In the second part of this month’s water potential series (see part 1), we discuss the separate components of a water potential measurement. The total water potential is the sum of four components: matric potential, osmotic potential, gravitational potential, and pressure potential. This article gives a description of each component. Read the article here…
Six short videos teach you everything you need to know about soil water content and soil water potential—and why you should measure them together. Plus, master the basics of soil hydraulic conductivity.
Next week learn about the four components of water potential—osmotic potential, gravitational potential, matric potential, and pressure potential.
Take our Soil Moisture Master Class
Six short videos teach you everything you need to know about soil water content and soil water potential—and why you should measure them together. Plus, master the basics of soil hydraulic conductivity.
University of Georgia researcher, Shuyang Zhen, wanted to find out if she could optimize greenhouse irrigation with reference evapotranspiration calculated from environmental factors and a crop coefficient, using NDVI measurements to adjust for canopy size (see part 1). Learn the results of the experiment and how fast growth and flowering caused problems with the NDVI measurement.
Shuyang’s experimental setup.
Fast Growth Causes Problems
Shuyang says because the plants grew so large, the canopy filled in beyond what the sensor could see. That meant there was additional leaf area that participated in vapor loss which wasn’t identified by the NDVI sensor. As the canopies approached moderate-to-high canopy densities, Shuyang observed that the NDVI readings became less responsive to increases in canopy size. To work around this problem, Shuyang tried to calculate a vegetation index called the Wide Dynamic Range Vegetation index with the spectral reflectance outputs of the two wavebands measured by the NDVI sensor. Shuyang says, “This index was supposed to improve the sensitivity at higher canopy density, so I transformed all my data and was surprised that it actually improved the sensitivity when the canopy density was lower. But at a higher canopy density it wasn’t as effective.”
The red flowers reflected a lot of red light compared to the leaves, which confused the NDVI measurement.
Plant flowering also caused problems with the NDVI measurement. Shuyang explains, “We had one cultivar of petunia with red flowers which formed on top of the canopy. The red flowers reflected a lot of red light compared to the leaves, which confused the NDVI measurement. The NDVI value gradually decreased when the plants started to flower. There was no way I could get around that issue, so in some of the replicates, I removed the flowers, and in some I kept the flowers so I could compare the different responses and characterize why it happened.”
The NDVI was very sensitive to the increase in crop size when the canopy was relatively small, but when you reach a certain canopy size and the canopy closure was nearly complete, then the sensitivity decreased.
Summary and Future Studies
During the early stages of growth, the research team saw a linear relationship between NDVI and crop coefficient. However, when the crop coefficient reached higher values, the response leveled off. Shuyang says, “The response failed to change with further increases in the crop coefficient. The NDVI was very sensitive to the increase in crop size when the canopy was relatively small, but when you reach a certain canopy size and the canopy closure was nearly complete, then the sensitivity decreased.”
Lack of NDVI sensitivity during canopy closure and flowering translated to a problem with under-irrigation,
Shuyang adds that the lack of NDVI sensitivity during canopy closure and flowering translated to a problem with under-irrigation, so the team is thinking about developing separate models for different canopy stages. She explains, “When the canopy reaches high canopy closure we may have to add an additional coefficient to compensate for that underestimation, but it’s difficult to evaluate what kind of coefficient we should use without more data. We need to do more studies to get an idea of what kind of adjustments will make the prediction more precise.”
Learn more about Shuyang’s work on the University of Georgia horticulture blog.
Greenhouse growers need irrigation strategies to maintain high plant quality, but it’s difficult to obtain quantitative information on exactly how much water will produce the highest-quality growth.
Greenhouse plant canopies are highly variable.
Estimating irrigation needs by using reference evapotranspiration calculated from environmental factors and a crop coefficient is standard for controlling field crop irrigation, but in a greenhouse this method can be challenging. Greenhouse plant canopies are highly variable, and there’s limited information on the crop coefficient values for ornamental crops.
Researchers used a sensor-controlled automated irrigation system with soil moisture sensors.
