Top Of Canopy Research Articles

Optimal greenhouse lighting scheduling using canopy light distribution model: A simulation study on tomatoes

Light is considered the most important environmental factor for greenhouse crops, and lack of lighting limits the productivity of greenhouses. Many studies have focused on improving the energy efficiency of lighting in agriculture. In this study, LED lights were mounted in the top, middle and bottom of the plant canopy to provide additional photosynthetically active radiation. Assuming that 100% of the predicted outdoor radiation is available, the canopy light distribution model was used to determine the relationship between the light energy consumption and the optimal yield. Decision-making on lighting scheduling may be considered as a constrained optimisation, and the status of the LEDs in a period acts as the decision variable. Besides, there are two objective functions: one is the energy consumed by the LED device; the other is the expected yield calculated by the crop model of tomatoes. The optimal solution of the multi-objective optimisation is not unique but corresponds to an infinite number of yield/energy combinations. Farmers can select these combinations according to additional restrictions such as energy efficiency ratio or photoperiod. The yield of our strategy was increased by 12.3% under the constraint of equal energy consumption compared to the greenhouse threshold control strategy. Given the same output as the constraint, the energy consumption was reduced by 30.1%.

The structure of turbulent flow through submerged flexible vegetation

The hydrodynamics of turbulent flow through submerged flexible vegetation is investigated in a flume using acoustic Doppler velocimetery (ADV) measurements. The flow characteristics such as the energetics and momentum transfer derived from conventional spectral and quadrant analyses are considered as the flow encounters a finite vegetation patch. Consistent with numerous canopy flow experiments, a shear layer and coherent vortex structures near the canopy top emerge caused by Kelvin–Helmholtz instabilities after the flow equilibrates with the vegetated layer. These instabilities are commonly attributed to velocity differences between non-vegetated and vegetated canopy layers in agreement with numerous experiments and simulations conducted on dense rigid canopies. The power-spectral density function for vertical velocity turbulent fluctuations at different downstream positions starting from the edge of the vegetation layer are also computed. For a preset water depth, the dominant dimensionless frequency is found to be surprisingly invariant around 0.027 despite large differences in vegetation densities. The ejection and sweep events significantly contribute to the Reynolds stresses near the top of the vegetation. The momentum flux carried by ejections is larger than its counterpart carried by the sweeps above the canopy top. However, the momentum flux carried by sweeps is larger below the top of the canopy.

Effects of daytime intra-canopy LED illumination on photosynthesis and productivity of bell pepper grown in protected cultivation

In the past decade, light-emitting diodes (LEDs) have been replacing most other types of light sources. One emerging use for LEDs in horticulture is ‘intra-canopy illumination’ or ‘LED-interlighting’, feasible owing to their low heat output and small physical size. The interlighting technique, typically implemented in environmentally-controlled greenhouses also supplemented with overhead lighting, is mostly relevant for high-wire vegetable cultivation, in which self-shading results in light limitations for a large fraction of the canopy. Interlighting has been shown to increase the yield and/or improve fruit quality in all-year round greenhouse crops such as tomato, cucumber and sweet pepper. In this work, we utilized daytime supplemental intra-canopy LED illumination for sweet pepper grown in high-density ‘Spanish’ trellis systems within passive high tunnels in the Jordan Valley, Israel (latitude ˜32 °N). While canopy top at these conditions is not light-limited, extensive deep shading of the inner canopy is a disadvantage. In two experiments carried out in two separate seasons, the supplemental lighting, which enhanced the photosynthetic rates of the inner canopy foliage by 3.5- to 5.7-fold, resulted in significant increase (˜30%) of the fruit yield during the spring season. The added yield was attained by higher fruit numbers, with no notable effects on fruit size or weight. Our results raise the prospect that LED-interlighting may be a useful practical tool for maximizing fruit production, even in geographical regions of ample sunlight.

