68 research outputs found
Multiscale remote sensing of plant physiology and carbon uptake
This study investigated the use of optical remote sensing for estimating leaf and
canopy scale light use efficiency (LUE) and carbon exchange. In addition, a new
leaf level model capable of predicting dynamic changes in apparent reflectance
due to chlorophyll fluorescence was developed.
A leaf level study was conducted to assess the applicability of passive remote
sensing as a tool to measure the reduction, and the subsequent recovery, of
photosynthetic efficiency during the weeks following transplantation. Spectral
data were collected on newly planted saplings for a period of 8 weeks, as well
as gas exchange measurements of LUE and PAM fluorescence measurements. A
set of spectral indices, including the Photochemical Reflectance Index (PRI),
were calculated from the reflectance measurements. A marked depression in
photosynthetic rate occurred in the weeks after outplanting followed by a gradual
increase, with recovery occurring in the later stages of the experimental period.
As with photosynthetic rate, there was a marked trend in PRI values over the
study period but no trend was observed in chlorophyll based indices. The study
demonstrated that hyperspectral remote sensing has the potential to be a useful
tool in the detection and monitoring of the dynamic effects of transplant shock. Relationships between hyperspectral reflectance indices, airborne carbon exchange
measurements and satellite observations of ground cover were then explored
across a heterogeneous Arctic landscape. Measurements were collected
during August 2008, using the University of Edinburgh’s research aircraft, from
an Arctic forest tundra zone in northern Finland as part of the Arctic Biosphere
Atmosphere Coupling at Multiple Scales (ABACUS) study. Surface fluxes of CO2
were calculated using the eddy covariance method from airborne data that were
collected from the same platform as hyperspectral reflectance measurements. Airborne
CO2 fluxes were compared to MODIS vegetation indices. In addition, LUE
was estimated from airborne flux data and compared to airborne measurements
of PRI. There were no significant relationships between MODIS vegetation indices
and airborne flux observations. There were weak to moderate (R2 = 0.4 in
both cases) correlations between PRI and LUE and between PRI and incident
radiation.
A new coupled physiological radiative transfer model that predicts changes in the
apparent reflectance of a leaf, due to chlorophyll fluorescence, was developed. The
model relates a physically observable quantity, chlorophyll fluorescence, to the sub
leaf level processes that cause the emission. An understanding of the dynamics
of the processes that control fluorescence emission on multiple timescales should
aid in the interpretation of this complex signal. A Markov Chain Monte Carlo
(MCMC) algorithm was used to optimise biochemical model parameters by fitting
model simulations of transient chlorophyll fluorescence to measured reflectance
spectra. The model was then validated against an independent data set. The
model was developed as a precursor to a full canopy scheme. To scale to
the canopy and to use the model on trans-seasonal time scales, the effects of
temperature and photoinhibition on the model biochemistry needs to be taken
into account, and a full canopy radiative transfer scheme, such as FluorMOD,
must be developed
Spatial Variation of Leaf Optical Properties in a Boreal Forest Is Influenced by Species and Light Environment
Leaf Optical Properties (LOPs) convey information relating to temporally dynamic photosynthetic activity and biochemistry. LOPs are also sensitive to variability in anatomically related traits such as Specific Leaf Area (SLA), via the interplay of intra-leaf light scattering and absorption processes. Therefore, variability in such traits, which may demonstrate little plasticity over time, potentially disrupts remote sensing estimates of photosynthesis or biochemistry across space. To help to disentangle the various factors that contribute to the variability of LOPs, we defined baseline variation as variation in LOPs that occurs across space, but not time. Next we hypothesized that there were two main controls of potentially disruptive baseline spatial variability of photosynthetically-related LOPs at our boreal forest site: light environment and species. We measured photosynthetically-related LOPs in conjunction with morphological, biochemical, and photosynthetic leaf traits during summer and across selected boreal tree species and vertical gradients in light environment. We then conducted a detailed correlation analysis to disentangle the spatial factors that control baseline variability of leaf traits and, resultantly, LOPs. Baseline spatial variability of the Photochemical Reflectance Index (PRI) was strongly influenced by species and to a lesser extent light environment. Baseline variability of spectral fluorescence derived LOPs was less influenced by species; however at longer near-infrared wavelengths, light environment was an important control. In summary, remote sensing of chlorophyll fluorescence has good potential to detect variation in photosynthetic performance across space in boreal forests given reduced sensitivity to species related baseline variability in comparison to the PRI. Our results also imply that spatially coarse remote sensing observations are potentially unrepresentative of the full scope of natural variation that occurs within a boreal forest.Peer reviewe
