Unlocking Nature’s Water Secrets for Greener Futures, Part 1
TL;DR Plant hydraulics unravels the journey of water within plants, aiding tree health, nursery production, urban forest management, and climate resilience. 🌿🌿
Last month, we delved into the fascinating world of soil hydraulics, exploring how water moves beneath our feet. In this post, we’re staying within the realm of water movement but shifting our focus to a different dimension of nature – plants. Prepare to journey through the intricate pathways of plant hydraulic physiology, where we uncover the secrets of how trees and other woody plants manage water, adapt to challenging conditions, and ultimately contribute to a greener, more sustainable world.
Plant hydraulic physiology is all about how water moves through plants. Scientists study this to understand how trees and other woody plants react when they have enough water or not enough. This knowledge helps us figure out how different ways of growing plants in nurseries affects their growth. People have known for a while that this field is important for plants in forests. But now, thanks to recent discoveries by this lab and others, this amazing field of science is being applied to nurseries and other horticultural production systems. In this summary, I will explain the basic ideas about how water moves through plants , how it connects to their structure and how they work. With this knowledge, scientists, nursery workers, and people who care for forests can ensure they grow strong, healthy trees that can handle harsh conditions when planted outside.
UNDERSTANDING WATER MOVEMENT IN PLANTS Let’s start by talking about how water moves in plants. Imagine it’s like water moving through a hose in your garden. We can measure this flow of water using something called “flow rate,” which is just how much water moves in a certain amount of time.
We use units like gallons per minute or liters per minute to measure it. For example, think about a water hose in your garden. If you want to know how much water it sprays out in a minute, that’s its flow rate. Now, here’s something interesting: the size of the pipe or hose matters. A big hose can let a lot more water flow through than a tiny one. In fact if you double the diameter of a hose it can allow 4 times the flow of the smaller diameter hose. Plants have tiny water pipes called “xylem.”
XYLEM CONDUCTANCE Okay, now let’s talk about “conductance.” Think of it as how easy or hard it is for water to move through something. For plants, this refers to how easily water can travel through their pipes. We usually keep the pressure the same, like when you use a hose with a constant water pressure. This helps make sure the plants get water evenly. Lastly, there’s something called “conductivity.” It’s like a fancy version of conductance but scaled to the size of different parts of the plant. It helps us compare how different parts of the plant move water. For example, we might want to know how water moves through the stem compared to the roots.
Now, here’s where it gets cool: in plants, water doesn’t get pushed like in your garden hose. It gets pulled up by something called “tension.” This happens because plants lose water from their leaves when it evaporates. Imagine a plant sipping water through a straw from the soil. When the water evaporates from the leaves, it creates tension, like a vacuum pulling water up the plant. This is how water can move up the tallest trees. So, we measure something called “water potential” to understand this tension. It tells us how much “pull” the plant has on the water. When there’s a difference in water potential between different parts of the plant, it’s like a driving force that makes water move from where there’s less pull to where there’s more. This helps water move up from the roots to the leaves, even against gravity. We call this whole process the “Soil-Plant-Atmosphere Continuum,” but you can just think of it as how plants drink water.
And that’s the basics of how water moves in plants!
Nursery science researchers have embarked on a journey to harness the principles of soil hydraulics to reshape container production practices. In the past couple of years, Dr. Chris Criscione and Dr. Jeb Fields and others released a series of articles that showed that stratifying pine bark can serve as a substitute for peat-based media in floriculture and bark-based woody plant production. Through layering premium floriculture media over cost-effective pine bark within containers, they reduced reliance on peat. In their study focused on Petunia hybrid ‘Supertunia Honey’, the stratified substrates yielded crops of comparable size and quality, along with enhanced root productivity. Their work also showed superior performance of bark:coir substrates in a stratified setup, even when subjected to reduced irrigation when producing Loropetalum chinense ‘Ruby’ liners. Positive microbial communities in stratified systems further aided in mitigating water stress. Furthermore, Red Drift® rose plants grown in stratified substrates exhibited equal or superior crop growth despite receiving less controlled-release fertilizer. This suggests potential for reducing fertilizer and irrigation rates while upholding crop quality, offering a sustainable avenue for containerized crops. This blog post aims to shed light on stratified substrates and also provide insights into ongoing projects at the Nackley Lab that are delving into this innovative frontier of nursery science.
