Hello, my name is Colin Campbell. I’m a senior research scientist here at METER Group. And today we’ll talk about how to estimate the transpiration from a single leaf. Occasionally we get this question: Can I estimate the transpiration from a leaf by measuring its stomatal conductance? Unfortunately, you can’t. And I want to show you why that’s true and what you’ll need to do to estimate the total conductance, and therefore, the evaporation of a leaf.
The calculation of transpiration (E) from a leaf is given by Equation 1
where gv is the total conductance of vapor from inside the leaf into the air, Cvs is the concentration of vapor inside the leaf and Cva is the concentration of vapor in the air.
Trees are merchants; they sell water to the atmosphere in exchange for the CO2 they need to photosynthesize sugars. The exchange rate or ‘water-use efficiency’ that drives the plant carbon-water market place is a function of atmospheric CO2 concentrations. Thus, theoretically human carbon emissions, which have increased atmospheric CO2 by 40% since 1850, should increase plant water use efficiency, resulting in “CO2 fertilization” of our forests and crops.
La Plata Mountains, where the study gradient is located
However, evidence for CO2 fertilization is extremely mixed. That’s why Leander Anderegg, postdoctoral fellow at UC Berkeley, and his research team are performing a two-step experiment to determine if increased atmospheric CO2 conditions increase plant water-use efficiency. The team is leveraging a natural elevation gradient in temperature, vapor pressure deficit, and precipitation on a southwestern Colorado mountain to understand:
How much physiological variation is on a single mountain slope between two species
How that variation ultimately affects the potential for CO2 fertilization and differential vegetation responses to rising CO2
For five years, the team has worked on quantifying how two tree species (Ponderosa Pine and Trembling Aspen) can shift their physiology going from low elevation (hot, high vapor pressure deficit environments) up to high elevation (wet, cooler, low vapor pressure deficit environments). They want to understand what that means for each species’ water relations, drought vulnerability, and biogeography in a drying and warming climate.
Quantifying weather parameters
Anderegg and his team use METER all-in-one weather stations to quantify exactly how much the local environment changes from the bottom to the top of the mountain. Anderegg says he’s been surprised at how influential vapor pressure deficit changes are on the tree species. He says, “When we compare Aspens to Ponderosas, we’ve found that the difference in atmospheric demand is a big part of the story, particularly in how they respond to drought stress. There is more atmospheric demand at the bottom of the mountain. So one key objective was to quantify how much drier the air was during the peak mid-summer dry down for most of the species. This was critical then to infer how stomata were responding to that gradient and water stress. It’s really difficult in these wide field plots to actually measure transpiration. But with physiological measurements of leaf water potential, hydraulic conductivity, and the vapor pressure deficit from relative humidity sensors, we could then infer how open the stomata were.”
Coring a ponderosa to measure its growth rate
Anderegg used a pressure chamber to measure leaf water potential and also did a lot of shotgun sampling to measure the hydraulic conductivity in twigs. He describes the process, “I went out at 3 am with a 20 gauge shotgun loaded with birdshot to shoot off branches. We pulled water through the branches by applying a vacuum to a pressure chamber and then inserting one end of the branch. To get the hydraulic conductivity of the branch, we measured how quickly the water moved into the chamber.” (Kolb et al 1996)
Vapor pressure deficit was surprising
Anderegg says vapor pressure deficit changes across the elevation gradient were much stronger than he expected. He says, “It was pretty impressive that as you drove up this elevation gradient, the vapor pressure deficit differed by approximately 1.5 kPa. In the crop realm, a vapor pressure deficit of 2 kPa is pretty intense, but we went from a bit over 1 kPa near the top of the mountain to more like 3 kPa down at the drier bottom, which translates to a remarkably different water-use efficiency.
Two species—two adaptation strategies
When asked what they’ve learned, Anderegg says the difference between the two tree species is pretty amazing. “We’ve seen that the two species have extremely different responses to drought stress. Aspen keeps its stomata open, even at the bottom of the mountain where it’s really dry. It just alters its hydraulic system to try and keep up with it. The Ponderosa, however, does not alter its hydraulic system. It just closes its stomata until it rains in the fall.”