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Model for the prediction of nail withdrawal stiffness
An important factor affecting the strength and ultimate load of wood structures is the strength, stiffness and durability of its joints. Therefore, the behavior of nailed joints must be given proper consideration when designing and analyzing wood structures. Presently,formulas for determining the strength of nailed joints under withdrawal loads, as outlined in the National Design Specifications and the Uniform Building Code, are based on the statistical regression between maximum withdrawal load (W) and the material properties of the wood and nail.New models were developed which predict W, withdrawal resistance (WR) and withdrawal stiffness (WK) based on the actual forces responsible for holding the nail in the wood. These include a normal force to the nail due to the elastically compressed wood around the nail and the resulting friction between the wood and nail surfaces. The accuracy of these models were compared to the accepted formulas in the design codes and were found to be superior in predicting W, WR and WK
Leaf-Level Spectral Fluorescence Measurements: Comparing Methodologies for Broadleaves and Needles
Successful measurements of chlorophyll fluorescence (ChlF) spectral properties (typically in the wavelength range of 650–850 nm) across plant species, environmental conditions, and stress levels are a first step towards establishing a quantitative link between solar-induced chlorophyll fluorescence (SIF), which can only be measured at discrete ChlF spectral bands, and photosynthetic functionality. Despite its importance and significance, the various methodologies for the estimation of leaf-level ChlF spectral properties have not yet been compared, especially when applied to leaves with complex morphology, such as needles. Here we present, to the best of our knowledge, a first comparison of protocols for measuring leaf-level ChlF spectra: a custom-made system designed to measure ChlF spectra at ambient and 77 K temperatures (optical chamber, OC), the widely used FluoWat leaf clip (FW), and an integrating sphere setup (IS). We test the three methods under low-light conditions, across two broadleaf species and one needle-like species. For the conifer, we characterize the effect of needle arrangements: one needle, three needles, and needle mats with as little gap fraction as technically possible. We also introduce a simple baseline correction method to account for non-fluorescence-related contributions to spectral measurements. Baseline correction was found especially useful in recovering the spectra nearby the filter cut-off. Results show that the shape of the leaf-level ChlF spectra remained largely unaffected by the measurement methodology and geometry in OC and FW methods. Substantially smaller red/far-red ratios were observed in the IS method. The comparison of needle arrangements indicated that needle mats could be a practical solution to investigate temporal changes in ChlF spectra of needle-like leaves as they produced more reproducible results and higher signals
Simulation-Based Evaluation of the Estimation Methods of Far-Red Solar-Induced Chlorophyll Fluorescence Escape Probability in Discontinuous Forest Canopies
Peer reviewe
Measuring solar-induced fluorescence from unmanned aircraft systems for operational use in plant phenotyping and precision farming
Demand for high spatial and temporal resolution measurements has triggered a rapid development of unmanned aircraft systems (UAS) for plant phenotyping and precision farming purposes. Similarly, recent progress in low-altitude remote sensing of solar-induced chlorophyll fluorescence (SIF) resulted in several studies aiming at the development of SIF proximal sensing approaches. Although first experimental results are promising, the requirements for reliable and repeatable measurements in agricultural experiments still constrain applicability of these platforms. In this study, we analyze current capabilities and potentials of SIF measuring UAS for operational use. We highlight existing challenges and outline how UAS SIF sensing could be used more frequently and reliably in precision agriculture applications in the near future.Peer reviewe
Nocturnal Light Emitting Diode Induced Fluorescence (LEDIF) : A new technique to measure the chlorophyll a fluorescence emission spectral distribution of plant canopies in situ
Solar-induced chlorophyll a Fluorescence (SIF), which is distributed over a relatively broad (similar to 200 nm) spectral range, is a signal intricately connected to the efficiency of photosynthesis and is now observable from space. Variants of the Fraunhofer Line Depth/Discriminator (FLD) method are used as the basis of retrieval algorithms for estimating SIF from space. Although typically unobserved directly, recent advances in FLD-based algorithms now facilitate the prediction (by model inversion) of the canopy emitted fluorescence spectrum from the discrete-feature FLD retrievals. Here we present first canopy scale measurements of chlorophyll a fluorescence spectra emitted from Scots pine at two times of year, and also from a lingonberry dominated understory. We used a high power mul-tispectral Light Emitting Diode (LED) array to illuminate the respective canopies at night and measured under standardised conditions