Image: mixing coarse and fine substrates in containers to address soil stratification
Soil hydraulics delves into the intricate world of water movement within soils—how water traverses the soil matrix, its distribution, and its intricate interactions with soil particles and structure. It’s a field dedicated to understanding the physics governing water’s journey through soils, and this knowledge carries significance for agriculture, environmental science, hydrology, and civil engineering. Soil hydraulics guides irrigation strategies, shapes drainage designs, and fuels sustainable land management practices.
Image: water in saturated and unsaturated soils
Central to soil hydraulics is the concept of matric potential, a critical factor in irrigation management and plant-available water. Matric potential captures the degree to which soil particles retain water due to molecular attractions—it’s essentially the “stickiness” of water to soil particles. When soil isn’t completely saturated, minuscule air-filled spaces exist between particles. Water molecules adhere to these particles, creating capillary forces that coax water upwards. The strength of the bond between water and soil particles determines the matric potential, effectively influencing water availability to plants.
Image: the gaseous, liquid and solid phases of soils
Two other essential players in the realm of soil hydraulics are soil texture and structure. Envision soil as an intricate mosaic, with particles coming together to form aggregates, shaping distinct pathways and chambers. This arrangement, referred to as soil structure, creates macropores— akin to express highways—through which water can flow rapidly. At the same time, micropores, reminiscent of narrow alleyways, gently retain water against gravity’s pull, acting as reservoirs for plant roots during dry spells. Additionally, soil structure influences permeability, determining how efficiently water infiltrates and the risk of surface runoff.
Soil texture, on the other hand, hinges on the proportions of sand, silt, and clay particles in the soil. Each particle type comes with distinct traits that shape water dynamics. Coarser-textured soils with more sand have ample space between particles, allowing water to move quickly with less retention. Finer-textured soils, rich in silt and clay, boast smaller gaps between particles, leading to slower water movement and higher water-holding capacity.
Image: The Soil Texture Triangle
However, when soil is confined to pots or containers, it undergoes transformations in its natural structure due to various container-related factors. The restricted space of a pot, contrasting with the expanse of natural soil, can lead to the erosion of soil structure. Aggregates—clusters of particles forming pores and pathways—can deteriorate over time due to limited expansion room. This confined space often results in compaction, where soil particles compress closely, reducing essential air-filled pores needed for root growth and water movement. The watering practices specific to potted plants can contribute further by compacting soil particles as water occupies gaps. The absence of natural soil organisms and plant roots within containers hampers the maintenance of soil structure. Repeated disturbances, like transplanting or repotting, can exacerbate these structural changes. To counter these effects, selecting appropriate potting mixes that retain structure, incorporating organic matter for improved aeration and water retention, and being mindful of compaction during planting and watering are recommended.
Image: Hydrangea in pot with drip emitter irrigation
Traditionally, the nursery field has focused on creating homogeneous potting mixes that maintain structure while offering suitable hydraulic properties. Classic blends often comprise bark, coir, peat, perlite, vermiculite, and pumice. However, a recent shift in focus has led scientists to explore how layering media can simulate natural soil hydraulics—an approach known as stratified substrates.
Coarse substrate
Fine substrate
Stratified substrates involve arranging potting media of varying textures in layers within a single container. This structured layering entails placing coarser-textured substrates at the bottom and finer-textured ones on top, mimicking natural soil layers. This technique aims to influence water movement, nutrient distribution, and hydraulic behavior within the confined environment of a container. By borrowing from the stratification seen in the ground, stratified substrates strive to optimize resource efficiency, plant growth, and root development in controlled settings like potted plants.
