Title: “Our Changing Menu: Using the Power of Food to Confront Climate Change”
This will take place beginning at 4 p.m. in the Putah Creek Lodge and will include a social, lecture and dinner. Reservations closed. (See more)
Monday, Oct. 21, 4:10 to 5 p.m.,122 Briggs
Andrew Corbett
Research Affiliate, UC Davis Department of Entomology and Nematology (formerly with the lab of UC Davis distinguished professor Jay Rosenheim, now emeritus)
Title: "In Silico Experiments with the Effect of Natural Habitats on Biological Control in Agricultural Landscapes."
Monday, Oct. 28, 4:10 to 5 p.m., 122 Briggs
Jolene Saldivar
UC Davis Chancellor's Postdoctoral Fellow, lab of Professor Louie Yang
Title: "Disturbance in Coastal Sage Scrub and the Implications for Migratory Butterflies”
Monday, Nov. 4, 4:10 to 5 p.m., 122 Briggs
Eliza Litsey (exit seminar)
Litsey, a former graduate student in the honey bee lab of Elina Niño, UC Davis Department of Entomology, received her master's degree in entomology in June 2024 and is now a laboratory technician at the lab of research entomologist Julia Fine, USDA/ARS, Davis. Litzey also holds a bachelor's degree from UC Davis.)
Monday, Nov. 18, 122 Briggs (in-person only; will not on Zoom)
Andre Custodio Franco
Assistant Professor, Indiana University Bloomington
Title: "Deciphering the Soil Macrobiome: Belowground Communities Driving Ecosystem Responses to Global Change”
Monday, Nov. 25, 4:10 to 5 p.m., 122 Briggs
Christine Sprunger
Associate Professor of Soil Health at Michigan State University
Title: "Nematodes as Bioindicators of Soil Health and Climate Resiliency”
Monday, Dec. 2, 4:10 to 5 p.m., 122 Briggs
Inga Zasada
Research Plant Pathologist, USDA-ARS
Title: "How an Applied Nematolgist Uses Genomic Tools to Address Plant-Parasitic Nematode Research”
For more information, contact Hodson at akhodson@ucdavis.edu
- Author: Kathy Keatley Garvey
On Sept. 6, 2016, it happened.
A monarch fluttered into our pollinator garden in Vacaville and touched down on a Mexican sunflower, Tithonia rotundifola.
It wasn't just "any ol' monarch"--if there's ever such a thing as "any ol' monarch."
This one, tagged with my alma mater, Washington State University, came from Ashland, Ore., as part of a migratory monarch research project launched by entomologist David James.
The tag's serial number read “Monarch@wsu.edu A6093.” It hung around for about five hours and then left.
James, an associate professor at Washington State University, studies the migration routes and overwintering sites of the Pacific Northwest Monarch population, which overwinter primarily in coastal California. (Access his Facebook page, Monarch Butterflies in the Pacific Northwest, for his latest research.)
When we emailed him, we learned that citizen scientist Steven Johnson of Ashland tagged and released the monarch on Sunday, Aug. 28.
"So, assuming it didn't travel much on the day you saw it, it flew 285 miles in 7 days or about 40.7 miles per day," James told us. "Pretty amazing. So, I doubt he broke his journey for much more than the five hours you watched him--he could be 100 miles further south by now."
Repeat: 285 miles in 7 days, or 40.7 miles per day. Incredible.
Fast forward to today. It's the anniversary of the sighting of A6093.
Any sightings today? Not. A. Single. One.
And not a single sighting of a tagged monarch since Sept. 6, 2016.
- Author: Grace Nguyen-Sovan Dean
California's forests have long been adapted to fire, where the presence of regular, low-severity fires helped maintain forest health. After decades of fire suppression, many private forest landowners are interested in reintroducing fire to their landscape through prescribed burns. When planning for a prescribed fire, landowners must consider a variety of factors, including the age of their trees.
A new study from Hunter Noble (University of Nevada, Reno) and Rob York (UCCE) sheds insight on how prescribed fire affects stands of varying ages. The 2024 paper is a continuation of research conducted at Blodgett Forest Research Station following a 2018 prescribed burn. The new findings provide crucial information for Sierra Mixed-Conifer (SMC) land managers who seek to implement prescribed fire in young forests.
For the tree species in the SMC forest type (Douglas fir, ponderosa pine, sugar pine, incense cedar, white fir and giant sequoia), low-severity fire is a natural part of the ecological process. Reintroducing fire to young, reforested SMC stands can help protect areas burned by high-severity fires from future “reburn” fires by reducing fuel. This study seeks to help answer the question: what is the earliest age you can burn a stand of trees?
In this study, tree mortality rates among 12, 22, and 32-year old stands at UC Berkeley's Blodgett Research Station were observed two years post-burn. There is little known about the effects of prescribed fire on young trees, as prescribed burns are often used to treat older trees with more fire-resistant characteristics. However, understanding when fire can be reintroduced to young stands is critical for those in California managing reforested, post-wildfire landscapes.
When surveying trees in each age class, researchers found that the 32-year old stands experienced the lowest rate of tree mortality (78% of trees survived), whereas the 12-year old stands experienced the highest (31% of trees survived). The 22-year old trees had a 63% survival rate.
An important consideration is that burn conditions may have greatly contributed to the recorded high mortality rate among the 12-year stands. The 2018 burn was conducted at the end of the burn prescription, meaning conditions were hotter and drier than is typical. York and Noble described these mortality results as a “worst case scenario”, referencing a previous study that described a 0-24% mortality rate for a similarly aged stand. However, the authors note that a high mortality rate may not necessarily be undesirable if one's management goal is to create a “low-density, high-complexity stand...similar to historic conditions.”
For those managing post-fire landscapes, utilizing prescribed fire is beneficial towards preventing reburns and can work in harmony with reforestation treatments. However, as outlined in the study, burning under different conditions can significantly affect tree mortality, yielding higher or lower rates. York and Noble conclude that when land managers seek to implement prescribed fire, identifying an acceptable level of tree mortality is key, and burning under the right conditions can lessen fuel loads without sacrificing tree survival in the years to come.
Read the full article here: https://doi.org/10.3733/001c.117485