Imaging canopy temperature: shedding (thermal) light on ecosystem processes

Still, Christopher J.; Rastogi, Bharat; Page, Gerald; Griffith, David R.; Sibley, Adam; Schulze, Mark; Hawkins, Linnia; Pau, Stéphanie; Detto, Matteo; Helliker, Brent R.

doi:10.1111/nph.17321

Cited by 54 publications

(53 citation statements)

References 55 publications

(87 reference statements)

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“…The different components of the leaf energy balance, which can help us to understand how leaf temperature is regulated, can be described to a high degree of accuracy under controlled conditions (Michaletz & Johnson, 2006; Gutschick, 2016; Prashar & Jones, 2016; Schymanski & Or, 2016), but this task can prove more challenging under field conditions due to environmental fluctuations, complex effects of canopy structure, and complications relating to the operation of sensitive instrumentation. However, identification of the range of such variations, and the limits to leaf temperature control resulting from energy balance requirements under field conditions, is critical to assessing the response of vegetation to climate change, and to scaling processes from leaf to canopy to ecosystem, and to even larger scales (Bonan, 2008; Still et al ., 2021).…”

Section: Introductionmentioning

confidence: 99%

“…High-precision direct measurements of leaf temperature under field conditions are a prerequisite to expanding our knowledge in this area, but they remain rare (see reviews in Still et al, 2019Still et al, , 2021. This is particularly true for forests, due to the vegetation height and because of the requirement for the combined measurement of leaf surface temperatures and surrounding air temperatures (Kim et al, 2018).…”

Section: Introductionmentioning

confidence: 99%

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Evidence for efficient nonevaporative leaf‐to‐air heat dissipation in a pine forest under drought conditions

et al. 2021

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Summary The drier climates predicted for many regions will result in reduced evaporative cooling, leading to leaf heat stress and enhanced mortality. The extent to which nonevaporative cooling can contribute to plant resilience under these increasingly stressful conditions is not well known at present. Using a novel, high accuracy infrared system for the continuous measurement of leaf temperature in mature trees under field conditions, we assessed leaf‐to‐air temperature differences (ΔTleaf–air) of pine needles during drought. On mid‐summer days, ΔTleaf–air remained < 3°C, both in trees exposed to summer drought and in those provided with supplemental irrigation, which had a more than 10‐fold higher transpiration rate. The nonevaporative cooling in the drought‐exposed trees must be facilitated by low resistance to heat transfer, generating a large sensible heat flux, H. ΔTleaf–air was weakly related to variations in the radiation load and mean wind speed in the lower part of the canopy, but was dependent on canopy structure and within‐canopy turbulence that enhanced the H. Nonevaporative cooling is demonstrated as an effective cooling mechanism in needle‐leaf trees which can be a critical factor in forest resistance to drying climates. The generation of a large H at the leaf scale provides a basis for the development of the previously identified canopy‐scale ‘convector effect’.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Evidence for efficient nonevaporative leaf‐to‐air heat dissipation in a pine forest under drought conditions

et al. 2021

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“…( 12) and Eq. (7). The obtained range of diurnal T s and observed T a with perturbation is used to calculate the uncertainity in ∆T ; an example for July 15 is shown in Fig.…”

Section: /19mentioning

confidence: 99%

Challenges and uncertainty in plot-scale emissivity and surface temperature estimation using flux tower measurements

Thakur

Schymanski

Trebs

et al. 2021

Preprint

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Land surface temperature (LST) is a preeminent state variable that controls the energy and water exchange between the Earth’s surface and the atmosphere. At the landscape-scale, LST is derived from thermal infrared radiance measured using space-borne radiometers. At the plot-scale, the flux tower recorded longwave radiation components are inverted to retrieve LST. Since the down-welling longwave component was not measured routinely until recently, usually only the up-welling longwave component is used for the plot-scale LST retrieval. However, we found that ignoring reflected down-welling longwave radiation for plot-scale LST estimations can lead to substantial error. This also has important implications for estimating the correct surface emissivity using flux tower measurements, which is needed for plot-scale LST retrievals. The present study proposes a new method for plot-scale emissivity and LST estimation and addresses in detail the consequences of omitting down-welling longwave radiation as frequently done in the literature. Our analysis uses ten eddy covariance sites with different land cover types and found that the LST values obtained using both up-welling and down-welling longwave radiation components are 0.5 to 1.5 K lower than estimates using only up-welling longwave radiation. Furthermore, the proposed method helps identify inconsistencies between plot-scale radiometric and aerodynamic measurements, likely due to footprint mismatch between measurement approaches. We also found that such inconsistencies can be removed by slight corrections to the up-welling longwave component and subsequent energy balance closure, resulting in realistic estimates of surface emissivity and consistent relationships between energy fluxes and surface-air temperature differences. Landscape-scale daytime LST obtained from satellite data (MODIS TERRA) was strongly correlated with our plot-scale estimates for most of the sites, but higher by several Kelvin at two sites. We also quantified the uncertainty in estimated LST and surface emissivity using the different methods and found that the proposed method does not result in increased uncertainty. The results of this work have significant implications for the combined use of aerodynamic and radiometric measurements to understand the interactions and feedbacks between LST and surface-atmosphere exchange processes.

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“…Fourth, thermocouples sample only a small part of each given leaf, and large temperature gradients have previously been shown (Leigh et al, 2017). Owing to these difficulties, many recent studies have chosen to use infrared thermography for leaf temperature measurements (Still et al, 2019(Still et al, , 2021.…”

Section: Introductionmentioning

confidence: 99%

‘Dual‐reference’ method for high‐precision infrared measurement of leaf surface temperature under field conditions

et al. 2021

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Summary Temperature is a key control over biological activities from the cellular to the ecosystem scales. However, direct, high‐precision measurements of surface temperature of small objects, such as leaves, under field conditions with large variations in ambient conditions remain rare. Contact methods, such as thermocouples, are prone to large errors. The use of noncontact remote‐sensing methods, such as thermal infrared measurements, provides an ideal solution, but their accuracy has been low (c. 2°C) owing to the necessity for corrections for material emissivity and fluctuations in background radiation Lbg. A novel ‘dual‐reference’ method was developed to increase the accuracy of infrared needle‐leaf surface temperature measurements in the field. It accounts for variations in Lbg and corrects for the systematic camera offset using two reference plates. We accurately captured surface temperature and leaf‐to‐air temperature differences of needle‐leaves in a forest ecosystem with large diurnal and seasonal temperature fluctuations with an uncertainty of ± 0.23°C and ± 0.28°C, respectively. Routine high‐precision leaf temperature measurements even under harsh field conditions, such as demonstrated here, opens the way for investigating a wide range of leaf‐scale processes and their dynamics.

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Imaging canopy temperature: shedding (thermal) light on ecosystem processes

Cited by 54 publications

References 55 publications

Evidence for efficient nonevaporative leaf‐to‐air heat dissipation in a pine forest under drought conditions

Evidence for efficient nonevaporative leaf‐to‐air heat dissipation in a pine forest under drought conditions

Challenges and uncertainty in plot-scale emissivity and surface temperature estimation using flux tower measurements

‘Dual‐reference’ method for high‐precision infrared measurement of leaf surface temperature under field conditions

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