Plant Area Density Research Articles

Mapping plant area index of tropical evergreen forest by airborne laser scanning. A cross-validation study using LAI2200 optical sensor

Leaf area index estimates in dense evergreen tropical moist forest almost exclusively rest on indirect methods most of which being of limited accuracy or spatial resolution. In this study we examine the potential of full waveform Aerial Laser Scanning (ALS) to derive accurate spatially explicit estimates of Plant Area Index (PAI).A discrete representation of the forest canopy is introduced in the form of a 3D voxelized space. For each voxel (elementary volume, typically one cubic m) a first estimate of local transmittance of vegetation is computed as the ratio of the sum of energy exiting a voxel to the sum of energy entering the same voxel. A spatially hierarchical model is subsequently applied to refine estimates of individual voxel transmittance. Plant area density (PAD) profiles are then computed from the local transmittance values by applying Beer Lambert's turbid medium approximation. PAI values are obtained from vertical integration of PAD profiles. The model is shown to be robust to low sampling intensity and high occlusion rates.We further compared simulated values of gap fraction obtained by ray tracing for 5 angular sectors with in situ LAI2200 measurements taken at 135 positions in a 0.5ha forest plot located in the center of the scene. The overall patterns of simulated and measured values (average value per inclination and pattern of variation along a 70m transect line) were highly consistent. A slight but systematic discrepancy was observed along the inclination gradient, gap fractions derived from ray tracing in the voxelized scene being slightly lower than the measured values. This difference might be the consequence of multiple reflections which have been found to bias gap fractions estimates produced by LAI2200.PAI estimates derived from LAI2200 measurements (either simulated 6.8 or observed 5.9) are much lower than the PAI derived from vertical integration of local PAD (13.6). This large difference reflects the fact that distribution of foliage is strongly spatially structured and that this structural information is not properly accounted for in PAI estimates derived from mean gap fraction per elevation angle. After adjusting local transmittance to match mean LAI 2200 profiles the PAI at plot level was found to be 13.2m2·m−2.We conclude that Aerial Laser Scanning can produce accurate maps of Plant Area Index over large areas with unmatched efficacy, accuracy and ease. This should be of major relevance for many forest ecological studies.

Large-Eddy Simulation Comparison of Neutral Flow Over a Canopy: Sensitivities to Physical and Numerical Conditions, and Similarity to Other Representations

The representation of a neutral atmospheric flow over roughness elements simulating a vegetation canopy is compared between two large-eddy simulation models, wind-tunnel data and recently updated empirical flux-gradient relationships. Special attention is devoted to the dynamics in the roughness sublayer above the canopy layer, where turbulence is most intense. By demonstrating that the flow properties are consistent across these different approaches, confidence in the individual independent representations is bolstered. Systematic sensitivity analyses with the Dutch Atmospheric Large-Eddy Simulation model show that the transition in the one-sided plant-area density from the canopy layer to unobstructed air potentially alters the flow in the canopy and roughness sublayer. Anomalously induced fluctuations can be fully suppressed by spreading the transition over four steps. Finer vertical resolutions only serve to reduce the magnitude of these fluctuations, but do not prevent them. To capture the general dynamics of the flow, a resolution of 10 % of the canopy height is found to suffice, while a finer resolution still improves the representation of the turbulent kinetic energy. Finally, quadrant analyses indicate that momentum transport is dominated by the mean velocity components within each quadrant. Consequently, a mass-flux approach can be applied to represent the momentum flux.

Estimation of Vertical Plant Area Density Profiles of a Rice Plant by High Position Measurements Using a Middle Range Portable Scanning Lidar

Estimation of Vertical Plant Area Density Profiles of a Rice Plant by High Position Measurements Using a Middle Range Portable Scanning Lidar