Measuring Crop Size
University of Georgia researcher, Shuyang Zhen, wanted to find out if she could solve this problem for greenhouse growers using NDVI measurements to adjust for canopy size. In a greenhouse setting, she and her team planted four types of fast growing herbaceous plants in small containers on top of greenhouse benches. They set up a small weather station to monitor environmental parameters and used that data to calculate reference evapotranspiration.
NDVI measurements are a non-destructive, continuous monitoring method to get information as to how big a crop is.
Using a sensor-controlled automated irrigation system with soil moisture sensors, the team determined the amount of water the plants used, which allowed them to calculate a crop coefficient on a daily basis. They then used NDVI measurements to monitor crop size. Shuyang says, “It’s easy to monitor environmental factors such as light, temperature, relative humidity, and wind speed, but it’s much harder to determine how big the crop is because many methods are destructive and time-consuming. We chose NDVI measurements as a non-destructive, continuous monitoring method to get information as to how big our crop was. We were specifically interested in looking at how NDVI changes with the crop coefficient and how those two parameters correlate with each other.”
Some species were more upward growing and some more sprawling.
Shuyang mounted multiple NDVI sensors on top of the benches, approximately four feet from the plants. Each sensor had a field of view of about .6 square meters and tracked the changes in plant size and NDVI values for over 8 weeks. Shuyang says, “Each species had different growth habits. Some species were more upward growing and some more sprawling. They also had different leaf chlorophyll content. Over the course of my study, three species reached reproductive stages, producing flowers. All of these factors had an effect on the NDVI measurements.”
Next week: Learn the results of the experiment and how fast growth and flowering caused problems with the measurement.
In part 2 of our PAR Measurement Series (read part 1), Dr. Gaylon S. Campbell discusses the impact of leaf arrangement, measuring light in a canopy, and why we measure PAR.
Vertical leaves absorb less radiation when the sun is at a high angle, and more radiation when the sun is at a low angle; the converse is true for horizontal leaves.
Leaf Arrangement
Leaf display (angular orientation) affects light interception. Strictly vertical or horizontally oriented leaves are extreme cases, but a large range of angles occurs. Vertical leaves absorb less radiation when the sun is at a high angle, and more radiation when the sun is at a low angle; the converse is true for horizontal leaves. The greatest photosynthetic capacity can be achieved by a change from nearly vertical to nearly horizontal leaves lower down. This arrangement leads to effective beam penetration and a more even distribution of light.
The highest LAI’s usually occur in coniferous forests.
Leaf area index (LAI), a measure of the foliage in a canopy, is the canopy property that has most effect on interception of radiation. LAI usually ranges between 1 and 12. Values of 3-4 are typical for horizontal-leafed species such as alfalfa; values of 5-10 occur in vertical leafed species such as grasses and cereals, or in plants with highly clumped leaves, such as spruce. The highest LAI’s usually occur in coniferous forests, which have overlapping generations of leaves. These forests have a photosynthetic advantage due to the longevity of individual needles.
PAR must be measured at a number of locations and then averaged.
Measuring Light in a Canopy
Variability of leaf distribution in canopies results in wide variations in light. To determine light at any height in the canopy, PAR must be measured at a number of locations and then averaged. Direct methods of measurement include using the horizontal line sensors whose output is the spatial average over the sensor length. The appropriate sensor length or number of sampling points depends on plant spacing.
Indirect methods for measuring canopy structure rely on the fact that canopy structure and solar position determine the radiation within the canopy. Because it’s hard to measure three-dimensional distribution of leaves in a canopy, models for light interception and tree growth often assume random distribution throughout the canopy; however, leaves are generally aggregated or grouped.
Models for light interception and tree growth often assume random distribution throughout the canopy; however, leaves are generally aggregated or grouped.
Why Measure Photosynthesis or PAR?
The ability to measure PAR assists with understanding the unique spatial patterns that different plants have for displaying photosynthetic surfaces. Since effective use of PAR influences plant production, knowledge of the structural diversity of canopies aids research on plant productivity. One result: researchers can use information about different plants’ abilities to intercept and use PAR to engineer canopy structure modifications that significantly improve crop yield.
