GERARDO SANCHEZ | Universidad Autónoma de Tamaulipas (original) (raw)
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Papers by GERARDO SANCHEZ
Remote Sensing of …, Jan 1, 2004
The role of tropical forests in sustainable development mechanisms and payments for environmental... more The role of tropical forests in sustainable development mechanisms and payments for environmental services is becoming increasingly important. Therefore, there is a greater need for accurate and detailed information about their biophysical characteristics (e.g., Leaf area index -LAI) along different stages of ecological succession. Remote sensing offers the possibility of providing relatively accurate estimations of such biophysical characteristics at a reasonable cost for most regional projects. The objectives of this study are to (1) document the variability of LAI in different stages of secondary growth in a tropical moist forest, (2) estimate LAI from spectral vegetation indices (SVIs), and (3) link LAI to the estimation of other canopy physiognomic characteristics. We found that segregation of LAI measurements by successional stage (early, intermediate, late) contributed to a better definition of the relationship between LAI and the SVIs. In addition, we conclude that the propagation of errors of precision through the SVI formulas must be taken into consideration along with intra-site and radiometric variability when uncertainty terms are calculated. From a linear regression analysis, we found that there is only a minimal difference between the nonparametric Theil -Sen and classical least-squares regressions. We also found that not only does the Lorentzian cumulative transition function describe the relationship between LAI and the SVIs, it also provides an estimate of the range of LAI values to which each index is sensitive. D
Geomorphology, Jan 1, 1994
The morphology, and architecture of the tussocks of alpha grass (Stipa tenacissima) as well as it... more The morphology, and architecture of the tussocks of alpha grass (Stipa tenacissima) as well as its associated micro topography have been described in several sites on the south versant of the Filabres Range, in southern Spain. The annual rainfall of this area is between 225 and 275 mm, and plant cover ranges from 20 to 45%. The results show the formation of micro topographic mounds upslope of the tussocks which are interpreted as deposition bodies originating from the interception of downslope sediment movement by the vegetated patches. The tussock architecture shows an outer growing zone, a middle mature zone and a central senescent zone. The outward expansion of the tussock is performed by tillers in the growing zone which bend at the nodes and root when they come into contact with the soil surface. The mat of dead leaves inside the tussock may prevent the stems from bending and this factor plays a significant role in controlling tussock growth. A similar effect on slopes is produced by the mound of sediments which accumulate upslope of the tussock and press upon the growing stems. A simple simulation model has been built from the above field observations. The model is able to reproduce the tussock shapes found in nature and predicts that at low plant growth rates, the tussocks become unstable and collapse, while at higher levels of plant vigour and sediment movement rates, sedimentation occurs upslope of the tussock and erosion downslope. These foci of sedimentation and erosion are crescent shaped and their relative importance depends on the splash factor.
Remote Sensing of …, Jan 1, 2004
The role of tropical forests in sustainable development mechanisms and payments for environmental... more The role of tropical forests in sustainable development mechanisms and payments for environmental services is becoming increasingly important. Therefore, there is a greater need for accurate and detailed information about their biophysical characteristics (e.g., Leaf area index -LAI) along different stages of ecological succession. Remote sensing offers the possibility of providing relatively accurate estimations of such biophysical characteristics at a reasonable cost for most regional projects. The objectives of this study are to (1) document the variability of LAI in different stages of secondary growth in a tropical moist forest, (2) estimate LAI from spectral vegetation indices (SVIs), and (3) link LAI to the estimation of other canopy physiognomic characteristics. We found that segregation of LAI measurements by successional stage (early, intermediate, late) contributed to a better definition of the relationship between LAI and the SVIs. In addition, we conclude that the propagation of errors of precision through the SVI formulas must be taken into consideration along with intra-site and radiometric variability when uncertainty terms are calculated. From a linear regression analysis, we found that there is only a minimal difference between the nonparametric Theil -Sen and classical least-squares regressions. We also found that not only does the Lorentzian cumulative transition function describe the relationship between LAI and the SVIs, it also provides an estimate of the range of LAI values to which each index is sensitive. D
Geomorphology, Jan 1, 1994
The morphology, and architecture of the tussocks of alpha grass (Stipa tenacissima) as well as it... more The morphology, and architecture of the tussocks of alpha grass (Stipa tenacissima) as well as its associated micro topography have been described in several sites on the south versant of the Filabres Range, in southern Spain. The annual rainfall of this area is between 225 and 275 mm, and plant cover ranges from 20 to 45%. The results show the formation of micro topographic mounds upslope of the tussocks which are interpreted as deposition bodies originating from the interception of downslope sediment movement by the vegetated patches. The tussock architecture shows an outer growing zone, a middle mature zone and a central senescent zone. The outward expansion of the tussock is performed by tillers in the growing zone which bend at the nodes and root when they come into contact with the soil surface. The mat of dead leaves inside the tussock may prevent the stems from bending and this factor plays a significant role in controlling tussock growth. A similar effect on slopes is produced by the mound of sediments which accumulate upslope of the tussock and press upon the growing stems. A simple simulation model has been built from the above field observations. The model is able to reproduce the tussock shapes found in nature and predicts that at low plant growth rates, the tussocks become unstable and collapse, while at higher levels of plant vigour and sediment movement rates, sedimentation occurs upslope of the tussock and erosion downslope. These foci of sedimentation and erosion are crescent shaped and their relative importance depends on the splash factor.