Thelma Hansen Symposium to Explore the Future of Water in Agriculture
Webinar Series

Presented by The Hansen Agricultural Research & Extension Center

Three Consecutive Days of Discourse
Each webinar will open to participants 5 minutes before 4:00 PM.

See the Full Schedule: https://ucanr.edu/sites/PSU/files/383979.pdf

REGISTER HERE

Tuesday, May 23: Water Availability
4:00 PMWelcome—Annemiek Schilder, PhD, Director UCCE/HAREC Ventura County
4:05Opening remarks—Kelly Long, Supervisor Ventura County District 3
4:15Long-term forecasting of trends in California water management—Erik Porse, PhD, Director,
UC ANR, California Institute for Water Resources
4:50Groundwater level changes and water well drilling along California's Central Coast and
around the globe—Scott Jasechko, PhD, Bren School of Environmental Science &
Management, UC Santa Barbara
5:30Closing—Annemiek Schilder

Wednesday, May 24: Water Quality
4:00 PMWelcome—Annemiek Schilder, Director UCCE/HAREC Ventura County
4:05Focus on regional water quality—Norma Camacho, Chair, Los Angeles Regional Water
Quality Control Board
4:20Nitrate, pesticides, and sustainable groundwater quality management in agricultural
landscapes—Thomas Harter, PhD, Department of Land, Air and Water Resources,UC Davis
5:05New regulations affect water quality management in Ventura County—Jodi Switzer, Ventura
County Agricultural Irrigated Lands Group
5:30Closing—Annemiek Schilder

Thursday, May 25: Water Economics
4:00 PMWelcome—Annemiek Schilder, Director UCCE/HAREC, Ventura County
4:05Economics of water in California agriculture—Richard Howitt, PhD, Emeritus Professor,
Center for Watershed Sciences, UC Davis
4:50Water Markets: the importance of good design—Matthew Fienup, PhD, Executive Director,
Center for Economic Research & Forecasting, California Lutheran University
5:30Closing—Annemiek Schilder

To maximize profits in the early navel orange market, growers need to have large fruit size and sufficient yellow-orange color and a high enough sugar-acid ratio to meet or exceed the legal minimum harvesting standards. Growers of early-maturing navel oranges in Kern County use different strategies to produce these oranges. Some growers irrigate at full evapotranspiration rates nearly up to harvest with the belief this will maximize fruit size, while others begin deficit irrigating a month or two prior to harvest to maximize development of sugar and color to promote earlier maturity. Little information exists in the literature to assist growers in making decisions related to producing early maturing navels such as Beck, Fukumoto and Thompson Improved. To determine the effects of late season irrigation stress, I, along with two University of California co-researchers Blake Sanden and Dr. Mary Lu Arpaia, participated in an experiment to elucidate some of the trade-offs that relate to irrigation strategies and early navel fruit production. The research was conducted from 2006 through 2008 in a cooperating grower's Beck orchard at the extreme southern end of the San Joaquin Valley. Our generous and patient cooperating growers were George and Colby Fry.

Three different irrigation treatments, defined as low, mid and high, were developed based on the relative amounts of irrigation water applied to the test plots. Each plot consisted of 10 trees in a central row, bordered by ten similarly irrigated trees in the two adjacent rows. Each treatment was replicated five times. The same irrigation treatment was applied to the same plots for the first two years, while in the third year the low treatment was changed to the high treatment to provide information on how rapidly the trees would recover from stress. The different irrigation treatments were administered by using irrigation emitters with different flow rates and by differentially shutting off water to some treatments as needed to achieve desired stress levels. Between growing seasons, the top three feet of soil profile was refilled with water during the winter and differential irrigation began in early August. Measurable differences in tree shaded stem water potential among treatment usually were noted by early September. In the second year of the experiment (2007), the low and mid-irrigation treatments applied approximately 38 and 71 percent, respectively on average, of the water of the high treatment. Water potential measurements made mid-day on shaded, interior leaves demonstrated that good separation was achieved among the three differential treatments. In 2007, for example, shaded stem water potential measurement in early September were about -9, -12, and -18 bars for the high, mid and low irrigation treatments, respectively and at harvest in mid-October were -12, -18, -24, respectively. Neutron probe measurements also demonstrated that trees differentially depleted available water stored in the soil as the season progressed (data not shown). In 2007, differences in applied water among the treatments were large. Including the increased quantity of water applied to refill the soil profile in the winter, 3.55, 2.58 and 2.11 acre feet of water on a per acre basis, were applied to the high, mid and low irrigation treatments respectively, from October 30, 2006, to harvest, October 15, 2007. Rainfall was minimal.

