Mapping Tree Water Deficit with UAV Thermal Imaging and Meteorological Data

Remote Sensing in Earth Systems Sciences https://doi.org/10.1007/s41976-023-00094-9 RESEARCH Mapping Tree Water Deficit with UAV Thermal Imaging and Meteorological Data Stuart Krause1,2 · Tanja GM Sanders1 Received: 25 August 2022 / Revised: 25 September 2023 / Accepted: 8 October 2023 © The Author(s) 2023 Abstract The mapping of forest stands and individual trees affected by drought stress is a crucial step in targeted forest management, aimed at fostering resilient and diverse forests. Unoccupied aerial vehicle (UAV)-based thermal sensing is a promising method for obtaining high-resolution thermal data. However, the reliability of typical low-cost sensors adapted for UAVs is compromised due to various factors, such as internal sensor dynamics and environmental variables, including solar radiation intensity, relative humidity, object emissivity and wind. Additionally, accurately assessing drought stress in trees is a complex task that usually requires laborious and cost-intensive methods, particularly in field settings. In this study, we investigated the feasibility of using the thermal band of the Micasense Altum multispectral sensor, while also assessing the potential for modelling tree water deficit (TWD) through point dendrometers and UAV-derived canopy temperature. Our indoor tests indicated that using a limited number of pixels (< 3) could result in temperature errors exceeding 1 K. However, enlarging the spot-size substantially reduced the mean difference to 0.02 K, validated against leaf temperature sensors. Interestingly, drought-treated (unwatered) leaves exhibited a higher root mean squared error (RMSE) (RMSE = 0.66 K and 0.73 K) than watered leaves (RMSE = 0.55 K and 0.53 K), likely due to lower emissivity of the dry leaves. Comparing field acquisition methods, the mean standard deviation (SD) for tree crown temperature obtained from typical gridded flights was 0.25 K with a maximum SD of 0.59 K (n = 12). In contrast, a close-range hovering method produced a mean SD of 0.09 K and a maximum SD of 0.1 K (n = 8). Modelling the TWD from meteorological and point dendrometer data for the 2021 growth season (n = 2928) yielded an R2 = 0.667 using a generalised additive model (GAM) with vapor pressure deficit (VPD), wind speed, and solar radiation as input features. A point dendrometer lag of one hour was also implemented. When predicting individual tree TWD with UAV-derived tree canopy temperature, relative humidity, and air temperature, an RMSE of 4.92 (μm) and R2 of 0.87 were achieved using a GAM. Implementing leaf-to-air pressure deficit (LVPD) as an input feature resulted in an RMSE of 6.87 (μm) and an R2 of 0.71. This novel single-shot approach demonstrates a promising method to acquire thermal data for the purpose of mapping TWD of beech trees on an individual basis. Further testing and development are imperative, and additional data from drought periods, point dendrometers, and high-resolution meteorological sources are required. Keywords Drought stress · Tree water deficit · UAV · Thermal imagery · Beech 1 Introduction Trees have evolved strategies to endure moderate drought episodes through physiological and morphological adaptations. These adaptations help maintain a balance between * Tanja GM Sanders 1 Thünen Institute of Forest Ecosystems, Alfred‑Möller‑Str. 1, Haus 41/42, 16225 Eberswalde, Germany 2 Department of Geography, University of Bonn, Meckenheimer Allee 166, 53115 Bonn, Germany cooling mechanisms in the crown while preventing excessive water loss and carbon starvation. Such adaptions involve the regulating stomatal conductance, reducing leaf surface area and solar tracking [1, 2]. Additionally, certain wood traits enable xylem to withstand hydraulic failure [3]. Even moderate drought periods increase the likelihood of mortality [4] and can lead to reduced growth [5, 6] regardless of climate change [7]. Recent extreme drought events, such as those in Europe in 2018 and 2019 [8, 9] caused by climate change-induced warming and shifts in precipitation patterns [10], have raised concerns about amplified tree mortality and die-off across various climate zones [11]. 13 Vol.:(0123456789) Remote Sensing in Earth Systems Sciences European beech (Fagus sylvatica), known for its high shade tolerance, has historically outcompeted other tree species in many parts of Europe [12]. However, in recent years, beech has shown declining growth throughout Europe [13–16]. Nevertheless, it may gradually acclimatise to drought over time [17]. Typical drought episodes can result in reduced carbon uptake due to decreased stomatal conductance, premature leaf senescence [18, 19], and a decrease in foliage in the following year due to reduced bud availability [20]. Prolonged drought episodes, especially for anisohydric plant species, can cause irreversible damage, including xylem embolism [15], resulting in permanent damage to the hydraulic system [21, 22]. One approach to assess drought-tolerant species involves classifying a tree’s hydraulic strategy along the anisohydric and isohydric spectrum [18, 23]. Despite numerous studies on tree hydric behaviour, there is no mathematical model describing this trait and is typically categorised based on the relationship between stomatal conductance gs and leaf water potential Ѱ1 [24]. Isohydric plants are known for reducing transpiration by closing stomata during water shortages, which reduces CO2 assimilation [25]. Anisohydric plants, on the other hand, keep stomata open for longer periods during water shortage, making them more vulnerable to hydraulic failure but maintaining higher C O2 uptake during drought episodes [8, 26]. The anisohydric strategy, in essence, necessitates more water to keep leaves cool during extreme heat and relies on significant fluctuations in tree stem (xylem) water content, often relying on nocturnal refilling [27]. Within each species, variations in hydric behaviour can also occur due to genetic variation in terms of drought stress tolerance [8, 10]. Species with high phenotypic plasticity may allow individuals to adapt to changing climate conditions [15]. Categorizing tree species, and even specific provenances [10], into hydric behavioural classes through quantification of stomatal conductance with gas exchange measurements and leaf temperature can assist in assessing drought stress tolerance in the face of climate change. However, it is important not to assume such categorization and to adopt a comprehensive holistic approach [8] particularly in terms of whole-tree carbon balance [21]. In practical terms, central European species are rarely strictly either anisohydric or isohydric, but rather they are typically evaluated in reference to other species. For instance, Quercus species tend to be more anisohydric than Fagus, while Pinus is often more isohydric than Fagus. A better understanding of hydric behaviour among species and individuals at the regional scale could significantly (...truncated)