UCCE Ventura County- Blogs

Figuring the Loss of Trees

UC Davis Ag Econ Department has put together a set of crop loss calculators in Excel.  They are generic, but also specific to lemon and avocado.  If you have had damage from the recent fire, take a look at them.  At this point it may be too early to actually know the extent of the damage, but keep these in mind for the future.

Generic calculator: https://coststudies.ucdavis.edu/tree-and-vine-loss-calculators

Avocado:  Avocado Trees, San Diego County (xls)

Lemon:  Lemon Trees, Ventura County (xls)

 

scorched tree

Fire in the Orchard

fire coming into orchard

The Soil Microbiome

It promises to be an outstanding seminar.

André Custodio Franco, assistant professor, Indiana University, Bloomington, will speak on "Deciphering the Soil Macrobiome: Belowground Communities Driving Ecosystem Responses to Global Change" at a seminar hosted Monday, Nov. 18 by the UC Davis Department of Entomology and Nematology.

It's set or 4:10 p.m. in Room 122 of Briggs Hall, UC Davis campus. It also will be on Zoom. The Zoom link: 
https://ucdavis.zoom.us/j/95882849672.

His seminar will then be archived on the ENT seminar page.

"Growing scientific evidence indicates that soil fauna and the ecosystem functions and services they provide are threatened by global environmental changes such as those related to climate and land use," Franco says in his abstract. "These threats affect species diversity and functional groups in belowground food webs, impacting ecosystem services that all life depends on. A better understanding of global change impacts on interactions among soil fauna communities and plants, as well as on physical and geochemical processes driven by soil fauna will enhance our ability to predict changes at the ecosystem level."

He will discuss "recent evidence indicating that soil fauna responses may challenge aboveground-based predictions of ecosystem functioning under global change."

Franco joined the Paul O'Neill School of Public and Environmental Affairs, Indiana University in 2012 after serving as a research scientist at the Department of Biology, Colorado State University, from 2018 to 2022, where he was an International Presidential Fellow.  Other honors include Senior Global Futures Fellow, Julie Ann Wrigley Global Futures Laboratory.

Franco focuses his research on how modern global environmental changes are affecting soils from tropical crop fields and rainforests to temperate grasslands, according to a faculty post. "His work has contributed to unraveling fundamental roles of microscopic soil animals, such as nematodes, and larger ones such as earthworms and termites, in influencing ecosystem responses to changing climate and land-use regimes."

His areas of interest include soil ecology, soil-plant interactions, ecosystem functioning, global environmental changes, sustainability and sustainable development.

Franco holds a bachelor's degree in agronomy (2008) from Universidade Estadual de Santa Cruz, Brazil; a master's degree (2012) in bioenergy, Universidade Estadual de Londrina, Brazil; and a doctorate (2015) in soil science from the University of São Paulo, Brazil. His dissertation: "Soil Engineering by Macroinvertebrates: Controls on Soil Organic Matter Storage Across Land Use."

Nematologist Amanda Hodson, assistant professor of soil ecology and pest management, is coordinating the ENT seminars. The full list is here. For more information or for technical issues, contact Hodson at akhodson@ucdavis.edu.

handfull of dirt

Irrigation in a Freeze

Confronted with approaching freezing conditions a grower has several options to mitigate the potential cold damage. There are foliar sprays like copper that can reduce the incidence and need to be applied several weeks in advance of cold. There are orchard heaters that are still allowed in some growing areas but tend to be expensive to run due to fuel costs. There are wind machines that are capable, but less effective on avocado hillsides that have natural air movement. Then there is irrigation.

Of course you need healthy, well-watered trees to protect, otherwise, it is probably not worth the effort. And you need water that is dispersed in the air. Drip is much reduced in efficacy compared to microsprinklers and much less than high pressure overhead sprinklers. And you need water volumes that can be used continuously over the protected area during the freezing period.

Using irrigation water in frost control is a delicate balance among different physical characteristics of water. When it freezes, from a liquid state to the solid ice state, heat is released. More than what is actually making up the temperature of the liquid water. But then, in its liquid state, it starts evaporating turning to a gas which cools the surroundings. Heat from liquid to solid and cooling from liquid to gas. There's another property of water which is that it conducts heat really well, better than solid earth. So, wet soil heats up better from the daytime sun than a dry one.

So, you want to make sure the soil is moist during the day to soak up the heat. Then, you don't want water being applied after sunset to avoid evaporative cooling. Then when the trigger temperature for freezing occurs, the system should be run continuously so that heat is released during the freezing cycle. If you stop the water, then evaporative cooling kicks in and it could be colder than it would be if you had not run the water. And sometimes the emitters freeze up if they are turned off, and then they don't function when you try to come back around.

So, with that, you need to decide how much water you can run continuously in a given area. So, what is the coldest spot, and can you cover that area continuously for the cold period? Or the reverse is, if you know the cold area is going to get really cold and it may not make it through the cold even with irrigation, what is the area you want to protect that you know can be helped with irrigation? The water needs to run continuously. You don't want to be turning it on and off in order to roll it over to other irrigation blocks. When the water is turned off, the air starts cooling from evaporation. So decide how much area can be watered with the given volume.

Knowing all this you start watching for the cold with low temperatures that show 32 or lower. Older trees with canopies to the ground can handle more cold than young trees with little canopy to retain heat. Watch for the dew point. If it shows something much below 32, like 25, that means the air is dry and there will be a rapid temperature drop once temperatures start heading for the low temperature.