TurbEFA: an interdisciplinary effort to investigate the turbulent flow across a forest clearing

the atmosphere within turbulence closure models is mainly limited by a realistic three-dimensional (3D) representation of the vegetation architecture. Within this contribution we present a method to record the 3D vegetation structure and to use this information to derive model parameters that are suitable for numerical flow models. A mixed conifer forest stand around a clearing was scanned and represented by a dense 3D point cloud applying a terrestrial laser scanner. Thus, the plant area density (PAD) with a resolution of one cubic meter was provided for analysis and for numerical simulations. Multi-level high-frequency wind velocity measurements were recorded simultaneously by 27 ultrasonic anemometers on 4 towers for a period of one year. The relationship between wind speed, Reynolds stress and PAD was investigated and a parametrization of the drag coefficient CD by the PAD is suggested. The derived 3D vegetation model and a simpler model (based on classical forest assessments of the site) were applied in a boundary layer model (BLM) and in large-eddy simulations (LES). The spatial development of the turbulent flow over the clearing is further demonstrated by the results of a wind tunnel experiment. The project showed, that the simulation results were improved significantly by the usage of realistic vegetation models. 3D simulations are necessary to depict the influence of heterogeneous canopies on the turbulent flow. Whereas we found limits for the mapping of the vegetation structure within the wind tunnel, there is a considerable potential for numerical simulations. The field measurements and the LES gave new insight into the turbulent flow in the vicinity and across the clearing. The results show that the zones of intensive turbulence development can not be restricted to the locations found in previous studies with more idealized canopies.

Large-Eddy Simulation Study of the Effects on Flow of a Heterogeneous Forest at Sub-Tree Resolution

The effect of three-dimensional plant heterogeneity on flow past a clearing is investigated by means of large-eddy simulation. A detailed representation of the canopy has been acquired by terrestrial laser scanning for a patch of approximately $$328\,\mathrm {m}$$ length and $$172\,\mathrm {m}$$ width at the field site “Tharandter Wald”, near the city of Dresden, Germany. The scanning data are used to produce a highly resolved, three-dimensional plant area distribution representing the actual canopy. Hence, the vegetation maintains a rich horizontal and vertical structure including the three-dimensional clearing. The scanned plant area density is embedded in a larger domain, which is filled with a heterogeneous forest generated by the virtual canopy generator of Bohrer et al. (Tellus B 59:566–576, 2007). Based on forest inventory maps and airborne laser scanning, the characteristics of the actual canopy are preserved. Furthermore, the topography is extracted from a digital terrain model with some modifications to accommodate for periodic boundary conditions. A large-eddy simulation is performed for neutral atmospheric conditions and compared to simulations of a two-dimensional plant area density and an one-year-long field experiment conducted at the corresponding field site. The results reveal a considerable influence of the plant heterogeneity on the mean velocity field as well as on the turbulent quantities. The three-dimensional environment, e.g., the oblique edges combined with horizontal and vertical variations in plant area density and the topography create a sustained vertical and cross-flow velocity. Downstream of the windward forest edge an enhanced gust zone develops, whose intensity and relative position are influenced by the local canopy density and, therefore, is not constant along the edge. These results lead us to the conclusion that the usage of a three-dimensional plant area distribution is essential for capturing the flow features inside the canopy and within the mixing layer above.

Multiscale Simulation of a Prescribed Fire Event in the New Jersey Pine Barrens Using ARPS-CANOPY

AbstractSmoke prediction products are one of the tools used by land management personnel for decision making regarding prescribed fires. This study documents the application to a prescribed fire of a smoke prediction system that employs ARPS-CANOPY, a modified version of the Advanced Regional Prediction System (ARPS) model containing a canopy submodel, as the meteorological driver. In this paper, the performance of ARPS-CANOPY in simulating meteorological fields in the vicinity of a low-intensity fire is assessed using flux-tower data collected prior to and during a low-intensity prescribed fire in the New Jersey Pine Barrens in March 2011. A three-dimensional high-resolution plant area density dataset is utilized to define the characteristics of the canopy, and the fire is represented in ARPS-CANOPY as a heat flux to the atmosphere. The standard ARPS model is compared with reanalysis and upper-air data to establish that the model can simulate the observed synoptic-mesoscale and planetary boundary layer features that are salient to this study. ARPS-CANOPY profiles of mean turbulent kinetic energy, wind speed/direction, and temperature exhibit patterns that appear in the flux-tower observations during both the preburn phase of the experiment and the period of time the flux tower experienced perturbed atmospheric conditions due to the impinging fire. Last, the character and source of turbulence in and around the fire line are examined. These results are encouraging for smoke prediction efforts since transport of smoke from low-intensity fires is highly sensitive to the near-surface meteorological conditions and, in particular, turbulent flows.