Quantifying 3D structure and occlusion in dense tropical and temperate forests using close-range LiDAR

Terrestrial laser scanning (TLS) has emerged as a reference for three-dimensional measurements of forest structure as well as forest reconstruction and modeling. Ground-based measurements can be complemented by new light-weight sensors on unmanned aerial vehicles (UAVs) or laser scans from canopy cranes or towers. However, it is still largely unknown how much of the forest canopy volume can be sampled and how occlusion is spatially distributed. We present an approach for highly detailed 3D structure measurements based on TLS on the ground and above canopy measurements from a canopy crane or UAV platform, and assess their spatial sampling in terms of occlusion. Comparing the application in a dense tropical and temperate forest, we demonstrate the ability to sample the complete canopy volume with <2% occlusion at very high spatial resolution when combining ground and above canopy measurements. This is necessary for a full canopy reconstruction. Ground-based TLS can provide sufficient coverage when no sampling of leaves and branches at top of canopy is required, whereas UAV or tower-based measurements show considerable occlusion in the mid- and understory. We therefore recommend to perform above canopy measurements under leaf off conditions, in sparse forests, or as an addition to ground measurements if a full representation of the whole canopy is required at very high spatial resolution. The latter can pave the way for studies on light availability, micrometeorology, sensor simulations and algorithm testing and development.

Effects of variable fetch and footprint on surface renewal measurements of sensible and latent heat fluxes in cotton

Understanding crop evapotranspiration (ET) is important for efficient irrigation management. The eddy covariance (EC) technique is useful in measuring whole canopy latent heat flux from field crops. However, it requires expensive equipment and complex data analysis; hence, is relevant for research only. With the aim of developing a low-cost and simple method, we investigated here the surface-renewal (SR) method. This method estimates sensible heat flux from high frequency temperature measurements by using a fine wire thermocouple (TC). Next, latent heat flux is derived from the energy balance closure. Since fine wire thermocouple measurements of air temperature can be performed near the canopy top, it was hypothesized (Castellvi, 2012) that fetch requirements can be relaxed relatively to those for EC, and a relatively small fetch is sufficient for reliable flux measurements by SR. In the present study, the SR technique was examined in a cotton field in southern Israel. Seven fine wire thermocouples were installed at various distances from field edges and at different heights above the ground, providing variable fetch from 50 to 200 m, depending on field geometry and wind direction. An EC system was installed within the field at a position that provided sufficient fetch (400 m) for reliable reference values of sensible and latent heat fluxes. Excellent energy balance closure of 0.96 (R2 = 0.95) was obtained from a non-continuous period of 31 days of measurements. SR data from fine-wire thermocouples at all heights were classified by either available fetch or by 90% flux footprint. Only cases in which footprint was smaller than the available fetch were included in the analysis. Two footprint models were examined, KJ (Kljun et al., 2015) and HS (Hsieh et al., 2000). Results of fetch classification showed that the SR weighting factor varied between 0.67 and 1.34 and was independent of fetch. Footprint classification provided weighting factors between 0.46 and 1.18 for the KJ model and between 0.8 and 1.26 for the HS model. LE derived from SR sensible heat flux and energy balance closure, was regressed against LE reference values measured by the eddy covariance. Results showed deviations of up to 15% and 30% between SR and EC, for the KJ and HS model data, respectively. We conclude that in the cotton field under study the SR technique was reliable in estimating sensible and latent heat fluxes and the weighting factor was essentially independent of the geometrical fetch and the flux footprint of the sensors.

Tropopause Evolution in a Rapidly Intensifying Tropical Cyclone: A Static Stability Budget Analysis in an Idealized Axisymmetric Framework

Abstract Upper-level static stability (N2) variations can influence the evolution of the transverse circulation and potential vorticity in intensifying tropical cyclones (TCs). This paper examines these variations during the rapid intensification (RI) of a simulated TC. Over the eye, N2 near the tropopause decreases and the cold-point tropopause rises by up to 4 km at the storm center. Outside of the eye, N2 increases considerably just above the cold-point tropopause and the tropopause remains near its initial level. A budget analysis reveals that the advection terms, which include differential advection of potential temperature θ and direct advection of N2, are important throughout the upper troposphere and lower stratosphere. These terms are particularly pronounced within the eye, where they destabilize the layer near and above the cold-point tropopause. Outside of the eye, a radial–vertical circulation develops during RI, with strong outflow below the tropopause and weak inflow above. Differential advection of θ near the outflow jet provides forcing for stabilization below the outflow maximum and destabilization above. Turbulence induced by vertical wind shear on the flanks of the outflow maximum also modifies the vertical stability profile. Meanwhile, radiative cooling tendencies at the top of the cirrus canopy generally act to destabilize the upper troposphere and stabilize the lower stratosphere. The results suggest that turbulence and radiation, alongside differential advection, play fundamental roles in the upper-level N2 evolution of TCs. These N2 tendencies could have implications for both the TC diurnal cycle and the tropopause-layer potential vorticity evolution in TCs.

