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"Strahler, Alan H"
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Terrestrial Laser Scanning for Plot-Scale Forest Measurement
Plot-scale measurements have been the foundation for forest surveys and reporting for over 200 years. Through recent integration with airborne and satellite remote sensing, manual measurements of vegetation structure at the plot scale are now the basis for landscape, continental and international mapping of our forest resources. The use of terrestrial laser scanning (TLS) for plot-scale measurement was first demonstrated over a decade ago, with the intimation that these instruments could replace manual measurement methods. This has not yet been the case, despite the unparalleled structural information that TLS can capture. For TLS to reach its full potential, these instruments cannot be viewed as a logical progression of existing plot-based measurement. TLS must be viewed as a disruptive technology that requires a rethink of vegetation surveys and their application across a wide range of disciplines. We review the development of TLS as a plot-scale measurement tool, including the evolution of both instrument hardware and key data processing methodologies. We highlight two broad data modelling approaches of gap probability and geometrical modelling and the basic theory that underpins these. Finally, we discuss the future prospects for increasing the utilisation of TLS for plot-scale forest assessment and forest monitoring.
Journal Article
Assessing general relationships between aboveground biomass and vegetation structure parameters for improved carbon estimate from lidar remote sensing
Lidar‐based aboveground biomass is derived based on the empirical relationship between lidar‐measured vegetation height and aboveground biomass, often leading to large uncertainties of aboveground biomass estimates at large scales. This study investigates whether the use of any additional lidar‐derived vegetation structure parameters besides height improves aboveground biomass estimation. The analysis uses data collected in the field and with the Laser Vegetation Imaging Sensor (LVIS), and the Echidna® validation instrument (EVI), a ground‐based hemispherical‐scanning lidar data in New England in 2003 and 2007. Our field data analysis shows that using wood volume (approximated by the product of basal area and top 10% tree height) and vegetation type (conifer/softwood or deciduous/hardwood forests, providing wood density) has the potential to improve aboveground biomass estimates at large scales. This result is comparable to previous individual‐tree based analyses. Our LVIS data analysis indicates that structure parameters that combine height and gap fraction, such as RH100*cover and RH50*cover, are closely related to wood volume and thus biomass particularly for conifer forests. RH100*cover and RH50*cover perform similarly or even better than RH50, a good biomass predictor found in previous study. This study shows that the use of structure parameters that combine height and gap fraction (rather than height alone) improves the aboveground biomass estimate, and that the fusion of lidar and optical remote sensing (to provide vegetation type) will provide better aboveground biomass estimates than using lidar alone. Our ground lidar analysis shows that EVI provides good estimates of wood volume, and thus accurate estimates of aboveground biomass particularly at the stand level.
Journal Article
Radiometric Calibration of a Dual-Wavelength, Full-Waveform Terrestrial Lidar
Radiometric calibration of the Dual-Wavelength Echidna® Lidar (DWEL), a full-waveform terrestrial laser scanner with two simultaneously-pulsing infrared lasers at 1064 nm and 1548 nm, provides accurate dual-wavelength apparent reflectance (ρapp), a physically-defined value that is related to the radiative and structural characteristics of scanned targets and independent of range and instrument optics and electronics. The errors of ρapp are 8.1% for 1064 nm and 6.4% for 1548 nm. A sensitivity analysis shows that ρapp error is dominated by range errors at near ranges, but by lidar intensity errors at far ranges. Our semi-empirical model for radiometric calibration combines a generalized logistic function to explicitly model telescopic effects due to defocusing of return signals at near range with a negative exponential function to model the fall-off of return intensity with range. Accurate values of ρapp from the radiometric calibration improve the quantification of vegetation structure, facilitate the comparison and coupling of lidar datasets from different instruments, campaigns or wavelengths and advance the utilization of bi- and multi-spectral information added to 3D scans by novel spectral lidars.
Journal Article
Evaluation of Moderate Resolution Imaging Spectroradiometer land surface visible and shortwave albedo products at FLUXNET sites
We evaluated the accuracy of Moderate Resolution Imaging Spectroradiometer (MODIS) Collections 4 and 5 shortwave and visible albedo products at 18 FLUXNET sites globally from 2000 to 2007. While the shortwave product has been evaluated at a number of field sites, this represents the first attempt to assess the visible product. The surface visible albedo product relies more on quality of the atmospheric correction because atmospheric molecular scattering and aerosol absorption and scattering have larger impact on visible radiation than that on infrared radiation. The results show that in general the MODIS Collection 4 visible albedo product has an overall negative bias of −0.015, with a standard deviation (SD) error of 0.014 and a correlation coefficient of 0.88. The numbers are −0.010, 0.015, and 0.87 for Collection 5 visible albedo products. MODIS Collection 4 shortwave albedo product has an overall negative bias of −0.009, with a SD error of 0.023 and a correlation coefficient of 0.82. The numbers are −0.008, 0.023, and 0.80 for Collection 5 shortwave albedo products. A comparison of the site−averaged albedos shows that Collection 4 visible (shortwave) albedo has a SD of 0.009 (0.017) and a correlation coefficient of 0.94 (0.90), similar to the Collection 5 SD of 0.008 (0.017) and the correlation coefficient of 0.95 (0.89).
Journal Article
Declining insolation induces synchronous flowering of Montanoa and Simsia (Asteraceae) between Mexico and the Equator
We analyze the latitudinal shift in the onset of synchronous flowering in the woody genera Montanoa and Simsia (Asteraceae) between Mexico (28° N) and the Equator, where it cannot be caused by declining day length. Synchronous flowering of >100 Montanoa quadrangularis trees was observed during two consecutive years near Cali, Colombia (4° N). Analysis of herbarium specimens yielded flowering periods for 21 Montanoa species and 18 Simsia species between 4 and 28° N. Daily insolation is a function of day length and the angle at which the sun's rays strike the earth. Between Mexico and Colombia (4° N), the maximum of insolation gradually shifts from the summer solstice to the autumn equinox. In parallel, flowering of Montanoa and Simsia starts progressively later between July and November, during the period of declining insolation. Near the Equator, there are two periods of declining insolation, and correspondingly, two flowering periods. Thus, at all tropical latitudes, flowering time of Montanoa and Simsia is highly correlated with declining insolation. The seasonal decline in daily insolation, rather than in photoperiod, apparently induces synchronous flowering of Montanoa and Simsia at the same time each year.
Journal Article
Remote estimation of crown size, stand density, and biomass on the Oregon transect
In this paper, we invert a canopy reflectance model using multispectral satellite data in order to estimate remotely the parameters of crown size, tree count density, cover, foliage biomass, and total standing biomass for nine coniferous forest stands on a transect of western and central Oregon (USA). Issues involved in the inversion of the model are topographic correction, component signature estimation, spatial pattern analysis, and direct biomass estimation. Retrieved parameters correlate well with observed values from ground measurements of the test sites. The model inversion technique is a useful tool for mapping and timber inventory of forests at large scales.
Journal Article