The ultimate source of all energy on earth is the sun. Availability of this energy to most organisms occurs through photosynthesis, the conversion of CO2 and H2O to carbohydrates (stored energy) and O2. Photosynthesis occurs when pigments in photosynthesizers absorb the energy of photons, initiating a chain of photochemical and chemical events. Where does this energy and material exchange occur? In plant canopies. The amount of photosynthesis that occurs in canopies depends on the amount of photosynthetically active radiation (PAR) intercepted by leaves in canopies.
In canopies, leaves function collectively.
It’s More Complicated Than You Might Think
The rate at which photosynthesis occurs in one leaf might be calculated, but in canopies, leaves function collectively. Extrapolating photosynthesis from individual leaves to entire canopies is complex; the sheer numbers of leaves and their arrangement in the canopy structure can be overwhelming. Leaf area, inclination, and orientation all affect the degree to which light is captured and used in a canopy.
Average light level decreases exponentially downward through the canopy.
What Happens to Light in a Canopy?
Light varies dramatically both spatially and temporally through canopies. The average light level decreases more or less exponentially downward through the canopy, as the amount of leaf surface encountered increases. For some canopies, the greatest amount of leaf area occurs near the center. Therefore, canopy structure analysis becomes increasingly complex as one proceeds from a single plant to stands of the same plant, or to plant communities because of the variety of plants and growth forms.
Photosynthesis depends on leaf orientation.
Absorption of radiation and resulting photosynthesis depend on leaf orientation, sun elevation in the sky, spectral distribution and multiple reflections of light, and the arrangement of leaves. Patterns of light and shaded areas can be complicated and change with the sun’s position. In addition, seasonality of foliage can result in fairly small canopy interception of PAR for much of the year. PAR might also be intercepted by non-photosynthetic parts of plants (bark, flowers, etc).
In two weeks:Dr. Campbell discusses the impact of leaf arrangement, measuring light in a canopy, and why we measure PAR.
As a young university student, Dr. Y. Osroosh, now a researcher at Washington State University, wanted to design the most accurate soil moisture sensor. Over the years, however, he began to realize the complexity and difficulty of the task. Inspired by the work of Jackson et al. (1981) and researchers in Bushland, TX, he now believes that plants are the best soil moisture sensors. He and his team developed a new model for interpreting plant canopy signals to indirectly determine soil moisture.
The team measured microclimatic data in an apple orchard.
How Can Plants Indicate Water in Soil?
Osroosh and his team wanted to use plant stress instead of soil sensors to make irrigation decisions in a drip-irrigated Fuji apple tree orchard. But, the current practice of using the crop water stress index (CWSI) for detecting water stress presented some problems, Osroosh comments, “Currently, scientists use either an empirical CWSI or a theoretical one developed using equations from FAO-56, but the basis for FAO-56 equations is alfalfa or grass, which isn’t similar to apple trees.” One of the main differences between grass and apple trees is that apple tree leaves are highly linked to atmospheric conditions. They control their stomata to avoid water loss.
There is high degree of coupling between apple leaves and the humidity of the surrounding air.
So Osroosh borrowed a leaf porometer to measure the stomatal conductance of apple trees, and he developed his own crop water stress index, based on what he found. He explains, “We developed a new theoretical crop water stress index specifically for apple trees. It accounts for stomatal regulations in apple trees using a canopy conductance sub-model. It also estimates average actual and potential transpiration rates for the canopy area which is viewed by a thermal infrared sensor (IRT).”
Fuji apple orchard (Roza Farm, Prosser, WA) where Osroosh performed his research.
What Data Was Used?
Osroosh says they established their new “Apple Tree” CWSI based on the energy budget of a single apple leaf, so “soil heat flux” was not a component in their modeling. He and his team measured soil water deficit using a neutron probe in the top 60 cm of the profile, and they collected canopy surface temperature data using thermal infrared sensors. The team also measured microclimatic data in the orchard.
Neutron probes were problematic, as they did not allow collection of data in real time.
Osroosh comments, “The accuracy of this approach greatly depends on the accuracy of reference soil moisture measurement methods. To establish a relationship between CWSI and soil water, we needed to measure soil water content in the root zone precisely. We used a neutron probe, which provides enough precision and volume of influence to meet our requirements. However, it was a labor and time intensive method which did not allow for real-time measurements, posing a serious limitation.”