A Ground Elevation and Vegetation Height Retrieval Algorithm Using Micro-Pulse Photon-Counting Lidar Data

The Ice, Cloud and Land Elevation Satellite-2 (ICESat-2) mission employs a micro-pulse photon-counting LiDAR system for mapping and monitoring the biomass and carbon of terrestrial ecosystems over large areas. In preparation for ICESat-2 data processing and applications, this paper aimed to develop and validate an effective algorithm for better estimating ground elevation and vegetation height from photon-counting LiDAR data. Our new proposed algorithm consists of three key steps. Firstly, the noise photons were filtered out using a noise removal algorithm based on localized statistical analysis. Secondly, we classified the signal photons into canopy photons and ground photons by conducting a series of operations, including elevation frequency histogram building, empirical mode decomposition (EMD), and progressive densification. At the same time, we also identified the top of canopy (TOC) photons from canopy photons by percentile statistics method. Thereafter, the ground and TOC surfaces were generated from ground photons and TOC photons by cubic spline interpolation, respectively. Finally, the ground elevation and vegetation height were estimated by retrieved ground and TOC surfaces. The results indicate that the noise removal algorithm is effective in identifying background noise and preserving signal photons. The retrieved ground elevation is more accurate than the retrieved vegetation height, and the results of nighttime data are better than those of the corresponding daytime data. Specifically, the root-mean-square error (RMSE) values of ground elevation estimates range from 2.25 to 6.45 m for daytime data and 2.03 to 6.03 m for nighttime data. The RMSE values of vegetation height estimates range from 4.63 to 8.92 m for daytime data and 4.55 to 8.65 m for nighttime data. Our algorithm performs better than the previous algorithms in estimating ground elevation and vegetation height due to lower RMSE values. Additionally, the results also illuminate that the photon classification algorithm effectively reduces the negative effects of slope and vegetation coverage. Overall, our paper provides an effective solution for estimating ground elevation and vegetation height from micro-pulse photon-counting LiDAR data.

Using reflectance to explain vegetation biochemical and structural effects on sun-induced chlorophyll fluorescence

The growing availability of global measurements of sun-induced chlorophyll fluorescence (SIF) can help in improving crop monitoring, especially the monitoring of photosynthetic activity. However, variations in top-of-canopy (TOC) SIF cannot be directly interpreted as physiological changes because of the confounding effects of vegetation biochemistry (i.e. pigments, dry matter and water) and structure. In this study, we propose an approach of using radiative transfer models (RTMs) and TOC reflectance to estimate the biochemical and structural effects on TOC SIF, as a necessary step in retrieving physiological information from TOC SIF. The approach was assessed by using airborne (HyPlant) reflectance and SIF data acquired over an agricultural experimental farm in Germany on two days, before and during a heat event in summer 2015 with maximum temperatures of 27°C and 34°C, respectively. The results show that over 76% variation among different crops in SIF observations was explained by variation in vegetation biochemistry and structure. In addition, the changes of vegetation biochemistry and structure explained as much as 73% variation between the two days in far-red SIF, and 40% variation in red SIF. The remaining unexplained variation was mostly attributed to the variability in physiological status. We conclude that reflectance provides valuable information to account for biochemical and structural effects on SIF and to advance analysis of SIF observations. The combination of RTMs, reflectance and SIF opens new pathways to detect vegetation biochemical, structural and physiological changes.