Some may draw parallels between stratified substrates and the practice of placing rocks or gravel at the bottom of larger plant pots. While both concepts involve layering materials, there are distinctions. The practice of adding gravel or rocks to enhance drainage in larger pots shares a kinship with stratified substrates. However, it doesn’t replicate the comprehensive layering dynamics seen in stratified substrates. Adding gravel mainly addresses drainage concerns without fully incorporating the layered hydraulic principles inherent to stratified substrates.
Image: fine (l) and coarse (r) substrate, side by side
Dr. Chris Criscione, in collaboration with the Dr. Jeb Fields group at Louisiana State University, has been at the forefront of investigating stratified substrates in containerized plant growth. Their research delves into how layering different potting media textures can enhance water retention, nutrient availability, and overall plant performance. The studies highlight promising outcomes, such as heightened root productivity, improved growth, and enhanced quality under stratified conditions compared to conventional substrates. This technique holds potential for bolstering sustainable crop cultivation within controlled environments.
Image: potted hydrangeas on the gravel pad at NWREC
Nevertheless, it’s crucial to acknowledge that findings from studies conducted in one geographic region—such as the Southeastern USA—may not seamlessly extrapolate to other areas with distinct climates and environmental conditions, like the Pacific Northwest. Climate, temperature, humidity, and other factors can significantly influence plant growth and water dynamics. Given this variation, research in regions like the Pacific Northwest, such as Oregon, is crucial. The unique environmental factors there, including cooler temperatures and higher rainfall, can impact water movement, nutrient availability, and plant response to stratified substrates. Bark-based substrates, common in some areas, may behave differently in terms of water retention and drainage in regions with distinct soil compositions. To address this, the Nackley Lab initiated a collaboration with Dr. Fields and others in 2022, planning to explore the impacts and benefits of stratifying substrates in Nursery production. The first stratified substrate experiment was launched in 2023 at Oregon State University North Willamette Research and Extension Center (NWREC), aiming to reduce resource demand and provide insights into the effectiveness of stratified substrates in that context, contributing to more informed decision-making for nursery production and horticulture practices in the region.
The Gravel Pad update you’ve been waiting for, and more!
There’s so much going on in the season of plenty around NWREC! Enjoy this virtual tour of a few projects around the nursery.
Dalyn has been continuing her work with mini-lysimeters that control irrigation in shade trees – these tiny scales weigh the potted plants and use the change in weight as they dry to determine when to turn on the water. The lysimeters are gathering data on plant weight along with an on-site weather station to better understand the relationship between heat and irrigation in gravel pad production. Read more about this project here.
Young red maple trees on the gravel pad are using lysimeters to monitor water loss in conjunction with a dedicated weather station (left).
The Willamette Valley has had a few HOT summers in a row, even though lately this one has been pretty mild. Nevertheless, we haven’t given up on finding solutions for heat mitigation – including growing ornamentals under drought conditions to see which are the most “climate-ready” to meet changing needs. We’ll be asking the public to evaluate those plants in the upcoming Climate-Ready Field Day, come along and see how the plants are progressing (click the link above for more info).
In addition, we’re evaluating different means of mitigating the heat and the resultant high rates of evapotranspiration (basically ways to reduce plant sweat), from misting the young plants to covering tissues with kaolin, introducing fungicides that may be beneficial in managing water loss, using white pots instead of the traditional black, and even growth inhibitors – it’s been a pretty amazing feat to monitor the effects as you can see- check out this monitoring station!
Dalyn, Lloyd and summer hire Jaiden show off the monitoring station in the heat stress/evapotranspiration mitigation study.
Does the color of the pot change the heat stress for these water-loving shrubs on the gravel pad?
A small project growing marigolds for festivals and holidays – like Dia de los Muertos – is also underway. Growing the marigolds has certainly brightened up the Nursery Zone at NWREC, and we’ve progressed into evaluating passive means to dry the flowers, saving energy and resources while preserving the gorgeous summer color.
