1,721,622 research outputs found
Indirect Estimates of Canopy Gap Fraction based on the Linear Conversion of Hemispherical Photographs - Methodology and Comparison with Standard Thresholding Techniques
No full textAbstract
One of the crucial steps in image processing is the thresholding, meaning the image segmentation needed to separate the foreground (canopy) from the background (sky). Due to the plethora of factors affecting image grey levels (lens vignetting, gamma correction, heterogeneity of sky irradiance, etc.), with current thresholding procedures the estimation of canopy gap fraction is rather uncertain. Both manual and automatic segmentation methods are highly dependent on exposure, operator experience or assumptions of thresholding algorithms, and therefore produce rather subjective and non-repeatable results. However, since CCD and CMOS sensors used in digital cameras respond linearly to light, it is nowadays possible to obtain images in which digital numbers are proportional to canopy brightness. For this purpose the standard in-camera logarithmic conversion (gamma correction) has to be replaced with a linear conversion of the sensor analog signal. In this work, the potential of the linear conversion of digital images in the estimation of canopy gap fraction is assessed and discussed, comparing the estimates obtained with digital cameras and with the LI-COR LAI-2000. In addition, the performance of the linear conversion is compared with standard automatic or manual thresholding methods, both in terms of canopy gap fraction and of plant architectural parameters. The ability to acquire hemispherical photographs with digital numbers proportional to radiance opens new possibilities in the analysis of canopy structure and microclimate, with important feedbacks in ecology and remote sensing. In particular, using two synchronized cameras above and below the canopy, it is possible to calculate high-resolution gap fraction data, which provide an accurate and objective estimation of the intercepted radiation and of typical architectural parameters, such as LAI. In addition, the effectiveness of this
methodology for the estimation of linear and logarithmic averages of canopy gap fraction, makes this technique an important step forward in the estimation of leaf area index of non-random canopies.JRC.H.2 - Climate chang
Vertical Foliage Distribution Determines the Radial Pattern of Sap Flux Density in Picea Abies
No full textUnderstanding the causes determining the radial
pattern of sap flux density is important both for improving
knowledge of sapwood functioning and for up-scaling sap flow
measurements to canopy transpiration and ecosystem water
use. To investigate the anatomical connection between whorls
and annual sapwood rings, pruning-induced variation in the radial
pattern of sap flux density was monitored with Granier
probes in a 35-year-old Picea abies (L.) Karst tree that was
pruned from the crown bottom up. Modifications in the radial
pattern of sap flux density were quantified by a shape index
(SI), which varies with the relative contribution of the outer and
inner sapwood to tree transpiration. The SI progressively diminished
during bottom up pruning, indicating a significant reduction
in sap flow contribution of the inner sapwood. Results
suggest that the radial pattern of sap flux density depends
mainly on the vertical distribution of foliage in the crown, with
lower shaded branches hydraulically connected with inner
sapwood and upper branches connected with the outer rings.JRC.DDG.H.2 - Climate chang
Diurnal and seasonal variability in radial distribution of sap flux density: implications for estimating stand transpiration
No full textDaily and seasonal patterns in radial distribution of sap flux density were monitored in six trees differing in social
position in a mixed coniferous stand dominated by silver fir (Abies alba Miller) and Norway spruce (Picea abies (L.) Karst) in the Alps of northeastern Italy. Radial distribution of sap flux was measured with arrays of 1-cm-long Granier probes. The radial profiles were either Gaussian or decreased monotonically toward the tree center, and seemed to be related to social position and crown distribution of the trees.
The ratio between sap flux estimated with the most external sensor and the mean flux, weighted with the corresponding annulus
areas, was used as a correction factor (CF) to express diurnal and seasonal radial variation in sap flow. During sunny days, the diurnal radial profile of sap flux changed with time and accumulated photosynthetic active radiation (PAR), with
an increasing contribution of sap flux in the inner sapwood during the day. Seasonally, the contribution of sap flux in the inner xylem increased with daily cumulative PAR and the variation of CFwas proportional to the tree diameter, ranging from 29% for suppressed trees up to 300% for dominant trees. Two models were developed, relating CF with PAR and tree diameter at breast height (DBH), to correct daily and seasonal estimates of whole-tree and stand sap flow obtained by assuming uniform sap flux density over the sapwood. If the variability in the radial profile of sap flux density was not accounted for, total
stand transpiration would be overestimated by 32% during sunny days and 40% for the entire season
Spatial Variability and Optimal Sampling Strategy of Soil Respiration
No full textSoil respiration is the second largest flux of carbon between terrestrial ecosystems and the atmosphere and is affecting climate sensitivity and vulnerability of the terrestrial carbon stock. Monitoring soil carbon dioxide efflux is a complex task, due to the high spatial and temporal variability of the fluxes. For this reason, more than 30 sampling points are required to attain reliable estimates of ecosystem soil respiration. However, the number of sampled points is often limited by labour, time and budget constraints. Stratified sampling is an alternative to random sampling as a method to reduce the number of sampling points when an effective proxy variable is available for the definition of the strata.