Wave-driven flow induced by suspended and submerged canopies

Coastal canopies deliver a wide range of ecosystem services through the flow and transport processes within and around the canopies. This study presents a numerical investigation on the effect of suspended/floating canopies on wave-driven current around canopy using a non-hydrostatic model SWASH. The model results by SWASH for a submerged canopy are first validated with experiment measurements and compared with predictions from two Volume-of-Fluid (VOF) based free surface flow models. Then the effects of suspended/floating canopies on wave attenuation and vertical mean flow structures are examined using SWASH. The model results show that a strong shoreward wave-driven current is generated at the top of a suspended canopy in the same direction as the wave, same as that at the top of a benthic canopy. In contrast, a seaward mean current is generated at the bottom of a suspended canopy in the opposite direction to the wave. The predicted particle trajectories indicate that due to flow attenuation within canopy, at the bottom of the canopy, the fluid particles travel faster at the bottom half of their orbits outside the canopy in the opposite direction of wave propagation than at the top half of their orbits within the canopy in the direction of wave propagation. This leads to open particle orbits and a time‐averaged mean current in the opposite direction of wave propagation. The present study is the first attempt that reveals and examines this physical phenomena that would play an important role in particulate transport and exchange across the suspended canopies. The maximum magnitudes of wave induced currents at the top and bottom of canopy are consistent with a recent empirical formula extracted from the observations for submerged canopy. The predicted wave decay agrees well with a three-layer analytical solution for suspended and submerged canopy and indicates that the canopy vertical location plays an important role in wave attenuation and canopy induced currents. The spatial distribution patterns of the wave-driven current clearly support the notion that the current strength generated at the canopy interface is a function of horizontal oscillatory velocity, vertical orbital excursion and canopy density. The spanwise extent of the mean current is examined by a 3-D simulation where the canopy occupies only half the flume width. Numerical results show that the wave-driven current is mainly confined to the part of flume occupied by the canopy and that the current strength is soon attenuated to zero towards the other part of flume through a mixing-layer-like transition region at the canopy side interface.

Vertical patterns of photosynthesis and related leaf traits in two contrasting agricultural crops.

To include within-canopy leaf acclimation responses to light and other resource gradients in photosynthesis modelling, it is imperative to understand the variation of leaf structural, biochemical and physiological traits from canopy top to bottom. In the present study, leaf photosynthetic traits for top and bottom canopy leaves, canopy structure and light profiles, were measured over one growing season for two contrasting crop types, winter barley (Hordeum vulgare L.) and rape seed (Brassica napus L.). With the exception of quantum yield, other traits such as maximum photosynthetic capacity (Amax), dark respiration, leaf nitrogen and chlorophyll contents, and leaf mass per area, showed consistently higher (P<0.05) values for top leaves throughout the growing season and for both crop types. Even though Amax was higher for top leaves, the bottom half of the canopy intercepted more light and thus contributed the most to total canopy photosynthesis up until senescence set in. Incorporating this knowledge into a simple top/bottom-leaf upscaling scheme, separating top and bottom leaves, resulted in a better match between estimated and measured total canopy photosynthesis, compared with a one-leaf upscaling scheme. Moreover, aggregating to daily and weekly temporal resolutions progressively increased the linearity of the leaf photosynthetic responses to light for top leaves.

Novel Twig Sampling Method by Unmanned Aerial Vehicle (UAV)

Biophysical and biochemical traits of foliage and twigs at the top of tree canopies provide essential information on trees’ ecophysiology. Conventional methods used for canopy sampling are typically time consuming and costly, while the very canopy tops are still out of reach unless a canopy crane is used. Thus, we developed a novel twig sampling method using a device attached to a drone that allows to grasp and cut small twigs and bring them to the ground for immediate analysis. This ``Flying Tree Top Sampler'' (FTTS) complements existing methods used in tree canopy research. This paper describes the method to make it openly available to interested users, provides the necessary technical details, and reports on a proof-of-concept application by probing a single tree during senescence. In this demonstration example we show that the ability to investigate the top part of the tree leads to the clear finding that chlorophyll content of the top leaves is significantly lower than that of leaves in the lower canopy. Without the samples collected by the FTTS, the decrease of chlorophyll content with height would not have been of statistical significance using a p = 0.05 significance threshold. Besides its advantages, the existing limitations of the FTTS are discussed, and suggestions for future developments are provided.

Retrieval of land surface properties from an annual time series of Landsat TOA radiances during a drought episode using coupled radiative transfer models