An aspen that died following a drought while it’s neighboring ponderosa lived
Anderegg adds that the two different water relations strategies line up with the type of biogeographical shifts occurring in the two species as the Southwest dries out. He says, “Aspen is sort of a ‘grin-and-bear-it’ species that toughs out drought while Ponderosa is a ‘sit it out’ sort of species. For the last 15 years, the Aspen have been creeping uphill but not gradually. Intermittent droughts are slowly trimming the driest Aspen up the hill in fits and starts. Ponderosa are better at dealing with extreme droughts because they preserve their hydraulic systems. We have not seen mortality pushing the Ponderosa uphill. However, there’s essentially no Ponderosa recruitment (new tree starts) at the bottom of the hill, and the growth rates of adults are a quarter of the rates at the top of the hill. So we think the Ponderosa will move uphill following mean climate change and not in fits and starts. They’ll gradually die off at the bottom and not be replaced by young recruits which will cause them to move uphill in a more gradual manner.”
Dying aspens in the middle of a low elevation aspen stand
Transitioning toward the future
In the years ahead, Anderegg hopes to move into the second phase of the experiment: testing how these two species will respond to CO2 fertilization. He says, “We need to make these measurements over multiple years and many environmental conditions to start to get at how much plasticity any individual plant can manifest (plasticity is the amount that a plant can change its physiology in response to climate change. So if this condition happened, how likely is the plant to respond in a particular way over time) and what the long term trajectories are in these hydraulic traits. We’ve gotten measurements at the height of a significant drought and then another medium year following that drought. We want to transition toward a long-term monitoring perspective that hopefully will give us the information we need to start thinking about how CO2 then plays in.”
You can learn more about Leander Anderegg’s research here: ldlanderegg.com
Kolb KJ, Sperry JS, Lamont BB (1996) A method for measuring xylem hydraulic conductance and embolism in entire root and shoot systems. Journal of Experimental Botany. 47:304, pg 1805-1810
This week, guest author Dr. Michael Forster, of Edaphic Scientific Pty Ltd & The University of Queensland, writes about new research using irrigation curves as a novel technique for irrigation scheduling.
Growers do not have the time or resources to investigate optimal hydration for their crop. Thus, a new, rapid assessment is needed.
Measuring the hydration level of plants is a significant challenge for growers. Hydration is directly quantified via plant water potential or indirectly inferred via soil water potential. However, there is no universal point of dehydration with species and crop varieties showing varying tolerance to dryness. What is tolerable to one plant can be detrimental to another. Therefore, growers will benefit from any simple and rapid technique that can determine the dehydration point of their crop.
New research by scientists at Edaphic Scientific, an Australian-based scientific instrumentation company, and the University of Queensland, Australia, has found a technique that can simply and rapidly determine when a plant requires irrigation. The technique builds on the strong correlation between transpiration and plant water potential that is found across all plant species. However, new research applied this knowledge into a technique that is simple, rapid, and cost-effective, for growers to implement.
Current textbook knowledge of plant dehydration
The classic textbook values of plant hydration are field capacity and permanent wilting point, defined as -33 kPa (1/3 Bar) and -1500 kPa (15 Bar) respectively. It is widely recognized that there are considerable limitations with these general values. For example, the dehydration point for many crops is significantly less than 15 Bar.
Furthermore, values are only available for a limited number of widely planted crops. New crop varieties are constantly developed, and these may have varying dehydration points. There are also many crops that have no, or limited, research into their optimal hydration level. Lastly, textbook values are generated following years of intensive scientific research. Growers do not have the time, or resources, to completely investigate optimal hydration for their crop. Therefore, a new technique that provides a rapid assessment is required.
How stomatal conductance varies with water potential
There is a strong correlation between stomatal conductance and plant water potential: as plant water potential becomes more negative, stomatal conductance decreases. Some species are sensitive and show a rapid decrease in stomatal conductance; other species exhibit a slower decrease.