Next week:Learn the results of Dr. Osroosh’s experiments, the future of this research, and about other researchers who are trying to achieve similar goals.
In the conclusion of our three part series on the reforestation of Banguet province in the Philippines, we asked Dr. Anthony S. Davis, Tom Alberg and Judi Beck Chair in Natural Resources at the University of Idaho, Loreca Stauber, one of the visionaries behind the project, and Kea Woodruff, former U of I Nursery Production and Logistics Associate, now at Harvard University, to explain some challenges associated with teaching reforestation to different cultures.
Even with increased environmental awareness, we’re still losing almost thirty million acres of forest globally every year.
What are some of the cultural challenges?
Anthony: As I spend more and more time looking at international forests, I realize that we’re losing forests at a phenomenal rate. Even with all of our awareness about where we get supplies, where trees come from, where wood comes from, and where paper comes from, we’re still losing almost thirty million acres of forest globally every year. That’s terrifying to me. What’s even worse is that most of it comes from countries that don’t have environmental controls. They don’t have systems in place that keep them from cutting down all the trees. Often, when we cut trees down for forestry, we replant. But, when you start to work in countries where that’s not valued or not part of the culture or the system, then a huge problem emerges.
How do you teach people to grow trees that can survive in their native terrain?
Anthony: There isn’t a lot of knowledge globally about how to grow high-quality tree seedlings. I’ve gotten really interested in the question of how to take a tree seedling which is grown in a nursery, where it essentially has all of the water and all of the nutrients it could possibly ask for, and get it into a condition where it’s likely to survive somewhere extremely harsh: with limited nutrients and water. How do you get it to the point where it’s able to overcome those challenges?
There are two ways to look at that. One is to get more water to that seedling after it’s planted. The other is to make sure that the seedling you’re planting has its best possible chance of developing a root system that can access water that might not normally be available in those six inches where healthy roots are located when it’s first planted. Based on work that’s be done here at the University of Idaho in graduate student projects over the years, we found that if you can grow a seedling in a healthy manner in the nursery, it’s more likely to grow roots or access water that previously they might not have been able to access.
Working on one of the water tanks that will supply water to the Benguet nursery in the Philippines. The project is proceeding nicely after a series of setbacks: a destructive typhoon, slides that had to be cleared, 2 deaths, 1 funeral, and electrical power interruptions.
What challenges the plants after they leave the nursery?
Anthony: If that seedling can get roots down and access water, it starts to grow. The beauty of reforestation, in general, is that it’s very simple; it can be very easy to get trees to grow. However, what often happens is you have a social element that overlaps the biological element. Some of it could be a lack of education, where people don’t understand that a large amount of foliage or leaves on a tree means that you need more water. You think about that image of success: people want to plant the biggest tree possible. That might work in a yard, but it really doesn’t work in a reforestation situation.
What are the challenges of establishing a nursery in a place like the Philippines?
Kea: In the place like the Philippines where resources aren’t necessarily as available, it becomes a huge challenge just finding the right kind of media or container. Also, there’s a decentralization of the knowledge resource itself. While we were there, we had the opportunity to meet with different government agencies, and there are definitely people who know a lot about the species that are available and how to grow them, but in terms of that information being disseminated and widely available to the public, that’s a challenge. The techniques that will be needed to actually produce a seedling resource need to be addressed.
Loreca: The basic thing is a good nursery. That has been a problem. In the past, the government, in an effort to green the Philippines, has given seedlings, but oftentimes, these seedlings are so poor in quality that they don’t survive in out planting.
Coffee beans will thrive in the tropical Philippines.
How can you help other cultures to succeed at reforestation?
Anthony: During some work I was doing in the Middle East, in Lebanon, we found that communicating to people what a high-quality seedling became really important. You teach them about quality, defining it in terms of how much water a plant needs to survive, or how a plant has to grow in order to colonize a site. We had a lot of success with the project there, getting people to understand that there was a problem in only looking at above ground information in terms of what makes a high-quality seedling. Really, when the roots are what’s driving survival, they’re looking at the wrong part of the picture.