The accurate retrieval of land surface vegetation properties under varying environmental conditions from time series of moderately high spatial resolution satellite observations is challenging. By coupling various Radiative Transfer (RT) models one can describe the soil, vegetation and atmosphere contributions in a “bottom-up” approach and, thereby, simulate top-of-atmosphere (TOA) spectral radiance data comparable to satellite-observed TOA radiances. This makes it possible to retrieve vegetation properties directly from TOA radiances rather than from atmospherically corrected top-of-canopy (TOC) reflectance data. The advantages of this approach are that a separate atmospheric correction of the satellite images is not necessary, and that the anisotropic surface reflection can also be taken into account effectively. In this study, we coupled various RT models, including the brightness – shape – moisture (BSM) reflectance model of the soil, the optical radiative transfer (RTMo) model of vegetation and the ‘MODerate resolution atmospheric TRANsmission’ (MODTRAN) model of the atmosphere, to simulate an annual time series of Landsat satellite TOA radiances observed during a drought episode in California Mediterranean grasslands in 2004. The inversion of this coupled system through an optimization technique against Landsat TOA radiances resulted in direct retrieval of vegetation properties. We accommodated the surface anisotropic reflection in our coupled modeling and also defined a novel anisotropy index to quantitatively express the importance of this phenomenon in satellite image analysis for the first time. Our study showed that the coupled use of RT models was able to accurately reproduce the time series of observed TOA radiances collected under varying soil moisture contents during an extended drought episode. The proposed coupling approach is useful for successful retrieval of vegetation properties from time series of satellite TOA radiance data to produce maps of land surface properties and to monitor vegetation properties variations in an operational straightforward way. The approach can also be easily adapted for conducting multi-sensor time series studies, creating a much denser temporal sampling than would be possible for separate single sensors.

Controls on gross production in an aspen–sagebrush vegetation mosaic

AbstractThe critical zone (CZ; defined as the zone between the top of the vegetation canopy and the groundwater) mediates the impact of precipitation amount and timing on water availability and plant productivity. However, CZ structure, including soil and subsurface properties, are almost always unknown, leading to considerable uncertainty in the links between precipitation, plant water availability, and gross production. Using multiyear records of gross ecosystem CO2 exchange (GEE), micrometeorology, and streamflow, we examined the sensitivity of GEE to environmental controls in a sagebrush shrubland and an aspen forest. The sites were within 500 m of each other and had similar precipitation and temperature but marked differences in CZ structure. Cumulative growing season GEE was approximately two times greater at the aspen compared with the sagebrush, underscoring the importance of CZ properties in structuring spatial “hot spots” for carbon cycling. Larger soil water holding capacity and topography that produced lateral subsurface flow to the aspen enabled marked difference in plant functional type. Annual variability in growing season GEE within each site was not driven by annual precipitation; this lack of relationship was attributed to the CZ's limited ability to store water and an associated increase in water yield. Instead, both seasonal timing and cumulative growing season GEE for the sagebrush varied with spring and summer rain, whereas aspen GEE responded to spring snowpack conditions. These results emphasized how mapping and understanding CZ structure will help predict the spatial variability in plant functional type and temporal sensitivity of growing season GEE.

Assessment of cross-sensor vegetation index compatibility between VIIRS and MODIS using near-coincident observations

Cross-sensor compatibility of spectral vegetation indices (VIs) between Suomi National Polar-orbiting Partnership Visible Infrared Imaging Radiometer Suite (VIIRS) and Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) was investigated using their near-coincident observation pairs obtained along overlapped orbital tracks across the globe for the year 2015. The “top-of-atmosphere (TOA)” and “top-of-canopy (TOC)” normalized difference vegetation indices (NDVIs), TOC-enhanced vegetation index (EVI), and TOC two-band EVI (EVI2) were investigated. For all four VIs, VIIRS and MODIS VIs were subject to systematic differences in which VIIRS VIs were higher than their MODIS counterparts. The overall systematic differences and uncertainties (measured as mean differences and root mean square differences, respectively) were small (0.010 to 0.020 VI units and 0.015 to 0.022 VI units, respectively). TOA NDVI cross-sensor differences were neither seasonally nor view zenith angle dependent, whereas TOC NDVI cross-sensor differences slightly varied seasonally, but were not view zenith angle dependent. TOC EVI and TOC EVI2 cross-sensor differences were view zenith angle dependent, where systematic differences increased with increasing view zenith angle and, for large view zenith angles, they were higher during the summer seasons. These results support the normalization of view zenith angles as a required step to extend the MODIS VI record with VIIRS data.

