Use of thermal imaging to determine leaf conductance along a canopy gradient in European beech (Fagus sylvatica)

Abstract

Using an infrared camera, we measured the leaf temperature across different canopy positions of a 23-m-tall deciduous forest tree (Fagus sylvatica L.) including typical sun and shade leaves as well as intermediate leaf forms, which differed significantly in specific leaf area (SLA). We calculated a temperature index (I(G)) and a crop water stress index (CWSI) using the surface temperatures of wet and dry reference leaves. Additional indices were computed using air temperature plus 5 °C (I(G) + 5, CWSI + 5) as dry references. The minimum temperature of the wet leaf and the maximum temperature of the dry leaf proved to be most suitable as reference values. We correlated the temperature indices with leaf area-related conductance to water vapor (g(L)) using porometry at the leaf level and using xylem sap flow at the branch level. At the leaf and at the branch level, I(G) and CWSI were equally well suited as proxies of g(L), whereas the relationships of I(G) + 5 and CWSI + 5 with g(L) were only weak or even insignificant. At the leaf level, the correlations of I(G) and CWSI with g(L) were significant in all parts of the crown. The slopes of g(L) vs. I(G) and CWSI did not differ significantly among the crown parts; this indicates that they were not influenced by SLA or irradiance. At the branch level, close correlations (r > 0.8) were found between temperature indices and g(L) across the crown. These results demonstrate that satisfactory relationships between temperature indices and g(L) can be established in tall trees even in those canopy parts that are exposed to relatively low levels of irradiance and exhibit relatively low values of g(L).

Selecting Canopy Zones and Thresholding Approaches to Assess Grapevine Water Status by Using Aerial and Ground-Based Thermal Imaging

“…Several researchers have found high correlations between the sunlit canopy temperature and the stem water potential and stomatal conductance [19,24,25]. Conversely, other studies have shown that the shadow canopy side has better correlations with g s and Ψ stem due to less temperature variation within shadow zone [7,21].…”

Continuous, long-term, high-frequency thermal imaging of vegetation: Uncertainties and recommended best practices

“…These sensors are also small and affordable enough to deploy to field sites. Recent work with thermal cameras has demonstrated the power to measure species-specific responses to leaf energy balance, but did not capitalize on the continuous monitoring capability of these instruments, nor did the work assess measurement error (Leuzinger and Körner, 2007;Leuzinger et al, 2010;Reinert et al, 2012;Scherrer et al, 2011). Additional work has utilized thermal cameras for characterization of stomatal conductance and closure, irrigation schedules, and plant stress, but focused exclusively on laboratory or crop field environments where some external factors can be controlled, and also did not take advantage of the continuous monitoring capabilities of the technology (Ballester et al, 2013;Berger et al, 2010;Grant et al, 2006;Jones, 2004Jones, , 1999Jones et al, 2009).…”

Measuring Stress Reactions of Beech Seedlings with PRI, Fluorescence, Temperatures and Emissivity from VNIR and Thermal Field Imaging Spectroscopy

“…Broadband thermal imaging was employed by Reicosky et al [1994], who used temperature measurements to characterize diurnal trends in photosynthesis and evapotranspiration in wheat canopies, and Zarco-Tejada et al [2012] who used a UAV platform to detect water stress in an orchard with a micro-hyperspectral imager and a thermal camera. Reinert et al [2012] used thermal imaging to determine leaf conductance in European beech. They found significant correlations between thermal indices and leaf conductance, relying on the position of the considered leafs in the canopy (upper, middle, or lower part).…”