Posts from the ‘Canopy’ Category

Nov 1

Spectral Reflectance and Water Content in the Wasatch Plateau Experiment

We chose to collaborate with Brigham Young University in an experiment on the Wasatch Plateau in 2009 because a scientist friend of ours had been working in that area the previous five years, and he noticed there were big grazing responses. The plants growing in the long-term grazed areas were all drought tolerant, while ungrazed plots had plants that were often found only in wetter areas. The only difference was the fence that kept sheep on one side and not on the other. The big question was: how does water influence plants in this ecosystem that we understand relatively well? The story had always been the influence of grazers, when in fact, maybe the indirect consequence of grazing was mediated by water.

The Wasatch Plateau above Ephraim Canyon, UT, USA.

METER donated some sensors in order to set up an experiment where we changed the amount of water in various plots of land. We had rain exclusion plots, and we had treatments where we collected all incoming rainfall and reapplied it either once a week or every three weeks. This allowed us to say to what extent this system was controlled by water during the growing season. To do this, we took measurements with our prototype NDVI Spectral Reflectance Sensor to measure canopy greenness. We also used our prototype volumetric water content sensors to measure soil moisture (this was a few years ago and the sensors were prototypes at the time). Using these sensors, we found that water is critical in a system people have dismissed as being climate-controlled because it’s at the top of a mountain.

A very early prototype of a NDVI sensor measuring canopy greenness in experimental plots on the Wasatch Plateau.

It turns out the amount and timing of precipitation makes a big difference. We were able to directly connect plant survival, not just to the grazing treatment, but to the actual amount of water that was in the soil. Also, using continuous NDVI data, we were able to look closely at the role of grazing on plant canopies. When we looked at our NDVI data, we were able to see a seasonal signal, not just a single snapshot sample in time. So by having the richer data from the data loggers, we obtained a more nuanced understanding of the impact of land use on these important ecological processes.

One of the mistakes we made was failure to include redundancy in the system. We only had two replicates, so when one of them went down we ended up having just one little case study. However, that mistake gave us new ideas on how to set up a better system using the right sensors for the job, and it generated a new idea on how to get real-time analysis of data. In our new Desert FMP project, we have a much better-replicated system where more is invested in the number of sensors that we’re putting out. Each treatment combination will have five to ten water potential sensors. We are also developing a system where we can analyze data in real-time, so this time we will know when a sensor goes out if a student accidentally kicks it.

For more details on the Wasatch Plateau Experiment, watch for our published paper that we’ll link to when it comes out.

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Dec 11

Burn victim research leads to new method for measuring stomatal conductance

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.