Plant physiologist refer to P50 as a value that clearly defines a species’ tolerance to dehydration. One definition of P50 is the plant water potential value at which stomatal conductance is 50% of its maximum rate. P50 is also defined as the point at which hydraulic conductance is 50% of its maximum rate. Klein (2014) summarized the relationship between stomatal conductance and plant water potential for 70 plant species (Figure 1). Klein’s research found that there is not a single P50 for all species, rather there is a broad spectrum of P50 values (Figure 1).
Figure 1. The relationship between stomatal conductance and leaf water potential for 70 plant species. The dashed red lines indicate the P80 and P50 values. The irrigation refill point can be determined where the dashed red lines intersect with the data on the graph. Image has been adapted from Klein (2014), Figure 1b.
Taking advantage of P50
The strong, and universal, relationship between stomatal conductance and water potential is vital information for growers. A stomatal conductance versus water potential relationship can be quickly, and easily, established by any grower for their specific crop. However, as growers need to maintain optimum plant hydration levels for growth and yield, the P50 value should not be used as this is too dry. Rather, research has shown a more appropriate value is possibly the P80 value. That is, the water potential value at the point that stomatal conductance is 80% of its maximum.
Irrigation Curves – a rapid assessment of plant hydration
Research by Edaphic Scientific and University of Queensland has established a technique that can rapidly determine the P80 value for plants. This is called an “Irrigation Curve” which is the relationship between stomatal conductance and hydration that indicates an optimal hydration point for a specific species or variety.
Once P80 is known, this becomes the set point at which plant hydration should not go beyond. For example, a P80 for leaf water potential may be -250 kPa. Therefore, when a plant approaches, or reaches, -250 kPa, then irrigation should commence.
P80 is also strongly correlated with soil water potential and, even, soil volumetric water content. Soil water potential and/or content sensors are affordable, easy to install and maintain, and can connect to automated irrigation systems. Therefore, establishing an Irrigation Curve with soil hydration levels, rather than plant water potential, may be more practical for growers.
Example irrigation curves
Irrigation curves were created for a citrus (Citrus sinensis) and macadamia (Macadamia integrifolia). Approximately 1.5m tall saplings were grown in pots with a potting mixture substrate. Stomatal conductance was measured daily, between 11am and 12pm, with an SC-1 Leaf Porometer. Soil water potential was measured by combining data from an MPS-6 (now called TEROS 21) Matric Potential Sensor and WP4 Dewpoint Potentiometer. Soil water content was measured with a GS3 Water Content, Temperature and EC Sensor. Data from the GS3 and MPS-6 sensors were recorded continuously at 15-minute intervals on an Em50 Data Logger. When stomatal conductance was measured, soil water content and potential were noted. At the start of the measurement period, plants were watered beyond field capacity. No further irrigation was applied, and the plants were left to reach wilting point over subsequent days.
Figure 2. Irrigation Curves for citrus and macadamia based on soil water potential measurements. The dashed red line indicates P80 value for citrus (-386 kPa) and macadamia (-58 kPa).
Figure 2 displays the soil water potential Irrigation Curves, with a fitted regression line, for citrus and macadamia. The P80 values are highlighted in Figure 2 by a dashed red line. P80 was -386 kPa and -58 kPa for citrus and macadamia, respectively. Figure 3 shows the results for the soil water content Irrigation Curves where P80 was 13.2 % and 21.7 % for citrus and macadamia, respectively.
Figure 3. Irrigation Curves for citrus and macadamia based on soil volumetric water content measurements. The dashed red line indicates P80 value for citrus (13.2 %) and macadamia (21.7 %).
From these results, a grower should consider maintaining soil moisture (i.e. hydration) above these values as they can be considered the refill points for irrigation scheduling.
Further research is required
Preliminary research has shown that an Irrigation Curve can be successfully established for any plant species with soil water content and water potential sensors. Ongoing research is currently determining the variability of generating an Irrigation Curve with soil water potential or content. Other ongoing research includes determining the effect of using a P80 value on growth and yield versus other methods of establishing a refill point. At this stage, it is unclear whether there is a single P80 value for the entire growing season, or whether P80 shifts depending on growth or fruiting stage. Further research is also required to determine how P80 affects plants during extreme weather events such as heatwaves. Other ideas are also being investigated.