Marigold blooms
There’s even more in the works – stay tuned for information about fall workshops and PACE courses created specifically for nursery and greenhouse production for topics covering drone sprays, integrated pest management, and more.
At the Western Region International Plant Propagators Society (IPPS), the Pacific Northwest Insect Management Conference (PNWIMC), and the Orchard Pest and Disease Management Conference (OPDMC) last month, we presented cutting-edge research and advancements in our field. Our presentations at the Western Region IPPS and PNWIMC focused on the latest developments in sensor-controlled irrigation, and flatheaded borer management, respectively.
Dr. Melissa Scherr Presents at the PNWIMC in Portland
At the Orchard Pest and Disease Management Conference, we discussed the latest techniques in IPM for managing powdery mildew with biological fungicides applied by our laser-guided Intelligent Sprayer system. Through our presentations at these conferences, we aim to advance the knowledge and understanding of plant health in our field and to promote collaboration among professionals. By sharing our research and engaging in discussions with our peers, we strive to advance the science of horticultural production to support the growth and success of the horticulture in the Pacific Northwest region.
Grower tour visits the olive grove
The buses meet our Horticulture Team at NWREC
At NWREC, we have been working on our new hydroponic greenhouse project. However, since October we have encountered construction challenges in connecting the natural gas heaters, which has impacted the growth of crops such as lettuce, tomatoes, and cucumbers. As a result, lettuce growth has been slow and plagued by Botrytis, and warmer-growing crops like tomatoes and cucumbers have fared even worse. We are working to resolve the permitting issues with the heaters as soon as possible and look forward to updating you on the progress of the greenhouse project in the coming year.
For the past few years we’ve limited gatherings on the farm due to COVID-19 restrictions. In the summer of 2022, however, we were finally able to welcome the public back for Nursery Program Field Days. We’d like to take this opportunity to boast about a few of our highlights from the last several months.
Sadie Keller discusses shade tree physiology
For the first time, the Nackley Nursery Production team was an official stop on the Oregon Association of Nurseries Farwest Innovative Production Grower Tour. Our portion of the tour at NWREC showcased sensor-controlled irrigation, heat-stress mitigation techniques, LiDAR smart-sprayer systems, and practices that can reduce boxwood blight spread, and methods of scouting and monitoring insects in nurseries and greenhouses. These projects offer a wide range of savings for growers.: up to 80% improvement in irrigation efficiency, up to 70% reduction in sprayed pesticides, and a significant reduction in boxwood blight infection.
Stakeholders evaluating climate-readiness of various landscape ornamentals
The second big event was an open house for our Climate Ready Landscape Plant trial, the largest coordinated landscape plant irrigation trial in the Western US. Plant professionals from around the region came to rate plants and discuss how we, as a society, are going to maintain healthy landscapes while faced with increasing extreme weather.
Ongoing projects that will continue this year include, research by our graduate student Sadie Keller, who is investigating Oak and Maple drought tolerance. This summer, Sadie shared her preliminary findings with scientists at the American Society for Horticultural Science, in Chicago.
Sadie Keller and Lloyd Nackley at the ASHS Meeting in Chicago.
In addition, Dr. Melissa Scherr continues our research on the Pacific Flatheaded beetle, with the anticipation of a grower event hosted at NWREC discussing current research on Flathead Borer biology and control this coming April – 2023.
I am originally from the great city of Tacoma, Washington. I went to an outdoors based high school where my love for plants and environmentalism blossomed. My favorite class was our version of “PE”, where we hiked through Point Defiance Park identifying native species. This passion drove me to pursue a bachelor’s degree in Environmental Science at the University of Redlands. Moving to Redlands in Southern California was a stark contrast to my home in Puget Sound, but I grew to love most parts of it!