In order to evaluate different sampling strategies we tested, with a Monte Carlo simulation, the effectiveness of random and stratified samplings, using experimental data collected in three alpine ecosystems (two forests and one grassland). We evaluated an innovative method for defining the strata to be sampled. The method is based on an initial sampling of soil respiration from a large number of candidate points in order to account for the spatial variability. The minimum number of sampling points required to adequately represent soil respiration for the entire area were then selected using stratified statistical sampling.We show that this method is unbiased and that it reduces considerably the uncertainty in the sampling process compared to random sampling. The method was highly effective in the two forest ecosystems, characterized by a high spatial variability in soil respiration and by a high temporal correlation of the fluxes. On the contrary the method was not so effective in the grassland site, where fluxes have lower spatial variability and temporal correlation. However, the stratified sampling offered a consistent reduction of the error (%) of the estimated annual soil CO2 efflux in all the ecosystems. At the grassland ecosystem the average reduction of the error (%) of the annual CO2 efflux was about 12%, while at the forest ecosystems the average reductions were 55% and 57%, respectively.
# 2007 Elsevier B.V. All rights reserved.JRC.H.2 - Climate chang
Notes on the host plant, larval features and life history of Cephalcia hartigii (Bremi) (Hym., Pamphiliidae).
No full textIndirect partitioning of soil respiration in a series of evergreen forest ecosystems
No full textA simple estimation of heterotrophic respiration can be obtained analytically as the y-intercept of the linear regression between soil-surface CO2 efflux and root biomass. In the present study, a development of this indirect methodology is presented by taking into consideration both the temporal variation and the spatial heterogeneity of heterotrophic respiration. For this purpose, soil CO2 efflux, soil carbon content and main
stand characteristics were estimated in seven evergreen forest ecosystems along an elevation gradient ranging from 250 to 1740 m. For each site and for each sampling date the measured soil CO2 efflux (RS) was predicted with the model RS = a x SC + b x RD±e, where SC is soil carbon content per unit area to a depth of 30 cm and RD is the root density of the 2–5 mm root class. Regressions with statistically significant
a and b coefficients allowed the indirect separation of the two components of soil CO2 efflux. Considering that the different sampling dates were characterized by different soil temperature, it was possible to investigate the temporal and thermal dependency of autotrophic and heterotrophic respiration. It was estimated that annual autotrophic respiration accounts for 16–58% of total soil CO2 efflux in the seven
different evergreen ecosystems. In addition, our observations show a decrease of annual autotrophic respiration at increasing availability of soil nitrogen
Structural acclimation and radiation regime of silver fir (Abies alba Mill.) shoots along a light gradient
No full textShoot architecture has been investigated using the ratio of mean shoot silhouette area to total needle area (Ss) as a
structural index of needle clumping in shoot space, and as the effective extinction coefficient of needle area. Although can be used effectively for the prediction of canopy gap fraction, it does not provide information about the withinshoot radiative regime. For this purpose, the estimation of
three architectural properties of the shoots is required: needle area density, angular distribution and spatial aggregation. To estimate these features, we developed a method based on the inversion of a Markov three-dimensional interception model. This approach is based on the turbid medium approximation for needle area in the shoot volume, and assumes an ellipsoidal angular distribution of the normals to the needle area. Observed shoot dimensions and silhouette areas for different vertical and azimuth angles (AS) are used as model inputs. The shape coefficient of the
ellipsoidal distribution (c) and the Markov clumping index (l0) are estimated by a least square procedure, in order to minimize the differences between model prediction and
measurements of AS. This methodology was applied to silver fir (Abies alba Mill.) shoots collected in a mixed fir–beech–spruce forest in the Italian Alps. The model worked
effectively over the entire range of shoot morphologies: c ranged from 1 to 8 and
l0 from 0·3 to 1 moving from the top to the base of the canopy. Finally, the shoot model was applied to reconstruct the within-shoot light regime, and the potential of this technique in upscaling photosynthesis to the canopy level is discussed
Experimental analysis of flux footprint for varying stability conditions in an alpine meadow
No full textEddy-covariance measurements, which were performed in the summer 2003 in an alpine grassland site, were used to determine