For more information on Irrigation Curves, or to become involved, please contact Dr. Michael Forster: [email protected]
Klein, T. (2014). The variability of stomatal sensitivity to leaf water potential across tree species indicates a continuum between isohydric and anisohydric behaviours. Functional Ecology, 28, 1313-1320. doi: 10.1111/1365-2435.12289
Henry Adams, a PhD student at the University of Arizona, is studying the effect of climate change and drought on Piñon Pines in the university’s Biosphere 2 lab (see part 1). This week, find out how the researchers made comparisons at leaf level, transplanted the trees, and future implications for the Piñon Pine.
The Piñon Pine, a conifer with an extensive root system, grows at high elevations in the Southwest. (Image: naturesongs.com)
Sensitivity to Dry Conditions
Another part of the drought study involved a hydrologist who was interested in using weighing lysimeter data to parameterize some models used by hydrologists to model water loss during drought. “The lysimeters are a pain to run, but they’re pretty sensitive,” says Adams. “They can measure with a 0.1 kg precision, so that sounds like a good way to quantify water loss. It turns out that stomatal conductance from the porometer actually appears more sensitive than the weighing lysimeter data. Water loss from the scale hits zero pretty quickly, and we can’t measure any loss after a couple of weeks, but we can still see water loss with our porometer data from the morning and the evening.”
The Piñon Pine’s root system makes it remarkably drought tolerant, but an extended drought in combination with a bark beetle outbreak killed 12,000 hectares of the trees in 2003. (Image: naturesongs.com)
Expanding the Experiment
At the peak of the experiment, Adams had undergraduates and lab techs running up to three porometers at a time all day long, and although he’s still buried in data from the first experiment, he’s looking forward to accumulating even more data. “One limitation of our study is that the trees had pretty small root balls when they arrived. We’ve transplanted some trees [at different elevations at a site] in northern Arizona using a full-sized tree mover to get as big a root to shoot ratio as possible in the transplant. We’ll be using the porometers to try to understand the physiology of how these trees die and to predict their temperature sensitivity in the light of global climate change, using elevation change as a surrogate for temperature. We also have trees at the site that are not transplanted to serve as a control for the transplants.”
Adams acknowledges that not everyone in the Southwest is worried about the Piñon Pine. “We work in a system that doesn’t have a lot of economic value. A lot of the ranchers are happy to see the pines go. They just think there will be a lot more grass for the cattle, and firewood cutters are out there cutting up the dead trees and selling them.” But if temperature alone makes trees more susceptible to drought, the implications go far beyond economics. Adams puts it succinctly, if somewhat mildly: “It’s kind of scary.”
In the name of science, Henry Adams has killed a lot of trees. Adams, a PhD student at the University of Arizona, is studying the effect of climate change and drought on Piñon Pines. The Piñon Pine, a conifer with an extensive root system, grows at high elevations in the Southwest. Its root system makes the Piñon Pine remarkably drought tolerant, but in 2002- 03, an extended drought in combination with a bark beetle outbreak killed 12,000 hectares of the trees. It was a 100 year drought, the driest period on record, and interestingly it coincided with temperatures 2 to 3˚C above recorded averages.
Adams and his advisors wondered if increasing temperatures due to climate change might exacerbate the effects of drought and accelerate tree die-off. The University of Arizona has an unusual opportunity to test drought conditions and temperature change in its Biosphere 2 lab. Biosphere 2, a unique 3-acre enclosed “living laboratory” in the high Arizona desert, once hosted 8 people for two years of self-contained survival living. Now it hosts research projects, and Adams was able to use space inside to induce drought in two separate treatments of transplanted Piñon pines, one at ambient temperatures and one at temperatures 4˚C above ambient.
Sobering Outlook for the Piñon Pine
“Obviously, the warmer trees should die first,” says Adams. “But we want to test whether temperature change, independent of other factors, accelerates mortality.” If that acceleration in fact occurs, a shorter drought, the kind the Piñon Pine has historically been able to wait out, might cause a significant die-off.