Scout collects samples
I went on many abroad terms and saw amazing parts of the world such as Peru, Ethiopia, and Iceland. Each time I returned I wished I had been there longer and itched to immerse myself even more in a different country. Once I graduated from Redlands, the natural next step was to join the Peace Corps. I spent two years in the Southeast corner of Senegal, West Africa. I lived in a 100-person village in the region of Kedougou where I learned to speak Jaxanke. As an Agroforestry Extension Agent, I helped with various agriculture and agroforestry projects. We created small-scale nurseries, collected seeds, showcased new and improved agroforestry techniques, and outplanted trees and shrubs around the village. I loved my time in Senegal and miss being there constantly.
After returning to the United States, I moved to Eugene, OR to work at Dorena Genetic Resource Center. I assisted the lead horticulturist in end-to-end native plant restoration, collecting/processing seed, and producing native plants to restore areas affected by fires, floods, and construction. I became the lead irrigator, which was a new problem-solving and damp adventure, and led seed collection trips across Oregon. I also helped develop a seed collection mapping application to track plant populations and store seed collection data.
This leads me to OSU! I just started at OSU this fall to pursue my Masters in Horticulture and work in the Nackley Lab. I am partnered with Sadie Keller on a project looking at stem hydraulics and how it relates to drought in shade trees. I am new to this type of research but am so eager to learn more! I am excited to get our stem hydraulics lab up and running and start the journey of data collection.
Shade tree growers need to be prepared for the effects of climate change in Oregon.
In order to equip growers with the tools necessary for production success, we aim to determine critical shade tree stress thresholds, characterize plant responses to drought conditions, and correlate remotely collected spectral images with ground based plant water stress measurements.
Previous studies have sought drought response measurements for Acer rubrum (Red Maple) and Quercus rubra (Red Oak), but never in a nursery production setting.
We aim to disseminate this information to Oregon shade tree growers at the completion of this experiment with the hope to aid growers in making data driven irrigation decisions and demonstrate the use of these technologies in nursery production settings.
Sadie Keller in the shade trees equipped with soil moisture sensors and a weather station.
The Problem:
In Oregon’s Willamette Valley, the heart of the nursery country, rainfall is scarce during the summer and humidity is low. Oregon’s dry summer conditions can lead to low moisture stress conditions for maples and oaks in normal years. Plant stress resulting from low soil moisture, high heat, and low relative humidity have been exacerbated in recent years with the increasing frequency of heatwaves and drought. Drought and heat stress scorch the maple and oak canopies, which can lead to decreased plant quality and economic losses for shade tree growers. Sensor-based technologies can be used to model plant responses to environmental gradients to develop warning systems to help growers prevent stress and bridge a knowledge gap in the nursery production industry regarding drought responses.
How are we studying plant stress responses?
Starting late June 2022, we will implement two irrigation treatments (well-watered and drought) in our shade tree planting with each row having independent irrigation control. The well-watered rows will be maintained at a soil water potential of >-1.0 mPa. The drought treatment rows will be allowed to naturally dry down to a soil water potential of -4 mPa. If during the experiment, our metrics (stomatal conductance and stem water potential) do not show considerable responses at -4 mPa tension, we will allow the drought treatment to continue to dry down progressively (-1 mPa) until stress is evident.
Why and how do we measure stem water potential?
Plant water status is commonly defined in terms of water potential or the ability of the water to do work. In most cases, well watered plants have “high” water status and drought conditions lead to a “low” water status (Levin and Nackley 2021). Using the pressure chamber, we will take midday stem water potential measurements twice weekly from 12pm-3pm. This time frame is important because it represents the time of day where leaf transpiration is at its maximum.
First, we will cover the leaf and stems to be measured with an opaque bag for at least 10 minutes before pressurization to allow the plant to stop transpiring. Once we excise the sample from the tree it should be placed into the pressure chamber or “pressure bomb” within 30 seconds (Levin 2019). Once the stem is placed into the chamber and pressure is applied, the amount of pressure that it takes to cause water to appear at the cut surface tells us how much tension the stem is experiencing.
