How to Measure Canopy Stomatal Conductance.

 

Canopy stomatal conductance is a measure of water vapour transfer from the plant to the atmosphere. How to measure canopy stomatal conductance is explained in this article along with methods, equations and statistics on its analysis.

Conductance is the ease with which water can transfer from the plant to the atmosphere with higher values indicating easier transfer. Canopy conductance is a fundamental parameter in plant hydraulics and its understanding is important for plant water relations.

Canopy conductance (denoted by G_c or G_t) and canopy stomatal conductance (denoted by G_s) are often used interchangeably but have the same definition and methodology. Canopy stomatal conductance is measured via sap flow sensors and integrates water flux of the entire canopy. In contrast, stomatal conductance, or leaf stomatal conductance, is a leaf level measurement of conductance and is typically measured with a porometer (e.g. SC-1 leaf porometer) and is usually denoted with the symbol g_s.

 

Why is canopy conductance important?

Canopy conductance measurements can be used to understand plant physiology, water relations and hydraulic architecture. Canopy conductance can also be used to gain insights into energy exchanges between vegetation and the atmosphere as well as to identify ecological interactions and evolutionary adaptations.

The inverse of conductance is resistance. Therefore, knowledge about canopy conductance can be used to gain insights into resistance of water flow within and from plants. The most important variable influencing canopy resistance is stomatal opening where open stomata offer less resistance and partially closed stomata creating higher resistance.

Canopy conductance is useful for understanding the hydrologic cycle as well as energy exchanges between vegetation and the atmosphere. Canopy conductance from sap flow measurements is routinely used in combination with Eddy Correlation techniques to understand water vapour exchanges between vegetation and the atmosphere.

Canopy conductance and resistance can also be used to compare species hydraulic architecture. For example, the canopy conductance of coexisting species within a habitat, or phylogenetic independent contrasts (PIC) between closely related species, can be compared to gain insights into the ecological and evolutionary significance of varying hydraulic architecture. Differences in canopy conductance or resistance will likely reflect evolutionary adaptations of the hydraulic pathways within the plant (e.g. xylem diameter, xylem length, xylem density, stomatal density, wood density, and more). Therefore, accurate measurements of canopy conductance is useful for plant physiologist and evolutionary biologists.

 

Required equipment to measure canopy conductance.

An Implexx Sap Flow Sensor and an ATMOS 41 Weather Station is required to measure canopy conductance.

The Implexx Sap Flow Sensor is needed to measure water (sap) flux within the plant.

The Implexx Sap Flow Sensor is installed on the trunk or stem of the plant or tree. Typically, the sensor is installed lower on the trunk beneath the crown. However, some experiments or hypotheses may test canopy conductance at various locations along the trunk such as upper versus lower branches or northern versus southern aspects.

The ATMOS 41 Weather Station is needed to measure atmospheric evaporative demand variables including vapour pressure deficit (VPD), wind, solar radiation, temperature, and relative humidity.

The weather station should be installed as close as possible to the measured plants while still following standard protocols on where to install a weather station (i.e. in the open and away from obstructions).

 

 

Canopy conductance calculated with an inverse Penman-Monteith equation.

Paudel et al (2015) proposed a method to calculate canopy conductance using an inverse Penman-Monteith evapotranspiration equation. The following equation was proposed by Paudel et al (2015) to calculate canopy conductance (G_c):

 

 

where F is sap flux density (kg m⁻² s⁻¹) measured with the Implexx Sap Flow Sensor, λ is the latent heat of vaporization of water (J kg⁻¹), G_a is canopy aerodynamic resistance (m s⁻¹), Δ is the slope of the relationship between water vapor pressure and temperature [KPa °C⁻¹], ρ is the density of dry air (kg m⁻³)], C_p is specific heat of dry air at constant pressure (J kg⁻¹ C⁻¹), D is vapor pressure deficit (KPa), γ is the psychrometric constant (KPa °C⁻¹), R_n is net radiation (MJ m⁻² s⁻¹) and G is soil heat flux (MJ m⁻² s⁻¹).