Naturally, Adams and his colleagues did more than just watch how fast trees would die without water. They also studied the trees physiological response to drought, measuring gas exchange, water potential, and stomatal conductance. To measure stomatal conductance, they used a leaf porometer, making almost 9,000 separate measurements in sessions that lasted from sunup to sundown on one very long day once each week.
Stomatal Conductance in Conifers
There isn’t much guidance in the porometer manual for people who want to use it on conifers, so Adams “played around with it a little bit” on non-drought stressed trees before he started his study. He found that the best way to get good readings was to cover the aperture with a single layer of needles. “Needles are this three-dimensional thing,” he explains. “They have stomata on several sides, depending on the species. If you imagine that the fingers on your hand are needles sticking up from a branch, we just took those and pushed them together to make sure that there was just a one needle thick covering over the aperture. If you spread your fingers, that’s what it would be like if you didn’t totally cover the aperture-then you underestimate the conductance. We also found that if we stuck several layers in there, we could drive the conductance number up.
Next week:Find out how the researchers made comparisons at leaf level, transplanted the trees, and future implications for the Piñon Pine.
Dr. Y. Osroosh, now a researcher at Washington State University, believes that plants are the best soil moisture sensors (see part 1). He and his team have developed a new model for interpreting plant canopy signals to indirectly determine soil moisture in a Fuji apple orchard. Below are the results of their efforts and what he sees as the future of this research.
Could plants be the best indicators of soil moisture?
Osroosh says they expected to see correlations, but such strong relationships were unexpected. The team found that soil water deficit was highly correlated with thermal-based water stress indices in drip-irrigated apple orchard in the mildly-stressed range. The relationships were time-sensitive, meaning that they were valid only at a specific time of day. The measurements taken between 10:00am and 11:00am (late morning, time of maximum transpiration) were highly correlated with soil water deficit, but the “coefficient of determination” decreased quickly and significantly beyond this time window (about half in just one hour, and reached zero in the afternoon hours). Osroosh says this is a very important finding because researchers still think midday is the best time to measure canopy water stress index (CWSI). He adds, “The apple trees showed an interesting behavior which was nothing like what we are used to seeing in row crops. They regulate their stomata in a way that transpiration rate is intense late in the morning (maximum) and late in the afternoon. During the hot hours of afternoon, they close their stomata to minimize water loss.”
Researchers have found good relationships between CWSI and soil water content in the root zone near the end of the season at high soil water deficits in row crops.
Osroosh points to other efforts which have tried to correlate remotely-sensed satellite-based thermal or NIR measurements to soil water content. He says, “The closest studies to ours have been able to find good relationships between CWSI and soil water content in the root zone near the end of the season at high soil water deficits in row crops. Paul Colaizzi, a research agricultural engineer did his PhD research in part on the relationship between canopy temperature, CWSI, and soil water status in Maricopa, Arizona; also motivated by Jackson et al. (1981). Steve Evett and his team at Bushland, Texas are continuing that research as they try to develop a relationship between CWSI and soil water status that will hold up. They are using a CWSI that is integrated over the daylight hours and have found good relationships between CWSI and soil water content in the root zone near the end of the season when plots irrigated at deficits begin to develop big deficits.”
Osroosh wants to study other apple cultivars, tree species, and perhaps even row crops, under other irrigation systems and climates.
What’s The Future?
In the future, Osroosh hopes to study the limitations of this approach and to find a better way to monitor a large volume of soil in the root zone in real-time (as reference). He says, “We would like to see how universal these equations can be. Right now, I suspect they are crop and soil-specific, but by how much we don’t know. We want to study other apple cultivars, tree species, and perhaps even row crops, under other irrigation systems and climates. We need to monitor crops for health, as well, to make sure what we are measuring is purely a water stress signal. One of our major goals is to develop a sensor-based setup which, after calibration, can be used for “precise non-contact sensing of soil water content” and “stem water potential” in real-time by measuring canopy temperature and micrometeorological parameters.”
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.
Due to controversy over the growing number of high capacity wells in the Wisconsin Central Sands, University of Wisconsin PhD student, Mallika Nocco, is researching how agricultural land use, irrigation, and climate change impact the region’s water-energy balance. She and her team have uncovered some surprising results.
