Drone-based remote sensing shows no effect of stand density on canopy temperature in semi-arid pine forest during drought

High radiation, low albedo, and limited evaporative cooling greatly affect canopy temperature in many semi-arid ecosystems. This makes the dissipation of excess energy essential to tree survival. Remote sensing has the potential to optimise management and better understanding of tree survival mechanisms in this zone. Stand density is thought to affect canopy and soil temperature through shading, change in overall albedo, evapotranspiration, and its effect on convective cooling through wind penetration into the canopy layer. Our objective was to assess the effect of stand density on the canopy and the mean plot temperature as a basis to optimize energy management of a severely water-limited forest.

We used a drone equipped with RGB, thermal (FLIR) and multispectral cameras (Parrot Sequoia, bands: 550nm, 660nm, 735nm & 790nm) alongside independent Lidar measurements in a set of five replicate plots of three different stand density treatments (100, 200 & 300 trees/ha) alongside ground-based measurements. Drone flights were performed during midday throughout the peak of the summer drought (lasting ~8 months) in our semi-arid Aleppo Pine forest research site in southern Israel (sun near NADIR & midday solar radiation >800 W·m^-2, air temperature >30°C). Finally, a set of techniques were developed to automatically identify and extract data of individual tree canopies from the aerial images.

Initial results highlight the importance of partitioning the forest into exposed and shaded soil and tree canopy: The canopy-to-air and exposed soil-to-air temperature differences reached up to 5°C and 35°C, respectively, while shaded soils were in the same temperature range as canopies. Ground-based measurements of DBH and photosynthetic activity increased with decreasing stand density. This is in spite of up to 30% more longwave radiation reaching the canopies through exposure to the hot soil and lack of shading from neighbouring trees in the lower density plots. Unexpectedly, there was a lack of significant canopy temperature differences among density plots, indicating that trees in all treatments dissipated the excess energy equally efficiently. Therefore, mean plot-scale forest surface (skin) temperatures (including both soil and canopy) were affected by the fraction of canopy cover rather than canopy temperature differences among different stand density plots. The results highlight the limitation of interpreting low-resolution satellite data in open canopy forests. Our results will allow us to assess the stand density effects on the balance between carbon sequestration (biogeochemical effects) and surface energy balance (biogeophysical effects).