using a field spectrometer mounted in the nadir position above the canopy. We refer to the technique, which facilitates the in situ upscaling of a commonly measured leaf scale quantity to the canopy, as nocturnal LED-Induced chlorophyll a Fluorescence (LEDIF). The shape of the LEDIF spectra was dependant on the colour of the excitation light and also on the dominant species. Because we measured pine at two different times of year we were also able to show an increase in the canopy scale apparent quantum yield of fluorescence which was consistent with leaf-level increase in fluorescence yield recorded with a monitoring PAM fluorometer. The automation of the LEDIF technique could be used to estimate seasonal changes in canopy fluorescence spectra and yield from fixed or mobile platforms and provide a window into functional traits across species and architectures. LEDIF could also be used to evaluate FLD and inversion-based retrievals of canopy spectra, as well as different irradiance normalisation schemes typically applied to SIF data to account for the dependence of SIF on ambient light conditions.Peer reviewe
Uncrewed aircraft system spherical photography for the vertical characterization of canopy structural traits
The plant area index (PAI) is a structural trait that succinctly parametrizes the foliage distribution of a canopy and is usually estimated using indirect optical techniques such as digital hemispherical photography. Critically, on-the-ground photographic measurements forgo the vertical variation of canopy structure which regulates the local light environment. Hence new approaches are sought for vertical sampling of traits. We present an uncrewed aircraft system (UAS) spherical photographic method to obtain structural traits throughout the depth of tree canopies. Our method explained 89% of the variation in PAI when compared with ground-based hemispherical photography. When comparing UAS vertical trait profiles with airborne laser scanning data, we found highest agreement in an open birch (Betula pendula/pubescens) canopy. Minor disagreement was found in dense spruce (Picea abies) stands, especially in the lower canopy. Our new method enables easy estimation of the vertical dimension of canopy structural traits in previously inaccessible spaces. The method is affordable and safe and therefore readily usable by plant scientists.Peer reviewe
Investigating Forest Photosynthetic Response to Elevated CO2 Using UAV-Based Measurements of Solar Induced Fluorescence
The response of ecosystems to increasing atmospheric CO2 will have significant, but still uncertain, impacts on the global carbon and water cycles. A lot of infounation has been gained from Free Air CO2 Enrichment (FACE) experiments, but the response of mature forest ecosystems remains a significant knowledge gap. One of the challenges in FACE studies is obtaining an integrated measure of canopy photosynthesis at the scale of the treatment ring. A new remote sensing approach for measuring photosynthetic activity is based on Solar Induced Fluorescence (SIF), which is emitted by plants during photosynthesis, and is closely linked to the rates and regulation of photosynthesis. We proposed that UAV-based SIF measurements, that enable the spectrometer field of view to be targeted to the treatment ring, provide a unique opportunity for investigating the dynamics of photosynthetic responses to elevated CO2. We have successfully tested this approach in a new FACE site, located in a mature oak forest in the UK. We flew a series of flights across the experiment arrays, collecting a number of spectra. We combined these with ground-based physiological and optical measurements, and see great promise in the use of UAV-based SIF measurements in FACE and other global change experiments.Peer reviewe
Do all chlorophyll fluorescence emission wavelengths capture the spring recovery of photosynthesis in boreal evergreen foliage?
Chlorophyll a fluorescence (ChlF) is closely related to photosynthesis and can be measured remotely using multiple spectral features as solar-induced fluorescence (SIF). In boreal regions, SIF shows particular promise as an indicator of photosynthesis, in part because of the limited variation of seasonal light absorption in these ecosystems. Seasonal spectral changes in ChlF could yield new information on processes such as sustained nonphotochemical quenching (NPQ(S)) but also disrupt the relationship between SIF and photosynthesis. We followed ChlF and functional and biochemical properties of Pinus sylvestris needles during the photosynthetic spring recovery period to answer the following: (a) How ChlF spectra change over seasonal timescales? (b) How pigments, NPQ(S), and total photosynthetically active radiation (PAR) absorption drive changes of ChlF spectra? (c) Do all ChlF wavelengths track photosynthetic seasonality? We found seasonal ChlF variation in the red and far-red wavelengths, which was strongly correlated with NPQ(S), carotenoid content, and photosynthesis (enhanced in the red), but not with PAR absorption. Furthermore, a rapid decrease in red/far-red ChlF ratio occurred in response to a cold spell, potentially relating to the structural reorganization of the photosystems. We conclude that all current SIF retrieval features can track seasonal photosynthetic dynamics in boreal evergreens, but the full SIF spectra provides additional insight.Peer reviewe
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