A class 1 trout stream has sufficient natural reproduction to sustain populations of wild trout at or near carry capacity.
Water Use Debate
There are class 1 trout streams in the Central Sands region, and some people worry that the increasing number of high capacity wells used for agriculture will reduce the water levels in those streams. “Lake Huron has lost about 11 feet of water since 2000,” says one resident of the Central Sands area, “and water levels are continuing to drop.” In 2008, the small well he used to pump drinking water went dry, and he blames the high capacity wells.” (Aljazeera America) On the other side of the debate, agriculture irrigated by these wells is extremely valuable to the state, and growers have taken quite a bit of time to understand the water cycle and their role in it. You can read about their water management goals and accomplishments here.
Updating Former Research
Irrigated agriculture wasn’t prevalent or profitable in the Wisconsin Central Sands until groundwater irrigation with high capacity wells became feasible in the 1950s. Since then, this relatively small ecological region has gone from 60 high capacity wells in 1960 to over 2,500 today.
Mallika Nocco is studying potential groundwater recharge from irrigated cropping systems that use the wells, hoping to understand if the irrigation water is lost or returned to the groundwater. She says, “Until now, we’ve been relying on models validated by two lysimeters in the 1970s. Champ Tanner (one of the fathers of environmental biophysics) designed the weighing lysimeters, and they were very accurate, but we wanted to do a larger scale study with multiple crops to get a handle on interannual variability and to improve our understanding of recharge in the region so we can do a better job of managing irrigation and groundwater.”
Lysimeter installation into actively managed fields presented challenges that the research team had to overcome.
Nocco used twenty-five drain gauge lysimeters to capture vadose zone flux under potato and maize cropping systems. She monitored soil water (and temperature) flux by stratifying water content sensors from the soil surface to a depth of 1.4 meters. She also estimated evapotranspiration (ET) using a porometer to measure stomatal conductance, in addition to obtaining micrometeorology, leaf area index, and gas exchange measurements.
Nocco and her team had to put their sensors in to avoid cultivation, so they extended the drain gauge PVC that comes up to the soil surface and removed it any time there was major fieldwork, whether it was tillage or planting, so that the area over the lysimeter got the same treatment as the rest of the agricultural fields.
Below the Root Zone
Nocco says getting the lysimeters below the root zone was a challenge. Next week, read about how she solved that challenge, how she used a GPS system to find the lysimeters within a half-inch of accuracy, and about her surprising conclusions.
Want to develop an appreciation for Gore-Tex? All you need is five minutes in a rubber raincoat. But how do you know whether the North Face knock-off you’ve just purchased in China for a ridiculously low price is Gore-Tex or rubber? If you’re a METER researcher, you dash back to your hotel room and clamp a porometer onto the fabric.
The leaf porometer was designed to measure stomatal conductance in leaves. It’s typically used by canopy researchers to relate stomatal resistance to canopy attributes like water use, water balance, and uptake rates of herbicides, ozone, and pollutants. Yet, from the beginning, Dr. Gaylon Campbell, the porometer’s designer, saw the possibilities: “Give this to someone with only a passing interest in research, a ten-year-old kid for example, and they’ll go around the garden and come back with some really interesting observations,” he said. “There are lots of questions about what loses water and what doesn’t that you can answer with this instrument.”
SC1 Leaf Porometer
Dr. Campbell was probably thinking the questions would be about organic material—but it hasn’t always turned out that way. By putting a wet paper towel on one side of an inorganic material and clamping the towel and the material into the porometer head, you can measure how well water vapor diffuses through the material. Using this strategy, the researcher in China discovered that his raincoat was pretty much impermeable (unlike real Gore-Tex, which is a good vapor conductor). Spotting the fake North Face coat is now a favorite part of METER’s canopy seminar. And the coat is not the only leafless item that has been tested. “People will clamp the porometer on just about anything,” Doug Cobos, a METER research scientist, admitted. He himself grabbed it when a local contractor brought in a sample of some supposedly unique house wrap.