How do you teach people to think beyond the nursery?
Anthony: Our work in Lebanon coincided with a project in Haiti. In Haiti, we had a former student who had been here at the University of Idaho who asked for help starting a nursery. These same conversations occurred: what is a healthy seedling, what is likely to survive, where do you get your seed, how long do you grow it for, when do you plant it? We were able to have conversations around all of the elements that go into growing trees.
I remember clearly the “aha” moment where this young woman said, “We’ve been doing it wrong! We’ve always focused on growing as many seedlings as possible, and we haven’t worried about quality.”
See it live
Watch a video where Anthony talks about his work.
You can learn more about the reforestation programs that the University of Idaho nursery is involved with here.
Get more information on applied environmental research in our
In one of the first agroforestry efforts in mountainous terrain, Moscow, Idaho community leader Loreca Stauber, Dr. Anthony S. Davis, Tom Alberg and Judi Beck Chair in Natural Resources at the University of Idaho, and their partners have initiated a program where U of I students travel overseas to work with farmers of Banguet province in the Philippines to develop the skills needed to grow high quality tree seedlings. Local vegetable farmers have historically terraced the mountains that have been forested so they could grow monoculture crops, causing serious erosion (read about it here). The land has degraded so much that the Philippine government has stepped in: warning farmers to begin conservation techniques, or they will take away the land and manage it themselves.
Building a local nursery in Benguet.
Inspiring Students to Look at the Big Picture
One of the steps in helping local farmers to solve this problem is to create a local nursery where they can start growing native plants and trees. Fortunately, the University of Idaho has operated a tree nursery for over one hundred years, and they understand how to grow trees. Dr. Davis specializes in setting up native nurseries for growing native plants all over the world. He says, “I want our students to be exposed to this because we’re graduating students who should be problem solvers, who should be able to look at the biggest challenges and contribute their own ideas towards resolving those challenges.”
Loreca Stauber adds, “We are part of the world and the world is part of us. The students can do more than just get their degree and find a job. Anthony and Kea, when they do this, inspire students to look at a bigger world than they are currently living in.”
Training Students to Understand Native Terrain and Resources
Davis says a good plan needs to take local conditions into account: “The principles of growing trees are actually universal. It doesn’t matter whether you’re in Haiti, Lebanon, Idaho, or in the Philippines. Those principles are the same and they’re readily transferable. It’s how you adapt them to unique local situations that makes a difference.”
“It’s not really about the best way to grow a plant in a greenhouse environment; It’s about the best way to grow a plant that will also survive on its outplanting site.”
Kea Woodruff, former U of I Nursery Production and Logistics Associate, now at Harvard University, says they train the students who go overseas on the “target plant” concept: designing a growing regime based on what the plant is going to need in its future home. She says, “It’s not really about the best way to grow a plant in a greenhouse environment; It’s about the best way to grow a plant that will also survive on its outplanting site. Determining what the outplanting site is and what each species will need to survive on that outplanting site is what determines greenhouse operations.”
Dr. Davis says you need to consider native resources when doing these types of projects. “There could be plumbing there, but there’s no guarantee that when you turn the system on, the tap water will come out. That depends on the seasonality of the rains. It’s part of why we wanted the project partners (the farmers) to have data loggers: so we could look at the data together and get a better feel for when water is most abundant and when it’s most scarce, so it can be stored for later use.”
Overcoming Native Challenges with Remote Data
Decagon (now METER) donated data loggers to the program so that Dr. Davis and other people on the team could look at data with the farmers in the Philippines and advise them when to irrigate. Davis says, “One of the things that’s most important in trying to set up a very remote nursery and manage the production in that nursery from approximately four flights, twelve hours, and twelve time zones away, is knowing what’s going on. There are things that are really easy to ask, like could you send me a picture every Wednesday and Saturday of the nursery, or could you measure the height and the diameter of the seedlings? What’s much harder to tell is how much water is coming in, or what the temperature was during the day or night, because those require people to be monitoring things at a greater frequency than is often possible. If we know how much water is coming into the nursery from rainfall, we can build collection systems so that we can manage where that water goes later on.”