Here is how to calculate all the parameters in the canopy conductance equation:

• F – use data measured from an Implexx Sap Flow Sensor. Note that F is sap flux density which is kg of sap per m² of sapwood area per second. The Implexx Sap Flow Sensor outputs total sap flow (kg per hour) and this will need to be converted to F.
• λ =1000000*(2.5-(0.00236*T)) where T is air temperature (°C) measured by the ATMOS 41 weather station.
• G_a = 0.1*u where is wind speed (m s⁻¹) measured by the ATMOS 41 Weather Station.
• Δ =((λ/1000)*M_w*e_o)/(0.00813*(T+273)*(T+273))/(1000)
• M_w = molecular weight of water and is a constant value of 18.
• e_o = saturated vapour pressure (kPa) =0.6108*EXP((17.27*T)/(T+237.3))
• R_n – it is possible to use the solar radiation measurement from the ATMOS 41 Weather Station. Check the units of measurement, though, as most pyranometers measure in units W m⁻². There are many other methods to measure net radiation with other sensors, if required.
• G – soil heat flux can be measured with a sensor. Often, G is negligible, and it is conveniently given a value of 0 or is ignored.
• ρ is a constant value of 1.204.
• C_p is a constant value of 1010.
• D =(610.8*EXP(17.29*T/(T+237.3))/1000)*(1-RH) where RH is relative humidity and is measured with the ATMOS 41 Weather Station.
• γ is a constant value of 0.0667.

 

Canopy conductance of the “average” stomata.

Köstner proposed a method to calculate the canopy conductance of the “average” stomata across the entire canopy (Köstner et al 1992, 1996, 2001). Total water vapour transfer conductance (G_t) is defined as:

G_t = (G_v*TK)*(E_c/D)

where:

• G_v is the gas constant of water vapour and is a constant value of 0.462 (m³ kPa⁻¹ kg⁻¹ K⁻¹).
• TK is air temperature in Kelvins (i.e. T + 273.15 where T is air temperature measured by the ATMOS 41 Weather Station).
• E_c is canopy water flux (kg m⁻² s⁻¹) calculated as total tree sap flow, measured from the Implexx Sap Flow Sensor (l/h) divided by canopy projection area (m²) per second. Note that E_c is different to F as E_c is corrected for canopy projection area whereas F is corrected for sapwood area.
• D is vapour pressure deficit (kPa) =(610.8*EXP(17.29*T/(T+237.3))/1000)*(1-RH) where RH is relative humidity and is measured with the ATMOS 41 Weather Station.
• Köstner et al (1992, 2001) noted that the G_c is related to G_t by subtracting G_a, defined as: 1/G_c = 1/G_t – 1/G_a

 

Interpretation of data.

Researchers must carefully decide whether G_c or G_t, or both, is more appropriate parameter for their experiment and hypotheses. The interpretation of results will likely differ depending on whether data has been analysed via the G_c or G_t equation. As an example, below is presented a data set from 30 days of measurement for a 1.5m tall, potted, lilly pilly (Syzygium paniculatum Gaertn.) sapling growing in a common garden, Melbourne, Australia during summer. The data set includes sap flow measured with the Implexx Sap Flow Sensor, weather data measured via the ATMOS 41 Weather Station, and G_c and G_t calculated via the above equations.

Figure 1 is an example set of data over 7 days for sap flow, G_c and G_t. All variables show a cycle where maximums occur during the day and minimums at night. However, there are differences when maximums occur during the day. During the night, G_c is consistently low or zero whereas sap flow is relatively high during the early night and G_t is relatively high during the latter portion of the night. These differences may be related to physiological processes such as stem refilling or nocturnal transpiration. However, further measurements and analysis are required for a better understanding.

 

 

 

The relationship between G_c, G_t and atmospheric variables may also lead to difference in data interpretation. For example, Figure 2 shows the relationship between VPD, G_c and G_t. Figure 2A shows no relationship between VPD and G_c; whereas Figure 2B shows a negative relationship between VPD and G_t. Careful consideration of the G_c and G_t equation is required for appropriate interpretation of these results.

 

 

Conversely, the relationship between G_c, G_t and solar radiation yields a different interpretation. Figure 3A is the correlation between solar radiation (daytime only) and G_c with a positive linear relationship. Figure 3B is the relationship between solar radiation (daytime only) and G_t with no relationship.

 

 

Although these examples and analyses are simple, and more data is required for a comprehensive interpretation, it does emphasise that care needs to be exercised when choosing canopy conductance methods. The most appropriate method, with the correction interpretation, is required for a rigorous and thorough analysis.

 

References.

Köstner et al 1992. Oecologia 91, 350-359. https://www.jstor.org/stable/4220079

Köstner et al 1996. Theoretical and Applied Climatology 53, 105-113. https://link.springer.com/article/10.1007/BF00866415

Köstner 2001. Meteorology and Atmospheric Physics 76, 69-82. https://link.springer.com/article/10.1007/s007030170040

Pauldel et al 2015. Tree Physiology 35, 425–438. doi:10.1093/treephys/tpu113