Leaf and canopy temperatures
Observed Tleaf and leaf-to-air temperature difference,
ΔT, across species and between provenances, while under common
microclimatic conditions, were determined using two different methods:
first by tracking individual Tleaf with point
measurements taken over 10 days using an infrared thermometer, and
subsequently by determining whole canopy temperature
(Tcan) using UAV- based thermal imagery. Individual leaf
temperatures were recorded using a dual laser infrared thermometer
(MS6530, Mastech, Pittsburgh, Pennsylvania, US), with a spectral
response of 8-14 µm, a distance to spot size ratio of 12:1, and an
assumed emissivity at 0.95. Measurement campaigns for
Tleaf took place between the times of 10:45 and 14:45
during cloud-free periods with low windspeed. Measurements were
standardised across trees by taking measurements on 10 sun-exposed
leaves on branches that were perpendicular to the solar angle to ensure
maximum incoming radiation. This was repeated over multiple days and in
a randomised order. The total number of leaves measured on each plant
ranged from 30 to 50 (n = 1990). The difference between leaf and
air temperatures (ΔT) for each plant was calculated by subtracting
10-min average air temperature.
For Tcan, thermal images of whole tree canopies were
taken using a drone (DJI Mavic 2 Enterprise Advanced, DJI Technology Co.
Ltd, Shenzhen, China) fitted with a FLIR M2ED thermal camera (Teledyne
Flir, Wilsonville, OR, USA) with a thermal spectral band ranging from 8
to 14 µm, an uncooled Vox microbolometer thermal sensor (with automated
calibration), sensor resolution of 640 x 512 pixels, approx. 9 mm lens
with a 57° horizontal field of view, and 30 Hz frame rate. The flight
was completed on 11th December 2023 at 12:07 pm, with
a 15 min flight time. Both thermal (IR) and true colour (RGB) images
were taken simultaneously and flown with 50% overlap at 1.3 m
s-1. With a flight altitude of 27 m above the ground
the pixel resolution was 4.2 cm pixel−1. RGB images
were captured at the same times and locations as the thermal images,
with an image size of 5472 x 3648 pixels, 35 mm format equivalent lens
length of 24 mm, and a horizontal field of view of 84°. An RGB
orthophoto mosaic of the trial site with an average ground sample
distance of 2.18cm was created using Pix4Dmapper (Pix4D S.A., Prilly,
Switzerland).
Thermal images were converted to TIFF, with radiometric data converted
to temperatures using the mean air temperature (29 °C) and relative
humidity (38 %) measured on the crane weather station during the
flight, and an object distance of 27 m minus the target tree height,
resulting in object distances ranging from 18.00 m to 26.34 m (mean
22.54). Images were analysed using imageJ, with target tree canopies
determined manually using polygons, and background temperatures of the
ground excluded using threshold filtering. For each tree, calculations
of the mean, standard deviation, median, minimum, maximum, and skewness
of canopy temperatures were recorded. We used the mean of each
individual canopy for analysis of Tcan.