Estimation of vertical plant area density profiles in a rice canopy at different growth stages by high-resolution portable scanning lidar with a lightweight mirror

We used a high-resolution portable scanning lidar together with a lightweight mirror and a voxel-based canopy profiling method to estimate the vertical plant area density (PAD) profile of a rice (Oryza sativa L. cv. Koshihikari) canopy at different growth stages. To improve the laser’s penetration of the dense canopy, we used a mirror to change the direction of the laser beam from horizontal to vertical (0°) and off-vertical (30°). The estimates of PAD and plant area index (PAI) were more accurate at 30° than at 0°. The root-mean-square errors of PAD at each growth stage ranged from 1.04 to 3.33m2m−3 at 0° and from 0.42 to 2.36m2m−3 at 30°, and those across all growth stages averaged 1.79m2m−3 at 0° and 1.52m2m−3 at 30°. The absolute percent errors of PAI at each growth stage ranged from 1.8% to 66.1% at 0° and from 4.3% to 23.2% at 30°, and those across all growth stages averaged 30.4% at 0° and 14.8% at 30°. The degree of laser beam coverage of the canopy (expressed as index Ω) explained these errors. From the estimates of PAD at 30°, regressions between the areas of stems, leaves, and ears per unit ground area and actual dry weights gave standard errors of 7.9gm−2 for ears and 12.2gm−2 for stems and leaves.

VOXEL-BASED ESTIMATION OF PLANT AREA DENSITY FROM AIRBORNE LASER SCANNER DATA

Abstract. Three-dimensional distribution of plant area density (PAD) was retrieved using airborne laser scanning (ALS) data. The calculation of PAD requires the number of laser-pulses intercepted by plant materials and which are not intercepted by trees (i.e., passed laser-pulses) for the spatial unit. To estimate the passed laser-pulses at a voxel (1-m voxel in this study), we traced every laser-return by using its flight line information. The assumption was that every laser-pulse traveled on the orthogonal line to flight line, meaning that the sensor mounted in the aircraft scanned perpendicularly to its flight line. Our function based on this assumption allowed PAD to be calculated. Consequently, we successfully obtained the PAD profiles at every 1-m voxel for the canopy area of 56 trees, which could be useful in the quantitative assessment of canopy structure at a broad scale.

Measurements of Vertical Plant Area Density Profiles of a Rice Plant Using a Portable Scanning Lidar

Measurements of Vertical Plant Area Density Profiles of a Rice Plant Using a Portable Scanning Lidar

Wind effects on trees

The interactions between airflow and trees and forest stands are diverse. They include the reduction in nearsurface wind speed and production of turbulence by trees. Near-surface wind conditions affect physiological processes in trees, tree growth, and survival (Ennos 1997; Eugster 2008). Turbulent components of the flow field dominate the tree response behaviour (Mayer 1987; Gardiner 1995; Schindler et al. 2010) and drive the scalar exchange at the forest–atmosphere interface (Finnigan 2000). Wind–tree interactions take place at a wide range of temporal and spatial scales (de Langre 2008). Aerodynamic drag at all surfaces of the aerial parts of trees—from individual leaves (Vogel 1989) to whole tree crowns (Kane et al. 2008)—perturbs the airflow inside forest canopies (Shaw et al. 1974; Baldocchi and Meyers 1988; Turnipseed et al. 2003). Therefore, detailed information on forest structure is an essential precondition for understanding wind–tree interactions and the successful application of flow models to tall canopies. In flow models, forest structure is often represented by the mean vertical profile of the plant area density. Queck et al. (2011, this issue) present a method that can be used to record detailed 3D stand structure from terrestrial laser scanning. They investigated the relationship between wind speed, aerodynamic drag, and plant area density and show how 3D laser scanner data can be used to derive turbulence parameters for flow models. The structure of windward forest edges (Mitscherlich 1973; Dupont and Brunet 2008a, b) as well as the stand structure (Gardiner et al. 1997; Marcolla et al. 2003; Dupont and Brunet 2008b, c; Queck and Bernhofer 2010) affects the flow field within and above forests. In the nearedge region, pronounced gradients of flow quantities provoke high wind load on trees (Stacey et al. 1994; Peltola 1996; Gardiner et al. 1997), which may trigger damage in strong wind conditions. Although Gardiner and Stacey (1996) as well as Dupont and Brunet (2008a) report that tapered forest edges reduce wind loading and related bending moments of trees near the edge, it is still not completely clear, how changes in edge and stand structure affect the flow field at the canopy near forest edges, and whether these changes can contribute to the mitigation of damage in high wind conditions. In a detailed wind tunnel study, Ruck et al. (2011, this issue) investigated the effect of changes in taper angle of windward forest edges on the flow field for different stand densities. Their results demonstrate the impact of edge shape and stand density on quantities of the flow field near the canopy top, the region most relevant for tree failure in strong wind conditions. The aerial parts of trees start to vibrate in response to wind excitation (Sellier and Fourcaud 2005; Rodriguez et al. 2008). In forests, dynamic tree responses are not only observed at the single tree level (Mayer 1987; Gardiner 1995; Peltola 1996; Flesch and Wilson 1999; Rudnicki et al. 2008; Schindler et al. 2010) but also at the tree group level (Rudnicki et al. 2001, 2003; Schindler et al. 2011, this issue). At both levels, the resulting tree response patterns are complex and complicated. So far, even in horizontally homogeneous terrain, not all physical and biological This article belongs to the special issue ‘Wind Effects on Trees’.