the footprint of CO2 turbulent fluxes and to investigate its dependence on stability. The analysis is based on the spatial variability of carbon sinks generated by the difference in the time of the cutting of two concentric portions of the footprint, located outside and inside the fence surrounding the eddy tower, at approximately 30 m distance. Due to this time difference, turbulent flux
measurements were performed both in homogeneous (BEFORE and AFTER cutting) and heterogeneous (BETWEEN cuts) condition of spatial sink distribution. The analysis is based exclusively on daytime measurements. Half-hourly records ranked in two stability classes were used to calculate light response functions separately for three 10-day periods (BEFORE, BETWEEN and AFTER cutting). The contribution of the area inside the fence to the total flux was determined for different photosynthetic photon flux density (PPFD) values considering the BETWEEN cuts light response curve as a weighted average of the BEFORE and AFTER ones. The weight of the BEFORE cutting light response curve has been analytically determined and corresponds to the flux fraction which originates from inside the fence. The experimental estimates of the relative importance of the area inside the fence produced values ranging from 30%, during stable conditions, and up to 80% during unstable conditions. These values derived from observations were later compared with the predictions of three analytical footprint models. One of the models systematically underestimated experimental observations, while observations concurred with the other two, particularly in moderately unstable
conditions
Geometry of the hemispherical radiometric footprint over plant canopies
Full text linkRadiometric measurements of hemispherical surface reflectance and long-wave irradiance are required to quantify the broadband albedo and the outgoing thermal radiation. These observations are typically integrated with eddy covariance measurements of sensible and latent heat fluxes to characterize the surface energy budget. While the aerodynamic footprint has been widely investigated, the geometry of the hemispherical radiometric footprint over plant canopies has been rarely tackled. In the present work, the size and shape of the hemispherical radiometric footprint are formalized for a bare surface and in presence of a vegetation cover. For this purpose, four idealized canopies are analyzed and the dependency of the radiometric footprint on leaf area index and canopy height is explored. Besides, the radiometric footprint is compared with the aerodynamic footprint in conditions of neutral stability. It was observed that almost 100% of the hemispherical radiometric signal originates within a distance of a few radiometer heights, while only about 50–80% of the cumulative aerodynamic signal is generated within a distance of about 20 sensor heights. In order to achieve comparable extensions of the footprint areas, hemispherical radiometric measurements should therefore be taken about 6–15 times higher than turbulent flux ones, depending on the vegetation type. The analysis also highlights that the size of the radiative footprint decreases at increasing leaf area index, whereas the aerodynamic footprint shows an opposite behavior. For the abovementioned reasons, this work may support the interpretation of energy flux measurements and the optimal design of eddy covariance stations located in heterogeneous sites
Main determinants of forest soil respiration along an elevation/temperature gradient in the Italian Alps
No full textThe main determinants of soil respiration were investigated in 11 forest types distributed along an altitudinal and thermal gradient in the southern Italian Alps
(altitudinal range 1520 m, range in mean annual temperature 7.8 1C). Soil respiration, soil carbon content and principal stand characteristics were measured with standardized methods. Soil CO2 fluxes were measured at each site every 15–20 days with a closed dynamic system (LI-COR 6400) using soil collars from spring 2000 to spring 2002. At the same time, soil temperature at a depth of 10 cm and soil water content (m3m-3) were measured at each collar. Soil samples were collected to a depth of 30 cm and stones, root content and bulk density were determined in order to obtain reliable estimates of carbon content per unit area (kgCm-2). Soil respiration and temperature data were fitted with a simple logistic model separately for each site, so that base respiration rates and mean annual soil respiration were estimated. Then the same regression model was applied to all sites simultaneously, with each model parameter being expressed as a linear function of site variables. The general model explained about 86% of the intersite variability of soil respiration. In particular, soil mean annual temperature explained the most of the variance of the model (0.41), followed by soil temperature interquartlile range (0.24), soil carbon content (0.16) and soil water content (0.05)
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