Assessing the variability and uncertainty of two-flux FAPAR measurements in a conifer-dominated forest

Putzenlechner, Birgitta; Marzahn, Philip; Kiese, Ralf; Ludwig, Ralf; Sánchez‐Azofeifa, Arturo

doi:10.1016/j.agrformet.2018.10.007

Cited by 16 publications

(10 citation statements)

References 47 publications

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“…Based on these criteria, a measurement field 70 m to the southwest of the tower was chosen (Figure 1). There, 10 PAR sensors were mounted on tripods in 1.3 m height and arranged with 10 m distance in two hexagons to maximize the sensing area (Putzenlechner, Marzahn, et al., 2019) and one of these hexagons was also equipped with six R/FR sensors.…”

Section: Methodsmentioning

confidence: 99%

“…For calculating APAR, cases with PAR trans > PAR in were excluded as a sign of cloud cover only above the tower. High wind speeds can induce an increase of the sampling error of PAR trans measurements from a limited number of sensors during direct light conditions (Putzenlechner, Marzahn, et al., 2019). This sampling error is caused by the high spatial variability of forest canopies (Leuchner et al., 2011; Widlowski, 2010).…”

Section: Methodsmentioning

confidence: 99%

See 1 more Smart Citation

Environmental Drivers of Gross Primary Productivity and Light Use Efficiency of a Temperate Spruce Forest

Reitz

Bogena

Neuwirth³

et al. 2023

JGR Biogeosciences

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The gross primary productivity (GPP) of terrestrial ecosystems, of which forests are the dominant factor (Pan et al., 2011), is a key element of the global carbon cycle (Canadell et al., 2021). The resulting biomass further is important for human demands of food, energy, and construction materials (Taye et al., 2021). The assimilation of atmospheric CO 2 via photosynthesis is primarily driven by photosynthetically active radiation (PAR), though it is also sensitive to intertwined environmental and physiological variables, such as temperature, water, and nutrient availability, or chlorophyll content of the canopy (

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Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Environmental Drivers of Gross Primary Productivity and Light Use Efficiency of a Temperate Spruce Forest

Reitz

Bogena

Neuwirth³

et al. 2023

JGR Biogeosciences

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show abstract

“…Currently, most studies that have employed microclimate sensors in forest canopies have often used (a) a very small number of sensors per tree (minimum = 1 and maximum = 12 sensors/tree), (b) limited their measurements to only certain parts of the vertical canopy (e.g., lower canopy only), (c) neglected the horizontal canopy profile and/or d) only sampled one microclimate variable (e.g., radiation, RH or temperature) (Supplementary Table 2). For example, only nineteen of the studies (63%), we found in our literature search, used sensors in the forest understory, positioning them at most two meters above the ground (e.g., Pecot et al, 2005;Lindner, 2011;Putzenlechner et al, 2019). Studies by Feldhake (2002), Wieser et al (2002) and Awada et al (2013) used fixed sensor positions along the height of the canopy.…”

Section: Discussionmentioning

confidence: 99%

Quantification and Variation of Microclimatic Variables Within Tree Canopies - Considerations for Epiphyte Research

Murakami

Ramos

Durand

et al. 2022

Front. For. Glob. Change

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Forest canopies are incredibly complex self-maintaining biological structures. Conditions above and within the canopy can differ vastly, often resulting in a vertical gradient of microclimate conditions. Canopy epiphytic plants have to deal with climatic variability on much more variable scales compared to many other plant groups. The difficulty of sensor installation and their high cost can explain why it has been ignored in many studies on canopy epiphytes. Direct measurements of microenvironmental conditions are the only accurate way to assess specific intra-canopy environmental conditions, as there is also still a lack of methodologically and financially viable alternatives to allow the collection of this type of data. This study aims to make recommendations for the direct use of microclimate measurements in epiphyte research and to summarize key discussion points concerning the number and placement of sensors required for different types of epiphyte studies. In addition, we presented high-resolution field data from the United Kingdom, where we employed over 56 microclimate sensors, to demonstrate the spatial and temporal variability of radiation, temperature, and relative humidity (RH) in a tree canopy. Our data demonstrated that sensor height in the tree and leaf-set were the most important factors determining microclimate variability in the canopy. For the first time, we have made recommendations regarding the placement and number of sensors required in studies that specifically require the use of microclimate sensors in epiphyte studies in forest canopies.

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“…Newly-grown leaves, mature leaves, and senescent leaves have different absorption ability in the PAR band. Previous study show that the presence of yellow leaves leads to an overestimation when using fIPAR to approximate fAPAR because yellow leaves are characterized by a higher brightness than green leaves [54,58,59].…”

Section: The Uncertainty Of Using Fipar As a Proxy For Faparmentioning

confidence: 99%

Comparison of Absorbed and Intercepted Fractions of PAR for Individual Trees Based on Radiative Transfer Model Simulations

Wojnowski

Wei

et al. 2021

Remote Sensing

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The fraction of absorbed photosynthetically active radiation (fAPAR) is a key parameter for estimating the gross primary production (GPP) of trees. For continuous, dense forest canopies, fAPAR, is often equated with the intercepted fraction, fIPAR. This assumption is not valid for individual trees in urban environments or parkland settings where the canopy is sparse and there are well-defined tree crown boundaries. Here, the distinction between fAPAR and fIPAR can be strongly influenced by the background and large illumination variations due to multi-scattering and shadows of buildings. This study investigates the radiative budget of PAR bands using a coupled leaf-canopy radiative transfer model (PROSPECT-DART), considering a suite of tropical tree species over a wide range of assumed leaf chlorophyll contents. The analyses simulate hyperspectral images (5 nm bandwidth) of individual tree crowns for the selected background (concrete vs. grass) and illumination conditions. We then use an artificial neural network-based method to partition sunlit vs. shaded leaves within each crown, as the latter have lower fAPAR and fIPAR values. Our results show fAPAR of sunlit leaves decreases with the ratio of diffuse to direct scene irradiance (SKYL), while SKYL has minimal influence for shaded leaves. Both fAPAR and fIPAR decrease at more oblique solar zenith angles (SZA). Higher values of fAPAR and fIPAR occur with concrete backgrounds and the influence of the background is larger at higher diffuse ratio and solar zenith angles. The results show that fIPAR is typically 6–9% higher than fAPAR, and up to 14% higher for sunlit leaves with a concrete background at SKYL = 0. The differences between the fIPAR and fAPAR also depend on the health condition of the leaves, such as chlorophyll content. This study can improve the understanding of urban individual trees fAPAR/fIPAR and facilitate the development of protocols for fAPAR field measurements.

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Assessing the variability and uncertainty of two-flux FAPAR measurements in a conifer-dominated forest

Cited by 16 publications

References 47 publications

Environmental Drivers of Gross Primary Productivity and Light Use Efficiency of a Temperate Spruce Forest

Environmental Drivers of Gross Primary Productivity and Light Use Efficiency of a Temperate Spruce Forest

Quantification and Variation of Microclimatic Variables Within Tree Canopies - Considerations for Epiphyte Research

Comparison of Absorbed and Intercepted Fractions of PAR for Individual Trees Based on Radiative Transfer Model Simulations

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