Wind fields in heterogeneous conifer canopies: parameterisation of momentum absorption using high-resolution 3D vegetation scans

Applications of flow models to tall plant canopies are limited, amongst other factors, by the lack of detailed information on vegetation structure. A method is presented to record 3D vegetation structure and make this information applicable to the derivation of turbulence parameters suitable for flow models. The relationship between wind speed, drag coefficient (CD) and plant area density (PAD) was experimentally investigated in a mixed conifer forest in the lower part of the Eastern Ore Mountains. Essential information was gathered by collecting multi-level high-frequency wind velocity measurements and a dense 3D representation of the forest was obtained from terrestrial laser scanner data. Wind speed dependence or streamlining was observed for most of the wind directions. Edge effects, i.e. the influence of the here not regarded pressure gradient and the advective terms of the momentum equation, are assumed to cause this heterogeneity. Contrary to the hypothetic shelter effect, which would reduce the drag on sheltered plant parts, the calculated profiles of drag coefficients revealed an increasing CD with PAD (i.e. a dependence on canopy and plant structure).

Constructing wind profiles in forests from limited measurements of wind and vegetation structure

The aim of the advection initiative ADVEX within CarboEurope-IP (CEIP) was to evaluate the importance of advection on three coniferous sites. A prerequisite and crucial point for calculating the advective terms of CO 2 balance are the assumptions for the construction of wind fields. Wind measurements must be interpolated over complex control volumes. Here the plant area density is identified as a key variable. During ADVEX campaigns, the rectangular control volumes were marked by four towers at the corners, each carrying four sonic anemometers over the volume height (up to 30 m). An additional experiment was designed at the CEIP forest site Tharandter Wald to investigate the error made by interpolation between only four measurement levels. Thirteen sonic anemometers were installed on one tower up to a height of 42 m (canopy height is 30 m). In addition, plant area density profiles were measured to derive plant-specific parameterisation of the wind profiles. Profiles of velocity moments and nondimensional wind speed gradients are discussed as basic characteristics of the wind field. A new parameterisation scheme based on the mixing length approach was developed. The model was validated with the high-resolution data and applied to calculate horizontal advection for the ADVEX sites. In addition to improving the interpolation routine, this model provides the opportunity to reduce the number of large towers needed to calculate in-canopy wind fields.

Estimating vertical plant area density profile and growth parameters of a wheat canopy at different growth stages using three-dimensional portable lidar imaging