Again, using 2007 as an example, as the level of applied water decreased, soluble solids (i.e. sugars) and titratable acid, were greater at harvest, although the sugar acid ratio was not different (see Table 1).

Rows in the experimental orchard were oriented east and west. Fruit on the south side of the tree had higher soluble solids concentration and sugar/acid ratio than fruit on the north side of the tree, regardless of irrigation treatment. Fruit juiciness, either measured as weight of juice to weight of fruit (see Table 1) or volume of juice per weight of fruit (results not shown) were not different among irrigation treatments, suggesting the increase in sugars and acid was the result of osmotic adjustment and not fruit dehydration. We were also interested in seeing if the differential irrigation treatments influenced eating quality of the fruit. To test this idea, we provided fruit from the highest and lowest irrigation treatments of 2007 and 2008 to volunteer panelists at the UC Kearney Ag Center and asked if they could detect any differences between the fruit. Results from both years showed that the panelists could not detect differences between the two irrigation treatments. This suggests that the increase in soluble solids in the low irrigation treatment was not sufficient to influence eating quality.

In 2007, yield and grade decreased as the amount of applied water decreased (see Table 2).

Fruit in the high and mid irrigation treatments peaked on size 56 per carton and on size 72 per carton in low treatment (data not shown). The decrease in fruit grade at pack-out appeared to be largely due to a more oblong shape. The negative yield, fruit size and grade effects measured in the low and mid treatments in 2007 were probably the cumulative result of deficit irrigation in Years 1 and 2 and not just Year 2 alone. Reduced rates of irrigation hastened development of fruit color compared to the high irrigation treatment (see Table 3) and this occurred every year.

The deleterious effects on yield, and grade on the trees in the low-irrigation treatments suggested that not much would be gained by continuing this level of stress for a third season in the same plots. In 2008, the low irrigation treatment was replaced by a high irrigation treatment and, at harvest, yield by weight and fruit numbers were not different from the control high-irrigation treatment. This observation demonstrated that the Beck navels rebounded quickly from the low irrigation stress of 2006 and 2007. The mid-level irrigation stress of 2006 and 2008 was less severe than that of 2007, and yield and fruit quality was not as adversely affected as in 2007.

This study provides information on some of the trade-offs that might be expected among fruit yield, size, grade, sugar and color in relation to reduced irrigation as harvest approaches. More detailed information from the trial can be found at the following link: https://doi.org/10.21273/HORTSCI.46.8.1163. How growers respond to this information will depend on their approach to profiting in the early navel market and how much water will be available for irrigation. If reducing water use is the primary goal of the grower, while minimizing effects on yield and fruit quality compared to fully irrigated orchards, work by Dr. Goldhamer, UC irrigation specialist, demonstrated that regulated deficit irrigation in the mid-May through mid-July time period would be the best strategy. The authors gratefully acknowledge the Citrus Research Board for its financial support of this project.

So what happens to an avocado tree when it runs out of water? The stomata close and stops transpiring. When water no longer evaporates from the leaf surface, it heats up. If it gets hot enough, and it will, the leaf sunburns and dies. It stopped photosynthesizing long before that, because the process is temperature sensitive. Avocado is an “upper story, late successional” tree which is more affected by drought than faster growing, weedy species. So read about what happens to a forest as temperatures increase.

by: Olof Lönnehed
University of Gothenburg
olof.lonnehed@science.gu.se

Trees get overheated in a warmer rainforest

The ability of rainforests to store carbon can decrease in pace with climate change. This is due to photosynthesis rates in the leaves of rainforest species falling at higher temperatures and the trees' natural cooling systems failing during droughts. Increased heat threatens especially the species that store most carbon. This has been shown in a new thesis from the University of Gothenburg.

Some species of trees are able to handle rising heat in the tropics by sucking up large quantities of water to their leaves and transpiring through wide-opened pores in their leaves. These are mainly fast-growing trees that establish themselves early as a rainforest grows up. The same cannot be said for the trees that make up the canopy of rainforests in old growth forests. They grow slower, but get bigger and taller, and their leaves do not have the same ability to cool themselves via transpiration.

