Background

Sapflow technology is a tool in plant ecology that uses heat as a proxy of water flow within stems or other plant organs. Although a variety of sapflow methods have been developed (see review by McElrone and Bleby, 2011), external miniature sapflow gauges make use of the heat ratio method (HRM) (Burgess et al., 2001) for estimating plant sap velocity.

The HRM relies upon two thermocouples evenly spaced either side of a heating element along the same axis as the flow of sap (Burgess et al., 2001). (Note: Generally, water moves up through a plant from the roots towards the leaves, where it is lost through stomatal pores during evapotranspiration [E].) Marshall (1958) showed that for low rates of flow the ratio of the downstream temperature differential, T₁, to the upstream temperature differential, T₂, provides an accurate estimation of the heat pulse velocity, v_h:

v_h = α/x ln (δ T₁/δ T₂), in cm s^-1 (Eq. 1)

Where,
α is the thermal diffusivity (cm² s^-1),
x is the distance above or below the heating element (cm).

When there is no sapflow the ratio of δ T₁ to δ T₂ is equal to one and thus the logarithm of the ratio of the two temperature differentials is zero. When sapflow occurs, the ratio of δ T₁ to δ T₂ is less than or greater than 1, with values above 1 measuring flow towards the leaves and values below 1 indicating reverse flow towards the roots.

The thermal diffusivity, α, can be estimated by recording the temperature profile of one of the thermocouples following a heat pulse under conditions of zero flow (Clearwater et al., 2009). It is proportional to the amount of time it takes for the thermocouple to reach a maximum temperature, t_m, after a heat pulse:

α = x²/4 t_m, in cm² s^-1(Eq. 2)

For miniature external gauges, α is a property of the gauge material and the properties of the stem with which it is in contact. A significant proportion of heat is propagated through the gauge block and α varies little between individuals of a species (Clearwater et al., 2009). α can be estimated from Eq. 2 by installing gauges on excised stems of study species and recording heat pulses with no imposed xylem flow. Thereafter α can be assumed to be constant for a species and applied in Eq. 1 for other individuals of the same species. Vandegehuchte and Steppe (2012) showed recently that thermal diffusivity, α, of woody stems may vary throughout a growing season, which affects calculations of v_h. An improved estimate of α and how it varies throughout a growing season is desirable and may improve the fit between v_h and transpiration (E).


Figure 1. Miniature external sapflow gauge with 10 m lead cable connected to a small branch of a potted Umbellularia californica plant and a Campbell Scientific CR10X data logger