Vertical plant area density profiles of wheat ( Triticum aestivum L.) canopy at different growth stages (tillering, stem elongation, flowering, and ripening stages) were estimated using high-resolution portable scanning lidar based on the voxel-based canopy profiling method. The canopy was scanned three-dimensionally by laser beams emitted from several measuring points surrounding the canopy. At the ripening stage, the central azimuth angle was inclined about 23 ∘ to the row direction to avoid obstruction of the beam into the lower canopy by the upper part. Plant area density profiles were estimated, with root mean square errors of 0.28–0.79 m 2 m −3 at each growth stage and of 0.45 m 2 m −3 across all growth stages. Plant area index was also estimated, with absolute errors of 4.7%–7.7% at each growth stage and of 6.1% across all growth stages. Based on lidar-derived plant area density, the area of each type of organ (stem, leaves, ears) per unit ground area was related to the actual dry weight of each organ type, and regression equations were obtained. The standard errors of the equations were 4.1 g m −2 for ears and 26.6 g m −2 for stems and leaves. Based on these equations, the estimated total dry weight was from 63.3 to 279.4 g m −2 for ears and from 35.8 to 375.3 g m −2 for stems and leaves across the growth stages. Based on the estimated dry weight at ripening and the ratio of carbon to dry weight in wheat plants, the carbon stocks were 76.3 g C m −2 for grain, 225.0 g C m −2 for aboveground residue, and 301.3 g C m −2 for all aboveground organs.

Seasonal change of plant area density measured by permanently-installed laser scanner in larch forest

Seasonal change of plant area density measured by permanently-installed laser scanner in larch forest

Estimating the plant area density of a Japanese larch ( Larix kaempferi Sarg.) plantation using a ground-based laser scanner

The three-dimensional (3D) distribution of plant area density (PAD) is difficult and time-consuming to determine. In this study, we measured this distribution using a ground-based laser scanner consisting of a laser range finder and two rotation stages, and compared the results with those obtained based on the attenuation of solar radiation and litter traps. To determine the 3D distribution of PAD, we divided the forest horizontally and vertically to define volume elements (voxels). In each voxel, we calculated PAD from the gap fraction, which was defined as the probability that laser beams from the forest floor would pass through the voxel without interception. Measurements were obtained four or five times between the in-leaf and leafless periods in 45- and 46-year-old Japanese larch ( Larix kaempferi Sarg.) plantations with different stem densities after thinning. We found the following: (1) Because the largest tree height was greater than the highest point of interception of the laser beams, our method underestimated PAD in the upper layer of the forest. (2) Compared with estimates based on the attenuation of solar radiation, our method overestimated plant area index (PAI) by around three units throughout the season, but successfully captured the reduction of PAI between bud break and leaf-fall. (3) The horizontal distribution of PAI estimated by our method was well correlated with that estimated using litter traps and reproduced the horizontal non-uniformity of PAI. Despite some limitations, such as the diameter and divergence angle of the laser beam, the ground-based laser scanner successfully provided direct, non-destructive estimates of the forest's 3D distribution of PAD.

Momentum Transfer By A Mountain Meadow Canopy: A Simulation Analysis Based On Massman's (1997) Model

Using a mountain meadow as a case study it is the objective of the present paper todevelop a simple parameterisation for the within-canopy variation of the phytoelementdrag (Cd) and sheltering (Pm) coefficients required for Massman's model of momentum transfer by vegetation. A constant ratio between Cd and Pm is found to overestimate wind speed in the upper canopy and underestimate it in the lower canopy.Two simple parameterisations of Cd/Pm as a function of the plant area density and the cumulative plant area index are developed, using values optimised by least-squares regression between measured and predicted within-canopy wind speeds. A validation with independently measured data indicates that both parameterisations work reliably for simulating wind speed in the investigated meadow. Model predictions of the normalised zero-plane displacement height and the momentum roughness length fall only partly within the range of values given in literature, which may be explained by the accumulation of plantmatter close to the soil surface specific for the investigated canopies. The seasonal course of the normalised zero-plane displacement height and the momentum roughness length are discussed in terms of the seasonal variation of the amount and density of plant matter.

Effects of Vertical Profiles of Plant Area Density and Stomatal Resistance on the Energy Exchange Processes within a Rice Canopy.