Siding is supposed to protect a house from the elements, but most building codes now require that houses be wrapped under the siding. House wraps provide a secondary defense against liquid water and increase energy efficiency by preventing drafts. As with raincoats, high-performance house wrap needs to repel water and stop wind while remaining permeable to water vapor.
House under construction with protective wrap under the siding.
The practice of applying a sheathing of tar paper under siding is a hundred years old, but in the last fifteen years, high tech house wraps made from polypropylene in combination with a push towards energy efficiency have made the house wrap market big and competitive. Upstart wraps try to gain market share through innovation and the one brought in by the local contractor came along with an outlandish claim. According to the manufacturer’s rep, this plastic wrap would allow water vapor to diffuse out while preventing any from diffusing in. Some builders might have scratched their heads and moved on. Our local man decided to check it out. He brought a sample of the mystical wrap to Decagon. Out came the porometer and a quick scientific study of house wrap was born.
Dr. Cobos tested industry standard Tyvek house wrap along with the great one-way pretender. The results? “The vapor conductance of the new material was basically the same, regardless of which side of the material faced wet filter paper,” Dr. Cobos said. “And, in fact, the material didn’t diffuse well at all. Its conductance was similar to cheap perforated plastic. It didn’t come close to the performance of Tyvek.” Ultimately, the newfangled wrap was retested by the manufacturer and taken off the market.
Probably the porometer’s best and highest use is still in canopy research, but it still gets pulled out to measure whatever seems interesting, organic and inorganic alike. That dovetails with Dr. Campbell’s vision of it as a tool for routine use in canopy studies—and everywhere else. Can you use it on yourself? “Oh sure,” says Dr. Campbell. “People clamp the porometer on their fingers all the time. That’s a quick way to see if it’s working.” He grins. “Maybe you could use it as a lie detector on your kids.”
Measuring the stomatal conductance of a leaf should be a pretty straightforward problem. The conductance is just the flux density of water vapor divided by the concentration difference between the leaf and its surroundings. Common approaches to this problem involve either flowing air of known vapor concentration over the leaf and measuring how much water vapor is picked up, or sealing a cup of known capacity to the leaf surface and measuring how quickly the vapor concentration in the cup increases. Both of these, though simple in concept, require quite a bit of expensive equipment to pull off. We wanted a simpler approach. We put a humidity sensor in a small tube, the end of which could be pressed against the leaf. As vapor diffused through the tube the humidity in the tube increased. The conductance of the tube is easily calculated. It is the diffusivity for water vapor divided by the tube length. The leaf conductance could be computed from the tube length, the humidity in the tube and the ambient humidity. That worked, but it turned out that ambient humidity variations introduced too much error, so we later added a second humidity sensor toward the distill end of the tube. Our approach was very simple, and works well, but it wasn’t a new idea.
Cross section of METER’s Leaf Porometer
I read of a similar device in a conference proceedings (I don’t recall the name of the conference) in 1977 when I was on sabbatical at University of Nottingham in England. The device wasn’t for leaves. It was developed by a medical researcher to assess severity of burn injuries, and for use on neonatal infants. The skin of a non-sweating human is pretty impermeable to water. A typical conductance is around 5 mmol m-2 s-1. This is about half the value for a leaf with stomates closed, and about two orders of magnitude lower than leaves with open stomates. Burned skin, however, is much more permeable, and the permeability is related to the severity of the burn. A device that could measure the permeability of skin would therefore give information on the severity of the burn. The researcher built an apparatus, similar to our porometer, with two closely spaced humidity sensors in a diffusion tube. As I recall, it was somewhat successful, but I’m not aware of it ever having been commercialized or used much after that. The application for infants is also interesting. Full-term babies have low skin conductances. I haven’t seen measurements, but assume they are similar to adult conductances. The skin of premature infants, though, has a much higher conductance. I don’t know typical conductance values, but do know that, without intervention, the conductance can be so high that evaporative water loss from the baby will reduce body temperature to dangerously low levels, even in an incubator. I don’t know if later work has been done to measure skin conductance, but it is interesting that the first applications of the technology we now use in our porometer was for measuring conductance of the human epidermis, not the epidermis of leaves.