Estimation of Canopy Height Using an Airborne Ku-Band Frequency-Modulated Continuous Waveform Profiling Radar

An airborne Ku -band frequency-modulated continuous waveform (FMCW) profiling radar terms as Tomoradar provides a distance-resolved measure of microwave radiation backscattered from the canopy surface and the underlying ground. The Tomoradar waveform data are acquired in the southern Boreal Forest Zone with Scots pine, Norway spruce, and birch as major species in Finland. A weighted filtering algorithm based on statistical properties of noise is designed to process the original waveform. In addition, another algorithm of estimating canopy height for the processed waveform is developed by extracting the canopy top and ground position. A higher-precision reference data from a Velodyne VLP-16 laser scanner and a digital terrain model are introduced to validate the accuracy of extracted canopy height. According to the processed results from 127 765 copolarization measurements in 32 stripes of Tomoradar field test, the mean error of canopy height varies from −0.04 to 1.53 m, and the root-mean-square error approximates 1 m. Moreover, the estimated canopy heights highly correlate with the reference data in view of that the correlation coefficients maintain from 0.86 to 0.99 with an average value of 0.96. All these results demonstrate that Tomoradar presents an important approach in estimating the canopy height with several meters footprint and is feasible of being a validation instrument for satellite LiDAR with large footprint in the forest inventory.

Evaluation of PROBA-V Collection 1: Refined Radiometry, Geometry, and Cloud Screening

PROBA-V (PRoject for On-Board Autonomy–Vegetation) was launched in May-2013 as an operational continuation to the vegetation (VGT) instruments on-board the Système Pour l’Observation de la Terre (SPOT)-4 and -5 satellites. The first reprocessing campaign of the PROBA-V archive from Collection 0 (C0) to Collection 1 (C1) aims at harmonizing the time series, thanks to improved radiometric and geometric calibration and cloud detection. The evaluation of PROBA-V C1 focuses on (i) qualitative and quantitative assessment of the new cloud detection scheme; (ii) quantification of the effect of the reprocessing by comparing C1 to C0; and (iii) evaluation of the spatio-temporal stability of the combined SPOT/VGT and PROBA-V archive through comparison to METOP/advanced very high resolution radiometer (AVHRR). The PROBA-V C1 cloud detection algorithm yields an overall accuracy of 89.0%. Clouds are detected with very few omission errors, but there is an overdetection of clouds over bright surfaces. Stepwise updates to the visible and near infrared (VNIR) absolute calibration in C0 and the application of degradation models to the SWIR calibration in C1 result in sudden changes between C0 and C1 Blue, Red, and NIR TOC reflectance in the first year, and more gradual differences for short-wave infrared (SWIR). Other changes result in some bias between C0 and C1, although the root mean squared difference (RMSD) remains well below 1% for top-of-canopy (TOC) reflectance and below 0.02 for the normalized difference vegetation index (NDVI). Comparison to METOP/AVHRR shows that the recent reprocessing campaigns on SPOT/VGT and PROBA-V have resulted in a more stable combined time series.

Simulating arbitrary hyperspectral bandsets from multispectral observations via a generic Earth Observation-Land Data Assimilation System (EO-LDAS)

This paper presents results of using multi-sensor and multi-angular constraints in the generic Earth Observation-Land Data Assimilation System (EO-LDAS) for reproducing arbitrary bandsets of hyperspectral reflectance at the top-of-canopy (TOC) level by merging observations from multispectral sensors with different spectral characteristics. This is demonstrated by combining Multi-angle Imaging Spectroradiometer (MISR) and Landsat Enhanced Thematic Mapper Plus (ETM+) data to simulate the Compact High Resolution Imaging Spectrometer CHRIS/PROBA hyperspectral signal over an agricultural test site, in Barrax, Spain. However, the method can be more generally applied to any combination of spectral data, providing a tool for merging EO data to any arbitrary hyperspectral bandset.Comparisons are presented using both synthetic and observed MISR and Landsat data, and retrieving surface biophysical properties. We find that when using simulated MISR and Landsat data, the CHRIS/PROBA hyperspectral signal is reproduced with RMSE 0.0001–0.04. LAI is retrieved with r2 from 0.97 to 0.99 and RMSE of from 0.21 to 0.38. The results based on observed MISR and Landsat data have lower performances, with RMSE for the reproduced CHRIS/PROBA hyperspectral signal varying from 0.007 to 0.2. LAI is retrievedwith r2 from 0.7 to 0.9 and RMSE from 0.7 to 1.4. We found that for the data considered here the main spectral variations in the visible and near infrared regions can be described by a limited number of parameters (3–4) that can be estimated from multispectral information. Results show that the method can be used to simulate arbitrary bandsets, which will be of importance to any application which requires combining new and existing streams of new EO data in the optical domain, particularly intercalibration of EO satellites in order to get continuous time series of surface reflectance, across programmes and sensors of different designs.