A multilayer energy budget model within and above a rice canopy is developed to show physical and physiological effects of the vegetation on the energy exchange processes and to evaluate the characteristics of the energy budget. Micrometeorogical observations within and above a rice canopy are analyzed to optimize model parameters and to test the model. Model parameters of the inclination factors of a leaf Fl(z) and a panicle Fp for radiation, the effective drag coefficient cd, the transfer coefficient for sensible heat ch and stomatal resistance rs(z) are optimized by fitting the modeled vertical profiles of downward shortwave radiation S↓(z), mean horizontal wind speed, air temperature, plant surface temperature and water vapor pressure to the observations in the morning, near noon and in the afternoon on 4 September 1998, when the growing stage of rice was early ripening. The optimized model parameters demonstrate that the radiation exchange processes depend on the solar altitude and the vertical distribution of the plant area density and its components, i.e. the morphology of the canopy, and that panicles are as important as leaves for the energy exchange processes in the paddy field, while culms are unimportant. The optimized vertical profiles of the stomatal resistance rs(z) show inverse correlations with Fl(z)S↓(z), and positive correlations with vapor pressure deficit VPD(z). Thus, the model demonstrates that the stomatal resistance is also influenced by the morphology of the canopy, and interacts with VPD adjacent to the leaves. According to the calculated vertical profiles of sensible and latent heat fluxes, the negative sensible heat flux in the afternoon originates mainly from lower canopy layers, which occurs even under the condition of higher rs(z).

Intermittent wind close to the ground within a grass canopy

Wind speed was measured at a height of 1 cm above the ground and at several other heights in and above a canopy of tall fescue grass (Festuca arundinacea) using single hot-wire and triple hot-film anemometers. The plant area density in the canopy was concentrated close to the ground, with ∼75% of the plant area standing belowz=15 cm, wherez is height above the ground. The frequency distributions of horizontal wind speeds,s, were sharply skewed towards positive values at all measurement heights, but were most highly skewed near the ground where the coefficient of skewness ranged from 1.6 to 2.9. Above mid-canopy height, the frequency distribution ofs was described reasonably well by a Gumbel extreme value distribution. Average wind speed,S, decreased exponentially with depth into the canopy with an exponential scale length of abouth/2.8, whereh is the height of the canopy. Atz=1 cm, the value ofS was about 11% of the surface-layeru *. The standard deviation of the fluctuations of the vertical and horizontal components of the wind speed also decreased exponentially with depth inside the canopy with a scale length of abouth/2.5. Inside the canopy, the Eulerian integral time scales for the vertical (Γ w ) and horizontal (Γ u ) components of wind speed were about 0.1 s and 1.0 s, respectively, and were approximately constant with height. Above the canopy, these time scales increased sharply and, atz=2.25h, Γ w and Γ u were approximately 1.0 and 3.0s, respectively. Turbulence length scales in the vertical and downwind directions, Γ u and Γ w ·U, respectively, were approximately 1 cm for heights between 1 to 10 cm above the ground inside the canopy, while atz=2.25h, they were about 55 cm and 277 cm. Relatively quiescent periods (lulls) in the air close to the ground were interrupted frequently by gusts. The frequency of occurrence of gusts appears to be correlated with the value of the local shear near the top of the canopy.

Computational parameter estimation for a maize crop

During a whole growing season, the evolution of the displacement height, d, and roughness length, z 0, of a maize crop has been estimated by a measurement programme. The results have been used to check different types of existing models to calculate these parameters from canopy characteristics only; a simple geometric model and two matching models have been investigated. A geometric model is based on geometric features of the surface only. After a simple modification, the geometric model gives good results for the displacement height as well as for the roughness length.A matching model, based on gradient-diffusion theory, yields good results for the displacement height. The roughness parameter, however, is overestimated by 17%. By a simple modification, the model results could be improved considerably.A matching model, based on a second-order closure procedure, yields excellent results for the displacement height and good results for the roughness length. But it appears that, when applying this model, the plant density index and plant area density distribution as a function of height must be well known.

A Higher Order Closure Model for Canopy Flow

Abstract The equations of motion were used to develop a one-dimensional, nonbuoyant mathematical model of air flow within vegetative canopies. The model consists of equations for mean horizontal momentum, Reynolds stress, and for the three components of turbulent kinetic energy with closure achieved by parameterizing the higher order terms. This eliminates the need to model the Reynolds stress directly using an eddy viscosity. The closure schemes rely upon a prescribed length scale and have been used elsewhere in modeling the atmospheric boundary layer free of vegetation. The equations were solved numerically using specified boundary conditions. Using a profile of plant area density for a crop of corn (Zea mays L.) the model predicted mean wind velocity, Reynolds stress and turbulent intensities for the region from the soil surface to twice the canopy height that compare well with experimental measurements (Shaw et al., 1974a,b). The model is believed to overestimate the intensity of turbulence generated ...