Day Two: Independent Research

Today, I went over both of Chris Oishi's articles in depth, analyzing the detailed theory behind the Granier Thermal Dissipation Probe method to measure sap flux.  I then did some further searching and discovered another article relating to my research.  I coordinated with maintenance today in order to get the wiring mounted in the trees.  It was raining all day today, so the wiring has not gone up - however, some drilling has been done and everything is prepared.  I'm really excited to get everything into the tree and ready for data collection!  I'm a little worried about the wiring of the datalogger - it looks pretty complicated.  All the software is also designed for Windows XP or older, so it may be a little difficult to get everything to interface with each other in the beginning.  I'm still waiting on the phenology camera to arrive from Harvard and BU.  It may not arrive until earlier this week - there was some delay in getting everything packaged and sent.  Once it arrives, I'll have to coordinate with maintenance again in getting it installed on the roof of the art studio.  It'll measure visual physiological traits of the trees so that we can correlate and link it with the sap flux data that we'll hopefully begin recording tomorrow or the day after.  I have an appointment with Chris Oishi and his team (Henry, Poy) tomorrow.  I'm excited to be there!

Here are my article summaries for the two Oishi articles:

Estimating components of forest evapotranspiration: A footprint approach for scaling sap flux measurements: A. Christopher Oishi et al. 2008

This article described the methods used to resale sap flux methods in order to better match eddy covariance method measurements.  This is important because, only after understanding all the components of evapotranspiration can scientists begin to hypothesize and analyze fluctuations in specific components of the evapotranspiration.  Components included canopy interception, evaporation from soil, and transpiration from the canopy.  The equation for this is .  The sap flux sensors are the same thermal dissipation probes as described in Granier 1987.  Several trees of each species were outfitted with the probes and thens called first to the tree level, and then to the stand level.  The tree level is determined by the formula .   is the volume of the effective sapwood,  is the distance from the center of the tree to the centroid of the sap flux curve, and  is the integrated area beneath the fitted curve for a single tree.  This is further scaled o the stand level by comparing allometric relationships (size vs. sap flux) for the different trees.  This was the combined with LAI measurements to scale to the stand level.

There is a general decreasing trend in sap flux as we move from the outer edge of the xylem to the interior.This relationship is graphically displayed in Fig. 4 in the article.  The total amount of EVT that is actually transpiration is 54%.  This means that there is a significant amount of EVT that is related to either evaoporation from the soil or to the canopy interception.  Finally, the conversion from mV to sap flux can be quantified with the equation , where  is the maximum temperature differential where sap flux is zero.

The article reviewed focused a great deal on how to accurately apply Granier’s methods to the oak and pine trees found in the Duke Forest site. 

Interannual Invariability of Forest Evapotranspiration and Its Consequence to Water Flow Downstream: A. Christopher Oishi et al. 2010

This paper concerned itself with measuring the effect of drought conditions on transpiration and total evapotranspiration.  Data was taken from the same 2008 study that was described in Oishi et al. 2008.  This study revealed surprising results.  Transpiration rates, when integrated over a season or a year, actually did not significantly change over their non-drought counterparts.  That is to say, yearly transpiration remained around the same during years of drought and relative wetness.  The theory for these results is that the increased  in times of drought actually benefits transpiration compared to the low  in times of wetness.  So as a general trend, increasing Vapor Pressure Deficit results in an increased transpiration rate.

It appears that L. tulipifera showed the greatest sensitivity to drought conditions in this study, which concurs with earlier publications.  L. styraciflua showed low levels of sensitivity contrary to other studies.  Carya is one of the most drought resistant genera and reflected that in this study.  Finally, Quercus showed extreme sensitivity below a cutoff moisture level of 0.2m³m^-3.

Transpiration plays a significant role in the water budget, and this study shows that, at least for a short period of time, drought does not significantly affect transpiration.  The article notes that over a period of extended drought, however, transpiration rates would certainly fall.

I also reviewed another article:

Environmental controls on sap flow in a northern hardwood forest: Bovard, B. D. et al. 2005

The article describes how researchers empirically determined which of three factors affected sap flow in four different species of trees in Northern Michigan.  The three factors evaluated were photosynthetically active radiation (PAR), vapor pressure deficit (D), and soil water.  PAR is essentially a measure of the amount wavelength that a tree can use to photosynthesize, and has units .  The higher the value of PAR, the more light in these wavelengths is available to the tree. D describes the difference between the amount of water in the air and the maximum amount of moisture that the air can hold, and has units .  Soil water content is simply a measure of the saturation of soil, and is measured in percent.  The researchers monitored four species of trees within a certain radius of a measuring tower over two periods of drought, and looked for relationships between PAR, D, soil water, drought, and sap flux.  The researchers concluded that, day by day, soil water content only affected the sap flow rate if D was over a certain value (). 

The stand of trees was monitored for 61 days in 1999.  PAR was measured with a quantum sensor, air temperature with a shaded, ventilated thermocouple, water vapor partial pressure with an infrared gas analyzer, and precipitation with a tipping bucked rain gauge.  Wind speed and direction were measured, along with water vapor levels.  All data was taken in 30s means and put together. 

Researchers calculated aerodynamic conductance and then installed continuously heated thermocouples into the tree.  Continuously heated thermocouples are fundamentally different than heat-pulse thermocouples because the heat operates continuously and the reference thermocouple monitors differences in heat continuously.  The probes were inserted into the north side of the four species.  Measurements from the probe were extrapolated to the entire tree by finding the area of sapwood depth.  This was done by staining a tree core with 2% tetrazolium trimethyl-chloride, which stains the sapwood red. 

There was a significant difference in sap flow between species in the experiment.  There was an overall positive linear correlation between PAR and sap flow, along with D and sap flow.  However, there seemed to be little to no correlation between soil water concentration and sap flow in any species.  Soil water concentration seemed to only significantly affect sap flow when D was greater than 1.  The dominant controllers of sap flow at the individual level were the same as those at the ecosystem level.  The researchers determined that their calculations of sap flow for the entire stand were chronically undervalued.  They hypothesized that error in procedure could have resulted in a lower cross section area than was actually present. 

In conclusion, climate change that affects PAR, D, and soil water concentration will affect the ecosystem water use and the hydrologic cycle.  However, PAR and D are the two dominating factors.  As early successional species in the Northern Michigan stand are replaced with later succession species, the patterns of water use will also change.

Overall, I think that this was a successful day for research.  Not only did I read the three articles reviewed above, I looked over the phenology website and the Duke FACE site website.  These may all be places to look in once I begin getting data.  Tomorrow I'll be at Duke pretty much all day.