How stand tree motion impacts wind dynamics during windstorms

Understanding how wind and trees interact during wind storms is crucial for better predicting forest wind damage. The complexity of this interaction is enhanced by the fragmented environment of forests. Here, we present an unprecedented field experiment (TWIST) where both the wind dynamics and the tree motion in the edge region of a maritime pine forest have been recorded simultaneously during four non-destructive wind storms. For three of them, the instrumented trees were under stand flow while for one of them they were under an edge flow. Our measurements demonstrate that the well-known characteristics of stand-flow dynamics remain valid under high wind conditions. Only the sub-canopy flow appeared more intermittent as canopy-top turbulent structures penetrate easier within the canopy due to the tree foliage reconfiguration. Under similar storm intensity, the tree motions were lower under edge flow than under stand flow due to the lower turbulence of the former flow while the mean wind speed was higher. This result demonstrates the importance of considering both the turbulence and the mean wind speed in wind risk models. No impact of tree motion other than tree reconfiguration were observed on the stand flow dynamics. On the other hand, for the edge flow, our measurements reveal a peak in frequency on the wind velocity fluctuations related to the fundamental tree vibration mode. This peak was especially visible at canopy top and in the upper trunk space under high wind conditions. Compared to the stand flow, we suspect that the velocity fluctuations induced by the tree motion emerge in the edge flow due to the lower background turbulence. Our edge storm was nonetheless not strong enough for tree motion to enhance flow turbulence and for trees to enter into resonance. These findings may suggest a higher susceptibility of near-edge trees to reach resonance than stand trees due to the motion of upwind trees in a lower background turbulence.

Effects of leaf area index and density on ultrafine particle deposition onto forest canopies: A LES study

A framework to describe transport and deposition of ultrafine particles (UFP) within forests using large eddy simulation (LES) is presented. Comparison with measurements collected within and above a Scots pine stand in Southern Finland are used to explore the plausibility of the simulations. The numerical model is then employed to quantify the effects of canopy morphology (leaf area index and leaf area density), turbulence intensity, and particle size on the partitioning between upper canopy and subcanopy deposition and the overall deposition velocities. Results show a complex interplay between canopy morphology and turbulence, which is reflected on the particle flux profiles within the canopy. However, mean particle concentration profiles, total deposition, and deposition velocities at the canopy top are insensitive to the leaf area density profile but show dependence mostly on leaf area index, turbulence levels, and particle size. Finally, with the goal of understanding the sensitivity of the deposition velocity to all these parameters, an analytical model is developed that shows good agreement with LES results (within ±20%) for all conditions simulated here.

The Effect of Light Spectrum on the Morphology and Cannabinoid Content of Cannabis sativa L.

Cannabis sativa L. flowers are the main source of Δ-9-tetrahydrocannabinol (THC) used in medicine. One of the most important growth factors in cannabis cultivation is light; light quality, light intensity, and photoperiod play a big role in a successful growth protocol. The aim of the present study was to examine the effect of 3 different light sources on morphology and cannabinoid production. Cannabis clones were grown under 3 different light spectra, namely high-pressure sodium (HPS), AP673L (LED), and NS1 (LED). Light intensity was set to ∼450 µmol/m²/s measured from the canopy top. The photoperiod was 18L: 6D/21 days during the vegetative phase and 12L: 12D/46 days during the generative phase, respectively. At the end of the experiment, plant dry weight partition, plant height, and cannabinoid content (THC, cannabidiol [CBD], tetrahydrocannabivarin [THCV], cannabigerol [CBG]) were measured under different light treatments. The experiment was repeated twice. The 3 light treatments (HPS, NS1, AP673L) resulted in differences in cannabis plant morphology and in cannabinoid content, but not in total yield of cannabinoids. Plants under HPS treatment were taller and had more flower dry weight than those under treatments AP673L and NS1. Treatment NS1 had the highest CBG content. Treatments NS1 and AP673L had higher CBD and THC concentrations than the HPS treatment. Results were similar between experiments 1 and 2. Our results show that the plant morphology can be manipulated with the light spectrum. Furthermore, it is possible to affect the accumulation of different cannabinoids to increase the potential of medicinal grade cannabis. In conclusion, an optimized light spectrum improves the value and quality of cannabis. Current LED technology showed significant differences in growth habit and cannabinoid profile compared to the traditional HPS light source. Finally, no difference of flowering time was observed under different R:FR (i.e., the ratio between red and far-red light).