Managing data for both the short and long term is critical, says Davis, because it’s often whether there was rainfall in the predicted amount, and at the right time, that determines whether a seedling establishes or not.
Acknowledgements: The SEAGAA agroforestry project in Benguet is agro and forest; the farmers received a grant from the Rufford Foundation based in the UK to build a greenhouse and much of the water catchment system and auxiliary structure that go with a nursery facility. They also received a sizable grant from the Philippine government to launch mushroom growing as a necessary complement to help support long-term agroforestry. The project is beyond reforestation – it is the growing of trees, shrubs, ground cover, the restoring of watersheds, creating livelihoods, the rebuilding of soil fertility and integrity, the revival of springs which have vanished with the removal of perennial flora, and the restoring biodiversity to bring back the natural checks and balances of a natural ecosystem.
In the mountainous Benguet province of the Philippines, farmers grow up to three crops of vegetables a year. Their mountain vegetable farms exist at the expense of original forest cover, causing tremendous erosion difficulties. To counteract erosion and preserve the watershed as well as promote reforestation, the Philippine government issued a mandate: farmers must find alternatives that restore the watershed or lose their land.
Rice terraces in the Philippines
An Agroforestry Alternative
Loreca Stauber is no scientist, but she loves Benguet, and a letter from her friend, a scientist living in the Philippines, inspired her with the vision of teaching farmers to reforest the mountains and grow vegetables amongst the trees.
Her friend writes, “We envision mountain farms as forest ecosystems whose primary social responsibility to the communities around and below is to be part of responsible watersheds that court, catch, store and gradually share water. We see mountain farms that are not prone to soil erosion or leaching: cultivated with minimal chemical inputs and tillage that will allow the natural buildup of biomass, organic matter, helpful organisms and fauna. We think of forest ecosystems that may not make millionaires of its farmers for one generation and heavy debtors even before the next. Rather, we envision forest farm ecosystems that are self-sufficient and self-sustaining. We are working on demonstrating forest ecosystems that can substitute for monocrop vegetable farms that deplete and leach the soil, pollute watersheds and are self-destructing.”
Realizing the problem in the Philippines could be solved by reforestation, Loreca emailed Dr. Anthony S. Davis, Tom Alberg and Judi Beck Chair in Natural Resources in the University of Idaho’s Department of Forest, Rangeland, and Fire Sciences. The U of I operates a 100-year-old nursery specializing in growing hardy tree seedlings. Dr. Davis recalls, “The email she sent me said, “I think you should do something about this,” and I thought, “Actually I agree. I think we should do something about this. So we began to screen the idea, asking: are there partners? Is it a good idea? Does it fit with this little thing that we do really well, which is essentially teaching people how to grow tree seedlings, and is there an educational component that’s valuable for our students? When those check boxes lined up, then it was a matter of taking advantage of that opportunity and seeing where it could go.”
Forested mountains in the Philippines
Determining What Already Works
Together, they and other partners started a program in which U of I students went overseas to teach the people of Benguet how to grow trees, with the goal of moving the land toward agroforestry. They wanted to grow a forest ecosystem (trees, shrubs, and ground cover) along with annual crops. Kea Woodruff, former U of I Nursery Production and Logistics Associate, now at Harvard University, traveled to the Philippines with an interdisciplinary team of undergraduate and graduate students to look at what agroforestry projects were already working and to conduct a needs assessment. She says, “I saw a wide variety of landscapes in the areas that we were. One woman decided on her own that she was going to practice agroforestry, and people come and view her land as a demonstration site. It has mature bamboo, coffee trees, and mature Benguet pine. It really looks like what you would expect the native forest to look in an area like the Philippines.”
Kea said there were also intermediate sites where there are Benguet pines and some coffee with row crops blended in, such as strawberries and squash. She adds, “There’s clearly great potential to grow different species on these lands if we can help figure out the best way to use the resources that are available.”
Next week: Learn how partners in the project have been able to use native resources in the quest to reforest erosion-plagued Benguet.
Take our Soil Moisture Master Class
Six short videos teach you everything you need to know about soil water content and soil water potential—and why you should measure them together. Plus, master the basics of soil hydraulic conductivity.