Water powers the ‘air conditioning' of the leaves

“The tropics have not experienced Ice Ages and have thus had a relatively stable climate historically as well as seasonally. With climate change, it has started to get warmer and then we have seen that some species of trees are showing increased mortality rates, but we have not really known why before,” says Maria Wittemann, who wrote the thesis.

She has studied several species of tree that can be roughly divided into early successional species, which establish themselves early in a new rainforest, and late successional species, which grow slower but grow considerably bigger, and are thus a larger carbon sink over the long term. A clear difference is how the trees in the two groups handle heat. The early successional species open the pores wider in their leaves, through which they transpire large amounts of water, thus keeping down the temperature in their leaves – similar to an air conditioning system. The late successional species do not open their pores as much, and therefore it's more difficult for them to stay cool.

More sensitive to drought

“We found large temperature differences in the leaves in our measurements. There could be a difference of 10 degrees Celsius between late successional species and early successional species growing in the same location. The late successional species had more difficulty coping with abnormally high temperatures. These trees had a higher mortality rate,” says Maria Wittemann.

However, the early successional species' profuse transpiration through their leaves also requires a lot of water. During a period of drought, the researchers noted that early successional species became more vulnerable to the heat and dropped their leaves. Their reduced consumption of water meant that late successional species were more resistant to drought.

“Our results show that photosynthesis rates in rainforest trees falls when the temperature rises in their leaves, which occurs mainly in late successional species. The proteins and membranes in their leaves, that are essential for photosynthesis, fail, and eventually the trees die due to carbon starvation because they cannot convert enough carbon dioxide from the air. This affects the entire ecosystem. We know, for example, that some animals eat the fruits of the late successional species,” says Maria Wittemann.

Co-operation with a local university

Previous research shows that the situation is worst in the Amazon. It is estimated that this carbon sink will be transformed into a carbon source by as early as 2035. In African rainforests, climate change has not gone as far.

Research at the University of Gothenburg is being conducted in high-elevation forests in Rwanda in collaboration with the University of Rwanda. The trees have been studied in situ, but seeds have also been planted in climate chambers in Gothenburg to study their development at different temperatures.

“We are working with various stakeholders in Rwanda. There is not much rainforest left in Rwanda and when they plant new trees, they want to know which indigenous species will be able to survive in a warmer climate,” says Maria Wittemann.

Facts about the study: The sensitivity of trees to climate change was studied by planting tree species adapted to a cooler climate in Rwanda's elevated tropical rainforests at three locations with different climates at different altitudes. One step down the elevation gradient corresponds to a possible future climate. The field experiment is called Rwanda TREE (TRopical Elevation Experiment) and consists of 20 species and 5,400 trees. To learn more about Rwanda TREE, visit the website www.rwandatree.com or watch the film https://www.youtube.com/watch?v=EkDvbwisqlQ.

https://gupea.ub.gu.se/handle/2077/73318

Terrestrial laser scanning data show that trees move their branches in a diurnal pattern, settling down for the night – as if falling asleep. So far, however, researchers have been uncertain as to why this happens.

A new study utilising time-series of terrestrial laser scanning measurements shows that changes in the water status of leaves and branches causes branches to move downward at night, up to 20 cm depending on the tree species. Leaves and branches replenish their water storage during the night, increasing their weight and causing them to droop down. Terrestrial laser scanning is a remote sensing technique that can produce a 3D representation of the surroundings with millimetre accuracy. With repeated measurements, it is possible to study small structural changes in the environment, such as the movement of branches.

“By monitoring the movement of tree branches, we can gain insight into how water moves inside the tree. Climate change reduces the availability of water and increases drought stress, so it is important to understand the movement of water in trees in order to understand changes in forest health,” Postdoctoral Researcher and the lead author of the article Samuli Junttila from the University of Eastern Finland says.

In the laboratory, the researchers found that tree branch position followed changes in tree water status also over a longer time period. These findings also have practical applications. For example, laser scanning could be used to monitor plant water status in a greenhouse to automate watering regimes and save valuable resources.

The study was conducted at the University of Eastern Finland in collaboration with the Finnish Geospatial Research Institute and the University of Helsinki. The study was conducted within the UNITE Flagship Programme funded by the Academy of Finland.

https://www.mdpi.com/1999-4907/13/5/728