So, you are forewarned. Get water on to the grove to make sure the trees are adequately hydrated. Then a couple of days before the freeze event, make sure the surface soil is wetted during the day to take advantage of daytime heat, but make sure the water is off before sunset. Then when the big night hits, when temperatures hit a trigger temperature like 33 or 34, you should start the water. You want to have the system going before it drops below 32. Run the system until sunrise, and then you can most likely shut down. And wait for the next night and follow the same drill. We often will have two to three nights in a row that need protection.

Hopefully there's enough water for successive nights. And hopefully, it's not so much that root rot becomes an issue. Isn't farming fun?

 

 

 

frost satsuma irrigation

Wild Fire and Soil

Low-severity wildland fires and prescribed burns have long been presumed by scientists and resource managers to be harmless to soils, but this may not be the case, new research shows.

According to two new studies by a team from the University of California, Merced (UCM) and the Desert Research Institute (DRI), low-severity burns - in which fire moves quickly and soil temperature does not exceed 250oC (482oF) - cause damage to soil structure and organic matter in ways that are not immediately apparent after a fire.

"When you have a high-severity fire, you burn off the organic matter from the soil and the impact is immediate," said Teamrat Ghezzehei, Ph.D., principal investigator of the two studies and Associate Professor of Environmental Soil Physics at UCM. "In a low-severity fire, the organic matter doesn't burn off, and there is no visible destruction right away. But the burning weakens the soil structure, and unless you come back at a later time and carefully look at the soil, you wouldn't notice the damage."

DRI researcher Markus Berli, Ph.D., Associate Research Professor of Environmental Science, became interested in studying this phenomenon while visiting a burned area near Ely, Nev. in 2009, where he made the unexpected observation that a prescribed, low-severity fire had resulted in soil structure damage in the burned area. He and several colleagues from DRI conducted a follow-up study on another controlled burn in the area, and found that soil structure that appeared to be fine immediately after a fire but deteriorated over the weeks and months that followed. Berli then teamed up with Ghezzehei and a team from UCM that included graduate student Mathew Jian, and Associate Professor Asmeret Asefaw Berhe, Ph.D., to further investigate.

Soil consists of large and small mineral particles (gravel, sand, silt, and clay) which are bound together by organic matter, water and other materials to form aggregates. When soil aggregates are exposed to severe fires, the organic matter burns, altering the physical structure of the soil and increasing the risk of erosion in burned areas. In low-severity burn areas where organic matter doesn't experience significant losses, the team wondered if the soil structure was being degraded by another process, such as by the boiling of water held within soil aggregates?

In a study published in AGU Geophysical Research Letters in May 2018, the UCM-DRI team investigated this question, using soil samples from an unburned forest area in Mariposa County, Calif. and from unburned shrubland in Clark County, Nev. to analyze the impacts of low-severity fires on soil structure. They heated soil aggregates to temperatures that simulated the conditions of a low-severity fire (175oC/347oF) over a 15-minute period, then looked for changes in the soil's internal pore pressure and tensile strength (the force required to pull the aggregate apart).

During the experiment, they observed that pore pressure within the soil aggregates rose to a peak as water boiled and vaporized, then dropped as the bonds in the soil aggregates broke and vapor escaped. Tensile strength measurements showed that the wetter soil aggregates had been weakened more than drier soil samples during this process.

"Our results show that the heat produced by low-severity fires is actually enough to do damage to soil structure, and that the damage is worse if the soils are wet," Berli explained. "This is important information for resource managers because it implies that prescribed burns and other fires that occur during wetter times of year may be more harmful to soils than fires that occur during dry times."Next, the research team wondered what the impact of this structural degradation was on the organic matter that the soil structure normally protects. Soil organic matter consists primarily of microbes and decomposing plant tissue, and contributes to the overall stability and water-holding capacity of soils.

In a second study that was published in Frontiers in Environmental Science in late July, the UCM-DRI research team conducted simulated burn experiments to weaken the structure of the soil aggregates, and tested the soils for changes in quality and quantity of several types of organic matter over a 70-day period.

They found that heating of soils led to the release of organic carbon into the atmosphere as CO2 during the weeks and months after the fire, and again found that the highest levels of degradation occurred in soils that were moist. This loss of organic carbon is important for several reasons, Ghezzehei explained.

"The loss of organic matter from soil to the atmosphere directly contributes to climate change, because that carbon is released as CO2," Ghezzehei said. "Organic matter that is lost due to fires is also the most important reserve of nutrients for soil micro-organisms, and it is the glue that holds soil aggregates together. Once you lose the structure, there are a lot of other things that happen. For example, infiltration becomes slower, you get more runoff, you have erosion."

Although the research team's findings showed several detrimental effects of fire on soils, low-severity wildfires and prescribed burns are known to benefit ecosystems in other ways -- recycling nutrients back into the soil and getting rid of overgrown vegetation, for example. It is not yet clear whether the negative impacts on soil associated with these low-severity fires outweigh the positives, Berli says, but the team hopes that their research results will help to inform land managers as they manage wildfires and plan prescribed burns.

"There is very little fuel in arid and semi-arid areas, and thus fires tend to be short lived and relatively low in peak temperature," Ghezzehei said. "In contrast to the hot fires and that burn for days and weeks that we see in the news, these seem to be benign and we usually treat them as such. Our work shows that low-severity fires are not as harmless as they may appear."

The study, "Soil Structural Degradation During Low?Severity Burns," was published on May 31, 2018 in the journal AGU Geophysical Research Letters and is available here: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018GL078053.

The study, "Vulnerability of Physically Protected Soil Organic Carbon to Loss Under Low Severity Fires," was published July 19, 2018 in the journal Frontiers in Environmental Science, and is available here: https://www.frontiersin.org/articles/10.3389/fenvs.2018.00066/full.

 

 

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