We measure stomatal conductance using a porometer that measures the degree of stomatal openness and the number of stomata (Licor.com). This indicates the plant’s physiological response to its current environment. If a plant is stressed, it will tend to close its stomata and lower the stomatal conductance rate. We will be using a combination of the LI-6800 Portable Photosynthesis System and the LI-600 Porometer/Fluorometer to make our measurements twice a week from 12pm-3pm.
For more information:
Please stay tuned in the coming months for more blog posts about how we will find plant stress thresholds by measuring the hydraulic conductivity of these shade trees. We will also correlate remotely collected spectral and thermal images with our ground based plant stress measurements to demonstrate how implementing a UAS equipped with a multispectral and thermal camera can be used to detect water stress in nursery production.
The pretty flowers of spring, shade providing leaves of summer, and fire like colors of fall help us know when the seasons are changing. We use calendars to plan everything in our lives. Nature does the same but not in the same way.
Shade tree seasonal progression from spring (top left) to fall (bottom right).
What causes trees to change?
We can all remember early spring weather that was warm and dry as well as those years when we hoped our 4th of July BBQ would not get rained out. These weather differences from year to year influence events in nature that are crucial for species to survive. Plants and insects go dormant over the cold winter and begin growing in the spring and do it without a single calendar. They use day length and temperature to schedule their life events. The day length in the Willamette valley changes from 8 hours and 46 minutes in winter to 15 hours and 36 minutes in summer. These changes are predictable because they are cause by the tilt of the Earth, which doesn’t change. Our calendars align with these dates (winter solstice and summer solstice).Temperature, however, is unpredictable because there are a vast number of factors that influence it.
What changes are we monitoring?
A large part of the nursery industry in Oregon is dedicated to growing shade trees. Best management practices require monitoring for signs of event changes throughout the tree’s life cycle. This is known as Phenology, defined as the study of cyclic and seasonal natural phenomena, especially in relation to climate and plant and animal life. The Nackley Lab has a small in ground tree nursery that we use for experiments. For the past 3 years we have been conducting a Phenology Study where we have tracked the dates our tree events happened.
Monitored events in our Phenology study
Bud Break: Date when the protective scale coating is shed from the bud exposing the tender new growth shoot
First leaf: Date the first leaves are completely unfolded on at least 3 branches
All Leaves Unfolded: Date when 90% of buds have reached first leaf
First Flower: Date the first flowers are opened and stamens are visible on at least 3 branches.
Full Flower: Date when half or more of the flowers are fully open
First Ripe Fruit: date when the first fruits become fully ripe or seeds drop naturally
Full Fruiting: Date when half or more branches have fully ripe fruit or have dropped seeds
50% Color: Date when half or more of the branches have leaves that have started to change color
50% Leaf Fall: Date when half or more of the leaves have fallen off the tree
Phenological stages of flowering trees from bud break (top left) to full flower (bottom right).
How can this information be useful?
As we collect data in year 3 (2022) of this study we are excited for how this data may be used in the future. Temperature data can be used to make degree day models which are based on heat units. The number of heat units per day are added together in a running total. This information is much better at predicting events in nature when compared to calendars. When growing shade trees in a production nursery setting, defending the crop from disease and predators is essential. Spraying a tree with a fungicide at bud break keeps them growing healthy. Spraying pesticides at first flower protects trees from insect attacks. By creating degree day models, growers can predict when to apply chemical protection for trees, eliminating double applications caused by calendar reliance.
Growers know the uncertainty caused by a changing climate impacts tree growth events. It’s hard not to trust the calendar dates which we plan everything else in our lives by. Future projects include modeling the events recorded over the last 3 years, and seeing how they align with degree day accumulation. The end goal is to use what we’ve learned to help keep the labor and pesticide costs down for the local nurseries while the produce the beautiful tree’s we all depend on.
