Divergence of Leaf and Air Temperature

It is an everyday experience that surface temperatures of sun exposed, non-heated objects diverge from actual ambient air temperature (ΔT). Simplified the phenomenon can be reduced to three main components (for more a detailed explanation, please take a look at our actual product catalog, Page 14 et seq.):
1. Different energy influx (specific absorption characteristics for electro-magnetic radiation, i.e. sun light)
2. Different heat storage capacity (specific capacity to store heat energy)
3. Different energy outflux (specific emission characteristics, convection parameters, latent heat flux via transpiration)

Relevance and fields of application

For the growth of green plants, leaves play a central role as energy harvesting organs (conversion of solar energy to chemical energy via photosynthesis). In order to meet their function, leaves have to be located at exposed positions of the plant body and are hence strongly affected by instantaneous climatic conditions. Thereby, diurnal temperature changes of tens of °C are no exception (Fig. 1, middle). Of special physiological relevance are extreme values (chilling/heat stress), that lead to necrosis of leaf tissue. Because of their strong impact on plant vitality, such threshold values and their exceedance are of high interest from an ecological perspective, as well as for plant production. But also between these lethal temperature thresholds, physiological processes depend, just as chemical reactions in general, on temperature (cf. Arrhenius-Equation). Actual leaf temperature is decisive for physiological functioning, at the leaf as well as the whole plant level and therefore a key parameter in plant ecological and physiological modelling. For lack of actual leaf temperature data, however, commonly mere air temperature together with other highly available climate data provides the basis for those models. The Neglection of input parameters, or the simple use of standard values (specific physical leaf properties, transpiration rate, etc.) for modelling the leaf energy balance, likely leads to a significant over- or underestimation of the actual leaf temperature, hence representing a significant source of uncertainty for all related downstream model results (cf. Tair and ΔT in Fig.1, above and middle)
Hence, following fields of application can significantly benefit from precise leaf temperature and the determination of lethal temperature thresholds at the basis of actual leaf temperature data:

Plant production

  • risk assessment
  • crop protection

Ecological research

  • Species distribution models,
  • Water balance models
  • Climate models (microclimate modelling in and below plant canopies, cooling effect of plants in urban modelling)
  • Physiological models related to stomatal conductance: O3-uptake, 13C und 18O isotope-signals in plant biomass

Fig_9__dTleaf-air_vs_Tair_vs_PPFD
Abb. 1: Comparison of temperature difference between leaf surface and ambient air (ΔT), air temperature (Tair) and solar radiation (PPFD).
Above: Diurnal variations in temperature difference between upper leaf surface and ambient air (ΔT, measured via ΔLA-B sensor) of a sun exposed leaf of a mature beech tree at the experimental site “Kranzberger Forst” of the TU Munich.
Middle: Diurnal variations in air temperature (Tair), measured at canopy height (27m above ground)
Lower: Diurnal variations in solar radiation above canopy, given in photosynthetic photon flux density (PPFD)
Comment: Highlighted are two periods which exemplify that mere air temperature or radiation data cannot be a sufficient basis to derive actual leaf temperature:
1. Similar air temperature on two consecutive days, diverging leaf-to-air temperature difference
2. Diverging air temperatures on three consecutive days, similar leaf-to-air temperature differences
A further important parameter that determines leaf temperature is the frequently unknown leaf transpiration rate (for more a detailed explanation, please take a look at our actual product catalog, Page 14 et seq.)

Measurement principle of the ΔLA Sensors

Air temperature is one of the central parameters in climate monitoring and existing measurement solutions are reliable and affordable. Hence, availability of air temperature data is usually high. Notwithstanding, the parameter cannot be readily employed as surrogate for leaf temperature (cf. explanation above). Additionally to air temperature data, only the temperature difference between ambient air and the leaf surface (ΔT Leaf-to-Air, in short ΔLA) has to be known to calculate the actual leaf temperature.
Thermocouples are electronic devices to precisely measure temperature differences. Based on a thermo-electronic effect between different types of metals, temperature differences between the two measurement points of the instrument are directly traduced into a thermo-electronic voltage. However the voltage that is induced in one single thermocouple is very low. Measurements of small temperature differences would hence produce a very weak signal and consequently result in a very low signal resolution and signal-to-noise ratio.
In order to enable high resolution measurements, including very small temperature differences, the ΔLA sensor captures the additive thermo-electronic voltage of a very thin chain (10-fold) of thermocouples.

Advantages

  • The additive signal of multiple thermocouples is strong enough for a direct recording with most of the available data loggers. Without the need for electronic or software based amplification, the measurement signal is obtained with the highest signal-to-noise ratio possible.
  • Multiple, spatially distributed measurement points in direct contact with the leaf, provide an integrative temperature signal of the leaf surface.
  • Consisting of very thin elements, the sensor stays lightweight and shading effects can be neglected, although the sensor may span a substantial part of the leaf.
  • Neglectable heat capacity and hence thermal inertia of the very thin thermocouples, enable measurements with high temporal resolution also under non-steady-state conditions.
  • Whereas unknown optical leaf properties (i.e. emissivity) affect the precision of optical temperature measurements, the direct measurement with the ΔLA sensor is free of such errors.

ΔLA leaf temperature sensors
Above: Type ΔLA-B on common hazel
Below: Type ΔLA-C on Norway spruce